Division of Focal Plane polarimeters (DoFP) operate by integrating an array of micropolarizer elements with a
focal plane array. These devices have been investigated for over a decade, and example systems have been built in
all regions of the optical spectrum. DoFP devices have the distinct advantage that they are mechanically rugged,
inherently temporally synchronized, and optically aligned. They have the concomitant disadvantage that each
pixel in the FPA has a different instantaneous field of view (IFOV), meaning that the polarization component
measurements that go into estimating the Stokes vector across the image come from four different points in
the field. In addition to IFOV errors, microgrid camera systems operating in the LWIR have the additional
problem that FPA nonuniformity (NU) noise can be quite severe. The spatial differencing nature of a DoFP
system exacerbates the residual NU noise that is remaining after calibration, and is often the largest source
of false polarization signatures away from regions where IFOV error dominates. We have recently presented a
scene based algorithm that uses frame-to-frame motion to compensate for NU noise in unpolarized IR imagers.
In this paper, we have extended that algorithm so that it can be used to compensate for NU noise on a DoFP
polarimeter. Furthermore, the additional information provided by the scene motion can be used to significantly
reduce the IFOV error. We have found a reduction of IFOV error by a factor of 10 if the scene motion is known
exactly. Performance is reduced when the motion must be estimated from the scene, but still shows a marked
improvement over static DoFP images.
Microgrid polarimeters, also known as division of focal plane (DoFP) polarimeters, are composed of an integrated
array of micropolarizing elements that immediately precedes the FPA. The result of the DoFP device is that
neighboring pixels sense different polarization states. The measurements made at each pixel can be combined to
estimate the Stokes vector at every reconstruction point in a scene. DoFP devices have the advantage that they
are mechanically rugged and inherently optically aligned. However, they suffer from the severe disadvantage
that the neighboring pixels that make up the Stokes vector estimates have different instantaneous fields of view
(IFOV). This IFOV error leads to spatial differencing that causes false polarization signatures, especially in
regions of the image where the scene changes rapidly in space. Furthermore, when the polarimeter is operating
in the LWIR, the FPA has inherent response problems such as nonuniformity and dead pixels that make the
false polarization problem that much worse. In this paper, we present methods that use spatial information from
the scene to mitigate two of the biggest problems that confront DoFP devices. The first is a polarimetric dead
pixel replacement (DPR) scheme, and the second is a reconstruction method that chooses the most appropriate
polarimetric interpolation scheme for each particular pixel in the image based on the scene properties. We have
found that these two methods can greatly improve both the visual appearance of polarization products as well
as the accuracy of the polarization estimates, and can be implemented with minimal computational cost.
Long-wave infrared (LWIR) imaging is a prominent and useful technique for remote sensing applications. Moreover, polarization imaging has been shown to provide additional information about the imaged scene. However, polarization estimation requires that multiple measurements be made of each observed scene point under optically different conditions. This challenging measurement strategy makes the polarization estimates prone to error. The sources of this error differ depending upon the type of measurement scheme used. In this paper, we examine one particular measurement scheme, namely, a simultaneous multiple-measurement imaging polarimeter (SIP) using a microgrid polarizer array. The imager is composed of a microgrid polarizer masking a LWIR HgCdTe focal plane array (operating at 8.3-9.3 μm), and is able to make simultaneous modulated scene measurements. In this paper we present an analytical model that is used to predict the performance of the system in order to help interpret real results. This model is radiometrically accurate and accounts for the temperature of the camera system optics, spatial nonuniformity and drift, optical resolution and other sources of noise. This model is then used in simulation to validate it against laboratory measurements. The precision and accuracy of the SIP instrument is then studied.
One of the most significant challenges in performing infrared (IR) polarimetery is the focal plane array (FPA) nonuniformity (NU) noise that is inherent in virtually all IR photodetector technologies that operate in the midwave IR (MWIR) or long-wave IR (LWIR). NU noise results from pixel-to-pixel variations in the repsonsivity of the photodetectors. This problem is especially severy in the microengineered IR FPA materials like HgCdTe and InSb, as well as in uncooled IR microbolometer sensors. Such problems are largely absent from Si based visible spectrum FPAs. The pixel response is usually a variable nonlinear response function, and even when the response is linearized over some range of temperatures, the gain and offset of the resulting response is usually highly variable. NU noise is normally corrected by applying a linear calibration to the data, but the resulting imagery still retains residual nonuniformity due to the nonlinearity of the photodetector responses. This residual nonuniformity is particularly troublesome for polarimeters because of the addition and subtraction operations that must be performed on the images in order to construct the Stokes parameters or other polarization products. In this paper we explore the impact of NU noise on full stokes and linear-polarization-only IR polarimeters. We
compare the performance of division of time, division of amplitude, and division of array polarimeters in the presence of both NU and temporal noise, and assess the ability of calibration-based NU correction schemes to clean up the data.
The results of experiments demonstrating the first amplified retro-modulated free-space optical communications link are presented. The amplifier increases the effective area of the retro-modulator by a factor of 318. The first experimental demonstration of a retro-modulator operating at a data rate of 2.5-Gbps is also presented. We will present the details of the experimental system, a simple theoretical model explaining the system performance, and the results of the first amplified retro-modulated link experiments.