Learning Library · Thermal imaging · 2
Thermal resolution and image size
A thermal camera's detector is a grid of pixels, and each pixel reports the average temperature of the patch of scene it sees. Point it at a target smaller than that patch and the pixel blends the hot spot with the cooler surroundings. The reading comes out low, and nothing on the screen warns you.
A 90 °C hot spot on a 35 °C panel, seen at the chosen distance. Left: the whole image. Right: the 15 × 15 pixels around the hot spot, with the true target size outlined.
IFOV: the angle one pixel sees
The instantaneous field of view, IFOV, is usually quoted in milliradians (mrad). Multiply it by distance and you get the size of the patch one pixel covers: 1 mrad is 1 mm at 1 m, or 10 mm at 10 m.
Resolution and lens work together. A narrower lens puts the same pixels on a smaller area and lowers the IFOV, just as more pixels do. That is why a camera's IFOV, often called spatial resolution on datasheets, is a better guide to detail than the pixel count alone. For example, the UNI-T UTi384G's 384 pixels across a 42° field work out to 1.91 mrad, the figure on its datasheet.
The three-pixel rule
To measure a temperature, not just see a warm spot, the target should cover at least three pixels across, and ideally three by three. That gives at least one pixel that sees only the target, with no blending at its edges. Some manufacturers quote a measurement spot ratio for the same purpose.
Try this
- Start with the 320 × 240 detector, a 20 mm hot spot, and 1.5 m. The spot covers several pixels and the hottest one reads close to 90 °C.
- Back away to 10 m. The spot shrinks to a fraction of a pixel and the reading drops toward the panel's 35 °C.
- At that same distance, switch to 640 × 480 with its narrower 24° lens. The IFOV falls by more than a factor of four, the spot covers about three pixels again, and the reading is back at 90 °C.
Super-resolution modes on some cameras combine several slightly shifted frames to double the displayed pixel count. They improve detail in the image. For a temperature reading, use the native detector figures and the three-pixel rule.
UNI-T examples
| Model | Detector | Field of view | IFOV (calculated) |
|---|---|---|---|
| UTi256G | 256 × 192 | 28° × 20° | 1.91 mrad |
| UTi730E | 320 × 240 | 56° × 42° | 3.05 mrad |
| UTi384G | 384 × 288 | 42° × 30° | 1.91 mrad |
| UTi740X | 400 × 300 | 27.7° × 20.6° | 1.21 mrad |
| UTi760H | 640 × 480 | 24° × 19.3° | 0.65 mrad |
Detector and field of view from UNI-T product data. IFOV is the field of view divided by the horizontal pixel count; datasheet values can differ slightly.
Put enough pixels on the target. Thermal imagers from 256 × 192 to 640 × 512.
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