Pyrometer

A pyrometer is the simplest instrument in the infrared-measurement family: it aims at a single spot, measures the infrared radiance arriving from within its field of view, and converts that into one temperature reading. It shares its basic physics with an infrared camera — both convert radiance into an apparent temperature using an assumed surface emissivity — but a pyrometer has no spatial dimension at all: it cannot show where the hot or cold region of a joint is, only that the one point it is aimed at reads a given value.

That single limitation shapes where a pyrometer is, and is not, the right tool. A welder checking interpass temperature with a handheld pyrometer between runs gets a fast, cheap, genuinely useful spot reading, at the cost of two known gaps: the point reached may not be the hottest region of the joint, and a single check between runs can miss a moment when the workpiece was briefly outside the required range, particularly on a fast multi-pass sequence. Neither gap means the pyrometer reading is wrong — it means it answers a narrower question than the one a full inspection needs answered.

A thermal camera closes both gaps by measuring the whole surface continuously rather than one point at one instant, which is why a fixed monitoring system is built around a camera and a pyrometer is more often the tool a person carries for a quick manual check, sometimes alongside a camera-based system rather than instead of one. Both instrument types depend on the same physics and the same emissivity assumption, but a spot pyrometer commonly specifies tighter absolute accuracy than an uncooled thermal-imaging array — the trade a fixed camera system makes is coverage for some of that point accuracy, not a free upgrade in both at once. The thermography and NDT testing page covers where a spot instrument and a full-field one are each the right call.