Radiance
The physical quantity that corresponds closely to the visual brightness of a surface. A simple radiometer for measuring the (average) radiance of an incident beam of optical radiation (light, including invisible infrared and ultraviolet radiation) consists of a cylindrical tube, with a hole in each end cap to define the beam cross section there, and with a photocell against one end to measure the total radiated power in the beam of all rays that reach it through both holes (see illustration). If A1 and A2 are the respective areas of the two holes, D is the length of the tube (distance between holes), and is the radiant flux or power measured by the photocell, then the (average) radiance is approximately given by the equation .
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A simple radiometer
McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.
radiance
[′rād·ē·əns] (optics)
The radiant flux per unit solid angle per unit of projected area of the source; the usual unit is the watt per steradian per square meter. Also known as steradiancy.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
radiance
The rate of radiant emission per unit solid angle and per unit projected area of a source in a stated angular direction from the surface.
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