Comparatively, CRI utilizes 15 test color samples (TCS) which do not represent the actual colors in the real world, and the general color rendering index only calculates the arithmetic mean value on the first 8 TCS.
The fidelity index Rf has been introduced to the Technical Report CIE 015:2018 and it is aiming to be a new standard for color evaluation. Similar as color rendering index, Rf is an accurate measure of average color fidelity and has a range of 0 to 100, with higher numbers indicating more similarity to the reference.
In addition to an improved color fidelity metric, the TM-30 method introduces an additional dimension to color quality through the gamut index (Rg). The Rg gamut index provides information about the relative range of colors that can be produced (via reflection) by a white light source. A score close to 100 indicates that, on average, the light source reproduces colors with similar levels of saturation as an incandescent bulb (2700K) or daylight (5600K/6500K). For LEDs of decent color quality, Rg can typically range between 80 and 120, with higher scores representing higher overall levels of saturation.
The color vector graphics are very intuitive and informative tools in informing us about a color source’s tendencies to reveal certain colors as appearing more vivid while others dull. For applications such as task lighting where color accuracy is required, a smooth, evenly distributed vector chart would be desired. On the other hand, for certain applications such as retail, over-saturation in certain colors may even be preferred in order to increase its vividness. YUJILEDS provides wide ranges of phosphors that allow for manufacture of LEDs with customized SPDs that can increase saturation in particular color.
The pure radical shift in the vectors is quantified in a series of 16 measures referred to as local chroma shift, with each value corresponding to one of the hue-angle bins.
Considering television lighting must have a minimum quality standard in order to satisfy the audience, the international exchange of programs and the archive, TLCI (The Television Lighting Consistency Index ) was developed by the EBU (European Broadcasting Union) Technical Committee in November 2012 in order to give technical aid to broadcasters who intend to assess new lighting equipment or to re-assess the colorimetric quality of lighting in their television production environment.
Knowing its performance limitations in advance can help in the choice of luminaire, identifying the potential extra cost of color correction in post-production as against the cost saving in power consumption of high-efficiency luminaires. Rather than assess the performance of a luminaire directly, as is done in the Color Rendering Index, the TLCI mimics a complete television camera and display, using only those specific features of cameras and displays which affect color performance. The TLCI is realized in practice using software rather than real television hardware. The only hardware that is required is a spectroradiometer to measure the spectral power distribution of the test luminaire, and a computer on which to run the software analysis program to perform the calculations. This mathematical calculation implemented software is the “TLCI-2012” which is specified in EBU Tech 3355 and available from the EBU as ‘TLCI-2012.zip’.
The Television Lighting Consistency Index averages 24 colors from the Macbeth ColorChecker chart: Dark Skin, Light Skin, Blue Sky, Foliage, Blue Flower, Bluish Green, Orange, Purplish Blue, Moderate Red, Purple, Yellow Green, Orange Yellow, Blue, Green, Red, Yellow, Magenta, Cyan, White, Neutral 8, Neutral 6.5, Neutral 5, Neutral 3.5, Black.
The maximum value is 100, with 100 being the best
The TLCI metric (Television Lighting Consistency Index) was developed as an alternative to address the shortcomings of the commonly used CRI when used with photography equipment. Because the way cameras interpret light is slightly different to how humans perceive it, the TLCI attempts to provide a more accurate prediction of color fidelity of a light source for broadcast use.CRI tells us about how a light source renders colors based on human perception. CRI, however, does not necessarily paint a complete picture about how the light source might render colors on camera or on film. Thus, a light source with a high CRI is not necessarily a light source fit for use in film and photography. To this end, the TLCI (Television Lighting Consistency Index) was developed. The metric is analogous to CRI in that a high TLCI metric tells us that a light source will render colors well in a film or photography context.
LED phosphors are a crucial material in the manufacture of white LEDs. Phosphor chemistry and composition largely determine an LED’s efficiency, light quality and stability. At our subsidiary Nakamura Yuji, a joint venture with investment from Mitsubishi Chemical Corporation, we understand phosphor technology at a fundamental chemical level, allowing us to pursue the best chemical formulations and manufacturing processes that result in the highest quality. Yuji is one of the very few LED companies in the world that has entire LED phosphors and packaging technologies. Multi-variety LED phosphors are the basis for YUJILEDS spectrum simulation.
Despite a seemingly ubiquitous LED market, high brightness and stability are difficult to come by, particularly for high CRI LED products. Our consistent focus on improving the brightness and stability of red nitride phosphors has allowed us to become an industry leader in providing high CRI LEDs at efficiency levels and stability in line with standard CRI level LEDs on the market.
UV phosphor can be effectively excited by a UV chip with an emission peak wavelength in the 250-350nm range. The main application of UV phosphor is to replace 260nm low pressure mercury lamp, to realize sterilization and skin disease treatment.
Phosphate LED phosphor can be effectively excited by a 400nm violet LED chip with an emission peak wavelength in the 445-475nm range. Combined with β-SiAlON green phosphor and nitride red phosphor, a full spectrum white light with R1-R15 all above 90 can be achieved.
Yttrium Aluminum Garnet LED phosphor is efficient and suitable for creating high correlated color temperature (CCT). Absorption peaks at 450 nm, and dominant emission wavelengths range from 540-560nm.
YAG LED phosphors can be effectively excited by a 450nm blue LED chip with an emission peak wavelength in the 540-560nm range. It is mainly used for increasing luminous efficiency. By adding a small amount of YAG yellow phosphor to an Ra80 LED, the luminous flux will increase dramatically.
Lutetium Aluminum Garnet LED phosphor offers comparable performance to YAG phosphors. We offer LuAG phosphor with dominant emission wavelengths ranging from 520nm to 540 nm. It is generally used in conjunction with red phosphors for high CRI full spectrum coverage. LuAG LED phosphor can be effectively excited by a 450nm blue LED chip with an emission peak wavelength in the 510-540nm range. Combined with nitride red phosphor, a high CRI spectrum with Ra above 95 can be achieved.
Nitride LED phosphor is crucial for high CRI and low correlated color temperature (CCT) white LEDs. Dominant emission wavelengths range from 605 nm to 660 nm. It can also be applied in plant growth light LEDs where wide and stable red spectrum coverage is required. nitride red LED phosphor can be effectively excited by 400nm-460nm violet LED or blue LED with an emission peak wavelength in the 600-660nm range. When combined with blue LED chip and aluminosilicate green phosphor, it can produce a white LED light with Ra 95 or above. With violet LED chip, phosphate blue phosphor and β-SiAlON green phosphor, a full spectrum of white light with R1-R15 all above 95 can be achieved.
LED Oxynitride phosphor can be used to package high CRI LEDs which require full spectrum coverage. Dominant emission wavelengths range from 500nm to 650nm.
Nitride green LED phosphor can be effectively excited by a 400-460nm violet or blue LED chip, emitting in the range of peak wavelength 525-545nm, full width at half maxima (FWHM) < 60nm with high color purity. Excited by a blue LED chip, combined with β-SiAlON green phosphor in a backlight, NTSC can be improved to above 100%. Excited by a violet LED chip, combined with phosphate blue phosphor and nitride red phosphor, a full spectrum of white light with R1-R15 all above 90 can be achieved.
KSF LED phosphor can be effectively excited by a 460nm blue LED chip with a strongest emission peak wavelength at near 631nm, full width at half maxima (FWHM) < 60nm with comparatively high color purity. Combined with β-SiAlON green phosphor for a backlight, NTSC can be improved to above 100%.
Infrared LED phosphor can be excited by a 450-460nm blue LED chip with an emission peak in the 710-730nm range. It can be used in plant growth light. Combined with blue light (460nm) + red light (660nm) + near-infrared light (730 nm), the plant growth, especially the radial growth can be regulated and improved.
These products, also referred to as RGB, RGBA, RGBW, spectrally tunable, or color changing, usually have three or more different LED primaries that can be individually varied in output to create a mixture of light that is white, a tint of white, or a saturated hue. The individual LEDs used in a full-color-tuning mixture can be very narrow band LEDs (producing a narrow range of blue or red, for example), or also monochromatic but with phosphor coatings that produce a slightly wider spread of color (e.g., a “mint” green LED is a phosphor-coated blue) or white PC LEDs (W) produced by phosphor-coating a blue- or violet-pump LED. Usually the different monochromatic LED colors include red, green, and blue (RGB, the primary colors of light), but these can be augmented with amber (A), one or more white PC LEDs (W), and other monochromatic colors. The minimum number of LED colors is three for full-color tuning, but four-, five-, and seven-color systems are also on the architectural lighting market, and some sophisticated color systems use even more unique colors of individual LEDs.
Ne unique advantage of this type of color-tuning is the ability to move the color point off the blackbody locus or, put more simply, to move beyond different CCTs of white light toward light with a distinct color. For example, such a product could provide 4000K light in an office during the day and then be tuned for a purple-themed party in the evening. This makes full-color-tunable products well-suited for such applications as theaters, theme parks, and restaurants.
Another advantage of full-color tuning is the ability to match the chromaticity of any other light source. Light from fluorescent lamps, for example, is difficult to match with LEDs, because “3500 K” can be created by dozens of different combinations of spectra, and the chromaticity can appear distinctly green or pink while still legitimately calculating to 3500 K. The only way to closely match the chromaticity of a light source is by manipulating the output of individual LEDs.
White matching the color rendering of different sources can be even more difficult, controlling the colors of individual LEDs introduces the option of tuning the spectrum to enhance colors for retail applications – for example, making a floral arrangement really “pop” in appearance.
The wide variability of full-color-tuning requires a user interface that is more complicated than a simple slide dimmer. A control protocol such as DMX, DALI, or wireless with high resolution is required, and the luminaire must be powered separately from the intensity and color control signals.
White-tunable products require a minimum of two independent LED primaries, with the most basic configuration being a mix of warm-white and cool-white phosphor-coated (PC) LEDs. The ratio of the two can be adjusted to mix the light to CCTs anywhere in between the minimum and maximum CCT. Mixing only two LED primaries results in a linear range of chromaticity; therefore the nomenclature linear white tuning is used in this report. However, the blackbody locus, which serves as a reference for CCT calculations, is not linear in a chromaticity diagram. Accordingly, two-primary white-tunable products will not follow the blackbody locus (i.e., will not have the same Duv) throughout their color range; instead, they may take on a purple/pink tint in the middle of the available range (Figure 1). This deviation from the blackbody becomes larger with a wider range of possible CCTs, although it may or may not be noticeable or objectionable.
Other types of white-tunable luminaires combine more LED primaries, which allows more flexibility for changing color. All of the products tested for this investigation with more than two primaries attempted to follow the blackbody locus, giving rise to the classification nonlinear white tuning or blackbody white tuning (also shown in Figure 1). One approach seen in this round of testing was the combination of two white LEDs (warm-white and cool-white) with a red LED. Other products used three, four, or five independent LED primaries and preprogrammed calibrations/control-response algorithms.
White-tunable and full-color-tunable products vary in their type of control, generally using 0–10 V, DMX, or DALI protocols. While each method allows the user to adjust the color and/or output of the product, they can be implemented in a number of ways. Some manufacturers provide proprietary control devices, which often rely on an existing protocol but provide a customized user interface/hardware. Other color-tunable lamps and luminaires rely on controls from third-party manufacturers, which provide a greater range of options but may also lead to compatibility issues.
The products tested for this investigation were controlled using DMX software, 0-10 V “dimmers”, or a proprietary control device. This provided variety, but the exact type of control system was not a focus, as long as the applicable range of output could be achieved. Nonetheless, the control interface is an important aspect of implementation for color-tunable lighting systems, and may ultimately play a large role in their acceptance by end users.
Beyond the user experience, the control system used may have some effect on the performance of the luminaire. For example, some LED drivers expect either a linear or a logarithmic signal over the dimming range, and performance can vary if the appropriate signal is not provided. For this report, all products were tested on an appropriate control, but the investigation did not test a given luminaire with multiple controls.
]]>The advent of SSL technology has already brought substantial change to the lighting industry, and the evolution of products is ongoing. One recent intriguing development is color-tunable luminaires. Although versions of this product type have been around for years, LEDs make color-tunable luminaires much more practical, even though they remain a niche market segment. With potential benefits including improved health and wellbeing, increased productivity, enhanced mood or alertness, and higher occupant satisfaction, there is reason to believe that color-tunable luminaires will gain market share. At this point, however, it is important to understand the tradeoffs, limitations, and issues, so that the industry can work together to maximize the rate of product maturation.
In order to have a best performance when using white tunable LED products, we need to understand the different kinds of white tunable LED products. Include the available types of color tunable products and the types of control for color tunable products.
Before an effective use of the right kinds of color-turntable product, it is important to consider the types of color-tunable products that are available, and how they might be classified. The following is a brief description of three distinguishable product classes, with additional detail on what is and is not covered in this article:
These products, also referred to as warm dim, blackbody dimming or incandescent-like dimming, mimic incandescent or halogen dimming performance, usually designed for 2700-3000K at full output with a decrease in correlated color temperature (CCT) as the output is reduced, down to as low as 1800K (the color of candlelight). As with incandescent lamps, the light color becomes increasingly warm in appearance (i.e., more yellow and red) as the product dims.
The light color and dimming quality of incandescent / halogen products are prized in such settings as restaurants, hotel lobbies and guestrooms, ballrooms, theaters, and residential spaces.
Because the dimming of this type of product is linked to the color change, there is only one control signal and thus one controller per group of luminaires that dim together. Some systems can achieve this function with a phase-cut dimmer, in which the dimming information is carried in the voltage waveform, but this approach may not have as much dimming resolution or smoothness as a control system using 0-10V, DALI, or DMX protocols. The latter three require separate wiring for the intensity / color signal and luminaire power. Alternatively, dim-to-warm luminaires can be equipped with a wireless receiver for control by a wireless transmitter using Zigbee, Wifi, Bluetooth, or another protocol and hard-wired to building power.
Some white-tunable products, also called tunable white or Kelvin changing, have two sets of controllable phosphor-coated (PC) LEDs: one with a warm-white color (usually around 2700K) and the second with a cool-white color (usually 5000K to 6500K). By individually raising and lowering the output of the two colored “LED primaries”, white colors between the two color points can be created along the straight line that connects them on a chromaticity diagram (this is called linear white tuning). When only two white PC LEDs are selected, the manufacturer must choose where the mixed colors of white will lie, relative to the blackbody curve. Will the color of white light appear pinkish or purplish as it tracks from one CCT to the next? The blackbody line is curved, so two colors of white cannot track along the blackbody and, the wider the range of CCTs, the greater the maximum deviation from the blackbody (Duv). There are also white-tunable products that use three or more LED primaries, in which case they may have the capability to produce a wider range of colors than just different CCTs of white. However, such products may operate in a mode that allows only color change along the blackbody locus. The advantage of white-tuning products with three or more LED primaries is that they track the curve of the blackbody (called nonlinear white tuning). Some of these products closely track the blackbody curve throughout their tuning range (i.e. their Duv values can be very small), meaning they will not appear green or pink compared to a reference light source whose chromaticity falls right on the blackbody curve.
