Abstract

A calibration method is described which makes it possible to measure the brightness or steradiancy of an extended source with a photometer or radiometer which may have nonuniform directional response. A uniform extended source of limited size is successively observed, first as a point source and then as an infinite extended source, with the same radiometer. A circular aperture, slightly larger than the radiometer lens, is illuminated through diffusing sheets of opal glass. The brightness of this source is computed from the total intensity measured with the radiometer at a distance large enough to permit use of a “point-source” calibration scale. Then the radiometer is brought up to the source, where both aperture and field are completely filled, to obtain the response to an “infinite” extended source of known brightness.

© 1953 Optical Society of America

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