Optical Pyrometer - Construction, Principle, Working, Advantages & Disadvantages

An optical pyrometer is a device used for the measurement of temperature. It is a non-contact type temperature measuring device i.e., it measures temperature without making any contact with the body whose temperature is being measured. Hence, it can measure higher temperatures, where there is a chance of damaging the sensors that measure high temperature by making physical contact.


It measures the temperature of hot bodies by measuring the heat radiations emitted by that body. The optical pyrometer is also referred to as the Disappearing Filament Type of Pyrometer. Let us see its construction and principle of operation.


Principle of Optical Pyrometer :

The measurement of temperature using optical pyrometers is based on the principle of brightness comparison. The brightness produced by the hot object (whose temperature is being measured) is compared with the brightness of the reference temperature lamp.


The brightness of the reference temperature lamp is adjusted until it becomes equal to the brightness of the hot body. The value of current passing through the filament of the lamp when both the brightnesses becomes equal gives the temperature of the hot body under measurement.


Construction of Optical Pyrometer :

The construction of optical pyrometer or disappearing filament type of pyrometer is quite simple. It consists of a reference temperature lamp, absorption screen, objective lens, red filter, and eyepiece. The below figure shows the systematic arrangement of the optical pyrometer.

Optical Pyrometer

It is a cylindrical inside with an objective lens and absorption screen on one side and a red filter and eyepiece on the other side. The reference temperature lamp is placed between the absorption screen and red filter which is connected to a battery source through a rheostat and PMMC meter.


The intensity of the lamp can be controlled by varying rheostat i.e., by varying the current through the filament of the lamp. The red filter allows only a narrow band of wavelength to the observer observing from the eyepiece. The absorption screen increases the temperature measuring range of the instrument.

Working of Optical Pyrometer :

In order to measure the temperature of a body, the radiation emitted from that body is made to fall on the reference temperature lamp filament. Now by observing the lamp's filament through the eyepiece, the brightness of the lamp is varied until the brightness of both the reference temperature lamp and radiation emitted from the hot body becomes same. When the brightness of both the filament of the lamp and the hot body becomes equal the filament gets disappeared.


Since the radiations emitted from the hot body i.e., the brightness of the hot body depends upon its temperature, and the brightness of the lamp depends upon current passing through the filament. The temperature can be determined by calibrating the optical pyrometer in terms of current flowing through the filament of the lamp. Therefore, the reading indicated by the PMMC meter that measures the flow of current through the lamp's filament gives the temperature of the hot body.

Optical Pyrometer

Figure (iii) above shows the disappearance of the filament of the lamp when the brightness of the hot body and reference temperature lamp becomes equal. When the brightness of the lamp is low than the hot body's brightness, the filament of the lamp appears dark as shown in figure (i) above. Similarly, when the brightness of the hot body is low than the brightness of the reference temperature lamp, the filament of the lamp appears bright as shown in figure (ii).


Advantages of Optical Pyrometer :

  • The accuracy of the optical pyrometers is quite high.
  • Capable of measuring high temperatures.
  • It measures the temperature without making any contact with the body whose temperature is being measured.
  • It is a portable instrument with lightweight.
  • The operation is simple.

Disadvantages of Optical Pyrometer :

  • Cannot be used in the continuous measurement of temperature.
  • Exhibits less sensitivity at low temperatures.
  • The cost of optical pyrometers is high.
  • The accuracy can be affected by incorrect adjustment of rheostat and other thermal background radiations.

Applications of Optical Pyrometer :

  • It can be applied to measure the temperature of furnaces.
  • It can be applied in the measurement of the temperature of heated materials and also molten metals.


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