Emissivity, and why your pyrometer shows the wrong temperature
An infrared thermometer measures radiation. Turning it into temperature takes one number that you have to supply.
A pyrometer that reads 60 degrees too low usually isn’t faulty. It is almost always being asked to convert radiation into temperature using an emissivity value that doesn’t match the surface it is pointed at.
What emissivity is
Emissivity is how efficiently a surface radiates compared with a perfect black body. Matt oxidised steel sits around 0.95. Polished aluminium can drop below 0.1. The same instrument reading both at the same true temperature will report two very different numbers — unless it is told which is which.
How to set it in practice
The reliable method is comparison: bring the target to a known temperature measured with a contact probe, then adjust the emissivity until the pyrometer agrees. For a process running a single material, this is a one-off exercise.
About one in five instruments sent to us as faulty works perfectly and is simply set to the wrong emissivity.
