Infrared detection (infrared radiation detection) principle and infrared thermometer

Infrared detection (infrared radiation detection) principle and infrared thermometer

The principle of infrared detection (infrared radiation detection)

The essence of infrared detection (infrared radiation detection) in the non-destructive testing technology method is to use non-contact infrared temperature recording method by utilizing the characteristics of infrared radiation of the object.
Infrared is an electromagnetic wave with the same nature as radio waves and visible light. The wavelength is between 0.76 and 100 μm. It can be divided into four categories: near-infrared, mid-infrared, far-infrared, and far-infrared, depending on the wavelength range. It is a continuous spectrum of electromagnetic waves. The location in the middle is the area between radio waves and visible light. Infrared radiation is one of the most widespread electromagnetic radiations in nature. It is based on the random movement of any molecule and atom that produces its own environment and constantly radiates out thermal infrared energy, movement of molecules and atoms. The more intense, the greater the energy of radiation, and conversely, the smaller the energy of radiation.

All objects whose absolute temperature is above -273.15K° will have infrared rays radiated into the surrounding space due to their own molecular motion. The infrared radiation energy of the object and its distribution according to wavelength and its surface temperature There is a very close relationship. After the infrared radiation detector converts the power signal radiated by the object into an electrical signal (measurement of the infrared energy emitted by the object itself), the surface temperature thereof can be accurately measured, or the output signal of the imaging device can be completely Correspondingly, the spatial distribution of the surface temperature of the scanned object is simulated, processed by the electronic system, and transmitted to the display screen to obtain a thermal image corresponding to the thermal distribution of the surface of the object. Using this method, we can achieve long-distance thermal state image imaging and temperature measurement and analysis and judgment of the target, which is the basic principle of infrared radiation detection.
Planck's Law of Blackbody Radiation: Blackbody is an idealized radiator that absorbs all wavelengths of radiation energy, no energy is reflected and transmitted, and its surface has an emissivity of 1. Although there is no real black body in nature, in order to understand and obtain the infrared radiation distribution law, we must choose a suitable model in the theoretical study. This is Planck's quantum oscillator model of body cavity radiation, which leads to the Langack's law of blackbody radiation, ie, the spectral radiation of blackbody expressed in terms of wavelength, is the starting point of all infrared radiation theory, so it is abbreviated as the law of blackbody radiation.
The actual objects that exist in nature are hardly black. The radiation amount of all actual objects depends on the wavelength of the radiation and the temperature of the object, and also relates to the types of materials constituting the object, the preparation method, the thermal process, and the surface conditions and environmental conditions. Therefore, in order for the law of blackbody radiation to apply to all real objects, a proportional coefficient, namely the emissivity, must be introduced in relation to the properties of the material and the surface state. The coefficient represents the degree of closeness of the thermal radiation of the actual object to blackbody radiation, with values ​​between zero and less than one. According to the law of radiation, as long as the emissivity of the material is known, the infrared radiation characteristics of any object are known.
According to the material's thermal diffusivity (emissivity): a = k / (ρ · c), where: thermal conductivity of the k-material; ρ-mass bulk density; c-specific heat of the material known thermal diffusivity (emission Rate) is related to the nature of the material. For a uniform, defect-free material, a is a constant. When a defect exists in a uniform material, the defect corresponds to a material having another thermal diffusivity, and thus the thermal state of the defective portion and the non-defective portion is different, and the surface of the material is different. The difference in heat conduction forms a temperature gradient in time and space on the surface of the material, ie temperature disturbance: ΔT = Tf - T, where: ΔT - temperature disturbance; Tf - surface temperature of the material at the defect; T - no defect The surface temperature of the material. △T is not only related to the thermal diffusivity of the material, but also related to the geometric dimensions of the defect and the depth of burial.
When the temperature difference of the material surface is greater than the minimum measurable temperature of the infrared thermal imager, the thermal image of the surface temperature distribution of the test piece can be observed on the thermal imager, and the presence or absence of defects in the material can be analyzed and judged so as to achieve the purpose of detection. In other words, the main factors that affect the emissivity are related to the type of material, surface roughness, physical and chemical structure, and material thickness.
