Analysis on Modern Application of Infrared Thermometer Technology

The temperature measurement principle of an infrared thermometer is to convert the radiant energy of the infrared emitted by an object into an electrical signal. The magnitude of the infrared radiant energy corresponds to the temperature of the object itself. According to the magnitude of the converted electrical signal, the temperature of the object can be determined. Infrared temperature measurement technology has been developed to be able to scan and measure the temperature of a thermally changing surface, determine its temperature distribution image, and quickly detect the hidden temperature difference. This is an infrared thermal imager. Infrared thermal imaging cameras were first used in the military. American TI company developed the world’s first infrared scanning reconnaissance system in 19″. Later, infrared thermal imaging technology has been used in aircraft, tanks, warships and other weapons in Western countries. , As a thermal sighting system for reconnaissance targets, it greatly improves the ability to search and hit targets. The infrared thermal imaging camera produced by Swedish AGA is in a leading position in civilian technology.

Infrared thermometer is composed of optical system, photodetector, signal amplifier, signal processing, Display output and other parts. The optical system concentrates 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 of the thermometer and its position. 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 being corrected in accordance with the internal therapy algorithm of the instrument and the target emissivity.

In nature, all objects whose temperature is higher than absolute zero are constantly emitting infrared radiation energy to the surrounding space. The size of the infrared radiation energy of an object and its distribution according to wavelength have a very close relationship with 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 on which infrared radiation temperature measurement is based.

A black body is an idealized radiator, which absorbs all wavelengths of radiant energy, without energy reflection and transmission, and its surface emissivity is 1. However, the actual objects that exist in nature are almost not black bodies. In order to clarify and obtain the distribution of infrared radiation, an appropriate model must be selected in theoretical research. This is the quantized oscillator model of body cavity radiation proposed by Planck. The Planck’s law of black body radiation is derived, that is, the black body spectral radiance expressed in wavelength, which is the starting point of all infrared radiation theories, so it is called the law of black body radiation. In addition to the radiation wavelength of all actual objects and the temperature of the object, the radiation of all actual objects is also related to factors such as the type of material that constitutes the object, the preparation method, the thermal process, and the surface state and environmental conditions.

Infrared temperature measurement adopts a point-by-point analysis method, that is, the thermal radiation of a local area of ​​the object is focused on a single detector, and the radiation power is converted into temperature through the known emissivity of the object. Due to the different objects to be detected, measurement range and use occasions, the appearance design and internal structure of infrared thermometers are not the same, but the basic structure is roughly similar, mainly including optical system, photodetector, signal amplifier and signal processing, display output And other parts of the composition. Infrared radiation emitted by the radiator. Entering the optical system, the infrared radiation is modulated by the modulator and transformed into the corresponding electrical signal by the detector. The signal passes through the amplifier and signal processing circuit, and is converted into the temperature value of the measured target after being corrected according to the algorithm in the instrument and the target emissivity.

There are three major categories of infrared thermometers: (1) Human infrared thermometer: Forehead infrared thermometer is a thermometer that uses infrared receiving principles to measure the human body. When in use, it is only necessary to conveniently align the detection window to the forehead position, and the human body temperature can be measured quickly and accurately. (2) Industrial infrared thermometer: Industrial infrared thermometer measures the surface temperature of an object, its light sensor radiates, reflects and transmits energy, and then the energy is collected and focused by the probe, and then other circuits convert the information into reading display On the machine, the laser light equipped with this machine is more effective in aiming at the measured object and improving the measurement accuracy. (3) Animal Infrared Thermometer Thermometer for Animal Husbandry: Based on the Planck principle, the veterinary infrared non-contact thermometer can correct the temperature difference between the body surface temperature and the actual temperature by accurately measuring the body surface temperature of a specific part of the animal body surface. Can accurately show the individual body temperature of the animal.

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 are low or varying emissivity. In the high temperature area, the best wavelength for measuring metal materials is near infrared, and the wavelength of 0.18-1.0 μm can be used. Wavelengths of 1.6μm, 2.2μm and 3.9μm can be selected for other temperature zones. Since some materials are transparent at a certain wavelength, infrared energy will penetrate these materials, and a special wavelength should be selected for this material. For example, the internal temperature of the measuring glass should be 10μm, 2.2μm and 3.9μm (the glass to be measured should be very thick, otherwise it will pass through) wavelengths; the internal temperature of the measuring glass should be 5.0μm; the wavelength of 8-14μm should be used for measuring the low area. ; Another example is the measurement of polyethylene plastic film with a wavelength of 3.43μm, and polyesters with a wavelength of 4.3μm or 7.9μm.

