Conceptually, these types of the devices are fairly easy to understand, everyone is familiar with the term red hot. In the radiation thermometry arena, all we do is apply numerical values correseponding to temperature readings based upon the radiation which is emitted. With a disappearing filament optical pyrometer, one visually looks at the color of a hot surface, and compares that to a thin heated wire, where in the temperature of the heated wire is a known value. When the heated wire is at the same temperature as the surface one is looking at, it effectively blends into the background.
In fact, to those skilled in the field, one can even come pretty close (a few hundred degrees F) to estimating the temperature of a known material just by viewing it with the naked eye. I blew away the Minardi F1 guys, by telling them their brake rotor temperatures via observation, back when I worked with them years ago.
Granted, instrument design, calibration, and accurate temperature measurements are a lot more complex than this simple explanation, but as an initial post on the subject, I thought it best to start off with a very simplified layman’s approach.
For a more detailed explanation, the guys over at Spectrodyne have a nifty graphic. Way back when, I did a lot of work with them, they are one of the ultimate calibration houses out there, albeit their focus is limited to a few specific manufacturers. They also repair and calibrate Radiamatic sensors, which played a large part of my life for a number of years.
Granted, these types of instruments have pretty much all disappeared save the retrofit market, and the Spectrodyne model. In part, being they require an operator to visually make a call, wide variances can exist from operator to operator, but also different materials require different spectral regions for measurment, and being the human eye is limited to the visual spectrum, significant error can be introduce. These factors combined with a need for tighter and tigheter measurement accuracy really limit their application. Yet, for ease of explantions, the basic operation is something most everyone can relate to.