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The measurement of radiometric signatures in the Long Wave Infra-Red (LWIR) 8μm to 14μm, is an essential endeavour in many different contexts including: the design of threat warning systems; the measurement of own platform emission and stand-off temperature measurement of individuals for healthcare and bio-security. Recent advancement in uncooled micro-bolometer LWIR imagers afford a potential alternative lower cost approach to the measurement of LWIR radiometric signatures. Micro-bolometers have been predominantly designed for imaging applications, with considerable development devoted to the improvement of imaging quality. Good imaging performance relies on a response function which is linear with irradiance observed at the imaging aperture plane. It is therefore possible to determine the aperture plane irradiance using a-priori knowledge of this linear response function. During the development of a system designed to measure the temperature of individual people using micro-bolometer imagers, Thales UK observed that the response function of these devices was indeed linear. The parameters of this response function however, drifted considerably with time. Therefore to understand the response function at any given time, two data points of known irradiance are required in the scene. The conventional methodology for the measurement of radiometric signatures in the LWIR relies on the use of cooled imaging radiometer devices which can be very expensive with devices typically costing on the order of several hundred thousand dollars for one device, with many devices also requiring a supply of liquid nitrogen to function. These limitations make these devices ill-suited for portable low cost devices for radiometric measurements in the LWIR. This paper will present the approaches taken and results observed by Thales to effectively use micro-bolometer imagers to measure radiometric quantities for close-in (less than 5m) objects as well as outlining what further considerations need to be made for the radiometric signature of distant (hundreds of metres) objects at known range.
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