Patent ID: 8169594

Claim:
An illumination system having a pupil plane and a field plane, the illumination system comprising: a transmission filter having a different transmittance at least at two positions, the transmission filter being between the pupil plane and the field plane, wherein: the illumination system is configured to be used in a microlithographic projection exposure apparatus; a mask is displaced along a scan direction during use of the microlithographic exposure apparatus; a light field is illuminated on the transmission filter during use of the illumination system; the light field has shorter dimensions parallel to the scan direction of the illumination system than perpendicularly thereto; the transmission filter is positioned so that, during use of the illumination system, a bundle of light rays, all of which pass through a point in the field plane, passes through the transmission filter with a maximum diameter which is less than L x /2 and greater than L y /30; and L x is a length of the light field perpendicularly to the scan direction; and L y is a length of the light field along the scan direction.