Screen extension linkage for a video real-screen projection apparatus
This is a continuation of application Ser. No. 731,332, filed May 6, 1985, now abandoned, which is a continuation of application Ser. No. 623,599, filed June 22, 1984, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a video rear-screen projection apparatus in which pictures are projected from a cathode-ray tube through one or more mirrors onto an enlarged picture screen. The various components of known apparatus are in part accommodated movably in a housing in such a manner that, when the apparatus is out of use, all the components are located inside the housing, while, when the apparatus is in use, one or more of the components are moved outwards in order to obtain a system which optically operates correctly. 
     SUMMARY OF THE INVENTION 
     The invention has for its object to provide a video rear-screen projection apparatus of the above kind comprising a movement mechanism for the various components wich mechanism is of simple construction and which mechanism, in a folded condition, yields an apparatus of very small dimensions. 
     In order to achieve this object, the apparatus according to the invention comprises a housing accommodating a fixedly arranged cathode-ray tube which has a projection lens arranged with the optical axis in a plane of symmetry, the components of the movement mechanism being preferably arranged symmetrically about the plane. The tube and lens are secured to a frame which, adjacent two opposite side walls of the housing, has two members in which a shaft coupled to a motor is journalled at its ends, which shaft carries a plate or crank member at each of its ends. The inventive apparatus further comprises a first mirror which extends in the direction of width of the housing and is connected at two opposite sides via hinges which define a first pivot axis perpendicular to the plane of symmetry, arranged approximately in alignment with its wide side to said side walls of the housing, the first mirror being pivotally linked by a bar on each of said two sides to an associated one of said plates so that, in one position of the plates, the mirror is located entirely inside the housing and in another position it is extended with its reflecting surface facing the projection lens. The inventive apparatus further comprises a transparent screen which extends in the direction of width of the housing and parallel to the front side of the housing and is provided at each of two opposite sides with a pair of pivots which are located a certain distance above one another and which are connected by first and second links or bars which cross one another to a pivot on the frame, and a pivot on an associated one of said plates, respectively, each of the first bars being provided between its ends with a pivot which is connected via a third link or bar to the associated one of said plates, and a second mirror being supported between the second bars in such a manner that, upon rotation of said plates, the screen and the two mirrors are extended so that the rays from the cathode-ray tube strike the transparent screen and, in the folded condition, said components are all located inside the housing. 
     The invention will be described more fully with reference to the drawing which shows in sectional view an embodiment of a video projection apparatus in two conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a side elevational view of the apparatus in the extended condition, while 
     FIG. 2 is a similar view of the apparatus in the folded condition. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawing, housing 1 includes a bottom 2, a rear wall 3, an upper wall 4 and two side walls. In the housing 1, a cathode-ray tube unit 5 is connected by means (not shown) to a frame 14. The tube unit is provided with a lens 6 which projects the rays onto a first mirror 7. This mirror 7 extends over a part of the width of the housing 1, the width of the housing extending in a direction perpendicular to the plane of the drawing. The mirror 7 is provided at two opposite sides with side plates 8 and with pivots 9 by means of which the mirror 7 is pivotally connected to the bottom 2 of the housing 1. The side plates 8 are each pivotally linked by a bar 10 to one of two plates 11. The plates 11 are fixedly secured on the ends of a shaft 12 which is pivotable about a second axis perpendicular to the symmetry plane, and is journalled in lugs 13, which are secured to the frame 14 adjacent the side walls of the housing. The shaft 12 is coupled to a controllable electric motor not shown. 
     The apparatus is further provided with a transparent screen 15 which extends throughout the width of the housing and substantially parallel to the front side of the housing. The transparent screen 15 is provided at two opposite sides with side plates 16. Each of these side plates 16 carries two pivots 17, 18 which are connected via bars 19, 20 which cross one another to pivots 21, 22, respectively, on the frame 14 and an associated one of the plates 11, respectively. The pivots 21 thus define a third axis fixed with respect to the frame and perpendicular to the symmetry plane. The pivots 22 define a movable fourth axis, parallel to the other defined axes, which is thus spaced from the second axis about which the plates 11 pivot, while the bars 19 and 20 serve as first and second rigid links supporting and positioning the screen 15. Each bar 19 is further provided between its ends with a pivot 23 which is connected by a third link or bar 24 to a pivot 25 on the associated plate 11. The pivots 35 thus define a fifth parallel axis, also spaced a given distance from the second axis. Further, the pivots 17 and 18 define sixth and seventh parallel axes, spaced from the third and fourth axes respectively by distances defined by the lengths of the first and second links, which are moved to define the screen 15 position. 
     A second mirror 30 is supported between the two bars 20. The path of the light rays 26,27,28 from the cathode-ray tube is indicated in FIG. 1 by dotted lines parallel to or symmetrical about a plane of symmetry to which each of the pivot axes is perpendicular. 
     By actuation of the electric motor (not shown), the shaft 12 can be rotated through a given angle, whereby in position 1 the situation shown in FIG. 2 is reached. In this position, the mirrors and the transparent screen are folded so that they are in a stowed position located entirely within the confines of the housing 1. 
     In the other position, the situation shown in FIG. 1 is obtained. In this case, the mirror 7, like the transparent screen 15 and the second mirror, is extended. The transparent screen has not only moved forwards but has also moved slightly upwards, which has the advantage that a better eye level for the spectator is obtained. 
     In a practical embodiment, the dimension of the apparatus from front to back was 67 cm in the extended condition and only 42 cm in the folded condition. This ensures that the apparatus, when it is out of use, occupies only a very small space so that it can be used conveniently in smaller rooms.