SCREEN WITH REFLECTIVE MICROSTRUCTURES

Description

[0001] This patent application claims priority from French patent application FR19 / 13341. Technical field

[0002] The present application relates to the field of retroreflective screens. It relates more particularly to a screen provided with reflective microstructures in the shape of cube corners, a method for producing such a screen, and a mold for producing such a screen. It relates more specifically to a method for manufacturing a mold for manufacturing a retroreflective screen. Prior art

[0003] The applicant has already proposed, in patent applications US2017059862 and US2017197338, screens provided with retroreflective microstructures in the shape of cube corners, and methods for manufacturing such screens. The solutions described in these patent applications make it possible in particular to produce partially transparent and partially retroreflective screens, suitable for applications for projecting images onto transparent surfaces such as vehicle windshields, in particular motor vehicles.

[0004] It would be desirable to improve at least in part certain aspects of the solutions described in the above-mentioned patent applications. Summary of the invention

[0005] The invention is set out in the attached set of claims. According to the invention as defined in claim 1, one embodiment provides a method of manufacturing a mold for manufacturing a retroreflective screen, comprising the following successive steps: forming parallelepiped cavities on one side of a substrate; filling the cavities with a photosensitive resin; exposing the resin using a light beam inclined relative to the bottom of the parallelepiped cavities, so that part of the resin does not see the exposure beam; and removing the exposed part of the resin. Variations of the manufacture of this mold are defined in the dependent claims.

[0006] Another embodiment provides a method of manufacturing a retroreflective screen comprising a step of molding said face of the film using a mold as defined above, followed by a step of depositing a reflective metal layer over the entire face of the film.

[0007] According to one embodiment, the method further comprises a step of removing the reflective metal layer outside the micro-recesses, by chemical-mechanical polishing. Brief description of the drawings

[0008] These and other features and advantages will be set forth in detail in the following description of particular embodiments given without limitation in relation to the attached figures, among which: there Figure 1A is a perspective view schematically illustrating an example of a microstructure of a retroreflective screen according to one embodiment; Figure 1Bis a front view of the microstructure of the Figure 1A ; there Figure 1C is a sectional view of the microstructure along plane 1C-1C of the Figure 1B ; THE Figures 2A to 2E illustrate several successive steps of a method of manufacturing a retroreflective screen incorporating the method of manufacturing a primary mold according to the invention; the figure 3 is a schematic and partial top view of an example of a retroreflective screen according to one embodiment; figure 4 is a schematic and partial top view of another example of a retroreflective screen according to one embodiment; Figure 5 is a schematic and partial top view of another example of a retroreflective screen according to one embodiment; figure 6 schematically illustrates a system for projecting an image onto a retroreflective screen according to one embodiment; the figure 7schematically illustrates an exemplary embodiment of a mold for manufacturing a retroreflective screen according to one embodiment; and the figure 8 illustrates in more detail an example of the embodiment of a retroreflective screen of the type described in relation to the figure 6 . Description of the embodiments

[0009] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0010] For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been shown and are detailed. In particular, the different uses that can be made of the retroreflective screens described have not been detailed.

[0011] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures.

[0012] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%, and even more preferably to within 1%, or, when it comes to angular or similar values ​​(for example, orientation qualifiers such as the terms parallel, orthogonal, vertical, horizontal, etc.) to within 5 degrees, preferably to within 1 degree, and even more preferably to within 0.1 degree.

[0013] The aforementioned patent applications US2017059862 and US2017197338 describe exemplary embodiments of a retroreflective screen comprising a film, one face of which comprises a plurality of micro-recesses, each micro-recess having a bottom substantially parallel to the mean plane of the screen and first and second side walls substantially orthogonal to each other and substantially orthogonal to the bottom, the first and second side walls and the bottom of each micro-recess meeting at the same point and forming a trihedron. These screens are designed to have maximum retroreflection efficiency for an angle of incidence of approximately 60 degrees.

