Scripps Institution of Oceanography

A submerged object of reflectance R forms a contrast C with a water background of reflectance Rw. The magnitude of this contrast depends upon the altitude of the sun and the relative amount of sunlight in the daylight impinging on the surface of the water. Known optical principles can be used to predict the apparent contrast seen by the observer whenever the water surface is perfectly calm. If the water surface is roughened by wind, the apparent contrast of submerged objects is reduced.

Many factors combine to govern the visual detectability of submerged objects, even when the observer is stationed beneath the water surface. Detection in such a case requires that the luminance or the color of the object differ sufficiently from that of its background for the optical signal reaching the observer to exceed his contrast threshold despite attenuation by the intervening water. Optical principles govern the generation of contrast by a submerged non-self-luminous object and the transmission of the resulting optical signal to any point within the water, under natural lighting conditions.