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In radiometry, a measure of the intrinsic radiant intensity emitted by a radiator in a given direction. It is the irradiance (radiant flux density) produced by radiation from the source upon a unit surface area oriented normal to the line between source and receiver, divided by the solid angle subtended by the source at the receiving surface. It is assumed that the medium between the radiator and receiver is perfectly transparent; therefore, radiance is independent of attenuation between source and receiver. </dd>
If the radiant source is a perfectly diffuse radiator (that is, emits exactly according to Lambert law), then its radiance is equal to its emittance per unit solid angle. The radiance of a light source is termed luminance (formerly, brightness). </dd>


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use