White tuning allows for changing the color of light from warm to neutral to cool in appearance, which may be desirable for a range of reasons, from aesthetic to medical. Such tuning of white light can be used to:
Provide apparent cooling or warming to a room. This can create psychological effects – for example, by using cooler-colored light to make occupants feel cooler on a sweltering summer day, or the reverse.
Match room finishes, especially when they change on a seasonal basis, or when a space undergoes an interior-design remodeling or a branding / theme change.
Suit the preferences of a new tenant, owner, or user. Simulate daylight or candlelight to set a mood, or match gallery lighting to the works of art on display. Match the color of daylight in a windowed lobby by, for example, tuning the light to be warmer during early morning and late afternoon and cooler at mid-day.
Assist with behavior control. Some classroom studies suggest that the color and intensity of light can be modified to calm or invigorate students, or to focus their attention.
Support the human circadian system. Light plays a key role in setting and regulating the body’s biological clock. Both the intensity and the spectral content of light can be used to stimulate or suppress the secretion of melatonin and other hormones that in turn affect our mood, alertness, and health. Although the exact mechanisms and effects are not yet fully understood, this may be an important consideration for industrial and medical spaces as well as senior- living facilities, prisons, dormitories, and high-density housing.
Correct circadian misalignment. Varying the light color and intensity may be used by medical professionals to treat jet lag, sleep disorders, and other conditions.
Controls are a critical element in white-tuning systems. The most successful products have clever algorithms for tuning from one CCT to another without doubling light levels in the middle ranges. These algorithms are built in to the driver or an interface between the controller and the driver. A white-tuning system requires separate power for the LED driver and for the control signal, so it’s common to see a dimmer for each control signal: one for power and intensity level, and a second for color. The two separate controls can also be combined into a user interface that has separate dials, sliders, or buttons for controlling color and intensity independently. The most common control protocols are 0-10V, DMX, DALI, a proprietary control protocol, or a system where the luminaire is powered through the building circuits but the luminaire is equipped with a wireless (or power-line-carrier) receiver that interprets wireless Zigbee, Wifi, Bluetooth, or other protocol signals. Each of these dimming protocols has its advantages and disadvantages.
]]>MDER is the abbreviation of Melanopic Daylight Efficacy Ratio, which is defined by the CIE and introduced in the CIE S 026/E:2018 document. Also called the Melanopic DER, this metric is invented for evaluating the circadian effect compared to natural light.
The MDER is a mathematical calculation and the formula of is complicated, however, we can simplify it for easier understanding.
Generally, the MDER can be considered as the circadian effect comparison between your artificial light and the natural light, the higher the MDER result is, the more circadian effect your light is.
After research, it is known that the work efficiency is related to the sunlight at different times of a day. People are less productive in the early morning and dusk, but are more productive in midday, this phenomenon is called the circadian effect. There is a metric called M/P ratio for evaluating the circadian effect, the higher the M/P ratio is, the more stimulatory effect will be, and the more excitement or focus you will gain.
Natural light is generally recognized as the sunlight, but to evaluate the circadian effect quantificationally, we should choose a standard and consistent reference, hence, the CIE D65 (Daylight 6500K) is selected to be this reference. (Why choose D65?)
During the use of the M/P ratio, it is found that it lacks an objective comparison. The M/P ratio cannot intuitively show the actual impact of light on the biological clock. Even if a person is skilled in this art, like a lighting designer, cannot use the M/P ratio to quickly design indoor lighting that matches the biological clock. The MDER is to solve this by applying the ratio of circadian effect which helps with designing the lighting simply and intuitively.
This means the circadian effect of your lights is equivalent to the real sunlight. It is an ideal situation, but difficult to achieve by the common CCTs for general lighting.
This means you need more lights to achieve an equivalent circadian effect to the sunlight, but there might be challenges:
Generally speaking, the various CCTs of common LED will not be greater than 1 unless significantly increasing the color temperature, but a high CCT will always cause uncomfortableness and increase the risk of high blue light hazards. After calculation, for the standard LED technology, the CCT should be higher than 7500K to achieve the MEDR > 1, but 7500K is much higher than 4000K which is widely accepted and used in the workplace.
The MDER is only for evaluating the work or study environment considering the concentrating effect, therefore it is not related to the design for sleep or relaxed lighting, after understanding this, there are the following steps to utilize the MDER metric during the lighting design:
During the lighting design for a specific space, the designers always need to calculate and confirm the lux demand. For example, the illuminance in the office should be >500 lux according to the CIE 008/E-2001, here we take this 500 lux as the calculation factor for the next steps.
The circadian effect lux you need =
Assuming the MDER value of the light is 0.8, the calculated lux is >500, then the circadian effect lux you need is 500/0.8 = 625 lux, therefore, the actual visual lux you need is at least 625 lux than 500 lux.
With the calculated 625 lux and the necessary photometric data (IES/LDT file), designers can figure out how many lights should be used in the space. And the people who live in this space will benefit from the specific circadian effect design to help them concentrate to be more productive.
A high MDER light can simplify the lighting design and keep a balance between the circadian effect and lux. We can simulate a comparison below when the CCT and circadian effect lux are the same, different MDER values mean different lights.
MDER value |
The lights you will need |
0.5 |
2 lights |
0.8 |
1.25 lights |
1.0 |
1 light |
The key factor to generate the circadian effect for concentrating is the enhanced 480nm lighting output in the spectrum. It is found in the 2000s that the energy around 480nm determines the secretion of the melatonin and the melatonin determines our mental condition for focusing or relaxing. It is from a third photoreceptor in addition to rods and cones that “sees” differently compared to human eyes.
The Yuji Lighting Well24™ Day technology increases the intensity of light near 480nm significantly than the common LEDs on the market, restraining the secretion of the melatonin effectively to make our minds clearer and more focused.
The tailor-made spectrum increases the 480nm and reduces the 450-460nm energy which is recognized as the main wavelength of blue hazard, therefore the Yuji Lighting Well24 Day™ series is not only helping with concentrating, but improving the healthy lighting simultaneously.
The tailor-made spectrum makes the MDER of Yuji Lighting Well24™ Day series can reach 0.967 or even higher whereas a standard LED is only 0.756 when they are the same CCT. Hence, it also saves the number of lights you will need but can reach the light intensity required by the human biological clock.
With high MDER, the Yuji Lighting Well24™ Day can save 30% cost on the number of lights and maintain the same circadian effect, and it is not only about the energy and cost saving, but also meaningful in avoiding unnecessary lux to cause the uncomfortableness. The Yuji Lighting Well24™ Day technology can help with concentrating but just enough lux and appropriate lighting cost.
]]>Blue light is a kind of high-energy short wave visible light (HEV), which refers to the blue part of visible light. Its harm is mainly concentrated in the 380 nm ~ 450 nm band. For people who often engage in outdoor activities, the main source of blue light is sunlight, while indoor man-made sources include fluorescent lamps, LED lamps and display screens of electronic equipment. The 450 nm ~ 500 nm blue light can bring us certain benefits. An appropriate amount of blue light can also let us enjoy health and vitality. But going beyond the limit is as bad as falling short, too much blue light will bring some danger. Excessive blue light radiation can cause oxidative stress response of retinal cells, cause irreversible photochemical damage and aggravate cell damage in macula. This is what we usually call "blue light hazard".
The standard of blue light hazard can be judged according to the optical biosafety test standard IEC 62471. IEC 62471 is a comprehensive parallel standard that describes all potential health hazards, including the ultraviolet part of the spectrum to the visible and infrared parts. According to the retinal blue light hazard exposure limit defined in iec62471, the size of blue light injury is mainly determined by the blue light weighted irradiance, the blue light weighted irradiance and the duration of exposure to blue light, and the blue light weighted irradiance is equal to the value of multiplying the spectral irradiance of the blue light band by the blue light hazard weighting function, and then integrating the wavelength:
In order to prevent retinal photochemical damage caused by long-term blue light radiation, the maximum allowable irradiation time:
T(max) = 1000000/LB(s) (t<<10000s)
The greater the weighted radiance of blue light, the less the maximum irradiation time allowed.
Due to the length of blue light wave, if you want to see objects clearly under the stimulation of blue light, your eyes will be in a tense state for a long time, causing visual fatigue and inability to concentrate, which directly affects our study and work efficiency.
Excessive blue light will lead to the decline of retinal epithelial pigment cells, cell death in light sensitive areas, macular disease, fundus disease, vision loss and even blindness.
Blue light stimulates the brain, inhibits melatonin secretion and improves the production of adrenocortical hormone, so as to regulate heart rate, alertness, sleep, body temperature and gene expression, disrupt human physiological rhythm, and the consequence is naturally unable to sleep at night and get up in the morning. It also increases the risk of diseases, especially breast cancer and colorectal cancer.
According to scientific research, the retina of mammalian eyes includes three types of photoreceptors: cone cells that are responsible for seeing during the day, rod cells that are responsible for seeing at night, and endogenous light sensitive retinal ganglion cells (ipRGCs) that are not responsible for seeing. These ipRGCs are not idle and have powerful functions. They sense the blue-green light components (460-550 nm) in the light source, are responsible for reporting the brightness of light to the brain, regulating the circadian rhythm and pupil contraction, and affecting our sleep, mood and learning ability. This is why blind people can also feel day and night changes, and there is also pupil reflex to light.
460-500 nm blue light is beneficial! Blue light can stimulate the spirit and reduce depression, so it is used to treat seasonal mood disorders (winter depression). Blue light can improve sleep quality. With the growth of age, the lens of the eye turns yellow, the absorbed blue light increases, the blue light entering the eye during the day decreases, and the secretion of melatonin is affected. Therefore, the elderly are prone to sleep disorders.
Blue light is an indispensable element of natural light and white light lighting. Since the birth of human beings, they have been living under the natural light with blue light. What we should do is to understand the characteristics of blue light, so as to effectively avoid harm without affecting our full use of it.
Based on the Yuji Lighting SunWave™ technology, we can utilize the blue light adequately but avoid the hazard efficiently. The tailor-made spectrum increases the 480nm and reduces the 450-460nm energy which is recognized as the main wavelength of blue hazard, therefore the Yuji Lighting SunWave™ series is not only helping with concentrating, but improving the healthy lighting simultaneously.
As shown in the spectral power distribution above, SunWave™ series are designed at the spectral level to limit disruption to restful sleep by utilizing a proprietary blend of long wavelength phosphors. The result is a spectral distribution that produces natural light, but simultaneously limits the impact on melatonin suppression. With a 2700K color temperature and 98 CRI rating, objects appear natural and vivid.
]]>Do you sometimes find it difficult to tell a deep navy blue and black apart under a dim light? You thought maybe you need to change the light to a brighter one. No, it is not about the brightness. It is probably just about the CRI.
In general terms, CRI is a measure of a light source's ability to show object colors "realistically" or "naturally" compared to a familiar reference source, either incandescent light or sunlight.
High-CRI LED lighting is a light-emitting diode (LED) lighting source that offers a high color rendering index (CRI).
If you have been browsing lighting and light fixtures, you may have come across the acronym CRI, usually accompanied by a number. CRI has a maximum score of 100. The more a light source gets closer to 100, the more it shows the realistic and true colors of an object.
On a general scale, CRI values that fall between 0 and 55 are considered poor and damaging to the eyes. Values between 60 and 85 are considered good and can be used for everyday activities. However, values that fall within the range of 90 to 100 are excellent light sources.
Testing for CRI requires special machinery designed specifically for this purpose. During this test, the light spectrum of a lamp is analyzed onto 15 different colors (or “R values”), termed R1 through R15.
There are 15 measurements which can be seen below, but when we talked about CRI, for most circumstances, we refer in particular to CRI(Ra), which only uses the first 8 colors. Of course, R9-R15 are also important, see Why Understanding CRI(Re), Matters to You?
The lamp receives a score from 0-100 for each color, based on how natural the color is rendered in comparison with how the color looks under a “perfect” or “reference” light source (either incandescent light or sunlight) at the same color temperature (CCT) as that lamp.
For example,
R1 |
R2 |
R3 |
R4 |
R5 |
R6 |
R7 |
R8 |
R9 |
R10 |
R11 |
R12 |
R13 |
R14 |
R15 |
99 |
98 |
92 |
97 |
99 |
96 |
97 |
98 |
100 |
92 |
97 |
89 |
99 |
95 |
99 |
Ra(R1~R8) = (R1+R2+…+R8)/8=97
Re(R1~R15) = (R1+R2+…+R15)/15=96
In ancient times, people didn’t have lights. Their behaviors were based on the sun’s ups and downs. The sunlight will tell you when to work and when to sleep. Then, lights were created and modeled on what the sun was doing. (HCL) So, improving the artificial lighting as close to the sunlight is the eternal pursuit for the top LED manufacturers like YUJI. That is why the CRI is so important. The higher CRI the light is, the closer it is to the sunlight.
The major qualities to look for when it comes to choosing the right lighting for your home: the brightness, quality, and safety. So much has to be weighed when choosing the perfect lighting. Color Rendering Index (CRI) is another big one, and it has a significant impact on especially artists, filmmakers, and crafters.
Whether you’re an artistic person or not, our eyes are sensitive to light quality and color. A red shirt lit directly with noontime sunlight will render much different than lit under a fluorescent bathroom light. The High CRI of our LEDs will help identify objects. Imagine you are playing with the puzzles under a low CRI light source, and you will probably go mad when you are trying to distinguish between similar colors. It may also affect your eyesight when you're doing certain tasks like reading a book, watching television or preparing food in the kitchen.
While all of our strips have 95+ CRI ratings, our VTC (Very True Color) strips boast 98+ CRI ratings. These lights have important applications especially for photography, retail display and grocery lighting, where accurate color presentation is key. High CRI lighting is equally valuable for in-home use, as it can transform a room by highlighting design details and creating a comfortable, natural overall feel.
Students already have to worry about protecting their eyes because of the heavy duty in paper works. Constant use of bright light without protection causes retinal damage, which is a major source of concern for their parents.
Lamps sources with high CRI, like YUJILEDS Full Spectrum Lightings , can differentiate close colors from each other perfectly, so they don't need to strain their eyes further to separate close colors from each other.
