Indirect illumination to reduce veiling luminance in seawater

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Abstract
The distance one can see using an underwater lamp is limited by veiling luminance from the lamp's beam. The simplest and least expensive way to reduce this luminance is geometrical. One separates the lamp as far as possible from the imaging aperture (eye or camera) and looks through as little of the light beam as possible. For a downlooking geometry, e.g., searching the sea floor, a geometry that directly illuminates only the extreme lateral edges of the field of view and lets scattered light suffice to illuminate the interior of the field is investigated. Image intensity and veiling luminance were calculated as functions of distance for two geometries assuming typical numbers for scattering and absorption of light in the sea. Regarding theoretical factors presented here, indirect illumination is competitive with conventional direct illumination. Regarding practical factors such as quantum-limited image detectors for extended range, the indirect scheme has merit in keeping excess light out of the detector.

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