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.