7 follows a long circle segment with large radius 8 the cross section formed by these circle segments of screen opening 6 possesses three axes of symmetry 12 the screen surface is hereby formed by several in this case parallel rows of adjacently arranged screen openings 6 whereby adjacent screen openings 6 are arranged offset to each other by approximately 180 u00b0 and axis of symmetry 12 of screen opening 6 respectively progresses perpendicular to orientation 13 of the appropriate row for optimum space usage always two screen openings 6 located opposite each other of adjacent rows have mirror contours in contrast hereto screen openings 6 in fig6 are formed respectively by two long circle segments with large 8 radius and two short segments with small 7 radius in this case too large and small radii 7 8 alternate in a circumferential direction of screen opening 6 to influence screen characteristics elongated screen openings 6 are oriented differently on the screen surface as an example screen openings 6 in this case are arranged in pairs parallel adjacent to each other whereby however screen openings 6 of adjacent pairs are oriented always perpendicular to each other screen openings 6 according to fig7 are formed by four long circle segments with large radius 8 9 and four small circle segments with small radius 7 here too large radius 8.