Emissivity, also known as specific emissivity or emissivity, refers to the ratio of the radiant flux emitted by a feature to the blackbody radiant flux at the same temperature. The emissivity of a feature is related to the nature of the feature, surface conditions (such as roughness, color, etc.) and is a function of temperature and wavelength.

The emissivity / emittance refers to the ratio of the radiation ability of an object to the radiation power of a black body at the same temperature. It is called the emissivity or blackness of the object, also called the emissivity and the specific emissivity.

This is for all wavelengths and should therefore be referred to as full emissivity, often referred to simply as emissivity. English emissivity refers to the physical properties of a single substance, interpenetrating with epsilon in the radiation heat transfer formula, and emittance refers to the emissivity of a sample.

The emissivity of an actual object is related to the surface state of the object (including the surface temperature of the object, the surface roughness, and the presence of surface oxides, surface impurities, or coatings).

The emissivity of a metal increases as the surface temperature increases, and the emissivity of a non-metal generally decreases as the surface temperature increases. The emissivity of metals is much smaller than that of non-metals.

The thermal radiation energy emitted by the black body in unit time is revealed by the Stefan-Boltzmann law as: Φ=AσT4 (temperature fourth power)

This term is often used in infrared detection activities. It is the ratio of the energy of the target measured by the instrument during the test to the energy collected.