Abstract:
A poster display system for displaying a plurality of different images which includes an image grid with interlaced strips from the various images, masking grid adjacent the image grid having spaced apart masking strips alternating with spaced apart window strips, a mounting system for mounting the image grid and the masking grid for movement relative to one another, a drive mechanism for producing relative movement between the image grid and the masking grip between successive display positions in which the window strips of the masking grid are aligned with image strips of a particular image, the image strips of the other images being obscured by the masking strips, and compression means for urging opposite edge portions of the image grid and of the masking grip towards each other whereby the image grid and the masking grid become curved and the image grid and the masking grid are brought into intimate contact with one another under pressure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to poster display systems and to methods of displaying poster images. 
     2. Description of Related Art 
     It is often desired to display more than one poster image at a particular site and there already exist poster display systems in which the poster image is displayed on a series of juxtaposed elongate elements of polygonal cross-section. One face of each element carries a respective part of the poster image and the image displayed can be changed by rotating all of the elements in unison so that they each present another face. Such display systems involve the use of a multiplicity of polygonal elements each of which must be rotated during use of the systems and to each of which must be attached respective parts of each poster image. Thus the manufacture of such a system is expensive and the changing of the set-up of the system to enable a different set of poster images to be displayed is cumbersome and expensive. 
     It has been proposed to provide a poster display system for presenting a plurality of different images by mounting a masking sheet over a poster image sheet and moving the masking sheet small distances relative to the poster image sheet. The poster image sheet is formed with a multiplicity of image strips with adjacent strips defining portions of different images. The masking sheet has alternate opaque and transparent strips so as to reveal selected ones only of the poster image strips. In one position of the masking sheet relative to the image sheet, strips of a first image remain exposed while other image strips are concealed behind opaque strips of the masking sheet. In another position of the masking sheet relative to the image sheet strips of the first image are concealed behind opaque strips whilst the image strips relating to another image that were previously concealed behind the masking strips become exposed. Provided the width of the masking strips is not too great, a human eye will fill in the masked parts of the image and see an entire poster image. The image seen will, however, vary according to which image strips are exposed. Thus the display system may be used to display a plurality of unrelated static images, or a plurality of closely related images which, when displayed in the correct sequence, provide an animation, as for example proposed in U.S. Pat. No. 3,918,185. 
     For the proper functioning of a poster display system of the kind just described, in which image strips relating to a plurality of poster images are provided on a common poster image sheet, it is very important that the relative positions of the poster image sheet and the masking sheet are very precise. That requirement applies and is relatively simple to meet if only two images are included on the poster image sheet but is more onerous if more images are included. Furthermore, it is very desirable that it should be easy to replace one poster image sheet by another so that a different set of images can be displayed by the system. Further still, it is very important that the poster image sheet and the masking sheet maintain overall contact with each other between the surfaces of the composite material upon which the grids are printed. 
     SUMMARY OF THE INVENTION 
     Such requirements have proved very onerous and prevented widespread commercial use of such display systems. 
     According to the present invention there is provided a poster display system for displaying a plurality of different images, the system comprising 
     (a) an image grid carrying a first set of image strips comprising a multiplicity of first strips spaced apart from one another and presenting different regions of a first image, and a second set of image strips comprising a multiplicity of second strips spaced apart from one another and presenting different regions of a second image, said second strips being distributed over the grid between the said first strips; 
     (b) a masking grid adjacent to the image grid and carrying a set of masking strips comprising a multiplicity of masking strips spaced apart from one another and a set of window strips comprising a multiplicity of window strips spaced apart from one another, said window strips being distributed over the grid between said masking strips; 
     (c) a mounting system for mounting the image grid and the masking grid for movement relative to one another; 
     (d) drive means for producing relative movement between the image grid and the masking grid between a first display position, in which said window strips are aligned with said first image strips and said second image strips are obscured by said masking strips, and a second display position, in which said window strips are aligned with said second image strips and said first image strips are obscured by said masking strips; and 
     (e) compression means for urging opposite edge portions of the image grid and of the masking grid towards each other whereby the image grid and the masking grid become curved and the image grid and the masking grid are brought into intimate contact with one another under pressure. 
     The use of further compression of the image grid or the masking grid enables them to be brought into close proximity as a result of the substantially complete exclusion of air between these grids, and this can result in particularly good registration of the window strips of the masking grid with image strips of the image grid, thereby enabling only image strips of one image and no other to be viewed at a time. 
     The image grid preferably carries three or more sets of image strips. It is then possible to have three or more images displayed sequentially by the system. The more sets of image strips that are provided, the greater the width required for the masking strips relative to the image strips. As a result, accurate mounting of the masking strips relative to the image strips, which can be facilitated by the present invention, becomes more advantageous. 
     Using a masking grid having masking strips alternating with window strips, the width of each window strip may be less than one fifth of the width of adjacent masking strips and a good image can still be displayed. It is advantageous that the masking strips between adjacent window strips have widths of less than about 2 mm so that the images appear continuous to the eye. 
     The width of each window strip is preferably less than the width of each image strip with the respective window strip is aligned. Such extra width of image strip provides a small tolerance for the relative positions of the window strips and image strips, and it also allows for oblique viewing of the display. 
