Variable coherence tomography (VCT) was recently developed by Baleine and Dogariu for the purposes of
directly sensing the second-order statistical properties of a randomly scattering volume. In this paper we
generalize the theory of VCT to include polarized inputs and scatterers. The measurement of the scattered
coherency matrix or Stokes vector is not adequate in general to describe the surface, as these quantities depend
on the coherence state of the incident beam. However, by controlling the polarized coherence properties of
the beam with polarized VCT, we are able to design a method that can measure analogous information to the
polarimetric BRDF, but do it from monostatic data. Such a method has potential impact on both polarimetric
and scalar active remote sensing.
There are many acceptable ways to construct an imaging polarimeter, each with its own benefits and drawbacks.
The most common systems involve rotating elements, but use of these systems puts limitations on the dynamic
nature of the scene. Division of Aperture (DoAP) and Division of Amplitude Polarimeters (DoAmP) solve the
temporal synchronization issue by using multiple light paths, each of which has its own set of polarization optics.
These systems can provide real-time imagery, but there are significant challenges surrounding optomechanical
alignment and sensitivity to vibrations. Division of Focal Plane devices (DoFP) use an integrated array of
micropolarizers to solve the temporal and mechanical alignment issues, but suffer from exactly 1 pixel of IFOV
error that cannot be compensated for at the full resolution of the system. Recently we presented a concept
that creates a highly parallel array of non-imaging DoAP devices. The design uses two microlens arrays to
relay the image at an intermediate focal plane through a microgrid polarizer. The microlenses are configured
such that each lens in the first array feeds four lenses in the second array, so as to create a non-imaging DoAP
polarimeter. Our previous work was only a conceptual design. In this paper, we will present a design and
ray-tracing analysis of a proposed system. We quantify the principal drawbacks of vignetting and crosstalk,
and give expected performance parameters of a final device.
An imaging variable retardance, Fourier transform spectropolarimeter (VRFTSP) is presented that is capable of creating spectropolarimetric images of scenes with independent characterization of spatial, spectral, and polarimetric information. The device is used to image simple, canonical targets such as spheres and cylinders in a laboratory setup. The resulting images demonstrate the capability of developing systems to collect spectropolarimetric data of field images using the concept of pushbroom scanning and serial collection of polarimetric information, with the possibility of developing a parallelized collection strategy allowing the collection of near-real-time images of real-world targets.
We have developed a laboratory breadboard spectropolarimetric hyperspectral imaging sensor for operation in the visible-near IR waveband to demonstrate its potential for future airborne and spaceborne systems. An existing spatially modulated imaging Fourier transform spectrometer was modified by the addition of polarization analyzer components. Images collected by the focal plane array vary spatially in one dimension and spectrally in the other. A scanning slit simulates a pushbroom scanning mode for the second spatial dimension. Polarimetric sensing is accomplished using two liquid crystal variable retarders in tandem followed by a linear polarizer. To recover the complete Stokes vector four images, each with distinct combinations of retardances, are collected for each slit position. Sample images are presented.
The Kestrel Corporation visible-near IR band (525 to 1016 nm) airborne Fourier Transform Hyperspectral Imager was modified to include measurement of the polarization characteristics of several ground cover classes. The polarization contrast of typical terrestrial background and target objects was characterized. First, the t statistic was used as an index of class separation to determine whether polarized images were more useful for discriminating several cover classes than unpolarized images. Second, the information present in polarized images which is not present in unpolarized images was identified and described. This was done by regressing polarized and unpolarized images, generating images of predicted values for the polarized images using the regression coefficients, generating images of residuals by subtracting the actual values from the predicted values, and analyzing the statistical separation of cover classes in the residual images. A single polarized image was not more useful for identifying the cover classes than an unpolarized image. A residual image derived from a single polarized image and an unpolarized image provided a mean maximum statistical separation of t equals 18.3 for all cover class combinations. The sum of two orthogonal polarized images provided slightly greater separation, with a mean maximum separation of t equals 23.7.
