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Everything You Need To Know Optical Pyrometer

Due to a unique optical process, the pyrometer has very safe features for testing temperatures without contact. Its infrared properties make the pyrometer an ideal tool to utilize when a regular thermometer cannot measure extremely hot or extremely cold temperatures safely. A good pyrometer can be highly accurate and provide data that is consistently correct every time. A pyrometer can be used for multiple applications and numerous thermometers, so it is crucial to take advantage of this feature. A pyrometer can also measure humidity and barometric pressure.

Optical pyrometer

The principle of temperature measurement by the brightness comparison is used in an optical pyrometer. A colour variation with the temperature growth is taken as an index of temperature. This optical pyrometer compares the brightness of the image produced by the temperature source with the reference lamp temperature. The current in a lamp is adjusted until the brightness of the lamp equals the brightness of the image produced by that temperature source. Since the intensity of lamplight of any wavelength depends on the temperature of radiating object, a current passing the lamp becomes measurable for the temperature source when calibrated.

Operation of optical pyrometer

When a temperature source is measured, the radiation from the source is focused on a filament of the reference lamp temperature using an objective lens. Now the eyepiece is adjusted so, reference lamp temperature filament is in sharp focus, and the filament is seen superimposed on the image of the temperature source. Now the observer starts controlling lamp current, and a filament will appear dark. If the filament is lesser than the temperature source, the filament will appear bright. If the filament’s temperature is more than the temperature source, then the filament will not be seen if the filament and temperature source are at the same temperature. Hence the observer needs to adjust the lamp current until both the filament and source temperature have the same brightness.

Benefits of Optical pyrometer

  • Physical contact with the object is not required to measure the temperature of the temperature source.
  • Accuracy is high + or – 5’C.
  • Provided an adequately sized image of the source temperature is obtained in the device, the distance between the instrument and the source does not matter.
  • Optical pyrometers are used to measure the temperature of molten metals or heated materials.
  • The instrument is easy to operate.


If the high temperature of the body has to be measured, contact‐type temperature measurement devices will not be suitable because they may get damaged When they come in contact with the hot body. An optical pyrometer is typically used to measure the furnace temperature and hot bodies. Hence, the use of non‐contact type temperature measurement devices becomes imperative. Such devices are employed for high-temperature measurement. The distance between the source temperature and the instrument does not affect the measurement. These non‐contact type temperature devices are called pyrometers. Measurements of temperature are carried out either by measuring energy. Radiated by a hot body or by colour comparison.

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