emissivities

Emissivity, denoted by epsilon, is a property of a surface that quantifies how efficiently it emits thermal radiation compared with a perfect blackbody at the same temperature. It is a dimensionless number between 0 and 1. A blackbody has emissivity of 1, while most real materials have emissivities less than 1. Emissivity depends on wavelength, temperature, viewing direction, and surface characteristics such as roughness and coatings.

Emissivity can be described in several ways: spectral emissivity, which varies with wavelength; directional or angular emissivity, which depends on the observation direction; and hemispherical or total emissivity, which integrates emission over all directions. In simple calculations, a constant value known as the graybody emissivity is sometimes assumed when the emissivity is roughly independent of wavelength or temperature.

Kirchhoff's law of thermal radiation states that, at thermal equilibrium for a given wavelength and direction, emissivity equals absorptivity. Thus a surface that strongly absorbs radiation at a certain wavelength will also be a strong emitter at that wavelength. This relation underpins many radiative heat transfer analyses and aids in predicting a surface's thermal behavior.

Measuring emissivity typically involves infrared radiometry techniques, including integrating-sphere systems, reflectance-based methods converted to emissivity, or spectroradiometric measurements for spectral emissivity. Emissivity is central in calculating radiative heat transfer, designing thermal insulation and coatings, infrared thermography, astronomical observations, and climate models.

Common examples: polished metals often exhibit low emissivity, while oxidized or painted surfaces have higher values. Emissivity can be engineered via surface treatments to tailor thermal emission for energy efficiency, cooling, or sensing applications.