[0014] Here we propose to modify the shape of the micro-recesses to, on the one hand, improve the retroreflection efficiency of the screen, and on the other hand, control the angle of incidence for which the retroreflection efficiency is maximum so as to enable more varied angles of incidence to be addressed.

[0015] Studies carried out by the inventors have shown that the provision of micro-recesses in the form of truncated pyramids makes it possible to increase the retroreflection efficiency compared to screens of the type described in the aforementioned patent applications.

[0016] THE Figures 1A, 1B And 1C schematically represent an example of a micro-recess 101 of a retroreflective screen 100, in the shape of a truncated pyramid. The Figure 1A is a perspective view, the Figure 1B is a front view, and the Figure 1C is a sectional view along plan 1C-1C of the Figure 1B .

[0017] The screen 100 comprises a film 103 comprising a first main face 103a called the front face and a second main face 103b opposite the face 103a, called the rear face. The micro-recess 101 is formed in the film 103, on the side of the face 103a of the film. In practice, the screen 100 may comprise a large number of micro-recesses 101, for example identical or similar, formed on the side of the face 103a of the film 103. The screen 100 is intended to be illuminated on the side of the face 103a of the film 103. For example, the film 103 is made of a plastic material, for example of the polymethyl methacrylate type. In this example, the film 103 is made of a transparent material. The face 103b of the film 103 is for example substantially flat. Each microrecess 101 is surrounded laterally and separated from the other microrecesses by a portion 105 of the film 103 in which the face 103a of the film is substantially flat and parallel to the face 103b.The portions 105 define transparent regions of the screen, while the microrecesses 101 define retroreflective regions of the screen. Thus, the screen 100 is partially transparent and partially retroreflective. The rate of occupation of the surface of the screen by the microrecesses 101 defines the transparency of the screen. To obtain good transparency allowing the viewing of a scene through the screen, the rate of coverage of the screen 100 by the microrecesses 101 is for example less than 50%, and preferably less than 20%. In this example, the side walls and the bottom of each microrecess 101 are coated with a reflective metal layer 107. For the sake of clarity, the metal layer 107 has not been shown in the . Figures 1A and 1B and is only visible on the Figure 1C . As it appears on the Figure 1C, to preserve the transparency of the screen in the regions 105, the layer 107 does not cover the face 103a of the film 103 at the level of the regions 105.

[0018] The screen 100 may further comprise a transparent protective film (not shown) glued to the side of the upper face 103a of the film 103, for example by means of a layer of transparent glue (not shown) filling the micro-recesses 101. Preferably, the protective film and the transparent glue have substantially the same refractive index as the film 103.

[0019] In the example of the Figures 1A, 1B And 1C , the micro-recess 101 has the shape of a truncated pyramid with a quadrilateral base. Hereinafter, the four vertices of the quadrilateral forming the base of the truncated pyramid will be called E, F, G and H respectively, and the four vertices of the quadrilateral forming the summit face of the truncated pyramid, that is to say its face opposite the base EFGH, will be called A, B, C and D respectively.

[0020] The microrecess 101 comprises an opening defined by the base EFGH of the truncated pyramid. The base EFGH is parallel to the mid-plane of the screen 100, which can be defined as a plane parallel to the face 103b of the film 103. More particularly, in this example, the base EFGH is located in the plane of the front face 103a of the film 103.

[0021] The micro-recess 101 further comprises a bottom defined by the top face ABCD of the truncated pyramid. The bottom ABCD of the micro-recess 101 is also parallel to the mid-plane of the screen.

[0022] The micro-recess 101 further comprises four side walls defined respectively by the faces ABFE, ADHE, DCGH and BCGF of the truncated pyramid. Among these four side walls, the walls ABFE and ADHE are substantially orthogonal to each other and substantially orthogonal to the bottom ABCD. The walls ABFE and ADHE and the bottom ABCD meet at point A and define a trirectangle trihedron or cube corner having point A as its vertex. The side walls DCGH and BCGF of the micro-recess 101 are inclined, that is to say that the angle between the wall DCGH and the bottom ABCD of the micro-recess 101 and the angle between the wall BCGF and the bottom ABCD of the micro-recess 101 are both greater than 90 degrees.

