Abstract:
A curved projection screen having a one-piece seamless substrate having a three-dimensional curvature. The screen is arranged for being bent to a certain degree around one of its axes so that the smallest deformation of the screen is elastic, where the edge of the screen has been shaped so that its deformation during the above mentioned degree of bending is also elastic.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to curved projection screens, in particular projection screens that are curved in three dimensions. 
     Projection screens that are shaped like a sphere or part of a sphere, so-called dome screens or immersive screens, can be used in a variety of fields, such as simulation, training, high end gaming, immersive display for virtual and augmented reality, etc. It is crucial for the quality of the projected image to have a smooth and bulge-free surface in order to provide a high image quality. This can be obtained by implementing the screen in a rigid material that resist deformation that could arise from draft, non-uniform tension between fixation points, its own weight etc. It is common to implement such rigid screens by dividing them into smaller segments, in order to facilitate logistics. In this way it is possible to use standard trucks or containers to carry the screen, or to be able to pass through a standard door or window when bringing the screen into a building. The dimensions of the opening width of a “standard door” are typically in the range 750 to 800 mm. For example, the Housing Standards Review—Illustrative Technical Standards Developed by the Working groups from the UK Department for Communities and Local Government dated August 2013 details requirements for communal entrances, both internal and external A communal entrance door to a building containing flats (or other dwelling types served by a communal entrance or doors) should have a clear opening width of at least 800 mm. All external gates, openings or doorways within an approach route should provide a minimum clear opening width of 850 mm. Secondary entrance doors, for example within a lobby or porch, should provide a minimum clear opening width of 800 mm. For a private entrance area a door should have a clear opening width of at least 800 mm. The doorway to any habitable room, the kitchen, and the room containing the WC should provide a minimum clear opening width of 750 mm. 
     At the installation site, the screen is mounted by putting the segments together. A visible seam between the screen segments would disturb the projected image, therefore these seems are concealed with additional fill up and painting. To assemble and finish the screen surface on-site requires a substantial amount of labour and equipment. The final coatings or paints should also be applied by specially trained people. 
     U.S. Pat. No. 5,724,775 discloses a multi-pieced portable projection dome that is constructed from a plurality of rigid panels. 
     U.S. Pat. No. 6,712,477 discloses a non-deployable hemispherical projection dome. 
     CN2932438 discloses a method of manufacture of a cylindrical rollable projection screen. 
     SUMMARY OF THE INVENTION 
     The present invention therefore seeks to provide a screen that can be curved in three dimensions and that is suitable for high quality image projection. The screen comprises a one-piece seamless curved substrate that can be rolled or bent for better suiting logistic requirements. The edge of the screen is shaped so that it can deform elastically when the screen is bent to a certain degree. 
     According to an aspect of the present invention there is provided a curved projection screen comprising a one-piece seamless substrate having a three-dimensional curvature. Said screen is arranged for being bent to a certain degree around one of its axes so that the smallest deformation of the screen is elastic, wherein the edge of said screen has been shaped so that its deformation during the above mentioned degree of bending is also elastic. 
     In one aspect the present invention provides a curved projection screen comprising a self-supporting seamless substrate, having a three-dimensional curvature and with at least one pair of truncations on two opposing truncated edges, each pair being positioned at the ends of an axis, wherein said screen is rolled around one of the axes so that the smallest diameter of the screen in its rolled state is about 30% of the diameter in its deployed state. 
     The shape depends on the degree of bending. This shape can comprise at least two truncations of the curved surface whereof two truncations are placed opposite to each other and perpendicular to the axis of bending. In this way the radius of curvature in the bended state is increased and the deformation and corresponding induced stress is decreased so that it is elastically deformed. Further can the substrate be coated, before shipping and installation, with at least one optical coating on the concave or convex surface or on both surfaces. This avoids the additional step of applying the coating at the installation site. 
     According to another aspect of the invention there is provided a curved projection screen comprising a one-piece seamless substrate having a three-dimensional curvature. Said screen is arranged for being bent to a certain degree around one of its axes so that the smallest deformation of the screen is elastic, wherein the edge of said screen has been shaped so that its deformation during the above mentioned degree of bending is also elastic. 
