The luminous intensity is a quantity in photometry for characterizing a light source. It is defined as the luminous flux per unit solid angle. The luminous intensity takes into account the spectral response of the human eye – typically for photopic vision, i.e., assuming sufficiently high light intensity (luminance) for color vision. The term is mostly applied with the approximation of a point source, i.e., in distances which are large to the extent of the source.
The SI units of the luminous intensity are the candela = lumen per steradian (cd = lm/sr). One candela approximately corresponds to the luminous intensity of an ordinary candle.
Cases With Uniform Omnidirectional Emission
In the simplest case, where the luminous intensity is independent of the direction (uniform omnidirectional emission), the luminous intensity is the luminous flux divided by 4π sr. If the light emission is restricted to a smaller solid angle, e.g. by a lamp housing containing a reflector, the luminous intensity can become correspondingly higher for the same luminous flux. On the other hand, the size of the emitting volume is irrelevant.
For uniform omnidirectional emission, the luminous intensity is independent of the distance from the light source – in contrast to the irradiance, which scales with the inverse square of the distance from the light source.
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