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16 Human eye sensitivity and photometric quantities 16.2 Basic radiometric and photometric units The luminous intensity of a light source can thus be characterized by giving the number of standardized candles that, when combined, would emit the same luminous intensity. Note that candlepower and candle are non-SI units that are no longer current and rarely used at the present time. The luminous flux, which is also a photometric quantity, represents the light power of a source as perceived by the human eye. The unit of luminous flux is the lumen (lm). It is defined as follows: a monochromatic light source emitting an optical power of (1/683) watt at 555 nm has a luminous flux of 1 lumen (lm). The lumen is an SI unit. A comparison of the definitions for the candela and lumen reveals that 1 candela equals 1 lumen per steradian or cd = lm/sr. Thus, an isotropically emitting light source with luminous intensity of 1 cd has a luminous flux of 4π lm = 12.57 lm. The illuminance is the luminous flux incident per unit area. The illuminance measured in lux (lux = lm/m2). It is an SI unit used when characterizing illumination conditions. Table 16.1 gives typical values of the illuminance in different environments. Table 16.1. Typical illuminance in different environments. Illumination condition Full moon Street lighting Home lighting Office desk lighting Surgery lighting Direct sunlight Illuminance 1 lux 10 lux 30 to 300 lux 100 to 1 000 lux 10 000 lux 100 000 lux The luminance of a surface source (i.e. a source with a non-zero light-emitting surface area such as a display or an LED) is the ratio of the luminous intensity emitted in a certain direction (measured in cd) divided by the projected surface area in that direction (measured in m2). The luminance is measured in units of cd/m2. In most cases, the direction of interest is normal to the chip surface. In this case, the luminance is the luminous intensity emitted along the chip-normal direction divided by the chip area. The projected surface area mentioned above follows a cosine law, i.e. the projected area is given by Aprojected = Asurface cos Θ , where Θ is the angle between the direction considered and the surface normal. The light-emitting surface area and the projected area are shown in Fig. 16.5. The luminous intensity of LEDs with lambertian emission pattern also depends on the angle Θ 278PDF Image | Human eye sensitivity and photometric quantities
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