Diffusing material and production method

Abstract

The invention relates to a diffusing material (110), the material having a layer (10) comprising microcavities (5) included between the two main faces of the layer, the microcavities (5) having a largest dimension greater than 50 micrometers in a longitudinal direction and a small dimension of between 0.5 micrometers and 130 micrometers, the longitudinal direction of the microcavities (5) being parallel or inclined with respect to a normal (11, 12) to a main face (1, 2), a ratio between the largest dimension and the distance separating the microcavities being greater than a predetermined threshold, the material (110) having a cut-off angle dependent on said ratio, the material (110) having a percentage of diffused light greater than or equal to 30% for a light beam (40) incident on said main face (1, 2) at an angle of incidence greater than or equal to the cut-off angle.

Description

Description Title of the invention: Diffusing material and manufacturing method Technical field

[0001] The present invention relates to the technical field of optical diffusers. The present invention also relates to a method for treating a transparent layer by laser to obtain an optically diffusing material under certain conditions of use. In particular, the present invention relates to the technical field of glazing for buildings, for example residential or office windows. The present invention also relates to the technical field of optical privacy filters for display devices. The present invention also relates to the technical field of projection screens for display devices.

[0002] More particularly, the invention relates to a material with angularly selective light transmission and diffusion and a laser treatment method for transforming a transparent layer into such an angularly selective diffusion material. Prior art

[0003] In the field of window materials, materials are known that consist of a filter arranged on a translucent substrate. Diffusing filters are particularly known that block direct transmission through the transparent substrate. Filters based on the principle of Venetian blinds are also known. Such a filter comprises, for example, micro-shutters or micro-slits arranged horizontally in the plane of the material. The spacing and height of these micro-shutters determine a blocking angle for direct light incident on the material depending on the angle of incidence in a vertical plane. This blocking angle is, for example, of the order of 30 degrees above a horizontal line. Beyond this angle, the light is blocked by the micro-shutters and the filter appears black. However, the micro-shutters exhibit absorption in transmission along the normal to the screen, which depends on the thickness and spacing of the micro-shutters.The absorption of such a filter under normal incidence as well as under oblique incidence greatly reduces the brightness of the material.

[0004] In the field of display devices, privacy filters are known which are arranged on or in front of a display screen. Such a privacy filter comprises generally micro-shutters or micro-slits arranged vertically and perpendicular to the screen. The spacing and height of these micro-shutters determine the viewing angle available laterally in relation to the normal to the screen. This viewing angle is, for example, of the order of 30 degrees on each side for a computer screen. Beyond this angle, the light is blocked by the micro-shutters and the screen appears black. However, the micro-shutters exhibit absorption in transmission along the normal to the screen, which depends on the thickness and spacing of the micro-shutters. The absorption of such a privacy screen under normal incidence reduces the brightness of the display screen.

[0005] In the field of projection screens for display devices, a white, opaque, diffusing support is generally used to project an image generated by a video projector system. The visual rendering of the projected image depends on the uniformity of the support.

[0006] There is a need for a material that exhibits excellent transmission in one direction, for example normal to its surface, and that scatters light beyond a determined angle relative to the transmission direction.

[0007] There is also a need for an optical privacy filter that exhibits excellent transmission along the normal to its surface and laterally blocks vision beyond a determined angle relative to the surface normal.

[0008] There is a need for a projection screen for a display device that allows simultaneous display on both sides of the screen.

[0009] There is also a need for a manufacturing process for such materials or screens that is reproducible and industrializable. Statement of the invention

[0010] To this end, the present invention provides a diffusing material, in particular with angularly selective light transmission and diffusion, the material comprising a transparent and diffusing layer having two main faces, preferably parallel.

[0011] According to the invention, the layer (in particular sheet, in particular flat or curved) comprises at least one zone comprising microcavities included between the two main faces of the layer, the microcavities having an elongated shape having a larger dimension in a longitudinal direction and a small transverse dimension at the most large dimension, the largest dimension being greater than the small dimension, the largest dimension being greater than 50 micrometers, the small dimension being between 0.5 micrometers and 130 micrometers, the longitudinal direction of the microcavities being parallel to a normal (for example Z) to one of the main faces of the layer or inclined with respect to said normal, at an angle of less than 90°, preferably less than 85°, and even less than 50°, the microcavities having in a plane of incidence including the normal (for example XZ or YZ, where XYZ forms an orthonormal reference frame) on said zone a spatial distribution determined by a separation distance between microcavities, a ratio between the largest dimension and the separation distance of the microcavities being greater than a predetermined threshold.

