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The need for improved vision in various undersea applications has been recognized for decades. Improvements in underwater cameras and lenses have followed similar improvements in land-based cameras, but limitations associated with the image transfer characteristics of the medium have been primary obstacles in developing high performance systems. The application of lasers to the imaging formation process has provided a means of acquiring long range images and has, at least partially, formed the basis for the development of high power blue- green laser technology. Unlike atmospheric applications, underwater images can be formed only over small distances -- perhaps only tens of meters. Distance is primarily determined by the optical power available, the system geometry, the detection means, and (primarily by) the optical attenuation of the medium. The attenuation is governed by absorption and scattering processes which act to either remove or redirect photons originating from the source of illumination. In spite of the limitations imposed by these effects, considerable progress has been made over the past several years toward increasing the image formation range and image quality. These gains have been made through the use of nonconventional image formation techniques and laser technology. This paper reviews the classically described methodologies for underwater image formation, as well as their limitations, and discusses recent developments that extend imaging capability beyond the construction of 2-dimensional reflectance maps. For example, a system under development produces three-dimensional maps of the image space using a scanning laser configuration. The scene is viewed from a separate location to provide depth information via triangulation. The detector provides an estimate of position of the apparent landing spot of the laser beam for each scan angle from which a depth estimate is calculated. The system is designed to scan a 20 X 20 degree field-of-view at distances from 0.5 to 1.5 meters with a resolution of about 1 millimeter.
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