We present a comparative study involving five distinctly different polarimetric imaging platforms that are designed
to record calibrated Stokes images (and associated polarimetric products) in either the MidIR or LWIR spectral
regions. The data set used in this study was recorded during April 14-18, 2008, at the Russell Tower Measurement
Facility, Redstone Arsenal, Huntsville, AL. Four of the five camera systems were designed to operate in the LWIR
(approx. 8-12μm), and used either cooled mercury cadmium telluride (MCT) focal-plane-arrays (FPA), or a near-room
temperature microbolometer. The lone MidIR polarimetric sensor was based on a liquid nitrogen (LN2) cooled
indium antimonide (InSb) FPA, resulting in an approximate wavelength response of 3-5μm. The selection of
cameras was comprised of the following optical designs: a LWIR "super-pixel," or division-of-focal plane (DoFP)
sensor; two LWIR spinning-achromatic-retarder (SAR) based sensors; one LWIR division-of-amplitude (DoAM)
sensor; and one MidIR division-of-aperture (DoA) sensor. Cross-sensor comparisons were conducted by examining
calibrated Stokes images (e.g., S0, S1, S2, and degree-of-linear polarization (DOLP)) recorded by each sensor for a
given target at approximately the same test periods to ensure that data sets were recorded under similar atmospheric
conditions. Target detections are applied to the image set for each polarimetric sensor for further comparison, i.e.,
conventional receiver operating characteristic (ROC) curve analysis and an effective contrast ratio are considered.
We present a comparative study involving five distinctly different polarimetric imaging platforms that are designed
to record calibrated Stokes images (and associated polarimetric products) in either the MidIR or LWIR spectral
regions. The data set used in this study was recorded during April 14-18, 2008, at the Russell Tower Measurement
Facility, Redstone Arsenal, Huntsville, AL. Four of the five camera systems were designed to operate in the LWIR
(approx. 8-12μm), and used either cooled mercury cadmium telluride (MCT) focal-plane-arrays (FPA), or a near-room
temperature microbolometer. The lone MidIR polarimetric sensor was based on a liquid nitrogen (LN2) cooled
indium antimonide (InSb) FPA, resulting in an approximate wavelength response of 3-5μm. The selection of
cameras was comprised of the following optical designs: a LWIR "super-pixel," or division-of-focal plane (DoFP)
sensor; two LWIR spinning-achromatic-retarder (SAR) based sensors; one LWIR division-of-amplitude (DoAM)
sensor; and one MidIR division-of-aperture (DoA) sensor. Cross-sensor comparisons were conducted by examining
calibrated Stokes images (e.g., S0, S1, S2, and degree-of-linear polarization (DOLP)) recorded by each sensor for a
given target at approximately the same test periods to ensure that data sets were recorded under similar atmospheric
conditions. Target detections are applied to the image set for each polarimetric sensor for further comparison, i.e.,
conventional receiver operating characteristic (ROC) curve analysis and an effective contrast ratio are considered.
Accurate calibration of polarimetric sensors is critical to reducing and analyzing phenomenology data, producing
uniform polarimetric imagery for deployable sensors, and ensuring predictable performance of polarimetric algorithms.
It is desirable to develop a standard calibration method, including verification reporting, in order to increase credibility
with customers and foster communication and understanding within the polarimetric community. This paper seeks to
facilitate discussions within the community on arriving at such standards.
Both the calibration and verification methods presented here are performed easily with common polarimetric equipment,
and are applicable to visible and infrared systems with either partial Stokes or full Stokes sensitivity. The calibration
procedure has been used on infrared and visible polarimetric imagers over a six year period, and resulting imagery has
been presented previously at conferences and workshops.
The proposed calibration method involves the familiar calculation of the polarimetric data reduction matrix by
measuring the polarimeter's response to a set of input Stokes vectors. With this method, however, linear combinations
of Stokes vectors are used to generate highly accurate input states. This allows the direct measurement of all system
effects, in contrast with fitting modeled calibration parameters to measured data. This direct measurement of the data
reduction matrix allows higher order effects that are difficult to model to be discovered and corrected for in calibration.
This paper begins with a detailed tutorial on the proposed calibration and verification reporting methods. Example
results are then presented for a LWIR rotating half-wave retarder polarimeter.
Data from a recent “first-look” at using Long Wave InfraRed Imaging Polarimetry (LWIR-IP) to detect surface swimmers is presented and discussed. A significant increase in detection SNR over conventional IR imaging techniques was discovered. The physical phenomena that produces the increased SNR is discussed along with data that shows range effects and their degradation on the SNR. Most significantly, a method to classify the detected object using the same dataset is discussed. Augmenting current swimmer detection systems using this technique will likely significantly decrease the false alarm rates of the system, thus saving manpower resources and preserving force readiness.
Polarimetric imagery that is collected from time-sequential and multiple image format sensors all have potential for image misregistration. Since polarization is usually measured as small differences between radiometric measurements, it is highly sensitive to misregistration, especially at regions of high contrast. The general consensus in the polarization community is that image misregistration on the order of 1/10th of a pixel can introduce artifacts in polarization images. If the registration is not achieved and maintained to this resolution, the data must be registered in software. Typically, rotation and translation (horizontal and vertical) are the main transformations that need to be corrected. It is desirable to have a registration algorithm that determines rotations and translations to 1/10th of a pixel, does not require user intervention, takes minimal computation time, and is based on analytical (non-iterative), automated calculations. This paper details an analytical, automated registration algorithm that corrects for rotation and translations by using a Fourier transform technique. Examples of images registered with this algorithm, and estimates of residual misregistrations are presented. Typical processing times are also given.
Recent interest in the utility of polarimetry for military and commercial applications has led to the development of many different imaging polarimetric systems. Much of the attention has focused on time-sequential acquisition systems. These systems use a rotating retarder or polarizer with data acquisition times that range anywhere from 10 to 100 seconds. This length of time is needed to capture the required frames of data used to construct a polarimetric image. Consequently, scene changes during data collection will either induce false polarization effects or induce mis- registration effects. These can occur in scenes where the target, background or sky is changing intensity rapidly or in cases where the target or sensor is in motion. To overcome these limitations SY Technology Inc. is developing and testing a 4 camera polarimetric imaging system capable of simultaneously capturing the needed frames to produce a full-stokes polarimetric image. The system operates in the visible to near IR spectrum and is capable of a 25Hz Stokes image frame rate. This paper will summarize the design, components and calibration of a 4 camera polarimetric system.
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