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Analysis of non-contact infrared thermography temperature measurement technology

2020-06-11 Mr. Chen Hongqiang from Zhejiang Dali Technology C 529 people follow

一、Background

"Temperature" is a word everyone knows and understands what it stands for. Every object has a temperature, but in the Spring Festival of 2020, the word appears frequently in the major TV broadcasts, and people are vigilant about this word, especially sensitive words such as "human body temperature", "37.0 ℃". A new outbreak of novel coronavirus has swept the globe. The contactless infrared thermometer is standing on the front line of the epidemic prevention war. It has screened out a large number of suspected fever patients, reducing the probability of cross infection among the people, and ensuring the safety of medical staff and security personnel, and effectively preventing the spread of the virus. This leads to a question: can non-contact infrared thermal imaging technology create greater value in the field of civil security, protect people's life and property security, and promote the development of social economy?

 

二、Introduction to relevant principles

1. Basic principle of infrared thermal imaging

In nature, any object whose temperature is higher than absolute zero (- 273 ℃) will produce infrared radiation. The infrared thermal imager collects the radiation signal and converts it into electrical signal, simulates the heat distribution field on the surface of the object, transforms it into video image signal through image processing and other technologies, and intuitively displays the image. Infrared thermal radiation has strong conductivity. Infrared thermal imaging cannot be affected by light, smoke, weather and other factors, and can be used in a wide range of scenes.

2. Basic principles of blackbody

Generally speaking, blackbody has the following three characteristics:

(1) An object that completely absorbs foreign radiation of any wavelength without any reflection under any conditions.

(2) An object with an absorption ratio of 1.

(3) An object that absorbs radiation of any wavelength at any temperature.

 

Any object has the ability to radiate, absorb and emit electromagnetic wave continuously. The radiated electromagnetic wave is different in each wave band, so it has a certain spectral distribution. This spectral distribution is related to the characteristics of the object itself and its temperature, so it is called thermal radiation. In order to study the law of thermal radiation that does not depend on the specific physical properties of matter, physicists defined an ideal object (i.e. blackbody) as the standard object of thermal radiation research. The so-called blackbody means that all the incident electromagnetic waves are absorbed, neither reflected nor transmitted. According to Kirchhoff's radiation law, the ratio of radiant energy to absorptivity of a body in thermal equilibrium is independent of its physical properties, only related to wavelength and temperature. According to Kirchhoff's radiation law, at a certain temperature, blackbody must be the object with the largest radiation ability, which can be called a complete radiator.

3. Improve the accuracy of temperature measurement

For scenes with high temperature accuracy requirements, blackbody can be used for temperature calibration to improve the temperature measurement accuracy of infrared thermal imager. Of course, all thermal imagers will be calibrated and calibrated with blackbody before delivery. The higher the blackbody accuracy is, the higher the temperature measurement accuracy of thermal imager is. The performance of blackbody determines the accuracy of thermal imager. In order to improve the temperature measurement accuracy of the thermal imager, different specific radiation coefficients should be adjusted according to the different materials of the object to be measured. Taking this epidemic as an example, because the temperature of human body is normal at about 36.5 ℃, the temperature measurement accuracy of temperature detector is an important key indicator to accurately determine whether the temperature is normal. At present, we use blackbody correction technology to improve the temperature measurement accuracy.


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4. Temperature measurement correction

The measuring temperature of the blackbody at 37 ℃ is 36.9 ℃, and the measuring error is 0.1 ℃, which is within the error range of the instrument. Taking blackbody as the reference object, the temperature deviation of human body measured under the same condition can be ensured not to exceed ± 0.3 ℃. As shown in figure (3) and figure (4): under the condition of the same environmental state and measurement distance, the forehead measurement temperature is 33.1 ℃ (for men) and 34.2 ℃ (for women), respectively. 

 

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The body temperature of human is constant between 36 ℃ and 37 ℃, but the temperature of forehead is usually lower than that of body temperature in recent actual measurement, which needs to consider the difference of body surface temperature and body temperature. Because of the direct heat dissipation, the body surface temperature of forehead is lower than that of armpit. Secondly, the temperature of forehead is very easy to be affected by the environment temperature: the temperature of forehead in low temperature environment is lower than that in vivo, similarly, the temperature of forehead (mostly the temperature of hair) of the tested personnel in the environment of sunlight is higher than that in vivo.

