MRTD
Minimum Resolvable Temperature Difference (MRTD) is a measure of an infrared camera’s ability to resolve a target. Conceptually this measurement is similar to that done with a resolution test chart in a visible system. A typical test chart such as the 1951 U.S. Air Force resolution test chart, presents white bars against a black background at various sizes (frequencies).
A simple measurement determines the smallest feature (highest line frequency) that can be resolved.
In the Infrared regime, black and white bars are created not by variation in reflectivity, but by variation in temperature. For a high temperature difference (∆T=5°C) bars can be resolved to a high frequency. As the temperature difference is reduced the bar contrast will fade-out. The highest frequencies will fade out first, until the whole image is lost in a grey background.
Figure 1.1 1951 USAF resolution test chart, high contrast white bars on black background at increasing frequencies. Figure 1.2 The IR bar target is analogous to the visible test chart, it consists of hot bars on a cold back ground.
The temperature contrast at which the bars disappear is recorded. The temperature contrast is then inverted, cold bars on a hot background, and the measurements are repeated and averaged together. Figure 2 depicts a typical MRTD curve.
Figure 2 A typical MRTD curve. System 1 can resolve all frequencies at a smaller temperature difference then System 2. (i.e at 1.5 cycles/mrad system 1 can resolve a 0.125 °K temperature difference, System 2 requires a 0.36 °K difference to resolve the same frequency)