We show that the cross-spectral density in the far zone of a homogeneous spherical source can be described as a lowpass filtered version of that existing across the source surface. We prove that, to an excellent approximation, the corresponding filter with respect to a (normalized) spatial frequency ζ has a functional structure of the form √1-ζ2, for 0 ≤ ζ ≤ 1. The cases of spatially incoherent and Lambertian sources are treated as significant examples.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics