In a lithographic projection apparatus, a space between an optical element is filled with a first fluid and a second fluid separated by a transparent plate. The first and second fluids have first and second indices of refraction, respectively, that are different from one another. The first fluid is provided...http://www.google.es/patents/US6954256?utm_source=gb-gplus-sharePatente US6954256 - Gradient immersion lithography
