A projection optical system is a catoptric system in which a field of view region and an imaging region are located spaced from an optical axis, in which a numerical aperture of light reaching each point on an image plane is substantially uniform regardless of an image height and a direction. An aperture...http://www.google.com/patents/US7483122?utm_source=gb-gplus-sharePatent US7483122 - Projection optical system, exposure apparatus, and exposure method
