Since 1995 the Global Ozone Monitoring Experiment (GOME) is measuring ozone (total column and profile), nitrogen dioxide and other minor trace gases on-board of the European Space Agency (ESA) ERS-2 satellite. The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and ESA decided to fly an advanced GOME-2 instrument on the METOP satellites. Within the EUMETSAT Polar System (EPS), the GOME-2 measurements will provide the input for the ozone data record in the timeframe 2005 to 2020. The radiometric calibration of the polarisation sensitive GOME-2 instrument is significantly improved by the simultaneous measurement of s- and p-polarised light at moderate resolution and high temporal resolution. The Polarisation Monitoring Unit (PU) measures the spectral range between 312 and 790 nm in 15 narrow bands. The ground pixel size in the 960 km swath is 40 * 5km2. The paper describes in detail the polarization measurement devices and their technical capabilities.
We present the implementation of snapshot imaging spectropolarimetry in a short-wave infrared (SWIR) system. It is the first of its kind to provide imaging spectropolarimetry with no moving parts and snapshot capability. This has applications in many fields, such as mining, biomedical imaging, and astronomy. The SWIR Computed Tomographic Imaging Channeled Spectropolarimeter (CTICS) is a snapshot imaging spectropolarimeter with 54X46 pixel spatial resolution and 10-band spectral resolution from 1.25-1.99 μm for the purpose of object identification. First, we present the design of the two main parts: the Computed Tomography Imaging Spectrometer (CTIS) and the channeled spectropolarimetry components. A discussion follows on the reconstruction technique. We then present the final assembled system and testing results.
A high speed Mueller matrix imaging polarimeter is presented. The instrument enables measurement of the full Mueller matrix in transmission, reflection, or retro-reflection. The Mueller matrix provides a complete description of the polarization transforming properties of the sample. The retardance, diattenuation, polarizance, and depolarization are all characterized by the polarimeter. The polarimeter is able to measure the polarization properties of samples ranging from sub-millimeter optical components to large optics. The imaging capabilities can be modified to measure the polarization properties across the surface of the sample or as a function of the angle through the sample. The dual rotating retarder polarimeter provides up to sixty-two full Mueller matrix images per second. Instrument details, measurement techniques, example data, and applications are presented.
The recent development of channelled spectropolarimetry presents opportunities for spectropolarimetric measurements of dynamic phenomena in a very compact instrument. We present measurements of stress-induced birefringence in an ordinary plastic by both a reference rotating-compensator fixed-analyzer polarimeter and a channelled spectropolarimeter. The agreement between the two instruments shows the promise of the channelled technique and provides a proof-of-principle that the method can be used for a very simple conversion of imaging spectrometers into imaging spectropolarimeters.
The paper presents a new approach to description of the partial polarization in a fiber-optic system. Basing on the investigation of interference phenomenon in a fiber-optic Sagnac interferometer the depolarization process is defined applying statistical point of view. The introduced model is treating depolarization as a some distribution of different states of polarization (SOP). The parameters of such a SOP distribution function are connected with spectral character of a light source, which may give depolarization process different from a linear one in an optical fiber system. Such description of light propagation in an optical fiber can be useful for investigation of the polarization mode dispersion. This paper presents also results of the degree of polarization measurement application of the introduced approach in the fiber-optic interferometric polarization analyzer. This is the fiber-optic Sagnac interferometer which uses a measurement technique based on the fourth Fresnel-Arago's condition of polarized beams interference.
In this paper, we measure the polarization state of various fibers with varying the bending factors. We also analyze the result of bending fiber with various bending numbers. The usage of fiber bending effect to achieve fiber-optical waveplate in the optical communication systems are reported. The results of our experiment have the similar characteristics as retarder and can be turned by varying the wrapping number. Our experimental contributions can help the design of future advance fiber-optic wave plates for a lot of polarization control applications.
For imaging systems the polarization of electromagnetic waves carries much potentially useful information about such features of the world as the surface shape, material contents, local curvature of objects, as well as about the relative locations of the source, object and imaging system. The imaging system of the human eye however, is “polarization-blind”, and cannot utilize the polarization of light without the aid of an artificial, polarization-sensitive instrument. Therefore, polarization information captured by a man-made polarimetric imaging system must be displayed to a human observer in the form of visual cues that are naturally processed by the human visual system, while essentially preserving the other important non-polarization information (such as spectral and intensity information) in an image. In other words, some forms of sensory substitution are needed for representing polarization “signals” without affecting other visual information such as color and brightness. We are investigating several bio-inspired representational methodologies for mapping polarization information into visual cues readily perceived by the human visual system, and determining which mappings are most suitable for specific applications such as object detection, navigation, sensing, scene classifications, and surface deformation. The visual cues and strategies we are exploring are the use of coherently moving dots superimposed on image to represent various range of polarization signals, overlaying textures with spatial and/or temporal signatures to segregate regions of image with differing polarization, modulating luminance and/or color contrast of scenes in terms of certain aspects of polarization values, and fusing polarization images into intensity-only images. In this talk, we will present samples of our findings in this area.
