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Principles and Issues of Using Thermal Imaging Technology to Measure and Screen Human Body Temperature

Because of COVID-19, all parts of the world are in panic and disorder. In daily production and life, the testing standard for people's physical health is 37.3℃. As a result, various infrared temperature measurement devices have gradually demonstrated their role in epidemic prevention, and their status has become more and more important. However, some related questions have also emerged: Why are the human body temperatures measured by different infrared temperature measurement devices so different? Is infrared temperature measurement real and reliable? Is it really helpful for epidemic prevention?

To know the truth of the matter, we must first understand the principle of infrared temperature measurement.

Ⅰ. The principle of infrared temperature measurement

As long as the temperature of any object is higher than absolute zero (-273.15°C), it will emit heat radiation to the outside and also absorb the heat radiation emitted by other objects. The temperature of the object is different, the energy it radiates is also different, and the wavelength of the radiation wave is also different, but it always contains infrared radiation. For objects below 1000°C, the strongest electromagnetic wave in the thermal radiation is infrared waves, so the measurement of the infrared radiation of the object itself can accurately determine its surface temperature.

Kirchhoff's law (the law of thermal radiation) believes that the ratio of the energy radiated by an object in thermal equilibrium to the absorptance has nothing to do with the physical properties of the object itself, but only with wavelength and temperature.

Black body

If there is an object and the ratio of its radiant energy to the absorbed energy is known, then use this object as a standard to detect the heat radiated by a nearby target object, and we can infer its surface temperature (the process and methods of reverse deduction are as follows: a lot of). This standard object is called absolute black body, or black body for short.

Thermal Imaging TechnologyDefinition of black body: It can absorb all the energy of any wavelength radiated to its surface at any temperature. This means that the ratio of the radiant heat to the absorbed heat of the black body is 1.

Because of this characteristic of black body, in actual temperature measurement, black body can be used as a standard to measure the temperature of other objects.

The calibration of the black body can be carried out in the production process, and it can also be calibrated online in the on-site environment. But this is only in theory. In practice, there are many reasons that will affect the accuracy of temperature measurement.

Ⅱ. Factors affecting the accuracy of human body temperature measurement:

Through the above analysis, if you want to measure a person's body temperature, the following three aspects will affect the accuracy of the temperature measurement: the human body itself, the heat transfer process, and the infrared detector.

1. The temperature of the human body:

The ambient temperature will affect the body temperature of the human body.

Different exercise status will affect people's body temperature. The temperature of the human body varies in the morning and in the evening. The human body temperature is the lowest in the early morning, reaches the highest in the afternoon, and then gradually decreases. The temperature of women is generally slightly higher than that of men. Different parts of the human body have different temperatures. The surface of the forehead is unobstructed, the capillaries are densely distributed, and the temperature distribution is relatively uniform, which is a good temperature measurement point.

As mentioned above, the emissivity of a black body is 1. Using a black body as a standard, the human body's is about 0.98, which is similar to a black body (the actual black body is also difficult to reach 1). Therefore, in some actual temperature measurement, you can directly treat a person as a black body.

Human Skin


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2. Heat transfer process

The heat energy emitted by an object is only related to temperature and wavelength, and the infrared band concentrates most of the radiant energy. To detect temperature, we don't need to detect the energy of the whole band, only the infrared band.

The infrared radiation of an object has a lot to do with the penetrability and wavelength of the atmosphere. Fortunately, God has opened several windows (scientific name: infrared atmospheric window). The main atmospheric windows include 2μm -2.6μm, 3μm~6μm and 8μm~14μm. Infrared thermal imaging technology is using the "atmospheric window" of infrared radiation. (Many infrared detectors choose germanium glass lenses because germanium glass has good light transmission performance in the 2-16μm band, and its chemical properties are stable.)

Thermal Imaging Technology

(透射比 Transmittance, 波长 Wave Length, 吸收分子 Absorbing Molecule, 大气透射光谱 Atmospheric Transmission Spectrum)

Although it is a window, the transmittance still does not reach 1, so the thermal radiation will be attenuated to a certain extent due to the distance. When the distance is close, we simplify the problem and ignore the impact. When the distance is long, the infrared electromagnetic wave propagating in the air will be absorbed by gases such as carbon dioxide, causing energy attenuation, and corresponding temperature compensation needs to be considered.

3. Infrared detector

There are two types of detectors commonly used to convert thermal radiation into electrical signals: detectors based on thermopile (thermocouple); detectors based on the principle of resistance micro-radiation measurement.

Thermopile (thermocouple)-based detectors: forehead guns and face temperature measuring panels mostly use this scheme. Common manufacturer brands include Melexis, Heimann, Amphenol, Oriental System, and Shanghai Sunshine Technologies.

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