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In photometry, a measure of the intrinsic luminous intensity emitted by a source in a given direction; the illuminance produced by light from the source upon a unit surface area oriented normal to the line of sight at any distance from the source, divided by the solid angle subtended by the source at the receiving surface. Also called brightness but luminance is preferred. See Lambert law. Compare luminous emittance. </dd>
It is assumed that the medium between source and receiver is perfectly transparent; therefore, luminance is independent of extinction between source and receiver. The source may or may not be self-luminous. Luminance is a measure only of light; the comparable term for electromagnetic radiation in general is radiance. </dd>


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