<title>Passive target detection using polarized components of infrared signatures</title>

Abstract

Oneofthe limiting difficulties facing infrared systems seeking to detecttargets isbackground clutter. In many interesting situations viewing dim targets, detection ispossibleon thebasisofsystem sensitivity,butpreventedbylow signal-to-clutterratios.Overthepastseveral years, we have investigatedthepotentialofinfraredpolarizedsignaturecomponents for suppressing background clutter and enhancing the detection of dim targets. In the course of this work, we have built an imaging infrared polarimeter. participated in field experiments, and compared measured results with theoretical predictions.This paper seeks to summarize this work. It includes a brief theoretical description of the effect, a description of our measurement instrument, samples of our empirical findings, and comparisons with simple theoretical predictions. We conclude with a summary of what we have learned and what issues remain unresolved. BACKGROUNDIn military applications ofinfrared technology (e.g., thermal imagers, spectrometers, seekers), polarization has largely been ignored. Although there have been priorattempts to exploitlRpolarization in the 1960's and 70's [e.g., 1-4], the efforts seem to have been relatively short lived.Recently, there has been renewed community interest [e.g., 5-8] in JR polarization. Beginning in 1985, we have studied polarization in passive LR applications. Our motivation has been to auak low contrast and clutter-limited target detection problems where radiometric conirast is an insufficientdiscriminate. This work has included a number ofstudies [8][9][10][11][12][13][14][15][16] involving laboratory measurements, field measurements, and theoretical analyses; these are summarized in this paper. BASICS OF PASSIVE INFRARED POLARIMETRIC SIGNATURESIt is well known that unpolarized light reflected from smooth surfaces (at non-normal incidence) acquires an elliptical polarized component whose major axis is oriented perpendicular to the surface normal. Said differently, the surface reflectance is greater for electric field components oriented perpendicular to the surface normal. It isless welirecognized thatKirchoff's Law applies so that infrared radiation emitted from smooth surfaces is also partially polarized, but in this case with a preference to electric field components oriented parallel to the surface nonnal. At infrared wavelengths, it is the partial polarization of emitted radiation that is of primary (but not exclusive) interest.The observed polarized signature component depends on many factors including the view angle, material refractive index, surface roughness, temperature, the radiative environment, and the surface shape seen within a single viewing resolution element. First, no effect is seen when viewing a surface at normal incidence. In practice, the greatest polarized signature components are generally observed at viewing angles in the 60-80 degree range (measured from nonnal), while useful signature levels can be expected from approximately 30 to nearly 90 degrees. Second, the material complex refractive index...

Testing smooth surface characteristics based on thermal infrared polarization

“…The reason is that the gray number is very small in the three images of each scene. When we use the equations (3,4) to calculate the DOP, the few changes of the gray number (such as noise) will bring large changes in DOP image, and likewise we use the equations (3,5) to calculate the AOP, we would meet with the same problem. But these problems can be ignored, because our attention is focused on the target itself, but not the background.…”

<title>Measurements of midwave and longwave infrared polarization from water</title>

“…(4) As the incidence angle 9 is increased from zero at nadir (straight down), the surface emissivity becomes increasingly p polarized, while the surface reflectivity becomes increasingly s polarized. The net state of polarization seen by the sensor depends upon the relative magnitudes of the reflected and emitted radiance terms.…”

Measured Degree of Infrared Polarization for a Variety of Thermal Emitting Surfaces

“…), it has been suggested that by examining the polarization state of the IR image forming radiation, one can significantly improve the ability to see objects that are obscured by background clutter (4,5,6).…”