Abstract
This paper describes invariance of the normalized optical spectrum for a particular class of optical fields propagated in the far zone from a secondary, spatially, partially coherent source. The optical field across the secondary planar source is inhomogeneous to give the complex degree of spectral coherence such that h{αω(ρ1-ρ2)}exp{iεω(ρ12-ρ22)}, where α, ε are constants, ω is optical frequency, and ρ1, ρ2 denote two points in the secondary source. This expression for is the same as obtained in the Fresnel zone from a primary, spatially incoherent source. The invariance law does not hold for the spectrum of the light propagated from the primary source.
