11 October 2005 Atmospheric effects on infrared imaging systems
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Abstract
Radiometric infrared camera systems are often used at test ranges to characterize the IR signature of targets such as aircraft or rockets through significant air columns that reduce the received signal through a combination of absorption and scattering. The dominant effect in clear air is molecular resonant absorption which is particularly strong in the midwave IR band (3-5 microns) for carbon dioxide and water vapor. Tactical targets can be imaged at standoff distances up to 1000km or more, but there are many cases where these targets are within a 1km range, as is the case with a close-in flyby at a test range. Therefore it is useful to model the short-range atmospheric transmission to predict its effect on radiometric measurements. Many industrial processes that occur in large outdoor facilities also lend themselves to radiometric measurement for standoff ranges of tens or hundreds of meters. This paper compares experimental radiometric data taken at ranges under 1km to a theoretical model of the atmosphere, and describes a simple method for correcting for air column effects at these relatively short ranges. The data were collected in the 3-5 micron band using an indium antimonide staring-array camera and a long focal length lens combined with radiometric analysis software. The system was calibrated to measure target radiances, but can also be used to estimate target temperatures in cases where the in-band emissivity of the target is well understood. The radiometric data are compared to a model built on MODTRAN code, with conclusions about the attenuation introduced by the atmosphere for standard medium-range imaging systems in "typical" observing conditions. Effects caused by the MTF of the lens system are studied briefly, and used to set limits on the minimum number of pixels the target can subtend and still have an accurately measurable radiance.
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Austin Richards, Austin Richards, Greg Johnson, Greg Johnson, } "Atmospheric effects on infrared imaging systems", Proc. SPIE 5987, Electro-Optical and Infrared Systems: Technology and Applications II, 598701 (11 October 2005); doi: 10.1117/12.631758; https://doi.org/10.1117/12.631758
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