Patent ID: 8625071

Claim:
An optical system having an optical axis, the optical system comprising: a first element which is partially translucent for light of a working wavelength of the optical system, the first element having at least one partially reflecting first surface that is rotated at a first angle about a first axis of rotation relative to a plane perpendicular to the optical axis of the optical system; a second element in succession to the first element along the optical axis of the optical system so that there is no optical element between the first and second elements along the optical axis of the optical system, the second element being partially translucent for light of the working wavelength, the second element having at least one partially reflecting second surface that is rotated at a second angle about a second axis of rotation relative to the plane perpendicular to the optical axis of the optical system; and an intensity measuring device configured to measure an intensity of light reflected at the first surface and an intensity of light reflected at the second surface, wherein: the second axis of rotation is not parallel to the first axis of rotation; the optical system is a microlithography illumination system or a microlithography projection objective; and during use of the optical system: the first element splits light entering the first element into no more than two light beams; light that is transmitted through the first element enters the second element; and the second element splits light entering the second element into no more than two light beams.