Abstract

Problems of radiative transfer in seawater are unique because the volume scattering function σ(θ) is sharply peaked in the forward direction. Consequently, angles of interest vary over 3.5 orders of magnitude (1 rad to 180°). The smallest angles blur vision, while the larger ones affect illumination, photosynthesis, signalling by light beam, etc. Not surprisingly, no one theory of radiative transfer is convenient for such a broad range of angles, nor is one class of meter appropriate for all measurements. We have developed unconventional instruments for both the small angels (cos θ ~ 1) and the large ones (θ > 29°). The former is based on MTF theory, the latter on the spherical harmonic technique for radiative transfer. The small-angle meter is based on a resolution test pattern (bar charts). These spatial frequencies have a simple dependence on distance: they decay exponentially. For large angles, we have built a radiometer that measures ten integral moments of radiance as a function of depth. From these data we infer nine spherical moments of σ(θ). Data reduction inverts the equations of radiative transfer, which avoids some limitations of conventional instruments. Two voyages have yielded satisfactory results and suggested improvements using lamps and simultaneous readings of the ten moments.

© 1991 Optical Society of America