A device presenting a disk supporting two prisms and rotating about an axis offset in relation to the path of an incident beam. One of the two prisms is in the shape of part of a circle, and presents a series of faces in the form of parallel portions of the circle, and which are inclined, in relation to a plane perpendicular to their axis, by an angle increasing gradually towards the peripheral face of the prism. The disk and the two prisms are rotated by two electric motors via respective gear assemblies; and the disk is fitted to a shaft also fitted with a gear of the respective gear assembly. Each of the two prisms is integral with a ring gear, and the respective gear assembly presents a gear rotating on the shaft of the disk and meshing simultaneously with both ring gears.