True to color is key to helping people produce great work. This is why they need lighting that renders colors properly while protecting their eyes.
The best thing about LED lights with excellent, high CRI ratings is that they don’t cost much more than those with standard good CRI ratings. Considering the benefits of natural color rendition, it can pay in many ways to spend a little more for a higher CRI.
With so many benefits, it’s easy to see why you should take notice of CRI when making your next lighting decision.
Obviously, CRI is not the stand-alone method for measuring lighting color quality, we also recommend the combined use of TM-30 Gamut Area Index (Will TM-30 Replace CRI Metric?).
]]>In order to understand this, we process relevant experiments and calculations for analyzing the reason behind this requirement to figure out if it is meaningful.
Conclusions –
First, it is necessary to clarify and emphasize some basic concepts which will be mentioned and used.
CRI evaluates the ability of the light source that renders the original color of an object, with a full score of 100 (Ra), this metric picks Munsell test color samples, taking different illuminants as reference sources and comparing the chromatic aberration result under the test measured and reference light sources. The more aberration the lower color rendering scores. Taking the arithmetic mean value of the first 8 test color samples, we get the Ra as the general color rendering index, taking the 15 test color samples we get the extended color rendering index. In this calculation, tristimulus which is for the human eye’s adaption is also involved, CRI is a kind of visual perception.
Figure 1. 15 Munsell test color samples for CRI calculation.
Currently, on the market, we can find different CRI grade (Ra) LED products from generally 80 to 98. For the foreshadowing of the following content, we will talk a bit more regarding the method to improve CRI.
At the spectral level, there could be different recipes of phosphor combination to increase the CRI score, but the most effective and practical way is to enhance the long wave of the red spectrum. Figure 2 shows the comparison between Ra 80 and Ra 98, taking 3000K and 6000K spectra as examples. It can be concluded that shifting the red spectrum helps with increasing the CRI significantly.
Figure 2. Spectra of different CRI.
LED is famous for its remarkable energy-saving features compared to conventional light sources, and this also helps to win a Noble Prize. Energy-saving, from another perspective, means efficient. There are different measurements for efficiency and the luminous efficacy, which is a visual perception, is measured based on photopic vision with peak sensitivity at 555nm (Figure 3).
Figure 3. Human visual sensitivity.
According to the theory of calculating luminous flux, when other conditions are the same, the more the spectrum overlaps the photopic vision, the higher luminous efficacy can be obtained. This theory also answers that generally, high CRI LED is always lower luminous efficacy relatively.
Wall-plug efficiency evaluates the efficiency of converting electrical to optical power. Unlike to CRI, the WPE calculates the physical characteristic without any addition of visual perceptions. Understanding and analyzing WPE also help with calculating how to find a suitable heatsink.
Figure 4. The energy conversion process when LED works.
Photosynthetically active radiation defines the wavelength of 400nm - 700nm within which the photosynthesis corresponds most sensitively. It is similar to the human eye’s visual range, but the sensitivities are quite different (Figure 5).
Figure 5. Photosynthetic sensitivity.
The photosynthetic photon flux is the calculation based on the PAR, and essentially it is the photon generated by the light source in every second, measured with micromole (µmol), and the calculation involves the Avogadro's constant (Na = 6.022 × 1023), Planckian constant (h = 4.14 × 10-15 eV∙s), light speed (c = 3 × 108 m/s) and the spectral power distribution (discrete in a certain of nanometer intervals, measured as W/nm). Therefore, we can calculate the photosynthetic photon flux at each nanometer:
Obviously, all of the constants and variables used for calculating the PPF are “pure” physical values without extra photosynthetic or biological weightings. This also means that as long as there is a certain of spectral power distribution within the PAR, it is possible to get the same PPF whatever the shape of the spectrum.
It would be easy to calculate the photon efficacy when we have the photon flux, the PPE measures how efficiently the light source generates photons, with the based denominator of electrical power, the PPE is generally described as µmol/J (J = Joule, equals to W·s). Likewise, the calculation of PPE is about “pure” physical value. This is quite different from the calculation of luminous flux and efficacy.
In order to compare the experimental results equally, we try to make the conditions as same as possible, and the experimental conditions are based on:
The die and package are kept as the same (package 2835), considering the adjustment on CRI, which means the wavelength of the spectrum will be changed, the phosphor will be applied differently as well.
Test condition.
With the industrialized standard integrating sphere and spectrometer, with standard calibration procedure, the test condition is kept as constant to avoid any instrumental error.
As mentioned above, improving CRI is essentially about improving the spectral power distribution at the longer wavelength (600-650nm), and improving luminous efficacy, at the spectrum level, is to adjust the spectral power distribution within the photopic vision as far as possible. Therefore, in the following experiments with different spectrum design, the switch between CRI and luminous efficacy is considered as primary.
Accordingly, four different CRI experiments are processed to match the conditions above, they are Ra 95+, Ra 90+, Ra 85+ and Ra 80+, and pick the average result out of 10 samples to avoid bias effect, the list of optical parameters from the actual test is presented in figure 6.
Figure 6. Average test data of different CRI at 4000K, based on the same condition.
The result of the actual test matches the theory – with the improvement of CRI, the luminous flux and efficacy reduce gradually.
In Figure 7 (a) the four spectra are compared as normalized in the same coordinate axis. It can be found the peak wavelength of the red spectrum shifts to the right when increasing the Ra.
In Figure 7 (b) the absolute irradiances of Ra 80 and Ra 95 are compared in the same coordinate axis. Apparently, the irradiance of the blue peak of Ra 80 is stronger than Ra 95, this is because the longer wavelength of red phosphor absorbs more energy from the blue chip or the phosphors excited by the blue chip. Therefore, the energy from the spectrum is more evenly distributed.
Figure 7. Spectral comparison with different CRI.
As widely recognized, luminous flux is not suitable for assessing the performance for horticultural lighting because plants see different compared to a human, instead, the PPF and PPE should be adopted. According to the algorithm, the converted PPF and PPE are also presented in Figure 6.
Strangely, from Ra 80 to Ra 95, the luminous efficacy is different obviously, but the PPF and PPE are the same (considering the instrumental error during the tests), which means the CRI and luminous efficacy do not influence PPF and PPE, why?
As mentioned above, the experiments are kept as same conditions as possible, besides the same package 2835, the LED dies are the same as peak wavelength of 450nm, radiant power of 230mW, and phosphors used in the experiments are:
As introduced in the basic concepts, luminous flux and efficacy are visual perceptions for human eyes, the calculation of the flux is integral with the visual sensitivity as:
It can be concluded that the shape of the spectrum is one of the crucial factors affecting flux and efficacy. From Ra 80 to Ra 95, by reducing the distribution within the visual function, flux and efficacy are reduced synchronously. Figure 6 indicates these four LEDs generate similar radiant power (considering the instrumental error), but the results get different when converting to luminous flux.
An additional phenomenon of these different CRIs is that, as widely understood, long-wavelength spectrum mainly affects the rendition of saturated red color, which is R9 in the CRI 15 TCS (Test Color Samples), however, in these experiments apparently, the long-wavelength spectrum also affect the Ra – the average R1 to R8 significantly. Indeed, from the CRI results, R9 is the most variable parameter, from Ra 80 to Ra 95, R9 is calculated as -7.36, 19.12, 62.49 and 91.55 respectively. Figure 8 indicates the specific Ri from the four CRI LEDs
Figure 8. Specific Ri of different CRIs.
To explain this phenomenon, we could check with the reflectance factor of the CRI TCS 1-8 (Figure 9). From 600nm to 650nm, the TCS1 – 8 have the corresponding reflectance that fluctuates from 14.8% to 67.6%, which means the distribution within 600nm-650nm affects the TCS 1-8 effectively, that is to say the long-wavelength affects the Ra effectively.
Figure 9. The reflectance factors of CRI TCS 1 – 8.
As for PPF and PPE, although they start with “Photosynthetically” or ”Photosynthetic”, none of any specific botanical factor is involved in the calculation. Besides the relevant constants, the spectral power distribution , which essentially is the radiant power, determines the PPF and PPE. The process of energy conversion in this 4000K LED and relevant key factors that affect the conversion can be described as:
The experiments pick same LED die of 230mW radiant power, 450nm wavelength and 2.9V, and driving at 150mA, with the same EQE of green and red phosphors that are used for different CRI, which means all of the variables used for calculating the LED radiant power and PPF/PPE are the same, then we get this result and conclusion that CRI and luminous efficacy do not affect PPF and PPE, but remind that this conclusion is based on same experimental conditions – same constants and variables in the calculation, and based on the general recognition that improving CRI means improving the long-wavelength of the spectrum.
Although it is concluded that CRI and luminous efficacy do not affect the PPE, the calculation of PPF/PPE measures the amount of photon within the PAR, but this does not include the weighting of different photosynthetic sensitivities. Figure 10 indicates the comparisons of Ra 80 and Ra 95 spectra with the sensitivities of Chlorophyll a absorption, Chlorophyll b absorption, Phytochrome red (Pr) and Phytochrome far-red (Pfr), and the long-wavelength (>600nm) range which is the most different part between Ra 80 and Ra 95 is rendered in green and purple in Figure 10.
Figure 10. Comparisons of spectra and different photosynthetic sensitivities.
Apparently, the long-wavelength (>600nm) is involved in the peaks of photosynthetic sensitivities, and compare the different CRI spectra, the peak of Ra 95 is closer to the peaks of photosynthetic sensitivities, which means when considering the absorption of Chlorophyll a, Chlorophyll b, Pr and Pfr, high CRI spectra should perform better, and the “high CRI” is not based on a better visual color rendition, but is more sufficient spectral power distribution in long-wavelength (>600nm). As for the actual performance for specific plants at a specific growth stage, we leave this to the biologists.
]]>LED luminous flux is the amount of light emitted per unit time, and LED luminous efficacy is the LED luminous flux per unit power. They both indicate the luminous intensity of LED devices, which is an important parameter for the luminous performance of the LED devices. However, they are used on different occasions. For applications require high illumination, the luminous flux (absolute value) is important; while for high performance areas, it demands high light efficacy.
The color gamut is the range of light colors that LED devices can achieve. With larger gamut, the LED devices can achieve more amounts and more saturated color. Taking the display field as an example, the early TV sets were only black and white with a small color gamut. As the technology development, the cathode ray display could express 72% of NTSC color standard and LCD is 90%. Now the mainstream LED backlit display has a number of 105%. The latest quantum dot display technology can increase the color gamut value to over 110%. However, it has the disadvantages of high cost, high toxicity, and low yield, and thus currently cannot dominate the market. In 2018, Beijing Yuji Xinguang Optoelectronic Technology Co., Ltd. increased the gamut value to the level of quantum dot technology based on GaN blue chip + nitrogen oxide (β-Sialon) green phosphor + silicon fluoride (KSF) red phosphor and other auxiliary materials, taking the lead in the field of backlit technology.
A color rendering index (CRI) is the quantity describing the degree of differences between the color of an object perceived by human eyes under the sample light source and that under a standard light source (sunlight) with the same CCT. The value is up to 100 and could be negative. A higher CRI indicates the sample light source rendering the color more similar to the standard light source and thus is considered to have better light quality.
a. Average color rendering index(Ra)The average of CRIs using the first eight test color samples specified in CIE (1995).
b. Special color rendering indexThe CRI of the last six test color samples.
The above standard color cards correspond to R1-R15. Ra is the average value of R1-R8 and R9-R15 are special color rendering indexes. The average value of R1-R15 is called CRI. For LED devices, Ra or CRI can be used to indicate how consistent the light spectrum emitted by the device is with a standard light source. For example, in the field of white LED illumination, R9 represents the light quality for rendering the color strong red, and the light spectrum is more saturated in the red light part if R9 is higher. Under the joint efforts of Beijing Yuji Xinguang Optoelectronic Technology Co., Ltd. and Beijing Nakamura Yuji Technology Co., Ltd., the R12 blue-green portion is supplemented by phosphate blue phosphor, and the R9 part is supplemented by nitride (SCASN) phosphor. Under new packaging process, the R1 ~ R15, Ra and CRI of LED are more than 95 under the excitation by 450-460 nm blue light (without purple light part), achieving true high color rendering with a spectrum extremely close to the daylight spectrum. The development of the violet LED chip and its packaging technology further improves Ra, CRI, R1~R15, which will be the trend of the future market and the heavyweight technical advantages of the Yuji Group.
The stability is essential for LED phosphors and LED devices. Higher stability refers to longer lifespan and thus better practicality and applicability. The stability refers not only to the lifetime of the phosphor and the device, but also to the influence of the environment. Normally, the stability test will be carried out under extreme conditions like high temperature (over 200 °C) and high humidity (over 90%). Among them, the “double 85 tests” (85 °C and 85% humidity) is the most widely used and the most representative method. The product is qualified if after tested under ‘double 85 tests’ for 1000h, its luminous flux is more than 90% of the initial state, and the color coordinate shift is less than 10%. The product is considered superior if its luminous flux is more than 95% and the color coordinate shift is less than 5% after the test. For the MPR series nitride red phosphors and their LED devices from Beijing Nakamura Yuji Technology Co., Ltd., the luminous flux is more than 97% of the initial state and the color coordinate shift is less than 3%, leading the domestic LED phosphor market with the ultra-high performance and stability.
]]>The performance of LED phosphors mainly can be divided into the powder properties, particle size distribution, and stability (aging performance), etc. Usually, luminescent performance of LEDs mainly refers to the quantum efficiency of phosphors, spectrum, relative brightness, relative intensity, CIE1931 color coordinate, peak wavelength, FWHM (full width at half maximum) and so on, with some correlations among these parameters. In practical applications, different parameters tend to be chosen as the key performance indicators in different fields. In general, the parameters like (1) the quantum efficiency, (2) the CIE1931 color coordinate, (3) peak wavelength, and (4) FWHM are considered as the main optical performance parameters; the equivalent particle size, size distribution and discrete degree represent the powder particle properties; the light attenuation evaluates the stability in marketing and research analysis.
The ratio of emission energy (photon number) from Luminescent materials to excitation energy (photon number)
In the actual process, not all energy can be absorbed, and a certain proportion of energy is reflected or escaped. Therefore, the absorption rate is a parameter that is indispensable for the actual luminescence process. The above formula can be modified by using parameters like external quantum efficiency (eQE), internal quantum efficiency (iQE), and absorption rate (Abs).
