Well, this is a reasonable question but with no simple answer. There are too many factors that would affect the results, such as the attenuation in different climatic conditions, the sensitivity of thermal detector, the imaging algorithm, dead-point and back ground noises, and the target background temperature difference. For example, a cigarette butt is more clearly to see than the leaves on a tree at the same distance even if it is much smaller, because of the target background temperature difference.
The detection distance is the result of a combination of subjective factors and objective factors. It is related to the observer’s visual psychology, experience and other factors. To answer “how far can a thermal camera see”, we must find out what it means firstly. For example, to detect a target, while A thinks he can see it clearly, B may not. Therefore, there must be an objective and unified evaluation standard.
Johnson’s criteria
Johnson compared the eye detection problem with the line pairs according to the experiment. A line pair is the distance subtended across parallel light and dark line’s at the limit of the observer’s visual acuity. A line pair is the equivalent of two pixels. Many studies have shown that it is possible to determine the target recognition ability of the infrared thermal imager system by using line pairs without considering the nature of the target and image defects.
The image of each target in the focal plane occupies a few pixels, which can be calculated from the size, the distance between the target and the thermal imager, and the instantaneous field of view (IFOV). The ratio of the target size (d) to the distance (L) is called the aperture angle. It can be divided by IFOV to obtain the number of pixels occupied by the image, that is, n = (D / L) / IFOV = (DF) / (LD). It can be seen that the larger the focal length, the more prime points occupied by the target image. According to Johnson criterion, the detection distance is farther. On the other hand, the larger the focal length, the smaller the field angle, and the higher the cost would be.
We can calculate how far a specific thermal image can see based on the minimum resolutions according to Johnson’s Criteria are:
Detection – an object is present: 2 +1/-0.5 pixels
Recognition – the type object can be discerned, a person vs. a car: 8 +1.6/-0.4 pixels
Identification – a specific object can be discerned, a woman vs. a man, the specific car: 12.8 +3.2/-2.8 pixels
These measurements give a 50% probability of an observer discriminating an object to the specified level.
Post time: Nov-23-2021