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How far can thermal camera see?
- Jun 07, 2018 -

To correctly determine the range of a thermal imaging camera requires some sophisticated modeling. There are many variables to consider including the type of thermal imaging camera you are using, the type of lens you are using, the nature and size of the object you want to detect, the atmospheric conditions and the very definition of what it means to “see” a target.

 “Seeing” an object - To define what is meant by “seeing a target”, the so-called Johnson’s criteria can be used. John Johnson, a Night Vision & Electronic Sensors Directorate scientist, developed criteria that relate to the effective range of infrared cameras. Although developed for the military (hence the use of the term “target” to refer to the object of interest), the Johnson criteria are widely used in the commercial marketplace to characterize thermal imaging systems. According to these criteria a distinction needs to be made between degrees of “seeing” a target:

A.  Detection:

In order to detect if an object is present or not, its critical dimension needs to be covered by 1.5 or more pixels. 1.5 pixels in a staring array is equivalent to 0.75 “cycles”, which is the unit of system resolution originally used in Johnson’s definition.

B. Recognition:

Recognizing an object is defined as seeing what type of object it is. It means being able to make the distinction between a person, a car, a truck or any other object. In order to recognize an object it needs to be subtended by at least 6 pixels across its critical dimension.

C. Identification:

This term is often used in the military sense of the word, which means seeing if someone is “friend or foe”. In order to do this, the critical dimension of the object in question needs to be subtended by at least 12 pixels."

All of the above data are obtained with a probability of 50%, that is, just finding the target, and the contrast between the target and the background is 1. As can be seen from the Johnson criterion above, how far a set of infrared thermal imagers can see is determined by the target size, lens focal length, detector performance and other factors.

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Factors that determine the detection distance

1.  Focal length

The most important factor that determines the detection distance of thermal imager is the focal length of lens. The focal length of the lens directly determines the size of the target image, i.e. the number of pixels in the focal plane. Usually this is using the spatial resolution (IFOV), It represents the Angle at which each pixel opens in physical space, also is the minimum Angle system can distinguish, generally calculated  by the ratio of the pixel size (d) and the focal length (f), namely: IFOV = d/f.

The image formed by each target in the focal plane takes up several pixels, which can be calculated by the target size, the distance between the target and the thermal imager, and the spatial resolution (IFOV). The ratio between the target size (D) and the distance between the target and the thermal imager (L) is the span Angle of the target, and the number of pixels occupied by the image is obtained by dividing the span Angle of the target with IFOV, i.e., n=(D/L)/IFOV=(Df)/(Ld). It can be seen from this that the bigger the focal length, the more pixels the target occupies. According to Johnson's criteria, the detection distance is farther. On the other hand, the bigger the focal length, the smaller the field of view, and the higher the cost.

Here's an example. The pixel size of the focal plane of the thermal imager is 38μm, and with 100mm focal length lens, the spatial resolution IFOV is 1.38mrad. If the target is 2.3 miles away, the span Angle of the target is 2.3mrad, and the image of the target takes up 2.3/0.38=6 pixels. According to Johnson's criteria, the recognition level is reached.

2.  Performance of detector

The focal length of lens determines the detection distance of thermal imager theoretically. Another factor that plays an important role in practical application is the detector performance. The focal length of the lens only determines the size of the image, the number of pixels occupied, and the detector performance determines the image quality, such as the degree of blur, signal-to-noise ratio, etc. The detector performance can be analyzed from the aspects of pixel size, thermal sensitivity and signal processing.

The smaller the pixel size is, the smaller the spatial resolution (IFOV) is. As can be seen from the previous discussion, the larger the detection distance is. A typical example is the FLIR uncooled thermal imager Photon320 pixel size is 38 μm, Photon640 pixel size is 25 μm, with 100 mm lens, observations of 2.3 m target, according to the Johnson’s criteria, the recognition distance is 1 km, 1.5 km respectively.

The detector’s thermal sensitivity and signal processing determine the image sharpness. If the thermal sensitivity and signal processing ability of the detector are not good, then the image formed is fuzzy and cannot be recognized. As a result, some thermal detector sensitivity is not high, increase the lens aperture method is taken to improve the effect of image, this not only increased the cost, but also increased the use of inconvenience.

3.  atmospheric environment

Although the thermal radiation has a better atmospheric penetration than visible light, but the atmospheric absorption, heat dissipation have a certain impact on the thermal imager imaging, especially the fog and rain weather environment, which affect the detection range of thermal imager.

Above all, “how far a thermal imager can see” is a question hard to say, it is influenced by several aspects: the detector, lens, target, the atmospheric environment and other objective factors and the common man's subjective factors.

Computational formula :

n = (D/L)IFOV=(Df)/(Ld)=[Target size(D)*Focal length(f)]/[Detection distance(L)*Pixel size(d)]


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