WO2019185949A1

WO2019185949A1 - Inflatable light diverging mirror and method for making the same

Info

Publication number: WO2019185949A1
Application number: PCT/EP2019/058210
Authority: WO
Inventors: Johan Cyriel DECOCK
Original assignee: Barco N.V.
Priority date: 2018-03-30
Filing date: 2019-04-01
Publication date: 2019-10-03
Also published as: GB201805799D0; GB2572648A; GB201805349D0

Abstract

The present invention describes an inflatable light diverging mirror for projection applications and a method for making the same. The inflatable light diverging mirror comprises a rollable sheet sandwiched between a first and second board, and the first board has an opening. When the edges of the sandwich structure is sealed, the rollable sheet can be inflated with gas through the second board so that the sheet adopts a convex shape. The gas has an overpressure and can keep the sheet in the convex shape.

Description

Inflatable light diverging mirror and method for making the same

The present invention relates to a kit of parts to be assembled into an inflated light diverging mirror, to an inflatable light diverging mirror and to a method for making the inflatable light diverging mirror.

Background

The projection of image- or video content onto a curved projection screen or dome (or in general projection with a wide field of view) may be implemented e.g. with a projector having a fish-eye lens. Alternatively it is possible to use a projector having a conventional lens in combination with a convex mirror of e.g. coated glass or acrylic. In many situations these solutions bring high monetary cost or they do not provide sufficient quality. For example, the amount of light that a fish-eye lens can receive is often lower than what is required to fill the surface of a dome.

BoPET polyester films are hereafter referred to as“Mylar” (even though there can be several suppliers and products other than e.g. Mylar™, Melinex™ or Hostaphan™). Mylar can be coated with reflective materials and configured for radiation concentrating applications such as e.g. solar collectors or astronomy instrumentation. Construction and Optical Testing of Inflatable Membrane Mirror Using Structured Light Technique (International Journal of Photoenergy, vol 2015, Patino Jimenez et. al) discloses a concave solar collector which is parabolically shaped in order to collect the incoming solar radiation in a focal point. Other types of applications are decorative structures such as toy balloons or art creations.

Summary of the invention

It is an objective of the present invention to provide a kit of parts for making an inflatable light diverging mirror for projection applications comprising a seamless rollable sheet having a reflective layer and a first and second board, and means for sealing. The first board can have a circular or elliptical opening and the opening can have an edge. The sheet can be dimensioned so that it can be sandwiched between the boards so that it extends across the opening, and the reflective layer can face the first board. Means for sealing can be provided for sealing The sheet and the boards to each other outside the opening, so that the sheet and the second board are able to enclose a cavity and a supply of gas for being fed into the cavity which has a pressure higher than the ambient atmospheric pressure, so that the sheet can have a convex shape with respect to the second board after filling and can be pressed against the edge of the opening.

Additionally or alternatively, the board can comprise a circular or elliptical opening where any diameter is at least 40 mm, up to at least 700 mm or up to at least 2m or up to 5m. Additionally, if the opening is circular, the convex part of the sheet can have a shape of a spherical cap with a diameter of at least 40 mm, up to at least 700 mm or up to at least 2m or up to 5m and an apex angle of at least 90 degrees.

Additionally or alternatively, the gas supply can provide gas at a pressure of at least 0.1 bar, up to 0.5 bar up to 2 bar above the ambient air pressure. The gas can be air, Nitrogen, Argon or any inert gas.

Additionally or alternatively, the mirror can comprise a seamless rollable sheet which can be pre-shaped in a convex shape with respet to the second board. Additionally or alternatively, the part of the second board that is aligned with the opening of the first board can be pre-shaped in a convex shape with respect to the remaining part of the second board. This is a high quality feature that is suitable for projection applications. Further parts of a kit are detailed below.

It is alternatively an objective of the present invention to provide an inflatable light diverging mirror for projection applications comprising a seamless rollable sheet having a reflective layer and a first and second board, and means for sealing as well as to provide an inflated light diverging mirror of this type. The first board can have a circular or elliptical opening and the opening can have an edge. The sheet can be sandwiched between the boards so that it extends across the opening, and the reflective layer can face the first board. The sheet and the boards can be sealed to each other outside the opening with means for sealing, so that the sheet and the second board enclose a cavity comprising a gas which has a pressure higher than the ambient atmospheric pressure, so that the sheet can have a convex shape with respect to the second board and can be pressed against the edge of the opening. This has the advantage of providing an inflatable mirror having a smooth surface with high quality that is suitable for projection applications. These features also assist in no wrinkles propagating along the convex shaped sheet. This is also a high quality feature that is suitable for projection applications.

