A pulse modifying unit is provided in the illumination system of the lithographic apparatus to reduce the degradation of the expensive lens elements by billions of the high intensity ultraviolet pulses from the laser is configured to receive an input pulse of radiation along a first optical axis and further configured to emit one or more corresponding output pulses of radiation along a second optical axis, including a divider disposed along the first optical axis and configured to divide the incoming pulse into a first and a second pulse portion, wherein the divider is further configured to direct the first pulse portion along the second optical axis. A first and a second mirror, each with a radius of curvature, are disposed facing each other with a predetermined separation, configured to receive the second pulse portion and to redirect the second portion along the second optical axis. The optical path of the second portion through the pulse modifier is longer than that of the first portion, and the separation is less than radius of curvature.