All the thermal imaging cameras effect distance conforms to Johnson’ s criteria.  — An international standard to describe outdoor thermal camera performance.

How far can the thermal camera detect a target? Usually the definition is as below:  

•  “D” : Detection, means distinguish an object from the background. ( defined as finding a target within the field of view. At this time, the image formed by the target must occupy 1.5 pixels or more in the critical dimension direction)

•  “R” : Recognition, classify the object class (animal, human, vehicle, ship etc…), The target can be classified, ie the target can be identified as a tank, a truck or a person. This is because the image formed by the target must occupy 6 pixels or more in the critical dimension.

•  “I” : Identification, describe the object in details, e.g. If a human is wearing a hat. This is an image of the target must occupy more than 12 pixels in the critical dimension direction.