Patent ID: 8817233

Claim:
An illumination optical system, comprising: a first facet mirror comprising a plurality of first facets including a group of first facets, each of the first facets having a first reflection face; and a second facet mirror comprising a plurality of second facets, each of the second facets having a second reflection face, wherein: for each of the first facets of the group of first facets: when the first facet is in a first position, the first reflection face of the first facet guides a part bundle of illumination light along a first channel to a corresponding second reflection face; when the first facet is in a second position, the first reflection face of the first facet guides the part bundle of illumination light along a second channel to a corresponding second reflection face which is different from the second reflection face to which the first reflection face guides the part bundle of illumination light when the first reflection face is in its first position; when the first facet is in its first position, the first reflection face reflects the part bundle of illumination light with a first reflectivity; when the first facet is in its second position, the first reflection face reflects the part bundle of illumination light with a second reflectivity; the second reflectivity coincides with the first reflectivity within a tolerance range of +/−10%; the corresponding second reflection faces of the first and second channels are located on a conic section portion of the second facet mirror; the conic section portion is limited by a first conic section line and a second conic section line; the first conic section line is defined by a first angle of incidence of the part bundle illumination light onto the first facet; the second conic section line is defined by a second angle of incidence of the part bundle illumination light onto the first facet; and absolute values of the first angle of incidence and the second angle of incidence differ by at most 10°; and the illumination optical system is configured to be used for projection lithography.