Abstract: This invention relates to an optical system, adapted to be disposed between a source capable of emitting a beam of light and a target. Near the upstream focal point of the first optics for focussing the beam, the beam provides an image which is at least substantially in section, but which, due to characteristics inherent in the beam of light, produces an image which is oblong in shape at the downstream focal point of the first optics for focusing the beam. The optical system successively comprises, in the direction of propagation of the beam, downstream of the first optics, first diaphragm disposed near the focal point of the first focussing optics, capable of removing a variable part of the area of the section of the beam at the focal point, and second focussing optics. The optical system may further include a second diaphragm disposed upstream from the first optics for focusing the beam for further limiting the ellipticity of the beam.