The characteristics of aerosol optical depth (AOD), Ångström exponent α, water vapor content and turbidity coefficient were analyzed by using the latest observation data of CE318 Sun-photometer in the southern suburb of Xi’an between December 2019 and November 2020. In addition, the frequency distribution and aerosol types were studied, and the influencing factors were analyzed briefly. The annual mean value of AOD in the southern suburb of Xi'an was 0.529±0.358, and the distribution of frequency was unimodal. The values of AOD in winter and spring were higher than that in summer and autumn. The annual mean value of α was 0.894±0.373, and the distribution of frequency was multipeak distribution. The number of the α value lower than 0.5 in spring was significantly higher than that in the other three seasons, which was caused by the influence of coarse mode particles, while the other seasons were affected both by coarse and fine mode particles. It can be seen that different types of aerosols are mixed in Xi’an. The influence of Desert Dust Aerosols in spring is greater than that in other seasons. Biomass Burning/Urban Industry Aerosols (BB/UI) are the main types in autumn and winter, and all types of aerosols are relatively balanced in summer.
This paper presents a 9-channel, spatially modulated partial Mueller matrix polarimeter that uses a spatial light modulator (SLM) to create the polarization state generator (PSG) modulation and a division of focal plane (DoFP) polarimeter as the polarization state analyzer (PSA) with a coventional 2×2 spatial modulation pattern. We demonstrate here how adapatation of the PSG modulation to the spectral structure of the scene can have significant benefits in reconstruction accuracy.
The theoretical operation and experimental demonstration of a Fourier-transform Stokes imaging spectropolarimeter are presented. It is composed of two birefringent crystal retarders with equal thickness (the frontal retarder is rotatable) and a Fourier-transform spectrometer based on Savart polariscope. The polarized light enters the spectrometer to create three sets of interferograms, where the spectral Stokes parameters can be calculated and acquired. Compared with previous instruments, the significant advantages of the described sensor are no spatial aliasing in the polarized spectra and it can be used in wider spectral coverage with low cost, ultra-compact size and a simpler common-path configuration.
The basic principle of tempo-spatially mixed modulated Fourier transform imaging spectrometer (FTIS) based on savart plates is outlined. A calibration method of pixel response non-uniformity of charge-coupled device (CCD) camera in such type of instrument is presented. The method which uses column-flat-fields can avoid the influence of interference fringes. The use of polychromatic calibration source can solve the problem of the slant of the fringes in large optical path difference areas. The procedure of calibration experiment and the algorithm of data processing are detailed described. Two groups of relative radiometric calibration coefficient are obtained through the method of least-square. The original images are corrected by using the coefficients to validate its calibration effect. The results indicated that the method can obviously improve the uniformity of pixels and the vignetting artifacts and defect of the instrument can be well corrected. This study provides a theoretical guidance for study, design, modulation, experiment and engineering of FTIS.
Snapshot imaging spectropolarimetry is emerging as a powerful tool for mapping the spectral dependent state of
polarization across most of scenarios (stable and variable), owing to its capability of real-time parallel acquisition. In this
paper, two schema of snapshot full-Stokes imaging polarimeters (SFSIP) based on division-of-aperture polarimetry are
presented firstly. In compliance with the definition of Stokes parameters, the first SFSIP consists of three Wollaston
prisms with superior extinction ratio and simultaneously measures six polarimetric intensities (I0, I90, I45, I135, IL and IR)
of scene. However, the spatial resolution of each polarimetric image only occupy one-six of detector. To increase the
spatial resolution, the second SFSIP comprises a optimal four-quadrant polarization array and a pyramid prism is used to
simultaneously acquire four polarimetric intensities. Since the optimal four-quadrant polarization array consists of a
uniform linear polarizer and four 132º retarders with different azimuth of fast axis, the signal-to-noise ratio for each of
the recovered Stokes parameters will be balanced and enhanced. Finally, the four-quadrant polarization array and
pyramid prism are integrated into a integral field spectroscopy to construct a snapshot full-Stokes imaging
spectropolarimetry (SFSISP). It is used to map the spectral dependent full Stokes parameters across a scene in real time.
Three compact and static birefringent Fourier transform imaging spectropolarimeters are presented. They based on the different combinations of birefringent elements, including Savart polariscope, Wollaston prism, achromatic half-wave plate and quarter-wave plate. After acquiring several interferograms simultaneously for different polarization states with a single CCD, the spectral dependence of polarization states are recovered with Fourier transformation. The interference models are described theoretically, and the performances are demonstrated through numerical simulations and experiments. In contrast to the well-known channeled spectropolarimetry, the most important advantages are that the sampling interferograms have no channel aliasing and directly correspond to the maximum optical path difference of birefringent interferometer. That is say, they can recover the spectral variation of polarization state with the interferometer’s maximum spectral resolution.
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