Briefly describe the working principle of medical infrared thermometer
Infrared thermometer by the optical system, photodetector, signal amplifier and signal processing, display output and other components. Optical system convergence of its field of view of the target infrared radiation energy, the size of the field of view by the thermometer's optical components and their location to determine. The infrared energy is focused on the photodetector and transformed into a corresponding electrical signal. This signal is converted into the temperature of the target by amplifiers and signal processing circuits and corrected for the target emissivity according to the algorithms used in the instrument.
In nature, all objects with temperatures above absolute zero are constantly emitting infrared radiation energy into the surrounding space. The size of the object's infrared radiation energy and its distribution by wavelength - and its surface temperature has a very close relationship. Therefore, through the measurement of the infrared energy radiated by the object itself, it will be able to accurately determine its surface temperature, which is the objective basis for infrared radiation thermometry.
Blackbody is an idealised radiator, which absorbs all wavelengths of radiant energy, no energy reflection and transmission, the emissivity of its surface is 1 ． However, the existence of actual objects in nature, almost all are not a blackbody, in order to clarify and obtain the distribution of infrared radiation law, in the theoretical study must choose the appropriate model, which is Planck put forward by the body cavity radiation quantisation vibronic model, which led to Planck's law of the blackbody radiation, that is, the wavelength of the blackbody spectral radiance, which is the starting point for the theory of all infrared radiation, it is called the blackbody radiation The law of blackbody radiation. In addition to the wavelength of radiation and the temperature of the object, the amount of radiation of all real objects is also related to the type of material, preparation method, thermal process, and the surface state and environmental conditions and other factors.
Therefore, in order to make the blackbody radiation law applicable to all real objects, it is necessary to introduce a proportionality coefficient, i.e., the emissivity, which is related to the nature of the material and the state of the surface. This coefficient expresses the proximity of the thermal radiation of a real object to the blackbody radiation and has a value between zero and a value less than one. 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 using infrared radiation thermometer to measure the temperature of the target first of all to measure the target in its band range of infrared radiation, and then by the thermometer to calculate the temperature of the target to be measured. Single-colour pyrometers are proportional to the amount of radiation in the band; two-colour pyrometers are proportional to the ratio of the amount of radiation in the two bands.
