What is an Optical Pyrometer : Construciton, Working & Its Applications

A pyrometer is a noncontact device and it is also known as a radiation thermometer. The main function of this instrument is to detect the surface temperature of an object by measuring the temperature of the electromagnetic radiation generated from the object. So, thermal radiation can be measured by using this non-conductive device. By using this, we can determine the temperature of the surface of the object. There are different types of pyrometers available in the market like infrared and optical pyrometers. So this article discusses an overview of what is an optical pyrometer, construction, and working.

What is an Optical Pyrometer?

Definition: A temperature measuring device that is used to measure the temperature of molten metal’s, overheated material, furnaces otherwise liquids. It is one kind of measuring device of temperature with a non-contact. The working principle of this optical pyrometer is to match the object’s brightness of the filament within the device. By using contact type instruments, measuring the temperature of the highly heated body is not possible. So this non-contact type device is used to measure the temperature. The optical pyrometer diagram is shown below.


Optical Pyrometer Construction

The shape of the pyrometer is cylindrical and inside parts of the optical pyrometer mainly include eyepiece, power source, absorption screen, and red filter.

  • An eyepiece and the lens of an object are arranged at both sides of the device.
  • A battery, millivoltmeter & rheostat are connected to a temperature bulb.
  • An absorption screen is arranged in the middle of the reference temperature lamp and the objective lens to increase the temperature range and his range can be measured with the help of the device.
  • The red filter is placed in between the lamp and eyepiece so that the lamp allows simply a narrow band with 0.65mui wavelength.

Operating Principle

In an optical pyrometer, the temperature measurement principle is used by comparing the brightness. A color disparity with the increase in temperature can be taken as an index of the temperature. This type of pyrometer contrasts the intensity of the generated image through a source of the temperature of the lamp.

The current within the lamp is regulated until the lamp’s brightness is equivalent to the image brightness generated through the source of temperature. When the light intensity of any wavelength depends on the temperature of the radiating object, then the flow of current through the lamp becomes a measure of the temperature source when adjusted.

Optical Pyrometer Working

The optical pyrometer diagram is shown below. It includes the lens to focus on the generated energy from the heated object and aims at the filament of the lamp. The filament in the lamp mainly depends on the flow of current through it. Therefore the changeable current can be supplied throughout the lamp.


The magnitude of the flow of current can be changed until the filament’s intensity is similar to the intensity of the object. As the intensity of both the filament and object are the same, then the filament outline can vanish completely.

The filament in the bulb seems intense as its temperature is higher compare with the temperature of the source. Similarly, the filament seems dim if their temperature is lower than that necessary for equivalent brightness

Optical Pyrometer Advantages

The advantages of this pyrometer include the following.

  • It is used for high temperatures.
  • It is used to check the distant objects as well as moving the object’s temperature.
  • Accuracy
  • It can be measured without connecting with the target.
  • Less weight
  • It is flexible and portable.

Optical Pyrometer Disadvantages

The disadvantages of this pyrometer include the following.

  • Due to the radiation of thermal background, dust, and smoke, the accuracy of this device can be affected.
  • These do not apply to the temperature measuring of burning gases because they do not emit visible energy.
  • It is expensive.
  • Manual type pyrometers are not suitable for evaluating the object’s temperature under 8000C because, at less temperature, the generated energy will be too low.


The applications of optical pyrometer include the following.

  • It is used to measure the temperature of highly heated materials
  • It is useful to measure furnace temperatures.
  • It is used in critical process measurements of semiconductor, medical, induction heat treating, crystal growth, furnace control, glass manufacture, medical, etc.


1). What is a pyrometer?

A remote-sensing thermometer that is used to gauge the temperature of a surface is known as a pyrometer.

2). What is the function of an optical pyrometer?

It is used to gauge non-contact high-temperature measurements in several industrial applications.

3). What is the working principle of pyrometer?

The working principle of the pyrometer is to measure the temperature of an object through detecting the energy or heat generated from the target object without contact.

4). What is the optical pyrometer range?

The operating range of a typical pyrometer ranges from 700°C – 4,000°C

5). What is the function of a red filter?

A red filter is used between the eyepiece as well as the reference bulb to help in narrowing the wavelengths band.

Thus, this is all about an overview of the optical pyrometer. From the above article, finally, we can conclude that this is one kind of noncontact type measuring device used for temperature. The working principle of this device is to match the intensity of an object to the intensity of the filament that is used in the pyrometer. Here is a question for you, what are the types of pyrometer?