How They Work
And Who (Still*) Makes Them
The Optical Pyrometer is a highly-developed and well accepted noncontact temperature measurement device with a long and varied past from its origins more than 100 years ago.
In spite of the fact that more modern, automatic devices have nearly displaced it, several makers still produce and sell profitable quantities each year.
In general, opticals, as they are often called, can be described as fitting into two seperate types, according to the two USA companies that produce them.
However, there are actually several different types that vary in complexity and cost. A quick review of the descriptions below will provide some of the differences and a check of the web sites of the two companies will yield even more information.
We suspect that there are other makers overseas and we are looking to find more details about them and their web presence.
How Optical Pyrometers Work
Optical Pyrometers work on the basic principle of using the human eye to match the brightness of the hot object to the brightness of a calibrated lamp filament inside the instrument.
The optical system contains filters that restrict the wavelength-sensitivity of the devices to a narrow wavelength band around 0.65 to 0.66 microns (the red region of the visible spectrum).
Other filters reduce the intensity so that one instrument can have a relatively wide temperature range capability. Needless to say, by restricting the wavelength response of the device to the red region of the visible, it can only be used to measure objects that are hot enough to be incandescent, or glowing.
This limits the lower end of the temperature measurement range of these devices to about 700 °C.Some experimental devices have been built using light amplifiers to extend the range downwards, but the devices become quite cumbersome, fragile and expensive.
Modern radiation thermometers provide the capability to measure within and below the range of the optical pyrometer with equal or better measurement precision plus faster time response, precise emissivity correction capability, better calibration stability, enhanced ruggedness and relatively modest cost.
This Illustration graphically how it looks to a user via a html pagesin Microsoft Power Point presentation format, with basic artwork courtesy of Spectrodyne, Inc.
The entire slide show also can be downloaded in a self-extracting compressed format and then viewed using a free Power Point viewer available from Microsoft.
To extract the slides from the compressed file, copy the file to its own subdirectory and then run it. It will extract the files into PowerPoint format, i.e. having file extension names of *.ppt.
Then run the viewer and open the show “OPYRO”, in the appropriate subdirectory or the individual sldes by their file names.
If you have some comments or questions, let us know.