The invention relates to a roller blind, particularly for use as a blackout shade, comprising a spring-biassed roller bar designed to be mounted at the top of a window opening and a blind rolled on the roller bar and having along its lateral edges guide members guided in tracks provided in guide rails disposed at both sides of the window opening, a bottom bar for the blind being likewise provided, at both ends, with guide means engaging guideways in said guide rails, and being further controlled by means of a cord arrangement with a cord which from a fixture means at the bottom of one guide rail is passed through the part of said guide rail positioned beneath the bottom bar, the bottom bar itself, and the part of the other guide rail positioned above the bottom bar, to a fixture means at the top of the other guide rail.
In roller blinds to be mounted in connection with skylights in inclined roofs use is made of a spring-biassed roller bar ensuring that the blind is kept tight in all positions between the completely raised position and the maximum drawn bottom position.
In order to enable the bottom bar to be arrested in its bottom position and in a number of intermediate positions it is known to mount side rails along the longitudinal sides of the window frame, the side rails being provided with downwards facing recesses for engagement with pins or the like at the ends of the bottom bar. This makes it possible to retain the bottom bar in a limited number of intermediate positions.
Such comparatively simple side rails are, however, not usable in connection with blinds for blackout shades of the above mentioned type, in which the side guide rails must be designed so as to ensure light-proofness at the edges of the window. At the top and at the bottom of the window opening light-proofness is obtained by providing the roller bar, which is most frequently enclosed in a cassette, as well as the bottom bar with appropriate sealing strips.
In U.S. Pat. No. 785,806 a roller blind of the above mentioned type is disclosed in which the movement of the bottom bar is controlled by a double cord arrangement ensuring parallel guidance of the bottom bar in relation to the roller bar, thereby preventing the bottom bar from getting jammed due to careless operation.