What is the function of a blackbody in infrared thermometers?
(1) An object that completely absorbs incident radiation of all wavelengths without any reflection under any conditions.(2) An object with an absorptivity of 1.(3) An object that fully absorbs incident radiation of all wavelengths at any temperature.
All objects continuously radiate, absorb and emit electromagnetic waves. The electromagnetic waves radiated vary across different wavebands and present a specific spectral distribution. This distribution is related to an object's inherent properties and temperature, and is defined as thermal radiation. To study the laws of thermal radiation independent of material characteristics, physicists have defined an ideal object - the black body, which serves as the standard reference for thermal radiation research. A black body absorbs all incident electromagnetic waves with no reflection or transmission, while still emitting radiation outward.
According to Kirchhoff's Law of Radiation, in a state of thermal equilibrium, the ratio of an object's radiant energy to its absorptivity is independent of its material properties and depends only on wavelength and temperature. In accordance with this law, a black body features the strongest radiation capacity at a given temperature and is also known as a full radiator.
In infrared thermal imaging, electromagnetic waves radiated by different substances differ distinctly. The black body acts as a reference benchmark, equivalent to a zero point. Since an ideal black body does not exist in reality, emissivity is defined to quantify such differences. For any given wavelength, emissivity refers to the ratio of the radiant energy of a real object within a narrow wavelength interval to that of a black body at the same temperature. Emissivity is a positive value between 0 and 1, generally affected by material properties, environmental conditions and measurement parameters. Objects with wavelength-independent emissivity are called grey bodies, while others are classified as selective radiators.
Long-term operation of infrared thermal imagers is susceptible to external interference, which may cause measurement errors. Therefore, blackbodies are adopted for instrument calibration.
