Luminance describes the intensity of the light impression perceived by the human eye when looking at light produced directly by a source or reflected from a surface. Therefore, it can be defined as the luminous intensity or luminosity ascribed to a given surface.
It depends on the intensity of illumination on the observed object, the reflective properties of the object's surface (color, degree of roughness) and its apparent luminous surface area. The apparent illuminating surface is the size of the surface of the illuminating plane perceived by the observer, depending on the direction of its observation.
The apparent illuminating surface is both a plane illuminating directly - a lighting fixture, and a plane illuminating indirectly, e.g. a wall, through reflection of light. When the angle between the perpendicular to the luminous surface and the viewing direction is 0 °, the area of the apparent luminous surface is equal to the area of the luminous surface. As the above-mentioned angle, the area of the apparent luminous surface decreases according to the cosine of that angle, down to an angle of 90 °, when it is zero.
Luminance is expressed by the formula: