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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.
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