Measurement of Polarized Volume Scattering Function of Particles in Water in a Wide Angle Range

Objective The polarized volume scattering function of particles in water is the most basic and complete parameter describing their scattering characteristics as it reflects the types, particle size spectra, shapes, and refractive indexes of the molecules and large particles in water. Therefore, the polarized volume scattering function of particles in water is of great research significance. This function is the most critical inherent optical parameter in the study of active and passive ocean optical remote sensing. Nevertheless, it is also the inherent parameter most difficult to measure. To solve the problem that no instrument is currently available in China for measuring the polarized volume scattering function of particles in water in a large angle range, this paper develops a measurement system for the polarized volume scattering characteristics of particles in water on the basis of a periscope-like optical path structure and the detection method of the rotating polarization detector. It further verifies the applicability of an output prism obtained by the half attenuation bonding method to the measurement of the polarized volume scattering characteristics in a large angle range and achieves the measurement of the 3x3 scattering Mueller matrix of particles in water in the range of 10 degrees-170 degrees. Methods In this study, pure water was used for baseline measurements, and the scattering characteristics of particles were determined by removing the contribution of the pure water signal to the total signal. According to the characteristics of standard particles in the Mie scattering theory, a standard particle (diameter of 0. 2 mu m) with relatively gentle Mie scattering results was used for amplitude calibration and calibration coefficient determination, and another one (diameter of 2 mu m) with salient angle characteristics was used for angle calibration. Polarization was performed with a Stokes meter. The scattered light received by the detector suggests that the scattering optical paths detected at all angles are not the same, and the attenuation transmission distances in water are also different, which necessitates the normalization of the scattering optical path and the correction of the attenuation optical path. Results and discussions A new type of exit prism with simpler processing was designed in this paper. The prism was developed by bonding a neutral density filter with a prism cut into a specific shape. The new prism designed could effectively reduce the influence of stray light on the measurement of large-angle backscattering and small-angle forward scattering. In Fig. 3, the measurement accuracy of the new prism is verified by comparing the measurement results of the old and new prisms. Fig. 5 compares the measured results of 3-mu m diameter polystyrene standard particles with the theoretically calculated values and reveals that they are in good agreement with each other, which proves the reliability of the system in measuring the polarized volume scattering function. Finally, the system was tested in the natural water body of the Qiandao Lake, and the 3x3 scattering Mueller matrix of particles in water within the range of 10 degrees-170 degrees was obtained for the first time in China, as shown in Fig. 7 and Fig. 8. A comparison with the spectral shapes measured by HS6 indicates that the particle types at different depths at the same station are different, and the surface particle types at different stations are also different. The results show that polarized scattering characteristics can provide more abundant information of particle characteristics. Conclusions In this study, a measurement system based on a periscope-like optical path structure and the rotating polarization detector was designed to measure the polarized volume scattering function of particles in water, and the measurement of the 3x3 polarized volume scattering function in the range of 10 degrees-170 degrees was thereby achieved. Moreover, a new type of exit prism with simpler processing was designed, and the measurement accuracy of the new prism was verified. To obtain accurate measurement results, this paper proposes strict methods for data processing and calibration procedures of the instrument, including baseline measurement, angle and amplitude calibration, polarization calibration, and data correction. An accurate theoretically calculated value of the polarized volume scattering function was obtained by applying the Mie scattering theory and compared with the measurement results of the experimental instrument. The measurement results are in good agreement with the theoretically calculated value. The accuracy of the measurement results of the experimental prototype is thus ensured. The system was further tested in the Qiandao Lake, and a 3x3 scattering Muller matrix of particles in water was obtained in the range of 10 degrees-170 degrees in this lake.