luminous intensity (original) (raw)

Author: the photonics expert (RP)

Definition: luminous flux per unit solid angle

Category: article belongs to category light detection and characterization light detection and characterization

Related: photometryluminous fluxradiant intensity

Units: candela (cd = lm/sr)

Formula symbol: ($I_\textrm{v}$)

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DOI: 10.61835/kpa Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is a Luminous Intensity?

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 compared to the extent of the source.

The analogous quantity in radiometry (not considering 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.

Frequently Asked Questions

What is luminous intensity?

Luminous intensity is a quantity in photometry that describes the luminous flux emitted by a light source in a particular direction per unit solid angle. It is adapted to the spectral sensitivity of the human eye under bright conditions (photopic vision).

What is the SI unit for luminous intensity?

The SI unit for luminous intensity is the candela (cd), defined as one lumen per steradian (lm/sr). The luminous intensity of an ordinary candle is approximately one candela.

How does luminous intensity differ from luminance?

Luminous intensity characterizes a light source approximated as a point source, independent of its size. In contrast, the visual brightness of an extended source is determined by its luminance, which depends on both the luminous intensity and the apparent size of the source.

Does the luminous intensity of a source change with distance?

No, the luminous intensity is an intrinsic property of the light source in a given direction and does not depend on the distance to the observer. However, the irradiance received from the source decreases with the inverse square of the distance.

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