Determining the response time: The response time represents the reaction speed of the infrared thermometer to the measured temperature change. 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 photodetector, signal processing circuit and display system. The response time of the new infrared thermometer can reach 1ms. This is much faster than the contact temperature measurement method. If the speed of the target is fast or when measuring a rapidly heated target, a fast-response infrared thermometer should be selected, otherwise the signal response will not be adequate and the measurement accuracy will be reduced. However, not all applications require a fast-response infrared thermometer. When there is thermal inertia in a static 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 adapted to the conditions of the measured target.

The optical resolution is determined by the ratio of D to S, which is the ratio of the distance D from the thermometer to the target and the diameter S of the measuring spot. If the thermometer must be installed far away from the target due to environmental constraints, and you want to measure a small target, you should choose a thermometer with high optical resolution. The higher the optical resolution, the higher the D:S ratio and 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 are low or varying emissivity. In the high temperature area, the best wavelength for measuring metal materials is near infrared, and the wavelength of 0.18-1.0 μm can be used. Wavelengths of 1.6μm, 2.2μm and 3.9μm can be selected for other temperature zones. Since some materials are transparent at a certain wavelength, infrared energy will penetrate these materials, and a special wavelength should be selected for this material. For example, choose the wavelengths of 1.0μm, 2.2μm and 3.9μm to measure the internal temperature of the glass (the glass to be measured must be very thick, otherwise it will pass through); to measure the internal temperature of the glass, choose the wavelength of 5.0μm; to measure the low area, choose the wavelength of 8-14μm It is appropriate; for example, the wavelength of 3.43μm is used for the measurement of polyethylene plastic film, and the wavelength of 4.3μm or 7.9μm is used for the polyester type.

Determining the response time: The response time represents the reaction speed of the infrared thermometer to the measured temperature change. 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 photodetector, signal processing circuit and display system. The response time of Guangzhou Hongcheng Hongkong CEM brand infrared thermometer can reach 1ms. This is much faster than the contact temperature measurement method. If the speed of the target is fast or when measuring a rapidly heated target, a fast-response infrared thermometer should be selected, otherwise the signal response will not be adequate and the measurement accuracy will be reduced. However, not all applications require a fast-response infrared thermometer. When there is thermal inertia in a static 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 adapted to the conditions of the measured target.

Signal processing function: Measuring discrete processes (such as parts production) and continuous processes are different, and infrared thermometers are required to have signal processing functions (such as peak hold, valley hold, and average value). For example, when measuring the temperature of the glass on the conveyor belt, it is necessary to use the peak value hold, and the temperature output signal is transmitted to the controller.

Environmental conditions consideration: The environmental conditions in which the thermometer is located have a great influence on the measurement results, which should be considered and properly resolved, otherwise it will affect the temperature measurement accuracy and even cause damage to the thermometer. When the ambient temperature is too high, dust, smoke and steam are present, the manufacturer’s protective cover, water cooling, air cooling system, air purifier and other accessories can be selected. These accessories can effectively solve the environmental impact 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 and become tough, a two-color thermometer is the best choice. Under noise, electromagnetic field, vibration, or inaccessible environmental conditions, or other harsh conditions, the fiber optic two-color thermometer is the best choice.

In applications with sealed or hazardous materials (such as containers or vacuum boxes), the thermometer observes through the window. The material must have sufficient strength 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 choose the appropriate installation location and window material to avoid mutual influence. In low temperature measurement applications, Ge or Si materials are usually used as windows, which are opaque to visible light, and human eyes cannot observe the target through the window. If the operator needs to pass through the window target, optical materials that transmit both infrared radiation and visible light should be used. For example, optical materials that transmit 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, easy to operate, and easy to be used by the operator. Among them, the portable infrared thermometer is a small, light, portable, and portable infrared thermometer that integrates temperature measurement and display output. Temperature measuring instruments can display temperature and output various temperature information on the display panel, and some can be operated by remote control or computer software programs.

In the case of harsh and complex environmental conditions, you can choose a separate system for the temperature measuring head and the display to facilitate installation and configuration. The signal output form that matches with the current control equipment can be selected. Calibration of infrared radiation thermometer: The infrared thermometer must be calibrated to make it correctly display the temperature of the measured target. If the temperature measuring instrument used appears to be out of tolerance during use, it must be returned to the manufacturer or repair center for recalibration.