The statement that the mutual intensity function is equal to the two-dimensional spatial Fourier transform of the traditional radiance is analyzed within the framework of the theory of partial coherence and the generalized radiometry. It is found that this relationship, which plays a central role in the transport theory of mutual intensity function propagation in a multiple scattering medium, is valid (1) in the source plane if and only if the source is quasi-monochromatic, wide sense spatially incoherent, and Lambertian and (2) in the radiation field if and only if the region of interest lies in the Fresnel diffraction zone of a quasi-monochromatic, wide sense spatially incoherent Lambertian source. It is also shown in the Fresnel approximation that the aforementioned relationship is universally correct for the generalized radiance and that the traditional and generalized radiance functions are identical under conditions 1 and 2 above. The concept of mutual radiance is introduced, and a generalized radiance invariance principle is developed. An example that illustrates these results is worked out, and conditions for the validity of the small angle transport equation are developed.
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