Engineering tool
Thermal Imaging Calculator: DRI Range, IFOV & Field of View
Compare two field-of-view methods, estimate Johnson DRI range and check IFOV and scene coverage for your detector and lens.
Camera configuration
Quick configurationTarget dimensions
Dimension used for DRIDetection
910 mTarget presence can be detectedRecognition
228 mTarget class can be recognizedIdentification
114 mTarget details can be identifiedCompare FOV calculation methods
Compare theoretical full angles from the same detector and nominal focal length.
| Calculation method | Horizontal FOV | Vertical FOV | Diagonal FOV |
|---|---|---|---|
| Rectilinear geometry2 × atan(d / 2f) × 180 / π | — | — | — |
| Small-angle approximation(d / f) × 180 / π | — | — | — |
| DifferenceLinear minus rectilinear | —— | —— | —— |
The linear estimate is larger and approaches the rectilinear result at small angles. Neither result is a measured lens specification. Here, d is the active sensor width, height or diagonal in mm; f is focal length in mm. Both formulas are converted to degrees.
Wide field: the small-angle approximation differs substantially. Use rectilinear geometry for initial scene planning and confirm the actual lens specification.
Why do the two methods differ? Read the 640 × 512 / 15 mm exampleIFOV · near axis
—mrad / pixel · pitch / focal lengthProjected pixel size (GSD) · near axis
—mm / pixel at the working distanceScene width
84.40mScene height
67.52mScene diagonal
108.08mScene coverage uses rectilinear FOV: 2D × tan(FOV / 2). The linear-angle approximation above is for comparison.
Reference result. Engineering estimate — validate the final lens, detector, atmosphere and image-processing chain in real conditions.
Reference guide
Thermal camera formulas explained
Use this calculator to compare modules and lenses consistently. Results are geometric estimates, not guaranteed operational performance.
Johnson DRI range
R = (Target × Focal) ÷ (Pitch × Pixels)Range is estimated from focal length, target dimension, pixel pitch and the required pixels across the target. This tool uses 1.5 pixels for detection, 6 for recognition and 12 for identification.
IFOV and field of view
Rectilinear: 2 × atan(d / 2f) × 180 / πSmall-angle: (d / f) × 180 / πIFOV is pixel pitch divided by focal length. Compare rectilinear FOV = 2 × atan(d / 2f) with the small-angle estimate FOV ≈ (d / f) × 180 / π. The same sensor and focal length can give different results; neither is a measured lens specification.
Scene coverage
Width = 2D × tan(HFOV ÷ 2)Scene width and height use the rectilinear FOV at the stated distance. The linear-angle approximation is displayed for comparison and is not substituted into scene coverage.
Selection workflow
How to use the result for module selection
- 1Enter the detector resolution, pixel pitch and lens focal length from the module specification.
- 2Use realistic target dimensions and compare DRI thresholds rather than relying on detection range alone.
- 3Check scene coverage and GSD at your actual working distance, then validate with a sample.
Continue your thermal imaging evaluation
FAQ
Thermal imaging calculator FAQ
What is the difference between detection, recognition and identification?
Detection indicates that a target is present. Recognition estimates whether its general class can be distinguished. Identification requires more pixels and aims to reveal defining details.
Are Johnson DRI distances guaranteed?
No. They are geometric planning estimates. Atmosphere, target contrast, optics, detector sensitivity, stabilization, display scale and image processing can materially change usable range.
What does IFOV mean for a thermal camera?
Instantaneous field of view is the angle represented by one detector pixel. A smaller IFOV normally means finer spatial sampling at the same distance.
Should I choose a longer focal length for more range?
A longer focal length can improve target sampling and DRI range, but it narrows field of view. The correct lens balances range, coverage, payload size and tracking requirements.
Why do the two FOV methods give different results?
The small-angle method replaces the arctangent with a linear approximation. For 640 × 512, 12 μm and 15 mm, rectilinear geometry gives 28.72° × 23.15°, while the approximation gives 29.34° × 23.47°. The approximation becomes closer for narrower fields. Published lens specifications may differ from both models.
Compare lenses with the thermal imaging calculator
Use this thermal imaging calculator to compare scene coverage and target detail before choosing a module. Keep the detector and target settings unchanged when comparing focal lengths so the lens effect is clear.
For a 640 × 512 detector with 12 μm pixels and a 15 mm lens, the rectilinear model gives 28.72° × 23.15°, while the small-angle approximation gives 29.34° × 23.47°. The differences are approximately 0.617° horizontally and 0.320° vertically. Use the 15 mm preset and open the IFOV & FOV tab to compare both results.
At 100 m, rectilinear scene coverage is 51.20 × 40.96 m, with a near-axis projected pixel size of 80.00 mm. Scene coverage uses the rectilinear model; the linear-angle result is a comparison estimate.
To reproduce the distance-based results, enter 100 m in both the IFOV & FOV working-distance field and the separate Scene coverage distance field. These two settings are independent.
Target dimension also changes the DRI estimate. For the 15 mm example, a 1.8 m target height gives about 1.50 km detection range; a 0.5 m width gives about 417 m. Choose the visible target dimension and keep the threshold consistent.
These geometric estimates require validation with the actual optics, target contrast and environment. A product’s specified field of view may differ. Projected pixel size does not establish temperature-measurement accuracy or AI tracking performance.
Read the worked comparison of the two FOV formulas.
Optical background: Edmund Optics explains focal length and field of view.
Need help matching a module and lens?
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