perpendicular to this interface thus any refraction at the lower surface of optical element 31 is avoided independent of the various or varying refractive indices of the materials fig4 shows another embodiment of the invention this embodiment is advantageous if for any reason refraction at the optical element shall be minimized or completely avoided here optical element 41 is designed as meniscus lens whose surfaces have a common center of curvature this common center is the focal point 44 or focal line in case of a cylindrical lens lower surface of lens 41 is designed with radius r 1 centered at focal point 44 whereas the lens 39 s upper surface 48 is designed with radius r 2 centered at the same focal point 44 obviously the radii difference defines the thickness of meniscus lens 41 incident beam c 1 with its marginal rays 46 a and 46 b passes through the curved upper surface 48 and the curved lower surface 47 of lens 41 perpendicularly the same is true for the reflected beam c 2 with its marginal rays 49 a and 49 b thus there will be practically no refraction of the beams independent of differing refractive indices of the media neither refractive.