It is found that iQE is determined by the crystal structure of the LED phosphor itself. The better is the crystallinity, the higher is the iQE. The absorption rate is not only related to the crystal structure of the phosphor itself but is also extremely related to the particle size and particle size distribution of the powder. In the development and application of LED phosphors, the luminescent performance of phosphors is often judged by IQE. The corresponding ability of phosphors to excite energy is evaluated by absorption rate. EQE (the product of IQE and Abs) reflects the energy efficiency of the LED phosphor. It should be noted that the difference of QE between LED phosphors is large. There are also dramatic deviations if the LED phosphors are produced by different processes and different manufacturers even under the conditions of the same system, the same ratio, the same synthesis temperature and time. Taking MPR series nitride (1113) red phosphor as an example, which is produced by Beijing Nakamura Yuji Technology Co., Ltd., a subsidiary of Yuji Group, the IQE, absorption rate and other parameters of the series can reach double (both over 90%) as a result of the continuous improvement of process. The EQE determined by the two parameters is 2% higher than domestic and international competing products.
Spectra indicates the excitation wavelength and emission wavelength of the LED phosphors.
(1) Excitation Spectrum (Ex)
(2) Emission spectrum (Em)
(3) Stokes law: the emission wavelength is longer than the excitation spectrum wavelength.
Stokes law, also known as Stokes shift, analyzes the mechanism of photoluminescence from the perspective of the spectrum, and side proofs the concept of quantum efficiency. During the illuminating process, the excitation energy is not completely absorbed by the LED phosphor, and some of the energy is reflected or transmitted. While most of the absorbed energy will be released in the form of light, the rest of the energy may be converted into heat, mechanical energy or other forms of electromagnetic waves. For the luminescence process, the energy loss causes the emitted energy to be less than the absorbed energy, and thus the wavelength of emitted light is shorter than that of the excitation light. This is so-called the Stokes shift.
WLP (Peak Wavelength): the wavelength corresponds to the maximum of the luminous intensity or the radiant power. It is a pure physical quantity and can be expressed in λp. Generally, the peak wavelength refers to either the excitation peak wavelength or the emission peak wavelength, which are represented by λex and λem, respectively. The excitation peak wavelength is the most responsive wavelength of the LED phosphor to the excitation energy. The emission peak wavelength is the wavelength of the light with the highest intensity emitted by the LED phosphor. In the actual applications of LED phosphors, LED devices are the foundation of luminescence. For different types of devices, the excitation light and excitation wavelength are determined. Therefore, peak wavelength often refers to the emission peak wavelength; and the wavelength of the excitation peak is hidden in the performance of the LED device.
The FWHM is the linewidth at the half maximum height of the band, which objectively reflects the color purity and light area of the LED phosphor. Generally speaking, the narrower is the FWHM of the emission peak, the purer the color of the light it represents. And the wider is the FWHM of the emission peak, the broader the range of light it represents. In different areas, the requirements of FWHM are slightly different. For example, in the field of white LED illumination, the FWHM of the LED phosphor requires a certain width. The spectrum emitted by the LED device formed with phosphors and standard blue light can cover the entire visible region to achieve the "solar spectrum" effect. In the field of LED backlit display, the FWHM is the smaller the better, which leads to the purer emitted light color and is useful for color matching and color gamut distribution.
On the other hand, the FWHM also indicates the crystallinity of the LED phosphor. The better is the crystallinity of the phosphor, the less the lattice defects, and the narrower is the corresponding FWHM. Within the same system, the powder properties are better if the FWHM of the LED phosphor is narrower. And different systems have their own characteristic FWHM. For example, the FWHM of YAG yellow powder is 100-120 nm; the FWHM of nitride red phosphor (SCASN) is 70-90 nm; the FWHM of nitrogen oxide green phosphor (β-Sialon) is 40-60 nm; the FWHM of silicon fluoride red phosphor (KSF) is around 30 nm. Beijing Nakamura Yuji Technology Co., Ltd. (BNY) is made up of a strong team of domestic and foreign experts, relying on independent research and development to continuously improve the production processes and the quality of products. Under the same ratio and the same firing condition, the FWHM of the MPR series nitride red phosphor produced by BNY is 2% lower than that of the competing product due to the optimization of the process route, achieving the highest performance over the world.
Obviously, requirements are not the same in different fields, but BNY can provide customers with a full range of personalized customized products based on their strong research, development and production capacity.
The CIE 1931 chromaticity diagram is a CIE chromaticity diagram represented by nominal values, with x representing the red component and y representing the green component. Point E represents white light, whose coordinates are (0.33, 0.33). The colors surrounding the edge of the color space are the spectral colors, which are the most saturated colors. The numbers on the boundary represent the wavelengths of the spectral color, and the outline contains all perceived tones. All monochromatic light is located on the outline tongue curve, which is a monochromatic locus. And the actual colors in nature are located in the interior of the closed curve. The physical three primary colors selected in RGB system are on the tongue curve of the chromaticity diagram. In other words, each type of light could find a corresponding coordinate point representation in the CIE chromaticity diagram. If the connection of two coordinate points passes through the intermediate white light region, white light can be formed by the combination of the two phosphors. In practical LED phosphor applications, blue or violet light from semiconductor chips has a fixed color coordinate point, forming a triangle with the other two color coordinate points. If the geometric center of the triangle is in the white light region, it will achieve white LED illumination. In the development of white LED lighting, the earliest white light is formed by the combination of standard blue light and yellow phosphor for high CCT white light. However, despite of the advantages of LED, a low CRI (Ra) is presented at the same time. To promote CRI of LED devices, the process route gradually developed to standard blue light + green powder + nitride red powder (1113). The performance of LED devices based on the three-color solution is also constantly improving.
(1) Particle diameter and the equivalent diameter
Flaky, rod, Ellipsoid particles are approximated to sphere.
(2) The meaning of D10, D50, D90
D-values is a one of the methods for monitoring particle size distribution.
If we line up all the particles of the powder by particle diameter and cumulate the particles in a rising order. D10, D50 & D90 are the interception diameters for 10%, 50% and 90% of the cumulative mass.
In other words, D10 measures the fine particle diameter and D90 measures the coarse particle diameter of the powder. D50 is also called the median diameter.
(3) Distribution coefficient / Discrete degreeLED phosphor, in a narrow sense, is a kind of Photoluminescent materials can absorb the energy of violet and blue light from gallium nitride or other LED chip, and transform it into visible light. Phosphors are made of solid luminescent materials, which can absorb blue light (450-460 nm) and release green, yellow, orange or red light, on the present mainstream LED market. These kinds of emission can mix the blue light from LED chip at a certain proportion for application in lighting, backlit display or other fields. For example, Ra > 80 white light illumination is realized by 450-460 nm blue chip + 530 nm Ga – YAG green phosphor + 630 nm nitride (1113 system) red phosphors; and NTSC > 100 high color gamut LED backlit displays are obtained by 450-460 nm blue chip + 530 nm nitrogen oxides (β-Sialon) + 630 nm fluoride (KSF) red phosphors. With the growing demand of society, the concept of LED phosphors is expanding. Since 2018, the demand and research has been extending continuously in the field of solar spectrum, infrared, ultraviolet and vacuum ultraviolet, MINI LED and so on. Based on this, the generalized LED phosphors can be defined as the inorganic luminescent materials to absorb electromagnetic radiations at a certain wavelength, such as ultraviolet, visible and infrared light, and convert energy into radiations at required wavelengths. The phosphor luminescence process is shown in the Figure 1.
LED phosphors have several classified methods under different circumstances. According to applications, phosphors can be divided into phosphors for backlit display, white light lighting, medicine, security, agriculture and agricultural and sideline products, etc. On the basis of energy sources, phosphors can be divided into vacuum ultraviolet phosphors, near ultraviolet phosphors, violet light LED phosphors, blue LED phosphors, etc. In terms of luminescence wavelengths, phosphors can be divided into ultraviolet, blue, green, yellow, red and infrared phosphors, etc. According to raw materials, phosphors can be divided into silicates, phosphate, aluminate, oxide, nitride, fluoride, and sulfide phosphors, etc. Basically, phosphors are involved in every aspect of industry and our life, with differences in performance requirements under various circumstances. Overall, manufacturers and customers of LED phosphors commonly use optical properties (emission light colors, CIE coordinates, light wavelengths), granularity characteristics, and stability (aging performance) to distinguish the merits of the phosphor performance.
As the main source of LED phosphors excitation energy, electromagnetic radiations can be obtained by inert gas discharging, violet light semiconductor chips, blue light semiconductor chips, etc. Excitation energy can be made into light emitting device carrier by a specific process route. Thus, the LED phosphors can be applied on the device and the photoluminescence process can be realized. Take white LED lighting as an instance, LED lamp can be encapsulated by mixing up the red and green phosphors with silica-gel-based materials at a certain proportion and then coating them on gallium nitride LED blue chip. The blue chip emits 450-460 nm blue light when powering up. A part of blue light is absorbed by YAG powder (yellow), Ga - YAG powder (green), SCASN phosphors (such as nitrides 1113) and then the powders emit the corresponding color. Different CCTs (correlated color temperature) and CRIs (color rendering index) of white light illumination can be adjusted by mixing the color of light emitted by phosphors and the transmissive blue light at a certain proportion. Compared with the white LED devices, the processes of the LED phosphors are different for LED backlit display, UV, infrared and other applications. But the luminescent mechanism of these applications is basically the same, which is the excitation energy from a specific light-emitting device being absorbed by the LED phosphors and then being converted to the corresponding wavelengths of light (electromagnetic waves) to meet the lighting requirements. While the above luminous requirements depend on the part of the LED chips, LED powders, LED device packaging technologies and other LED related parts. Among them, the core of luminous performance depends on phosphors, chips and LED packaging technology. So far, the core technique and technology of the chips is still under the control of a few foreign companies. Compared with other factors, the differences of LED chip performance is negligible because of the monopoly of LED chip technology. In practical applications, encapsulation technology and LED phosphors become the determinants of LED device performance. Beijing Nakamura Yuji Technology Co., Ltd and Beijing Yuji Xinguang Photonics Technology Co., Ltd,affiliated with Yuji Group, possess the state-of-the-art technology of LED phosphors and LED packaging respectively, and occupy the leading status of market in their respective fields.
]]>In color management and print quality verification processes, visual assessment of colors is a key. In order to acquire color accuracy, a proper lighting condition becomes a must. The ISO3664:2009 standard specifies a set of five conditions which all must be present in order to assure the benefits of this standard in critical color viewings. The five conditions include Color Quality, Light Intensity, Evenness, Surround and Geometry.
It is a fundamental principal that human could see and perceive colors only under lighting. There are no colors without lights. Due to the specific structure of the human eye, our sensitivity to lighting varies at different wavelengths. Because of this, we can perceive various colors. And this ability to perceive colors depends on both the quality and quantity of lighting.
Consistent light intensity is fundamental to consistent color reproduction. The ISO3664:2009 standard proposes a standard intensity to allow full tonal visibility of shadow details in an image without washing out highlights.
In particular, ISO3664:2009 applies to:
Based on above, ISO3664:2009 covers five major types of viewing conditions:
The perceived tonal scale and colors of a print or transparency can be significantly influenced by the chromaticity and luminance of other objects and surfaces in the field of view. For this reason, ambient conditions, which will possibly affect the state of visual adaptation, need to be designed to avoid any significant effects on the perception of color and tone, and immediate surround conditions also need to be specified.
Experience in the industries covered by this international standard has revealed the need for two levels of illumination:
The following specifications of light intensity in critical color viewing are provided in ISO3664:2009.
Figure 1. Light intensity specified by ISO3664:2009 standard.
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Yuji LED, the industry leader of high CRI LED phosphor and lighting, developed CRI 99 white LED, the world’s highest lumen density LED COB, full spectrum LED, and five-colors-in-one packaged LED for various lighting applications. After two years of R&D work, Yuji released its Standard Illuminant CIE D50 LED lamp that fully complies with ISO 3664:2009 and perfectly replaces the conventional fluorescent lamp in printing industry.
To know more about YUJILEDS, welcome to visit https://www.yujiintl.com and https://store.yujiintl.com
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While color and density measurements play important roles in the control of color reproduction, they cannot replace the human observer for final assessment of the quality of complex images. Color reflection artwork, photographic transparencies, photographic prints and photomechanical reproductions such as on-press and off-press proofs or press sheets, are commonly evaluated for their image and color quality or compared critically with one another for fidelity of color matching.
There is no doubt that the best viewing condition for the visual assessment of color is that in which the product will be finally seen. Therefore, Technical Committee ISO/TC 42 Photography in collaboration with Technical Committee ISO/TC 130 Graphic technology introduced the ISO3664:1975 (the first edition) standard viewing conditions for graphic technology and photography in 1975. In 2009, the third edition (ISO3664:2009) was released as to cancel and replace the second edition (ISO3664:2000).
This ISO3664 standard specifies a set of five conditions which all must be present in order to assure the benefits of this standard in critical color viewings. The five conditions include Color Quality, Light Intensity, Evenness, Surround and Geometry.
Color quality matters in three aspects: Chromaticity, Color temperature, Spectral power distribution.
The reference spectral power distribution specified in ISO3664:2009 is CIE illuminant D50. According to the description from CIE 015:2018 (Colorimetry, 4th Edition), D50 is a relative spectral power distribution representing a phase of daylight with a correlated color temperature of approximately 5000K. D50 is a relative spectral power distribution representing a phase of daylight with a correlated color temperature of approximately 5000K, with the chromaticity coordinate of x=0.3457, y=0.3585 (CIE 1931 2° observer) and u’10=0.2102, v’10=0.4889 (CIE 1976 10° observer). Figure 1 shows the chromaticity coordinates in different color spaces. To understand CIE1931 and CIE 1976, please click here.
Normally CIE 1976 10° observer coordinates are widely used in color industries. The ISO 3664:2009 requires the chromaticity coordinates of a light source should be within a 0,005 radius of the D50 chromaticity coordinates.
In the ISO 3664:2009 standard, the color temperature of a light source is required to be relative to natural daylight with a correlated color temperature (CCT) of about 5000K. There is not a tolerance defined in standard. In fact, in real color viewing occasions, CCT is accepted when the chromaticity coordinates of a light source is within 0,005 radius of the D50 chromaticity coordinates.
The ISO 3664:2009 standard provides a specific spectral power distribution between 300 nm and 780 nm for the D50 standard light source as required. Since D illuminants are based on the daylight curve that runs above the blackbody, D50 has a spectral energy distribution that closely matches that of a blackbody at 5000K.
