An optical system for projecting a laser-beam on a mask to illuminate the mask includes a beam homogenizing arrangement including spaced arrays of microlenses. The beam homogenizing arrangement redistributes light in the laser beam such that the intensity of light in the laser-beam on the mask is nearly uniform along a transverse axis of the laser-beam. A stop extending partially into the laser-beam between the microlens arrays provides a more uniform light-intensity on the mask along the transverse axis than can be achieved by the microlens arrays alone.