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Emissivity and Kirchhoff's law

The emissivity tells you how well a surface radiates compared with a black body, which has ε=1\varepsilon = 1. Shiny, polished metals radiate poorly and have low emissivity, while matte, dark surfaces radiate well. Kirchhoff's law says a good radiator is also a good absorber.

α=ε\alpha = \varepsilonKirchhoff's law, gray surface in thermal equilibrium

Symbols

ε\varepsilonemissivity
α\alphaabsorptivity

Example

Two identical spheres at the same temperature, one painted black (ε≈0.9\varepsilon \approx 0.9) and one shiny (ε≈0.1\varepsilon \approx 0.1), radiate very differently: the black one loses about nine times as much heat by radiation.

Do not assume the same emissivity for every surface – shiny surfaces can have ε\varepsilon below 0.1, while matte surfaces often have ε\varepsilon above 0.9.
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Part of Heat Transfer: Radiation and transient heat.