The disclosed device is aimed at improving the light output of this type of device without impairing its homogeneity in colour and luminance while, at the same time, limiting the amount of space occupied. In such a device, three optical valves are placed before three faces of a recombiner cube. The means used to separate a beam of white light into three beams of coloured light consist of a divider cube. The intersections of the treated surfaces of the, two cubes are aligned, and these treated surfaces are in sets of two in one and the same plane. Furthermore, outside the cubes, the optical paths of the three beams are identical, so that the optical distances between the source of white light and the valves are themselves identical.