Additionally or alternatively, the board can comprise a circular or elliptical opening where any diameter is at least 40 mm up to at least 700 mm or up to at least 2m or up to 5m. Additionally, if the opening is circular, the convex part of the sheet can have a shape of a spherical cap with a diameter of at least 40 mm, up to at least 700 mm or up to at least 2m or up to 5m and an apex angle of at least 90 degrees.

Additionally or alternatively, the mirror can comprise the gas having a pressure of at least 0.1 bar up to 0.5 bar up to 2 bar above the ambient air pressure. The gas can be air, Nitrogen, Argon or any inert gas.

Additionally or alternatively, the mirror can comprise a seamless rollable sheet which can be pre-shaped in a convex shape with respet to the second board. Additionally or alternatively, the part of the second board that is aligned with the opening of the first board can be pre-shaped in a convex shape with respect to the remaining part of the second board. This is a high quality feature that is suitable for projection applications.

Pre-shaping of the sheet and the second board can have the advantage that the cavity will be smaller so that less amount of gas has to be used to inflate the sheet, making the system less sensitive to environmental variations in e.g. pressure and humidity.

Additionally or alternatively, the system can comprise means for sealing being glue, screws, bolts or a combination thereof.

Additionally or alternatively, the sheet can comprise a polyester foil.

Additionally or alternatively, the system can comprise the second board being opaque. Additionally or alternatively, the first or second board can comprise a light absorbing surface, which can reduce secondary reflexions.

Additionally or alternatively, the system can comprise an o-ring between the sheet and one of the boards, which can strengthen the seal.

Additionally or alternatively there can be provided a projection system comprising the above inflatable light diverging mirror, a light beam impinging on said mirror, a projection screen, a blocking screen and a position of a viewer, and where the blocking screen is placed between the inflatable light diverging mirror and the position of the viewer so that it blocks light that would otherwise have been reflected outside the projection screen. Such light could be reflected directly the eyes of a viewer, or be projected and reflected on surfaces outside the projection screen, which is normally unwanted.

Preferably, no wrinkles propagate along the convex shaped sheet. This is also a high quality that is suitable for projection applications.

Additionally or alternatively, the projection system can comprise the above inflatable light diverging mirror and a light beam impinging onto said mirror, an projection screen and an optical filter that blocks light that would otherwise have been reflected outside the projection screen. Preferably these features result in no wrinkles propagating along the convex shaped sheet. This is also a high quality feature that is suitable for projection applications.

In another aspect of the present invention there is provided a method for manufacturing an inflatable light diverging mirror for projection applications comprising a seamless rollable sheet and a first and second board, and means for sealing. The first board can have a circular or elliptical opening and the opening can have an edge. The second board can have a gas valve. The method can comprise the steps of

- sandwiching the sheet between the boards so that it extends across the opening and is facing the first board,

- sealing the sheet and the boards to each other outside the opening using the means for sealing,

inserting a gas through the gas valve so that the gas is expanding the sheet into a convex shape with respect to the second board, and pressing the sheet against the edge of the opening. Preferably, these features result in no wrinkles propagating along the convex shaped sheet. This is also a high quality feature that is suitable for projection applications .Additionally or alternatively, the method can comprise that before the step of sandwiching the sheet between the boards, the sheet can receive a reflective layer on the side facing the first board. By starting with a sheet that already has a reflective layer, this has not to be administered in a later step.

Additionally or alternatively, the method can comprise that before the step of sandwiching the sheet between the boards, the sheet can be pre-shaped in a convex shape. Additionally or alternatively, if the first board is aligned and positioned on top of the second board, the method can comprise pre-shaping the part of the second board that is aligned with the opening of the first board into a convex shape with respect to the rest of the second board (i.e. the part which is positioned outisde the opening of the first board). Pre-shaping is a high quality feature that assists in making an inflatable light diverging mirror suitable for projection applications.

