Whereas illuminance expresses the luminous power incident on a surface, the luminance describes the light given off by this surface. This light can be given off by the surface itself (e.g. when considering luminance of lamps and luminaires). Luminance is defined as the ratio of light intensity and the area projected perpendicularly to the emission direction. The light can also be reflected or transmitted by the surface however. For diffuse reflecting (matt) and diffuse transmitting (murky) materials, the luminance can be calculated from the illuminance and the reflectance or transmittance. Brightness correlates with luminance; although, the actual impression of brightness is still influenced by how well the eyes have adapted, by the surrounding contrast levels and by the information content of the viewed surface.