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The solid angular luminous flux density in a given direction from a light source. It may be considered as the luminous flux on a small surface normal to the given direction, divided by the solid angle (in steradians) which the surface subtends at the source of light. Since the apex of a solid angle is a point, this concept applies exactly only to a point source. The size of the source, however, is often extremely small when compared with the distance from which it is observed, so in practice the luminous flux coming from such a source may be taken as coming from a point. See Candlepower, Photometry
a fundamental photometric quantity characterizing a source of visible radiation. It is in general different for different directions from the source. The luminous intensity is equal to the ratio of the luminous flux emitted by the source, in an infinitesimal (that is, very small) solid angle containing the given direction, to the solid angle. The unit of luminous intensity in the International System of Units is the candela. The concept of luminous intensity is applicable only at distances from the source that greatly exceed the source’s dimensions.