Emissivity

Emissivity is the property that turns a thermal camera from a radiance sensor into a thermometer. The instrument measures radiation; converting that reading into a temperature requires knowing how efficiently the surface radiates compared with an ideal emitter. Get the value wrong and every absolute temperature in the record is wrong with it, in a consistent and entirely plausible-looking way.

For welding this is harder than for most applications, because the surface does not hold still. Bright rolled steel, mill scale, an oxidised heat-affected zone, spatter and a fresh weld bead have visibly different emissivities, and a single seam can present several of them within a few centimetres. Temperature changes it too, and so does viewing angle.

That is why serious thermal weld monitoring is built to be robust to it rather than to assume it away. Two approaches do the work. Relative features — gradients, cooling slopes, the shape and symmetry of the thermal signature, run-to-run repeatability — survive an uncertain emissivity because the error largely cancels. Calibrated absolute measurement is used where a real number is required, as when an interpass maximum has to be evidenced, and it depends on establishing the value for that surface and recording that this was done.

The practical rule follows from the physics: absolute temperature claims need a documented emissivity and a documented surface state, while anomaly detection and process-drift signals do not. A supplier quoting absolute weld temperatures without saying which value was used, and on what surface, is quoting a number with an unstated assumption inside it.