There is provided an illumination system. the illumination system includes (a) a source of light having a wavelength of less than or equal to 193 nm, and (b) an optical element in a path of the light, having a first raster element, a second raster element, a third raster element and a fourth raster element situated thereon. The second raster element is adjacent to the first raster element, and located a first distance from the first raster element. The fourth raster element is adjacent to the third raster element, and located a second distance from the third raster element. The second distance is different from the first distance.