In optical apparatus for illuminating a mask-plane with a line of light, a light source includes four fast-axis stacks of laser diode bars. One optical arrangement collects light beams from all of the diode-laser bar stacks, homogenizes and expands the beams in the fast-axis direction of the diode-laser bar stacks and partially overlaps the homogenized and expanded beams in the fast-axis direction. Another optical arrangement homogenizes the sum of the partially overlapped beams and images the sum of the beams as a line of light having a length in the fast-axis-direction and a width in the slow-axis direction.