Buying a used thermal imager: what to check before you pay

Thermal imagers hold their value better than almost anything else in the test equipment market, which means used prices stay high and the cost of a bad purchase stays high with them. The specification sheet tells you very little about whether a specific used camera is a good buy.

Resolution is not the whole story

Detector resolution gets quoted first because it is a single number, but thermal sensitivity (NETD) often matters more in practice. A 320x240 camera with sub-50 mK sensitivity will show a developing electrical fault more clearly than a 640x480 camera at 100 mK, because the fault signature is a small temperature difference rather than a small object.

Where resolution does matter is standoff distance. If you are inspecting switchgear from behind a barrier, or roofs from ground level, pixels on target is the limiting factor and higher resolution earns its price.

Check the detector for dead pixels

Point the camera at a uniform surface — a wall, a sheet of card — and look for pixels that stay consistently brighter or darker than their neighbours as you pan. A handful of dead pixels is normal on an older camera and does not affect measurement. A cluster does, particularly in the centre of frame.

Run the internal calibration (the shutter click) and check whether the artefacts persist. Non-uniformity correction should clear most of them. Anything that survives NUC is permanent.

The focus mechanism is the usual failure point

On manual-focus cameras, work the focus ring through its full travel. It should be smooth and evenly weighted. Grinding or a dead spot means grit in the mechanism, and the repair is often uneconomic.

On LaserSharp and MultiSharp models, verify the autofocus actually converges. A camera that hunts and settles on a soft image has a failed rangefinder, and that is a factory repair.

Ask specifically whether the camera has been dropped. Detector alignment is not user-serviceable, and a knock hard enough to shift it does not always leave a visible mark.

Calibration drift and what it costs

Microbolometer detectors drift with age. A camera that has not been calibrated in five years may still produce a good-looking image while reading several degrees off, which matters if you issue reports against threshold criteria.

Factory calibration is available for most Fluke Ti and TiX models and typically costs a meaningful fraction of a used camera's price. Establish before you buy whether the seller is including it, and if not, price it in.

For qualitative work — finding the hot connection rather than reporting its exact temperature — drift matters much less. Be honest about which kind of work you actually do.

Lenses, batteries and what should come in the box

Interchangeable smart lenses on the Ti450 and above are expensive and easy to lose track of. Confirm exactly which lens is fitted and whether any others are included, by part number.

Batteries are consumable. Expect a used camera to need at least one new pack, and check that the charger base is present — they are frequently separated from the camera during liquidation.

The SD card, the USB cable and the hard case add up. A camera at a low headline price with none of these is not the bargain it appears to be.

Common questions