the slits such that the expanded slits 30 39 now have a substantially higher light transmissibility than the closed slits 30 in the starting position illustrated in fig6 the light transmissibility with such an elastic sheet 29 is adjustable within an approximate range of 5 60 a suitable arrangement for stretching of the sheet 29 is described below in connection with fig1 an alternate to the previously described elastic film in accordance with fig8 amp 9 is a configuration of individual laminar strips 31 32 33 whereby the strips are vertically offset relative to each other and are partially overlapping with each successive strip arranged slightly higher than the respective adjacent strip for each pair of adjacent strips overlapping connection areas 34 for example where the adjacent strips are interconnected by welding points 35 alternate in the longitudinal direction of the strips with unconnected areas 36 furthermore the connection areas 34 at one longitudinal edge of each strip are offset relative to the connection areas at the opposite longitudinal edge thereof and the unconnected areas 36 are recessed relative to the connection areas as can be seen in fig9 application of a tensile force f in a direction transverse to the length of the strips results in a torque m at the middle strip 32 depicted there due to the vertical.