During the last three decades, several attempts have been made to establish a relation between radiometry and physical optics. These attempts have marked the beginning of a new theory that can be denominated as modern radiometry. The present state of this theory may be summarized as follows.
The fundamental primary quantity in classical radiometry is the spectral radiant fluxF(Î½) defined as the total power of radiation per unit frequency interval at frequency Î½ transported through some reference surface. The other radiometric quantities are defined as the corresponding spatial densities of the radiant flux on the basis of the fundamental law of radiometry.
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