In the thermal imaging system, MRTD is the main parameter for comprehensively evaluating the temperature resolution and spatial resolution of the system. It includes not only the system characteristics but also the subjective factors of the observer. It is defined as a standard blackbody pattern of four strips (7:1 aspect ratio) with a certain spatial frequency, observed by the observer on the display screen for an indefinite period of time. When the temperature difference between the target and the background gradually increases from zero until the observer confirms that the target pattern of the four strips can be resolved (50% probability), the temperature difference between the target and the background is called the minimum of the space. The temperature difference can be resolved. The MRTD is a function of the spatial frequency. When the spatial frequency of the target pattern changes, the corresponding resolvable temperature difference is different.
MRTD is not only an important basis for designing infrared imaging systems, but it can also be used to estimate the working distance of the system. For the gaze-type thermal imaging camera, which considers three-dimensional noise, its minimum resolvable temperature difference calculation model is
random spatiotemporal noise; is the noise correction function, subscript z stands for or ; is the threshold SNR for identifying 4 strip targets, where the recommended value of FLR92 is 2.6; is human The time integral function of the eye, and are the spatial integration functions of the human eye, and their expressions are respectively
In the formula, is the time sampling correlation degree, is the frame frequency, τE is the human eye integration time, is the horizontal sampling rate, is the vertical sampling rate, and is the system noise filter.
The test of the minimum resolvable temperature difference is usually done by different people looking at different infrared targets of different sizes and distances. During the test, the black body is placed behind the target, and a fixed temperature difference is maintained with the target, and then observed by the human eye. If the human eye can move just to distinguish the target pattern, the temperature difference is the minimum resolvable temperature difference of the angle of view.