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. 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 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. Similar, the luminance on 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 remain sufficient, so that the apparent brightness is maintained. That 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 for 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.