The introduction of white-light LEDs has increased interest in using LED lighting. But it also has raised an equal number of questions concerning what white-light LEDs can and can not illuminate, and how they can be used. To understand the potential of white-light LEDs, it is helpful to examine the two methods used to produce white light from LEDs—combining three LED chips (red, green, and blue) in one discrete package or as a cluster LED lamp, and coating blue indium-gallium-nitride (InGaN) LED chips with phosphorous (see the figure).
Designers use red, green, and blue (RGB) LED chips in signage because they can be combined to create 256 colors. This potential makes such a technique a cost-effective and efficient solution. If a lighting application requires a rainbow of colors from a single-point light source, then an RGB LED is the answer.
In the second method, blue light filters through the phosphorous coating and generates a cool-white or fluorescent-light appearance. The InGaN technology results in superior reliability and color integrity. InGaN LEDs could be used in elevator panel lights, for instance, because they only require one color. The brightness and color purity of InGaN white LEDs depends on the amount of phosphorus coating. Three shades of white (InGaN) LEDs exist: cool white, pale white, and incandescent white. Cool white has the least amount of phosphorous, incandescent white has the most, and pale white is somewhere between the two. Moreover, cool white is the brightest, incandescent is the dimmest, and pale white holds the middle ground.
Previously, the lack of white-light LEDs limited the integration of LEDs into a wide range of applications. Now, however, some designers wrongly believe InGaN white LEDs can illuminate a lens of any color, thereby simplifying lighting requirements and designs.
Since the color red isn't represented in the white LED, white LEDs can only be used behind a clear or milky white lens or panel. Place a white LED behind a red lens, and a pink color is produced. A yellow lens turns lemon-lime, a green lens shifts to aqua, and an orange lens becomes yellow. To maintain accurate and brilliant colors, it is imperative to match the LED color with the lens color. With that said, white LEDs can be used as an illuminator (area/task lighting) for all colors, producing a fairly consistent result. White LEDs made from a blue chip cannot be used as a general backlighting light source for different colored lenses and panels.