The methods for determining the probability of detection of a thermally camouflaged target at various ranges by the sensor includes determining an "inherent" temperature difference, .DELTA.T.sub.RSS, between the target and its immediate surroundings. Once .DELTA.T.sub.RSS is determined, a scaled N.sub.50 cycle criterion, or the number of resolvable cycles needed by the sensor for a fifty percent probability of detection, is determined according to the relationship N.sub.50=A/(.DELTA.T.sub.RSS).sup.B+C, where A, B and C are constants that are predetermined according to whether the immediate surroundings are of a woodland or a littoral nature. The scaled N.sub.50 cycle criterion is then used in an empirical Target Transfer Probability Function (TTPF) defined by P=(N/N.sub.50).sup.E/(1+(N/N.sub.50).sup.E), where E=2.7+0.7(N/N.sub.50), in order to determine the probability of detection of the thermally camouflaged target.