While the introduction of white-light LEDs has increased interest in using LED lighting, they have raised an equal number of questions concerning what white-light LEDs can and cannot 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.
White light from LEDs can be produced by two methods: (1) combining red, green, and blue LED chips in one discrete package or cluster LED lamp, and (2) coating blue Indium Gallium Nitride (InGaN) LED chips with phosphorus. The technique of mixing red, green and blue (RGB) LED chips is used in signage because RGB LEDs can be combined to create 256 colors making it a cost-effective and efficient solution. If a lighting application requires a rainbow of colors from a single-point light source then a RGB LED is the answer.
In the second method, blue light from an InGaN LED chip filters through the phosphorus and generates a cool-white or fluorescent light appearance. InGaN technology results in superior reliability and color integrity. InGaN LEDs would be used, for example, in elevator panel lights because illumination of only one color is needed. The brightness and color purity of InGaN-white LEDs depend on the amount of phosphorus coating. Three shades of InGaN-white LEDs exist: 1) cool white, 2) pale white and 3) incandescent white. Cool white has the least amount of phosphorus, incandescent white the most and pale white is somewhere between the two. Cool white is the brightest, incandescent the dimmest and pale white holds the middle ground.
Previously, the lack of white-light LEDs has limited the integration of LEDs into a wide range of applications; however, now, the misconception exists that InGaN-white LEDs can illuminate a lens of any color thereby simplifying lighting requirements and designs. Wrong! Since the color red is not 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 the light produced is a pink color, a yellow lens turns a lemon-lime, green lens shifts to aqua and orange lens becomes yellow. To maintain accurate and brilliant colors, it is imperative to match the LED color with the lens color. In short, white LEDs made from a blue chip cannot be used as a general backlighting light source for different colored lenses and panels. (see Figure Q)