When α is absorptivity, ρ is reflectivity and τ is transmittivity, then for a diathermanous body,

• α = 1, ρ = 0 and τ = 0
• α = 0, ρ = 1 and τ = 0
• α = 0, ρ = 0 and τ = 1
• α + ρ = 1 and τ = 0