     Although the image grid may form one face of a rigid body of substantial thickness, it is preferably formed on a flexible sheet which advantageously has a thickness of less than 1 mm and more particularly of less than 0.5 mm. The image strips are preferably printed on the image grid. 
     The masking grid should at least be translucent and preferably transparent. It is also preferred that the masking strips be printed on the masking grid. Forming the image strips and the masking strips by printing is very simple and modern printers can print such strips with sufficient accuracy. 
     The masking grid and the image grid are preferably made from the same material so that they will expand and contract in a similar way in response to temperature changes and the like. These two grids preferably have a low coefficient of expansion. 
     It is generally preferred to arrange the image grid and the masking grid with their respective printed faces immediately adjacent to each other as this can enable the accuracy of positioning the image strips and the masking strips to be improved and problems caused by oblique viewing can be reduced. 
     Whilst it is possible to have an intermediate translucent sheet or sheets between or laminated to the image grid and the masking grid, it is preferable to have these grids in direct contact with one another with substantially no air between them. The exclusion of air from between the grids not only enables them to be brought into contact with one another, it also tends to keep them in contact with one another since the grids can only separate as a result of air entering between them at their edges. If one or both grids are laminated with a laminating film to protect the ink used to print the respective grids, the lamination film is preferably not more than 13 μm thick. 
     In addition to using compression to bring the image grid and the masking grid into intimate contact, systems in accordance with the present invention can include additional means for increasing such contact. Electrically charging one of the grids is a particularly suitable method of achieving this. Employing the inherent static properties of the image and masking grids to attract one grid to the other is another such means. 
     Systems in accordance with the present invention preferably include adjustment means for setting up the relative positions of the image grid and the masking grid in a first display position. Such adjustment means preferably allow an operator to carry out fine adjustment of the relative positions of these grids after the installation of a new image grid, and it is preferred to use a screw-threaded adjuster element to effect such adjustment. 
     The adjustment means preferably includes means for altering the positions of a drive member for moving the moveable grid, and the drive means by which it is driven, relative to a member to which the static member is fixed. The drive member may be a rotatable cam which is preferably detachable without dismantling the display unit, it being particularly convenient to be able to change the cam and the grids without dismantling the whole system. Thus the position of the axis of rotation of the cam is preferably adjustable using adjustment means. 
     The material from which the image grid and/or the masking grid are made can also be significant. Preferably they are of a plastics material and more preferably polyethylene terephthalate or a polycarbonate. An especially preferred material is sold under the trade mark Melinex, the grade Melinex 506 being used in the specific embodiments of display systems in accordance with the present invention which are described herein. 
     Systems in accordance with the present invention preferably include means for lighting the image grid from behind both the image grid and the masking grid and this can be provided by providing a pathway for natural light and/or at least one artificial light source. 
     It might be thought that the masking grid would have to be in front of the image grid, but we have found that these grids can be mounted either way around, it being preferred to use back lighting when the image grid is nearest to the viewer. The mounting arrangement can, therefore, enable the two grids to be mounted with either in front. However, it is especially preferred to use a mounting arrangement for detachably mounting the image grid in front of the masking grid when the curve of the grids is concave as seen by a viewer, but behind the masking grid when the curve of the grids is convex as seen by the viewer. Replacement of the image grid can then be relatively simple. 
     The drive means can be attached to either the masking grid or to the image grid. The grid not so attached remains static. 
     It has been found that by curving the image grid and the masking grid, the retention of overall contact between the two adjacent faces of these grids can be greatly improved. Such a curve can be achieved by fixing a clear base panel, made of the same or similar material, and of the same or similar thickness as that of the image grid and the masking grid, along two opposite edges within a display housing to an angled section using clamping bars of an appropriate material. The clear base panel can then be fixed so that it forms a natural curve when the display housing is laid horizontally on its back. 
     When in this position, the lowest part of the curve formed by the clear base panel can, for example, be between 5 and 50 mm lower than its fixed edges. Once the base panel has been fixed in position, for example using clamping bars, adjustable compression bars of an appropriate material can then be positioned between the two clamping bars. A secondary clear panel can then be positioned between the two compression bars. 
     The curved width of the secondary clear panel is preferably a few millimeters less than the curved distance between the two compression bars when these two bars are fully adjusted away from each other. However, the curved width of the secondary clear panel is wider than the curved distance between the two compression bars when they are fully adjusted towards each other. The compression bars can therefore be adjusted to meet the corresponding and parallel edges of the secondary clear panel and then further squeezed together providing a compressive force upon the secondary clear panel forcing it into the curve of the clear base panel. In this manner, the curved clear base panel and the curved secondary clear panel become sufficiently rigid to retain a coherent curve when the display housing is raised into an upright and vertical position. 
     The masking grid, which has a width which is less than the curved distance between the compression bars and thereby allowing lateral registration of the masking grid in relation to the image grid, is placed over the secondary clear panel and fixed to this panel by adhesion or by the use of bolts along its top or bottom edge through the secondary clear panel to the clear base panel. 
     The image grid, which also has a width which is less than the curved distance between the compression bars but is greater than the width of the masking grid, is placed over the curved masking grid and it is attached to a mechanism for moving it up and down relative to the masking grid. Lateral registration of the image grid relative to the masking grid can then be achieved using off-set cams positioned either side of the image grid towards the edge of the image grid furthest away from the mechanism. These cams can then be rotated and fixed in position touching opposite side edges of the image grid once the latter has been laterally registered with the masking grid. Vertical registration of the image grid relative to the masking grid using the mechanism has been previously described. 