Although natural light sources produce depolarized light, partially linearly polarized light is naturally abundant in the scenes animal view, being produced by scattering air or water or by reflection from shiny surfaces. Many species of animals are sensitive to light's polarization, and use this sensitivity to orient themselves using polarization patterns in the atmosphere or underwater. A few animal species have been shown to take this polarization sensitivity to another level of sophistication, seeing the world as a polarization image, analogous to the color images humans and other animals view. This sensory capacity has been incorporated into biological signals by a smaller assortment of species, who use patterns of polarization on their bodies to communicate with conspecific animals. In other words, they use polarization patterns for tasks similar to those for which other animals use biologically produced color patterns. Polarization signals are particularly useful in marine environments, where the spectrum of incident light is variable and unpredictable. Here, cephalopod mollusks (octopuses, squids, and cuttlefish) and stomatopod crustaceans (mantis shrimps) have developed striking patterns of polarization used in communication.
We present an approach for imaging the polarization state of scene
points in a wide field of view, while enhancing the radiometric
dynamic range of imaging systems. This is achieved by a simple
modification of image mosaicking, which is a common technique in
remote sensing. In traditional image mosaics, images taken in
varying directions or positions are stitched to obtain a larger
image. Yet, as the camera moves, it senses each scene point
multiple times in overlapping regions of the raw frames. We rigidly attach to the camera a fixed, spatially varying polarization and attenuation filter. This way, the camera motion-induced multiple measurements per scene point are taken under different optical settings. This is in contrast to the redundant measurements of traditional mosaics. Computational algorithms then analyze the data to extract polarization imaging with high dynamic range across the mosaic field of view. We developed a Maximum Likelihood method to automatically register the images, in spite of the challenging spatially varying effects. Then, we use Maximum Likelihood to handle, in a single framework, variable exposures (due to transmittance variations), saturation, and partial polarization filtering. As a by product, these results enable polarization settings of cameras to change while the camera moves, alleviating the need for camera stability. This work demonstrates the modularity of the Generalized Mosaicing approach, which we recently introduced for multispectral image mosaics. The results are useful for the wealth of polarization imaging applications, in addition to mosaicking applications, particularly remote sensing. We demonstrate experimental results obtained using a system we built.
In this paper we present experimental measurements of optimized rotating retarder systems in the presence of noise and in the presence of experimental error in the angular positioning of the rotating elements. Previous studies have analyzed such effects theoretically and through numerical simulation. Our results agree with the theoretical predictions, but point out several practical factors that affect such systems. These factors include intensity fluctuations that cannot be modeled as additive noise, the balance between noise and error, which have always been considered separately, and the effect of input polarization state on the performance of the polarimeter systems. We present results for both passive (Stokes) and active (Mueller) polarimeters.
Accurately identifying and bounding error sources in imaging spectro-polarimeters is a challenging task. Here we present an error evaluation methodology intended as an organizational tool for both itemizing and quantifying sources of error in polarimetric instruments. Associated with each source of error are both a metric and test by which these errors may be quantified. Using this procedure, we examine the accuracy and precision of a particular imaging Stokes vector hyper-spectral polarimeter. A subset of the identified error sources are selected and propagated through the system. These measured error quantities are then used to put absolute error bounds on the data acquired by our instrument. These measured error quantities are further documented and presented in the form of an error evaluation sheet.
Polarization effects are typically assumed to be negligible in semi-conductor photo-detectors. However, effects on the order of a few percent or less have occasionally been reported. Such reports have been difficult to assess because they are typically at or near measurement limits. Because remote sensing technology currently under consideration requires calibration precision of 3 % or less, these effects can no longer be ignored. We have recently carried out a set of high-quality polarimetric measurements of semi-conductor photo-detector materials across appropriate wavelength ranges in order to develop a better understanding of polarization effects on photo-detector response. Samples studied include GaAs and Si. The wavelength range was chosen to encompass the transition between strongly absorbing and non-absorbing for these materials. In addition, GaAs was compared with Si in order to better understand the contribution of bi-refringence effects in the regime of high optical absorption as required for efficient photo-detection. Mueller matrix data were obtained in order to allow prediction of photo-detectors over a wide range of angles of incidence under polarized illumination. This understanding is of interest for applications that require very fast optics, which is often the case for systems that operate in “photon-starved” environments.
