Modulation transfer function of water

Particulate matter of the sizes, concentrations, and refractive indices found in situ renders the Optical Transfer Function of water a real quantity. Modulation Transfer Function (MTF) is measured in vitro by spatial filtering of the projected image of a slit in the Fourier Transform plane. Analysis is by Moire fringes with a smooth and continuous variation of spatial frequency (nominally 0-40,000 cycles/radian) obtained by counter-rotating Ronchi rulings. The analogue of convolution of impulse responses is tested as the cascadability in a scatter- ing medium of true sinusoidal MTF´s. Coulter Counter techniques are used to measure differential particulate count in 15 channels up to 100 μm. Experimental data for various ranges and particle distributions are compared to theoretical predictions based on volume scattering functions (VSF) obtained by Mie scattering calculations and the Fourier transform conversion relating MTF and VSF first obtained by Willard Wells. The equipment is being repackaged for in-situ measurements to accompany forthcoming flood-illuminated SEGAIP (SELF GATED IN-WATER PHOTOGRAPHY) trials.