Thermal imaging - how far can you see with it?

June 2008 Surveillance

This is one of the first questions asked by a potential buyer of this technology. It is a reasonable question to ask, but you can see the moon, ±250 000 km from earth, but it does not mean that you can detect, recognise or identify an object or person at that distance.

The 'range', as called in the thermal imaging industry, you can see with a thermal camera is determined by sophisticated modelling. Some of the variables to consider are the type of thermal imager used (cooled or un-cooled), focal length of the lens, atmospheric conditions the size of the object, do you want to detect, recognise or identify the object.

To define what it means to 'see' an object, the Johnson's criteria can be used. John Johnson was a night vision and electronic sensors directorate scientist. He developed criteria that relate to the effective range of infrared cameras. This was mainly developed and used for the military, but are widely used in the commercial market to characterise thermal imaging systems. There are three main distinctions between degrees of 'seeing' a target: detection, recognition and identification.


Cooled detector 640 x 480 with 15 micron pitch.

Pixels across 0,75 m critical dimension of a man-sized target versus range.

Red line – 490 mm lens Blue line – 735 mm lens.


In order to detect if an object is present or not, its critical dimension needs to be covered by 1,5 or more pixels. The unit of system resolution originally used in Johnson's definition, 1,5 pixels in a staring array is equivalent to 0,75 'cycles'.


Uncooled 320 x 240 detector with 38 micron pitch

Pixels across 0,75 m critical dimension of man-sized target versus range.

Purple – 140 mm lens.

Blue – 100 mm lens.

Green – 50 mm lens.

Orange – 35 mm lens.

Red – 19 mm lens.


The definition of recognition is seen as what type of object it is. This means to make a clear distinction between a person or car or truck or any other object. In order to recognise an object it needs to be subtended by at least 6 pixels across its critical dimension.


This term is normally used in the military to distinguish between friend or foe. In order to do this the critical dimension of the target in question needs to be subtended by at least 12 pixels.

In summary, there is no easy answer to the question, 'How far can we see with a thermal imaging camera?' It depends on a large number of environmental conditions, system variables, nature of the target, the background and atmospheric conditions. It also depends on the chosen camera and lens.

Acknowledgement to Flir - Technical Notes.

For more information contact Timeless Technologies, +27 (0)861 846 383, [email protected],

Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Pentagon appointed as Milestone distributor
Elvey Security Technologies News & Events Surveillance
Milestone Systems appointed Pentagon Distribution (an Elvey Group company within the Hudaco Group of Companies) as a distributor. XProtect’s open architecture means no lock-in and the ability to customise the connected video solution that will accomplish the job.

AI camera for all-around AI solutions
Surveillance Products & Solutions
VIVOTEK has expanded its AI security business with the AI entry-tier 9383-Series network camera, a cost-effective solution that allows users to easily recognise attributes of people and cars, thereby enhancing operation and management efficiencies.

Video Analytics Selection Guide 2024
The Video Analytics Selection Guide 2024 highlights a number of video analytics/AI solutions companies offer to enhance and optimise video surveillance operations.

Optimising your camera-to-operator ratio
Learning from critical data points in your security systems is the key to quality monitoring, effectively deploying resources, and scaling control room profitability. The golden equation is your true Camera-to-Operator ratio.

Storage Selection Guide 2024
Storage Selection Guide Surveillance
The Storage Selection Guide 2024 includes a range of video storage and management options for small, medium and large surveillance operations.

Directory of suppliers
The Directory of Suppliers and Solution Providers provides a selection of companies involved in various aspects of surveillance projects, from consulting to implementation and ongoing maintenance, as well as equipment suppliers.

Perspectives on personal care monitoring and smart surveillance
Leaderware Editor's Choice Surveillance Smart Home Automation IoT & Automation
Dr Craig Donald believes smart surveillance offers a range of options for monitoring loved ones, but making the right choice is not always as simple as selecting the latest technology.

The TCO of cloud surveillance
DeepAlert Verifier Technews Publishing Surveillance Infrastructure
SMART Security Solutions asked two successful, home-grown cloud surveillance operators for their take on the benefits of cloud surveillance to the local market. Does cloud do everything, or are there areas where onsite solutions are preferable?

Cloud or onsite, a comparison
Astrosec Surveillance
In the realm of electronic security, the choice between cloud-based and onsite software solutions for offsite CCTV monitoring can significantly impact operational efficiency, cost-effectiveness, and overall effectiveness.

On-camera AI and storage create added benefits
Elvey Security Technologies AI & Data Analytics Surveillance IoT & Automation
The days of standalone security systems are long past, and the drive is now to educate system integrators, installers, and end users on the return on investment that can be derived from intelligent, integrated BMS, IoT and security systems.