A computer program product is disclosed. The object-side surface and the image-side surface of the fourth lens element are aspheric. The illumination optical system illuminates the mask with linearly polarized light whose polarization direction extends along one of the scanning direction and a direction perpendicular to the scanning direction, without losses in quantity of light. The first flange has two through-holes sized and positioned for connection to the structural portion inside engine bay by existing body panel mounting screws.