I think it is simple because it is uniform over the surface of optical element. In optics things get "complex" when they are changing over the area of the optic

I think it is simple because it is uniform over the surface of optical element. In optics things get "complex" when they are changing over the area of the optical element. These tend to be a real PITA because it is difficult to produce anything that is not uniform to astonishingly exact specification as they tend to be in optics. They also tend to require separate design for every optical system.