The University of Maine Maine In-situ Sound & Color Lab
School of Marine Sciences
Acceptance Angle Effects on Beam Attenuation

For nearly six decades the beam attenuation has been measured in water. It has been used to estimate the concentration of suspended particles in aquatic environments by correlating it with particle volume, total particulate mass and particulate carbon.

The theoretical beam attenuation, that which account for ALL scattered light, is not possible to achieve as a finite detector is needed to measure the transmitted light from which the beam-attenuation is derived. In addition, scattering due to turbulence has been shown to occur at very small forward angles; if this scattering is included in the measurement (by using a very small acceptance angle), the interpretation of the measurement would be complicated.

Different manufacturer have built different beam transmission sensor having, among other differences, different acceptance angles. This issue is well known and different corrections have been suggested. For example, Jerlov (1976) concluded, based on Petzold’s measurements, that at maximum 7% of the beam attenuation of oceanic waters is due to scattering in the first 1degree.

Here we show that the measurements of beam attenuation with different commercially available transmissometers all with sub one degree acceptance angles deployed in the coastal ocean can vary by as much as 60% for the same water. Within a single tidal period, this variation could be less than 10% and as large as 60%. These measurements are closest at the highest shear suggesting scattering due to turbulence cannot explain these discrepancies. Unfortunately, unless the (time varying) volume scattering function in the near forward is measured, there is no way to reconcile these measurements.

These results imply that empirical formulations linking the beam transmission to biogeochemical parameters or underwater visibility need to be reformulated to include the acceptance angle of the transmissometer used.

Boss, E. and W. Slade, 2006. Acceptance Angle Effects on Beam Attenuation. Ocean Optics Conference 2006, Montreal, Quebec.

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