Light-emitting diodes (LEDs) are solid-state sources, where the emitted light depends on the characteristics of the semiconductor material. It is usual to call an LED a light-emitting diode prepared from inorganic semiconductors, in contrast to organic light-emitting diode (OLEDs), prepared from organic compounds. In this chapter, devoted to the measurement of optical properties, we will deal only with inorganic semiconductor LEDs.
Photometric and Colorimetric Properties of Light-Emitting DiodesTwo classes of semiconductors are used nowadays to manufacture LEDs: aluminum gallium indium phosphide (AlGaInP) for producing red to yellow LEDs and indium gallium nitride (InGaN) for building green to blue LEDs. The two classes of LEDs have slightly different photometric and colorimetric properties. For example, the emission wavelength and intensity of AlGaInP (or shortly GaPbased) diodes depend more strongly on temperature than that of GaN-based diodes. Figure 5.1 shows the temperature dependence of a red and of a blue LED. As can be seen, both the intensity of the radiation diminishes, and the spectrum changes to longer wavelength in both cases, but the changes are much larger in case of the red LED. Due to such temperature dependence, all photometric and colorimetric measurements have to be performed at well-defined constant temperature, and if white light is produced for example in a Red-Green-Blue (RGB) LED module by mixing the light of red, green, and blue LEDs, keeping the color constant needs some active control.
