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
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