The magnitude of target thermal contrast is used as a key parameter in determining whether observers using thermal-imaging systems can detect military targets. The effects of differences in target and background emissivities on thermal contrast estimates are evaluated, and the effects these differences have on the estimate of the transmittance attenuation thresholds required to obscure a target viewed with a thermal imager are examined. Target-background radiometric difference in the thermal infrared bands is first computed in terms of target-background emissivities and thermodynamic temperatures; the result is then used to compute a difference function for brightness temperature relative to thermodynamic temperature as a function of the target-background emissivity ratio. The effect of emissivity differences between targets and backgrounds on smoke/obscurant attenuation required to obscure a target viewed with a thermal imager is then examined using an expression derived from the Center for Night Vision and Electro-Optics Static Performance model. Results of the analysis show that smoke screen requirements for obscuring targets from thermal imager detection can change by factors of 10 as the ratio of target-to-background emissivity changes by less than 25%.