Temperature is a physical quality that can be used to describe how hot or cold an object or substance is. It can be measured in a variety of scales and units depending on the situation. Any material’s temperature can be determined using a variety of tools and techniques. The temperature can be measured using a variety of techniques, including thermometers (liquid in glass), electric resistance thermometers, radiation thermometers/infrared thermometers/pyrometers, thermocouples, silicon diodes, bimetallic devices, bulb and capillary devices, constant volume gas, and pressure gas thermometers, depending on the physical properties of the material. Temperature can also be measured in the Celsius and Fahrenheit(F) scales in addition to the SI unit of Kelvin (k) . This article discusses the definition of a pyrometer as well as its types, benefits, and drawbacks.

What is Pyrometer:

Pyrometers are tools for measuring temperature that are also used to track electromagnetic radiation that an object emits. These come in a variety of spectral ranges. Pyrometers are categorised into 1-color, 2-color, and high-speed pyrometers based on the spectral range. Pyrometers are used to measure an object’s surface temperature, which is determined by the radiation (infrared or visible) emitted from the object. Pyrometers are also referred to as infrared thermometers, radiation thermometers, or non-contact thermometers. Due to their capacity to absorb energy and measure the intensity of EM waves at any wavelength, pyrometers serve as photodetector devices.


High-temperature of furnaces are measured using pyrometer. pyrometer are able to measure temperature very quickly, precisely, visually, and accurately. Pyrometers are available in a variety of spectral ranges, including long wave ranges for non-metals and short wave ranges for metals.

To measure the radiation emitted from the object during the measurement of temperature, colour pyrometers are employed. These are very accurate at measuring the temperature of the object. Since these devices have such low measuring errors.

The ratio of two radiation intensities with two spectral ranges is calculated using colour pyrometers. These are offered in various versions in the Metis M3 and H3 series as well as the Capella C3 portable series.

More quickly and accurately than M3 devices, high-speed pyrometers are used to measure temperature. These can be used with 1-color and 2-color pyrometers, respectively. These tools can measure proper temperature and produce precise temperature profiles of rapidly moving objects.

Working Principle of Pyrometer:-

Pyrometers are tools for measuring heat and electromagnetic radiation that an object emits. These come in various spectral ranges. Pyrometers are divided into 1-color, 2-color, and high-speed categories based on their spectral range.

The pyrometer’s fundamental working principle is that it measures an object’s temperature by detecting the heat or radiation it emits without coming into direct contact with it. Depending on the radiation’s intensity, it measures the temperature. The optical system and detectors, which are used to measure the object’s surface temperature, make up the pyrometer’s two basic parts.

The optical system will capture the energy emitted from any object whose surface temperature is to be measured with the pyrometer. The detector, which is highly sensitive to the radiation waves, is then given the radiation. The detector’s output describes the object’s temperature as a result of radiation. Keep in mind that the temperature of the object is directly proportional to the temperature of the detector used to analyse the level of radiation.

Beyond the absolute temperature( -273.15 degrees Centigrade ), each targeted object emits radiation based on its actual temperature. Infrared, which is located above visible red light on the electromagnetic spectrum, is the name given to this radiation that is being emitted. The radiated energy is converted into electrical signals by a detector and used to measure the object’s temperature.

Types of Pyrometer:

Pyrometer Having Two Types

Differences between optical and radiation pyrometers

Below is a comparison of the radiation pyrometer and optical pyrometer.

Serial No. Optical Pyrometer Infrared/Radiation Pyrometer
1 These are a particular kind of pyrometer used to find visible spectrum heat radiation. The amount of visible light that hot items release will determine how hot they are when being measured. These pyrometers are made to pick up thermal radiation in the infrared spectrum, which typically occurs between 2 and 14 m. It uses the radiation emitted by the target item to determine its temperature.
2 Compared to radiation pyrometers, optical pyrometers are more expensive. It performs well and have reasonable price.
3 It is portable and easy to assemble. The radiation pyrometer has a complicated design.
4 Different conditions like dust, smoke, and thermal radiation can affect the device’s accuracy. The measurement could be impacted by the target material’s emissivity.
5 useful for measuring the temperature of moving objects. It responds with a quick speed.

