Colors
The color of the light emitted by single-color LEDs can be set through the selection of the semiconducting material. The wavelength range of the light extends from near-infrared through the visible spectrum to the ultraviolet range. The shorter the wavelengths become, the larger the band gap of this semiconductor and the higher the conducting voltage UD for the operation of the LED.
The following table shows sample values for colors with the associated wavelengths, possibly usable semiconducting materials and associated conducting voltages. This table merely contains example values, therefore characteristic values and materials are not fully applicable and not applicable to every LED.
Color | Wavelength λ in [nm] | Material | Conducting voltage UD in [V] |
|---|---|---|---|
Infrared | >760 | Gallium arsenide (GaAs) Aluminum gallium arsenide (AlGaAs) | <1.6 |
Red | 610 - 760 | Aluminum gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminum gallium indium phosphide (AlGaInP) Gallium phosphide (GaP) | 1.6 - 1.9 |
Orange | 590 - 610 | Gallium arsenide phosphide (GaAsP) Aluminum gallium indium phosphide (AlGaInP) Gallium phosphide (GaP) | 1.8 - 2.2 |
Yellow | 570 - 590 | Gallium arsenide phosphide (GaAsP) Aluminum gallium indium phosphide (AlGaInP) Gallium phosphide (GaP) | 2.0 - 2.4 |
Green | 500 - 570 | Indium gallium nitride (InGaN) Gallium nitride (GaN) Gallium phosphide (GaP) Aluminum gallium indium phosphide (AlGaInP) Aluminum gallium phosphide (AlGaP) | 2.2 - 2.7 |
Blue | 450 - 500 | Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) | 2.6 - 3.3 |
Violet | 400 - 450 | Indium gallium nitride (InGaN) | 3.2 - 3.6 |
Ultraviolet | 230 - 400 | Aluminum nitride (AlN) Aluminum gallium nitride (AlGaN) Aluminum gallium indium nitride (AlGaInN) | 3.5 - 4.2 |
LEDs can generally only generate light in a small wavelength range with a width of a few tens of nanometers. White light is the sum of all colors or the sum of all wavelengths in the visible range. Therefore, colors must be mixed additively in order to generate white light with an LED. There are various methods of doing this, of which two essential methods are described below.
- Combination of different colored LEDs
Red, green and blue LEDs (RGB LEDs) can be combined with one another in a housing so that the colors mix in order to generate white light. If the LEDs are controlled appropriately, the light appears to be white. In the RGB combination of LEDs it is also possible through appropriate control of the individual LEDs to generate light of a different color with continuous color transitions. - Luminescence
A short-wave LED (blue, violet, ultraviolet) is combined with photoluminescent dye. Photoluminescence describes the emission of light after excitation by light – usually blue or ultraviolet. The dye converts blue, higher-energy light into longer-wave light with a typically larger wavelength range. The dye used significantly influences the color temperature, so that different white tones (Cold White, Warm White) can be generated.
As the duration of use of LEDs increases, the color of the emitted light changes due to aging. These color changes proceed differently with each LED. In LEDs that emit white light by means of a photoluminescent dye, both the LED chip and the dye itself age.