How far can you 'see' with a thermal imager

A frequently asked question here at FOTRIC is “how far can this camera see”? We are here to answer it once and for all.

There are quite a few factors to consider when answering this seemingly simplistic question. First of all, it all depends on what do people mean by “see”. A thermal imager can pick up a heat source as long as the source occupies at least one pixel of space on the detector. To accurately assess the temperature of an object, however, it has to occupy at least a 3x3 pixel region on the detector/screen. For a human brain to register the type of object (A tree? An animal? A car?), it needs to occupy at least an 8x8 pixel region. And it takes a minimum of 12x12 pixel region on the screen for a person to be able to distinguish one individual from another (Bob? George? Lisa?).

Secondly, to answer this question, we must get acquainted with the concept of the D:S ratio. D:S ratio stands for “Distance to surface diameter ratio”, more specifically, the ratio of the distance between the object and the observer to the diameter a pixel covers in the aforementioned distance. Naturally, the bigger this ratio is for a camera, the more details it can obtain about the object it’s observing at a given distance. To put it another way, the bigger the ratio is, the farther the camera can be while meeting the minimum threshold of detection resolution it requires to identify an object.

Let’s explain this concept mathematically with an example.

Q: If a hunter were to use a thermal imager to scout for small animals, let’s say, hares from a 20-meter distance away, what D:S ratio would he need on his thermal imager?

A: If the hunter wishes to identify the type of the animal as well as to detect it, the animal would have to take 8x8 pixels of occupation on screen. Assuming a hare’s cross-section is 0.2x0.2 m^2, each pixel would cover 0.2/8=0.025 m by dimension. This warrants a camera D:S ratio of at least 20/0.025 = 800:1.

To sum up, we must first determine to what degree do we need the object to register on the thermal imager’s screen/detector, then divide the object’s physical dimension by a factor according to the identification degree and we can get the pixel surface dimension. Finally, multiplying the surface dimension with a thermal camera’s D:S ratio will yield the maximum distance it can still ‘see’ the object.

Here is a table of all FOTRIC handheld cameras with their respective D:S ratio.

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