Additionally or alternatively, the method can comprise that when the gas has expanded the sheet to a convex shape with respect to the second board, the sheet can receive a reflective coating on its convex side. By depositing the reflective layer after pre-shaping and/or inflating the sheet, there will be less induced stresses and/or density losses in the coating.

Additionally or alternatively, the method can comprise any diameter of the opening being at least 40 mm, up to at least 700 mm or up to at least 2m or up to 5m.

Additionally or alternatively, the method can comprise the opening of the first board being circular, and the convex part of the sheet has a shape of a spherical cap with a diameter of at least 40 mm up to at least 700 mm or up to 2m or up to 5 m, and an apex angle of at least 90 degrees.

Additionally or alternatively, the method can comprise the seamless Tollable sheet being pre-shaped in a convex shape with respect to the second board.

Additionally or alternatively, the method can comprise the part of the second board, which is aligned with the opening of the first board, being pre-shaped in a convex shape with respect to the remaining part of the second board. Pre-shaping is a high quality feature that assists in making an inflatable light diverging mirror suitable for projection applications.

Additionally or alternatively, the method can comprise the gas having a pressure of at least 0.1 bar up to 0.5 bar up to 2 bar above the ambient air pressure.

Additionally or alternatively, the method can comprise the sealing being by glueing, screwing, bolting or a combination thereof.

Additionally or alternatively, the method can comprise the sheet being a polyester foil.

Additionally or alternatively, the method can comprise the second board being opaque.

Additionally or alternatively, the method can comprise the first or second board having a light absorbing surface.

Additionally or alternatively, the method can comprise the gas can be air, nitrogen, argon or any inert gas.

Additionally or alternatively, the method can comprise the sealing being by an o-ring between the sheet and a board.

Brief description of drawings

Figure 1 shows an example of an embodiment of the present invention comprising an immersive projection system.

Figure 2 shows an example of an embodiment of the present invention comprising an inflatable mirror embedded in a frame.

Figure 3 shows an example of an embodiment of the present invention comprising an exploded view of mirror components.

Figures 4a) and 4b) show an illustration of the geometry of a cone inside a sphere.

Figures 5a) to 5d) show examples of generalized geometries.

Figure 6 shows an example of an embodiment of the present invention comprising a shaped lower board.

Definitions

BoPET polyester films are hereafter referred to as“Mylar” which includes any of several suppliers and products other than being actually named as Mylar™, Melinex™ or Hostaphan™). Mylar or sheet being a polyester foil can be coated with reflective materials and configured for radiation concentrating applications such as e.g. solar collectors or astronomy instrumentation. Construction and Optical Testing of Inflatable Membrane Mirror Using Structured Light Technique (International Journal of Photoenergy, vol 2015, Patino Jimenez et. al) discloses a concave solar collector which is parabolically shaped in order to collect the incoming solar radiation in a focal point.

Detailed description

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. Figure 1 shows an embodiment of the present invention comprising a projection system 1 for immersive projection of image or video content. In accordance with embodiments of the present invention, a kit of parts can be provided which can be installed on site or a compkete system can be provided. The system can comprise a light diverging mirror 10 comprising a frame 11 having supports 12 to create an angle 13 of the mirror 10 towards a projection dome 14. The projector 15 can provide a lightbeam 16 to the mirror 10, so that the lightbeam is reflected onto the interior of the dome 14 and the viewer 17 can receive an immersive experience. Optionally, there can be a physical screen placed between the viewer 17 and the mirror 10 to inhibit first order reflexions from the mirror 10 to reach the viewer 17, or to inhibit light to be reflected and projected outside the dome (or projection screen). Additionally or alternatively, masking of pixels can be made with a physical filter (e.g. a so-called alpha plate) in front of the lens, or with masking by image processing (e.g. a so-called alpha mask.) Additionally, the dome 14 may be tilted with respect to the viewer 17 in order to provide more immersive projection area to the viewer 17. The projector 15 and/or the projector mirror 10 may then be tilted accordingly.

