Algal fluorescence: impact and potential for retrieval from measurements of the underwater degree of polarization
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Algorithms for retrieving inherent optical properties (IOPs) in coastal waters from remote sensing of water leaving reflectance spectra, are increasingly focused on red and near infrared (NIR) spectral bands, since the simple blue - green ratio approaches, valid in open oceans, fail when in coastal waters with strongly scattering inorganic particles and colored dissolved organic matter (CDOM). NIR spectra can however be significantly impacted by overlapping chlorophyll a fluorescence, and considerable progress has been made to quantify its contribution, and hence achieve more accurate [Chl] retrievals. Recently we have been studying multiangular hyperspectral polarization characteristics of underwater scattered light, using our recently developed Stokes vector polarimeter to fully measure Stokes parameters. From these studies, information on IOPs, in particular the characteristics of non - algal particles (NAP), which are the primary source of underwater polarized elastic scattering, can be obtained. Multiangular hyperspectral polarization measurements, combined with those of IOPs collected in eutrophic waters of Chesapeake/Virginia and New York Harbor/Hudson River areas, showed that chlorophyll a fluorescence markedly impacts (reduces) the underwater degree of polarization (DOP) in the 650 - 700 nm spectral region. By noting the unpolarized nature of algal fluorescence and the partially polarized properties of elastic scattering, we are able to separate the chlorophyll a fluorescence signal from the total reflectance. The analysis is based on comparisons of experimental measurements with vector/scalar radiative transfer computations using measured IOPs as inputs. Relationships between change in observed DOP and fluorescence contributions are examined, and the possibility of using DOP measurements for underwater fluorescence retrieval is evaluated for different scattering geometries.Keywords:
Degree of polarization
Stokes parameters
Polarimeter
We present the Stokes polarimeter for the new Coronal Solar Magnetism Observatory K-coronagraph. The polarimeter can be used in two modes. In observation mode, it is sensitive to linear polarization only and operates as a "Stokes definition" polarimeter. In the ideal case, such a modulator isolates a particular Stokes parameter in each modulation state. For calibrations, the polarimeter can diagnose the full Stokes vector. We present here the design process of the polarimeter, analyze its tolerances with a Monte Carlo method, develop a way to align the individual elements, and measure and evaluate its performance in both modes.
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Stokes parameters
Coronagraph
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A Stokes polarimeter based on polarization-dependent hologram is demonstrated by matrix analysis. The mean square error of the orientation angle and the ellipticity angle are 4.45 degree and 3.19 degree respectively.
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Stokes parameters
Degree of polarization
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We present the initial results of an imaging polarimeter operating at 632.8 nm that simultaneously analyzes four polarization states on a single detector array. In a single snap shot, the polarimeter has the ability to characterize the polarization of a scene by determining the complete Stokes vector. Images are processed to show Degree of Polarization (DOP), Degree of Linear Polarization (DOLP), Degree of Circular Polarization (DOCP), ellipticity and the angle of linear polarization. Our approach utilizes a monolithic analyzer that allows us to avoid issues usually associated with division of amplitude polarimeters such as jitter and tight tolerance requirements. We discuss our optical design, calibration procedure, and test data.
Polarimeter
Stokes parameters
Degree of polarization
Polarization rotator
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The design and construction of a Full-Stokes Imaging polarimeter is presented. The device uses a telescope optical system and a polarization state analyzer (PSA) to obtain polarimetric images on a CCD. The PSA employs two liquid crystal variable retarders (LCVR) and a linear polarizer to measure the four Stokes parameters. The Stokes polarimetry method used in this paper is based on the application of six combinations of retardance values on the LCVRs. A well-known method is used to extract all the Stokes vector parameters from this intensity data. Due to experimental errors, a calibration is necessary. The calibration method used in this paper, also calculates the errors in the experimental set-up by fitting the experimental intensity measurements for the calibration samples to a theoretical polarimeter with errors. In this case, we used incident 45° polarized light to control the output polarization, and six calibration samples. The errors calculated in the method include the axes alignment errors and the errors in the retardance values of both LCVRs. The acquisition of Stokes images used a telescope optical system with a CCD camera.