[0012] In particular, the layer having in the plane of incidence a cut-off angle that is a function of said ratio, the layer having a percentage of light diffused by diffuse transmission less than a determined value of diffusion x for a light beam incident on said main face with a first angle of incidence less than the cut-off angle determined in said plane of incidence and the layer having a percentage of light diffused by diffuse transmission greater than or equal to the determined value of diffusion x for a light beam incident on said main face with a second angle of incidence greater than or equal to the cut-off angle in said plane of incidence, the determined value of diffusion x being greater than or equal to 10%, and preferably greater than or equal to 30%.

[0013] According to a particular and advantageous aspect, the determined threshold of the ratio is equal to the product of the determined value of diffusion x and a function of the (optical) refractive index of the layer.

[0014] Preferably, the predetermined threshold is greater than or equal to 0.1 — 1 where n2 is the index v n the refractive index of the layer and nl the refractive index of air and even is greater than or equal to 0.3 R - 1.

[0015] Preferably, the predetermined threshold is greater than or equal to 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 0.9, 1.0 or 1.1.

[0016] Advantageously, the microcavities exhibit an angular distribution (distribution of angles relative to the normal) with an angle standard deviation of less than 10 degrees, and even of at most 5 degrees or 2 degrees and / or with an angle value at the peak of the angular distribution of at most 50° and even of at most 30° or 15° or even 5°.

[0017] In this text the expression “between” two values ​​includes these two values ​​or limits.

[0018] The spatial distribution of the microcavities can be regular and even periodic or pseudo-periodic. The spatial distribution of the microcavities can be arranged according to a one-dimensional microcavity network, in particular according to a first direction (X or Y parallel to the main face of the layer and even to a lateral or longitudinal edge of the layer) or a two-dimensional microcavity network, in particular according to a first direction (X or Y parallel to the main face of the layer and even to a lateral or longitudinal edge of the layer) and a second direction (Y or X) normal to the first direction or is in concentric circles.

[0019] According to one embodiment, the one-dimensional network comprises lines of microcavities parallel to the main face of the layer.

[0020] According to another embodiment, the two-dimensional network comprises lines of microcavities and columns of microcavities parallel to the main face, lines perpendicular to the columns.

[0021] According to a particular and advantageous aspect, the microcavities have a first separation distance of between 20 micrometers and 5 mm along a first direction parallel to the main face of the layer and the microcavities possibly have another separation distance (identical, greater or less than the first separation distance) of between 20 micrometers and 5 mm along a second direction parallel to the main face (1, 2) of the layer and in particular perpendicular to the first direction or with an angle of at least 30° or 45°.

[0022] According to another particular and advantageous aspect, the microcavities have an aspect ratio between the largest dimension and the smallest dimension of at least 1.5 or 5 and even of at least 20, 50 or 100.

[0023] More specifically, the cut-off angle 6 C in a plane of incidence (XZ or YZ) can be determined by the ratio between the largest dimension L and the separation distance S (Sx, Sy) between microcavities in the plane of incidence, the (optical) refractive index r?2 of the layer, the (optical) refractive index of air and the determined value of diffusion x according to the relation: 6 C = arcsin

[0024] According to another particular and advantageous aspect, the transparent layer comprises or is a sheet, a mineral glass, in particular clear or extra-clear, based on (silico)soda-lime glass, aluminosilicate or borosilicate, in particular a thermally toughened glass, or comprises or is a sheet of transparent polymer in particular based on polymethyl methacrylate, polycarbonate, polyurethane, polyesters, in particular a lamination interlayer sheet (thermoplastic or crosslinked), the interlayer sheet preferably being based on polyvinyl butyral, a vinyl acetate-ethylene copolymer or thermoplastic polyurethane.

[0025] Advantageously, the layer has, under normal incidence, a haze value of less than 30% and a clarity value (C) of greater than 88%, these values ​​being measured with a haze meter.

[0026] According to a particular aspect of the invention, the intensity of the diffuse transmission is a function of said ratio in said plane of incidence.

[0027] The microcavities are empty or filled with a gas, with a refractive index close to 1, lower than the refractive index of the layer (in the core in the case of an index cladding).

[0028] Advantageously, one or more of the following characteristics can be provided: - the transparent layer, in particular a sheet of mineral or polymer glass, is part of monolithic, laminated or multiple glazing (double or triple glazing). - the transparent layer, in particular a sheet of mineral or polymer glass, is part of building glazing, in particular facade glazing, in particular a window, or partition, or even vehicle glazing - the microcavities comprise a sheath (of index), in particular with a thickness of between 5 and 50 pm, for example approximately 10 pm - the thickness of the layer is uniform or varies - the layer is planar, that is to say that the two main faces are flat and parallel or the two main faces are flat and form an angle or the two main faces extend along non-flat but parallel surfaces.

