When designing a transmissometer or a laser diffraction-based sensor for particle size analysis, an important consideration is the beam acceptance angle.

A transmissometer will measure a – generally small – portion of the forward scattered light: The light that is scattered away from the beam but still within the acceptance angle of the detector. Thus, the transmittance is overestimated, resulting in an underestimated beam attenuation coefficient. This error depends on the optical elements and design of the transmissometer. There is no standard for transmissometer acceptance angle, and  each manufacturer may use a different acceptance angle for a particular design. This begs the question: What determines the best detector acceptance-angle choice for a transmissometer design?

The quick student would probably immediately answer that the smaller the acceptance angle, the better the measurement. But that may not always be correct. As with any other measurement, one should think about WHAT is being measured and WHY when you consider if you need to worry about the acceptance angle.

• Density differences in the medium can steer the beam at random – completely independent from the ordinary scattering processes – and increase the beam attenuation coefficient. E.g., Mikkelsen et al., 2008 and this webinar. This effect – called Schlieren – becomes more pronounced the smaller the acceptance angle. Thus, if one is interested in correlating the beam attenuation to the total suspended particulate mass, one should choose a transmissometer with a larger acceptance angle, for example the LISST-Tau. In this manner, the observed variations in the beam attenuation coefficient will be mostly due to variations in suspended load and the effect of density induced small-angle forward scattering variations minimized.
• However, if one is interested in the impact forward scattering has on, e.g., underwater imaging or LIDAR systems it may be relevant to quantify the small-angle forward scattering somehow and one should choose a transmissometer with a small acceptance angle, e.g. the LISST-200X. Note that the LISST-200X will also give you the Volume Scattering Function, as will Sequoia’s LISST-VSF.
• If one is using the beam attenuation measurement as input to scattering models of any kind, it becomes relevant to consider the angular resolution of the models: If the transmissometer acceptance angle is smaller than the angular resolution of the model, there is little gain to be had, since the model can’t deal with the finer resolution anyway.
• Finally, there’s the manufacturability issue: the smaller the acceptance ange the more difficult and expensive it becomes to engineer and manufacture a transmissometer.

For much more on this see these references:

OCCG Protocol Series (2019). Beam Transmission and Attenuation Coefficients: Instruments, Characterization, Field Measurements and Data Analysis Protocols. Boss, E., Twardowski, M., McKee, D., Cetinić, I. and Slade, W. IOCCG Ocean Optics and Biogeochemistry Protocols for Satellite Ocean Colour Sensor Validation, Volume 2.0, edited by A. Neeley and I. Cetinić, IOCCG, Dartmouth, NS, Canada. http://dx.doi.org/10.25607/OBP-458.

Acceptance angle effects on the beam attenuation in the ocean. Emmanuel Boss, Wayne H. Slade, M. Behrenfeld and G. Dall’Olmo.

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