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

A transmissometer measures a—generally small—portion of the forward-scattered light: light that scatters away from the beam but remains within the detector’s acceptance angle. As a result, it overestimates transmittance and underestimates the beam attenuation coefficient. This error depends on the optical elements and design of the transmissometer. There is no standard for transmissometer acceptance angle. 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 you want to correlate beam attenuation with total suspended particulate mass, you should choose a transmissometer with a larger acceptance angle, such as 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 you want to understand how forward scattering affects underwater imaging or LIDAR systems, you should quantify small-angle forward scattering. Choose a transmissometer with a small acceptance angle, such as the LISST-200X. Note that the LISST-200X will also give you the Volume Scattering Function, as will Sequoia’s LISST-VSF.
• When you use beam attenuation measurements as input to any scattering model, you must consider the model’s angular resolution. If the transmissometer’s acceptance angle is smaller than the model’s angular resolution, you gain little benefit. This is because the model cannot resolve that finer detail.
• 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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