Beamsplitting polarizer cubes consisting of two right angle prisms cemented together after one hypotenuse is coated have become important optical components in many optical systems. Usually the coating stack is of the MacNeille design. We present and compare an alternative coating structure consisting of a very fine wire grid structure on the cube hypotenuse that has performance advantages of improved polarization purity over an extended range of wavelengths and angles. Modern lithography permits wire spacings and dimensions that are small enough for good polarizer performance at visible wavelengths as well as near infrared wavelengths.
We examine the effect of incident angle on the Liquid Crystal Variable Retarder (LCVR) and develop a Mueller matrix formulation for the LCVR that is a function of incident angle and applied voltage. By comparing the model with laboratory measurements for both linear and circular input polarization, we show that the mathematical model provides a credible representation of a real LCVR. The model can be used to determine the field angle effects of LCVRs in complex optical polarization systems.
Polarimetry technique allows one to study the changes induced by a physical system in the polarisation of electromagnetic waves. As its optical response is greatly affected by the polarisation state and wavelength of the incident light, a class of powerful polarimeters has been developed to measure the polarimetric response of a sample over a wide spectral band. Such a device, therefore, allows one to greatly increase the number of data about the sample of concern. The polarimeter of that sort we developed and implemented is operating with a pulse source. Moreover, by running a novel and theoretical model to describe the compensated waveplates used we focused on the reduction of systematic errors. This model takes into consideration the elliptical birefringence of each rotating device. In doing so, the precision currently given for the Mueller matrix elements is drastically improved. Simulations enabled us, first, to determine the measurement error on each element of the Mueller matrix without a sample and, second, to adopt a method of calibration. Experimental results and corrections highlight the interest of taking elliptical birefringence and dichroism into account. This calibration procedure led us to develop a compensated-waveplate characterisation bench. Then, a statistical study allowed us to greatly reduce and quantify the residual errors inherent in a measurement.
This paper reviews the history and applications of the laser backscatter depolarization technique for atmospheric remote sensing. A relatively simple method, polarization diversity was among the earliest laser-radar (lidar) applications tested in the field, and soon proved to be uniquely suited for the study of the shape and orientation of hydrometeors, and hence the discrimination of water and ice clouds. More recent research has focused on atmospheric aerosols and the exotic clouds of the middle atmosphere, and on enhancing the information content of observations from lidars based on more sophisticated technologies. Various findings from polarization lidar research will be presented.
An innovative system architecture for a real time Active Imaging Polarimeter has been developed. The system benefits from very few hardware components (all of which are off the shelf) and a high performance signal recovery algorithm. An electo-optic modulator imposes a waveform of a defined frequency onto the optical signal from a standard telecom laser diode and is transmitted with a known polarization. A unique polarization signature is reflected off a target and imaged through different polarization analyzers onto four quadrants of a high frame rate, near infrared, focal plane array. Using knowledge of the modulation frequency, lock-in amplifier algorithms enable measurement of the received beam intensity and therefore polarization with high SNR performance. Multiple signals (each at unique modulation frequencies) can be differentiated and manipulated (in waveform, wavelength and polarization) to serve many imaging applications. The active imager architecture operates in turbid atmospheres day or night. This technique and its variations provide the necessary tools for a new approach to active imaging, polarimetry, 3D ranging and trace gas imaging.
Traditional ellipsometric measurements are limited in their accuracy because of the use of an external reference sample for calibration, and because of the noise inherent in the source at low light levels. We demonstrate that these limitations can be circumvented by using a non-classical source of light, namely, twin photons generated by the process of spontaneous parametric downconversion, in conjunction with a novel polarization interferometer and coincidence-counting detection scheme. The twin-photon nature of the source is a unique feature of our scheme. We are guaranteed, on the detection of a photon in one of the arms of the setup, that its twin will be in the other. We present experimental results showing how the technique operates.
This paper describes initial results on the use of a laser radar (LADAR)system to measure the rotational properties of a spinning sphere coated with two different surface materials. Numerical simulations were carried out using the Time-Domain Analysis Simulation for Advanced Tracking (TASAT) toolkit and a specialized laser ranging module included within the toolkit. Assuming some of the surface materials on the sphere's surface depolarize the incident radiation from the laser, the rotational properties of the object can be deduced. Futhermore, polarization properties of materials can enhance the ability to extract information about the sphere's rotational rate and relative orientation in the case where speckle noise and tracking jitter is significant. Future work will involve the creation of a large database of simulated return signatures for many orientations and rotation rates. The database will be correlated against actual LADAR measurements in order to determine the rotational and orientation properties of spinning objects in low earth orbit.
