The minimum temperature difference perceived (MTDP) is a new device figure of merit for assessing undersampled thermal imagers. MTDP is based on the perception of the standard four-bar test pattern, like minimum resolvable temperature difference (MRTD). In undersampled imagers, the test pattern image is increasingly distorted toward higher pattern frequencies due to aliasing. Contrary to MRTD, MTDP takes into account such signal distortions to a certain extent. With MTDP the imager performance can be assessed beyond the Nyquist frequency. The modulation in the distorted four-bar pattern images is studied as a function of pattern frequency and position. We show that for each pattern frequency, an optimum phase position can be found at which the average modulation in the reproduced pattern image has a maximum. This modulation is defined as the average modulation at optimum phase (AMOP). The MTDP is the temperature difference at which four, three or two bars can be resolved by an observer, with the test pattern at the optimum phase position. The MTDP approach is implemented in the TRM3 model. Examples comparing measured and predicted MTDP data for several staring imagers are presented. The validity of MTDP to predict the range performance of undersampled imagers is examined and results are presented. © 1999 Society of Photo-Optical Instrumentation Engineers.