[0023] Preferably, to obtain better retroreflection efficiency of the microrecess 101, the base EFGH and the top face ABCD of the truncated pyramid are square. In addition, in a preferred embodiment, the length of the side of the square EFGH is substantially equal to twice the length of the side of the square ABCD. The studies carried out have in fact shown that this dimension ratio between the opening and the bottom of the microrecess makes it possible to maximize the retroreflection efficiency of the structure (this allows the inclined sides of the truncated pyramid not to obstruct vision. They are then substantially in the axis of the user and the projector in the configuration where the retroreflected intensity is maximum). More generally, the length of the side of the square EFGH is preferably less than or equal to the length of the side of the square ABCD.Under these conditions, and by denoting by d the depth of the micro-recess 101, that is to say the length of the edge EA of the truncated pyramid, by a the width of the opening of the micro-recess, that is to say the length of the diagonal EG of the base EFGH of the truncated pyramid, and by n the optical index of the material filling the cavity 101, it can be shown that the angle of incidence αi for which the retroreflection efficiency of the structure is maximum satisfies the following relation: . a = 2 d × tan sin − 1 1 n sin αi

[0024] For a fixed depth d, we can calculate by this formula the width a to be provided as a function of the target angle of incidence αi. Depending on the application considered, we could for example provide a screen comprising micro-recesses 101 of the same depth d but having different widths a, adapted to address different angles of incidence αi.

[0025] For example, for a depth d of the order of 60 µm and in the case where the metallized micro-recesses are coated with a protective layer of a material with an optical index of the order of 1.5, the angle of incidence αi can take a value ranging from 10 to 60 degrees for a diagonal length a ranging from 14 to 85 µm.

[0026] THE Figures 2A to 2E illustrate successive steps of an example of a method of manufacturing a retroreflective screen 100 of the type described in relation to the Figures 1A, 1B And 1C . On the Figures 2A to 2E , the production of a single micro-recess 101 of the screen has been shown, it being understood that, in practice, a large number of micro-recesses, for example identical or similar, can be produced simultaneously. Each of the Figures 2A to 2E comprises two views (1) and (2). View (1) is a front view of the structure shown, and view (2) is a sectional view along plane 2-2 of view (1).

[0027] The f Figures 2A, 2B And 2C illustrate steps of manufacturing, according to the invention, a primary mold 210 intended to be used during the manufacture of the screen 100 itself. The Figures 2D and 2E illustrate steps in manufacturing the screen 100 from the primary mold 210.

[0028] There Figure 2Aillustrates a step of etching a cavity 203 on the side of the upper face of a substrate 201 in which it is desired to form the primary mold 210. In this example, the substrate 201 comprises a stack of a lower support layer 201a, an intermediate layer 201b, and an upper layer 201c. In the example shown, the main faces of the layers 201a, 201b and 201c are substantially horizontal. The intermediate layer 201b is made of a material such that the material of the upper layer 201c is selectively etchable relative to the layer 201b. For example, the upper layer 201c is made of silicon and the intermediate layer 201b is made of silicon oxide. The upper layer 201c is arranged on and in contact with the upper face of the intermediate layer 201b. The intermediate layer 201b is for example arranged on and in contact with the upper face of the lower support layer 201a.For example, the lower support layer 201a is made of silicon.

[0029] The thickness of the upper layer 201c of the substrate defines the depth d of the microrecesses 101 of the screen 100. For example, the thickness of the upper layer 201c is in the range from 10 to 500 µm and preferably in the range from 50 to 200 µm.

[0030] The cavity 203 extends vertically from the upper face of the layer 201c and is interrupted on the upper face of the intermediate layer 201b. The etching method used to form the cavities 203 is chosen to etch the substrate selectively with respect to the material of the layer 201b. For example, the cavities 203 are produced by deep reactive ion etching (DRIE). The layer 201b forms an etch stop layer, i.e. the etching of the cavities 203 is interrupted on the upper face of the layer 201b. Thus, the bottom of each cavity 203 is formed by the upper face of the layer 201b. An etching mask, not shown, may be placed on the upper face of the layer 201c during etching, to define the shape of the cavities 203.