If the sample is injected with a certain amount of heat (active) by heating (artificial or natural heating) on ​​the back or front of the sample, the infrared detection is performed by active infrared detection. Infrared detection of passive imaging of its temperature field by means of its own thermal radiation is passive infrared detection. For example, using the heat source (passive type) existing in the test piece itself, when there is a defect inside the test piece, since the thermal conductivity of the defect is different from the thermal conductivity of the parent material, the difference can be measured to detect the defect.
At present, the most commonly used infrared detection method is still more passive infrared detection. In addition to passive infrared detection in the industry for equipment, components, etc. hot spot detection, military applications such as infrared night vision, infrared sight, etc., can be used in medicine to check abnormal human body temperature regions, such as the 2003 Sa During SARS epidemic period, it is a typical application example to monitor whether there is fever in the forehead of a person in a crowded place such as an airport or a station.
Infrared detection (infrared diagnosis technology) is an on-line monitoring detection technology. It integrates photoelectric imaging technology, computer technology, and image processing technology. It displays the thermal image on a fluorescent screen by receiving infrared rays (infrared radiation) emitted by an object. , In order to accurately determine the temperature distribution of the surface of the object, with the advantages of accurate, real-time, fast and so on. Because of the movement of its own molecules, any object constantly emits infrared heat energy to form a certain temperature field on the surface of the object, commonly known as “thermal image”. Infrared diagnostic technology precisely absorbs this infrared radiation energy, measures the temperature and temperature distribution of the surface of the device, and judges the heating condition of the device. At present, there are many test equipments that use infrared diagnostic technology, such as infrared thermometers, infrared thermal televisions, infrared thermal imagers, and so on. Infrared thermal television, infrared thermal imager and other devices can use thermal imaging technology to turn this invisible “thermal image” into a visible light image, so that the test effect is intuitive, high sensitivity, can detect the subtle thermal state changes of the device, accurately reflect The internal and external heat generation of the equipment is highly reliable, and it is very effective in detecting potential equipment hazards. Infrared thermal imaging systems have been widely used in power, firefighting, petrochemical, and medical applications.
Infrared temperature measurement technology plays an important role in product quality control and monitoring, online fault diagnosis, safety protection, and energy conservation.
The advantages of infrared detection technology are non-contact remote measurement, direct display of real-time images, high sensitivity, and high detection speed. The infrared thermal imager is simple in structure, safe in use, fast in information data processing, and can realize automatic detection and permanent recording, and the surface finish of the test piece has little influence on the test. Therefore, infrared detection has been widely used in metal, non-metallic components, especially for low thermal conductivity materials, such as the detection of composite materials, adhesive structures and laminated structure of holes, cracks, delamination and debonding defects, but also Can be used for quality inspection of polymers, rubber, nylon, plastic board, asbestos, plexiglass, cement products, ceramics, etc., for solid rocket motor monoliths or housings, aero-engine nozzles, turbine blades, and electronic equipment. Temperature monitoring (such as printed circuit boards, integrated circuit blocks, etc.) can check the quality, brazing quality and working conditions of components, and in the power equipment (such as the reversing contacts of the generator sets, transformers, high-voltage porcelain bottles, high-voltage switches and Inspection of hotspots such as contacts, power transmission lines, etc., detection of hot wheels of railway vehicles, structural abnormalities in walls of buildings and the quality of wall finishes, and applications in petrochemicals, heating, and energy conservation. .
The disadvantage of infrared detection is that the detection sensitivity is related to the thermal emissivity. Therefore, the interference of the surface and background radiation of the test piece is affected by the size of the defect and the depth of burial. The resolution of the original sample is poor, and the shape and size of the defect cannot be accurately determined. And location. The time-temperature relationship is strictly required in the test. It requires the use of a detector such as liquid nitrogen cooling (the new infrared camera does not require liquid nitrogen or high-pressure gas cooling, but thermoelectric cooling, battery-powered). The interpretation of test results is more complex and requires reference standards. Test operators need to be trained. A new generation of infrared thermal imager has been able to integrate temperature measurement, modification, analysis, image acquisition, and storage into a single unit. The weight of the infrared camera is less than 7 kilograms, and the function, accuracy and reliability of the instrument have been significantly improved.

