Spectra of light produced by sources with different coherence properties

Abstract

It is known that the spectrum of light emitted by a steady-state primary or secondary source depends, in general, not only on the source spectrum but also on the degree of spectral coherence between the source fluctuations. Moreover the spectrum of the emitted light may be different at different points of observation. These facts do not appear to have been generally appreciated in the past, probably because light produced by thermal sources and also light generated by lasers is rather exceptional in this respect. The theoretical prediction of noninvariance of the spectrum of light on propagation1,2 has been recently verified by experiments.3 We report results of an investigation regarding the changes in the spectrum of light produced by sources of a certain class, as the light propagates from the source into the far zone. Light produced by sources that obey the scaling law1 and also light generated by sources that do not satisfy it are considered. Particular attention is paid to the relationship between (a) the source spectrum of a completely incoherent source σ1, (b) the field spectrum generated by σ1, and (c) the field spectrum produced by a source σ2 that has the same source spectrum as σ1 but is partially coherent.