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.

NASA funded the development of the LISST‑VSF through its SBIR program. The LISST-VSF 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, the instrument measures VSF over angles from 15° to 150°. It measures scattering for two incident laser polarizations—horizontal and vertical. For each polarization, the instrument splits the collected scattered light into its two polarization components. It then uses the four measurements at each scattering angle to solve for the VSF (also called P₁₁), and matrix 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%.). The ring detector estimates the beam attenuation coefficient from the optical transmission measurement, using the same approach as the LISST‑200X instrument.

The instrument incorporates some novel features to make it field-worthy. For example, the instrument automatically controls the gain of the photomultipliers behind the eyeball so they operate at the highest possible sensitivity without saturating. The instrument expands the VSF dynamic range by first dimming the laser so the eyeball can measure stronger scattering from 15–40°, then increasing the laser output for larger angles. Ambient light rejection is implemented by chopping the laser at high frequency.

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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