One of the main issues concerning diver visibility is that it is directly
dependent on the availability of natural light. The theories developed and discussed in the last chapter, for the most part, are only applicable under ideal situations. In other words, they can be seen as a one-way propagation special case, where path radiance can be neglected or does not pose significant issues, such as those in the development of the new Secchi disk theory.
However, the convenience of natural light is not always available nor
dependable from an operational standpoint. Also, the limited range of detection of the passive approach negates the usefulness of underwater EO systems at times, despite its higher resolution. Active EO imaging systems, like those of active sonar systems, can take advantage of extended ranges of detection and identification by applying various approaches to reduce (or discriminate against) mostly backscattered photons that enhance the range. Because of the similarities in principle between active EO and acoustical systems, and the wide-spread use of acoustical systems in underwater sensing applications, both are discussed in this chapter.
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