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There are mainly the following considerations when choosing infrared thermometers :
(1) Performance indicators, such as:
Temperature measurement range: Each type of thermometer has its own specific temperature measurement range, neither too narrow nor too wide. Generally speaking, the narrower the temperature measurement range, the higher the output signal resolution of the monitoring temperature. Accuracy and reliability are easily solved. Too wide temperature measurement range will reduce the accuracy of temperature measurement. Operating wavelength: According to the law of black body radiation, the change of the radiant energy caused by temperature in the short wavelength band of the spectrum will exceed the change of the radiant energy caused by the emissivity error. Therefore, the temperature measurement It should be better to use shortwave as far as possible, but it must also take into account the emissivity factor of the tested object:
The emissivity and surface properties of the target material determine the corresponding wavelengths of the spectrometer's spectrum, and low or varying emissivity for high reflectivity alloy materials. In the high-temperature area, the most suitable wavelength for measuring metal materials is near-infrared, which can be 0.8 to 1.0 μm. Other temperature zones are available in 1.6, 2.2 and 3.9μm. Since some materials are transparent at a certain wavelength, infrared energy penetrates these materials. Special wavelengths should be selected for this material. For example, the internal temperature of the glass is measured at wavelengths of 1.0, 2.2 and 3.9 μm (the measured glass is very thick, Otherwise it will pass through); measuring the glass surface temperature selection of 5.0μm; measuring low temperature zone selection of 8 ~ 14μm is appropriate, and if the measurement of polyethylene plastic film is selected 3.43μm, polyester selection of 4.3 or 7.9μm, the thickness of more than 0.4mm selection 8~14μm, for example, the CO in the flame is measured with a narrow band of 4.64μm, and the NO2 in the flame is measured with 4.47μm.
Spot size: The area of ​​the measuring point of the pyrometer is called the “spot size”. In order to obtain an accurate temperature reading, the distance between the pyrometer and the test target must have a suitable range, and the farther from the target The larger the spot size. Therefore, attention should be paid to the ratio of the distance to the spot size, or D:S, in the application. When determining the measurement distance, care should be taken to make the target diameter equal to or greater than the spot size being measured. If the target is smaller than the measured spot size, the thermometer will measure the temperature of the background object at the same time, which reduces the accuracy of the reading.
Infrared thermometer based on the principle can be divided into monochrome thermometer and two-color thermometer (radiometer colorimeter). For monochromatic thermometers, when measuring temperature, the measured target area should be full of thermometer field of view. It is generally recommended that the measured target size exceed 50% of the size of the field of view. If the target size is smaller than the field of view, background radiation energy will enter the field of view of the thermometer to interfere with temperature readings, causing errors. For two-color pyrometers, the temperature is determined by the ratio of the radiant energy in two separate wavelength bands. Therefore, when the target to be measured is too small to fill the field of view, there is smoke, dust, and blockage on the measurement path. When there is attenuation of the radiation energy, it will not have a major impact on the measurement results. Targets that are small and in motion or in motion sometimes move within the field of view, or may partially move out of the field of view. Under these conditions, the use of a two-color pyrometer will be more appropriate. If it is impossible to aim directly between the thermometer and the target, and the measuring channel is bent, narrow, and obstructed, it is appropriate to use a bicolor optical fiber thermometer. This is due to its small diameter, flexibility, and ability to transmit optical radiation energy over bent, blocked, and folded channels, so that targets that are difficult to access, harsh, or near an electromagnetic field can be measured.