[0029] For example, in a curved screen application, one of the principal faces is concave and the other principal face is convex, with the two principal faces being locally parallel to each other.

[0030] Alternatively, the two main faces extend along non-planar and non-parallel surfaces.

[0031] Preferably, the material of the layer is clear and better extra-clear to limit absorption. By clear material, respectively extra-clear, is meant a material which has a light transmission TL, under normal incidence for visible radiation between 400 nm and 700 nm, of 90.0%, respectively 90.9% for a thickness of 6 mm and / or a light transmission TL of 88.8%, respectively 90.4% for a thickness of 10 mm.

[0032] The layer may have a thickness of between 0.1 mm and 12 millimeters, and preferably between 0.5 mm and 5 mm. In applications of diffusing material for buildings or automobiles, the thickness of the layer (sheet), in particular made of mineral glass or transparent polymer (such as those mentioned above), is preferably between 1 mm and 8 mm, and even preferably between 2 mm and 6 mm, in particular with a manufacturing margin of approximately 0.1 mm. In applications of diffusing material for computer or multifunction telephone screens, the thickness of the layer (sheet), in particular made of mineral glass or transparent polymer (such as those mentioned above), is preferably between 0.1 mm and 1 mm. In applications of diffusing material for projection screens of a display device, the thickness of the layer (sheet), in particular made of mineral glass or transparent polymer (such as those mentioned above), is preferably between 1 mm and 8 mm.

[0033] Preferably, the microcavities have an elongated shape along a longitudinal direction. The microcavities have in particular a symmetrical shape of revolution around their longitudinal direction, for example ellipsoidal or cylindrical with a circular section. L denotes the largest dimension of a microcavity along its longitudinal direction and T the small dimension along a direction transverse to the direction. longitudinal. The minor dimension T is also called the diameter of the microcavity. The minor dimension T is measured along a section of the microcavity, for example at mid-height of the largest dimension L. In other words, the largest dimension L is the length taken in a longitudinal section and the minor dimension T is the width taken in a transverse section or the diameter of a circular section.

[0034] Advantageously, the small dimension T is between 0.5 pm and 30 microns, preferably greater than 1 pm and less than 25 microns, and more preferably greater than 5 pm, for example between 5 pm and 10 pm. The small dimension T is preferably greater than the wavelength in the visible range, so as to limit the diffraction effects. And / or the largest dimension L is greater than or equal to 50 pm, and generally between 50 pm and a few millimeters, for example L is equal to 60 pm, 200 pm or 500 pm.

[0035] Advantageously, the microcavities are closed and have homogeneous (dimensions), i.e. they preferably have the same largest dimension L ± 30% and the same small dimension T ± 30%. The microcavities have an aspect ratio RA = L / T, defined as the ratio between the largest dimension L and the small dimension T. According to one aspect of the invention, the aspect ratio of a microcavity (of the majority or even at least 80% or 90% or 100% of the microcavities) is preferably greater than 1.5, or 2, or 5, or 7 and in particular less than 20, or 15 or 10 (in particular by a Gaussian beam) and even at least 20, 50 or 100 (in particular by a Bessel beam which allows larger aspect ratios).

[0036] In the case of microcavities comprising a cladding, the largest dimension of a microcavity is the largest external dimension of the cladding and the small dimension is the external dimension or diameter of the cladding perpendicular to the largest dimension. In this case, the small dimension may be between approximately 10 μm and 130 microns, preferably less than 50 microns, for example between 20 μm and 40 μm. And / or the largest dimension may be greater than or equal to 60 μm, and even between 60 μm and a few millimeters. The ranges of values ​​for the aspect ratio RA indicated above remain valid for microcavities with index cladding.

[0037] The (laser-treated) area covers at least 50%, 60%, 70%, 80%, and even at least 90%, 95%, 99% of the surface of the material (of the layer) in particular extends over the entire surface of the material. In one embodiment, the (laser-treated) area extends over a limited part (and in particular of predetermined shape) of the surface of the material. For example, the surface of the area extends over a surface of geometric shape or in the form of a pictogram, number, letter, logo or drawing or any other shape (suitable for the desired application). The surface of the area can range from 1 mm 2 at 18 m 2 depending on the applications, generally around 1.5 m 2 for a window or car glazing, and about 3-5 m 2 for a glass partition.

[0038] The present invention also relates to a display screen for a video projector comprising a material according to any one of the embodiments already described.

