Accurate knowledge of the relative image irradiance is important in the design of some opticalinstruments, e.g., IR systems, where it determines the apparent temperature of the object, andcamera systems employing irregular-shaped aperture stops or light-absorbing elements. We reviewthe quick Hopkins method for calculating the relative image irradiance of systems with circular orelliptical stops and then describe a modification that enables it to be used for the less conventionalsystems mentioned. The new method is used to calculate the relative irradiance of a camera objectivethat includes a neutral density wedge. We also present a brief discussion of the often misunderstoodcos{sup}4 law, which is of some importance in the design of optical instruments.
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