The ability to image objects underwater is limited by several physical processes which occur as light propagates through water. Light traveling underwater from an optical source to a distant receiver is attenuated by absorption (loss of photons) and scattering (photons scattered out of the field of view of the receiver) by water particulates and dissolved substances. Before the photon limit is reached, image contrast and resolution can be degraded by optical scattering in several ways. Some light which does not reach the underwater object is reflected by water particles and biological organisms into the receiver field of view. This backscattered light creates a background noise level which degrades the image contrast. Secondly, light which reaches and is reflected from an underwater target encounters small forward angle scattering on its travel to the receiver. This scattering degrades contrast by decreasing the image sharpness or resolution. Finally, scattered light originating from other optical sources, such as the sun, can also increase the detector noise level at shallow depths.
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