Reduction of skylight reflection effects in the above-water measurement ofdiffuse marine reflectance

Reflected skylight in above-water measurements of diffuse marine reflectanc e can be reduced substantially by viewing the surface through an analyzer t ransmitting the vertically polarized component of incident radiance. For ma ximum reduction of effects, radiometric measurements should be made at a vi ewing zenith angle of similar to 45 degrees (near the Brewster angle) and a relative azimuth angle between solar and viewing directions greater than 9 0 degrees (backscattering), preferably 135 degrees. In this case the residu al reflected skylight in the polarized signal exhibits minimum sensitivity to the sea state and can be corrected to within a few 10(-4) in reflectance units. For most oceanic waters the resulting relative error on the diffuse marine reflectance in the blue and green is less than 1%. Since the water body polarizes incident skylight, the measured polarized reflectance differ s from the total reflectance. The difference, however, is small for the con sidered geometry. Measurements made at the Scripps Institution of Oceanogra phy pier in La Jolla, Calif, with a specifically designed scanning polariza tion radiometer, confirm the theoretical findings and demonstrate the usefu lness of polarization radiometry for measuring diffuse marine reflectance. (C) 1999 Optical Society of America.