     Said screen has means for fastening attached along a surface that is not intentionally displaying a projected image, for example on the backside of a front projecting screen, or along the edge on a rear-projecting screen. The means for fastening can be magnets or Velcro or screws, any combination thereof or any other type of fastening. With the means for fastening, the screen can be attached to a supporting frame, to make sure the shape of the screen stays as intended. 
     According to another aspect of the invention there is provided a curved projection screen comprising a one-piece seamless substrate having a three-dimensional curvature. Said screen is arranged for being bent to a certain degree around one of its axes so that the smallest deformation of the screen is elastic, wherein the edge of said screen has been shaped so that its deformation during the above mentioned degree of bending is also elastic. 
     Said screen has a plurality of alignment magnets attached to the backside of the screen. Reference magnets can then be placed on the displaying side of the screen so that they are attracted to the alignment magnets and attached to the screen. This provides a non-electrical and simple method for obtaining reference points for the alignment of the projected images. 
     A removal magnet, being larger and stronger than the reference magnet can be positioned on top of the reference magnet to remove it. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a segmented curved display screen. 
         FIG. 2 a    shows a plane sheet bent into a cylindrical shape, as seen in  FIG. 2   b.    
         FIG. 3 a    shows a sheet curved in three dimensions and  FIG. 3 b    shows the sheet then bent along one of its axes. 
         FIGS. 4 a  and 4 b    show an embodiment of the present invention where the curved display has two truncations opposite each other. 
         FIG. 5 a    shows an example of a vertical-filed of view and  FIG. 5 b    shows a horizontal field of view. 
         FIG. 6  shows a support structure where a curved screen can be attached. 
         FIG. 7  shows an embodiment of the present invention in a rolled-up state. 
         FIG. 8  shows a cross section of an embodiment of the present invention. 
         FIG. 9 a    shows an embodiment of the present invention where an alignment aid is provided by means of magnets.  FIG. 9 b    shows an embodiment of the present invention rolled to a cigar shape 
         FIG. 10  shows a stress-strain diagram. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. 
       FIG. 1  shows an example of a curved rigid display screen  1  made out of different rigid segments  2  that are attached together, for example by glue and bolts. The curved rigid display screen  1  can be in the form of a dome or part dome, i.e. exhibiting at least a portion thereof having a three-dimensionally curved surface such as a spheroidal or spherical surface. To hide the seams on the inside display surface  3  a paint or coating can be applied on top of the assembled segments. This final coating is preferably applied at the site of installation, and is thus an expensive and time consuming solution. 
       FIG. 2 a    illustrates a thin flat sheet  10  extending along the axes  12  and  13 , which are perpendicular and cross each other in the center of the sheet  10 . As seen in  FIG. 2 b    the sheet  10  is then curved around the axis  12  and a cylindrical surface  11  is obtained. The line  14  (centered) is curved with a radius of curvature R. 
       FIGS. 3 a  and 3 b    illustrate how the surface  11  is curved around an axis parallel to  13  so that a three-dimensionally curved surface  20  is obtained. The line  14  is curved with radius of curvature R and axis  15  (centered) is curved with radius of curvature R′. If the curved surface  20  is now rolled around an axis parallel to  12  the result is a cigar shaped volume  21  with two lines  16  and  17  with radiuses of curvature R1 and R2 respectively. Note that R1 will be different from R and R2 will be different from R′. 
     The stress experienced by surface  11  will be substantially the same along the line  12  since the radius of curvature is the same. The stress experienced by volume  21  will be higher at the edges  22  and  23  since the radius of curvature is smaller at those edges than at the line  16 .  FIG. 10  illustrates a stress  91  strain (or deformation)  92  diagram with the elastic limit  90 . If the edges  22  and  23  are submitted to a stress higher than the elastic limit  90 , the material will enter the plastic region and deform and/or break. At this moment, the material in the vicinity of line  16  may still deform elastically. The number t of degrees a sheet can be bent or rolled will thus depend on its geometrical- and material characteristics. 
     The materials of interest for the present invention need to be rigid enough to sustain their pre-formed rounded shape, even without support. Such a material has a high E-modulus, thus the stress  91  increases fast with the strain or deformation  92 . Such a material may be a rigid plastic having a modulus of elasticity either in flexure or in tension greater than 700 MPa at 23° C. and 50% humidity, when tested in accordance with ASTM methods D747, D790, D 638, or D 882 (see Compilation of ASTM standard definitions, Fourth Edition 1979). The flexibility of a material is determined by a combination of its modulus in bending or tension, its thickness and its extension at break. However as weight is an important characteristic for handling a transport a higher modulus of a thinner material can be preferable. Thus the present invention allows a variety of combinations of material properties such as thickness, bending or tensile modulus and extension at break. 
     An example of a suitable material is given in this table: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Property 
                 Test standard 
                   
