An optical system is usable to form an illuminating strip on a surface of a material. The optical system includes an illuminating device which contains several light sources which are located at a distance from the surface of the material. The illuminating device uses its light sources to generate the illuminating strip on the surface of the material, which is displaced in relation to the device. The illuminating device includes a mirror which focuses the light emitted by the light source into the illuminating strip, a diffusion body on the light emission side facing the surface of the material. The light sources feed their light into the reflector module. That module, the diffusion body, and the mirror are configured as a single component.