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according to a cosine law. Thus the luminance of lambertian LEDs is a constant, independent of angle. For LEDs, it is desirable to maximize luminous intensity and luminous flux while keeping the LED chip area minimal. Thus the luminance is a measure of how efficiently the valuable semiconductor wafer area is used to attain, at a given injection current, a certain luminous intensity. There are several units that are used to characterize the luminance of a source. The names of these common units are given in Table 16.2. Typical luminances of displays, organic LEDs, and inorganic LEDs are given in Table 16.3. The table reveals that displays require a comparatively low luminance because the observer directly views the display from a close distance. This is not the case for high-power inorganic LEDs used for example in traffic light and illumination applications. Photometric and the corresponding radiometric units are summarized in Table 16.4. Table 16.2. Conversion between common SI and non-SI units for luminance. Unit 1 cd/cm2 (1/π) cd/cm2 1 cd/m2 Common name 1 stilb 1 lambert 1nit Unit (1/π) cd/m2 (1/π) cd/ft2 Common name 1 apostilb 1 foot-lambert Table 16.3. Typical values for the luminance of displays, LEDs fabricated from organic materials, and inorganic LEDs. Device Display Display Luminance (cd/m2) 100 (operation) 250–750 (max. value) Device Organic LED III–V LED Luminance (cd/m2) 100–10 000 1 000 000–10 000 000 279PDF Image | Human eye sensitivity and photometric quantities
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