Depolarization is the generation of partially polarized light or unpolarized light from a completely polarized incident beam. For example when polarized light scatters from rough surfaces, the degree of polarization is usually reduced from one. Depolarization figures of merit are discussed and difficulties with the depolarization index are highlighted. Interesting depolarization patterns are presented for a liquid crystal in reflection are measured with a commercial polarimeter. Depolarization data from rough metal surfaces and paint is also discussed.
We develop models for light scattering appropriate for glossy and matte paints. Volume scattering is treated in the single scattering regime and the diffusive scattering regime. In the single scattering regime, scattering is treated in the Rayleigh-Gans approximation, using a Henyey-Greenstein phase function. Interaction of the light with the smooth or rough interface is treated in the facet approximation. The theory for transmissive light scattering by a rough interface in the facet approximation is presented. To treat volume scattering under a rough interface, a Monte Carlo approach is used, where light is allowed to interact with the surface twice, once upon entering the material and once upon exiting. We compare the polarization and intensity predicted by the models with experimental data from glossy and matte paint samples. The results indicate that the new models are an improvement over the Maxwell-Beard model.
Current simulations of optical polarization scattering and emission for remote sensing applications employ geometric optics. The approach is mathematically simple but lacks soundness of physics as it relies upon artificial adjustment of polarized specular and unpolarized diffuse components in the scattered radiation to match experiments. In order to improve the current polarization scattering model, we are developing a model based on the vector Kirchhoff diffraction integral. The vector Kirchhoff diffraction model will simulate a main lobe and a diffraction pattern for each rough surface facet of a material. Predictioins of measurable polarization states will result through calculating the diffraction lobes of different facet orientations. The Kirchhoff approach will produce specular and diffuse components solely depending on surface characteristics and incident/scattering angles. Our mathematical model is an extension of Beckmann’s scalar rough surface scattering model. The roughness of the surface is treated statistically using the rms roughness height and the autocorrelation length and a Gaussian distribution is used for the roughness slope and facet normal. The shadowing by neighboring rough surface facets is also taken into account in the model. The results of the model are to be compared with published results of polarization scattering experiment.
The polarimetric characteristics are experimentally investigated for dense scattering media such as compacted powders. The Mueller matrix is measured in backscattering configuration which is of interest for remote sensing as well as for characterizing rough scattering surfaces. Significant differences in various Mueller matrix elements are observed for different samples and are analyzed in terms of surface and volume scattering from an inhomogeneous medium.
Off-diagonal Mueller elements indicate polarization transformations as occur in polarizers and retarders. Target scattering may also generate off-diagonal elements, which then provide information unavailable from passive polarimetry or active depolarization measurements. The target and observation parameters required in active, monostatic systems for the detection of off-diagonal Mueller elements due to target scattering are investigated. The dependences of off-diagonal elements on incident angle, surface roughness, material composition, and target symmetry are investigated through analysis and measurements from two polarimeters. Multiple scattering and anisotropic roughness, which may result either from innate surface anisotropy or oblique incidence, are found to generate off-diagonal elements in the monostatic geometry. Results from a polarized microfacet scattering model corroborated with polarimeter data reveal a particular application of off-diagonal Mueller elements in the discrimination of dielectric from metal targets of similar roughness.
The analysis of the experiment results of measurements of the Mueller matrices shows that the values of error of each matrix element do not equal each other and depend on the anisotropy types of studied objects. However, this fact did not receive an appropriate attention in the literature yet. At the same time, this feature can be used for polarimeter optimization in measuring of objects with specific polarization behavior (linear amplitude or phase anisotropy). For the estimation of the Mueller matrix measurement error is usually used the total error, that is a square root of sum of squares of all matrix element error). As analysis have shown this is not always justified. The goal of this work is to analyze error distribution over experimental Mueller matrix elements for the serial probing polarimeter in its four input polarization mode. The analysis is carried out for the three cases: (1) uniformly precise Stokes measurements, (2) uniformly precise Fourier measurements, (3) non-uniformly precise Fourier measurements. These results disclose new aspects of the Mueller matrix measurement tools optimization and the Mueller matrix measurement strategies.
Accurate multi-spectral, multi-angle polarimetric measurements are a key remote sensing tool for the determination of the burden and microphysical properties of atmospheric aerosols and, as an adjunct, for the correction of these atmospheric effects in spectroradiometric remote sensing applications. This paper describes the performance of the Research Scanning Polarimeter (RSP), provides examples of using the RSP for aerosol remote sensing, and assesses the potential synergy between spectroradiometric and polarimetric measurements.