Selection of operating wavelength for infrared thermometer
In nature, when the temperature of an object is above absolute zero, due to its internal thermal motion, it continuously radiates electromagnetic waves in all directions, including infrared radiation with a wavelength range of 0.75-100 μ m. Infrared thermometers are made using this principle.
The wavelength range of visible light is 0.38 to 0.78 microns, and the working wavelength of industrial infrared thermometers is in the range of 0.65 to 14 microns.
Choosing the working wavelength/band of an infrared radiation thermometer is the first consideration for instrument designers. The basis for selecting the working wavelength is multifaceted.
Determine the working wavelength based on the temperature measurement range
Planck's radiation law shows that for objects close to a blackbody, the higher the temperature, the greater the energy radiated, and the peak wavelength shifts towards shorter wavelengths with increasing temperature. Therefore, from the perspective of energy utilization, short wave is generally used for high temperature measurement, long wave is used for low temperature measurement, and medium temperature measurement falls between the two.
Determine the working wavelength based on the emissivity of the object
Generally speaking, the objects being measured are of various types. A thermometer cannot simultaneously meet the requirements of various objects being measured. For objects that are neither blackbody nor gray, the selection of their working wavelength cannot be determined solely based on the measurement range, but mainly depends on the variation of the emissivity of the measured object with wavelength.
Determine the working wavelength based on the atmospheric transmission spectrum
The infrared radiation thermometer receives the radiation energy of the measured object through the intermediate atmosphere. To reduce the impact of atmospheric absorption on measurement results, in addition to considering the above factors, attention should also be paid to selecting windows through which atmospheric radiation passes. The so-called atmospheric transmission window does not mean absolute non absorption. As the distance increases, the transmittance will decrease. Of course, the distance mentioned here is measured in hundreds of meters, and the distance of a few meters will not have a significant impact; The atmospheric window referred to here refers to the absence of a large amount of water vapor, smoke, and dust in the atmosphere.
Choose the working wavelength of the infrared radiation thermometer, which is 8-14 μ m for low temperatures and 0.8-1.0 μ m for high temperatures. Other temperature ranges can be selected as 1.6 μ m, 2.2 μ m, and 3.9 μ m. Due to the transparency of some materials at certain wavelengths, infrared energy can penetrate these materials, and special wavelengths should be selected for these materials.