[0031] In this example, the cavity 203 has, seen from above, a square shape, the diagonal length of which defines the upper diagonal length EG (distance a) of the micro-recess 101 that it is sought to produce. More particularly, the cavity 203 has the shape of a rectangular parallelepiped whose upper and lower faces, square, correspond respectively to the opening and the bottom of the cavity. The lateral faces of the rectangular parallelepiped correspond to the lateral walls of the cavity 203 and are substantially orthogonal to the upper and lower faces of the substrate 201.

[0032] There Figure 2B illustrates a step of depositing a layer of photosensitive resin 205 on the upper face of the structure obtained at the end of the etching step of the Figure 2AThe thickness of the resin layer 205 is greater than or equal to the depth d of the cavity 203, so that the resin 205 completely fills the cavity 203. In the example shown, the thickness of the resin layer 205 is greater than the depth of the cavity 203.

[0033] There Figure 2B further illustrates a step subsequent to the deposition of the resin layer 205, during which the resin 205 is exposed with an illumination angle corresponding to the angle of incidence αi for which it is desired that the retroreflection efficiency of the microrecess 101 that is sought to be achieved is maximal. As illustrated by Figure 2B , the angle of incidence of the light beam exposing the resin 205 associated with the opacity of the upper layer 201c of the substrate produces a shadow in the cavity 203, at the level of the two lower side walls (in the orientation of the view (1) of Figure 2B) of the cavity 203. Thus, a part of the resin 205 is not exposed inside the cavity 203. In the view (2) of Figure 2B the insolated region of layer 205 is represented with texturing, while the uninsulated region of layer 205 is represented without texturing, the two regions being separated by a broken line.

[0034] There Figure 2C illustrates the structure obtained at the end of a development step of the photosensitive resin layer 205. In this example, the photosensitive resin 205 is a negative resin. Thus, during the development step, only the non-exposed part of the resin is retained. The exposed part is dissolved and removed using a chemical development solution. The remaining part of the resin 205 partially fills the cavity 203.

[0035] At this stage, the cavity 203 has the same truncated pyramid shape as the micro-recess 101 that is sought to be produced. In particular, the remaining portion of the resin 205 defines the inclined faces BCGF and DCGH of the truncated pyramid.

[0036] The structure obtained at the end of the stages of Figures 2A, 2B And 2C corresponds to a primary mold 210 intended to be used for the manufacture of the screen 100.

[0037] There 2D figure illustrates a step during which the structures of the upper surface of the primary mold 210 are replicated, by molding, on the face 103a (corresponding to the upper face in the orientation of the 2D figure) of the film 103. The replication of the patterns of the primary mold 210 on the face 103a of the film 103 involves the formation, from the primary mold 210, of a secondary mold (not shown) having a shape complementary to that of the primary mold 210, that is to say having protuberances in the form of truncated pyramids complementary to the cavities 203 of the primary mold 210. The structures of the face 103a of the film 103 are then obtained, from the secondary mold, by thermoforming or by any other suitable molding technique. For example, the secondary mold may be made of metal or a plastic material having a deformation temperature higher than that of the film 103.

[0038] There Figure 2Eillustrates a step of forming a reflective metallization 107 on the side walls and on the bottom of the microrecess 101. The metallization 107 is for example made of aluminum. For example, to form the metallization 107, a reflective metal layer is deposited, by a conformal deposition method, for example by sputtering, on the entire upper surface 103a of the film 103, that is to say not only on the side walls and on the bottom of the microrecesses 101, but also on the upper face of the regions 105 of the film 103 laterally separating the microrecesses 101 from each other. The deposited metal can then be removed at the regions 105 by chemical-mechanical polishing. During this step, the portions of the metal layer coating the side walls and the bottom of the microrecesses 101 are retained, forming the reflective metallizations 107.

