Infrared Thermometer: How it Works? How to Use it?

Alan   2021-10-21 17:39:37

Topics Covered in this Article:

What are Infrared Thermometers?

Infrared thermometer is a non-contact instrument for measuring the surface temperature of an object. Affected by the COVID-19 coronavirus epidemic, the demand for infrared thermometers has greatly increased, and they have played a key role in epidemic prevention. As the demand for infrared body temperature detection equipment increases, the demand for infrared sensors-the core component of infrared body temperature detection equipment, is also increasing. One of the most commonly used is the thermopile infrared temperature sensor-MLX90614ESF. This sensor is a silicon-based infrared sensor based on a thermopile.

Infrared (IR) thermometers allow you to take temperature readings quickly and without having to contact the thing being measured. It's a sensor that uses a lens to focus infrared (IR) energy onto a detector, which then converts the energy into an electrical signal that may be displayed in temperature units after being corrected for ambient temperature variations.

Infrared Thermometer.jpg

Infrared Thermometer

Infrared temperature measurement technology plays an important role in production process,  product quality control and monitoring, equipment online fault diagnosis and safety protection, and energy saving. Non-contact infrared thermometers have advanced quickly in technology over the last 20 years, with constant improvements in performance, features, variety, and application breadth. Compared with contact temperature measurement methods, infrared temperature measurement has the advantages of fast response time, non-contact, safe use and long service life.

How do infrared thermometers work?

The IR thermometer operates by concentrating and funneling IR light from the item into a detector.

Infrared light can be focused, reflected, or absorbed in the same way that visible light can. Infrared thermometers use a lens to concentrate the infrared light emitted by an item onto a thermopile detector.

The thermopile is just a series or parallel connection of thermocouples. When infrared radiation strikes the surface of a thermopile, it is absorbed and converted into heat. The voltage output is proportional to the amount of incident infrared radiation. This output is used by the detector to calculate the temperature, which is displayed on the screen.

The amount of electricity generated by the rays emitted by the object in issue is ultimately what determines the temperature indicated on the thermometer. An infrared thermometer is a quick way to get a temperature reading in a variety of situations because the reading is generated in a matter of seconds.

While the entire procedure may appear complicated, the infrared thermometer captures the temperature in a matter of seconds and displays it in your choice unit.

Basic theory of infrared temperature measurement

It was found in 1672 that sunlight (white light) is made up of different colored light. Newton came to the renowned conclusion that monochromatic light is simpler in nature than white light around the same time. The sunlight (white light) is decomposed into red, orange, yellow, green, cyan, blue, purple, and other monochromatic light colors using a dichroic prism. When British physicist F. W. Huxel explored various colored lights from a thermal point of view in 1800, he discovered infrared. He claimed that, in addition to the sun's apparent rays of radiation, there is a "hot wire" that is unseen to the naked eye. This invisible "hot wire" is located outside of the red light and is called infrared light. Infrared is an electromagnetic wave, which has the same essence as radio waves and visible light.

Infrared wavelengths range from 0.76 to 1000μm. It is split into four categories based on wavelength range: near infrared, mid-infrared, far-infrared, and extremely far-infrared. Its position in the electromagnetic wave continuum lies between radio waves and visible light. Infrared radiation is one of nature's most common forms of electromagnetic radiation. It is based on the concept that in a typical environment, any object would produce its own irregular movement of molecules and atoms, as well as emit thermal infrared radiation, which is the movement of molecules and atoms. The higher the intensity, the more energy is radiated, and vice versa.

Due to the movement of their own molecules, objects having a temperature above absolute zero will emit infrared rays. The output signal of the imaging device can mimic the spatial distribution of the surface temperature of the scanned item in a one-to-one correspondence after the object's power signal is translated into an electrical signal by the infrared detector. The electronic system then processes data and sends it to the display screen, resulting in a thermal image of the heat distribution on the object's surface. Using this method, it can realize the long-distance thermal image imaging and temperature measurement of the target, and analyze and judge it.

Black-body radiation

How does the measured infrared radiation energy calculate the temperature of the object, and what is the relationship between them?

In the 19th century, scientists Stefan and Boltzmann obtained the law of black body radiation through experiments and calculations: MB(T) = σT4 (σ is a constant). This law tells us that the total amount of radiation emitted from a unit area of a black body per unit time. The radiant energy is proportional to the 4th power of its own thermodynamic temperature.

