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Optical Pyrometers

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 compexity 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 micons (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.

Good luck and best wishes.

If you have some comments or questions, let us know.

Suppliers of Optical Pyrometers

  1. Pyrometer Instrument Company, Inc. (Pyro)., Northvale, New Jersey, USA.
    Pyro has made several types of optical pyrometers, as well as other contact and non contact temperature sensors. The most popular optical device was the Pyro-Optical which was a version of the original German design. The principle of operation was that the lamp filament was operated at a constant current and the brightness variation was caused by a variable density filter that changed the apparent filament brightness in the operator's view when compared to the unchanging brightness of the object being measured. Pyro also made a research device called the Micro-Therm Optical Pyrometer in which the filament current was changed and the value read on an expanded scale meter or digital meter.
  2. Spectrodyne, Colmar, PA, USA .
    Spectrodyne, Inc. services, repairs and calibrates the famous Leeds and Northrup Optical Pyrometers and recently introduced to the market a new instrument that works on the same principle, that of adjusting the lamp filament current to change the lamp filament brightness to match the brightness of the hot object.

* Still Using Optical Pyrometers?

That's why there are organizations still making them.

If you are still using them for accuracy reasons, you may be a little behind the technology times. More than 20 years ago, the joint program by (then) Minolta Camera Company of Japan, (now Konica-Minolta) and (then) Land Pyrometers Ltd of the United Kingdom (now Land Instruments International Ltd) developed and sucessfully marketed extremely high quality, sensor-based short wavelength Infrared (IR) Thermometers that were the equal in performance to Optical Pyrometers in many parameters and superior in others. They were faster, had built-in emissivity correction and were far more capable in most uses than Optical Pyrometers. Their product line today is extensive with many models for special uses. Visit our RTVendors Page for an extensive list and links to sensor-based Infrared Radiation Thermometer vendors.

Other companies like Chino Instruments of Japan, Ircon, Inc and Raytek Corporation of the USA joined the club of sensor-based devices, but as far as we know, the Minolta-Land Cyclops led the way in virtually replacing the venerable Optical Pyrometer in industry and scientific locales.

NB: These devices are not to be confused with the general purpose, low cost handheld IR Thermometers that pervade the market today, led by Raytek designs. They are very different types of instruments with very different uses. Needless to say, there are portable IR Thermometers on the market that are used at the low temperature measurement capability of low-cost, popular devices. The Minolta-Land, Ircon-Chino, Raytek organizations offer advanced performance handheld for lower temperature uses but at significantly higher selling prices.

The one area where Optical Pyrometer has maintained an advantage over newer technologies is where one is looking at relatively hot, small incandescent objects in the field of view, such as tungsten wire during heat treating or annealing.

Other Resources

Actually other pages on the site, but some of the key related ones are the following:

Also note The Temperature Sensors Community Web Site, a companion site set up to enable direct inputs from temperature sensor users and makers. Vendors, do visit there. Sign in and enter your own company data, product and service offerings and news-It's been Improved! Users can post reports & reviews of companies, news, products and services. Note that inputs are moderated for propriety and excess zeal! Best of all, it is freely available and "self service".





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