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Abstract. The problem and various techniques for underwater imaging have been reviewed. The effect of various design parameters like pulse shape and duration, etc. has been brought out. System design considerations for underwater photography have also been discussed. IntrodoctionThe search and recovery of sunken ships, submarines, lost atmoic bombs has historically been a tedious, time consuming and expensive task. Using underwater imaging and photography, not only the above tasks but also monitoring tasks such as laying cable, amplifiers and pipe lines in safe positions could be simplified. A combined sonar/TV re-entry system for the offshore oil industry is much useful. The combination of television and sonar images on board the drilling vessel provide immediate and exact positioning information for guidance in the re-entry procedure. The present day side locking sonar system is inherently prone to data interpretation problems leading to a high false target rate and the corresponding necessity of later optical verification of the target contacted. Various techniques have been developed and used with advantage for underwater imaging and photography1-4. The paper reviews the various techniques used for the purpose. Problems of Underwater ViewingThere is not enough natural light for viewing at depths below 125 m, even under good conditions at sea. A system that provides light at the target is necessary but scatter makes this difficult. Viewing under water is like trying to see in a dense fog. The problem of viewing underwater can be summarised as one of attenuation accompained by contrast diluting scatter, The one limits range by cutting signal level while the other adds noise. The solutions to these problems are as follows :-

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A relationship between the image contrast and the object contrast for a general periodic or a truncated periodic object is derived. The result is illustrated for a periodic object of rectangular wave profile. It has been shown that for a truncated sine wave object the image contrast bears a nonlinear relationship to the object contrast. The contrast dependence of the measured or the apparent transfer function defined as the ratio of the image contrast to object contrast has been investigated.

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