     If desired, the image grid and the masking grid can be interchanged so that the masking grid is in front of the image grid. In such a case, the image grid is preferably fixed to the secondary clear panel or through this panel to the base panel, and the masking grid is then positioned between the off-set cams and fixed to the drive mechanism. 
     If desired, a front sheet of a clear material can be placed tightly between the compression bars and fixed through the static grid and the secondary clear panel to the clear base panel. The height of the moveable grid is then preferably sufficiently less than that of either the clear panels or the static grid to avoid the fixings of the clear panels and the static grid. 
     If a separate diffusion panel is not incorporated into the display housing to diffuse light from lamps, either of the clear panels behind the grids or the rear grid itself can be made from a semi-opaque material. 
     The compression bars can be used in place of the cams to achieve lateral registration of the grids though this generally requires very accurate cutting of whichever is the moveable grid. 
     If required, illumination for the display can be positioned on the concave or the convex side of the curved clear panels and grids with the display unit being viewed from the other side of the curved clear panels or grids. 
     If required, the drive mechanism can be fixed to the top panel of the display housing, or to the base panel of the display housing. When the drive mechanism is attached to a side panel of the display housing, so that the compression bars are at the top and the bottom of the display housing, a solid bar of clear material should in general be fixed tightly between the compression bars in front of the concave curve of the clear panels and grids so that this solid bar bends to match the curve of the clear panels and grids. Such a bar should be placed between both ends of the compression bars, and thus two such bars should be used. 
     If required, whichever grid is placed over the secondary clear panel can itself be attached to the protruding stud on the drive mechanism, thereby becoming the moveable grid. The grid placed over the moveable grid then becomes the static grid which is attached to the clear base panel through the secondary clear panel. 
     Whichever grid is the moveable grid, it should in general be wider than the static grid to allow retention of registration if off-set cams are used to act on the moveable grid. 
     If required, either grid can be placed directly on to the clear base panel and the secondary clear panel can then be omitted, and indeed the clear front panel can also be omitted. Furthermore, the static grid can replace the clear base panel, and the secondary clear panel can be replaced by the moveable grid. Thus if the clear front panel is also omitted, only the image and masking grids would form the semi-rigid viewing area. In this latter case, the moveable grid would need to be cut very accurately as the compression bars become the means of keeping the moveable grid in linear registration with the static grid. 
     To overcome friction between the moveable grid and the compression bars, which would require greater torque from the drive motor, the compression bars can be attached directly to the moveable grid. Compression can then be applied directly to the compression bars, for example using off-set cams or adjustable circular bearings attached to the angled section. These off-set cams or adjustable bearings can also be used to effect linear registration between the image grid and the masking grid. 
     If the images made up of image strips on the image grid making up the respective images differ from each other only slightly, it is generally unnecessary for the image strips of the image grid to be wider than the transparent strips of the masking grid in order to achieve a smooth visual sense of animation of the image, especially if the grids are moved fairly rapidly relative to each other. In addition, when the image strips of the image grid and the transparent strips of the masking grid are of substantially the same width, the number of image strips that can be displayed, for example in an animation, can be increased compared to arrangements where the image strips are wider than the transparent strips because the number of image strips masked by each masking strip of the masking grid can thereby be increased. As will be appreciated, the more images that make up an animated sequence the smoother and longer an animated sequence can be. However, if the image strips are narrower than the width of the transparent strips, the animation effect will usually appear blurred. 
     By way of example, the opaque strips on a masking grid of A2 size can be at least 2.00 mm with the image strips and the transparent strips of the masking grid each being as narrow as 0.10 mm. It is then possible to provide an animated sequence of twenty images, each image varying slightly from its adjacent images, by obscuring nineteen consecutive sets of image strips using masking strips on the masking grid which are 1.90 mm wide. The twentieth image strip is then visible from behind or in front of the transparent strips of the masking grid. 
     In some instances it might be desirable to dwell for a longer period of time on, say, the final image of an animated sequence, in which case the last two image strips of each set of image strips can be identical or can have a width greater than the window strip. Cams controlling movement of the moveable grid will in general need to be adapted accordingly. For example, if each image is displayed through the masking grid for 0.25 seconds, the whole animated sequence from the first to the twentieth and back to the first image would last for 10 seconds, i.e. a total of forty images times 0.25 seconds per image. In this manner, an animated display can be achieved, for example a bird in flight or advertising graphics that appear at first to be moving away from the viewer and then towards them. 
     As the size of the masking grid and the image grid is increased, the widths of the opaque strips on the masking grid can often be increased as the viewing distance increases. However, the width of the image strips on the masking grid do not necessarily have to be increased in the same proportion, and the number of image strips that make up an animation sequence can thereby be increased as a result of using wider masking strips. 
