Patent ID: 7982969

Claim:
A projection objective comprising: a refractive optical element having a volume and a material related optical property that varies over the volume, and a plurality of optical surfaces that are arranged in a volume that is optically conjugate with the volume of the refractive optical element, wherein: variations of the material related optical property of the refractive index refractive optical element cause an aberration during use of the projection objective; each of the plurality of optical surfaces comprises at least one correction mechanism that at least partially corrects the aberration; and the projection objective is configured to be used in a microlithographic projection exposure apparatus, wherein the at least one correction mechanism is located in or intersects a first plane perpendicular to an optical axis of the objective, the first plane is conjugate with a second plane perpendicular to the optical axis, and the second plane intersects the volume of the refractive optical element, and wherein a ratio R=Dp/Dm is at least substantially the same in the first and second planes, where: Dp is a distance between a principal ray, which emerges from a point on a border of a field which is imaged by the objective, and the optical axis; and Dm is a distance between a marginal ray, which emerges from a point on the optical axis in the field which is imaged by the objective, and the optical axis.