emissividad

Emissivity is a property of a material or surface that describes its efficiency in emitting thermal radiation compared with a perfect blackbody at the same temperature. It is defined as the ratio of the spectral or total radiant exitance of the surface to that of a blackbody, with values between 0 and 1. Emissivity can depend on wavelength (spectral emissivity), direction (angle of emission), and temperature, and real materials are often non-grey, meaning ε varies with wavelength.

For opaque bodies that do not transmit radiation, absorptivity equals the complement of reflectivity; Kirchhoff's law states that, at thermal equilibrium, ε(λ, θ) = α(λ, θ). Therefore, for a non-transmitting surface, ε = 1 − ρ. Transparent or semi-transparent materials require considering transmission as well.

In radiative heat transfer, the total emissive power of a surface is M = ε σ T^4 if treated as a gray body; more generally, M = ∫ ε(λ) B_λ(T) dλ, where B_λ is the blackbody spectral radiance. If ε is constant, the object is called a gray body; if ε = 1 it is a blackbody.

Measurement and selection: Emissivity is influenced by surface roughness, coatings, oxidation, and temperature. It can be measured with reflectometry using an integrating sphere, or with calorimetric methods, or by comparing radiance to a known standard.

Applications include designing thermal insulation and coatings for solar collectors, furnace linings, and heat exchangers; in climate science, Earth's surface emissivity affects the infrared radiation budget; in astrophysics, emissivity concepts apply to atmospheres and dust.