     Embodiments of display system in accordance with the present invention will now be described by way of example with reference to the accompanying diagrammatic drawings. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF DRAWINGS 
     FIG. 1 is a sectional side view of a mechanism for effecting relative movement between the image grid and the masking grid of a first embodiment of system of the present invention; 
     FIG. 2 is a front view of the mechanism of FIG. 1; 
     FIG. 3 is a sectional view along line III—III of FIG. 2; 
     FIG. 4 is a partially cut away view of a first embodiment of system in accordance with the present invention; 
     FIG. 5 is a perspective view, to an enlarged scale, of the portion of FIG. 4 defined by the broken lines X-Y and W-Z; 
     FIG. 6 is a similar view to that of FIG. 5 of a second embodiment having different compression means; 
     FIG. 7 shows the cam used with the embodiments described with reference to the preceding FIGS.; 
     FIG. 8A shows a cut away portion of an image grid positioned over a masking grid; 
     FIG. 8B shows a similar view to that of FIG. 8A but with the masking grid over the image grid; 
     FIG. 9 shows an alternative form of cam; 
     FIGS. 10A and 10B show an image grid and a masking grid respectively having image strips and window strips of the same width; 
     FIG. 11 is a perspective view of a portion of a slider arrangement of a yet further embodiment; 
     FIG. 12 is a front view of the embodiment of FIG. 11; 
     FIG. 13A is a section through an alternative slider arrangement to that shown in FIG. 11; 
     FIG. 13B is a perspective view of the slider arrangement of FIG. 13A; 
     FIG. 14A is a section through a further alternative slider arrangement to that shown in FIG. 13A; 
     FIG. 14B is a perspective view of the slider arrangement of FIG. 14A; and 
     FIG. 15 shows a masking grid over an image grid which include means for obtaining registration between the grids. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 to  3 , a drive mechanism is shown which has a fixed rear plate  1  and a front plate  2  movably mounted on the rear plate  1 . The front plate  2  has a horizontal, forwardly projecting, top flange  3  and it is secured on each side using slide bearings  4 A and  4 B in a manner (not shown in detail in the drawings) which facilitates precise vertical sliding of the plate  2  relative to the rear plate  1 . The rear plate  1  has a horizontal, forwardly projecting top flange  5  which extends over the front plate  2  and a tension spring  6  (shown in FIG. 1) is connected between the plates  1  and  2 , thereby biasing the front plate  2  upwardly. A bearing roller  7  is mounted on the rear of the front plate  2  and it is biased upwardly by the spring  6  into contact with a rotatable cam  8  in front of the rear plate  1 . The cam  8  limits upward movement of the plate  2  and it thus defines the position of the front plate  2  relative to the rear plate  1 . A stud  31  projects forward of the plate  2 . 
     The cam  8  is mounted on an output shaft  9  of a motor  10  which incorporates as a unit a motor and reduction gearing. Rather than mounting the motor  10  directly on the rear plate  1 , it is mounted in a position that is finely adjustable relative to the rear plate  1  as will now be described. 
     An adjustable arm  12  is pivotally mounted at one end by a pivot  13  to the rear plate  1  and it is finely adjustable at the other end by an adjusting mechanism  14  which, when operated, causes limited pivotal movement of the arm  12  about the pivot  13 . The motor  10  has two mountings, the first being pivotal on the rear plate  1  coincident with the pivot  13 , and the second being on a rod  15  which is behind the rear plate  1  and the arm  12 . The adjusting mechanism  14  causes limited pivotal movement of the arm  12  about the pivot  13  and, when it does so, it imparts the same pivotal movement to the motor  10  thereby causing the shaft  9  on which the cam  8  is mounted to move about the pivot  13 . Since the shaft  9  is at substantially the same vertical level as the pivot  13 , the arcuate movement of the shaft  9  and the cam  8  is effectively vertical for small amounts of pivoting. An opening  17  in the rear plate  1  provides the necessary freedom of movement for the rod  15  to move relative to the plate  1 . 
     The adjusting mechanism  14  consists of a vertical shaft  18  connected at an upper end to the rear plate  1  and at a lower end to the arm  12  as will now be described more fully. 
     In the region of the shaft  18 , the rear plate  1  is partially cut away and portions  19 A,  19 B and  19 C are bent out of the plane of the plate  1 , portions  19 A and  19 C projecting rearwardly and portion  19 B projects forwardly (see FIG.  3 ). The upper end of the shaft  18  (not shown in FIG. 2) passes in front of the portions  19 A and  19 C and to the rear of the portion  19 B, it being screw-threadedly in those portions. In the region where the arm  12  is connected to the lower end of the shaft  18  it is similarly formed with bent portions  20 A and  20 C which project forwards and portion  20 B which projects backwards. The lower end of the shaft  18  passes behind the portions  20 A and  20 C and in front of portion  20 B, the shaft  18  being rotatably mounted in the portions without any axial movement. A knurled disc  22  fixed to the shaft  18  midway along its length enables a user to rotate the shaft  18 , and compression springs  23  on each side of the knurled disc  22  between the disc  22  and the portions  19 C and  20 A respectively prevent the shaft  18  from rotating except when the disc  22  is positively moved by the user. 
     When the shaft  18  is rotated, the screw-threaded engagement of the shaft with the portions  19 A,  19 B and  19 C of the rear plate  1  causes the shaft  18  to move axially relative to the rear plate  1  thereby moving the end of the arm  12  to which the shaft  1  is connected in a substantially vertical direction to pivot the arm  12  about the pivot  13 . 
