The LISST-VSF is the first autonomous VSF instrument for use in the sea. It measures the Volume Scattering Function, the beam attenuation and scattering coefficients (c and b), and depolarization via the elements P12 and P22 of the scattering Mueller matrix.
The LISST-VSF was developed with funding from NASA under the SBIR program. It employs a unique optical design. Whereas past instruments, laboratory or field, employ geometries that look at a common sample volume via a set of discrete detectors (Figure 1 left), or rotate a single detector around a common volume (Figure 1 center), the LISST-VSF design employs a rotating eyeball (Figure 1 right).
Figure 1 – The two prior methods of measuring VSF (left, middle) and the LISST-VSF method (right).
The eyeball views scattering normal to its window. As the eyeball rotates, it views different parts of a laser beam, thereby viewing scattering at different angles. In this way, VSF is measured over the angles 15°-150°. Scattering is measured for two polarizations of the incident laser, ‘horizontal’ and ‘vertical’. For each polarization, the scattered light collected by the eyeball is split into its two polarizations. The four measurements for each scattering angle are then solved for the VSF (also called P11), and elements P12 and P22.
For small angles below about 15-degrees (forward scattering), we use concentric silicon ring detectors, identical to ones used in our LISST-200X instruments.
From the measured VSF out to 150-degrees, the total scattering coefficient b is computed (missing the contribution from 150-180°, which is <1%.). Beam attenuation coefficient is estimated by the optical transmission measurement obtained by the ring detector in a manner identical to our LISST-200X instrument.
The instrument incorporates some novel features to make it field-worthy. For example, the gain of photomultipliers behind the eyeball is automatically controlled to operate at highest possible sensitivity but without saturation. The dynamic range capability of VSF is expanded by first dimming the laser while the eyeball measures the stronger scattering from 15-40°, and then undimming the laser for larger angles. Ambient light rejection is implemented by chopping the laser at high frequency.
Data collection, storage and retrieval is supported by simple to use firmware and software. MATLAB software is provided for processing of data. The figure below shows the VSF measured by LISST-VSF produced from calibration data of an actual commercial unit, contrasted to Mie theory. Note that for spherical particles, P22=1 at all angles. The departures from unity occur in the vicinity of 45-deg and 135-deg, and are explained in the Manual.
Fig.2: Measurements with LISST-VSF compared with Mie theory (red) for 0.496 micron polystyrene beads.
For an article on measurements of Arizona test dusts made with the LISST-VSF, see the article below.
Slade, W.H., Agrawal, Y.C. and Mikkelsen, O.A., 2013, September. Comparison of measured and theoretical scattering and polarization properties of narrow size range irregular sediment particles. In Oceans-San Diego, 2013 (pp. 1-6). IEEE.
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