Proceedings PaperScattering Meters For Light In The Sea
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To solve radiative transfer problems in seawater, we need two inherent properties, the volume scattering function (VSF) and the absorption. The traditional direct way to obtain these quantities uses a transmissometer and a scattering meter. However, there are prob-lems with the small sample size and errors in obtaining absorption by integration of the VSF. An indirect method also shows promise. One measures the radiance field and then inverts the equations of radiative transfer to obtain the inherent properties from the apparent. The only serious shortcoming is that radiance must be a function of only one position coordinate (plus two angles). (This coordinate is depth in the case of sunlight, or distance from an isotropic lamp otherwise.) We discuss two practical implementations of this indirect approach. One would measure the radiance field with a set of fisheye cameras (following R. Smith's precedent). This very thorough method produces lots of data and requires extensive calibration and number crunching. A proposed alternate radiometer would measure certain spherical moments of the radiance field, the moments being selected to facilitate recovery of the inherent properties [Appl. Opt. 22, 2313 (Aug 83)]. This scheme would produce fewer data, but it permits recovery of absorption and moments of the VSF in (nearly) real time. Similar direct and indirect approaches apply to the measurement of very small-angle scattering, from a milliradian to a few degrees, the sort of angles that blur vision. The indirect method infers small-angle scattering from the loss of contrast in images of bar charts. In this case, the indirect method is clearly superior for the same reasons that bar charts and other test patterns are widely used (instead of point spread functions) to evaluate the performance of television and various optical systems. We built a seawater MTF meter on this principle before 1970, and its features are briefly reviewed.