Working of an Infrared Thermometer

A thermometer is a device which measures temperature or temperature gradient.The two important fundamental components of this device are the sensor and indicator. Sensor is the component which senses the temperature of a body and indicator shows the value of temperature in a calibrated scale. Infrared thermometers measure the temperature of an object by measuring the infrared energy emitted by that object.

How Does an Infrared Thermometer Work

The infrared (IR) energy occupies a certain portion of the electromagnetic spectrum. The frequency of the electromagnetic radiation of infrared rays, is below the frequency range of visible light, and that makes the infrared rays invisible to human eyes. The wavelength of infrared radiation is between 0.7 microns to 1000 microns, although these devices can measure the radiation having wavelength in the range between 0.7 microns to 14 microns. However these devices cannot measure the temperature of the air, because, the design is such that, when the sensor of this device senses the radiation from the target, the air between the sensor and the target should not cause any temperature variations in the final measurement.

A property called emissivity, is a major factor in the temperature measurements of such devices. Emissivity is defined as the "ratio of the intensity of radiation emitted by the surface at a specified wavelength and direction to that emitted by a black body under the same conditions". So, the emissivity of a black body is 1. The emissivity of all the objects range from 0 to 1. Considering this property, there are two main types of bodies.

• Gray bodies have same emissivity at all wavelengths.
• Non gray bodies have emissivity which varies according to the wavelength. This change causes accuracy problems in the temperature measurements.

The radiation from an object is sensed by the sensor and it captures the radiation. This sensor is very sensitive as it can even detect a radiation of about 0.0001 watts. The heat energy is converted into electrical energy, which is amplified and modified into output voltage. A central processing unit digitalizes the electrical signal using a 16-bit, Analog-to-Digital Converter, and the Arithmetic Unit inside the processor calculates the temperature value of the object, using Planck's Radiation Law, where the value of ambient temperature and emissivity of the object is substituted, which results in a temperature reading on the infrared display.

The components of this device are:

• Color liquid crystal display screen
• Function key
• Machine shell
• Handle cover
• Camera device
• Laser emission hole
• Infrared measure hole
• Hygrothermograph
• Measure switch
• SD (Secure digital) card
• K-shape receptacle
• USB output plug

The camera device captures the image of a target object. The laser emission hole focuses at a single point on the target object, wherein the temperature of the single point position needs to be measured. The infrared measure hole measures the single point temperature of the object, which is instantly displayed on the color liquid crystal display screen. The image and the data are simultaneously stored in the SD card.

This model can be more efficient in working because, the Field of View (FoV) is reduced as the single point temperature is measured, and the temperature variance due to other materials or mediums can be reduced considerably. This advantage is not there in other models.

Working Principle

The basic function of these devices is to capture the infrared rays energy, emitted by any body having the temperature value above absolute zero (0 kelvin). This is because everybody is made up of atoms and molecules. The higher the temperature of the body, the more will be the vibrations in the molecules of the body and the radiation emission. The above phenomenon is scientifically known as Stefan-Boltzmann law, which states that "total radiation energy emitted by a body is proportional to the fourth power of the absolute temperature".

EαT⁴where,

• E is the total radiation energy.
• T is the absolute temperature.

Wien's displacement law is also applied in the working of such devices. This law states that, "the wavelength carrying the maximum energy is inversely proportional to the absolute temperature of a black body".

λ_max x T = bwhere,

• λ_max is the maximum wavelength.
• T is the temperature of the black body in Kelvins.
• b is 2.8977685 ± 51 × 10^-3 meters.Kelvins

Applications Depending upon Wavelength

Wavelength (µm)	Applications
0.7 - 1.1	High precision measurements in industries
1.1 - 1.7	Medium range of temperature and is used in glass and metal industries
2.2 - 2.5	General purpose measurements
3.43	To measure the temperature of thin-film plastics, oil and paints, etc.
3.9	To measure high temperature gas and oil-fired furnaces
4.4 - 4.4	To measure flame temperature
4.8 - 5.2	Glass and Ceramics temperature measurements
7.9	To measure the temperature of glass, polyesters, polyamides, etc.
8 - 14	Long range measurements

A non contact infrared thermometer is another type which can measure the temperature of an object, without touching it. This application can be used to measure the temperature of the chips in the computers just by pointing the device above the CPU of the computer. This device employs a laser pointer to aim at the target. These devices are very used in many working sectors, for temperature measurements.