Figure 2 shows an embodiment of the present invention comprising an inflatable light diverging mirror 10 as the projector mirror which can be embedded into a frame 11 comprising an upper part 22, a lower part 23 and a convex mirror surface 24. The convex mirror surface 24 can have a convex side of curvature 20 and 21, which may be different or equal. As shown in Figure 2 the two sides of curvature 20 and 21 are orthogonal. Preferably, no wrinkles propagate along the convex shaped sheet. This is also a high quality feature that is suitable for projection applications.

Figure 3 shows an embodiment of the present invention comprising the frame 11 with an upper or first board 22 and a lower or second board 23. The upper board 22 can have an opening 30 with an inner edge 31. A rollable sheet 32 can be inserted between the boards 22 and 23 so that it fully covers the opening 30. The sheet 32 may have a reflective surface, e.g. comprising a metallic coating deposited onto the sheet. When the boards 22 and 23 and the sheet 32 are aligned, the structure can be sealed, e.g. by using a glue. Additionally or alternatively screws 33 can be distributed along the rim of the opening 30. Additionally, an o-ring (not shown) following the shape of the opening or seal, may be used to improve the seal. The lower board 23 can comprise means for gas inlet 34 e.g. a gas valve. The gas inlet can be placed on the board 23 so that in the final mirror 10, it is aligned with the interior of the opening 30. The sheet 32 can be inflated by inserting gas (air, Nitrogen, Argon, or any inert gas) through the valve 34 until the sheet 32 has adopted the wanted shape. Preferably, no wrinkles propagate along the convex shaped sheet. This is a high quality feature that is suitable for projection applications.

The sheet 32 may be made of metallized or uncoated foil, e.g. Mylar. The coated or uncoated foil may be (re-)coated after inflation. The boards 22 and 23 can be made out of any mechanically supporting material such as Polymethylmethacrylaat (PMMA), metal, wood, concrete, etc. Preferably this supporting material is light absorbing, e.g. dark in color and/or diffusing.

If the opening 30 is circular, the shape of the convex mirror surface 24 will be a spherical cap, as illustrated in figure 4a): The sphere 40 has a center point 41 and radius 42 and a central axis 49. The spherical cap 43 can be defined by the cone 44 having sides 45 and 46, a base rim 47 and an angle 48, in that the edges of the cap 43 coincide with the base rim 47 of the cone 44. The angle 48 can be referred to as the plain angle of the cone apex, hereafter referred to as“apex angle”. The cone is three-dimensional, but due to the spherical symmetry, it is possible to use the apex angle to describe the cone. Hence the curvature of the spherical cap 43 may be described with a sphere radius and an apex angle. Figure 4b) shows a side view of the cone 44 in figure 4a) where the height 51 of the cone is indicated as the distance between the cone base and its top or apex and the height 52 is then the height of the spherical cap 43 (which is the sphere radius 42 minus the cone height 51). Note that the diameter 53 of the spherical cap is different from the sphere diameter of two times the radius 42.

If the inflated mirror can be spherically shaped with a spherical cap radius of at least 20 mm and the apex angle of the corresponding cone can be between 1 and 180 degrees.

The cap geometry can be generalized into a spheroidal or ellipsoidal geometry, as illustrated in figures 5 a) to 5d). Figure 5 a) shows two spheroids and figure 5 c) shows the top view of the spheroids. The spheroids resemble flattened or elongated spheres with sustained rotational symmetry around their axis 60 with radius 61 or 62, respectively. Figure 5b) shows an ellipsoid and figure 5d) shows the top view of the ellipsoid, which has two radiuses 63 and 64 and thus no rotational symmetry around its axis 60. The opening of the upper board 22 can be circular or elliptical so that the inflated mirror can have the shape of an spheroidal or ellipsoidal cap. Alternatively, the convex mirror surface 24 may be described as a spheroidal or ellipsoidal cap. So in this text“apex angle” may also refer to the apex angle of the“largest” corresponding cone of an ellipsoidal cap, i.e. the cone having the largest apex angle. Preferably, these features result in no wrinkles propagating along the convex shaped sheet. This is also a high quality feature that is suitable for projection applications.

Additionally, the means for gas inlet may be configured to both insert or remove gas to adapt the pressure and shape of the convex mirror surface 24. This could be beneficial e.g. before and/or after transport by air freight or if the surrounding air pressure at operation is different from that of the location where the gas was first inserted.

