4⁢π⁢RF⁡(x,y,z)=∫02⁢π∫0πsin⁡θ⁢ⅆ⁡θ⁢ⅆ⁡ϕ(x⁢sin2⁡θ⁢cos2⁡ϕ+y⁢sin2⁡θ⁢sin2⁡ϕ+z⁢cos2⁡θ)1/2,