When purchasing an infrared camera, users often ask a question: how far can the infrared thermal camera I purchased see? Or should I identify a thermal imager with what parameters a person should choose at 500 meters?

In fact, this is a particularly important but difficult issue to clarify. For example, every thermal imaging camera can see the sun 150 million kilometers away, but it can’t be said that the detection distance of thermal imaging camera can reach 150 million kilometers. However, the detection distance must be made clear to users, because users buy thermal imagers to detect and monitor targets. Therefore, we need an objective and measurable standard to calculate the detectable target distance.

Johnson told us a rule at the first night vision image intensifier seminar held in October 1958. Let’s take a look at how to determine the target detection distance.

Johnson criterion: the detection distance is the result of the combined action of subjective and objective factors. Subjective factors are related to the observer’s visual psychology, experience and other factors. To answer “how far can the thermal camera see”, we must first find out “what is clear”. For example, when detecting a target, Person A thinks it is clear, but Person B may think it is not clear. Therefore, there must be an objective and unified evaluation standard. Johnson linked the problem of target detection with the equivalent fringe detection according to the experiment. Many studies have shown that it is possible to use the resolution of the target equivalent fringe to determine the target recognition ability of the infrared thermal imager imaging system without considering the nature of the target and image defects, which is “Johnson Criterion”.

Target detection can be divided into three levels: detection, recognition and recognition.

Detection

Detection is defined as finding a target in the field of view. At this time, the image of the target must account for more than 1.5 pixels in the critical size direction.

Recognition

Recognition is defined as: the target can be classified, and the target can be identified as tank, truck or person. This is that the image of the target must occupy more than 6 pixels in the critical size direction.

Identification

The definition of identification is that it can distinguish the model and other characteristics of the target, such as the enemy and ourselves. This is that the image of the target must account for more than 12 pixels in the critical size direction.

Calculation formula

After knowing the concepts and guidelines, you can start the actual calculation:

Generally speaking, the temperature measuring thermal camera can accurately measure the temperature, which generally needs 9 pixels, while the recognition only needs 6 pixels, so the approximate distance can be calculated by the following formula:

Longest side of target  ÷  [(detector pixel size ÷ lens focal length) × Critical dimension direction pixels]= χ km

Examples: thermal imager detector pixel spacing 12 μm. Select 20mm focal length lens:

The recognition distance for people about 1.7m is 1.7 ÷ [(12 ÷ 20) × 6)]=0.472km； The distance that can effectively measure temperature is 1.7 ÷[ (12 ÷ 20) × 9]=0.315km

The vehicle identification distance of 2.3m is 2.3 ÷ [(12 ÷ 20) × 6)]=0.639km； The distance that can effectively measure temperature is 2.3 ÷ [(12 ÷ 20) × 9]=0.430km

The identification distance for a 6-meter center vessel is 6.0 ÷ [(12 ÷ 20) × 6)]=1.667km； The distance that can effectively measure temperature is 6.0 ÷ [(12 ÷ 20) × 9]=1.111km

After understanding, can you calculate it? If you still don’t understand, you can contact sales@viewsheen.com and let experts calculate for you. It’s also a good choice!