It is very stable.

6 human error’s impact There is almost no chance of error.
7 In Optical Pyrometer the result is seen with the human eye In radiation Pyrometer The thermocouple immediately provides the output in terms of mvolt.
8 The components of an optical pyrometer are a hot body, lens, filament lamp, and red filter arranged in a straight line. The three main parts of a radiation pyrometer are the lens, the receiving components, and the recording device.
9 Operating range – Up to 1400 degrees Celsius, which can be increased to 3000 degrees Celsius by using an absorption-type screen positioned next to the lens. Operating range – Pyrometers that measure total radiation are used to measure temperatures between 700°C and 2000°C.

Advantages and Disadvantages of Pyrometer:

Pyrometers typically have some benefits and drawbacks when used, similar to thermometers.

Advantages of Pyrometer:

The pyrometer has the following benefits:

  • Its weight is light.
  • Pyrometers are helpful for measuring high temperatures.
  • Pyrometers are particularly well adapted for measuring the temperature of moving objects or any non-reachable or non-touchable surfaces. Modern multispectral pyrometers can accurately measure the high temperatures present inside gas turbine engines’ combustion chambers.
  •  It can measure an object’s temperature without coming into contact with it. The term for this is non-contact measurement.
  • It can measure the temperature of various types of objects at varying distances, has a quick response time, and good stability while doing so.

Disadvantages of Pyrometer:

The drawbacks of pyrometers include:

  • It contributes to human error.
  • The Radiation Pyrometer uses Stefan’s law, which holds true for perfectly black bodies, to determine the body’s temperature. In actual practise, the hot body might not be perfectly black, which causes measurement error in the temperature.
  • Because the image of a hot body is not clear at temperatures below 600, the optical pyrometer cannot be used to measure the temperature of a hot body below 600.
  • Different conditions like dust, smoke, and thermal radiation can affect the device’s accuracy.
  • Since clean burning gases don’t emit visible light, the optical pyrometer is useless for determining their temperature.
  • They are typically durable and expensive.

Applications of Pyrometer:

There are numerous uses for pyrometers, including obtaining a more accurate reading of the temperature of moving or stationary objects.

  • In the operation of metallurgical furnaces, temperature is a crucial variable. For the operation to be controlled effectively, the metal temperature must be measured accurately and continuously. Maximizing smelting rates, producing slag at the ideal temperature, reducing fuel usage, and maybe extending refractory life are all possible. The conventional tools used for this purpose were thermocouples, but they melt and deteriorate, making them unsuitable for continuous measurement.

Heat treatment is done in salt bath furnaces, which run at temperatures of up to 1300 °C. Precision is maintained by monitoring the temperature of the molten salt at extremely high working temperatures with significant heat transfer between it and the steel being processed. The majority of errors are brought on by surface slag, which is cooler than the salt bath. The tuyere pyrometer is an optical device used to monitor temperature through tuyeres(A tube, nozzle, or pipe called a tuyere or tuyère is used to blow air into a furnace or hearth), which are typically employed to inject air or other reactants into the furnace’s bath.

  • Throughout the smelting sector.
  • In order to keep the fabric from overheating, a hot air balloon is fitted with a pyrometer that measures the temperature at the top of the envelope.
  • The temperature of liquid, metals and other highly heated materials can be measured
  • In steam boilers to measure the temperature of steam.
  • To measure the surface temperature of the turbine blades, pyrometers can be installed in experimental gas turbine engines. These pyrometers can be used in conjunction with a tachometer to correlate their output with the location of each turbine blade. Engineers can pinpoint the temperature at precise locations on blades going past the probe using timing and a radial position encoder.

