Patent ID: 8194230

Claim:
An optical system, comprising: a plurality of elements arranged to image radiation at a wavelength λ from an object surface to an image surface, the elements including mirror elements, each mirror element having a reflective surface positioned at a path of the radiation, wherein: at least one first mirror element of the plurality of mirror elements is a pupil mirror having a pupil mirror surface arranged at or near to a pupil surface of the optical system; at least one second mirror element of the plurality of mirror elements is a highly loaded mirror having a mirror surface arranged at a position where at least one of a largest value of a range of angles of incidence and a largest value of an average angle incidence of all remaining mirrors occurs, the at least one second mirror element being different from the at least one first mirror element; the pupil mirror surface is formed by a reflective coating designed as a one-dimensionally graded coating including a multilayer stack of layers of different materials, the layers having a geometrical layer thickness which varies according to a first grading function in a first direction of the coating and which is substantially constant in a second direction perpendicular to the first direction; and the mirror surface of the highly loaded mirror is coated with a reflective mirror coating designed as a graded coating according to a second grading function.