An expression is derived for the mutual coherence factor for a plane electromagnetic wave propagating in a region of dielectric turbulence. It is assumed that the medium is locally homogeneous and isotropic and that the scale of turbulence l is much larger than the wavelength λ. The formula is valid within ranges L determined by assuming small scattering angles (L«l4/λ3) and [(2π/λ)2〈(Δn)2〉lL]2«1 where 〈(Δn)2〉 is the mean-square fluctuation of the refractive index. It is shown that this same expression can be derived within the limits of geometric optics for small separation distances.
LEONARD S. TAYLOR, "Decay of Mutual Coherence in Turbulent Media," J. Opt. Soc. Am. 57, 304-305 (1967)