Encyclopedia > letter L > luminous intensity

Luminous Intensity

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: luminous flux per unit solid angle

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

Units: candela (cd = lm/sr)

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

DOI: 10.61835/kpa Cite the article: BibTex plain text HTML Link to this page LinkedIn

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.

More to Learn

Encyclopedia articles:

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.