     Referring to FIG. 4, clamping bars  69  and adjustable compression bars  71  within the display housing  43  are attached to the angled section  67  (see FIG.  5 ). A clear base panel  68  is located under the compression bars  71  and it is fixed to the angled section  67  by clamping bars  69 . The arrangement of a front clear panel  74 , an image grid  30 , a masking grid  25  and a secondary clear panel  73  laid over the clear base panel  68  can also be seen. 
     All of the clear panels  68 ,  73 ,  74  and the masking grid  25  are cut to the same length, this being indicated by lines  76  and  77 , and they each have a slot  78  cut out of them to allow for movement of the single central stud  31  which projects forwards from the mechanism  79  and to which the image grid  30  is attached. 
     The image grid  30  is shorter than the clear panels  68 ,  73 ,  74  and the masking grid  25  to ensure that as the image grid  30  is moved up and down it is not blocked by fixings  80  that pass through the clear base panel  68 , the secondary clear panel  73 , the masking grid  25 , and the clear front panel  74  to hold the masking grid  25  in position between the secondary clear panel  73  and the moveable image grid  30 . Lines  76  and  81  indicate the top and bottom of the image grid  30  when in its highest position, and the broken lines  82  and  83  indicate its lowest position. 
     Lower side slots  84  cut from the front clear panel  74  allow off-set cams  85  fixed to the compression bars  71  to be swivelled to touch the sides of the image grid  30  and to keep the image grid  30  in lateral registration with the masking grid  25 . Vertical registration between the two grids  25  and  30  is effected by the adjuster  22  forming part of the mechanism  79  which has previously been described. 
     FIG. 1 is a side view corresponding to that of FIG. 4 along broken line  86 - 87  of FIG. 4, FIG. 1 showing the angled section  67  (see FIG. 5) and the relative positions of the clear base panel  68 , the secondary clear panel  73 , the masking grid  25 , and the clear front panel  74 , plus the image grid fixed to the stud  31 , the image grid being sandwiched between the masking grid  25  and the clear front panel  74 . Also shown is how a fixing  80  passes through the clear base panel  68 , the secondary clear panel  73 , the masking grid  25  and the clear front panel  74  to fix the masking grid  25  in position. 
     As can also be seen from FIG. 1, the image grid  30  is cut shorter at a point  88  so that it is not blocked by the fixing  80  when the image grid  30  is moved up and down. A spacer  89  between the masking grid  25  and the front clear panel  74  allows free movement of the image grid  30 . 
     When the masking grid  25  and the image grid  30  form the semi-rigid viewing area, registration between these grids can be facilitated by retaining the compression bars so that they act on the moveable grid. Linear registration can then be achieved by adjusting the compression bars to ensure that the image strips on the image grid  30  are parallel to the strips on the masking grid  25  whilst applying sufficient compression to ensure total overall contact between the respective grids. 
     Referring to FIG. 5, a “T” section bar  66  is attached to the interior of the display housing  43  to allow the angled section  67  to be set in position. The clear base panel  68  is attached to the angled section  67  by a clamping bar  69  and also to an opposing angled section (not shown) by bolts passing through apertures  70  (see FIG.  4 ). An adjustable compression bar  71  is positioned on the clear base panel  68  and it is held lightly in place by bolts which pass through apertures  72  (see FIG. 4) in the compression bars  71  and they screw into the angled section  67  through the clear base panel  68 . 
     The secondary clear panel  73  is placed on the clear base panel  68 , and the adjustable compression bar  71  is pushed downwardly and inwardly as indicated by the arrow  71   x  so that the secondary clear panel  73  is compressed between the adjustable compression bar  71  and the opposing adjustable compression (not shown). The bolts passing through the apertures  72  are then tightened to secure the adjustable clamping bars  71  in position. 
     FIG. 6, which is similar to FIG. 5, shows the static grid, in this case the masking grid  25 , attached to an angled section  67  by the clamping bar  69  in a similar manner to that described with reference to FIG.  5 . Circular adjustable bearings  130  which can be adjustably fixed towards or away from the clamping bar  69 , as indicated by arrows  131 , act upon the compression bar  71 A which is attached directly to the movable grid, in this case the image grid,  30  to compress the moveable grid  30  into overall contact with the static grid  25 . By adjusting the circular bearings  130 , appropriate linear registration can also be achieved between the two grids. If required, a clear sheet  74  can be positioned between the compression bars  71 A to protect the moveable grid  30 . 
     As the compressive force is increased to keep the grids together, so the static quality of the material used to make the grids tends to become of less value as an additional means of maintaining overall contact between the grids. 
     If required, the off-set cams or the circular bearings  130  can be used to exert a compressive force directly to the moveable grid  30  to keep it in contact with the static grid  25 . In such a case, the compression bars are usually unnecessary. Also in such cases it can be more desirable to effect parallel registration between the two grids by attaching a single bracket to the moveable grid, this bracket being adjustably attached to another bracket or block attached to a slideable plate of the mechanism for moving the associated grid. The bracket attached to the slideable plate is then preferably attached at as great a distance as possible from the protruding stud to which the moveable grid is attached. The moveable grid can then be attached to the protruding stud on the slideable plate using circular bearings to allow the moveable grid to swivel at the point at which the moveable grid is attached to the protruding stud. If required, the static grid can be sandwiched between a clamped clear or semi-opaque sheet and the moveable grid to which the compressive force is applied. 
