In infrared thermometers, what is the role of the blackbody?
(1) An object that completely absorbs any wavelength of external radiation without any reflection under any conditions.
(2) An object with an absorption ratio of 1
(3) An object that absorbs all incoming radiation of any wavelength at any temperature. Any object has the ability to continuously radiate, absorb, and emit electromagnetic waves. The electromagnetic waves radiated out have different spectral distributions in different bands. This spectral distribution is related to the characteristics of the object itself and its temperature, and is therefore called thermal radiation. In order to study the laws of thermal radiation that do not depend on the specific properties of matter, physicists have defined an ideal object - a black body - as the standard object for thermal radiation research. The so-called blackbody refers to the absorption of all incident electromagnetic waves, without reflection or transmission (of course, blackbody radiation still radiates outward).
Kirchhoff's radiation law states that the ratio of energy radiated by an object in thermal equilibrium to its absorption rate is independent of the physical properties of the object itself, and only depends on wavelength and temperature. According to Kirchhoff's radiation law, at a certain temperature, a blackbody must be an object with the highest radiation ability, which can be called a complete radiator. The electromagnetic waves emitted by different substances in infrared thermography never intersect, and this blackbody is just a reference point, equivalent to a zero point. But in the real world, there is no such ideal blackbody, so what can be used to depict this difference? For any wavelength, the emissivity is defined as the ratio of the radiation energy of a real object within a small wavelength interval of that wavelength to the radiation energy of a blackbody at the same temperature. Obviously, emissivity is a positive number between 0 and 1, and generally depends on material properties, environmental factors, and observation conditions. If emissivity is independent of wavelength, then an object can be called a grey body, Otherwise, it is called a selective radiator.