ISO3664:2009 has been technically revised by tightening the compliance tolerances on the ultraviolet portion of the D50 spectral power distribution. The metamerism index UV (MIuv) should be less than 1.5, compared with less than 4 in the ISO3664:2000 standard. This marks the very difference between the ISO 3664:2009 and ISO3663:2000. The metamerism index visible (MIvis) should be less than 1.0.
As for the CRI, the general CIE color rendering index of the observed surface should be measured above 90. In addition, according to CIE 13.3-1995, the specific color rendering index of each individual color sample 1-8 should be above 80. In Japanese JSPST-1998 standard for color viewing condition, it even strictly requires each Ri(i=1-15) should be above 90.
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Yuji LED, the industry leader of high CRI LED phosphor and lighting, developed CRI 99 white LED, the world’s highest lumen density LED COB, full spectrum LED, and five-colors-in-one packaged LED for various lighting applications. After two years of R&D work, Yuji released its Standard Illuminant CIE D50 LED lamp that fully complies with ISO 3664:2009 and perfectly replaces the conventional fluorescent lamp in printing industry.
To know more about YUJILEDS, welcome to visit https://www.yujiintl.com and https://store.yujiintl.com
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More than half of people in this environment will become depressed and lethargic, and looks like a vibrant different person from summer. 3% of people even need to go to doctors. Before the 1980s, people know nothing about the causes of the personality change occurs with seasonal variations. Later, Dr. Norman E. Rosenthal, who worked at the U.S. National Institutes of Health Psychology as senior researcher, and clinical professor of psychiatry at Georgetown University, linked the short and dark winter's day and the onset of seasonal depression. He and his colleagues began to study this phenomenon and defined it as Seasonal Affective Disorder (SAD).
No one knows the causes of SAD. Experts believe that the light is playing a certain role to SAD patients. With more full spectrum light, you can sometimes improve their mood. Melatonin is also considered being associated with SAD. Melatonin is secreted at night, There would be more melatonin in human blood in winter than in summer, making human body temperature drops and becomes lethargic.
Scientists have confirmed that SAD patients’ symptoms usually are improved after light therapy, which may owing to the light blocked the generation of melatonin. Most SAD patients need to spend 2-3 days in the bright sunlight to eliminate abnormal symptoms. Also, you may find that when people travel closer to the equator, the symptoms become lighter.
Back in ancient Greece, Hippocrates period, when doctors had learned many aspects of the human body - both physical and psychological aspects, are resonating with colors in spectrum. They found that the color will affect human emotions, and even lead to many psychological disorders. In the book < COLOR ALCHEMY >, America's chief color designer Jami Lin gave a detailed description of the psychological impact of color to human’s thoughts and feelings. And as preface for this book, Burton Jack now who has a PHD in The physical organic chemistry at Columbia University, had mentioned the incredible effects of light on human body and spirit, namely - Cure Effect. In a major test conducted by National Aeronautics and Space Administration (NASA), the researchers used colored LED light penetrate human tissue deeply to promote wound healing, accelerating the growth of human tissue structure, and eventually proved the color wavelengths can affect the chemical structure of the human body, stimulating natural growth in the healing process.
The best way to avoid SAD, is more exposure to full spectrum natural sunlight. If your mood is changing along with the sun in winter, you can seriously consider taking a vacation to a sunny place, it can dramatically better you healthy and mood throughout the winter. Dr. Norman Rosenthal said, when people receiving the ambient light of full spectrum, his body releases significantly reduced melatonin, by which the depression is eased, and the mood has been improved significantly. Compared to those receiving light with certain wavelength, are subject to SAD.
For those who live in the north as well as those who stay indoor all day, the sunshine is almost a luxury during the most of the time in year. However, general indoor lighting products have a serious lack of spectrum. For example, a primary school in Vermont's tried to installed ultraviolet (UVB) fluorescent lamp in three classrooms. Before using this yet, the absenteeism caused by illness of children in experimental group and the comparison group are the same, but after replacing the fluorescent, the sickness rate of those kids who enjoyed the light with ultraviolet was reduced by 40% than the comparing group. Children have been relatively easy to get sick particularly in winter and spring. However, this situation could be changed by using lighting products with part of ultraviolet. That is to say, our immune system could be significantly affected by the length and quality of our daily light we exposure in.
If you cannot go to a warm sunny place, you can also think of ways to make yourself at home, receiving the maximum amount of sunlight. Or change the lamps in your home and office into a full-spectrum lighting product.
If the situation is really bad, you might consider phototherapy, although this has to be studied on the optimal irradiation therapy sessions, irradiation time and irradiation intensity. But for SAD people, two hours a day, from six to eight in the morning would be effective. Of course, some take longer sunshine hours, or increase the time of being illuminated at night. Light intensity should be 2500 lux, which is approximately equal to the received luminosity of standing at the window in spring.
Daylight glasses for SAD patient, developed by Denmark engineer
Of course, just like the author of < Healthy Pleasures > Robert Ornstein, Ph.D. & David S.Sobel, M.D. said, most time it is not the light affecting your mood, but your attitude towards yourself and look at the world.
]]>Infrared light is a portion between the visible and microwave regions of the electromagnetic spectrum ranging from 700 nm to 1 mm. Infrared light was discovered by a German-British astronomer named William Herschel in 1800 while investigating the temperature difference among the colors in the visible spectrum by using thermometers. He perceived the value of increased temperature in the thermometer scale different from the red light of the visible region. Herschel assigned that region as infrared light and postulated that infrared light can be sensed as heat. Any object with a temperature of >268 °C (450 °F) can emit infrared radiations.
Ever since the NIR spectroscopy was introduced into the American agriculture during the 1950’s when Karl Norris successfully applied this technology to analysis the product, the development of NIR spectroscopy never stops. Benefited from the Chemometries and the great promotion of computing power of Central Processing Unit (CPU), the NIR spectroscopy today is able to provide reliable data analysis.
Near-infrared spectroscopy is based on the principle that every molecule is constructed by several atoms connected with the characteristic bonds between them. When such molecule is excited with a specific radiation of light, it undergoes short-term vibrations in terms of reflection, transmission, and absorption within the molecule based on elemental constituents and bond strength. The nature of such light behavior is unique for each organic molecule and acts as a characteristic spectral fingerprint.
The improper classification of infrared light is more common in practice. However, the International Commission on Illumination classified infrared light into three categories based on photon energy, as presented in Table 1. Alternatively, the International Organization for Standardization 20473 classified infrared light based on its wavelength, as shown in Table 2.
Traditionally, the commercially available light sources are tungsten halogen lamps, laser diodes, supercontinum lasers and globars. Although they may partially meet the luminescence properties required for laboratory usage, the severe heat generation and incapability of compact design hinders their application towards miniature size or portable spectrometers. One hand, in terms of optical properties, the unstable spectra stability still matters. One the other hand, physical properties such as short lifetime and high-power consumption are in a high demand of promotion. One of the primary issue to bring the laboratory technology in daily life for food analysis and healthcare is to narrow down the massive size of standard desktop laboratory spectrometers into portable size like smartphones, which requires the efforts not only devoted to the light detecting systems but also the NIR light sources. The prevailing and mature technology, solid-state lighting based on InGaN LED chips and phosphor-converted layers is a promising candidate, which meet the demand of miniature size, flexible design, long lifetime and some other physical properties. Now the key point is whether the phosphor-converted LED technology can provide the required optical properties for specified applications.
Phosphor-converted near-infrared light emitting diode (pc-NIR LED) is a promising alternative light source for miniature NIR or portable hand-held spectrometers because of its remarkable advantages of smaller size, longer lifetime, stable spectral stability, and lower cost than traditional light sources. The resulting spectral light distribution from pc-NIR LED light source should be considerably high for effective and efficient functions. Various organic elements present in foodstuffs and human body possess the absorption and reflection spectra of light in the blue region (450~600 nm) and infrared light (700~900 nm) of the electromagnetic spectrum, respectively. Hence, broadband near-infrared phosphors that are excitable by blue light are highly desirable to develop miniature spectrometers.
LED is a semiconductor device constructed by two semiconductor materials, which include both p-type and n-type semiconductors, as shown in Figure 1a. On the application of voltage in the forward bias condition, the electrons from n-side and holes from p-side are recombined in the depletion zone and photons are released. The wavelength of the emitted photons is substantially determined by the use of phosphor conversion materials, and the elements are included in the semiconductor device. The structure of pc-NIR LED optoelectronic component shown in Figure 1b consists of a blue semiconductor chip generally on the basis of aluminum indium gallium phosphide or indium gallium nitride mounted on the epitaxial layer sequence in emitting blue light. The semiconductor chips are connected to the cavity in the base housing through a bonding wire. The base housing is embedded in a recess. The weighted amount of inorganic phosphor conversion materials is mixed with the matrix materials (i.e., silicone) and filled the cavity in the base housing frame. The reflector is mounted on the base housing unit with suitable materials coated on the inner walls of the recess to attain the maximum reflection of primary radiations from the blue semiconductor chip and infrared luminescence from the conversion materials embedded in the matrix materials. During operation, the blue light emitted from the semiconductor device will act as an excitation source of light for the phosphor conversion materials and results in infrared luminescence. The near-infrared LED device appears bluish white in addition to invisible infrared LED during the operation due to the possible combinations of visible red and blue lights. The fabrication of a pc-NIR LED device follows the general principles of white LED devices. In brief, the emitted light from the blue LED chip is used to excite the near-infrared phosphor deposited over the blue-chip, which results in near-infrared luminescence.
The overview of some reported scientific articles shows that the luminescent center for nearinfrared light in the phosphor material can be rare earth elements (i.e., Pr3+, Nd3+, Tm3+, Eu2+) or transition metal elements (i.e., Cr3+, Ni2+, V2+, Mn4+). Among these NIR phosphors, the transition element Cr3+ showed higher efficiency and matched well with blue LED chips. Cr3+-doped materials are investigated well by the scientific community for laser and persistent luminescence applications for the past several decades. The phosphor host system for near-infrared light is classified into five types, as shown in Figure 2.
3.1 Zn-gallogermanate system
The spinel crystal structure of MGa2O4 (M = Zn, Mg) is focused on persistent luminescence studies due to the possible generation of antisite defects and O vacancies due to Cr3+ transition element doping. Although the emission bandwidth is high (650–770 nm), the presence of antisite defects may deteriorate the fluorescence performance and increase the decay time due to spin-forbidden transition alone, thereby making the Zngallates and -gallogermanate system as inferior candidates for broadband near-infrared spectroscopy applications.
3.2 Ca-gallogermanate system
The Ca-gallogermanate system is another category of a host system for the broadband near-infrared spectroscopy applications. The selection design principle and chemical composition of this system are almost similar to those of the Zn-gallogermanate system, but Ca is present in the host system instead of Zn. Most Ca-gallogermanate compounds are located in the weak crystal field region of the d3 Tanabe–Sugano diagram due to the presence of large divalent cation in the host. Hence, Cr3+ in Ca-gallogermanate compounds follows the spin allowed transitions and results in tunable broadband emission spectrum.
3.3 Garnet-type system
The garnet-type crystal structure with the chemical formula of A3B5O12 (A = Y, Gd, La, Lu and B = Al, Ga) is also investigated for Cr3+ doping in the B3+ site and focused mainly on persistent luminescence studies. Garnet-type persistent luminescent phosphors show uniform single broadband near-infrared luminescence in the range of 650–800 nm centered at 700 nm, which is in contrast to those of Zn-gallate and Zn-gallogermanate systems, and the emission bandwidth of which is extremely narrow for broadband near-infrared spectroscopy applications.
The rigid crystal structure promises the garnet family a good choice served as host material for emitting ions, for example the Y3Al5O12:Ce is a superior yellow phosphor with high quantum efficiency and small thermal quenching for white light pc-LEDs. It is also a good choice of Cr3+ activated host candidates despite the only one available octahedral site.
3.4 La-gallogermanate system
La-gallogermanate systems are also investigated for the persistent luminescence capability by doping Cr3+ and rare-earth elements as codopants or sensitizers. The emission spectrum of Cr3+-doped LaGaO3 single crystal reveals several narrow lines with the two maximum positions at 739 and 729 nm. The La-gallogermanate system may be a good potential candidate for fluorescence property and is inferior to phosphorescence.
3.5 Borate system
The borate-based host system is also focused on broadband near-infrared spectroscopy applications in recent times. The borate-based host system possesses few thermal quenching behavior, which is one of the main important parameters required for LED semiconductor. Meanwhile, the use of the borate host system eliminates the high-cost starting precursors, such as Ge and rare earths, in the host, which is an additional advantage considering its practical implementations.
Near-infrared spectrometers are nondestructive analytical characterization tools using near-infrared light to perform near-infrared spectroscopy function in the diverse applications, including agriculture, pharmaceutical, food industry, and noninvasive health monitoring.
For NIR pc-LEDs, the exclusive merit compared to the semiconductor chips is the broadband luminescence spectrum and the spectral tunability, hence the NIR pc-LEDs is developed fast to accelerate the approaching of NIR spectroscopy into daily life. What is more, the versatile choices of abundant NIR emitting ions provide phosphors that enable the NIR pc-LEDs replacing semiconductor chips, which are designed for special usage that requires determined wavelength.
]]>The CIE 1931 were created by the International Commission on Illumination (CIE) in 1931. And this is the first time human created an explanation between distributions of wavelengths in the visible spectrum, and colors can be perceived in human color vision. The mathematical relationships that define these color spaces are essential tools for color management, important when dealing with color comparison, illuminated displays, and digital cameras.
Now days The CIE 1931 color spaces are still widely used, same as the 1976 CIELUV color space.
William David Wright and John Guild have done a series experiments in the late 1920s. They have found the human eye has three different types of color cones and that every color is a mixture of those three types of cones, which roughly reflect the three different colors which are red, green and blue. These cone cells having peaks of spectral sensitivity in short (wavelength from 420 nm – 440 nm), middle (wavelength from 530 nm – 540 nm), and long (wavelength from 560 nm – 580 nm) wavelengths. These 3 wavelengths roughly reflect the three different colors red, green and blue. These 3 parameters corresponding to levels of stimulus of the 3 kinds of cone cells, it can describe any color that human sensate. The observer (also called CIE standard observer) observed not every color could be matched and sometimes some red had to be added to the test color to get a correct match. This was also happened at the mathematical equations and resulted in a red curve which including negative values. These three curves were standardized and are called the CIE RGB color matching functions r, g and b. The negative number is very hard for people to understand so in order to solve it , the commission based on the CIE RGB to create a new functions and it called x, y and z . The new CIE XYZ color space is everywhere greater or equipment to zero. This new color space is also known as CIE 1931 color space. This CIE XYZ color space was created to describe all visible colors which can be observed by human eyes and can be shown as a three dimensional cube .
