What is the role of a black body in a thermometer
(1) Under all conditions, an object that completely absorbs external radiation of any wavelength without any reflection.
(2) Objects with an absorption ratio of 1.
(3) An object that absorbs all incident radiation of any wavelength at any temperature. Any object has the ability to continuously radiate, absorb, and emit electromagnetic waves. The radiated electromagnetic waves are different in each band, that is, they have a certain spectral distribution. This spectral distribution is related to the characteristics of the object itself and its temperature, so it is called thermal radiation. In order to study the law of thermal radiation that does not depend on the specific physical properties of matter, physicists have defined an ideal object——black body (black body), which is used as a standard object for thermal radiation research. The so-called black body means that all incident electromagnetic waves are absorbed, neither reflected nor transmitted (of course the black body still radiates outward).
Kirchhoff's radiation law (Kirchhoff), the ratio of the energy radiated by an object in thermal equilibrium to the absorption rate has nothing to do with the physical properties of the object itself, but only with wavelength and temperature. According to Kirchhoff's radiation law, at a certain temperature, a black body must be the object with the largest radiation ability, which can be called a complete radiator. In infrared thermal imaging, the electromagnetic waves radiated by different substances never intersect. This black body is just a reference object, which is equivalent to a zero point. But such an ideal black body does not exist in the real world, so what is used to describe this difference? For any wavelength, within a small wavelength interval of the defined emissivity, the radiant energy of a real object is the same as that of a black body at the same temperature The ratio of radiant energy. Obviously, the emissivity is a positive number between 0 and 1, and generally the emissivity depends on material properties, environmental factors and observation conditions. If the emissivity is independent of wavelength, then the object can be called gray body (grey body), otherwise it is called selective radiator.
Infrared thermal imaging cameras will be affected by external factors during long-term use, resulting in errors, so it needs to be calibrated with a black body