The distance coefficient (optical resolution) is determined by the ratio of D:S, which is the ratio of the distance D from the probe to the target of the pyrometer to the diameter of the spot. If the pyrometer must be installed far away from the target due to environmental conditions, but also to measure a small target, you should choose a high optical resolution thermometer, the higher the optical resolution, which increases the D: S ratio. If the thermometer is far from the target and the target is small, a thermometer with a high distance coefficient should be selected. For a fixed-focus pyrometer, the minimum spot size is at the focal point of the optical system, and the spot near and far from the focal spot will increase. There are two distance coefficients. Therefore, in order to accurately measure temperature at a distance close to and away from the focus, the target size to be measured should be greater than the spot size at the focus, and the zoom thermometer has a minimum focus position that can be adjusted according to the distance to the target. If D:S is increased, the received energy will be reduced. If the receiving aperture is not increased, the distance coefficient D:S is difficult to increase.
Response time: It indicates the reaction speed of the infrared thermometer to the measured temperature change and is defined as the time required to reach 95% of the final reading. It is related to the time constants of the photodetector, the signal processing circuit and the display system. If the target's moving speed is fast or the target of rapid heating is measured, a fast-response infrared thermometer should be selected, otherwise, a sufficient signal response cannot be achieved and the measurement accuracy will be reduced. When there is thermal inertia for a stationary or target thermal process, the response time of the thermometer can be relaxed. Therefore, the choice of the response time of the infrared thermometer should be compatible with the situation of the target to be measured, mainly based on the target's movement speed and the target's temperature change speed. For static targets or target references in thermal inertia, or the speed of existing control equipment is limited, the response time of the thermometer can be relaxed.
Signal processing function: In view of different discrete processes (such as part production) and continuous process, it is required that the infrared thermometer should have multiple signal processing functions (such as peak hold, valley hold, average), such as when measuring the bottle temperature on the conveyor belt It is necessary to use the peak hold function to send the output signal of its temperature to the controller, otherwise the thermometer will read the lower temperature value between the bottles. If peak hold is used, set the thermometer response time slightly longer than the interval between bottles so that at least one bottle is always in the measurement.
(2) Environment and working conditions, such as:
Protection accessories: The environmental conditions in which the thermometer is located have a great influence on the measurement results. If not properly solved, it will affect the accuracy of temperature measurement and even cause damage. When the ambient temperature is high, dust, smoke, and steam are present, accessories such as protective covers, water cooling, air cooling systems, and air purifiers can be used. These accessories can effectively solve the environmental impact and protect the thermometer to achieve accurate temperature measurement. When noise, electromagnetic fields, vibrations or inaccessible environmental conditions, or other harsh conditions, smoke, dust or other particles reduce the measured energy signal, it is more appropriate to use fiber optic dual-color thermometer.
Window material: In sealed or hazardous material applications (such as containers or vacuum boxes), the thermometer needs to be observed through a window. The window material must have sufficient strength and pass the operating wavelength range of the thermometer used. It is also necessary to determine whether the operator also needs to observe through the window, so choose the proper installation position and window material to avoid mutual influence. In cryogenic measurement applications, Ge or Si material is usually used as a window, not visible light, and the human eye cannot observe the target through the window. If the operator needs to observe the target through the window, an optical material that transmits both infrared radiation and visible light, such as ZnSe or BaF2, should be used as the window material.
When there is an easy ran gas in the working environment of the pyrometer, a safe infrared thermometer can be used to measure and monitor safely in a certain concentration of lean gas environment.
In the case of harsh and complex environmental conditions, a separate system of temperature measuring heads and displays can be selected to facilitate the installation and configuration and to select the signal output format that matches the current control equipment.
(3) Other options, such as simple operation, easy use, maintenance and calibration performance, and price.
For example, a portable infrared thermometer, which is a small, lightweight, portable instrument for measuring temperature by integrating temperature measurement and display output, displays temperature and output various temperature information on the display panel, and some It can be operated by remote control or through a computer software program.
Note: The infrared radiation thermometer must be calibrated so that it can correctly display the temperature of the measured target. If the thermometer used in the use of temperature measurement error occurs, you need to return to the manufacturer or repair center to re-calibrate.