     The cam used in the embodiments of FIGS. 1 to  6  is shown in FIG. 7, and, as can be seen, the periphery of the cam  8  is divided into a plurality of arcuate portions  8 A to  8 F, each defining an arc of a circle centered on the axis or rotation of the cam  8  and having a radius R A  to R F . At the junction between adjacent arcs are inclined ramps  65 . There are six arcuate portions shown in FIG. 7, but particular image grids and masking grids may have different numbers of arcuate portions, for example eight sets of arcuate portions with four sets of image strips. 
     During use of the illustrated display systems, the cam  8  is rotated by the motor  10  and, as it rotates, the bearing roller  7  rests against and remains in contact with the bottom of the peripheral surface of the cam  8  due to the bias of the spring  6 . The vertical position of the roller  7  is then determined by the radius of whichever portion  8 A to  8 F of the cam periphery is in contact with the bottom of the cam  8 , and the difference in radius between adjacent cam portions corresponds to the center-to-center spacing of adjacent image strips as shown, for example in FIGS. 8A and 8B. 
     An effect of one image fading into another can be achieved by reducing the incline or increasing the decline of the steps  65 . 
     The cam  8  shown in FIG. 7 will result in the images changing almost immediately and then remain for a given length of time. However, if the inclination of the steps  65  is considerably decreased and the declination of the steps  65  is considerably increased, for example so that the steps  65  are about one third of the lengths of the arcs  8 A- 8 F, the images displayed will appear to fade into the next which will then appear for a shorter period of time. 
     FIG. 8A shows a cut away view of an image grid  30  positioned over a masking grid  25  in a system as described with reference to the preceding drawings. If these two grids are set up with this relative disposition with the roller on one of the portions  8 A to  8 F, rotation of the cam  8  of FIG.  7  through one eighth of a revolution will result in the roller  7  moving exactly the distance required to move the image grid  30  relative to the masking grid  25  by the exact width of one of the image strips, image strips  49 B,  50 B,  51 B and  52 B then becoming aligned with window strips  58 F,  59 F,  60 F and  61 F respectively. 
     Since movement of the roller  7  takes place in steps rather than gradually as it rolls over successive inclined ramps  65 , the image grid is moved accordingly. 
     Using a cam  8  with six arcuate portions  8 A to  8 F which form two groups of three, the first three in turn increasing in radius and the second three in turn decreasing in radius, if the cam  8  revolves in a clockwise sense, the image grid  30 , will first be moved by three steps downwardly from the first image visible through the masking grid as the cam  8  is rotated, and then three steps upwardly as it completes a revolution back to the first image. Each step brings the next of the four sets of image strips into alignment with the window strips  58 F,  59 F,  60 F and  61 F of the masking grid  25 , and it then reverses the sequence, by which time the cam  8  has completed a single revolution. 
     As will be appreciated, the cam  8  can be rotated continuously at a constant rotational speed and the individual strips of the image grid  30  will then be viewed for the time taken for the cam to effect approximately one eighth of a rotation of the cam  8 . 
     If it is desired to display one of the images on the grid  25  for a longer period than the others, the cam  8  can be provided with a longer arcuate portion so that the roller  7  remains in contact with the cam  8  for a longer period of time with that portion than on others. In such cases it is generally preferred to have diagonally opposite long arcuate portions. 
     A viewer of the display is usually unaware of the way in which the image grid  30  is moved relative to the masking grid  25 , the viewer simply seeing a first image which, as the bearing roller  7  passes over one of the inclined ramps  65 , changes smoothly into a second image, and so on. 
     It is, of course, desirable that the masking strips  53 E to  57 E are not too thick as this could adversely affect the resolution of the display. However, masking strips with a width of up to about 1.5 mm are generally satisfactory, but a noticeable reduction in resolution can be observed if the width of these strips is much wider than this. By way of example, the masking strips  53 E to  57 E can be 1.4 mm wide with the image strips being 0.2 mm wide, the center-to center spacing of the masking strips being 1.6 mm with that of the image strips being 0.4 mm. 12.5% of the image is then displayed through the window strips. In this case, the cam  8  will have adjacent arcuate portions  8 A to  8 F which differ in radius by 0.4 mm. 
     In the case just described, the image strips are twice as wide as the window strips, only one half of each image strip therefore being displayed. However, even in this case, the overlap of the image strips by the opposing edges of the windows in the masking grid is still only 0.1 mm which can be regarded as the tolerance in positioning the respective strips relative to each other. The image grid and the masking grid therefore need to be printed, positioned and moved very accurately. It will also be appreciated that any separation of the masking grid from the image grid of even less than 0.1 mm can have a markedly adverse effect on the performance of the system, and especially when the systems are viewed other than from directly in front. 
     Although FIG. 8A shows only relatively few image strips and masking strips, it will be appreciated that in practice a multiplicity of strips will be used. For example, the display area can be up to 0.7 m wide or more and up to 1.0 m high or more, and it is obviously important that the image grid and the masking grid be maintained in contact with each other throughout this area and that differences in expansion between these grids is kept to a minimum. 
     The arrangement of grids shown in FIG. 8B is substantially the same as that described with reference to FIG. 8A, but the masking grid  25  is in this case on top of the image grid  30 . 
     As will also be appreciated, it is possible to move the image grid and to keep the masking grid stationary or to move the masking grid and keep the image grid stationary, and in either case, the image grid can be behind or in front of the masking grid. 
