A uniformity correction module for improving the uniformity of a radiation distribution in a rectangular illumination slit having two longer sides and two shorter sides, including a plurality of movable blades arranged along each long side of the illumination slit and a chamber containing a fluid wherein said movable blades are at least partly immersed in said fluid, and wherein the difference between the refractive index of each blade and the refractive index of said fluid is sufficiently small to prevent significant reflection and refraction at the surface of each blade.