Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m²). A non-SI term for the same unit is the "nit". The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m².

Explanation

Luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye's pupil. Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between 50 and 7002300000000000000♠300 cd/m². The sun has luminance of about 7009160000000000000♠1.6×10⁹ cd/m² at noon.

In video, luma represents the brightness in an image (the "black-and-white" or achromatic portion of the image). Luma is typically paired with chrominance. Luma represents the achromatic image, while the chroma components represent the color information. Converting R'G'B' sources (such as the output of a 3CCD camera) into luma and chroma allows for chroma subsampling: because human vision has finer spatial sensitivity to luminance ("black and white") differences than chromatic differences, video systems can store chromatic information at lower resolution, optimizing perceived detail at a particular bandwidth.

Luma versus luminance

Luma is the weighted sum of gamma-compressed R'G'B' components of a color video – the prime symbols (') denote gamma-compression. The word was proposed to prevent confusion between luma as implemented in video engineering and luminance as used in color science (i.e. as defined by CIE). Luminance is formed as a weighted sum of linear RGB components, not gamma-compressed ones. Even so, luma is erroneously called luminance.SMPTE EG 28 recommends the symbol Y' to denote luma and the symbol Y to denote luminance.

Luminance is a photometric measure of the density of luminous intensity in a given direction, measured in candela per square metre (cd/m²).

Luminance may also refer to:

See also