     While the embodiments described above use vertical movement of one grid relative to the other, it will be appreciated that the grids can be moved relative to each other horizontally or in a sideways direction. In a yet further alternative, the relative motion between the grids can be in a vertical direction, but with a drive mechanism below the grids rather than above them. 
     FIG. 9 shows an alternative eight sided cam  104  having eight arcuate cam surfaces  105 N to  112 N which subtend equal angles to and are convex towards its center. The respective ends  114 P to  121 P of the arcs  105 N to  112 N are each 2.00 mm further away from the center  113  than are the centers  112 Q to  129 Q of the surfaces  105 N to  112 N. Thus when cam  104  is used in place of the cam  8  described in relation to FIG. 7, the masking grid  25 A or the image grid  30 A (see FIGS. 10A and 10B) controlled by the mechanism in the manner described in relation to FIGS. 1 to  3  is moved 2.00 mm down and then 2.00 mm up as the periphery of the cam  104  moves in a clockwise sense over the bearing  7  of the mechanism  79  from position  126 Q to  117 P and then on to position  125 Q. 
     Further rotation of the cam  104  results in the bearing  7  being forced downwardly again to arc end  116 P, and so on. In this manner, as cam  104  rotates by one eighth of a revolution, the masking grid  25 A or the image grid  30 A are moved by rotation of the cam  104  and will at first fall 2.00 mm and then rise by 2.00 mm. 
     Referring to FIG. 10A, each image strip  90 C to  94 E of the image grid  30 A is 0.20 mm wide, and since image strips B to J of each set  90  to  94  are obscured by opaque strips K 95  to K 99  of the masking grid, which themselves are each 1.80 mm wide, only image strips A 90  to A 94  are visible through transparent strips L 100  to L 103  which are themselves 0.20 mm wide. A similar situation applies with regard to FIG. 10B where the image grid  30 A and the masking grid  25 A are identical to those in FIG. 10A but the image grid is on top of the masking grid  25 A rather than on top of it. 
     Each of the 0.20 mm wide image strips  90 C to  94 E shown in FIG. 10A, or  90 A to  94 G shown in FIG. 10B, will then briefly appear twice adjacent to the corresponding transparent strips  100 L to  103 L of the masking grid  25 A, once as the masking grid  25 A or the image grid  30 A rises, and then once as it falls. 
     Using the cam  104  with the image and masking grids of FIGS. 10A or  10 B, if the cam  104  revolves once every forty seconds, the resulting animation sequence will be five seconds long from the first image to the tenth image and back to the first image again. This results from each sequence being one eighth of the time taken for a full revolution of the cam  104 . 
     As an alternative to the compression and slide system described with reference to FIGS. 5 and 6, FIG. 11 shows the clamping bar of FIG. 5 replaced by a slide housing  69 B to which a static grid is attached, for example a masking grid  25 . The bearings  130  of FIG. 5 are replaced by a self-lubricating plastic slider  130 A to which the moveable grid  30  is attached, the slide housing  69 B being fixed to the angled section  67 . 
     Studs  132  fixed to and projecting from the upper surface of the self-lubricating plastic slider  130 A correspond with accurately punched holes along the side edges of the moveable grid  30  so that when the moveable grid  30  is held in position by the projecting studs  132 , it is compressed into the static grid  25  when the static grid  25  and the moveable grid  30  are concave, the compressive effect being achieved by ensuring the distance between the punched holes along one side of the grid and those along the other side is greater than it would be if these punched holes were positioned to hold the grid so that it lay along the curve of the other fixed grid without being compressed into the fixed grid. However, if the grids  25  and  30  are convex, the static grid  25  is compressed into the moveable grid  30 , again by ensuring the distance between the fixings holding the static grid in position are appropriate to ensure the compressive force is effective. 
     Referring to FIG. 12, the static grid  25  is longer than the moveable grid  30 , thereby allowing the static grid  25  to be attached to the slide housing  69 B at points  133  without blocking movement of the moveable grid  30 . To provide a firm backing to facilitate wiping of the grids  30  and  25 , and to remove air pockets between them, a sheet of clear or semi-opaque material, say 3 mm thick, can be compressed between the two slide housings  69 B to create a rigid curve matching that of the grids  30  and  25 . However, great care should be taken to ensure that the changing curvature of the clear or semi-opaque material caused by expansion and contraction produced by temperature changes does not affect movement of the moveable grid  30 , for example by pushing the static grid  25  against the moveable grid  30  with such force that the moveable grid  30  is jammed against the static grid  25  and thus cannot move. 
     The upper surface of the self-lubricating plastic slider  130 A to which the projecting studs  132  are attached projects outwardly of the slide housing  69 B, for example by 300 μm if the static grid  25  attached to the extended upper surface of the slide housing  69 B is 250 μm thick. 
     For greater accuracy of movement of the moveable grid  30 , the self-lubricating slider  130 A can be replaced by an arrangement of linear or roller bearings at each end of both slide housings  69 B, the linear or roller bearing at one end of the slide housing being joined to the other at the other end of the slide housing by an aluminum bar to which projecting studs  132  are fixed corresponding to the holes punched along the side edges of the moveable grid  30 . 
