## Instrumentation to measure the backscattering coefficient bb for arbitrary phase functionsPublished in Applied Optics, Vol. 50 Issue 21, pp.4134-4147 (2011) |

**Spotlight summary:**What is

*b*? As light propagates through the ocean, it experiences an exponential attenuation due to absorption and scattering. The backscattering coefficient

_{b}*b*is that component of the total scattering coefficient at angles greater than 90°. This is important because light that is scattered at angles less than 90° tends to keep going in the general direction of the incident light. If the sun is directly overhead, light scattered at an angle less than 90° will continue deeper into the ocean, while light that is absorbed or scattered at an angle greater than 90° will not. If the sun is not directly overhead, the penetration of sunlight can still be approximated using

_{b}*b*and a simple correction using the cosine of the sun angle. Because of the importance of photosynthesis, penetration of sunlight is the primary application of

_{b}*b*, but the same argument applies to moonlight, wide laser beams, and the lights that squid fishers use to attract squid to the surface.

_{b}The direct measurement of

*b*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

_{b}*b*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 caseâ€”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.

_{b}Haubrich

*et al.*describe a new technique to directly measure

*b*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.

_{b}--James Churnside

**Technical Division:**Information Acquisition, Processing, and Display

**ToC Category:**Atmospheric and Oceanic Optics

OCIS Codes:
| (010.4450) Atmospheric and oceanic optics : Oceanic optics |

(120.5820) Instrumentation, measurement, and metrology : Scattering measurements | |

(290.1350) Scattering : Backscattering | |

(010.4458) Atmospheric and oceanic optics : Oceanic scattering | |

(010.1350) Atmospheric and oceanic optics : Backscattering |

Posted on August 05, 2011

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