paper reviewed by Dr. Wayne Slade
Extending LISST-VSF Measurements to Clear Open Ocean Measurements using an Absolute Calibration Approach
At long last, this month’s featured paper is “Calibration of the LISST-VSF to derive the volume scattering functions in clear waters” by Lianbo Hu et al., published in Optics Express (2019).
The volume scattering function (VSF) describes the angular scattering distribution of light. The VSF is a key inherent optical property particularly relevant to propagation of laser beams, underwater imaging, and ocean color. Along with the absorption coefficient, these optical properties define the interaction of light with seawater (without considering polarization).
The paper is particularly concerned with a rigorous, absolute calibration method for Sequoia’s LISST-VSF Instrument. The LISST-VSF is the only commercially-available in-situ instrument for measuring the VSF across a wide angle range from approximately 0.1° to 150°. The range from approximately 0.1° to 15° is covered by Sequoia’s 32-ring detector optics as used in the LISST-200X, with the remaining angles out to 150° measured by rotating “eyeball” optic scanning the laser beam within the sample volume. Sequoia’s default data processing uses the calibrated VSF measured by the near-forward ring detectors to scale the eyeball scattering measurements. A clean water background measurement is also applied to both the ring detector and eyeball measurements. A particle-free blank can be prepared in the laboratory (e.g., nanopure water), however, measurements in the field then include a combination scattering by salts and particles. An alternative is to use a filtered seawater background measurement, however this precludes measuring particles below the filter pore size (typically 0.2 µm), which may be of interest when examining links between biogeochemistry and optics.
Because the authors are specifically interested in studying backscattering and small particles, they developed an alternative, absolute calibration method for the eyeball measurements using NIST-traceable polystyrene microspheres with well-characterized size distribution and index of refraction. A thorough examination of the LISST-VSF eyeball measurement processing is presented, including aspects of the laser power switching, polarization, and PMT gain switching. Their calibration approach is then described compared with the default approach. Briefly, they performed calibration experiments with three different bead suspensions, assuming modeled index of refraction and particle size distribution, calculated VSFs according to Mie theory for homogenous spheres. A Monte Carlo approach was used to characterize uncertainties in the size distribution and refractive index (they only appear to consider uncertainty in the imaginary index), and the theoretical calculations were compared with lab measurements of concentration series of three different bead diameters: 0.2 µm for performing the calibration and two larger (0.5 µm and 11.0 µm) for validation. Their results with the validation measurements indicated overall average relative uncertainties of 11.1%.
The authors then applied their calibration method to data from three field experiments and compared results using their absolute and the default calibration. Their results suggest the two calibration methods agreed well except in the clearer waters where βp(15°)<0.1 m−1sr−1, below which the values derived from using the standard calibration were significantly larger (average difference of 83.2% from their LP-2017 and LP-2018 data) than the values derived from using their absolute calibration method. This difference is attributed to noise in the outer two detector rings (used to scale the eyeball measurement) that becomes significant especially in clearer waters. They also compared the results from LISST-VSF with the single angle ECO-BB3 instrument (Sea-Bird Scientific) measuring at a scattering angle of ~124° and found that the distribution of data derived from their absolute calibration matched the ECO-BB3 results better compared with the default calibration.
Finally, Hu and colleagues note that one advantage of the default approach of calibration using scaling of eyeball to ring data and subtraction of clean water background is that it is expected to compensate for drift in the LISST-VSF instrument over time. They recommend that their absolute calibration method be used for clear oceanic waters to overcome the limitation of low signal to noise in the outer ring detectors, and to avoid the use of a filtered seawater blank since it removes the contribution of very small particles to the measurement. For more turbid waters, they recommend using the default approach since ring detector signal is high and also because a lower PMT gain would likely be needed that is outside of the range used in the bead measurements. For intermediate waters, the methods produce similar results and either method is applicable.
The results presented show that their method improves the accuracy of results from the LISST-VSF in clear waters, and furthermore, the authors have since applied the method to explore key questions related to particle scattering in the ocean, with relevance to ocean color and biogeochemistry (see references below).
A word of caution: the calibration coefficients derived in the study are instrument-specific and are not expected to be applicable to other LISST-VSF instruments. However, the methodology presented in their work is presented clearly and can be easily applied to other instruments. Indeed, even for a specific instrument, the calibration will need to be periodically repeated to accommodate any drift in optics and electronics.
- Hu, L., Zhang, X., Xiong, Y., Gray, D. J., & He, M.-X. (2020). Variability of relationship between the volume scattering function at 180° and the backscattering coefficient for aquatic particles. Applied Optics, 59(10), C31.
- Zhang, X., Hu, L., Xiong, Y., Huot, Y., & Gray, D. (2020). Experimental Estimates of Optical Backscattering Associated With Submicron Particles in Clear Oceanic Waters. Geophysical Research Letters, 47(4).
Other recent work using the LISST-VSF to better understand particle scattering and how it relates to biogeochemistry and ocean color:
- Koestner, D., Stramski, D., & Reynolds, R. A. (2018). Measurements of the volume scattering function and the degree of linear polarization of light scattered by contrasting natural assemblages of marine particles. Applied Sciences (Switzerland), 8(12), 1–32.
- Koestner, D., Stramski, D., & Reynolds, R. A. (2020). Assessing the effects of particle size and composition on light scattering through measurements of size‐fractionated seawater samples. Limnology and Oceanography, 65(1), 173–190.
- Gomathisankaraprasad Harharasudhan, V., & Shanmugam, P. (2019). Modelling the Particulate Backscattering Coefficients of Turbid and Productive Coastal Waters. Ocean Science Journal, 54(2), 147–164.