Comparison of Emissive and Reflective Minimum Resolvable Temperature Difference for Measurements of Sensor Performance

The Minimum Resolvable Temperature Difference (MRT or MRTD) of an IR imaging sensor provides a measure of system performance in terms of sensitivity as a function of resolution. It's expressed as the temperature difference (δT) between a target and a background at which target features are just discernable as a function of spatial frequency. Traditionally, MRT has been measured in the laboratory by imaging a flat plate "blackbody" at slightly elevated and depressed temperatures through a sequence of 4-bar slot patterns ("targets") in high emissivity disks ("backgrounds") at ambient temperature. In this traditional, purely emissive MRT approach, the luminance modulation in the images of the smaller targets ride upon luminance pedestals against the ambient background that make the image modulation hard to discern. Consequently, in the laboratory, sensor gain and offset adjustments sometimes must be performed in order to see the modulation on the smaller, i.e., higher spatial frequency, MRT targets. Quite frequently this part of the procedure does not correspond to how the sensor is operated in the field. An alternative approach, called reflective MRT, uses a disk in which the target region consists of slots and highly reflective, low emissivity spaces and is surrounded by a high emissivity background. Two flat plate "blackbodies" are used, one in transmission through the slots and one in reflection from the spaces between the slots. Both are at slightly different temperatures controlled and regulated above and below ambient. This results in target luminance modulation that does not ride upon ambient luminance pedestals, thus allowing MRT to be measured at the same sensor gain and offset for all target spatial frequencies. The intent of this approach is to improve the accuracy of laboratory MRT measurements as predictors of field performance. This paper describes this problem with emissive MRT, reflective MRT as a possible solution, and the experimental research planned for calendar year 2003 to compare emissive and reflective MRT measurements of well sampled imaging sensors.