Loss of Resolution in Water as a Result of Multiple Small-Angle Scattering

Multiple scattering of light by particles suspended in liquid can be a cause of degraded optical resolution. This is especially significant when the particles are relatively large and numerous, but most of the scattered power is deflected through very small angles, owing to a near match between the refractive indices of the particles and the medium. Formulas which convert the volume scattering function to an image modulation transfer function are derived for this case. This conversion allows calculation of image degradation, given the scattering properties of the medium. The inverse transform is also derived so that the scattering properties can be calculated, given the loss of resolution. The latter is best measured by imaging a set of paralled bar patterns having a wide range of spatial frequencies. The conversion from scattering function to optical transfer function is interpreted in general and evaluated for one published scattering function measured in sea water. The results are compared to published image measurements in fresh water.