]> $$R_G\left(x,y,z\right)=\frac{1}{4\pi }{\int }_0^{2\pi }{\int }_0^{\pi }\sqrt{\left(x{\sin }^2\theta {\cos }^2\phi +y{\sin }^2\theta {\sin }^2\phi +z{\cos }^2\theta \right)}\sin \theta d\theta d\phi$$