As P tot = P free + P bound , and for a fluid with intrinsic angular momentum density S we have P bound = 1 2 ∇ ∇ × × S , these identities suggest that the spin density can be identified as either S = μ μ 0 2 i ω ω H ∗ ∗ × × H or S = ϵ ϵ 0 2 i ω ω E ∗ ∗ × × E . The two decompositions coincide when the field is paraxial. They also coincide when the field is a pure helicity state – i.e. when E = i σ σ c B where the helicity σ σ takes the values ± ± 1 for light that is right or left circularly polarized respectively. In other cases they may differ.