Minimum resolvable temperature difference (MRTD) is a measure for assessing the performance of infrared cameras, and is inversely proportional to the modulation transfer function.
A curve of MRTD against spatial frequency is obtained which characterises the performance of the imaging system.
Modern infrared imaging systems can have low spatial frequency MRTDs of tens of millikelvins.
For each target he/she adjusts the blackbody, (source of Infrared radiation), temperature up and down until the pattern is "just resolvable."
The MRTD curve is a plot of these arrays (just resolvable temperature difference versus target spatial frequency).