Kirchhoff's proof considered an arbitrary non-ideal body labeled i as well as various perfect black bodies labeled BB. It required that the bodies be kept in a cavity in thermal equilibrium at temperature T . His proof intended to show that the ratio ⁠ E (λ, T, i) / a (λ, T, i) ⁠ was independent of the nature i of the non-ideal body, however partly transparent or partly reflective it was.