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Luminous Intensity

Definition: luminous flux per unit solid angle

German: Lichtstärke

Category: light detection and characterization

Units: candela (cd = lm/sr)

Formula symbol: <$I_\textrm{v}$>


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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 analogous quantity in radiometry (not consider the spectral response of the human eye) is the radiant intensity.

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.

Note that the visual brightness of a light source is determined by its luminance, and not by its luminous intensity because it also depends on the size of the source.

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 <$4\pi$> 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.

See also: photometry, luminous flux, radiant intensity

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