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What Is a Radiation Pyrometer?

A radiation pyrometer, also known as an optical pyrometer, is an instrument used to measure the temperature of an object without needing to make physical contact. It does so by detecting the infrared radiation emitted by the object, which is directly related to its temperature. The device then converts this radiation into an electrical signal, which can be displayed as a temperature reading. This technology is particularly useful in industrial settings where objects may be too hot, hazardous, or inaccessible for traditional thermometers, such as in steel forging or glass making. According to the Stefan-Boltzmann law, the total energy radiated per unit surface area of a black body is proportional to the fourth power of the black body's temperature, which is the principle that underpins the operation of radiation pyrometers.


Modern radiation pyrometers are highly sophisticated and can measure temperatures ranging from about -50°C to over 3,500°C with remarkable accuracy. They are essential tools in industries that require precise temperature control and monitoring. For instance, in the semiconductor industry, where the production of integrated circuits requires tightly controlled temperatures, pyrometers play a critical role. The accuracy of these devices can vary, but some advanced models boast an error margin as low as ±0.3% of the reading. This precision ensures that processes dependent on specific temperature thresholds can be monitored and maintained effectively, safeguarding product quality and process efficiency.

Alex Newth
Alex Newth

A radiation pyrometer is a device that uses optical technology to estimate the thermal radiation of a surface, without the user ever having to directly interact with that surface. This is done by making a machine that is sensitive to the color red — the color of objects above incandescence — and measuring how much thermal radiation is pouring out from that surface. Most radiation pyrometer units are only sensitive to red, so objects below incandescence cannot be measured. These machines are durable, but the optical sensors must be cleaned regularly to ensure they can see the surface and deliver an accurate radiation estimate. The most common use of pyrometers is in metallurgy and the smelting industry, because these industries constantly use extreme heat.

When an object or surface heats up dramatically, it becomes incandescent, or hot enough that it begins glowing. While an incandescent surface is obviously very hot, some industries need to know just hot it is without having to risk injury by directly interacting with the surface. A radiation pyrometer is used for this purpose, because it can sense the level of thermal radiation from a distance.

Scientist with beakers
Scientist with beakers

Radiation pyrometers work by sensing thermal radiation from the red energy spectrum. The unit is made to see the incandescent surface via optical pieces, and then accurately estimates the temperature based on the color and light. The problem with this is that, if the surface is hot but not incandescent or if there are cooler spots because of slag or discolored substances, the radiation pyrometer may deliver faulty results. For this reason, some pyrometer manufacturers make the optical device able to sense thermal radiation below incandescence.

The radiation pyrometer device is generally durable, but there is a part that needs to be constantly maintained: the optical piece. If someone accidentally touches this piece or if dust gets on it, the pyrometer may sense colors incorrectly, leading to inaccurate results. The optical surface should be cleaned regularly to ensure all thermal radiation measurements are accurate.

Metallurgists, smelters and companies working with metal are the leading users of radiation pyrometers, because they work with incandescent material all the time. Workers need to constantly watch and maintain high temperatures to ensure the metals are alloying, melting or being worked with correctly. Hot air balloonists would use a radiation pyrometer to measure the air and balloon fabric temperature. They also are used by people who work with steam boilers.

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    • Scientist with beakers
      Scientist with beakers