| abstract |
The unit comprising a plurality of planar members (11, 11'), a flexible film (12) disposed between, and spaced from, two of said planar members (11,11'), and a stretching member (15) capable of developing elasticity for stretching the flexible film (12) taut by imparting thereto a force in a direction angularly displaced to the film surface; said flexible film (12) extending at least over that region of said planar members (11,11') which forms a window, and the planar member (11,11') present on at least one side of the flexible film (12) being transparent or semi-transparent. |