It is important to keep the convex mirror surface 24 free from disturbances, such as e.g. seams between segments or wrinkles in the sheet, so that the projected content is correctly reproduced. Non-imaging inflatable applications (such as e.g. solar collectors, toy balloons, etc.) may be constructed by assembling segments of mirroring sheets since the imaging quality is of less or no importance. Further, such applications will still work even in the presence of some wrinkles, while this is not acceptable in projection applications of the present invention. Construction and Optical Testing of Inflatable Membrane Mirror Using Structured Light Technique (International Journal of Photo energy, vol 2015, Patino Jimenez et al) shows the poor imaging quality of a (concave) solar collector where wrinkles were still present. CH671832A5 discloses an inflatable reflective (concave) surface where the foil is supported by a frame inside a weld seam between the two foils.

The present invention can comprise inflating the convex mirror surface 24 with a pressure of e.g. 0.1 bar or more, above surrounding atmospheric air pressure. If the sheet 32 is adopting a convex shape through the opening 30, the gas pressure can press the sheet uniformly against the opening edge 31 and prevent that wrinkles propagates to the convex mirror surface 24. Wrinkles can for example arise from inhomogeneities in the sealing that can lead to a nonuniform stretching of the sheet 32.

When inflating the sheet 32, it will be submitted to stresses, which in turn can create distortions in the projected image. In another embodiment of the present invention the sheet to be inflated, e.g. sheet 32 in figure 3, may be pre-shaped in a shape similar to the final inflated shape, e.g. by thermoforming or vacuum forming. In this way the sheet will be submitted to less stress during the inflating and the optical distortion can be decreased. Pre- shaping is a high quality feature that assists in making an inflatable light diverging mirror suitable for projection applications. To further reduce stresses or density losses in the reflecting layer or coating, it can be deposited after the sheet has been pre-shaped and/or after the (pre-shaped or not pre-shaped) sheet has been inflated. If the rollable sheet 73 is pre-shaped, it can be given an arbitrary shape with less rotational symmetry.

Exemplary embodiment

Referring to figures 3, 4a) and b), in another exemplary embodiment of the present invention, the boards 22 and 23 were made out of Polymethylmethacrylaat (PMMA) 12 mm thick and painted black. The opening 30 of the upper board 22 had a diameter of about 707 mm. The sheet 32 was a metallized smooth Mylar film, 0.023 mm thick, which was sealed to the lower board 23 using“Tec7, Glue, mount and seal” glue and an o-ring of Ethyleen-Propyleen-Dieen-Monomeer (EPDM) rubber. Additionally, the boards 22 and 23 were screwed to each other with 32 screws 33 evenly distributed along the rim of the opening 30. The sheet was inflated with air via a valve 34 in the board 23 until the inflated sheet 32 adopted a convex shape of a spherical cap 43 having a diameter 53 of about 707 mm, a height 52 of about 146 mm and an apex angle 48 of about 90 degrees. The diameter of the cap can be at least 40 mm, up to at least 700 mm or up to at least 2m or up to 5m Preferably these features result in no wrinkles propagating along the convex shaped sheet. This is also a high quality feature that is suitable for projection applications. A conventional automotive valve for tubeless tires of 49 mm length and 11.3 mm in diameter was used. The pressure inside the inflated sheet was 0.2 bar above the surrounding air pressure. The curvature of the convex mirror surface 24 corresponded to a sphere 40 with a diameter of about 500 mm.

Figure 6 shows a cross sectional view of another embodiment of the present invention comprising an inflatable light diverging mirror 70 having a lower (or second) board 71 which is pre-shaped in a convex shape and comprising a gas valve 72. Pre-shaping is a high quality feature that assists in making an inflatable light diverging mirror suitable for projection applications. The rollable sheet 73 can be put on top of the lower board 71 and clamped with an upper board 74 having an opening 75. The rollable sheet 73 may be pre-shaped. Pre-shaping is a high quality feature that assists in making an inflatable light diverging mirror suitable for projection applications. The upper board 74, the lower board 71 and the rollable sheet 73 may be connected or sealed with bolts or screws 76. The seal may comprise an o-ring 77. A gas (e.g. air, Nitrogen, Argon or any inert gas) can be entered via the valve 72 until the gas pressure is higher than the ambient atmospheric pressure. In this way, the rollable sheet 73 can be inflated and kept in a convex shape by the gas 78. The distance 79 between the lower board 71 and the rollable sheet 73 may be in the order of several decimeters or centimeters or even sub-millimeters. An advantage of the present embodiment can be that the system can comprise less amount of gas which could be affected by changes in the environmental pressure, e.g. during flight transport or if the location of the system is located at high altitude. Additionally, it may be less influenced by environmental humidity changes; e.g. if the gas is air, this influence may be significant. The curved or convex shape of the lower board can provide an intrinsic self-stabilizing effect. The lower board 71 can also provide an additional mechanical support to the rollable sheet, especially if the distance 79 is small.

