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PolVSM

 

Scientific context :

The volume scattering function (VSF) describes the angular distribution of the light scattering that results from an incident beam interacting with a small volume of water. Knowledge of the VSF of marine particles is of primary importance for remote sensing and marine biogeochemical applications. VSF measurements have been rarely carried out because of practical difficulties to build an appropriate instrumentation. The main challenging tasks in the development of such instruments consist in (i) the measurements of the VSF at small (<1°) and large (> 170°) scattering angles and (ii) the minimization of the influence of the photons scattered in the backward direction by the instrument itself on the photons backscattered by the sample. We are developing a new instrument (so-called POLVSM) capable of measuring the Mueller scattering matrix elements over a wide range of scattering angles (typically from 1° to 178°). The device is designed to drastically reduce the stray light induced by the instrument itself in the backward direction. The POLVSM instrument provides the polarized volume scattering function of the particles which has not been measured for two decades. Here, we present the development of the instrument, the calibration experiments and the preliminary tests carried out in a coastal environment. 

 

The PolVSM

The system is illuminated by a laser source (figure 1a). The water sample is introduced within a black cylindrical basin (volume of 1.5 l) rotating around a vertical axis which cross the direct beam at half distance of the total direct pathlength. The incident direct beam is introduced using a first periscope and finally received by a sensor (sensor C in figure 1a) using a second periscope, thus providing the attenuation coefficient of the water sample. The pathlength of the direct beam in the sample is 70 mm. The scattered signal is measured using a detector which is positioned behind a window interlinked to the basin. The optics consists of a rotating polarizer filter, a convergent lens, diaphragms and a sensor positioned in the optical focal spot of the lens. The scattering angle is determined by the angle of rotation of the system : basin + sensor (figure 1b).

Funding and development strategy:

The PolVSM instrument is designed by the team and build in our lab with the help of the S2M workshop. The concept of the instrument, and especially the design of both periscope is patented. This development was funded by the French space agency (CNES) and by the Université Pierre et Marie curie (UPMC)

 

People:

From LOV : Malik Chami (PI), Edouard Leymarie, Alexandre Thirouard, Eric Tanguy, Francis Louis

 

From S2M : Alain Roussel, Serge Bonhomme