Tips: Working Principle of Infrared Thermometer (Reprinted)

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Tips: Working Principle of Infrared Thermometer (Reprinted) [Copy link]

Working principle of infrared thermometer (reprinted from gkong.com)
Understanding the working principle, technical indicators, environmental working conditions, operation and maintenance of infrared thermometers is to help users correctly select and use infrared thermometers.
All objects with a temperature higher than absolute zero are constantly emitting infrared radiation energy to the surrounding space. The infrared radiation characteristics of an object - the size of the radiation energy and its distribution by wavelength - are closely related to its surface temperature. Therefore, by measuring the infrared energy radiated by the object itself, its surface temperature can be accurately determined, which is the objective basis for infrared radiation temperature measurement.
Blackbody radiation law: A blackbody is an idealized radiator that absorbs radiation energy of all wavelengths, has no energy reflection or transmission, and its surface emissivity is 1. It should be pointed out that there is no real blackbody in nature, but in order to clarify and obtain the law of infrared radiation distribution, a suitable model must be selected in theoretical research. This is the quantized oscillator model of body cavity radiation proposed by Planck, which leads to Planck's law of blackbody radiation, that is, the blackbody spectral radiance expressed in wavelength. This is the starting point of all infrared radiation theories, so it is called the blackbody radiation law.
The influence of object emissivity on radiation temperature measurement: Almost all actual objects in nature are not black bodies. The radiation of all actual objects depends not only on the radiation wavelength and the temperature of the object, but also on the type of material, preparation method, thermal process, surface state and environmental conditions that constitute the object. Therefore, in order to make the blackbody radiation law applicable to all actual objects, a proportional coefficient related to the material properties and surface state must be introduced, namely the emissivity. This coefficient indicates the degree of closeness between the thermal radiation of the actual object and the blackbody radiation, and its value is between zero and a value less than 1. According to the radiation law, as long as the emissivity of the material is known, the infrared radiation characteristics of any object are known.
The main factors affecting the emissivity are: material type, surface roughness, physical and chemical structure and material thickness.
When using an infrared radiation thermometer to measure the temperature of a target, the infrared radiation of the target within its band must first be measured, and then the temperature of the target to be measured is calculated by the thermometer. The monochromatic thermometer is proportional to the radiation within the band; the two-color thermometer is proportional to the ratio of the radiation between the two bands.
Infrared system: Infrared thermometers are composed of optical systems, photodetectors, signal amplifiers, signal processing, display output and other parts. The optical system gathers the infrared radiation energy of the target within its field of view. The size of the field of view is determined by the optical parts and their positions of the thermometer. The infrared energy is focused on the photodetector and converted into a corresponding electrical signal. The signal passes through the amplifier and signal processing circuit, and is converted into the temperature value of the measured target after correction according to the algorithm and target emissivity of the instrument.
The selection of infrared thermometers can be divided into three aspects: performance indicators, such as temperature range, spot size, working wavelength, measurement accuracy, response time, etc.; environmental and working conditions, such as ambient temperature, window, display and output, protective accessories, etc.; other options, such as ease of use, maintenance and calibration performance, and price, also have a certain impact on the selection of thermometers. With the continuous development of technology, the best design and new progress of infrared thermometers provide users with various functions and multi-purpose instruments, expanding the choice.
Determine the temperature measurement range: The temperature measurement range is the most important performance indicator of the thermometer. For example, Raytek products cover a range of -50℃- +3000℃, but this cannot be accomplished by one type of infrared thermometer. Each type of thermometer has its own specific temperature measurement range. Therefore, the user's measured temperature range must be considered accurately and comprehensively, neither too narrow nor too wide. According to the blackbody radiation law, the change in radiation energy caused by temperature in the short-wave band of the spectrum will exceed the change in radiation energy caused by emissivity error. Therefore, it is better to use short waves when measuring temperature.
Determine the target size: Infrared thermometers can be divided into monochrome thermometers and two-color thermometers (radiation colorimetric thermometers) according to the principle. For monochrome thermometers, when measuring temperature, the area of the target to be measured should fill the field of view of the thermometer. It is recommended that the size of the target to be measured should exceed 50% of the field of view. If the target size is smaller than the field of view, the background radiation energy will enter the field of view of the thermometer and interfere with the temperature reading, causing errors. On the contrary, if the target is larger than the field of view of the thermometer, the thermometer will not be affected by the background outside the measurement area.
For Raytek dual-color thermometer, the temperature is determined by the ratio of the radiation energy in two independent wavelength bands. Therefore, when the measured target is small and does not fill the scene, and there is smoke, dust, and obstruction in the measurement path that attenuates the radiation energy, it will not affect the measurement results. Even when the energy is attenuated by 95%, the required temperature measurement accuracy can still be guaranteed. For targets that are small, in motion or vibrating; sometimes moving in the field of view, or may partially move out of the field of view, under this condition, using a dual-color thermometer is the best choice. If it is impossible to aim directly between the thermometer and the target, and the measurement channel is curved, narrow, or blocked, a dual-color fiber optic thermometer is the best choice. This is because its small diameter and flexibility can transmit optical radiation energy on curved, blocked, and folded channels, so it can measure targets that are difficult to approach, in harsh conditions, or close to electromagnetic fields.
