Book excerpt: The irradiance pattern was measured for the propagation of a collimated beam of light underwater. A neodymium-doubled green laser was transmitted horizontally at a six foot depth in Lake Winnipesaukee, N.H. The irradiance was measured at distances from 0 to 100 feet and for off-axis angles from 0 to 58 degrees. The water had an attenuation length of 4.54 ft/ln and an absorption coefficient of 0.0446 ln/ft. The ratio of attenuation coefficient to absorption coefficient was 4.94. The fractional power contained within a cone of various angles was computed. At 20 attenuation lengths only 10% of the total power is contained within a cone of 4 degrees whereas at 4 attenuation lengths this same cone contains 50% of the power. (Author).