Light is associated with a very narrow region of the electromagnetic spectrum between about 380 and 780 nanometers (nm), and is formally defined as optical radiation that can evoke a visual response in humans. Two classes of photoreceptors-rods and cones-found in the human retina transduce electromagnetic radiation into neural signals that ultimately evoke visual responses. Interestingly, light is the only physical quantity defined in terms of the human condition. All other physical quantities, such as length, mass, and time would continue to exist if the human species were to become extinct (Bureau International des Poids et Mesures, 1983; 2005; 2006). Only the definition of light would have to undergo a major revision by the species that succeeds us.
Photometry is the measurement of light. Several orthodox photometric quantities are used to characterize light sources and light fixtures and to specify or regulate the application of light. The base unit in orthodox photometry is the candela (cd), which is a measure of the luminous intensity of a light source in a particular direction. The luminous intensity of a source varies with angle, so light sources will produce different luminous intensities in different directions. The luminous intensity distribution of a frosted incandescent lamp is nearly the same in any direction, whereas an automobile headlight will have a maximum intensity in the direction of travel with much lower luminous intensities orthogonal to the maximum.
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