Thermal Camera Lens and IFOV Selection Guide
The lens is the part of a thermal camera most often ignored at purchase time — and the part that most often decides whether you can measure a small target. This guide explains IFOV, field of view and focal length so you can choose the right lens for the distances you actually work at.
IFOV, FOV and Focal Length: The Basics
- FOV (field of view) — the total angle the camera sees, usually given as a diagonal or horizontal angle such as 25°.
- IFOV (instantaneous field of view) — the angular size seen by one detector pixel, usually in mrad (milliradians). This is the spatial resolution of the camera.
- Focal length — a longer focal length gives a narrower FOV and a smaller IFOV, which means more detail at distance, like a telephoto lens on a normal camera.
IFOV is the number that connects resolution to distance. It is often listed on datasheets as "IFOV" or "spatial resolution".
Why IFOV Sets the Smallest Target You Can Measure
One pixel sees the average temperature of everything within its instantaneous field of view. If a hot spot is smaller than the pixel footprint at your working distance, the reading is diluted by the cooler surroundings. For a defensible measurement, aim for the target to cover at least 3×3 pixels, and ideally more.
How to Calculate Required IFOV for Your Work
The linear size one pixel covers at distance D is approximately: IFOV (mrad) × D (m) = pixel footprint (mm).
Example: a 25 mm target lug at 5 m, with a camera IFOV of 1.3 mrad, gives a pixel footprint of 6.5 mm. The lug covers roughly 3.8 pixels — borderline but workable. The same lug at 10 m covers under 2 pixels with the same lens, which is not enough for an accurate reading.
For that 10 m case you would need a telephoto lens (smaller IFOV) or a higher-resolution detector to keep the target above 3 pixels.
Standard vs Telephoto vs Wide-Angle Lenses
| Lens type | Typical FOV | Best for |
|---|---|---|
| Wide-angle | 45° or wider | Large areas close-up: panels, building surveys |
| Standard | 24°-35° | General electrical inspection, switchgear at 1-5 m |
| Telephoto | 12°-20° | Substation busbars, overhead lines, distant targets |
| Very narrow | <10° | High-voltage transmission, very small targets at distance |
Interchangeable-Lens Platforms
A fixed-lens camera is fine if all your work is at one typical distance. Teams that inspect both indoor panels and outdoor substations should consider an interchangeable-lens platform: carry a standard lens for daily rounds and a telephoto lens for distance work, and change lenses on site as needed.
Unitech Tools Models with Interchangeable Optics
| Model | Detector | Optics |
|---|---|---|
| E Series (E600) | 640×512, 12 µm | Interchangeable-lens platform |
| SC-B Series | 640×512 | Autofocus, laser ranging options |
| PT II Series | up to 1280×1024 | HD, TwinView DFOV, interchangeable optics |
| G Series | up to 1024×768 | Rotating lens, interchangeable optics |
Lens Selection Checklist
- Smallest target size you must measure (mm).
- Maximum safe working distance (m).
- Required IFOV = target size / (distance × 3) for a 3-pixel reading.
- Choose a camera + lens combination whose IFOV meets or beats that number.
- Confirm interchangeable lens availability and autofocus if you scan at varying distances.
Choosing between standard and telephoto?
Tell us your smallest target and furthest working distance. We will calculate the IFOV you need and recommend the lens configuration.