Application of the small-angle approximation to ocean water types

The small-angle approximation to the radiative transport equation is applie d to particle suspensions that emulate ocean water. A particle size distrib ution is constructed from polystyrene and glass spheres with the best avail able data for particle size distributions in the ocean. A volume scattering function is calculated from the Mie theory for the particles in water and in oil. The refractive-index ratios of particles in water and particles in oil are 1.19 and 1.01, respectively. The ratio 1.19 is comparable to minera ls and nonliving diatoms in ocean water, and the ratio 1.01 is comparable t o the lower limit for microbes in water. The point-spread functions are mea sured as a function of optical thickness for both water and oil mixtures an d compared with the point-spread functions generated from the small-angle a pproximation. Our results show that, under conditions that emulate ocean wa ter, the small-angle approximation is valid only for small optical thicknes ses. Specifically, the approximation is valid only for optical thicknesses less than 3. (C) 2001 Optical Society of America.