Definition
An optical thermometer is an instrument that performs non-contact temperature measurement based on the thermal radiation characteristics of an object. It detects the electromagnetic energy radiated by the target object in a specific wavelength or band and converts it into the corresponding temperature reading according to the laws of radiation. This technology avoids direct contact with the object being measured and is suitable for temperature monitoring of moving objects, high temperatures, or hard-to-access scenarios.
Principle
The working principle of optical thermometers is mainly based on Planck's law of black-body radiation. Any object with a temperature above absolute zero radiates electromagnetic waves outward, and there is a definite relationship between the radiation energy density and wavelength and temperature. For an ideal black body, its spectral emission can be described by Planck's formula:
Mλ = (2πhc2/λ5) · [1/(ehc/λkT - 1)]
Among them, Mλis the spectral radiation, h is Planck's constant, c is the speed of light, λ is the wavelength, k is the Boltzmann constant, and T is the absolute temperature. In practical applications, the temperature value of an object can be inverted by measuring the radiation intensity at a specific wavelength (usually in the infrared band) and using this formula or its simplified form (such as the Venn approximation). For non-blackbody physical objects, an emissivity ε need to be introduced for correction.
Measurement method
Common methods of optical temperature measurement include luminance method, colorimetric method, and multispectral method. The brightness method determines temperature by measuring the radiant energy within a single wavelength or narrow band, and its structure is relatively simple but sensitive to changes in emissivity. The colorimetric method calculates temperature by measuring the ratio of radiation energy at two adjacent wavelengths, which can reduce the influence of emissivity uncertainty to a certain extent, and is suitable for environments where the emissivity is unknown or varied. The multispectral law extends to multiple wavelengths to invert temperature and emissivity simultaneously by fitting radiation curves, making it suitable for measurements of complex surfaces. In addition, according to the size of the field of view, there are measurement methods such as dot, line and area scan to meet the needs of different spatial resolutions.
Influencing factors
The measurement accuracy of optical thermometers is influenced by various factors. The emissivity of the target surface is a key parameter, which is affected by the material, surface roughness, oxidation state and temperature itself. Environmental factors such as water vapor, dust, smoke, etc. along the measurement path can absorb or scatter radiation, affecting signal strength. Interference from background thermal radiation, especially when the target temperature is close to ambient temperature, can introduce significant errors. The field of view of the instrument itself needs to match the size of the target, and if the target does not fill the field of view, the background radiation will be mixed with the measurement signal. In addition, the cleanliness and transmission characteristics of the optical window, the response stability of the detector, and the fluctuation of ambient temperature also affect the measurement reliability.
Application:
Optical thermometers have a wide range of applications in the fields of industry and scientific research. In the metallurgical industry, it is used for online monitoring of metal surface temperature during continuous casting, rolling, and heat treatment. In glass manufacturing, it is used for temperature control in furnaces, forming and annealing processes. In semiconductor manufacturing, the temperature distribution of wafers is monitored during processes such as deposition and etching. In power systems, it can be used to detect overheating hazards in electrical connection points, transformers, and other equipment. In materials research, it is used to analyze the phase transition or thermophysical properties of materials at high temperatures. In addition, optical temperature measurement is often used for non-contact quality control in food processing, ceramic sintering, plastic molding, and other processes.
Selection
When choosing an optical thermometer, multiple technical parameters and application conditions need to be comprehensively considered. The measurement range should cover the temperature range where the target may occur, with a certain margin. The selection of spectral response bands should consider the emissivity characteristics of the target material in this band and the transmission window of the environmental medium. Spatial resolution is determined by the field of view and the measured distance, and the object should be sure to adequately cover the field of view of the instrument. Response times need to meet the needs of monitoring process dynamics. The environmental tolerance of the instrument, such as protection level and cooling method, should be adapted to the temperature, dust, vibration and other conditions of the site. For cases where the emissivity is uncertain or variing, a colorimetric method or a model with automatic emissivity compensation can be considered. At the same time, the functions of the data interface, output format and supporting software must also be matched to the existing control system or data analysis process.