Background clutter and target signatures have traditionally been described by parameters derived form measurements of spatial structure and spectral ratios derived from fixed spectral bandpass images. The advent of hyperspectral imagery requires descriptions of background clutter in a mixed wavelength-spatial or Fourier-transformed (FT) spectral - spatial framework because the data stream may contain simultaneous spatial - spectral, or FT spectral - spatial clutter components. We have developed and tested analytical routines for characterizing the background clutter and target signatures observed by Fourier-transform instruments, without requiring production of a hyperspectral data cube having spectra wavelength and 2D spatial image dimensions. The Kestrel Fourier-transform hyper spectral imager, a Sagnac format interferometer produces a data stream consisting of the Fourier spectra of the background in the in-track focal plane dimension and the spatial information int he cross-track dimension. The temporal data stream thus consists of a time series of frames in FT- spectral vs. spatial dimensions. Spectral wavelength filtering and guard-band subtraction can be accomplished in FT space by binary shift and add algorithms without prior transformation of the data into a hyperspectral data cube. Spatial filtering in the cross track dimension can similarly benefit from efficient binary operations. This paper summarizes some of the target and background clutter characterization algorithms developed and their evaluation against an example atmospheric gas detection scenario.
Kestrel Corporation is designing and building the first Fourier transform hyperspectral imager to be operated from a spacecraft. Performance enhancements offered by the Fourier transform approach have shown it to be one of the more promising spaceborne hyperspectral concepts. Simulations of the payload's performance have indicate that the instrument is capable of separating a wide range of subtle spectral differences. The concept design for the payload has been completed and hardware is in fabrication for an engineering model.
A compact portable Fourier transform spectrometer is described. The simplicity of the instrument design and monolithic structure of the interferometer make it suitable for use in satellites or man-portable terrestrial measurements. Conceptual designs for hyperspectral imaging applications in the near-UV through near-IR, and in the mid-IR are presented. Imaging and spectral performance predictions are given using ray and beam tracing software to synthesize multispectral interferograms.
The high altitude balloon experiment (HABE) optical system is designed to address target tracking and pointing issues. The HABE optical system is based on a 60 cm telescope and involves an IR (4.4 micrometers ) tracking camera with a 2.3 mrad field of view and a visible (0.532 micrometers ) tracking camera with a 0.257 mrad field of view. An inertial reference alignment laser controls a fast- steering mirror to point a marker laser at the target. The basic optical layout and the optical design issues are discussed. Diffraction-limited performance was achieved for the design of the imaging cameras.
A variation of the Ritchey-Common configuration was applied to the subaperture testing of a 29-in diameter flat at about 65 degrees oblique incidence in the 12-in collimated beam of a Fizeau interferometer, yielding an elliptical beam footprint spanning the full diameter of the mirror under test. A set of subaperture samples was built up in a 'flower petal' pattern symmetric about the mirror center by in-plane rotation of the mirror in 30-degree increments. A key advantage of this method of sampling over raster methods is that the synthesis of the full surface map is greatly simplified by not having to keep track of individual piston and tilt terms because of the symmetry. An advantage over the Ritchey-Common configuration is that the cavity length can be made much shorter, thus greatly reducing atmospheric effects. The data reduction and surface synthesis processes simply consisted of fitting Zernike polynomial expansions to the (digitized) individual interferograms, subtracting the piston and tilt terms, then applying rotation and scaling transformations to the pupil coordinate grid to map the (circular) pupil surface data into the appropriate elliptical footprints.
The third-order vector aberration theory developed by Roland Shack, and later extended to fifth order by Kevin Thompson, is adapted to a personal computer spreadsheet program. Several examples are given to illustrate the ease of application of this approach to the analysis, and in some cases optimization, of a variety of tilted and decentered systems. These include an afocal pupil relay system using three off-axis parabolas, a similar system where the third parabola is replaced by an off-axis ellipse, and a tilted component IR lens. The results of the spreadsheet analyses are compared with similar results obtained from a commercial lens design program.