PASSIVE THERMAL INFRARED
G8T fabricates passive infrared targets that may be used as references for the dynamic calibration and image quality assessment of infrared imaging systems operating in the Mid-Wave Infrared (MWIR, 3-5µm) and Long-Wave Infrared (LWIR, 8-12µm or 8-14µm). These passive infrared panels are comprised of fabrics that present different levels of thermal emissivity (ελ). They are passive in that the infrared irradiance presented by the panels is dependent upon the thermal environment in which they are deployed – this irradiance determined by the sum of the emitted irradiance from the panel (a function of the panel temperature) plus the reflected irradiance from the surrounding environment (clouds, buildings, vehicles). Consequently, passive infrared targets are dynamic in terms of upwelling radiant energy emitted and reflected from the target and do not present a constant level infrared irradiance as a function of time to an overhead infrared sensitive sensor. It is recommended by G8T that a passive infrared target be accompanied by deployment of a smaller instrumentation panel made of the same fabric at the same location as the principle target array. The irradiance emitted and reflected by the instrumentation panel should be monitored continuously with a thermal infrared radiometer operating in the same infrared spectral domain as the overhead sensor. This monitoring will provide ground truth data of the total infrared irradiance being provided by the passive target at the time of acquisition.
G8T utilizes metallic foil laminated fabrics (aluminum) to achieve emissivity levels between 0.02 and 0.20 and metallic pigmented coatings to produce fabrics that have emissivity levels between 0.30 and 0.80. Higher emissivity levels are produced using fabrics that are coated using pigments that have a high infrared absorbance in the spectral domain of interest.
Special fabrication techniques are used by G8T in assembling passive infrared targets or seam lines which will print through target patterns or form a raster pattern within a large area emissivity calibration panel. These fabrication techniques include insulation of image assessment patterns (such as resolution bars) from the background panel and minimizing the overlap seams in a large area calibration panel.
Low emissivity panels or target patterns will have high infrared reflectivity (Rλ = 1- ελ) and the upwelling radiant energy will vary significantly if the target is facing and reflecting clear skies (space) and then subsequently a passing cloud. The apparent radiometric temperature swing of the panel or pattern can exceed 100̊K under these conditions. Passive infrared targets are always designed by G8T in close consultation with the system designer or image analyst to determine potential dynamic range of the passive target, the resolution or spatial frequency range to be presented by target patterns, and the target thermal polarity of the target (background/pattern – warm/cold or cold/warm). All passive infrared targets fabricated by G8T are custom designed to the specific system and test requirements.