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 3×3 scattering Mueller matrix of particles in water in the range of 10°-170°.
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 μm) with relatively gentle Mie scattering results was used for amplitude calibration and calibration coefficient determination, and another one (diameter of 2 μm) 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.
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 3×3 polarized volume scattering function in the range of 10°-170° 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 3×3 scattering Muller matrix of particles in water was obtained in the range of 10°-170° in this lake.