1. An optical system comprising: a plane anisotropic retroreflector portion which specularly reflects incident radiation components in a first plane of incidence, but retroreflects radiation components in a second plane of incidence, a first imaging portion which produces on the retroreflector portion a line-shaped intermediate image of an object point in an object plane in a specified position relative to the system, said image extending along a line in the second plane of incidence, and a second imaging portion that images the line-shaped intermediate image into an image point.