The Point Spread Function (PSF) is an importantproperty in predicting beam propagation and imaging system performance. An instrument to measure the in situ PSF of ocean water has been built and PSF profiles obtained. This instrument consists of two parts, a flashlamp with cosine emission characteristics, and an imaging solid state camera system. The camera system includes a thermoelectrically cooled CCD array with over 50dB of dynamic range. This allows the camera to
measure the steeply peaked PSF over short (lOm) to long (80 m) ranges. Measurements of the PSF in three different locations are presented. One location was a coastal station off San Diego where the water column exhibited a well defmed shallow (approximately 30 meter) mixed layer with a particulate maximum (defined by a maximum in beam attenuation) at the bottom of this layer. During these measurements the PSF was highly variable with depth, as was to be expected
due to the dependence of the PSF on particle concentration and size distribution. In the second example the water column was almost homogeneous (as evidenced in the beam attenuation profiles). Hence, the PSF showed very little dependence on depth. Measurements of the variation of the PSF with range are also presented. A simple relationship of the variation of the PSF with angle and optical path length is presented.