Yoshihiro OHTSUKA

Department of Engineering Science, Faculty of Engineering, Hokkaido University, Sapporo, 060 Japan

(Received June 30, 1995; Accepted August 1, 1995)

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 Eľ=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 Eľ 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.

Key words : spectrum invariance on propagation, inhomogeneous optical fields, particular complex degree of spectral coherence