The requirements are given for absolute calibration of a spectroradiometer by means of an extended standard source. In some cases the extended source may be used without auxiliary optics, but in many cases a concave mirror is necessary or desirable. For such a mirror, a relation between incident and reflected steradiancy is derived. Brandenberg’s focusing relations for a concave spherical mirror are extended and applied to four practical cases, and it is shown that no loss in accuracy of calibration will result from the use of such mirrors provided that the source and the mirror are large enough. Experimental verification is given.
R. W. PATCH, "Reflecting Optics for Calibration of Spectroradiometers," J. Opt. Soc. Am. 55, 392-395 (1965)