Performance reported efforts to calibrate a MWIR imaging polarimeter met with moderate success. Recent efforts to calibrate a LWIR sensor using a different technique have been much more fruitful. For our sensor, which is based on a rotating retarder, we have improved system calibration substantially be including nonuniformity correction at all measurement positions of the retarder in our polarization data analysis. This technique can account for effects such as spurious optical reflections within a camera system that had been masquerading as false polarization in our previous data analysis methodology. Our techniques will be described and our calibration results will be quantified. Data from field-testing will be presented.
Efforts to understand the potential for polarization information to improve our target acquisition capability have been extended from the mid and long wave IR regions into the short wave IR. A passive, complete rotating- retarder Stokes imaging polarimeter has been developed, calibrated, and field tested. A review of the calibration and some example phenomenology will be presented, including temporal variation of target polarization signatures. Spectral investigation of targets and backgrounds in the SWIR band have been promising. For daytime use, passive polarization imaging could provide additional information for improved detection or identification. The passive polarization imaging effort in the SWIR forms one half of an active polarization agile 1.54 micrometers imaging system. In this system, the polarization of radiation from an eye-safe 1.54 micrometers laser is controlled so that a target or background can be illuminated with a known polarization. Analysis of the reflected beam allows calculation of the Mueller Matrices will hopefully lead to better target-to-background discrimination. This Active Mueller Imaging Ellipsometer has been calibrated, and initial imagery of targets has been collected.
Accurately calibrating an infrared polarimeter presents several challenges. Some of these challenges include characterization of the polarization elements within the polarimeter, overcoming the effects of thermal emissions within the polarimeter, and developing a test setup and procedure for conclusively verifying the instrument calibration. We describe our efforts to absolutely calibrate our imaging Stokes polarimeter that operates in the mid-wave infrared band (3 to 5 microns). We have developed a generator that provides well-controlled polarization states for calibrating the polarimeter. This generator overcomes problems associated with thermal emissions, stray reflections, and narcissus effects. While the polarization state emitted by the generator is not known with extreme accuracy, we are able to rotate the generated state without affecting its degree of polarization or ellipticity. We show that we can create a complete set of input states to allow a full calibration of the polarimeter, and we describe a technique for optimizing the calibration based on a variance-minimization. Results of our calibration are presented, indicating that our polarimeter precision is better than 0.1%. This technique is not limited to infrared polarimeters and should therefore have broad applicability.
Previous infrared polarization imaging research has shown manmade objects to be sources of emitted and reflected polarized radiation while natural backgrounds are predominantly unpolarized. The prior work underscored the dramatic improvements in signal to clutter ratio that could be achieved in a typical target acquisition scenario using polarization sensing techniques. Initial investigations into the polarization signatures of surface scattered mines have also shown polarization techniques are able to provide strong cues to mine presence. The US Army Night Vision and Electronics Sensors Directorate has developed complete Stokes imaging polarimeters in the midwave infrared (MWIR, 3-5 µm)1,2, the short wave infrared (SWIR, 1-2 µm) and the visible wavebands in order to investigate polarization phenomenology and to quantify expected improvements to target acquisition and mine detection. This paper will review the designs of the polarization cameras and will summarize efforts to calibrate them. An example phenomenology study of MWIR polarization from surface scattered landmines will be presented.
An optically addressed spatial light modulator that is based on the chemically enhanced bacteriorhodopsin thin film is demonstrated. The underlying principle is the light-induced shift of absorption band of the protein. The device can be chemically or genetically modified for different applications. In our preliminary experiment, a resolution of about 100 line pairs per millimeter with a linear dynamic range of above 100:1 is obtained.
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