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16 Human eye sensitivity and photometric quantities fraction of the chip area is injected with current. Table 16.7 summarizes frequently used figures of merit for light-emitting diodes. Table 16.7. Summary of photometric, radiometric, and quantum performance measures for LEDs. Figure of merit Luminous efficacy Luminous efficiency Luminous intensity efficiency Luminance Power efficiency Internal quantum efficiency External quantum efficiency Extraction efficiency Explanation Unit Luminous flux per optical unit power lm/W Luminous flux per input electrical unit power lm/W Luminous flux per sr per input electrical unit power cd/W Luminous flux per sr per chip unit area cd/m2 Optical output power per input electrical unit power % Photons emitted in active region per electron injected % Photons emitted from LED per electron injected % Escape probability of photons emitted in active region % 16.6 Brightness and linearity of human vision Although the term brightness is frequently used, it lacks a standardized scientific definition. The frequent usage is due to the fact that the general public can more easily relate to the term brightness than to photometric terms such as luminance or luminous intensity. Brightness is an attribute of visual perception and is frequently used as synonym for luminance and (incorrectly) for the radiometric term radiance. To quantify the brightness of a source, it is useful to differentiate between point and surface area sources. For point sources, brightness (in the photopic vision regime) can be approximated by the luminous intensity (measured in cd). For surface sources, brightness (in the photopic vision regime) can be approximated by the luminance (measured in cd/m2). However, due to the lack of a formal standardized definition of the term brightness, it is frequently avoided in technical publications. Standard CIE photometry assumes human vision to be linear within the photopic regime. It is clear that an isotropically emitting blue point source and an isotropically emitting red point source each having a luminous flux of, e.g., 5 lm, have the same luminous intensity. Assuming linearity of photopic vision, both sources still have the same luminous intensity as the luminous fluxes of the sources are increased from 5 to, e.g., 5000 lm. However, if the luminous fluxes of the two sources are reduced so that the mesopic or scotopic vision regime is entered, the blue source will appear brighter than the red source due to 286PDF Image | Human eye sensitivity and photometric quantities
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