Thermal IR image of an armored personnel carrier in the urban canyon environment. Imagery was collected by Polaris personnel during a night-time surveillance field-test.
Why Consider Polarimetry?
In radiometric images, an apparent temperature difference between the object and the background must be present in order for the object to be visible.
In polarimetric images, a polarization difference between the object and the background is required.
Why consider polarimetery for object in or on water?
For unpolarized objects to be visible, the temperature difference between the target and background (sky, land, water) is the most important.
Water presents a homogeneous background as its signature. For other objects that may be polarized, their optical properties must differ significantly from the water.
Polarimetric images offer another mechanism by which water surface objects or aircraft may be detected and classified.
Polaris MWIR Imaging Polarimeters provide both standard thermal images and polarization images.
Vela offers MWIR polarization in up to 6 different wavebands and operating at 120 frames per second.
Indus is a division of aperture imaging polarimeter that is also offered in a multi-wavelength configuration.
Zero Contrast! Where are the targets?
These military targets were imaged by ARDEC from the Picatinny Precision Armaments Laboratory tower.
-the targets are placed at 1/2km (500m) standoff
-the tower itself is 215ft tall. (approximately 400ft elevation due to a mountain setting)
The Target 1 is visible in the IR thermal image, but could be easily confused with shrubs and other natural features surrounding Target 1. Target 2 completely blends into the roadway background and is not visible in the IR thermal image because of it's apparent temperature equals that of the surrounding roadway. Zero contrast conditions often occur when the target has been at rest for several hours ( not heat load from the engine) or the target is camouflaged.
Low contrast is typically observed during two period of the diurnal cycle, typically at cross-over points. Those times depend primarily on the ambient temperature and solar loading profiles of a given day, along with parameters such as the thermal capacity and emissivity of the targets and backgrounds. A significant strength of polarimetery for target acquisition is that targets often remain visible during cross-over points.
Illustration of why polarization can be important
(Data collected at the Russell Tower facility at Redstone Arsenal)
Polarimetery Enhances Contrast.
- Man-made objects tend to be polarized
- Natural backgrounds tend to be unpolarized
Polarimetery can also provide information about object shape.