Thermal Camera FOV & Detection Range Calculator
Field of view, IFOV, pixels on target, Johnson's criteria detection / recognition / identification ranges, lens finder, mounting and radiometric spot size.
Thermal sensor & lens
Target
Pixels per level (Johnson's criteria)
1.5 / 6 / 12 pixels across the critical dimension, as used by Axis, FLIR and Hikvision datasheets. Raise them for video analytics (VCA), which needs more pixels.
Installation
Temperature measurement (radiometric)
A reliable reading needs the hot spot to fill at least about 3 × 3 pixels. Check your camera's own measurement spec.
Detection ranges with this lens (Johnson's criteria)
| Target | Detection | Recognition | Identification |
|---|
Top view: coverage
Lens finder
Side view: mounting & vertical FOV
Temperature measurement
Common lenses for this sensor
| Lens | H × V FOV | IFOV | Detect | Recognise | Identify |
|---|
Ranges follow Johnson's criteria: a 50% probability that an operator can detect, recognise or identify the target, assuming at least 2 °C between target and background and clear air. Fog, rain, dust, humidity and low thermal contrast shorten real ranges, and video analytics usually work at shorter distances than the detection figure. Use the results as a design baseline and confirm on site. Techubox.com
How to use the Thermal Camera Calculator
Thermal camera datasheets often quote a single "detection range" that is hard to compare. This calculator works out detection, recognition and identification distances from the sensor resolution, pixel pitch and lens using the Johnson criteria, for a person or a vehicle.
Step by step
- Enter the thermal sensor resolution, for example 384 x 288.
- Enter the pixel pitch in micrometres, for example 17 um or 12 um.
- Enter the lens focal length in millimetres.
- Choose the target: person (about 0.75 m) or vehicle (about 2.3 m).
- Read the field of view and the three ranges. Adjust the pixel criteria if your specification uses different values.
Worked example
A 384 x 288 sensor with 17 um pixels and a 25 mm lens has an IFOV of about 0.68 mrad. For a person (0.75 m) that gives roughly 735 m detection, 184 m recognition and 92 m identification using 1.5, 6 and 12 pixels.
Frequently asked questions
What are the Johnson criteria?
They are rules of thumb for how many pixels across a target you need: about 1.5 for detection, 6 for recognition and 12 for identification, at about 50% probability.
Why is my real range shorter?
Weather, humidity, low temperature contrast and image processing all reduce range. Treat the result as a best case and add margin for perimeter designs.
Does a smaller pixel pitch help?
With the same lens, a 12 um sensor gives more range than a 17 um sensor because each pixel covers a smaller angle.