Small-angle (≾10-3 rad) scattering of light by ocean water is quantitatively analyzed. The two mechanisms that give rise to such scatterings are suspended biological particles having an index of refraction close to that of water and refractive effects due to large-scale (compared to the laser beam diameter) index of refraction variations. Results of recent experiments performed at the Stanford Research Institute are compared with the present analysis, and reasonable agreement is obtained. Also, the modulation transfer function (MTF) for the scattering mechanisms is derived. It is found that for values of the transverse distances ρ less than the size of the large-scale index of refraction fluctuations but larger than the size of the suspended biological particles, the MTF due to the two mechanisms has a different functional dependence on transverse distance. Thus, an experimental determination of the dependence of the MTF on ρ will be useful in determining if a dominant scattering mechanism is at play. Also, we conclude that for long propagation paths (≿ 10 m), suspended particles can degrade the transverse coherence properties of the laser beam much more than do large-scale refractive index variations.
H. T. Yura, "Small-Angle Scattering of Light by Ocean Water," Appl. Opt. 10, 114-118 (1971)