Spectral invariance in fields generated by quasi-homogeneous scaling law sources

The spectrum of a field generated by a planar, secondary, quasi-homogeneous source of any state of coherence that obeys the scaling law is investigated. It is shown that throughout the half-space into which such a source radiates, the normalized spectrum of the radiated field is, to a good approximation, equal to the normalized source spectrum, except perhaps at points at distances of the order of a wavelength or less from the source plane. (C) 2002 Elsevier Science B.V. All rights reserved.