Abstract: 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 with fast and slow axis collimating arrangements providing four collimated beams of diode-laser light. A combination of a lens element and two diffraction gratings collects the four collimated beams and spreads the beams in the fast-axis direction such that the spread beams overlap in the mask plane to form a line of light having a length in the fast-axis-direction and a width in the slow-axis direction of the diode-laser bar stacks.