The three dimension objects is very hard to understand to so a two dimension object must to be created .The Y of the tristimulus values is a measure of the brightness .
However, people have observed that the biggest issue with CIE 1931 is the uniformity with chromaticity, the three dimension color space in rectangular coordinates is not visually uniformed. Therefore the CIE 1976 was invented to fix that .The CIE 1976 (also called CIELUV) was created by the CIE in 1976. It was put forward in an attempt to provide a more uniform color spacing than CIE 1931 for colors at approximately the same luminance. The values of u' and v' can be calculated from the CIE 1931 tristimulus values XYZ or from the chromaticity coordinates xy according to the following formulas:
Where X, Y, Z are the Tristimulus values in the CIE 1931 systems, and x, y are the chromaticity coordinates.
Chromaticity coordinates are the best estimate of color gamut and LED suitability for a particular display since color differences will be noticeable between two neighboring modules.
The CIE XYZ tristimulus do not correspond well to the visual sense of a color. The Y from CIE XYZ is the only one easy to observer because the Y related to the brightness. In order to create a more understandable color system, the CIE released CIE chromaticity coordinate x, y and z. These three values are calculate from CIE XYZ. The value Yxy are often read as CIE chromaticity coordinate since the Y provides a brightness function. The sum of x, y and z is 1. On any of the chromaticity diagram, x means the horizontal axis and y is the vertical axis.
Converting the XYZ to chromaticity coordinate:
]]>Luminous flux is a measure of the total amount of visible light emitted by a lamp. It's different from the radiant flux. Radiation flux is the measurement of all electromagnetic radiation emitted (including infrared, ultraviolet and visible), which is the total amount of objective light. Luminous flux is the amount of light that the human eye senses. It reflects the sensitivity of the human eye by weighting each wavelength with a luminosity function. So that it is the weighted sum of all wavelengths of power in the visible light band, excluding infrared and ultraviolet.
The luminosity function describes the relative sensitivity of human eyes to light of different wavelengths by subjectively judging the brightness of light of different colors. It shouldn’t be considered perfectly accurate, but it is a good representation of visual sensitivity of the human eye and it is valuable as a baseline for experimental purposes.
Figure 1: Photopic (black) and scotopic (green) luminosity functions
The SI unit of luminous flux is the lumen (lm). The lumen is defined in relation to the candela which is the unit of luminous intensity as
1 lm = 1 cd ⋅ sr
That is, when the luminous angle of a light source is one solid angle and the luminous flux is 1 lumen, its luminous intensity is 1 candela. When the luminous flux of a light source is also 1 lumen, but the luminous angle becomes 1/2 solid angle, the luminous intensity of this light source is considered to be 2 candelas.
Conversely, when the luminous intensity of a point light source that emits light in all directions is 1 candela, as a full sphere has a solid angle of 4π steradians, the luminous flux of this light source is 4π lumens, or 12.56 lumens.
Figure 2: A graphical representation of 1 steradian.
In photometry, illuminance is the total luminous flux of light incident per unit area. In other words, luminous flux represents the total amount of light emitted by the source, while illuminance refers to the total amount of light received by an object.
The relationship between illuminance and luminous flux is similar to that between Irradiance and radiation flux, that is, the radiation flux received per unit area. However, the illuminance is weighted according to the sensitivity of human eyes to light of different wavelengths, which represents the light intensity perceived by human eyes.
The SI unit of illuminance is the lux (lx). It is equal to one lumen per square meter.
1 lx = 1 lm/m2 = 1 cd·sr/m2.
In photography, there is also a non-metric unit of illumination, foot-candle. Foot-candle means "the illumination of a candle source on a surface one foot away." Thus, one foot-candle is equal to one lumen per square foot or about 10 lux.
Illuminance is the number of lumens per square meter. It means that when a light source of 1000 lumens illuminate an area of 1 square meter, the illuminance on this plane is 1000 lx. When a light source of 1000 lumens illuminate an area of 10 square meters, the illuminance on the plane becomes 100 lx.
So that, when we buy light bulbs, we should not just choose them based on the number of lumens. This is because when bulbs of same lumen number are installed in sitting room and toilet, as a result of the different size of rooms, the illuminance distinction that eyes perception can be greatly.
]]>What’s more, with the develop of the scientific understanding, through HCL (Human Centric Lighting), we can also improve and enhance the conditions and quality of people's work, study and life, and promote mental and physical health, and adjust people’s daily rhythms and improve their motivation, well-being, and productivity.
In the long history of human development, people have evolved under the sun. In the process, humans have adapted to sunlight and its highly diverse effects, just as they have adapted to natural circadian rhythms.
As recently as the year 2000, a new type of photoreceptor cell was discovered in the eye, which plays an important role in the biological and non-visual effects of light. The "new" receptor, a type of light-sensitive ganglion cell that is not involved in the actual visual process, responds specifically to the short-wave "blue" region of the spectrum, which are about 480 nm. Since its discovery, science and industry have devote themselves to understand the effects of non-vision on humans, and the current state of knowledge is increasingly reflected in the use of light.
So, let's look at the photoreceptor cell in the retina:
The retina contains three types of photoreceptors: color-sensitive cone cells, light-sensitive rod cells and blue-sensitive ganglion cells.
Human eyes have three cone cell types, with each type dedicated to receiving either blue, green or red light. This allows most people to tell the difference between different colored objects.
Rod cells help measure brightness, recognize different wavelength. Rod cells are much more numerous than cone cells and are vastly more sensitive to light.
Like rods and cones, these ganglion cells are found in the retina. They respond specifically to the short-wave "blue" region of the spectrum, which are about 480 nm.
The ganglion cells especially sensitive to the blue light component are in the lower region of the retina. They are not involved in the visual process; however, they serve as signal transmitters for our "inner clock".
At daylight, Blue light can give you a stimulating effect, and give you a stimulating effect that keeps people focused during the day and during work hours. But blue light at the wrong time can also be extremely harmful to the body. At night, it can suppress the secretion of melatonin and wreak havoc on our circadian rhythms.
Therefore, it is the highest state of lighting product design, manufacturing and implementation to adjust reasonable spectrum and light color for special people and special application.
People's "biological clock" changes according to the earth's rotation, the circadian rhythm is a change of life activity with a cycle of about 24 hours. The physiological function of human body, learning and memory ability, mood and work efficiency also have obvious fluctuation in circadian rhythm. It is closely related to human activities. Disruption of circadian rhythms causes appetite descent, lower work efficiency and more accidents.
Therefore, the current people-oriented lighting applications have been more and more mentioned. People-oriented lighting is based on human behavior, visual effect and physiological and psychological research, and develop more scientific, more efficient, more comfortable and more healthy lighting products and solutions. HCL lighting is about being able to personalize and control the light to suit your needs and increase your wellbeing. LEDs' digital nature makes them relatively easy to tune to settings that might support circadian needs.
Scientists at the university of Toronto in Canada and northwestern university in the United States randomly assigned participants to a brightly room and a dark room.
The researchers found that the difference in emotional expression between the two groups is related to the perception of heat light. Psychologists believe that too much light makes it easier for both parties to detect and amplify aggression each other. It also makes people more sensitive to emotional speech. In the place with darker light, both sides see each other not quite clear, the precaution is less, people get closer to each other than place with brighter light.
Human Centric Lighting...
--- Helps the human body to synchronize with the “natural day/night rhythm”
--- Spectral quality of light modulates emotional responses in humans
--- Enhance people’s performance and productivity, make them feel wellbeing
--- A low carbon / no mercury/ quite sustainable
--- Better visual acuity and allow clearer vision for longer
Human-centered lighting provides full spectrum, no stroboscopic light, and has strict requirements for the distribution of light. According to circadian rhythms, people need light of different color temperatures at different time periods.
Based on Circadian Rhythms, light can be adjusted according to time. It can help stimulate employee's activity and create a comfortable, healthy and efficient working environment.
In the morning, the office and industry lighting system simulate natural daylight to refresh employees. Over time, the intensity of the lighting will gradually weaken. After lunch, the lighting intensity is strengthened again to enhance the work vitality of employees. The light source is 5000K cold color temperature and the illumination is 780lux.These subtle changes not only help to improve the level of energy saving, improve the comfort of employees, but also ensure the eye comfort of employees.
Recently published studies in the office environment even show potential for productivity increases of 10 percent if HCL is implemented as part of a holistic approach together with other improvements in the working environment.
Schools utilize HCL to both calm students and to keep them alert depending on the time of day.
In the early morning, the right light and better lighting environment can improve students' alertness and concentration in class and reduce drowsiness during the day.
To provide higher light intensity and color temperature at appropriate time can help improve sleep quality, thereby improving learning.
There can be no substitute for good lighting. Good lighting can assist doctors in their treatment and paramedics in their work, help the elderly improve their vision, help patients in their care, help them recover, balance their circadian rhythms, and help balance their mindsets and hormones.
Lighting in nursing homes, care facilities and rehabilitation centers can firstly consider safety and visibility. The emotional atmosphere created by the lighting is also important: the lighting needs to make the nursing home more comfortable. In rehabilitation centers, lighting can consolidate therapy and promote rehabilitation.
For the staff, bright lights aid diagnosis, and dim lights aid recovery. People-oriented lighting provides the best working lighting environment, which can help doctors and staff work, improve efficiency and bring happiness to employees.
Today, when LED products have become the mainstream in the lighting industry, although the harm of blue light, the glare and flicker are also still existing. People’s concern about LED lighting has also shifted from "energy saving" to "health and comfort".
The non-visual effect of LED is more closely related to human health. Its influence on human physiological rhythm and biological effect and led people to rethink the definition of lighting quality.
]]>Generally, in order to meet the new demand of solid-state lighting (SSL) market and communicate with consumers about the color quality of lighting products, NIST (National Institute of Standards and Technology) has developed the light color quality meter, which evaluates light sources with 15 new colors.
The calculation method of color quality scale (CQS) is derived from the calculating method of color rendering index (CRI). This method evaluates the color quality of light sources in the 15 colors in the following table (Pic 1.1) to analyze the color quality of current solid-state lighting sources more accurately.
Pic 1.1 CQS Q1-Q15
CRI was firstly designed for evaluation of fluorescent lamps, and it has no substantive changes in recent 30 years. With the rapid development of solid-state lighting (SSL), color rendering index (CRI) is no longer enough to evaluate new artificial light such as LED. Here is an example (Pic 2.1) below that shows high CRI value cannot prove good saturation in colors.
Pic 2.1 Limitation of CRI [1]
From the picture above, it is obvious to see that the spectrum is simulated by three LED dies at peak 457nm, 540nm and 605nm. Comparing the colors of reference light and LED light, the value of Ra (R1-R8) is very closed, however from R9 to R12, the colors are much distinctive. Hence, high CRI value sometimes is not able to present the color quality of LED, especially in saturated colors.
Pic 2.2 Limitation of CRI [2]
The picture shows another example of problem with the CRI. In this picture, the neodymium incandescent lamp has only CRI 77, however the normal incandescent is CRI 100. The neodymium incandescent lamp presents as nearly same color as the incandescent lamp, but the CRI value is much lower.
In that case, NIST developed CQS (Color Quality Scale) to evaluate light sources with 15 new saturated colors in order to:
Pic 3.1 New set of 15 saturated color samples of CQS
The CRI makes it possible for a lamp to score quite well, even when it renders one or two samples very poorly. This situation is more likely with SPDs having narrowband peaks, such as LEDs.
RMS (Root Mean Square)
Color difference of 15 samples:
Pic 4.1 Formula of Ra (RMS) [1]
Example:
Pic 4.2 Comparison of Ra (mean) and Ra (RMS)
Case A presents the case that color qualities are at the same level.
Case B presents the case that most of colors are consistent, but few colors are distinct.
The example above (Pic 4.2) shows algorithm of RMS precisely present color qualities than algorithm of mean under some extreme circumstances. It can be clear to see that if colors of sample are closed to each other, the mean and RMS would get the same result. However, if there are only few colors are extremely high than rest of them, the calculation by RMS can clearly present the color quality than calculation by mean.
From what have been shown above, it can be summarized as follows.
Reference
[1] Color Quality Scale (CQS), Measuring the color quality of light sources, Wendy Davis
]]>Color temperature is a unit of measurement that characterizes the color of white light. Theoretically, the color temperature refers to the color that the Blackbody appears after being heated from absolute zero (-273 °C). The color of the blackbody gradually turns from black to red, to yellow, to white, and finally to blue light after being heated. At a certain temperature, the light color radiated by the blackbody is characterized by the temperature at that moment, and the unit is "K" (Kelvin). If the light emitted by another light source is the same as the spectral component contained in the light emitted by the blackbody at a certain temperature, the color temperature of the light source could be defined by the temperature of the blackbody. For example, the light color emitted by the 100 W bulb is the same as that of the blackbody at 2527 K, thus the color temperature of the light emitted by this bulb should be 2527 K.
Artificial light sources, such as fluorescent lamps, LED lamps, etc., are not thermally radiated. The light color emitted by the artificial light sources is not exactly the same as that emitted by the blackbody at various temperatures. The concept of "correlated color temperature" was introduced. When the light color emitted by these light sources is closest to the one emitted by the blackbody at a certain temperature, the temperature of the blackbody is referred to as the correlated color temperature of these artificial light sources.
LEDs, nowadays, have gradually replaced traditional bulbs and fluorescent lamps because of their advantages of energy saving, environmental protection, lng lifetime and small size. They are widely used in indoor lighting, signal lights, indicator lights, vehicle lights, display screens, advertising screens, outdoor large-screen and other light-emitting devices, and are praised as the new generation of green energy light-emitting device of energy-saving and environmental-friendly in the 21st century solid-state lighting field. In lighting filed, different phosphor recipes are designed to make LEDs achieve white light with different color temperatures. However, people sometimes feel that LED lighting is uncomfortable, one of the reasons is that the color coordinate points of the light sources deviate too much from the Planckian locus (or the blackbody locus), resulting in color distortion. And "Δuv" is used to characterize the color point deviation.
The color of the light sources is not good or bad, but the light color used for illumination is an exception. The light color has an evaluation index: ±Δuv, named as the color point deviation. The smaller deviation, the better the illumination color, and the best at Δuv=0. Due to meteorological factors, natural sunlight average +Δuv 0.001 is defined as CIE recombination daylight, so color temperature over 4000K requires + Δuv 0.001.