Based on the influence of environmental factors on forehead temperature, two suggestions are put forward when using infrared thermal imaging rapid temperature screening instrument:

(1) It is not recommended to use this screening instrument in the open air.

(2) It is suggested to modify the original indoor waiting channel, increase the retention time of the tested personnel, and ensure that the forehead temperature is consistent with the change trend of body temperature as much as possible.

5. Forehead temperature as a basis for screening

(1)The function of the infrared thermal imaging temperature rapid screening instrument is to carry out long-distance, non-contact temperature measurement in the case of large flow of people, and quickly screen out the people with high temperature in many people. In the field, only the head can be exposed.

(2)The internal temperature of human body is called body temperature for short, which is the average deep temperature in clinic. It is mainly reflected to the body surface temperature through the central nerve and blood circulation to keep the body temperature constant. The blood flow is supplied by the main artery and the carotid artery through the temporal artery near the forehead to the heart. The fast blood circulation can reflect the change of body temperature in time and truly. When fever occurs, the brain is first affected. When the body temperature rises or falls, the brain first regulates the temperature. The temperature on the forehead surface can also be reflected quickly.

It should be noted that the body temperature data measured by frontal temperature gun and infrared thermal imaging can be used to screen personnel, but can not be used as the basis of medical diagnosis.


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三、Application of infrared thermal imaging technology

At present, the domestic infrared thermal imaging technology is still in the development stage, and the manufacturing cost of the thermal imager is still high. Although a few domestic companies (such as Dali Technology) have been able to independently produce high-performance detectors, their production capacity still cannot meet the large demand of the domestic market, which limits the popularity of infrared thermal imaging in the civil field, so that the ordinary people rarely have access to such thermal imaging Products.

The following are some typical applications of infrared thermal imaging technology in the civil market:

1. Medical clinical observation and inspection and quarantine

It is used for rapid screening of human body temperature in airport, wharf, port, subway, station, entry and exit, etc. With blackbody real-time calibration, the temperature measurement accuracy of thermal imager can reach ± 0.3 ℃. The body temperature data of hundreds of people can be obtained in one minute. With the visible light face recognition function, it can be linked with the information database of the public security department, accurately record the body temperature data of each person, screen out the suspected abnormal body temperature personnel, and conduct big data analysis and processing to find out the indirect or direct contact with the patients.


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Human body temperature measurement at the entrance of the company's gate (with blackbody calibration)

In clinical diagnosis, human body is a natural infrared radiation source, and the infrared radiation band of human skin is 3-50mm. When the human body is ill, the heat balance of human body is destroyed, so measuring the change of human body temperature is an important index for clinical diagnosis of diseases. The thermal imager can display and record the temperature distribution of human body, compare the thermal image of human body in pathological condition with that in normal physiological condition, and judge the pathological condition from the abnormal change of thermal image.

2、Fire fighting

(1)Fire monitoring: Real time automatic measurement of the gray value of the object's temperature in the field of view (the maximum range that can be observed by the camera), if it is found that the temperature rise exceeds the alarm set by the system, it can automatically alarm.

(2)Early warning of fire prevention: Infrared thermal imager can be used for long-distance and large-scale monitoring. If abnormal high temperature points are found, these hidden fires can be found quickly and effectively, and the fire can be eliminated at the beginning.

(3)Fire scene command: When a fire occurs, the smoke generated by the fire is very large, which often covers the real fire point and the spread trend of the fire. Infrared thermal imager has a strong ability to penetrate the smoke, which can effectively find the real ignition point and quickly search and rescue the trapped people and animals.

(4)After fire monitoring: After the fire is put out, there may be sporadic fires left, even underground fires with hidden nature. It is difficult to find them with human eyes and visible light cameras. The hidden fire infrared thermal imager can quickly and accurately detect the potential danger.

(5)Monitoring role: Monitor smoking and illegal use of fire in the national forest protection area and other non-smoking and non-fire areas. It can even be carried to the UAV to observe the forest in a large area at high altitude to eliminate the hidden fire. It can also be linked with the public security department, forestry bureau and other relevant departments to prevent all illegal use of fire to protect the national property security.

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Thermal image of forest fire scene


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Fire control of Forest Scenic Spot


3、 Assist driving in the night

How to ensure the safety of driving at night has always been a big problem. The visible light camera of the dash cam is greatly affected by light, weather and so on. Based on the application of infrared thermal imaging technology, vehicles and pedestrians can be clearly observed hundreds of meters in advance in case of poor light at night. The safety of driving at night can be effectively improved by enlarging the observation sight range of driving at night.

