Photon surface (definition) : A photon surface of (M, g) is an immersed, nowhere spacelike hypersurface S of (M, g) such that, for every point p∈S and every null vector k ∈ T p S, there exists a null geodesic γ γ :(-ε,ε)→M of (M, g) such that γ γ ˙ ˙ (0)= k, |γ|⊂S.