A method of operating a microlithographic apparatus comprises the steps of providing an illumination system comprising an array of tiltable mirrors, wherein a light irradiance distribution on the array varies by at least 50% along a first line; specifying a scan integrated target angular light distribution and a target light energy for a point moving through an illumination field along a second line that extends parallel to a scan direction and is an image of the first line; determining a group of those mirrors through which the first line extends; determining tilt angles of the mirrors of the group such that a real angular light distribution and a real light energy for the point approximate the respective target values; producing the illumination field by forming an image of the array on a mask; and imaging a portion of the mask on a surface while the mask moves along the scan direction.