     The moveable grid  30  can then be moved by spindles  134  controlled by linear stepper drive mechanisms  135 A and  135 B fixed to one end of each of the self-lubricating sliders  130 A. The drive mechanisms  135 A and  135 B are controlled by a control board  136 , and they can be arranged to drive the spindles  134  repeatedly up and down in appropriate increments, for example four increments of 0.40 mm in the case of a quadruple image system, three increments of 0.50 mm in the case of a triple image system, and a single up and down movement of 2.00 mm in the case of an animated system incorporating ten images each made up of image strips 0.20 mm wide. 
     The self-lubricating slider  130 A shown in FIG. 11 is moved by the linear drive mechanisms  135 A and  135 B, and the slide housing  69 B remains static. However, with the alternative profiles of slide housing  69 B and self-lubricating slider  130 B shown in FIGS. 13A and 13B, the slide housing  69 B is moveable by the linear drive mechanisms  135 A and  135 B and the slider  130 B remains static. The slide housings  69 B are attached by brackets  137  to the spindles  134  which are driven by the linear drive mechanisms  135 A and  135 B, the self-lubricating slide  130 B being fixed to the angled section  67 . 
     The window strips of the masking grid will then usually need to be centered along the center lines of the image strips of the image grid, and this can be effected using the linear drive mechanisms  135 A and  135 B to move the associated spindles  134  sufficiently to achieve registration. If the image strips of the image grid are not parallel with the window grids of the masking grid, the drive mechanism on one side of the moveable grid is then adjusted accordingly. Any vertical movement required to bring about registration of image and window grids which are parallel but not otherwise in registration can then be effected by moving both sides of the moveable grid by the same amount using the drive mechanisms  135 A and  135 B. 
     Movement of the moveable grid relative to the static grid to display images can then be effected using equal steps produced by the linear drive mechanisms  135 A and  135 B under the control of the control board  136 . Extra steps can be incorporated into the drive sequence to take account of back-lash in the drive mechanism that can occur when the driving force changes direction. 
     FIGS. 13A and 13B show an alternative slide housing  69 B and self-lubricating slider  130 A, the slide housing  69 B being attached to the linear drive mechanism  135  using brackets  137  attached to the spindles  134 , and the self-lubricating slider  130 A being fixed to the angled section  67 . 
     FIGS. 14A and 14B show a yet further alternative arrangement of slide housing  69 B and self-lubricating slider  130 A which is similar to that of FIGS. 13A and 13B but has different profiles for the slide housing  69 B and the self-lubricating slider  130 A, in particular to provide a groove in the angled section  67 A in which a portion of the studs  132  extending below the slide housing  69 A can run freely. 
     FIG. 15 shows an arrangement of a masking grid  25  overlaying an image grid  30  which include means for obtaining registration between the window strips of the masking grid  25  and the individual image strips of the image grid  30 , the masking grid  25  having been moved to the left to show the image strips below. 
     The masking grid  25  shown in FIG. 15 has opaque strips Ei to Evii and window strips Fi to Fvii, and the image grid  30  has image strips Bi to Dviii. 
     The window strip Fiv, which is between adjacent masking strips Eiv and Ev, has a portion G near to the edge of the masking grid which is narrower than at its center and narrower than the window strips between the other masking strips. 
     An edge portion of the image grid  30  beneath the masking grid  25  has four window strips AW, BW, CW and DW centered along the center lines of four adjacent image strips AV to AD, these being positioned so that the narrow portion of the window strip Fiv coincides with the windows AW, BW, CW and DW of the four adjacent image strips AV to DV when the respective sets of image strips A to D are in registration with the window strips Fi to Fvii of the masking grid  25 . Registration of the window strips Fi to Fvii of the masking grid  25  with the respective images of the image grid  30  can then be effected using a photocell to detect light passing through the narrow windows in both grids, and this can also be used to control the relative upward and downward movement of these grids during normal operation of the system. 
     The use of a photocell with narrow window strips at the edges of the masking and image grids to control the linear drive mechanisms  135 A and  135 B can enable a particularly accurate positioning of these grids relative to each other and it can be used to control the number of the steps taken between successive images to compensate for backlash in the drive mechanism or for minor dimensional inaccuracies in the grids. 
     As will be appreciated by those skilled in the art, registration of the image grid with the masking grid, both vertically and, to ensure the image strips are parallel to the window strips, horizontally, can be effected using arrangements of photocells, for example which respond to positional information determined from the opposite edges of the image and the masking grid such as have been described for the grids with reference to FIG.  15 . 
     Poster display systems in accordance with the present invention can be used in a wide variety of applications, for example: 
     1. as retail outlet window displays, the system in accordance with the invention being suspended against the window of the retail outlet and relying on external, natural ambient light to backlight the poster or on electric illumination directed from inside the retail outlet which backlights the poster images so that they can be viewed from outside the retail outlet; 
     2. as illuminated display case accessories in which the display system is positioned within the case to be backlit by a light source in the case; 
     3. as edge-lit display accessories in which the display system is positioned against the front face of an edge-lit display; 
     4. as controllable partition systems in which,the partition can be transparent or opaque, the image grid itself possibly being in the form of a masking grid to make the partition opaque when the window strips of the masking grid are obscured by what amount to masking strips of the image grid; and 
     5. as a vending machine accessory in which the display system is positioned within the machine so that is can be seen by potential users.