Abstract

It is generally taken for granted that the spectrum of light is invariant on propagation, i.e., that its (normalized) spectral distribution is the same at every point of the field; and that, moreover, it is equal to the normalized spectral density of the source itself. We show that, at least for a certain class of model sources which includes the usual thermal ones, invariance of the normalized spectrum throughout the far zone necessarily implies that the degree of spectral coherence of the source obeys a certain scaling law. The results are illustrated by reference to Lambertian sources.

© 1985 Optical Society of America