During the course of the investigation on the primary standard of light at the National Research Council of Canada [ Sanders and Jones, J. Opt. Soc. Am. 52, 731 ( 1962)] it was found that the illuminance distribution across the image of the opening of the blackbody radiator was not uniform. The nonuniformity was attributed to diffraction caused by the imaging lens. An experimental check on this assumption has been carried out in which the blackbody radiator was simulated by a source consisting of a projection lamp, condenser lens, and an integrating sphere, which provides a uniform luminance. The nonuniformity of illuminance across the image, as well as the decrease of illuminance in the center, was measured using two different lenses. The latter follows the prediction in accordance with the diffraction theory to within 0.1%. If stray light is considered the agreement is further improved. A method is discussed which eliminates errors of the primary standard of light caused by diffraction.
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