Thermal imaging: how far can you see with it?

October 6, 2015

Two questions we receive on a regular basis at Timeless Technologies: what is the difference between Thermal and Night Vision and then how far can you see with Thermal vision.  In order to address the second answer the best would be to refer to the  "FLIR Technical Note - Thermal Imaging, how far can you see with it?" 

"Often, the first question that people interested in buying a thermal imaging camera ask is “How far can I see?”


This is a very reasonable question to ask, but it defies any simple answer. All FLIR Systems thermal imaging cameras are able to see the sun which is more than 146 million kilometers away from Earth. But it would be totally wrong to say that all FLIR Systems thermal imaging cameras can detect security threats at this distance."

Thermal imaging is a technology that enables detection of people and objects in total darkness and in very diverse weather conditions. A typical application for thermal imaging is border security, where most threats occur at night. Watchtowers spaced at 4km intervals or more have to be able to detect threats at ranges up to 2km or more to guarantee full coverage of the border. Knowing how far you can see with a thermal imaging camera and at which distance you can detect a possible threat is of the utmost importance. The distance you can see a given target with a thermal imaging camera is called the “range” in the thermal imaging industry.


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:


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.


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.

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."


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