As is well-known that there is the Planckian locus in the chromaticity diagram. Different positions on the Planckian locus mean different color temperature of the illuminants. Deviation of the color coordinate for this light source to the Planckian locus is expressed by Δuv as shown in Fig. 1. The upper deviation is represented by +Δuv, and the lower deviation is represented by -Δuv. There is a standard with the color temperature at 6504 K. As the color temperature gets larger, the light color gradually changes from white → light blue → blue (6500 K~15000 K). On the contrary, the light color gradually changes from white → light yellow → yellow →orange → orange red → red (6500 K~1800 K). Generally, the white turns greenish when the Δuv increases positively, and turns magenta when Δuv increases negatively.
We selected two white light sources to show the color appearance at different color deviations which were nature white at 6500 K color temperature and warm white at 3500 K color temperature. The comparison is shown in Fig. 2.
Due to the content ratio distribution of red, green and blue phosphors, the color deviation of the LEDs even in the same batch will be inevitably generated during production process. We generally use color tolerance to determine the product criteria of the color consistency. The smaller color tolerance means the higher color consistency.
When evaluating different color temperature of the light sources, the standard light sources for reference are different (generally the detection equipment will automatically determine the standard light sources). In the lighting industry, because the human eyes have different perceptions of chromatic aberrations at different color temperatures, so the color tolerance requirements are different depend on the color temperature are different. The color coordinates would be different for the light sources with the same color temperature but different color deviation.
SDCM is an acronym which stands for Standard Deviation Color Matching. Usually, we could perceive difference of 5-7 SDCM (Fig. 3).
In order to ensure color consistency, the LED will be classified by color bin. The color tolerance at different color temperature have been defined by ANSI. The standard requirements are shown in Tab. 1. And the color bins at different color temperature are defined as shown in Fig. 4.
Table 1. The ANSI C78.377-2008 general CCTs and color tolerance for LED.
General CCT 1) |
Objective CCT and Color Tolerance (K) |
Δuv and Deviation Tolerance |
2700 K |
2725 ± 145 |
0.000 ± 0.006 |
3000 K |
3045 ± 175 |
0.000 ± 0.006 |
3500 K |
3465 ± 245 |
0.000 ± 0.006 |
4000 K |
3985 ± 275 |
0.001 ± 0.006 |
4500 K |
4503 ± 243 |
0.001 ± 0.006 |
5000 K |
5028 ± 283 |
0.002 ± 0.006 |
5700 K |
5665 ± 355 |
0.002 ± 0.006 |
6500 K |
6530 ± 510 |
0.003 ± 0.006 |
Flexible CCT (2700 - 6500 K) |
T 2) ± ΔT 3) |
Δuv 4) ± 0.006 |
Note: 1) There are 6 general CCTs corresponding to fluorescent lamps of 2700 K, 3000 K, 3500 K, 4100 K, 5000 K and 6500 K color temperatures; 2) T should be an integer of 100 K (e.g. 2800 K, 2900 K, ..., 6400 K), but excludes the 8 rated CCT values listed above; 3) ΔT = 0.0000108×T2+0.0262×T+8; 4) Δuv = 57700×(1/T)2-44.6×(1/T)+0.0085。 |
As shown Table 1, the ANSI standard defined the central coordinate on the Planckian locus for the color temperatures below 3500 K. But for high color temperatures, there are none zero Δuv which means the central coordinates deviated from the Planckian locus. Especially at 6500 K, the Δuv is 0.003 which is a large value. This means that the light color will be greenish base on this standard.
According to ANSI standard, the white light at 4000 K, 5000 K, 5700 K and 6500 K color temperature is greenish, which is disadvantageous for the color reproduction of the illuminants. Since the color deviation is tiny, so it is difficult to perceive the incorrect color appearance by human eyes. However, in the field of photography, film and television lighting, this tiny color deviation would be easily distinguished by camera or any other equipment.
To Figure out the fundamental reason of the color appearance for the light sources with the same color temperature, we fabricated two kinds of LEDs with the same color temperature but difference color coordinates. The color coordinate of one LED is just on the Planckian locus (0.3136, 0.3235), the spectrum is shown in Fig. 5 by orange curve. And the other is centered at (0.3123, 0.3282) which is the color coordinate of 6500 K white according to the ANSI standard, the spectrum is shown in Fig. 5 by blue curve.
As shown Figure 5, in the green light region at the wavelength from 480 nm to 600 nm, there are significantly more green content for the blue curve than the orange curve. Additionally, in terms of color rendering, the data show that the color rendering index Ra of the LED with orange spectral curve is higher than the other LED. Typically, for the special color rendering index R9, and in the special color rendering index R9, the value of the LED with orange spectral curve is higher about 10 than the other LED. These means that the color quality of the LED with the color coordinate on the Planckian locus is better. The differences will be more obvious from camera vison.
According to the above results, from the view of light quality, color bin scheme centered on the Planckian locus will be better than the bin scheme defined by ANSI-based. Therefore, YUJI developed own color bin scheme for photography lighting that the center color coordinate of each color temperature is just on the Planckian locus. In order to further improve the color consistency, YUJI also reduce the color tolerance of each sub-bin to 3SDCM. Fig. 6 shew the details of YUJI’s color bin scheme.
Lifetime is a critical index for evaluating lighting fixtures. Conventional lighting fixtures are relatively inexpensive, of which the most widely adopted type is the fluorescent light, with only 3000 hours’ longevity. Compared to that, LED lights are normally more costly, but are famous for their long lifetimes, normally over 50000 hours. When it comes to plant growth, a long-life light can greatly save the maintenance cost and help increase profits. This is a key factor for plant factory owners when they choose lighting fixtures.
|
Incandescent |
Fluorescent |
HID |
MHI |
LED |
Lifetime (avg.) |
<1000 h |
~ 3000 h |
~ 5000 h |
<20000 h |
>50000 h |
An LED’s lifetime is defined as the time it takes until its lumen output reaches 70% of the initial output. LEDs have long lifetimes which are usually tens of thousands of hours. It is unrealistic and uneconomical to measure by lighting it up for that long. Almost all the LED manufacturers are disclosing an estimated value based on certain driving and operating conditions, rather than an actual measured figure. This means, that if we alter any of the conditions, the longevity of the light will be changed accordingly.
Let’s first have a look at how driving conditions would affect an LED’s lifetime. In an experiment we did on an LED model with 60 mA rated current, the LEDs have been continuously lit up at 60 mA, 90 mA, 120 mA, separately. After 6000 hours, (Figure 1), the LED at 60 mA has a less than 3% luminous output attenuation after 6000 hours, while at 90 mA and 120 mA, the decay reaches 35% and 41% respectively. Overcurrent shortens lifetime.
Figure 1. Lumen decay trends of an LED model at current levels of 60mA (black), 90 mA (blue) and 120 mA (red).
Besides, operation conditions also make a big difference. High temperature will not only diminish the LED’s luminous efficacy, but also reduce the activity of the junction area, thus shrinking the LED’s lifetime. Figure 2 shows the lumen output decay trends of a same LED type at ambient temperatures of 85℃ and 105℃ respectively. It is apparent that lumen decay aggravates with the ambient temperature rising.
Figure 2. Lumen decay trends of an LED model at 85℃ and 105℃ ambient temperatures.
In one word, the lifetime of an LED is uncertain and highly dependent on its driving and operation conditions. Modern plant factories, especially vertical farms and plant chambers are normally quite enclosed with little exposure to fresh air. As such, thermal design plays a more critical role in ensuring a longer lifetime of the light.
]]>Before LED lighting technology came to maturity, greenhouses were the major form of artificial plant factories. In greenhouses, temperature condition can be better controlled than plants grown outside, and sunlight is obtained during the day through the transparent plastic. Also, by setting conventional lights in greenhouses, the shortage in sunlight can be covered, and the lighting hours can be extended, thus adjusting the plant’s photoperiodism and finally promoting plant growth and yield.
In traditional greenhouses (Figure.1), the lights are usually placed on the ceiling with quite a distance above the plants. In this case the lights can cover the largest area of plants with minimum number of lights, so the energy and the cost can be saved. However, the drawback is that the luminous intensity may not be high enough to reach the light compensation point, in which case plants cannot grow. In large greenhouses, this can be solved by setting more lights, so that the average luminous intensity per area is increased. Yet traditional lights have low conversion efficiency from electric energy to light energy and also short lifetimes, so more lights mean more cost in fixtures and electricity consumption. Besides, during the energy conversion, traditional lights generate heat not only from the energy loss, but also from the emission of invisible infrared radiation. The heat accumulates and raises the temperature within the greenhouse, which is harmful to plant growth. What’s more, due to the infrared part from traditional light’s radiation spectrum, the lights have to be placed away from the plants, in which case the luminous intensity cannot be increased by shrinking the distance. These properties determine that traditional lights can only play a supplementary role in greenhouse lighting.
Figure 1. Application of traditional lights in greenhouse
As explained in our last post on light quality, the LED differs from traditional light sources in how it illuminates. The optical properties of LEDs are decided by energy band of the semiconductor material, making it highly customizable in spectrum, which helps meet the requirements for plant growth. What’s more, the LED can be easily made as “cold light source” without any infrared radiation, so the surrounding temperature of plants is not affected too much even the LED is placed very close to the plants. Combined with high energy conversion efficiency and high customizability, LED lighting technology raises possibilities for plant factory design. Therefore, new types of factories such as vertical farms, urban farms and growing chambers emerge, which are totally isolated from the outside. With more artificial factors being controllable and more studies on plant growth being conducted, an agricultural revolution is firmly grounded and is about to come.
Figure 2. Future vertical farm based on LED lighting technology
(Source: http://edition.cnn.com/2016/09/05/world/aerofarms-indoor-farming/index.html)
Figure 3. Growing chamber based on LED light technology
(source: http://www.conviron.com/products/mtr30-reach-in-plant-growth-chamber)
]]>Compared with the factor of LED viewing angle we discussed last time, our topic today–light quality is a more significant index in terms of which LEDs have a revolutionary advantage over conventional light sources. The emergence and growth of LED technology has stimulated the development of modern agriculture and resulted in many new forms of agriculture, such as vertical farming.
Biologically, plants can perceive light within certain range of wavelength, just as the human eye does. The luminosity function is used to describe the human eye’s sensitivity to lights of different wavelengths. Plants also have their own “luminosity function”, which is hugely different from that of the human eye. Several decades ago, people found through studies that plants have certain “luminosity function” (to be precise: absorption function) towards light. Gates (1965) explained in “Spectral Properties of Plants” why plants absorb lights of different types. The absorbance properties of chlorophyll a, chlorophyll b and carotene are shown in Figure 1. Plants can well absorb blue (400 nm to 500 nm) and red light (600 nm to 700 nm), yet are less receptive to green light (600 nm to 700 nm). That is to say, not all the lights within the photosynthetically active radiation range (400 nm to 700 nm) are effective in plant growth in the same way or to the same extent. The most efficient light source is the one which emission property best matches the plant’s spectrum absorption property.
Figure 1. Absorbance spectrum of plants (Gates et al. “Spectral Properties of Plants”,1965).
Before LEDs, conventional light sources, including metal halide lights, fluorescent lights and high pressure sodium lights, have limited emission properties due to the way they illuminate. It is difficult to change or tune the spectrum when manufacturing them. For the plants, part of the light (ie.500 nm-600 nm) from conventional light sources may be useless, thus compromising the efficiency, and increasing the operation cost of the plant factory. Besides, conventional lights such as metal halide lights emit infrared light over 700 nm and produce heat, which can be harmful to plant growth. This way, more limitations are placed for the lighting design in plant factories, and growers will have to set the lights away from the plants.
Different from conventional light sources, LEDs illuminate in a much more flexible way. The emission spectrum of an LED is determined by the semiconductor material that makes up the LED chip, so different materials can be used to make LEDs of different spectrum properties (Figure 2). Therefore, LEDs are highly customizable because they can be made diverse to meet needs of different plants in different growing stages. In most cases, plants need combination of lights, such as blue light and red light. Packaged LEDs can be made according to the needs, with chips of different spectrum properties or chips combined with phosphor. Compared to the single color LED in Figure 2, the combined light source has almost the same manufacturing process (only except the added material cost). This greatly promotes the popularization of special spectrum LEDs.
To wrap up this chapter, LED lighting technology provides specialized light sources for plants’ needs, which can utilize the maximum light energy transferred from electric energy, and can help cut the production and operation cost for plant factories.
]]>In our last post, we talked about the significance of lighting for pant growth in terms of light intensity, photoperiodism and light quality. In plant growth, light intensity is expressed as photosynthetic photon flux density (PPFD). It depends on both the optical design of the fixture and the number of photons emitted by the light source.
So what are the advantages of LEDs over conventional lights in providing sufficient light intensity for plant growth?
Let’s suppose there is a plant growth environment, where the plant is growing in the nutrient solution right under the lighting fixture on the ceiling. Only the light downward can reach the plant surface and be absorbed, while the upward and sideward light cannot. Figure 1 describes the light paths of traditional light sources and LEDs without optical design. Let’s say these four lights emit the same amount of photons, and the light from a traditional fixture, due to its broad viewing angle, is largely compromised and wasted if without any reflection accessories. Moreover, the reflector itself has an absorption rate that causes light loss. If the fixture requires small viewing angle, the reflecting angle must be small too. So for a light bulb or a tube light, the light upward and sideward is reflected multiply between the reflector and the fixture, leading to huge light loss and low efficiency. While when it comes to the LEDs, it is different. The LED itself has a small viewing angle, making the utility rate much higher, even without any additional reflectors. Besides, the LED dissipates small amount of heat, so optics made of low-priced resin can be added directly onto the light source, instead of any additional reflectors. In this way, the original 120° viewing angle can be decreased to any angle smaller (which is normally 15° on the market).
Fig.1 Viewing angles of different types of light sources.
Apart from that, the diversity in the viewing angle enables the widespread application of the LED light sources in plant factories. Each plant needs its own lighting solution to meet the illuminance requirement for every stage of growth. In Figure 2, the three LED lights have different viewing angles but the same amount of photon flux. Finally they reach the same PPFD on the same illumination area. This means that for plant factories that may differ a lot in heights, it is much easier and more convenient to use LED light sources, because the flexible viewing angle choices can help reach the required photosynthetic photon flux level per unit area.
Fig.2 Three LEDs at different heights achieving the same level of illuminance.
Light, just like water, soil, temperature and air, is indispensable for plant growth. Man had realized the importance of water, soil and temperature in growing plants since a long time ago, and had begun to utilize irrigation, fertilization, and greenhouse technologies to artificially alter the conditions and environment for the plants. As a result, the yield was successfully improved. However, we’ve always had little knowledge of lighting and its influence on plant growth.
