How wide, how far: choosing a lens for a 640 × 512 thermal core
Two questions come up in almost every conversation about a thermal core: will it see the whole area, and how far away does it still work? The lens answers both more than the resolution does. Here is the arithmetic, with a 640 × 512, 12 μm core as the example. The same formulas work for any uncooled core if you change the pixel count and pitch.
Start from the scene, not the lens
Write down three numbers before you look at a lens list: the nearest and farthest working distance, the width you need to cover, and the size of the smallest thing that matters, such as a person, a hot bearing or a 10 cm junction box.
The width a lens covers at distance d is 2 × d × tan(HFOV ÷ 2). One pixel covers IFOV × d, where IFOV is the pixel pitch divided by the focal length.
Three lenses for the same core
| Lens | Field of view (H × V) | IFOV | Sharp range (fixed focus) | Width at 10 m (calculated) | One pixel at 10 m (calculated) |
|---|---|---|---|---|---|
| 9.1 mm F1.1 | 48.7° × 38.6° | 1.3 mrad | 1.2–5.5 m | About 9 m | About 1.3 cm |
| 13.5 mm F1.0 | 31.9° × 25.7° | 0.9 mrad | 3.8 m to ∞ | About 5.7 m | About 0.9 cm |
| 18 mm F1.1 | 24.2° × 19.5° | 0.7 mrad | 6.14 m to ∞ | About 4.3 m | About 0.7 cm |
Field of view, IFOV and sharp range are the values listed in the module manual; the 10 m figures are calculated geometry. Going from 9.1 mm to 18 mm doubles the detail on a target and halves the width you see. No lens gives you both. The 9.1 mm lens is standard; the 13.5 mm and 18 mm lenses are non-standard, with samples shipped within 15 working days of payment.
“How far” depends on the job
Seeing that something warm is there needs only a few pixels. A person about 0.5 m wide at 100 m covers about 5.6 pixels with the 13.5 mm lens and about 7.5 pixels with the 18 mm lens (calculated).
Measuring a temperature needs more. A common rule is that the target should fill at least 3 × 3 pixels, and more is better. That gives a maximum measuring distance of target size ÷ (3 × IFOV):
- 10 cm target: about 37 m (13.5 mm) or 50 m (18 mm)
- 30 cm target: about 110 m (13.5 mm) or 150 m (18 mm)
- 9.1 mm lens: measure within its sharp range of 1.2–5.5 m; at 5.5 m a target of about 2 cm fills 3 × 3 pixels
All of these are calculated from pixel pitch ÷ focal length. Datasheets also list detection, recognition and identification ranges for a person. Treat those as best-case figures, with clear air and good thermal contrast, and run your own pixels-on-target check with your real target.
Wide-angle lenses: ask “horizontal or diagonal?”
Robots, aisles and indoor monitoring can need views as wide as 80–100°. When a wide lens is quoted as “about 100°”, ask which angle that is.
For a 640 × 512, 12 μm sensor (7.68 × 6.14 mm) behind a 4 mm lens, the geometry gives about 88° horizontal, 75° vertical and 102° diagonal (calculated, rectilinear). If your requirement is 90° across a corridor, that difference decides whether the side walls are in the picture.
Two more points: real lenses with barrel distortion often cover a few percent more than the rectilinear figure, and the IFOV you calculate from the focal length is the value at the image centre.
On a radiometric module, the lens is part of the calibration
Temperature calibration is done with the lens fitted. If the lens changes, the module has to be recalibrated by its maker. You cannot buy a wider lens elsewhere, fit it and keep accurate temperatures, so decide the lens before you order samples for measurement work.
Check focus at your real distance
These lenses are fixed focus, set at the factory, and each is sharp over its own range: 1.2–5.5 m for the 9.1 mm lens, 3.8 m to infinity for the 13.5 mm and 6.14 m to infinity for the 18 mm (module manual). Choose the lens whose range covers your working distance, and check sharpness at that distance when the sample arrives.
Tell us what the camera has to see.
Send the working distance, the area to cover and the smallest target, and we will suggest a lens. We reply within 24 hours on working days.