Thermal emissivity of selective surfaces—New lower limits

Numerical calculations of the thermal emissivity ε of spectrally selective surfaces consisting of absorbing films on reflective metal substrates are reported. A physically realizable form for the absorption coefficient of the film α (ω), and a refractive index n (ω) derived from α (ω) via a Kramers-Kronig relation were used to obtain new lower limits for ε.