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flux of the light source divided by the electrical input power. Luminous efficiency = Φlum / (I V ) (16.4) where the product (I V) is the electrical input power of the device. Note that in the lighting community, luminous efficiency is often referred to as luminous efficacy of the source. Inspection of Eqs. (16.3) and (16.4) reveals that the luminous efficiency is the product of the luminous efficacy and the electrical-to-optical power conversion efficiency. The luminous efficiency of common light sources is given in Table 16.6. Table 16.6. Luminous efficiencies of different light sources. (a) Incandescent sources. (b) Fluorescent sources. (c) High-intensity discharge (HID) sources. 16.5 Luminous efficacy and luminous efficiency Light source Edison’s first light bulb (with C filament) (a) Tungsten filament light bulbs (a) Quartz halogen light bulbs (a) Fluorescent light tubes and compact bulbs (b) Mercury vapor light bulbs (c) Metal halide light bulbs (c) High-pressure sodium vapor light bulbs (c) Luminous efficiency 1.4 lm/W 15–20 lm/W 20–25 lm/W 50–80 lm/W 50–60 lm/W 80–125 lm/W 100–140 lm/W The luminous efficiency is a highly relevant figure of merit for visible-spectrum LEDs. It is a measure of the perceived light power normalized to the electrical power expended to operate the LED. For light sources with a perfect electrical-power-to-optical-power conversion, the luminous source efficiency is equal to the luminous efficacy of radiation. Exercise: Luminous efficacy and luminous efficiency of LEDs. Consider a red and an amber LED emitting at 625 and 590nm, respectively. For simplicity, assume that the emission spectra are monochromatic (Δλ → 0). What is the luminous efficacy of the two light sources? Calculate the luminous efficiency of the LEDs, assuming that the red and amber LEDs have an external quantum efficiency of 50%. Assume that the LED voltage is given by V = Eg /e = hν/e. Assume next that the LED spectra are thermally broadened and have a gaussian lineshape with a linewidth of 1.8kT. Again calculate the luminous efficacy and luminous efficiency of the two light sources. How accurate are the results obtained with the approximation of monochromaticity? Some LED structures attain excellent power efficiency by using small light-emitting areas (current injection in a small area of chip) and advanced light-output-coupling structures (see, for example, Schmid et al., 2002). However, such devices have low luminance because only a small 285PDF Image | Human eye sensitivity and photometric quantities
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