LED is the lighting of the future. As one of the world market leaders in the area of innovative lighting technologies, OSRAM place particular emphasis on the development of white LEDs. There are also LEDs in many other light colors such as red, green, yellow or blue.
The LED’s semiconductor material determines the dominant wavelength and thus the emitting light’s color.
The production of the LED light colors
The light emitted from a light-emitting diode has a specific wavelength and thus a specific color. The latter depends on the LED’s semiconductor material. LED semiconductors consist of combinations of elements such as phosphides or arsenides. There are various combinations, each of which releases varying amounts of energy according to the semiconductor material’s bandgap. When charge carriers are recombined, photons are emitted according to specific discrete energy levels. This specifies the particular light color. For example, blue light is produced if a high level of energy is released and it is red light if a lower level of energy is emitted. Thus monochromatic (single color) light is produced. And the following is LED’s special feature: Each LED light color is limited to a very narrow range of wavelength (keyword: dominant wavelength) which accordingly only represents a specific light color. The only spectrum that cannot be produced directly from the chip is the white light spectrum, since white light represents a mixture of all light colors.
There are two methods for producing white LED light: photoluminescence and additive color mixing.
Photoluminescence: blue LEDs become white
The more common procedure is the principle of photoluminescence. Here, a thin phosphorus layer is applied on top of a blue LED. The LED’s shortwave energy-rich blue light stimulates the phosphorus layer to light up and it emits lower-energy yellow light. Part of the blue light is thus transformed into white light. The white light’s color tone can vary with the metering of the phosphorus colorant. Different white tones, such as warm white, neutral white or cold white are thus produced.
White light from additive color mixing
The second method for producing white LED light is based on the principle of additive color mixing. In this case, the white light is produced by mixing red, green & blue light (RGB) at different wavelengths. The advantage of this method is as follows: The light color can be changed by specified control. White light as well as colored light may thus be produced if desired. This process is used, for example, in LED television sets where LEDs are used to produce a picture and background lighting.