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Figure 10 Driving vehicles on the highway at night


4、Maritime affairs

Using infrared thermal imager at sea can quickly find and locate targets in all-weather environment, and it is suitable to cooperate with radar to search and obtain evidence of targets at sea. Support the real-time return of the moving speed and orientation information of the target object, realize 24-hour observation, search and monitoring of the external conditions of the cabin (the environment before and after the ship, marine resources, marine vessels, floating objects, etc.), and conduct real-time video recording to provide detailed information for the relevant departments to process the situation. The infrared camera is often installed in the environment of offshore drilling platform, ocean cruises, Coast Guard patrol ships, etc. it plays an important role in marine production, border investigation, marine search and rescue, criminal smuggling, navigation night vision navigation.

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Maritime search and rescue 


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Ships at sea


5、Petrochemicals

Many important equipment in petrochemical industry work under high temperature and high pressure, and most of them need long-term operation. Due to the inherent danger (inflammable and explosive) of petrochemical production, the industrial production department has always attached great importance to the strict on-line monitoring of the production process, the inspection of the reliability and safety of the equipment itself and the regular shutdown and overhaul. With infrared thermal imaging technology and visible light monitoring, the two advantages complement each other to achieve the purpose of 24 hours monitoring and reduce the human cost, greatly improving the production efficiency and the safety of personnel in the production process.

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Thermal imager in petroleum production


6、Power industry

Electric equipment is heated due to the effect of current and voltage. If the equipment is abnormal, the heater will intensify the heat release or abnormal performance, and its heat distribution image is also different from the normal situation. Through the transformation of the use of infrared thermal imaging technology, from fixed to mobile robot equipped with infrared thermal imaging device and high-definition video linkage, data processing based on artificial intelligence algorithm and model effectively reduces the cost of human verification. Monitoring the working status of equipment, collecting data, analyzing historical data, finding problems and defects in high-voltage and high-risk places such as substations, and reporting them, so as to timely find and deal with them, improve the stability of transmission and transformation grid operation, and ensure the safety of national production and power consumption.

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Power equipment fault diagnosis (1)


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Power equipment fault diagnosis (2) 


7、Metallurgical material manufacturing

     In the manufacturing process of metallurgical materials, we should pay attention to the accuracy of temperature control, use infrared thermal imaging technology to observe the material temperature in real time, avoid the uneven distribution of material heating, control the internal temperature of metallurgical blast furnace, accurately control every link in the material manufacturing process, and improve the qualification rate in the material manufacturing process.

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Steel bar smelting and forming


8、Construction industry

In the construction industry, infrared thermal imaging technology is mostly used to detect the quality problems of buildings, such as layering, hollowing, heat preservation and energy saving, wall and house leakage, inside or underground pipelines and pipes. For some cracks, we can find the problem through the thermal image display.

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Image of window with poor thermal insulation


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Floor heating leakage damage


四、Future development plan of infrared thermal imaging technology

 (1) Because of the limitation of technology, the high cost of infrared thermal imaging technology makes it difficult to apply to the field of ordinary people. With the rapid development of science and technology in China, Zhejiang Dali Technology Co., Ltd has independently developed the world's leading 6 million pixel infrared detector. The primary task in the future is to reduce the cost so that it can be integrated into the civil field.

(2)With the advent of higher pixel level infrared thermal imager, the requirement of network transmission speed will be higher. 5g technology has the characteristics of high speed, large capacity, low delay and high reliability, which can improve the transmission speed of thermal imaging video picture and effectively solve this problem.

(3)In the aspect of security monitoring, we can consider the development of deep learning method of human motion recognition, further mining the data of thermal imaging video, and collecting effective information. In combination with the existing visible light security system, infrared thermal imaging is used to make up for the monitoring defects and deficiencies under the condition of no visible light, and the multi-dimensional connection characteristics of 5g technology are used to expand the scope of security monitoring, which can provide more dimensional and comprehensive reference data for the intelligent security cloud on the IT system, so as to help the security cloud to make more accurate, more effective and faster security decision.

(4)Many companies have developed a variety of robots based on the artificial intelligence technology of fire, which are applied to all walks of life. The form of thermal imager + visible light camera is used to replace the human eye to observe and monitor. The thermal imaging technology can get the information that the human eye cannot observe.


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