<title>Analysis of material emissivity effects on detection/recognition of targets viewed through smokes/obscurants with thermal imaging systems</title>

Abstract

Imaging systems operating in the thermal infrared bands (3-to 5-urn or 8- to 14-urn) are key elements in major electro-optical weapons systems. Imager response as a function of radiative difference betwen target and background is commonly expressed in terms of temperature difference, or thermal contrast, between the target and the background. This can be done since radiative difference and thermal contrast, under the assumption of identical target and background emissivities and nearambient temperatures, are linearly proportional. Thus FLIR detector response is commonly expressed in terms of "minimum detectable temperature difference" (MDT) and "minimum resolvable temperature difference" (MRT) . Models such as the CCNVEO Static Performance Model for thermal imaging systems, which uses thermal contrast as a target characteristic, have had mixed success in predicting FLIR performance in field tests. Experimentally measured smoke/obscurant transmittance thresholds required for obscuring targets from FLIRs have large standard deviations but tend to agree with the the Static Performance model. The differences between model predictions and experimental results generally have been ignored because a large number of variables in the tests (such as variation in human response) cannot be controlled. Smoke/obscurant countermeasures tests and calibrations of targets used in these tests have been based on the assumptions that the emissivities of targets and backgrounds were identical, constant with wavelength, and near unity. However, this paper shows that relatively small differences between target and background emissivities can lead to significant differences between thermal contrasts predicted using true target and background radiative differences and that predicted by brightness temperature difference. Thermal contrast incorrectly estimated using an emissivity of 1 for target and background (brightness temperature) and expected thermodynamic temperatures can lead to major errors in the Static Performance model's estimate of the transmittance level required to reduce the detection/recognition capability of target observers using FLIRs. This source of error may help explain the wide variation in existing smoke/obscurant threshold data for FLIR5. The purpose of this paper is to evaluate the effect of differences in target and background ernissivities on thermal contrast estimates of radiative difference between target and background, and to examine the effects such errors have in estimating the transmittance threshold required to obscure a target viewed with an imager.