                 Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Bending modulus 
                 ISO 178 
                 8.3 
                 GPa 
               
               
                   
                 Bending strength 
                 ISO 178 
                 235.1 
                 MPa 
               
               
                   
                 Tensile strength 
                 ISO 527 
                 91.8 
                 MPa 
               
             
          
           
               
                   
                 Extension at break 
                 ISO 527 
                 6.64% 
               
               
                   
                 Weight 
                   
                 1 layer of 450 g/m 2   
               
             
          
           
               
                   
                 Thickness 
                   
                 2 ± 0.5 
                 mm 
               
               
                   
                   
               
             
          
         
       
     
     The standards refer to those applicable on 16 Sep. 2013. A suitable thickness range can be between 1.5 and 5 mm average. If a sheet of such a material is very heavily curved, for example having the shape of a complete semi-sphere, it is not possible to bend it around its centre axis without breaking it. The deformation and the associated stress rapidly rises to values higher than the elastic limit  90 . A remedy is to cut out a piece of material of the sphere, as shown at  32  and  33  in  FIGS. 4 a  and 4 b    so that the radius of curvature in its deformed shape is increased. This will reduce the deformation and the associated induced stress so that the stresses experienced at the positions indicated by  32  and  33  will remain within the elastic range. In principle this remedy is also valid for a curved screen that is not perfectly spherical but has more than one radius of curvature. 
     A related remedy is to use a smaller field or fields of view  40  and/or  41  of the screen, vertically and/or horizontally (see  FIG. 5 ). 
       FIGS. 5 a  and 5 b    illustrate the field of view of an image. The field of view can be described as an angle between the lines originating from the observer to the edges of the image. It is described for the vertical, horizontal and sometimes diagonal extent of the image.  FIG. 5 a    illustrates the vertical field of view  40  of a flat object  44  experienced by the observer  42  and  FIG. 5 b    illustrates the horizontal field of view  41  for the flat object  44  experienced by the observer  42 . The line of sight  43  can be used to differentiate between the positive vertical (above) and negative vertical (below) field of view. In two- and three dimensions the line of sight will be a line or a curve respectively and can be called the horizon. The human limit for the total vertical field of view  40  is about 135 degrees. The distribution of the field of view does not have to be symmetrical around the reference line  43  (or the horizon). In one embodiment of the present invention, the horizontal field of view of the screen is at least 75 degrees and the vertical field of view of the screen is at least 75 degrees. 
     With embodiments of the present invention it is possible to have a pre-coated seamless screen that is easy to bring into a building and is fast to install. The installation can comprise the steps of deploying the screen and inserting it into a supporting frame  50  as shown schematically in  FIG. 6 , with means of fastening. As shown in  FIG. 6 , the support is built up of elongated structures  51  leaving empty spaces  52  between and should not be limited to the layout in  FIG. 6 . The means of fastening can be for example magnets, hook and loop fasteners such as Velcro, or screws, but should not be limited hereto. The means of fastening may be distributed over the surface of the screen that is not used for intentionally displaying an image, hereafter referred to as the non-displaying surface. The means for fastening can also be applied along the edges of the screen, for example directly attached on the screen. Alternatively there can be part of the support structure placed so that it clamps the screen edge and the means for fastening is fixing this clamp to the screen. It is then possible to apply the means for fastening only along the edges of the screen. A mixture of the two is also possible. Fastening the screen provides further stability and assures the projection surface remains in its intended shape. 
       FIGS. 7 and 9   a  and  b  show how the screen can be rolled whereby a reduction can be achieved from 240 cm to 750 mm, which fits into or passes through a standard door.  FIG. 7  and  FIG. 9 b    show an embodiment of the present invention where the curved display screen  30  is rolled up around one of its vertical axes,  61 , into a cigar-like shaped roll  60 . The diameter in the rolled-up state can be significantly decreased. For a smaller sized display it can be possible to pass through a standard door, e.g. having a width of clear space of at least 750 mm or at least 800 mm. For example, if the screen has a deployed diameter of about 240 cm it could be rolled-up to a diameter of about 750 mm which is consistent with the above dimensions of at least 750 mm, e.g. at least 800 mm. But larger screens with for example diameters like 500 cm can also be rolled in order to decrease the diameter. These should, however, not be seen as limiting examples. 
       FIG. 8  shows a cross section of an embodiment of the present invention. The curved display screen can be used for front- or rear projection. For a rear-projection screen the substrate  70  has to be transparent for visible light and the optical coating  71  can be designed to diffuse the transmitted light. For a front projection screen the substrate  70  can be transparent or non-transparent. The coating  70  may for front projection be a reflective coating. It is also possible to have coatings on both sides of substrate  70  (not shown here). The screen substrate can be implemented with various types of materials, for example glass fiber reinforced plastics or polycarbonate or any other transparent light weight material for the rear-projection embodiment. For the front projection embodiment it is also possible to use non-transparent materials like steel, aluminum or other alloys, or any other material that would give a light weight screen. 
       FIG. 9 a    shows an embodiment of the present invention intended for front projection. Magnets  81  (only one indicated) are distributed over the non-displaying screen surface  80 . These magnets will be referred to as alignment magnets. When the screen is installed, additional magnets, referred to as reference magnets (not shown), can be attached to the screen at the location of the reference magnets. For example a plurality of alignment magnets can be pre-mounted on the backside of the screen. If several images are tiled, these magnets can be used as reference points during the alignment of the projectors. After the alignment is done, a magnet that is bigger and stronger than one of the reference magnets can be used for the removal of the reference magnets. For example neodymium magnets can be used, or any other strong type of magnet. In contrast to additional complex electrical equipment for alignment, this is a simple and fast solution.