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Luminance: definition and unit of measure

The luminance L of a self-luminous surface results from the ratio of luminous intensity l and its projected area A_p.

Luminance (L) describes the brightness perceived by the human eye of a surface that, as a light source or through transmission or reflection, emits light. Luminance is defined as the ratio of luminous intensity (I) to the area projected perpendicular to the direction of observation (A_p). The photometric unit is candela per square metre (cd/m²).

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What is luminance?

Depiction of the luminance of reflecting surfaces.

Luminance of a diffusely reflecting illuminated surface. Horizontal illuminance E_h and vertical illuminance E_v in indoor spaces

While illuminance (E) is the light incident on a surface, the luminance (L) describes the light radiating from this surface. The light can radiate from the surface itself, for example from a lamp, but it can also be reflected or transmitted from a surface. For diffuse reflecting (matt) and diffuse transmitting (opaque) materials, the luminance can be calculated from the illuminance and the reflectivity- or transmittance factor.

Luminance thus forms the basis of the perceived brightness; however, the actual impression of brightness is still influenced by the state of adaptation of the eye, the peripheral contrast ratios and the information content of the surface seen.