[0039] A screen 100 is thus obtained having retroreflective portions regularly distributed over its entire surface, corresponding to the metallized micro-recesses of the film 103. Each retroreflective screen portion is surrounded and separated from the other retroreflective portions by a transparent screen portion. The transparent screen portions correspond to the unstructured regions 105 of the film 103.

[0040] Once the metallizations 107 have been deposited, it is possible to provide a step of bonding a transparent protective film (not shown) to the upper face 103a of the film 103, for example by means of a layer of transparent glue (not shown) filling the micro-recesses 101, as described in relation to FIGS. 9A and 9B of the aforementioned patent applications US2017059862 and US2017197338.

[0041] An advantage of the screen 100 described above is that it has very good retroreflection efficiency at the angle of incidence αi for which the microrecesses 101 have been dimensioned. In addition, controlling the angle of incidence αi for which the retroreflection efficiency is maximum is easy to achieve, since this angle depends directly on the diagonal length a of the entrance opening of the microrecesses. In particular, it will be easy to produce a screen comprising distinct retroreflecting portions having maximum retroreflection efficiency for distinct angles of incidence αi.

[0042] Another advantage of the screen 100 described above is that, for a given retroreflection efficiency, it makes it possible to reduce the surface area occupied by the retroreflective microrecesses and therefore to increase the transparency of the screen compared to screens of the type described in the aforementioned patent applications.

[0043] Another advantage of the screen 100 described above is that its production is facilitated by the fact that each micro-recess 101 has two inclined side walls BCGF and DCGH. This makes it possible in particular to facilitate the demolding of the film 103 during the molding step described in relation to the 2D figure .

[0044] In most applications, and in particular in applications for projecting an image onto a vehicle windshield, the light source illuminating the retroreflective screen is not placed exactly in line with the user's gaze. Thus, the screen 100 must preferably be adapted to diffuse the retroreflected light in a diffusion cone encompassing the user's pupils. The diffraction effects on the edges of the micro-recesses and / or the inevitable surface imperfections of the screen may be sufficient to obtain the required diffusion effect. To amplify and / or control this diffusion, it is possible, for example, to play on the roughness of the sides and the bottom of the cavities 203 of the primary mold 210. As a variant, it is possible to use a method of the type described in relation to FIGS. 11A to 11G of the aforementioned patent application US2017197338 to produce a controlled diffusion pattern at the bottom of the cavities 203 of the primary mold 210.

[0045] It will be noted that in the retroreflective structure defined by the microrecess 101 of the screen 100, the inclined walls corresponding to the faces BCGF and DCGH of the truncated pyramid do not participate in the retroreflection at the angle of incidence αi for which the structure was dimensioned. To avoid parasitic reflections, it may be possible to degrade the reflection on these faces. For this, an optional microstructuring step leading to an increase in the roughness of the surface of the resin 205 may be provided at the end of the steps described in relation to the Figures 2A, 2B And 2C .

[0046] THE figures 3, 4 and 5 are top views respectively illustrating three examples of distribution of the microrecesses 101 in a screen 100 of the type described above. For the sake of clarity, the reflective metallizations 107 have not been shown on the figures 3, 4 and 5 .

[0047] In the examples of the figures 3, 4 and 5, the microrecesses 101 all have the same dimensions. In top view, the screen can be decomposed into a plurality of cells 110 of the same dimensions arranged in a matrix according to rows and columns. Each cell comprises a single microrecess 101, surrounded laterally by an unstructured region 105. In this example, the cells 110 have, in top view, a square shape.

[0048] In the example of the figure 3 , the microrecesses 101 are distributed according to a regular matrix arrangement. More particularly, in this example, in top view, each microrecess 101 is arranged in the center of the cell 110 to which it belongs. In other words, the point C of the bottom of each microrecess 101 substantially coincides, in top view, with the center of the cell 110.

