Smith - This innovative design disclosed by Stanley Smith in US patent 4,179,219 replaces one of the spherical reflectors from the previous slide with the inner surface of a toroidal reflector (16) which can be rotated. A much higher range of useful retardation can be accommodated by arranging the focal length of the rotating reflector to vary with angle. This compensates the variation of pathlength which is also made to vary with angle. The main advantage of this design is that it can accommodate a very large input beam divergence. The disadvantages include the difficulty of fabricating the required mirror figure, as well as the fact that the mirror figure must be a compromise between the slope required to vary retardation and the curvature required to match the focal length for each retardation.