The minimum resolvable temperature difference (MRTD) and to a lesser extent the minimum detectable temperature difference (MDTD) are two of the primary criteria used in evaluating the performance of thermal imaging systems. They are used both at the design stage, where they are calculated from known parameters of the system, as well as in acceptance testing of production systems. The measurement of MRTD requires the operator to judge the point at which the temperature difference of a 4-bar target is sufficient for the target to be just resolved. It therefore contains a subjective element which results in measurements done by different observers frequently yielding significantly different values. There is considerable pressure from purshasers, manufacturers and users of thermal imagers to find an objective alternative to this subjective measurement. This paper describes two techniques for obtaining an objective measurement of MRTD which are currently being developed by the authors and discusses some of the difficulties associated with producing an acceptable approach to this problem.