Infrared Thermometer

What is Infrared Thermometer?

An infrared thermometer is a temperature measuring device which measures the infrared radiation energy emitting from a body to get its temperature. Infrared thermometers are sometimes called as laser thermometers as they use laser beam to help in aiming the thermometer at pin point for non-contact thermometers or temperature guns.

Infrared radiation is a type of electromagnetic radiation having wavelength below the wavelength of visible range of light.

The basic design of an infrared thermometer is consisting of a lens to focus the incident infrared thermal radiations on to a detector. This radiant energy is converted into an electric signal by use of thermocouples.

Infrared Radiation

Every material has moving atoms and molecules when its temperature is above absolute zero temperature. Speed of these atoms or molecules is a function of temperature. The higher the temperature, the faster will be the movement of molecules. These moving molecules emit electromagnetic energy in the form of infrared radiation.

The wavelength of infrared radiation is longer than that of visible light. Hence, we are not able to see it with naked eyes. At higher temperatures, this radiation goes to the visible level spectrum range.

The heat that we feel from sunlight, from a radiator, or from a distant fire, all are examples of infrared radiations. It is the heat that an infrared thermometer detects to measure the temperature of objects.

Application of Infrared Thermometer

Major applications of Infrared thermometers are given below –

  1. Heating and Air conditioning – It is used for detection of insulation breakdown, heat loss or heat gain in ducting etc.
  2. Industrial Application – It is for monitoring the motor or engine cooling systems performance, boiler operations, steam systems and detection of hot spots in electrical systems and panels.
  3. Agriculture – Monitoring plant temperatures for stress and animal bedding to detect spoiling.
  4. Contactless Application – Infrared thermometers are useful in measuring the temperature across a range of industrial and clinical thermometer for contactless applications.
  5. Remote operation – Infrared thermometers are essential to use while reading the temperature of a surface that’s too dangerous, and almost impossible to reach.
  6. Infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used.
  7. It is useful in circumstances where the object to measure is fragile and dangerous to go nearer to it, or when other kinds of thermometers are not practical to apply.

Working principle of Infrared Thermometer


A basic Infrared thermometers is shown in figure. It works on the principle of black body radiations.

Any material body having temperature above absolute zero level has molecules randomly moving within it. The higher the temperature, the faster will be the speed of these molecules. These fast moving molecules emit infrared radiations which are in the range of not visible to human eyes. As the temperature of body increases more, they start to emit radiations in the range of visible light. This is the reason that a too hot metal emits a red or white glow of light. Infrared radiations follow the laws of light reflection, refraction and absorption etc. just similar to the visible light waves.

An infrared thermometer consists of following main parts –

  1. A converging lens.
  2. Thermopile consisting of thermocouples.
  3. Amplifier.
  4. Microprocessor.
  5. Display.

An infrared thermometer employ a converging lens to focus the incidence infrared light emitting from any distant object onto a IR detector known as a thermopile. A thermopile is a combination of thermocouples connected in series and parallel. When the infrared radiation falls on its surface, it gets absorbed and the surface gets heated. A photoelectric effect produces photoelectric electromotive force which is produced in proportion to the incident infrared energy. The detector uses this output to determine the temperature, which gets displayed on the screen.

Working principle of infrared thermometer is shown in figure –


Infrared thermometers employ a lens which focus the incident infrared light from a distant object onto thermopile. This thermopile absorb radiations and convert it to heat energy. The thermopile gets hotter as it absorbs more and more infrared energy. The heat energy is converted into electric signals, which is transmitted to a detector which processes and determines the temperature of the object.

Properties of Infrared Thermometer

A good infrared thermometer should have following properties –

Accuracy of Device

Accuracy of an infrared thermometer depends upon its distance to spot ratio called D/S ratio. This is the ratio of the maximum distance from where the thermometer can accurately measure the temperature of a specific surface area.

Consider that an infrared thermometer has D/S ratio of 4 : 1. This means that, this thermometer can accurately measure the temperature of a surface having area of 4 square inch from a distance of 4 x 8 = 32 inches.

Range of Device

Range of an infrared thermometer effects it’s working efficiency. An IR thermometer should have wide temperature range so that it can record various processes occurring at different temperature controlled environment.

Sensitivity of Device

Sensitivity of an infrared thermometer is expressed in terms of Response Time. Response Time is the reading speed of an infrared thermometer. It is the time to quickly deliver an accurate reading after initiation of the measuring process. It should be as minimum as possible.

Sensitivity is of great importance when measuring the temperature of a moving object or measuring an object which heats up very quickly.

Display of Device

An infrared thermometer is used for versatile indoor and outdoor applications in various industries, laboratories, hospitals and offices. Hence, an infrared thermometer should have backlit screen display which makes it convenient to use and take the readings in bright light or even in adverse lighting conditions.

Emissivity of Device

Emissivity shows how much infrared energy a thermometer can put out at a time. Infrared thermometers with emissivity closer to 1.00 can read more materials than those with lower emissivity value. A thermometer is also available with an adjustable emissivity level for different types of measurement.

Types of Infrared Thermometers

There are many types of infrared temperature sensing devices available today.

Various types of devices are available depending upon configurations and designs for flexibility and portable hand held use as well as for mounting in a fixed position on wall to serve a specific purpose.

The most common types of infrared thermometers are –

  1. Spot infrared thermometers – These devices measure the temperature at a spot on a surface.
  2. Infrared scanning systems – These devices scan a larger area. They are widely used in manufacturing processes involving conveyor belts.
  3. Infrared thermal imaging cameras – These cameras are infrared radiation thermometers used for measuring temperature at many points over a large area to create a two-dimensional image called a thermogram.