Isotropic orientation fourth moment

For a uniformly distributed unit vector in three dimensions, $⟨u_i{u}_j{u}_k{u}_l⟩=({\delta }_{ij}{\delta }_{kl}+{\delta }_{ik}{\delta }_{jl}+{\delta }_{il}{\delta }_{jk})/15$. Contracting indices fixes the coefficient. This identity is useful for incoherent scattering averages and random-axis material models.