Determine the optical resolution (distance sensitivity)
The optical resolution is determined by the ratio of D to S, which is the ratio of the distance D between the thermometer and the target to the diameter S of the measurement spot. If the thermometer must be installed far away from the target due to environmental conditions, and a small target needs to be measured, a thermometer with high optical resolution should be selected. The higher the optical resolution, that is, the larger the D:S ratio, the higher the cost of the thermometer.
Determine the wavelength range: The emissivity and surface characteristics of the target material determine the spectral response or wavelength of the thermometer. For high reflectivity alloy materials, there is a low or variable emissivity. In high temperature areas, the best wavelength for measuring metal materials is near infrared, and a wavelength of 0.18-1.0μm can be selected. For other temperature zones, 1.6μm, 2.2μm and 3.9μm wavelengths can be selected. Since some materials are transparent at certain wavelengths, infrared energy will penetrate these materials, and a special wavelength should be selected for such materials. For example, to measure the internal temperature of glass, wavelengths of 1.0μm, 2.2μm and 3.9μm (the glass to be measured must be very thick, otherwise it will be transparent) are used; to measure the internal temperature of glass, wavelengths of 5.0μm are used; to measure low areas, wavelengths of 8-14μm are appropriate; for example, to measure polyethylene plastic film, wavelengths of 3.43μm are used, and for polyester, wavelengths of 4.3μm or 7.9μm are used. For thicknesses exceeding 0.4mm, wavelengths of 8-14μm are used; for example, to measure CO2 in flames, wavelengths of 4.24-4.3μm are used, for CO in flames, wavelengths of 4.64μm are used, and for NO2 in flames, wavelengths of 4.47μm are used.
Determine the response time: Response time indicates the speed at which the infrared thermometer reacts to changes in the measured temperature. It is defined as the time required to reach 95% of the energy of the final reading. It is related to the time constant of the photoelectric detector, signal processing circuit and display system. Raytek's new infrared thermometer has a response time of up to 1ms. This is much faster than the contact temperature measurement method. If the target moves very fast or when measuring a rapidly heated target, a fast-response infrared thermometer should be used, otherwise the signal response will not be sufficient, which will reduce the measurement accuracy. However, not all applications require a fast-response infrared thermometer. For static or thermal processes with thermal inertia of the target, the response time of the thermometer can be relaxed. Therefore, the choice of infrared thermometer response time should be adapted to the situation of the target being measured.
Signal processing function: Measuring discrete processes (such as parts production) is different from continuous processes, and infrared thermometers are required to have signal processing functions (such as peak hold, valley hold, average value). For example, when measuring the temperature of glass on a conveyor belt, peak hold should be used, and the output signal of the temperature is transmitted to the controller.
Environmental conditions consideration: The environmental conditions of the thermometer have a great influence on the measurement results, which should be considered and properly resolved, otherwise the temperature measurement accuracy will be affected or even the thermometer will be damaged. When the ambient temperature is too high, dust, smoke and steam are present, accessories such as protective covers, water cooling, air cooling systems, and air purges provided by the manufacturer can be selected. These accessories can effectively solve environmental impacts and protect the thermometer to achieve accurate temperature measurement. When determining accessories, standardized services should be required as much as possible to reduce installation costs. When smoke, dust or other particles reduce the measurement energy signal, a two-color thermometer is the best choice. In noise, electromagnetic fields, vibration or difficult-to-access environmental conditions, or other harsh conditions, a fiber optic two-color thermometer is the best choice.
In sealed or hazardous material applications (such as containers or vacuum boxes), the thermometer is observed through a window. The material must be strong enough and pass the working wavelength range of the thermometer used. It is also necessary to determine whether the operator also needs to observe through the window, so the appropriate installation location and window material should be selected to avoid mutual influence. In low temperature measurement applications, Ge or Si materials are usually used as windows, which are not transparent to visible light, and the human eye cannot observe the target through the window. If the operator needs to see the target through the window, an optical material that is transparent to both infrared radiation and visible light, such as ZnSe or BaF2, should be used as the window material.
Simple operation and convenient use: The infrared thermometer should be intuitive, simple to operate, and easy to be used by operators. Among them, the portable infrared thermometer is a small, lightweight instrument that integrates temperature measurement and display output. It can display the temperature and output various temperature information on the display panel. Some can be operated by remote control or computer software programs.
In the case of harsh and complex environmental conditions, a system with separate temperature measuring head and display can be selected for easy installation and configuration. The signal output form that matches the current control equipment can be selected.
Calibration of infrared radiation thermometer: The infrared thermometer must be calibrated to enable it to correctly display the temperature of the target being measured. If the temperature measurement error of the thermometer used is exceeded during use, it needs to be returned to the manufacturer or maintenance center for recalibration.

xinzai

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ldk123

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tkjlx2006

Passing by

tmstd

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skyjeffery

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凯哥

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zxopenljx

Thank you for sharing

一杯星巴克

Well said. Thanks for sharing. Hahaha. Not bad