While the invention has been described hereinabove with reference to specific embodiments, this was done to clarify and not to limit the invention. The skilled person will appreciate that various modifications and different combinations of disclosed features are possible without departing from the scope of the invention.

Claims

1. An inflatable light diverging mirror for projection applications comprising a seamless rollable sheet having a reflective layer and a first and second board, means for sealing, the first board having a circular or elliptical opening, the opening having an edge,

wherein

the sheet is sandwiched between the boards so that it extends across the opening,

the reflective layer is facing the first board,

the sheet and boards are sealed to each other outside the opening with means for sealing so that the sheet and the second board enclose a cavity comprising a gas and wherein the gas has a pressure higher than the ambient atmospheric pressure, so that the sheet has a convex shape with respect to the second board and is pressed against the edge of the opening, and

no wrinkles propagate along the convex shaped sheet .

2. An inflatable light diverging mirror according to claim 1, wherein any diameter of the opening is at least 40 mm.

3. An inflatable light diverging mirror according to claim 1 or 2, wherein the opening of the first board is circular,

and the convex part of the sheet has a shape of a spherical cap with a diameter of at least 40 mm up to at least 700 mm or up to 2 m or up to 5 m and an apex angle of at least 90 degrees .

4. An inflatable light diverging mirror according to any of the claims 1 to 3, wherein the seamless rollable sheet is pre-shaped in a convex shape with respect to the second board.

5. An inflatable light diverging mirror according to any of the claims 1 to 4, wherein the part of the second board, which is aligned with the opening of the first board, is pre-shaped in a convex shape with respect to the remaining part of the second board .

6. An inflatable light diverging mirror according to any of the claims 1 to 5, wherein the gas has a pressure of at least 0.1 bar, or at least up to 0.5 bar or up to 2 bar above the ambient air pressure.

7. An inflatable light diverging mirror according to any of the claims 1 to 6, wherein the means for sealing is glue, screws, bolts or a combination thereof.

8. An inflatable light diverging mirror according to any of the claims 1 to 7, wherein the sheet is a polyester foil.

9. An inflatable light diverging mirror according to any of the claims 1 to 8, wherein the second board is opaque.

10. An inflatable light diverging mirror according to any of the claims 1 to 9, wherein the first or second board has a light absorbing surface.

11. An inflatable light diverging mirror according to any of the claims 1 to 10, wherein the gas is air, Nitrogen, Argon or any inert gas .

12. An inflatable light diverging mirror according to any of the claims 1 to 11, comprising an o-ring between the sheet and a board .

13. A projection system comprising the inflatable light diverging mirror according to any of the claims 1 to 12, a light beam impinging on said mirror, a projection screen, a blocking screen and a position of a viewer, wherein the blocking screen is placed between said mirror and the position of the viewer so that the blocking screen blocks light that would otherwise have been reflected outside the projection screen.

14. A projection system comprising the inflatable light diverging mirror according to any of the claims 1 to 13, and a light beam impinging onto said mirror, a projection screen and an optical filter that blocks light that would otherwise have been reflected outside the projection screen.

15. A method for manufacturing an inflatable light diverging mirror for projection applications comprising a seamless rollable sheet and a first and second board, means for sealing, the first board having a circular or elliptical opening, the opening has an edge, the second board having a gas valve, the method comprising the steps of sandwiching the sheet between the boards so that it extends across the opening and is facing the first board, sealing the sheet and the boards to each other outside the opening using the means for sealing,

inserting a gas through the gas valve, the gas expanding the sheet into a convex shape with respect to the second board and pressing the sheet against the edge of the opening, so that no wrinkles propagate along the convex shaped sheet.