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Polarizer
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In this paper we outline and address some of the challenges encountered in calibrating a visible 4-channel imaging reduced Stokes polarimeter. Specifically we expose the errors associated with the retardance of the optical elements in the system and show how the presence of circular polarization can affect the measurement of the linear states of polarization. These errors are analyzed in some detail and several possible solutions are proposed.
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Stokes parameters
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Polarimeter
Stokes parameters
Mueller calculus
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The polarization properties of radiation can contain additional information beyond what is available with only an intensity measurement. A full-Stokes polarimeter is capable of measuring the four Stokes parameters which completely characterizes the polarization of detected radiation. A division of time full-Stokes polarimeter often uses a rotating polarizing element to measure all four Stokes parameters and this rotation can introduce artifacts due to wobbling. In this paper a system is proposed which uses an electrically controlled phase bias instead of a rotating element to create a full-Stokes polarimeter for a millimeter-wave system which utilizes optical up-conversion.
Polarimeter
Stokes parameters
Extremely high frequency
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This paper describes a full Stokes polarimeter employing a monolithic off-axis polarizing interferometric module and a 2D array sensor. The proposed passive polarimeter provides a dynamic full Stokes vector measurement capability of around 30 Hz. As the proposed polarimeter employs no active devices and is operated by employing an imaging sensor, it has significant potential to become a highly compact polarization sensor for smartphone applications. To show the feasibility of the proposed passive dynamic polarimeter scheme, the full Stokes parameters of a quarter-wave plate are extracted and displayed on a Poincare sphere by varying the polarization state of the measured beam.
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Stokes parameters
Waveplate
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The polarimetric imaging system can acquire not only the target's two-dimensional light intensity distribution information but also the Stokes polarization vector information corresponding to the target light intensity distribution. The Stokes vector information can enhance the target's contrast, highlighting the target from the complex background information. The LCVR-based Stokes polarimeter uses a Liquid Crystal Variable Retardation (LCVR) instead of the rotating waveplate in conventional measurement techniques, which enables polarization measurements without moving parts. This paper first introduces the measurement principle of the Stokes vector method, the working principle of LCVR, and the parameters of commonly used evaluation indicators in polarimetric measurement systems. Then, a two-channel single LCVR-type polarimeter measurement scheme is proposed, which uses a split beam element (prism PBS, Savart plate, or Wollaston prism) and a single LCVR to obtain the full Stokes parameters of the target. Then the measurement matrix and light intensity calculation formulas for the polarimeter and polarization spectrometer were derived, and the evaluation parameters of the system (CN, RAD, EWV) were calculated. The study and evaluation analysis provide some theoretical references for studying the LCVR-based Stokes polarimeter.
Polarimeter
Stokes parameters
Prism
Mueller calculus
Waveplate
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A new technology for performing high-precision Stokes polarimetry is presented. One traditional Stokes polarimetry configuration relies on mechanical devices such as rapidly rotating waveplates that are undesirable in vibration-sensitive optics experiments. Another traditional technique requires division of a light signal into four components that are measured individually; this technique is limited to applications in which signal levels are sufficient that intensity reduction does not diminish the signal-to-noise ratio. A new technology presented here is similar to the rotating waveplate approach, but two liquid crystal variable retarders (LCVR’s) are used instead of waveplates. A Stokes polarimeter instrument based on this technology has been made commercially-available. The theory of operation is detailed, and an accuracy assessment was conducted. Measurement reproducibility was verified and used to produce empirical estimates of uncertainty in measured components of a Stokes vector. Uncertainty propagation was applied to polarization parameters calculated from Stokes vector components to further the accuracy assessment. A calibrated polarimeter measures four Stokes components with 10-3 precision and average predicted uncertainties less than ±2x10-3. An experiment was conducted in which the linear polarization angles were measured with a LC polarimeter and with a photodiode for comparison. Observed discrepancies between polarization angle measurements made with a polarimeter and those made with a photodetector were nominally within ±0.3°.
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Stokes parameters
Waveplate
Photodiode
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