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The approximate spectral sensitivity functions of the rods and three types or cones are shown in Fig. 16.3 (Dowling, 1987). Inspection of the figure reveals that night-time vision (scotopic vision) is weaker in the red spectral range and thus stronger in the blue spectral range as compared to day-time vision (photopic vision). The following discussion mostly relates to the photopic vision regime. 16.2 Basic radiometric and photometric units The physical properties of electromagnetic radiation are characterized by radiometric units. Using radiometric units, we can characterize light in terms of physical quantities; for example, the number of photons, photon energy, and optical power (in the lighting community frequently called the radiant flux). However, the radiometric units are irrelevant when it comes to light perception by a human being. For example, infrared radiation causes no luminous sensation in the eye. To characterize the light and color sensation by the human eye, different types of units are needed. These units are called photometric units. The luminous intensity, which is a photometric quantity, represents the light intensity of an optical source, as perceived by the human eye. The luminous intensity is measured in units of candela (cd), which is a base unit of the International System of Units (SI unit). The present definition of luminous intensity is as follows: a monochromatic light source emitting an optical power of (1/683) watt at 555 nm into the solid angle of 1 steradian (sr) has a luminous intensity of 1 candela (cd). The unit candela has great historical significance. All light intensity measurements can be traced back to the candela. It evolved from an older unit, the candlepower, or simply, the candle. The original, now obsolete, definition of one candela was the light intensity emitted by a plumber’s candle, as shown in Fig. 16.4, which had a specified construction and dimensions: one standardized candle emits a luminous intensity of 1.0 cd . 16.2 Basic radiometric and photometric units 277PDF Image | Human eye sensitivity and photometric quantities
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