16. A method according to claim 15, wherein, before the step of sandwiching the sheet between the boards, the sheet is receiving a reflective layer on the side facing the first board.

17. A method according to claim 15 or 16, comprising, before the step of sandwiching the sheet between the boards, pre-shaping the sheet in a convex shape with respect to the second board.

18. A method according to any of the claims 15 to 17, comprising, if the first board is aligned and positioned on top of the second board, pre-shaping the part of the second board that is aligned with the opening of the first board into a convex shape with respect to the rest of the second board.

19. A method according to any of the claims 15 to 18, comprising, when the gas has expanded the sheet to a convex shape with respect to the second board, the sheet receiving a reflective coating on its convex side.

20. The method according to any of the claims 15 to 19, wherein any diameter of the opening is at least 40 mm.

21. The method according to any of the claims 15 to 20, wherein the opening of the first board is circular,

and the convex part of the sheet has a shape of a spherical cap with a diameter of at least 40 mm up to at least 700 mm, or up to 2 m or up to 5 m and an apex angle of at least 90 degrees .

22. The method according to any of the claims 15 to 21, wherein the seamless rollable sheet is pre-shaped in a convex shape with respect to the second board.

23. The method according to any of the claims 15 to 22, wherein the part of the second board, which is aligned with the opening of the first board, is pre-shaped in a convex shape with respect to the remaining part of the second board.

24. The method according to any of the claims 15 or 23, wherein the gas has a pressure of at least 0.1 bar, or at least up to 0.5 bar or up to 2 bar above the ambient air pressure.

25. The method according to any of claim 15 to 24, wherein the sealing is by glueing, screwing, bolting or a combination thereof .

26. The method according to any of the claims 15 to 25, wherein the sheet is a polyester foil.

27. The method according to any of the claims 15 to 26, wherein the second board is opaque.

28. The method according to any of the claims 15 to 27, wherein the first or second board has a light absorbing surface.

29. The method according to any of the claims 15 to 28, wherein the gas is air, Nitrogen, Argon or any inert gas.

30. The method according to any of the claims 15 to 29, wherein the sealing is by an o-ring between the sheet and a board.

31. A kit of parts of an inflatable light diverging mirror for projection applications, the kit comprising a seamless rollable sheet having a reflective layer and a first and second board, means for sealing, the first board having a circular or elliptical opening, the opening having an edge,

the sheet being such that it can be cut or dimensioned so that it can be sandwiched between the boards so that it can extend across the opening, the reflective layer facing the first board, means for sealing the sheet and boards to each other outside the opening so that the sheet and the second board enclose a cavity, and

a gas supply having a pressure higher than the ambient atmospheric pressure.

32. The kit according to claim 31, wherein any diameter of the opening is at least 40 mm.

33. The kit according to any of the claim 31 to 32, wherein the opening of the first board is circular ,

and the convex part of the sheet has a shape of a spherical cap with a diameter of at least 40 mm up to at least 700 mm, or up to 2 m or up to 5 m and an apex angle of at least 90 degrees.

34. The kit according to any of the claims 31 to 33, wherein the seamless rollable sheet is pre-shaped in a convex shape with respect to the second board.

35. The kit according to any of claims 31 to 34, wherein the part of the second board, which is aligned with the opening of the first board, is pre-shaped in a convex shape with respect to the remaining part of the second board.

36. The kit according to any of the claims 31 to 35, wherein the gas supply has a pressure of at least 0.1 bar, or at least up to

0.5 bar or up to 2 bar above the ambient air pressure.

37. The kit according to any of claims 31 to 36, wherein the means for sealing is glue, screws, bolts or a combination thereof.

38. The kit according to any of the claims 31 to 37, wherein the sheet is a polyester foil.

39. The kit according to any of the claims 31 to 38, wherein the second board is opaque.

40. The kit according to any of the claims 31 to 39, wherein the first or second board has a light absorbing surface.

41. The kit according to any of the claims 31 to 40, wherein the gas is air, Nitrogen, Argon or any inert gas.

42. The kit according to any of the claim 31 to 41, comprising an o-ring for use between the sheet and a board.