History of Pyrometer:

The London Science Museum’s Hindley Pyrometer, made in 1752 for the Royal collection, is thought to be the earliest pyrometer specimen still in existence. The pyrometer was a widely used device by the time the mathematician Euler wrote a detailed description of it in 1760.

Josiah Wedgwood, a potter, created a distinct kind of pyrometer to gauge the temperature in his kilns. Originally comparing the colour of clay fired at established temperatures, the pyrometer later advanced to gauge the clay’s dependence on temperature-dependent shrinkage. Later examples made use of a metal bar’s expansion.

William and Werner Siemens, brothers, created a platinum resistance thermometer in the 1860s–1870s initially to monitor the temperature in underwater cables, but they later modified it to detect temperatures in metallurgy up to 1000 C, earning it the name pyrometer. By L. Holborn and F. Kurlbaum, the first disappearing-filament pyrometer was created in 1901. The only barrier between the observer’s eye and the hot item in this apparatus was a tiny electrical filament. It was calibrated to allow temperature to be deduced from the current through the filament by adjusting the current through the filament until it was the same colour (and hence temperature) as the object and no longer visible. The emissivity of the object affects the temperature that the vanishing filament pyrometer and other brightness pyrometers of its sort return. With increased usage of brightness pyrometers, it became clear that there were issues with depending just on emissivity value knowledge. It was discovered that emissivity varied, frequently significantly, with surface roughness, bulk and surface composition, and even temperature.

An old pyrometer from 1852. When a metal bar is heated, it presses against a lever, moving the pointer (c) along a scale that acts as a measuring index. An immovable object (e) holds the bar in place. The index will drop back as the bar cools due to a spring on (c) pushing on (b). Image Credit- Wikipedia

The two-color pyrometer, also known as the ratio pyrometer, was created to overcome these challenges. They rely on the notion that if Planck’s statement of the intensities at two different wavelengths is split, Planck’s law, which connects temperature to the intensity of radiation emitted at certain wavelengths, may be solved for temperature. The emissivity is assumed to be the same at both wavelengths and to cancel out in the division in this solution. The grey body assumption is what is meant by this. In essence, ratio pyrometers are two brightness pyrometers combined into one device. The ratio pyrometers’ working theories were created in the 1920s and 1930s, and they became widely available in 1939.

It was discovered that many materials, including metals as an example, do not have the same emissivity at two wavelengths as the ratio pyrometer gained popularity. These materials’ emissivity does not cancel out, resulting in inaccurate temperature readings. The emissivities and wavelengths used for the measurements affect how much inaccuracy is there. The emissivity of a substance cannot be determined by using a two-color ratio pyrometer.

– At the Raytheon transistor plant in 1956, a technician used a disappearing-filament pyrometer to measure the temperature of molten silicon at 2,650 °F (1,450 °C) in Czochralski crystal growing equipment. Image Credit- Wikipedia

Multiwavelength pyrometers were developed at the US National Institute of Standards and Technology and first described in 1992 to more precisely estimate the temperature of actual objects with unknown or fluctuating emissivities. Multiwavelength pyrometers aim to detect temperature accurately even when the emissivity is unknown, variable, and varies at each wavelength by using three or more wavelengths and statistical manipulation of the findings.


1.What distinguishes a pyrometer from a thermometer?

A Pyrometer is a remote-sensing thermometer and a non-contact measuring device for high temperatures. A thermometer is a device for measuring temperature (contact measurement).

2.Describe the optical pyrometer.

Devices that use the brightness of the target object and the brightness of the filament inside the pyrometer as the basis for their non-contact temperature measurement.

3.What tools are used to measure temperatures?

Thermometers, pyrometers, thermocouples, and Thermometers (liquid in glass)

thermometer with electric resistance

Infrared thermometers and radiation thermometers


the silicon diode

Metal-based devices

Capillary and bulb devices

Gas thermometers with constant volume and pressure

4.Which SI unit of temperature is used?

Kelvin (K). is the SI unit of temperature

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