  Accurate optical modeling is important for the design and characterisation of
current and next-generation experiments studying the Cosmic Microwave
Background (CMB). Geometrical Optics (GO) cannot model diffractive effects. In
this work, we discuss two methods that incorporate diffraction, Physical Optics
(PO) and the Geometrical Theory of Diffraction (GTD). We simulate the optical
response of a ground-based two-lens refractor design shielded by a ground
screen with time-reversed simulations. In particular, we use GTD to determine
the interplay between the design of the refractor's forebaffle and the
sidelobes caused by interaction with the ground screen.
