Different Kinds of White Tunable LED Part.2
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.
Types of Controls for Color-Tunable Products
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.