In short: Theoretical range = focal length × target size / (pixel pitch × number of pixels). For detection we use 1.5 pixels across the target height, 6 for recognition and 12 for identification. The result is theoretical – in the field it is reduced by weather and low thermal contrast.
Thermal imager theoretical range calculator
| Theoretical range | |
|---|---|
| Detection (1.5 px) | |
| Recognition (6 px) | |
| Identification (12 px) |
The result is theoretical (Johnson criteria). Manufacturers use different criteria, so their figures may differ. Real-world range is reduced by weather, humidity and low thermal contrast.
How the calculator works
range (m) = focal length (mm) × target size (m) / (pixel pitch (mm) × number of pixels). Pixel pitch in mm = µm / 1000. Target size is the height: person 1.7 m, wild boar 0.7 m, roe deer 0.8 m. Pixels: detection 1.5, recognition 6, identification 12.
Examples (checked by hand)
| Lens | Pitch | Target | Detection | Recognition | Identification |
|---|---|---|---|---|---|
| 35 mm | 12 µm | person 1.7 m | 3,306 m | 826 m | 413 m |
| 50 mm | 17 µm | wild boar 0.7 m | 1,373 m | 343 m | 172 m |
| 19 mm | 12 µm | roe deer 0.8 m | 844 m | 211 m | 106 m |
Why the result differs from manufacturers’ figures
- Manufacturers use different criteria (pixel counts and target size), so their figures can’t be compared directly.
- The theoretical calculation ignores weather, humidity, rain and fog.
- Thermal contrast matters: a target at a similar temperature to its surroundings is harder to see.
- The final image also depends on sensor sensitivity (NETD) and the quality of the lens and display.