An optical system has a light source having an original etendue of less than 0.1 mm2 for an illumination system for projection lithography. An optical assembly serves for simultaneously increasing the etendue of a used emission of the light source. The optical assembly is embodied such that an increase in the etendue by at least a factor of 10 results. A component of the optical assembly that is impinged on is displaced relative to the light source such that an impingement region of the emission of the light source on the optical component of the optical assembly varies temporally.