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.

Underwater lighting at virtually all distances (including long ranges) produced by submerged lasers (pulsed and continuous) was measured on-axis and at off-axis angles out to 900 in seven types of water. All of these data are summarized by a single closed form equation for engineering use. Computer studies using this equation have extrapolated the data to other types of water. The equation can predict the performance of submerged lasers operating at any wavelength. Methods for measuring the applicable optical constants of natural waters are described. Techniques for long range underwater photography using conventional light sources are illustrated and supplemented by engineering equations.