[0049] In the example of the figure 4, the micro-recesses 101 have a random distribution within a fixed grid. More particularly in this example, in top view, the positions of the different micro-recesses 101 within their respective cells 110 vary according to a random or pseudo-random distribution.

[0050] In the example of the Figure 5 , the screen has micro-recesses 101 of different orientations, which makes it possible, for a given angle of incidence, to address distinct planes of incidence. More particularly, in this example, in top view, each micro-recess 101 is arranged in the center of the cell 110 to which it belongs, but the orientation of the micro-recess relative to the cell varies depending on the position of the cell in the matrix.

[0051] To produce a large screen, a plurality of molds of the type obtained by the process described in relation to the Figures 2A to 2EDepending on the constraints of the application, it may be possible to juxtapose molds with different properties, in particular different sizes and / or orientations of micro-recesses, in order to address different angles of incidence or planes of incidence.

[0052] There figure 6 schematically illustrates a system for projecting an image onto a retroreflective screen 100 of the type described above.

[0053] Screen 100 of the system figure 6 has for example a generally rectangular shape. The system of the figure 6 comprises a projector 601 arranged to project an image over substantially the entire surface of the screen 100.

[0054] In the example of the figure 6, the screen can be decomposed into a plurality of juxtaposed zones 120. In the example shown, the zones 120 all have substantially the same hexagonal shape and substantially the same dimensions, and are juxtaposed in a honeycomb distribution.

[0055] Each zone 120 can be associated with a preferred angle of incidence and a preferred orientation of incidence. Within each zone 120, the micro-recesses 101 all have substantially the same dimensions and the same orientation, chosen as a function of the preferred angle of incidence and orientation of incidence of the zone 120. The micro-recesses 101 of different zones 120 can, however, have different dimensions and / or different orientations.

[0056] There figure 7 schematically illustrates an example of the production of a mold for the manufacture of a screen of the type described in relation to the figure 6. In this example, the mold is made from a substrate 201 of generally circular shape, according to a method identical or similar to that which has been described in relation to the Figures 2A to 2C . In this example, a plurality of basic cells are defined in the mold, corresponding respectively to the different zones 120 of the screen 100. In the example shown, 19 basic cells numbered from 1 to 19 are produced. In this example, the basic cells 1 to 19 all have substantially the same hexagonal shape and the same dimensions, corresponding to the shape and dimensions of the zones 120 of the screen 100 of the figure 6 .

[0057] For example, the micro-recesses of the basic cells all have the same orientation but have different dimensions, which allows, in this example, to address 19 distinct angles of incidence αi.

[0058] The substrate 201 is then cut so as to separate the different basic cells from each other. On the figure 7 , the cutting paths were represented by straight solid lines.

[0059] After cutting, each basic cell can be oriented in 6 different orientations, which allows 6 different incidence orientations to be addressed for each incidence angle αi.

[0060] The basic cells can thus be assembled to create projection surfaces suitable for various applications.

[0061] There figure 8 illustrates in more detail an example of a retroreflective screen 100 of the type described in connection with the figure 6 , formed by means of a mold made by assembling basic cells of the type described in relation to the figure 7. In this example, 11 basic cell types were used, corresponding to the 120 areas designated respectively by the letters a, b, c, d, e, f, g, h, i, j and k on the figure 8 . On each zone 120, a black dot has been represented, the size of which schematizes the dimension of the micro-recesses 101 in the zone. In addition, on each zone 120, a black arrow has been represented, the orientation of which schematizes the orientation of the micro-recesses 101 in the zone.

[0062] It will also be possible to associate specific diffusion properties with each zone 120. For example, within each zone 120, the micro-recesses 101 all have substantially the same diffusion properties, chosen according to the position of the zone 120 on the screen. The micro-recesses of different zones 120 may, however, have different diffusion properties.

