The direct measurement of bb in the ocean is difficult, because it requires measurement of the scattered light integrated over the 2π steradians of backscatter solid angle for a small length along the incident propagation direction. The most common solution is to measure the volume scattering coefficient at a single angle and estimate bb assuming that the shape of the volume scattering function is known. Scattering angles of 120° and 140° have been used with some success in regions where the volume scattering function is similar to the one that is assumed. But this is not always the case—especially in dense plankton blooms. The volume scattering function may even have an azimuthal dependence as when, for example, highly elongated diatom chains are aligned by current shear.
Haubrich et al. describe a new technique to directly measure bb in the ocean. It is not as simple as using a detector with a large collecting aperture behind the scattering volume; that geometry produces a weighting function proportional to the cosine of the scattering angle, while the correct weighting function is proportional to the sine. They have devised a very clever way to produce the desired weighting function by wrapping the detector collection aperture in a cylinder around the optical axis of the illuminating beam. The normal to the aperture is perpendicular to the illuminating beam rather than parallel, and a sinusoidal weighting function results. In the rest of the paper they relate a very careful theoretical and experimental investigation into the calibration and the performance of such an instrument. The final conclusion is that measurement accuracy of a few percentage less is possible under a wide range of conditions.
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