Man used to believe that plants could obtain all the necessary nutrition from the soil until the year 1880, when American scientist G. Engelmann found through experiments that green leaves could produce starch and release oxygen only with light. It was only after then that the lighting condition appeared in plant research and cultivation.
Normally the human eye can respond to wavelengths from 380nm to 780nm, (deep violet to near-infrared). Similar to the human eye, plants can absorb wavelengths from 400nm to 700nm, which is defined as photosynthetically active radiation (PAR). That is to say, only lights (wavelengths) from 400nm to 700nm can be perceived and absorbed for photosynthesis by plants.
Studies find that light influence plant growth in three aspects:
It is known that plants can absorb CO2, produce chemical energy, and release O2 only with light. In darkness, they respire, takes in O2, consume chemical energy, and release CO2. When the light intensity is 0, they only photosynthesize, and the higher the light intensity is, the faster the photosynthesis occurs. When photosynthesis and respiration happen at the same rate, the CO2 generated from latter is entirely absorbed by the former, and the light intensity at this point is defined as the light compensation point. Only when the light intensity is higher than the light compensation point can plants grow healthily. The rate of photosynthesis grows as light intensity does, but it stops when light intensity reaches a certain point, which is defined as the light saturation point.
Figure 1. Relationship between illumination intensity and photosynthetic rate
Photoperiodism is the physiological reaction of plants to the length of day and night. According to their requirements for length of light and darkness in the 24-hour time cycle, plants are categorized as short day plants, long day plants and day-neutral plants. Photoperiodism is formed during the plant’s long-term adaption to the earth environment. Studies find that decreasing or increasing the lighting period can stimulate or postpone blooming.
Figure 2. Effect of dark interruption on flowering
(Source: http://knowledgeclass.blogspot.com/2015/01/photoperiodism.html)
During the last several posts, we’ve analyzed the leading factors of LED lead frames, chips and encapsulants in terms of their qualities and costs. Now, we are moving to the finale of our “high quality LED” series, the LED phosphor.
Most LED phosphors are inorganic photoluminescent powder made of rare earth materials. Under radiance of high energy light, the electrons of the ground state absorb the energy of the exciting light and transit to the excited state of a higher-energy level. The instability of the excited state makes the electron transit back to the ground state in nanosecond time scale. During this process, the excited electrons are accompanied by the radiation of energy, in which a part of the energy is converted into photons and is perceived by us.
From the previous post on LED chips we know that different materials are used to make LEDs of different colors, such as red, yellow, green, blue, purple and so on, and a white LED can be made by mixing several colors of chips. Yet they all have different luminance efficiencies, and even for chips made of a same material, the wavelengths can hardly be adjusted to the same level. Besides, the full width at half maximum (FWHM) of the LED chip is normally quite narrow. This means producing white light by mixing different chips is costly. What’s more, the mixed light is not even close to natural light our eyes are used to, making such an approach totally inefficient.
If we add phosphor to the recipe, a great number of chips will be saved in the first place. Besides, the light will be much softer because the phosphor’s FWHM is wider than the chip’s. So let’s have a look at some of the most commonly used phosphors in the industry.
Normally phosphors can be classified by colors, such as green phosphor, orange phosphor and red phosphor. By the type of material they can also be classified as types of YAG, silicate, LuAG, GaYAG, nitride, phosphate and so on. Of course every type has its pros and cons in different application scenarios.
YAG phosphors are aluminate compounds that are both physically and chemically stable. They have wide emission FWHM and high brightness. This type is widely adopted because of the mature processing technique and the cheap raw materials. Nonetheless the narrow excitation wave band and spectral lack in the red light give the YAG type LEDs low CRI values.
This type of phosphor has wide tunable emission wavelength but poor chemical stability. It is vulnerable to moisture and high temperature, and has narrow FWHM. These properties confine the application range to small power LED packages.
The LuAG phosphor has stable chemical and thermal properties. With high luminous efficacy and little decay, it can be used together with other types of red phosphors to generate high quality white light. However, the raw materials are expensive and the manufacturing process is highly complex, making this type of phosphor costly.
GaYAG phosphor is green YAG phosphor containing Ga element. Similar to LuAG, it is chemically and thermally stable with high excitation efficiency and little decay. High CRI white LED can be realized through combination of GaYAG phosphor and other red phosphors. The drawback lies in that it has narrow excitation wave band, which raises the requirements for the LED chip.
Nitride phosphor has stable thermal performance, wide excitation wave band and wide tunable emission wavelength, thus it is commonly adopted in high CRI white LEDs. Just like LuAG phosphor, this type is also expensive due to the high price of the raw materials and the complex manufacturing process.
This is a blue phosphor normally used in full spectrum white LED together with other types of phosphor. It has limited applications and one of the highest prices owing to its unique characteristics.
Currently there are three common ways to produce white light with phosphors excited by LED chips.
The combination of violet chip and three colors of phosphors has the spectrum closest to natural light, and provides the highest light quality. That being said, large amount of phosphor is used, making the efficacy rather low. This method is applied to produce the most expensive white LEDs, for currently violet chips are much more expensive than blue chips, and the packaging technique is less developed. Applications are limited in certain fields where extremely high light quality is needed.
During the production of an LED, the phosphor is uniformly mixed with encapsulant and applied onto the chip. The phosphor is dominant in the light quality of an LED.
Here are several perspectives from which the light quality can be affected.
The particle shape is critical for the LED’s initial luminance output and decay characteristics. As showed in Fig.1 below, spherical phosphor has better dissipation in the encapsulant and can be applied more uniformly. Besides, the phosphor is a surface emitter. That is to say, the less surface defects there are, the higher luminous efficacy it will have. Spherical phosphor has less surface defects, therefore it possesses better initial luminous output.
Fig.1. a) Particle morphology of spherical phosphor. b) Particle morphology of irregular phosphor.
Particle size is the average diameter of phosphor particles, which usually ranges from a few micrometers to a dozen. The phosphor sedimentation during the dispensing process affects the LED’s spectrum characteristic. Larger size particle has greater sediment speed and is more uncontrollable. Another key factor is particle size distribution (PSD), which tells the uniformity of the particles’ sizes. So, in order to produce light sources of high color consistency, the requirements for particle size and PSD are high and strict.
Stability includes chemical stability, thermal stability and moisture resistance ability. These are all influential in the spectrum change and output decay in use. The stability of the phosphor is determined by the purity of the raw materials, the integration and the processing techniques. Phosphor manufacturers usually add a special process into the whole production procedure so as to increase the stability. Fig. 2 and Fig. 3 show the coating treatment conducted by a Yuji owned phosphor manufacturer. As can be seen, the uncoated phosphor will agglomerate after absorbing moisture, undermine the uniformity of the phosphor and finally decrease the total luminous output. While after treatment of coating, an apparent improvement in moisture resistance can be perceived.
Fig. 2. a) Uncoated phosphor before absorption of moisture. b) Uncoated phosphor after absorption of moisture.
Fig. 3. a) Coated phosphor before absorption of moisture. b) Coated phosphor after absorption of moisture.
Through this “high quality LED” series on LED lead frames, chips, encapsulants and phosphors, we can see that each part of an LED is critical for the quality of the light source in different ways. The lead frame determines heat dissipation performance, just as the chip does to the brightness, the encapsulant to the sealing performance, and the phosphor to the color quality. Any flaw in any part may cause major performance defect of an LED.
As a provider of high quality LED light source, Yuji LED always applies rigorous standard on material selection and only selects the most suitable materials to ensure the best quality and user experience. We are and always will be devoted to developing high quality light source and providing the ideal light to the market.
After analyzing LED lead frames and chips in terms of their qualities and costs, this time we will talk about another component in all LED products—the encapsulant.
As we know, the chip is the core component of an LED emitter. In order to ensure the chip’s stable performance, the packaging encapsulant acts as a key role within. The function of packaging encapsulant mainly consists of three aspects: (1) Sealing. The encapsulant prevents water, sulfur and other elements from penetrating into the LED, blocking moisture as well as dust. (2) Immobilization. It mechanically fixates the wires which connect the chip and the frame, thus preventing vibration of the wires. (3) Light extraction efficiency. The high refractive index of the encapsulant is favorable for decreasing light loss resulted from boundary reflection. Besides, it evenly mixes the phosphors to generate white light with blue chips and helps expand color diversity of the LEDs.
In general, most LED packaging companies utilize epoxy resin, methyl silicone resin and phenyl silicone resin to encapsulate LEDs. The table below shows the comparison among these three types of encapsulants in five dimensions.
Parameter |
Epoxy |
Methyl silicone |
Phenyl silicone |
Refractive index |
1.50~1.52 |
1.50~1.54 |
1.41 |
Operating temperature (℃) |
-40~130 |
-50~180 |
-60~230 |
UV prevention |
Weak |
Good |
Excellent |
Moisture& oxygen permeability |
Low |
Medium |
High |
Hardness |
High |
Medium |
Low |
The advantages of epoxy include high sealability, high shock resistance, excellent protection performance and low cost. While it also has flaws, for instance, the weakness in heat and weather resistance, high possibility to crack and yellow, and so on. Therefore, except for low-end market, there are almost no LEDs packaged in epoxy resin now.
Compared with epoxy resin, silicone resin shows some excellent merits, such as good UV prevention, high heat resistance and good heat dissipation performance. With the development of LED industry, low power LEDs cannot meet the market’s needs for brightness anymore, which means medium and high power LEDs will become increasingly popular in the near future. When being applied, medium and high power LEDs generate much more heat than low power ones. That’s why the packaging encapsulant must be more reliable in terms of heat resistance and heat dissipation performance.
Silicone resins, according to their chemical structures, can be divided into two types: methyl silicone resin and phenyl silicone resin. Both of them have their own pros and cons. For example, methyl silicone resin has better UV resistance, thus it is widely adopted for violet-chip LEDs to promote products’ reliability. And the latter is better than the former in refractive index. That is why most medium and high power LEDs adopt phenyl silicone resin for improvement of light efficiency.
There are two sides to every coin. Both epoxy resin and silicone resin can bring positive effects to LEDs, while there are also side effects due to their physical properties. Therefore, a superior LED packaging encapsulant must overcome the drawbacks. The primary function of packaging encapsulant is providing a well-sealed, safe and reliable environment for the chip to work. Now, we will explain the influences on packaged LEDs caused by the physical properties of the encapsulant.
The major function of packaging encapsulant is to create a totally sealed working circumstance for LED chips, in case that water, sulfur or other elements penetrate into LEDs and result in malfunction. Different from tungsten, the beam angle of LED chips is around 120°. Yet the light coming out from the chip get reflected, most of which cast back onto the frame. To offset such a loss in light efficacy, the surface of the frame is always coated with a layer of silver to reflect the light again. If the sealability of the encapsulant is imperfect, the sulfur in the air will penetrate into the LEDs and react with the silver. As a consequence, the reaction product-- silver sulfide is formed and the light efficacy is largely compromised. Figure 1 below shows the process of sulfidation where the color of function area gradually changes from silver to black.
Figure 1. Sulfidation of the silver coating on the frame due to bad sealabilty of the encapsulant.
Apart from this, bad sealability may also bring about the penetration of other organic compounds. Any kind of volatile compounds are likely to permeate into the porous structured organic silicone and occupy the gaps among the molecules. When being heated, the compounds will change in color and absorb light selectively. As a result, the overall performance of LEDs, including luminous flux, light efficacy, and color temperature reliability, will be influenced.
When an LED is working, it generates vast amount of heat especially for most white light LEDs. Even though there are some heat management solutions, including lead frames that conduct the heat out, there are still heat inside generated by the phosphor. Bad heat dissipation of the encapsulant keeps the heat within and creates a high temperature environment. After long hours of work, the heating problem could easily lead to crack and carbonization of the encapsulant, which can cause performance degradation and eventually luminous decay. More seriously, the crack can break the conduct wires apart and lead to LED failures.
As for moisture resistance, it shows the encapsulant’s ability to keep the moisture outside the LED. As mentioned before, bad heat resistance generates cracks in the encapsulant, leading to moisture penetration and failure of the LEDs. Furthermore, the encapsulant has its own property that takes up and retain the moisture in the air. Consequently, the moisture expansion and atomization will occur and impact LEDs function. So, the resistance abilities of heat and moisture of the encapsulant are crucial to an LED’s performance and quality. Figure 2 shows the atomization of an LED due to moisture penetration.
Figure 2. Atomization of and LED resulted by deficient encapsulant.
In terms of cost, the encapsulant takes up a small but non-negligible portion. A high-end packaging encapsulant of a well-known brand is about 450 USD/kg, while you can also get one kilogram of low-end encapsulant for ten bucks on the market with uncertain quality. We can take SMD 2835 for instance, which is the most typical 0.2 watt SMD on the market. According to the statistics from LEDinside, in August 2016, the average price of SMD 2835 is 7 USD and the lowest 4 USD. Assume that 1000 pieces of SMD 2835 need 5g packaging encapsulant, then the cost of high quality encapsulant will be more than 2.25 USD. However, by contrast, the cost of low quality encapsulant can be negligible.
Yuji LED aims to provide high quality high CRI LEDs. To keep the CRI value over 95, mixture of blue or violet chip and phosphor are used. Compared to LEDs with lower CRI, high CRI LEDs will compromise a part of luminous efficacy and generate more heat. Therefore, when developing high CRI LEDs, Yuji applies packaging encapsulants from well-known manufactures to meet the demand for heat resistance. In order to evaluate the products’ reliability, we conduct LM80 aging test on all our products. This test is done at temperatures of 55℃,85℃and 105℃ and humidity within 45%-65%, with the LEDs on for 10,000 hours successively. Figure 3 below is the LM80 test result of Yuji SMD 2835.
Figure 3: LM80 test result of Yuji SMD 2835 (Copyright @ 2016 Yuji International Co.,Ltd.)
The result shows that the lifespan of an LED is impacted by external temprature. After 10,000 testing hours, the luminous attenuation is 3.5% at 55℃. While at 105℃, it only increases 2.5% to a 6% decay. Even at a temperature as high as 105℃, the estimated average lifespan of Yuji LEDs reaches 52,000 hours, which far exceeds the industry standard---30,000 to 50,000 hours.
After long term use, the light color will inevitably change due to phosphor attenuation, and this change can be measured by chormaticity coordinates shift. Yuji packaged LEDs have the shift about just 0.0032 to 0.0038 even after 10,000 hours, standing out in the industry.
Lead frames, chips and encapsulants are all key components of packaged LEDs. Any breakdown of these parts will lead to overall degradation of the LED’s performance and will eventually compromise user experience. Hence, we must choose high quality packaged LEDs when producing light fixtures.
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