[0063] Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants could be combined, and other variants will occur to those skilled in the art. In particular, the described embodiments are not limited to application to the projection of an image onto a transparent surface. In particular, the described embodiments may have applications in various fields using retroreflective surfaces, not necessarily transparent, for example for signage purposes. In certain cases, it may indeed be desirable to be able to have a surface having good retroreflection efficiency for various angles of incidence. In the case where transparency is not desired, it will preferably be sought to maximize the coverage rate of the screen by the retroreflective portions.Furthermore, the material of the film 103 may be non-transparent. Furthermore, if the transparency of the screen is not required, the reflective metallizations 107 are not necessarily located on the side walls and on the bottom of the micro-recesses 101, but may form a continuous layer produced by conformal deposition, coating the entire face 103a of the film 103.

[0064] It will be noted that in the present description, the term film has been used to designate the element 103 of the screen 100. This term must however be understood broadly, and notably includes elements similar to films such as sheets, plates, etc.

Claims

1. Method for manufacturing a mold (210) for manufacturing a retroreflective screen (100), the mold comprising, on one face side, a plurality of cavities, each cavity having the shape of a truncated pyramid, the method comprising the following consecutive steps: a) forming parallelepiped openings (203) on the side of one face of a substrate (201); b) filling the openings (203) with a photosensitive resin (205); c) insolating the resin (205) by means of a light beam inclined relative to the bottom of the parallelepiped cavities, so that part of the resin (205) does not see the insolating beam; and d) removing the insolated portion of the resin (205) and retaining the non-insolated portion of the resin in the openings so as to form said cavities in the shape of a truncated pyramid.

2. Method according to claim 1, wherein the substrate (201) comprises a stack of a lower support layer (201a), an intermediate layer (201b), and a upper layer (201c), the thickness of the upper layer (201c) defining the depth of the cavities.

3. Method according to claim 2, wherein the material of the upper layer (201c) is selectively etchable relative to the intermediate layer (201b).

4. Method according to claim 2 or 3, wherein the upper layer (201c) is made of silicon, and the intermediate layer (201b) is made of silicon oxide.

5. Method according to any one of claims 1 to 4, wherein, at the end of step d), each cavity includes a bottom substantially parallel to the mean plane of the screen, corresponding to the summit face (ABCD) of the truncated pyramid, and two first sidewalls substantially orthogonal to each other and substantially orthogonal to the bottom, corresponding to two side faces (ABFE, ADHE) of the truncated pyramid.

6. Method according to claim 5, wherein, at the end of step d), each cavity (101) further includes two second sidewalls inclined relative to the mean plane of the screen, corresponding to two other side faces (BCGF, DCGH) of the truncated pyramid.

7. Method according to any one of claims 1 to 6, wherein, at the end of step d), each cavity has an inlet opening corresponding to the base (EFGH) of the truncated pyramid.

8. Method according to any one of claims 1 to 7, wherein the base (EFGH) of the truncated pyramid is square.

9. Method according to claim 8, wherein the summit face (ABCD) of the truncated pyramid is square.

10. Method according to claim 9, wherein the length of the base side (EFGH) of the truncated pyramid is substantially equal to twice the length of the side of the summit face (ABCD) of the truncated pyramid.

11. Method according to any one of claims 1 to 10, wherein, at the end of step d), the cavities all have substantially a same depth (d).

12. Method according to any one of claims 1 to 11, wherein each cavity is surrounded and laterally separated from the other cavities by an unstructured region of the substrate film (201).

13. Method according to any one of claims 1 to 12, wherein the substrate is divided into a plurality of cells having same dimensions arranged in a matrix in rows and columns, each cell comprising a single cavity and the cavities all having same dimensions, the positions of the cavities within their respective cells varying according to a random or pseudo-random distribution, or the orientations of the cavities within their respective cells varying according to the position of the cell within the matrix.

14. Method for manufacturing a retroreflective screen (100) including a step of molding one face of a film (103) by means of a mold produced by a method according to any one of claims 1 to 13, followed by a step of depositing a reflective metal layer over whole said face (103a) of the film (103).

15. Method according to claim 14, further including a step of removing the reflective metal layer outside micro-recesses (101) corresponding to the cavities of the mold, by chemical-mechanical polishing.