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The luminous intensity of any surface in a given direction per unit of projected area of the surface viewed from that direction. The International Commission on Illumination defines it as the quotient of the luminous intensity in the given direction of an infinitesimal element of the surface containing the point under consideration, by the orthogonally projected area of the element on a plane perpendicular to the given direction. Simply, it is the luminous intensity per unit area. Luminance is also called photometric brightness.
Since the candela is the unit of luminous intensity, the luminance, or photometric brightness, of a surface may be expressed in candelas/cm2, candelas/in.2, and so forth.
The stilb is a unit of luminance (photometric brightness) equal to 1 candela/cm2. It is often used in Europe, but the practice in America is to use the term candela/cm2 in its place.
The apostilb is another unit of luminance sometimes used in Europe. It is equal to the luminance of a perfectly diffusing surface emitting or diffusing light at the rate of 1 lumen/m2. See Luminous intensity, Photometry
(L), a photometric quantity equal to the ratio of luminous flux d2ф and geometric factor dΩdA cos θ:
L = d2ф/dΩdA cos θ
where dΩ is the solid angle filled with radiation, dA is the area of the emitting or illuminated surface, and θ is the angle between the normal to the surface and the direction of the radiation.
Two special definitions of great practical interest follow from the general definition of luminance. First, luminance may be defined as the ratio of the luminous intensity I of a surface element and the area of the element’s projection perpendicular to a given direction: L = dI/dA cos θ. Second, luminance may be defined as the ratio of the illuminance E at a point of a plane that is perpendicular to the direction to the source and the unit solid angle occupied by the flux produced by the illuminance:
L = dE/dΩ cos θ.
Luminance is measured in candelas per m2. It is the photometric quantity that is most directly associated with visual sensations, because the illuminance of the image of an object on the retina is proportional to the luminance of the object.
In the system of radiometric quantities, the quantity that is analogous to luminance is called radiance. Radiance is measured in watts per steradian per m2.
D. N. LAZAREV