Different Kinds of White Tunable LED Part.1
Tunable-white lighting is one of the biggest trends in LED lighting. LED developers have taken a serious grip on the photo-biological research being produced by university departments and other groups. We know more about the way that humans function than ever before and you might say that it’s fortunate for the LED community that the science appears to support a practical technology that is perfectly suited to LED exploitation.
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.
Types of LED Luminaires with Variable Color Output
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.