Emissivity (ε)
Emissivity is how effectively a surface radiates heat, from 0 to 1 relative to a perfect black body. Polished bare metal has low emissivity; oxidised, painted or fabric surfaces are high. It strongly affects both radiant heat loss and thermal-camera (FLIR) accuracy.
Emissivity governs the radiative part of surface heat loss (q = ε·σ·(Tₛ⁴−Tₐ⁴)) and must be set correctly when reading surfaces with an infrared camera — using the wrong ε gives wrong temperatures. Typical values: polished steel ~0.1–0.3, oxidised steel ~0.7–0.8, insulation fabric ~0.9. Thermography surveys record the ε used for each surface.
Related terms
Thermal Conductivity (λ) · U-value (Thermal Transmittance)