When people shop around for a thermal imaging device, one of the most frequent questions they ask is “how far can I see with it?” It may sound strange, but this question is extremely complex and many-sided. To get a precise answer one needs to take into account a variety of factors that could be separated into two groups: inner and outer.

Before we identify these factors, let us first define the terms “Detection”, “Recognition” and “Identification”. These definitions are based on work by J. Johnson who introduced a criteria to assess performance of optical equipment back in 1950s.

Detection: you can distinguish an unknown object from the background.

Recognition: you can tell what kind of object this is (e.g. human, car, etc.)

Identification: you can describe the object in details (e.g. a male civilian or a four-door sedan).

The example on the below illustrates the idea of detection, recognition and identification of a Canadian goose.

Inner factors that determine the range are related to the characteristics of the thermal system you are using. These parameters are: detector sensitivity, lens focal length, lens f-number, thermal detector’s resolution, display resolution and type (small or large).

Outer factors involve object size, environmental and weather conditions, the difference between the object and the background.

It now becomes quite obvious that one cannot give a definitive and universal answer to that “how far?” question. Some manufacturers bluntly state detection, recognition and identification figures, or DRI for short, without even mentioning what these performance figures depend on.

Have a look at this video - Its about UAV cameras, however the principal is the same