Of course, the radiation law of actual objects (non-black bodies) is generally more complicated. It needs to be studied with the help of the radiation law of black bodies. It is mainly affected by the emissivity of the object. The emissivity of different objects is different, which can be obtained by looking up tables or experiments. The temperature gun can compensate infrared radiation errors caused by the object's material, structure, thickness, and other factors, allowing it to more correctly estimate the object's surface temperature.

A blackbody is an idealized radiator, it absorbs all wavelengths of radiant energy, there is no reflection or transmission of energy, and the emissivity of its surface is 1. It should be pointed out that there is no real black body in nature, but in order to clarify and obtain the infrared radiation distribution law, a suitable model must be selected in theoretical research. This is the quantized oscillator model of body cavity radiation proposed by Planck, which is derived Planck’s law of black body radiation. That is, the spectral radiance of a black body expressed in wavelength, is the starting point of all infrared radiation theories, so it is called the law of black body radiation.

What is the accuracy of infrared thermometer?

The accuracy of infrared thermometers is generally ±2°C. In order to obtain accurate temperature readings, the distance between the thermometer and the test target must be within a suitable range. The so-called "spotsize" is the area of the measuring point of the thermometer.

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Object distance ratio

The farther away from the target, the larger the spot size. The ratio of distance to spot size is called D:S. On the laser sight type thermometer, the laser spot is above the target center with an offset distance of 12mm (0.47 inches).

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measurement distance

When determining the measurement distance, make sure that the target diameter is equal to or larger than the measured spot size. The distance between the object and the measuring instrument is positive, because the target is slightly larger than the spot size to be measured. If target is smaller than the measured spot size, the accuracy of the reading will be reduced.

Influencing Factors of Infrared Thermometer Measurement Accuracy

  • Measuring angle

  • Ambient temperature

  • Air quality

  • Electromagnetic interference

  • Environmental radiation

  • Field of view and target size

How to Use an Infrared Thermometer?

Understanding the principle of operation, technical indications, environmental working conditions, operation, and maintenance of infrared thermometers is the foundation for users to correctly select and utilize infrared thermometers. The optical system collects the infrared radiation energy of the target in its field of view, and the size of the field of view is determined by the optical parts and position of the thermometer. The photodetector is focused on the infrared energy, which is then transformed into an electrical signal. The signal is converted into the temperature value of the measured target after being corrected by the amplifier and signal processing circuit in accordance with the internal algorithm of the instrument and the target emissivity. In addition, the target and the environmental conditions where the thermometer is located should also be considered, such as the influence of temperature, atmosphere, pollution and interference on performance indicators and correction methods.

1. Pay attention to the working environment

The temperature range for infrared forehead thermometers is 16°C to 35°C, with the average being approximately 20°C. Because the epidemic occurs in winter, the outdoor is relatively cold, and the epidemic prevention and control monitoring points are generally located outdoors. The acquired values are prone to variations if the infrared forehead thermometer is used in a low-temperature outside setting for a long time. As a result, it's best to utilize it as much as possible indoors, where there's no wind. If this is not possible, some temperature protection for the infrared forehead thermometer is recommended, such as placing a warm baby in a cotton-padded jacket or in certain regions, and ensuring that its temperature does not fall below 16 degrees.

2. Pay attention to the measurement mode

The infrared thermometer has two modes, object temperature mode and body temperature mode. When working, put it in the body temperature mode. The body temperature mode is to convert the surface temperature to the armpit temperature to display. If it is messed up, it must be restarted according to the instructions in the manual to return to the body temperature mode.

3. Pay attention to the measurement location

In the absence of wind indoors, it is recommended to measure the forehead, the center of the eyebrows and the middle of the hairline as much as possible. According to the requirements of the instructions, the distance is generally 1-3 cm, which should be done according to the instructions of different manufacturers. Also you should make sure that there is no obstruction by hair, sweat, hats, etc. If the measurement is inaccurate once, when you want to repeat the measurement, first remove the infrared thermometer probe, and take the next measurement after an interval of 5 or 6 seconds.

Is it more accurate if you stick it on your forehead? This is not necessarily. The design of infared thermometer is non-contact, we’d better keep it 1-3 cm away from your forehead. If it is less than 1 cm, it may not measure accurately. Some people say that there are many prevention and control monitoring points that do not have indoor temperature measurement conditions, and what should be done if there is wind in the low temperature outdoor? At this time, you can measure warmer parts such as wrists and neck instead.