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In this work we discuss low brightness contrasts anticipated from bottom reflection changes due to water depth and other factors. Even under good lighting conditions it is necessary to expose film at full aperture and boost it in processing to an equivalent film speed of 1000. The best near term solution to this available light problem appears to be to turn to photoelectric sensors which also have inherent data handling advantages. The second and a closely related problem is depth calibration for different water spectral trans-missities. A red-green ratioing method was early devised by Moore (1945) for this problem - his "optical transparency method." In Southern California coastal waters there is a seasonal as well as individual storm change in coastal water turbidity due to run-off and dumping of terrigenous sediment and its mobilization by coastal currents. The problems of solar brightness variations on the water surface and the spatially complex calibration for inhomogeneous suspended sediment loads are apparently well-handled by the technique of electronically ratioing the difference of red-blue and red-green channels of the 9-channel Bendix EMSIDE scanner. The basic concept is that the red channel (6492 A) does not penetrate the water and hence, contains only surface infor-mation (e.g., solar glitter patterns, sea state variations, surface sediment plumes), whereas the blue and green channels contain a combination of surface and depth penetration information (e.g., blue light absorp-tion, green light scattering, bottom reflectances). Thus, differencing of these separate bands against the red band produces images free of surface effects. Ratioing of the differenced blue and green bands electronically produces a synthetic image whose density values are automatically calibrated in the same way as suggested by Moore in his optical transparency method. As a result we find the corresponding ratioed images of Catalina and Little Harbors, free of sun glitter and density changes. This synthetic image may then be isodensity traced and produce even depth density differences despite sharp differences in suspended sediment load.
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