luminance (original) (raw)
Author: the photonics expert (RP)
Definition: luminous flux per unit solid angle and unit projected area
Category: 
light detection and characterization
Related: photometrybrightnessradiance
Units: candela per square meter (cd/m2 = lm sr−1 m−2)
Formula symbol: ($L_\textrm{v}$)
Page views in 12 months: 899
DOI: 10.61835/k0u Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What is Luminance?
The luminance is a photometric quantity which may be applied to light sources, but also to light which is reflected or passes through a particular area. The luminance is the luminous flux per unit solid angle and unit source area. It can also be defined as the luminous intensity per unit emitting area.
The SI units of the luminance are the candela per square meter (cd/m2 = lm sr−1 m−2).
A high luminance of a light source is achieved if it produces a high luminous flux from a small emitting area and emits into a small solid angle. For example, a high intensity discharge lamp, having a quite small light-emitting volume, can produce a much higher luminance than a long-arc lamp generating the same luminous flux. Particularly high luminance values are achieved with lasers having a high beam quality.
For an observing eye, the luminance of a light source more or less determines its visual brightness. If the light source could shrink while maintaining its luminous flux, it could send the same amount of light through the eye's pupil, but that light would be concentrated to a smaller area on the retina and therefore appear brighter — which would be consistent with its increased luminance. Similarly, the luminance of a surface, which is caused by some illumination, is what determines how bright the surface will appear. If the observer moves away from the light source, less light will enter the pupil, but the image of the source on the retina will become smaller, as long as the angular resolution of the eye remains sufficient, so that the apparent brightness is maintained. This way, one can understand why the luminance is independent of observation distance.
However, for large observation distances, where the angular resolution of the eye becomes insufficient, the apparent brightness is reduced despite the constant luminance.
Some typical examples of luminance values:
- the Moon: 2.5 · 103 cd/m2 (seen through the clear atmosphere)
- the Sun: 1.6 · 109 cd/m2
- filament of a tungsten incandescent lamp: 107 cd/m2
The analogous quantity in radiometry is the radiance. Just like the radiance, the luminance is based on a relatively sophisticated concept; see the article on radiance for many aspects which can be directly applied to the luminance as well.
Frequently Asked Questions
What is luminance?
Luminance is a photometric quantity describing the luminous flux emitted or reflected from a certain area per unit solid angle. It can also be defined as the luminous intensity per unit of emitting area.
What are the SI units of luminance?
The SI unit for luminance is candela per square meter (cd/m2), which is equivalent to lumens per steradian per square meter (lm sr−1 m−2).
How does luminance relate to the perceived brightness of an object?
For an object that the eye can resolve, its luminance determines its perceived brightness. Apparent brightness remains constant with distance because as less light enters the pupil from further away, the image on the retina becomes proportionally smaller.
Which light sources have a particularly high luminance?
What is the radiometric equivalent of luminance?
The analogous quantity to luminance in radiometry is radiance. Luminance is a photometric quantity, meaning it is related to the perception by the human eye, whereas radiance is a purely physical quantity.
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