One of the important indicators of infrared imaging systems is the distance of observation, recognition, and recognition of specific targets under specified meteorological conditions, which are called observation distance, recognition distance, and recognition distance. The working distance is the main tactical index of the infrared imaging system and plays a decisive role in the design of the system. The working distance consists of two computational models: the point target model and the extended source model. When the target is not enough to fill a pixel, the target can be regarded as a point target. In theory, as long as the radiant energy of the target reaching the system is greater than the detection threshold of the system, the system responds and the target can be detected. In fact, the main purpose of detecting the target is to obtain the exact information of the target, such as the type and model of the target, so the point target model has no practical significance. At this point, the goal should be considered as an extended source target. When studying the distance of the infrared imaging system to such targets, it is not only possible to consider the radiant energy of the target, but also the influence of the size and shape of the target on the estimation of the line of sight.