Introduction to the development, theoretical basis, principle, advantages of infrared thermometers

From: Shanghai Zhengqianwang  Date:2018-06-05 18:13:08  Hits:715  Belong to:company news
The most basic principle of infrared thermometer's temperature measurement is to convert the radiant energy of infrared radiation emitted by an object into an electrical signal. The amount of infrared radiation energy corresponds to the temperature of the object itself. According to the size of the converted electrical signal, the object's temperature can be determined. temperature.

Development of infrared thermometers:
As early as the end of the nineteenth century, people have begun to have a preliminary conceptual understanding of the design of infrared thermometers. Various concepts were proposed in the "Temperature" book published by Charles A. Darling in 1911, but the technology available until the 1930s transformed these concepts into practical measuring instruments. Since then, design has evolved over a long period of time and has accumulated a wealth of measurement and application expertise. At present, this infrared temperature measurement technology has been widely accepted and widely used in industry and research.

Theoretical basis of infrared thermometer:
Most infrared thermometer users are unlikely to need these formulas, but an understanding of the formulas will help you understand the interdependence of certain variables and help clarify the above. The formula on which the infrared temperature measurement is based is well established and well proven. The formula is as follows:
Kirchhoff's law when an object is in thermal equilibrium, the amount of absorption will equal the amount of emission.
Stephan Boltzmann Law The hotter an object is, the more infrared energy it emits.
With the increase of temperature, the Wien shift law shortens the wavelength of maximum energy emission.
The Planck equation describes the relationship between spectral emissivity, temperature, and radiant energy.

Principle of infrared thermometer:
Infrared thermometer is composed of optical system, photodetector, signal amplifier, signal processing and display output. The optical system gathers the target infrared radiation energy 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 according to the algorithm and target emissivity inside the instrument.
In nature, all objects whose temperature is above absolute zero constantly emit infrared radiation energy to the surrounding space. The amount of infrared radiation energy of an object and its distribution by wavelength are very closely related to its surface temperature. Therefore, by measuring the infrared energy radiated by the object itself, it can accurately determine its surface temperature, which is the objective basis on which infrared radiation temperature measurement is based.

The black body of the infrared thermometer is the basis for checking whether the temperature of the infrared thermometer is accurate (reference object).
   What is black body? Materials that do not reflect or transmit any infrared energy are called black bodies and do not exist naturally. However, for the purpose of theoretical calculations, the true black body value is 1.0. The closest approximation to blackbody emissivity 1.0 that can be achieved in real life is an IR opaque spherical cavity with a small tubular entrance.
    The black thermometer is an ideal radiator. It absorbs radiant energy of all wavelengths, there is no reflection and transmission of energy, and its emissivity on the surface is 1. However, practical objects existing in nature are almost not black bodies. In order to understand and obtain the distribution pattern of infrared radiation, a suitable model must be selected in theoretical research. This is the quantized oscillator model of body cavity radiation proposed by Planck. The Law of Planck Blackbody Radiation, that is, the blackbody spectral radiance expressed by wavelength, is derived. This is the starting point of all infrared radiation theories, so it is called the law of blackbody radiation. In addition to the radiation wavelength and temperature of the object, the amount of radiation of all actual objects is also related to the types of materials, preparation methods, thermal processes, surface conditions and environmental conditions. Therefore, in order to make the law of blackbody radiation applicable to all practical objects, it is necessary to introduce a proportionality factor related to material properties and surface conditions, that is, emissivity. The coefficient indicates how close the thermal radiation of the actual object is to the radiation of the black body, and its value is between zero and a value less than 1.
Different kinds of materials and gases have different emissivities, so they will emit at different intensities for a given temperature. The emissivity of a material or gas is a function of its molecular structure and surface characteristics. Unless the source of color is a substance that is fundamentally different from the body of the material, it is usually not a function of color. A practical example is metal coatings doped with large amounts of aluminum. Regardless of the color, most paints have the same emissivity, but the emissivity of aluminum varies widely, thus changing the emissivity of metallized paints.
Just like visible light, the more polished a surface is, the more infrared energy it reflects. Therefore, the surface characteristics of the material will also affect its emissivity. This is especially important in temperature measurement for infrared opaque materials, which inherently have low emissivity. Therefore, the emissivity of highly polished stainless steel sheets will be much lower than stainless steel sheets with rough, machined surfaces. This is because the grooves created by machining prevent reflection of as much IR energy as possible. In addition to the molecular structure and surface conditions, the third factor affecting the apparent emissivity of a material or gas is the wavelength sensitivity of the sensor, which is the spectral response of the sensor. As mentioned earlier, actual temperature measurements use only IR wavelengths between 0.7 and 20 microns. Within this total band, each sensor can only work in a narrow part of the band, such as 0.78 to 1.06 or 4.8 to 5.2 microns, the reason will be explained later.
According to the law of radiation, as long as the emissivity of a material is known, the infrared radiation characteristics of any object are known. The main factors affecting emissivity are: material type, surface roughness, physical and chemical structure, and material thickness.
When measuring the temperature of a target with an infrared radiation thermometer, first measure the amount of infrared radiation in the target's band, and then calculate the temperature of the target with the thermometer.

The advantages of infrared thermometer application technology include:
• Take measurements in hard-to-reach areas, such as built-in areas, transformers or production ovens.
• Non-contact infrared temperature measurement does not damage the product, does not pollute the environment, and is used for product quality control.
• Determine the temperature of moving objects, such as products on a conveyor belt.
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