Catadioptric reduction objective

Claim 1

A catadioptric reduction objective for transmitting radiation from an object plane to an image plane defined by the objective, the catadioptric reduction objective defining an optical axis and comprising: a first lens group for receiving the radiation and transmitting the radiation along said optical axis; a concave mirror arranged on said axis; a second lens group for focussing the radiation into said image plane; a beam splitter mounted on said axis between said first lens group and said concave mirror for passing the radiation from said first lens group to said concave mirror and for deflecting the radiation returning from said concave mirror into said second lens group; said beam splitter defining an exit/reentry surface through which the radiation passes to and returns from said concave mirror; said exit/reentry surface and said concave mirror conjointly defining a clear uninterrupted lens-free space therebetween thereby facilitating centering of said concave mirror to said first and second lens groups; said concave mirror and said second lens group conjointly defining a pupillary distance and said second lens group having a focal length; said pupillary distance divided by said focal length defining a retrofocus ratio of less than one; said concave mirror having a substantial reduction ratio; and, said catadioptric reduction objective having an image-side aperture of at least 0.50 and a working distance greater than 4 mm.