Thermal images result from temperature differences and/or emissivity differences (apparent temperature differences) in a scene or target. It is the function of a thermal-imaging system to reproduce an acceptable visible image of the scene or target from its thermal content. Thus, a thermal-imaging system is required to resolve spatial differences of temperature and emissivity. The performance of a thermal-imaging system may be specified by means of the fundamental performance measures, noise-equivalent temperature difference (NEAT), minimum-resolvable temperature difference (MRTD), and/or minimum detectable temperature difference (MDTD). Noise-equivalent temperature difference is a convenient measure of an imaging system's thermal sensitivity to a broad-area target (extended source); however, by itself it is no indicator of imaging capability. On the other hand, both minimum-resolvable temperature difference and minimum-detectable temperature difference address the imagery, which includes the human factor; i.e., the image observer. Thus, MRTD is a measure of the compound system-observer capability to spatially resolve temperature differences in a standard periodic-bar target by observing its display on a video monitor. The minimum-detectable temperature difference is similar to the MRTD, except the target geometry differs in being an aperiodic square. The measurement and the significance of each of these performance measures is discussed.