The Volume Scattering Function
The optical Volume Scattering Function(VSF) is an ‘inherent optical property’ of water that is used by optical oceanographers to predict light propagation, image degradation, remote sensed ocean color, biological environment etc. in water.
Sequoia offers 3 instruments for this application. The LISST-100X (superseded by -200X), LISST-Stokes, and LISST-VSF. Respectively, these are for small-angle VSF, small-angle depolarization, and full-angle VSF with depolarization. All measure beam attenuation.
Small Angle VSF with the LISST-100X
The LISST-100X measures a weighted form of the VSF, covering the small forward range of angles, from 0.05 to 10° or 0.1 to 20°. No polarization information is available. The VSF measurements are offered at 32 log-spaced angles, i.e. successive angles increase by the factor 1.1809, beginning at the smallest values of 0.05 or 0.1 degree, depending on the model of instrument used, respectively C or B.
All measurements are made at the 670 nm wavelength of the laser. Measurements have been found to agree to few percent with Mie theory (see e.g. Agrawal (2005) in our library). Systems for the 532 nm green wavelength have been produced as custom units and remain available.
Small-Angle Depolarization with the LISST-STOKES
The LISST-STOKES from Sequoia is the world’s first commercially available submersible instrument for measuring small-angle depolarization in situ. Forward scattered light is divided into two polarizations; one part scatters onto a ring detector from where the VSF is obtained. The other part of the light passes through a crossed analyzer and is detected by a CCD array. From this measurement the depolarization ratio is obtained. Beam attenuation is also measured.
The instrument is designed for use in situ and can be submerged to a depth of 300m. Temperature and depth Power is applied externally, either via an external power cable or an external battery pack (included).
The first LISST-STOKES was shipped in August 2010.
Wide Angle VSF with Polarization, using the LISST-VSF
The LISST-VSF is an instrument that combines the small-angle VSF capability of the LISST-100X with a wide angle capability, and with additional information on polarization properties of the scattered light.
Like the LISST-100X, the LISST-VSF measures the VSF at small angles, 0.1 to 15-degrees with identical optics. Additionally, with a novel new ‘roving eyeball’ optics, it obtains a measure of the depolarization element P12.The roving eyeball covers the angles 15-160º in 1-degree intervals.
It describes the angular dependence of the fraction of incident light that scatters into different directions:
Integrating the VSF from 0 to 180º yields the total scattering coefficient, b [m-1].
Integrating the VSF from 0 to 90º yields the forward scattering coefficient, bf [m-1].
Integrating the VSF from 90 to 180º yields the backscattering coefficient, bb [m-1].
The VSF is identical to the P11 element of the scattering Mueller matrix of water.
Sequoia offers 3 instruments that measure the VSF: The LISST-100X, the LISST-STOKES, and the LISST-VSF.
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