Radiation Pyrometer

What is Radiation Pyrometer or Infrared Pyrometer:

In most cases, a radiation pyrometer type thermometer is utilised to measure any temperature more than 750 deg C. This form of temperature sensor or thermometer does not need to be in contact with the object whose temperature is being monitored, which is the main explanation. Conventional thermometers are not at all ideal for touching hot objects at very high temperatures since there is always a danger that the thermometer will sustain significant damage.

Additionally, sometimes a hot object is situated in a system with such a small passageway that it is hard to get a thermometer close to it. The radiation pyrometer is successfully utilised to measure the temperature in this instance as well. The primary idea of these devices is that they detect heat radiation from a target hot body and, depending on the strength of the radiation, read and record the temperature of the body.

Operating Range of Radiation Pyrometer | Temperature range of Radiation Pyrometer:

Radiation or Infrared Pyrometer can be used for measure the temperature above 750 Deg C. There is no high limit for radiation Pyrometer.

Working Principle of Radiation Pyrometer:

According to the idea that “Temperature of a body can be detected by measuring radiant energy radiated by that hot body,” radiation pyrometers operate. The total thermal radiation that a body emits can be computed using Stefan Boltzmann’s law using the formula Q = σ • T4 (in Wm2)

Where, T = Absolute Temperature in Kelvin

and σ = Stefan Boltzmann’s constant in Wm-2K-4

Construction of Radiation Pyrometer:

Three main parts make up a radiation pyrometer:


This device is used to direct the body’s radiated radiation, which is used to determine the temperature of the body, onto a receiving element.

Receiving element:

Thermocouple or a resistance thermometer can be employed as the receiving element.

Recording device:

To get a reading, a temperature indicator, recorder, or controller is connected to the receiving end.

Radiation Pyrometer

How a Radiation Pyrometer Works:

The pyrometer collects and focuses the total energy radiated by a heated body through a lens onto the receiving element (thermocouple). An emf develops when heated radiation strikes a thermocouple. A millivoltmeter, which is calibrated to immediately display the value of the observed temperature, is used to measure this emf. To prevent the reflection of radiation in undesirable directions, concave mirrors are utilised. As seen in Fig. 1, concave mirrors reflect them back to the receiving element.

Types of Radiation Pyrometer | Types of Inrared Pyrometer

Radiation pyrometers come in essentially two different types: fixed focus and variable focus.

Fixed Focus Type Radiation Pyrometer:

The major component of it is a long tube with a concave mirror attached at the end, as depicted. A sensitive thermocouple is positioned in front of the concave mirror at an appropriate distance so that heat radiation entering the tube through the tube’s narrow opening at the frond end is reflected by the concave mirror and concentrated on the thermocouple’s hot junction. No matter how far a hot object is from the instrument, the radiation is always directed at the thermocouple thanks to its permanent concave mirror. This is the basis for the name “fixed focus type radiation pyrometer” given to the instrument. The thermocouple’s emf is then measured using a millivoltmeter or galvanometer, which can be directly calibrated with temperature to provide quick temperature readings.

Variable focus Type Radiation Pyrometer:

In contrast to fixed focus radiator pyrometers, the instrument’s attached adjustment knob can be used to change the location of the concave mirror. The device is sometimes referred to as a variable focus radiation pyrometer because of its movable concave mirror. Steel that has been finely polished concave mirror. The concave mirror receives the heat rays from the hot object being targeted before reflecting them onto the blackened thermo junction, which consists of a tiny copper or silver disc on which the wires making the junction are soldered.

Through the eyepiece and the hole in the centre of the primary concave mirror, the visible image of the hot body is visible on the little metallic disc attached to the thermo-couple junction. To align the focus with the smaller metallic disc attached to the thermocouple junction, the main concave mirror’s position is modified. An electromotive force is created when the thermo junction is heated as a result of the thermal picture on the tiny metal disc. By using a millivoltmeter or a sesative galvanometer to monitor this thermally generated emf, the temperature of the object can be determined.

Difference Between Optical Pyrometer and Radiation Pyrometer:


Sr No A Comparison Point Optical Pyrometer Radiation Pyrometer/Infrared Pyrometer
1 Range of Temperature Measurements It can measure the temperature from 700°C -3000°C Radiation Pyrometer are useful to measure the temperature greater than 750 °C.
2 Sensitivity Good better
3 Calibration In contrast to the common tungsten strip light. In contrast to a common optical pyrometer.
4 Accuracy Have Less accuracy More Accurate
5 working The colour of the hot body in contrast to the heating filament. On the heated junction of the thermocouple, lenses concentrate radiant energy.
6 Stability Better Good
7 Output a colour contrasted with a heated filament whose temperature is known. Emf, a device that can measure a heated body’s temperature.
8 Cost Optical Pyrometer are Costly. Comparatively low cost.

Advantages and Disadvantages of Radiation Pyrometer:

Following are the Advantages of Radiation Pyromter-

Advantages of Radiation Pyrometer:

  • The thermocouple’s inability to measure high temperatures due to the possibility of melting, Radiation or infrared pyrometer are suitable to measure high temperature.
  • No direct contact with the object whose temperature is being measured;
  • Quick response time.
  • High productivity at reasonable cost.
  • Corrosive atmosphere has less of an impact on it.
  • A high accuracy of 2% of the complete scale.

Disadvantages of Infrared Pyrometer:

  • Nonlinear scalability
  • Mistakes that could occur as a result of intervening gases or vapours that absorb radiation.
  • Measuring temperature is affected by a hot body’s emissivity.
  • The instrument reads a lower temperature than the body’s real temperature because of debris or dust on the mirror or lens.

Applications of Radiation Pyrometer:


  • It is preferred for measuring temperature in hard-to-reach components or applications like furnaces, interiors, boilers, etc.
  • It measures the temperatures of objects or areas that are difficult to reach.
  • It is employed to measure the typical temperature of broad surfaces.
  • It is used to assess invisible radiation waves as well as the temperature of moving targets and objects.
  • Temperature measurement equipment may be contaminated when employed under corrosive ambient conditions.



1. Exactly what is a pyrometer?

A remote-sensing thermometer that measures surface temperature is referred to as a “pyrometer.”

2.What is the purpose of an optical pyrometer?

It is used to detect non-contact high temperatures in a variety of industrial applications.

3.What is the pyrometer’s work ?

The main function of a pyrometer is to determine an object’s temperature by detecting the heat or energy it emits without coming into contact.

4.What is the measurement range of the optical pyrometer?

The operating range of typical pyrometers is between 700°C and 3,000°C.

5.What is the purpose of a red filter?

By using a red filter, which is positioned between the eyepiece and the reference bulb, the wavelength band is narrowed.


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