[0039] The invention also relates to a method for manufacturing a diffusing material, in particular with angularly selective light transmission and diffusion, comprising a diffusing layer, the method comprising the following laser treatment step: - applying a laser beam to a transparent layer, such as a sheet of glass or polymer (flat or curved), having two main faces, preferably parallel, the laser beam being incident on one of the two main faces, the laser beam being focused in the layer between the two main faces, preferably at a distance of at least 10pm or 50pm from the main faces (for closed microcavities), the laser beam scanning at least one area of ​​the layer with a determined scanning speed or point by point, the laser beam comprising laser pulses of duration less than or equal to 100 picoseconds, the laser beam having an energy per pulse and a repetition frequency adapted to generate microcavities between the two main faces, the microcavities having an elongated shape having a largest dimension in a longitudinal direction and a small dimension transverse to the largest dimension,the largest dimension being greater than the small dimension, the largest dimension being greater than 50 micrometers, the small dimension being between 0.5 micrometers and 130 micrometers, the longitudinal direction of the microcavities being parallel or inclined relative to a normal to one of the two main faces, the scanning of the laser beam being adapted to generate in a plane of incidence on said zone a spatial distribution of the microcavities determined by a separation distance between adjacent microcavities, and a, ratio between the largest dimension and the separation distance of the microcavities being in particular greater than a predetermined threshold.

[0040] According to a particular and advantageous aspect, a solid envelope or sheath surrounds each microcavity, the solid envelope having a structure and / or a refractive index different from the microcavity and / or the refractive index of the transparent layer.

[0041] Advantageously, the energy per pulse is between 2 microjoules and 25 microjoules and / or the repetition frequency is less than or equal to 10 megahertz, for example between 250 kHz and 2 MHz.

[0042] Preferably, the laser beam operates in an operating range between a refractive index modification range of the transparent layer and an ablation range of the transparent layer.

[0043] In a particular aspect, the scanning speed is adjusted to adjust the first alignment axis.

[0044] The method may include a step of spatially shaping the beam to adjust the shape of the laser beam, generally Gaussian, for example to form a Bessel beam (focused on a focal line) or to focus the beam (on a focal point), for example using a standard lens or an f-theta lens generally used in scanner systems.

[0045] According to a particular embodiment, the method comprises a heat treatment step following the laser treatment step, in particular of at least 500°C, 550°C or 600°C for a layer which is a sheet of glass, in particular monolithic glazing. The heat treatment may be a thermal tempering of the glass.

[0046] According to a particular embodiment, the layer is an interlayer of lamination, the application of the laser beam is in said layer forming part of a laminated glazing.

[0047] In this document, a femtosecond pulse is understood to mean a pulse with a duration less than or equal to 100 picoseconds, preferably less than or equal to 1 picosecond, generally between Ifs and 800 fs, for example about 300 fs. The laser pulses are emitted at an inter-pulse repetition frequency or frequency of repetition, denoted Frep. The Frep repetition frequency is generally between 1 kHz and several MHz, for example 1.5 MHz. Alternatively, the femtosecond pulses are emitted in bursts, with a repetition frequency between pulse packets lower than the inter-pulse repetition frequency. Each pulse packet can include from 1 to 20 pulses, preferably between 1 and 5 pulses.

[0048] Adjusting the number of pulses, the repetition frequency Frep, the packet repetition frequency, the scanning speed and the focusing plane allows the regularity of the microcavities to be controlled. It is thus possible to generate microcavities having in a plane parallel to one of the main faces of the layer a random spatial distribution, or in lines, circles (to not favor any direction) or points on a two-dimensional network, following a rectangular mesh grid for example.

[0049] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive. Brief description of the drawings

[0050] In addition, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

[0051] [Fig. 1] is a schematic sectional view of a material according to the present disclosure,

[0052] [Fig. 2] is a schematic view of the manufacturing process according to the present disclosure;

[0053] [Fig. 3] is a diagram illustrating different laser-matter interaction domains between a pulsed laser and a layer as a function of the energy per pulse (on the abscissa) and the repetition frequency (on the ordinate);

[0054] [Fig. 4] is a microscope image of a first example of a material comprising an arrangement of microcavities in top view (left) and in section (right);

[0055] [Fig. 4'] is a microscope image of another example of a material comprising an arrangement of microcavities in sectional view;

[0056] [Fig. 5] is a microscope image of a second example of material comprising an arrangement of microcavities in top view;

[0057] [Fig. 6] is a schematic cross-sectional representation of a material comprising an arrangement of microcavities modeled as cylinders, and illustrating the cut-off angle;

[0058] [Fig. 7] illustrates an example of a material with variable diffusion depending on the lateral viewing angle;

[0059] [Fig. 8] is a schematic view of a display screen for a video projection system;

[0060] [Fig. 9] illustrates an example of a screen seen in normal transmission in video projector mode off (left) and the same projection screen illuminated by the video projector (right);

[0061] [Fig. 10] illustrates another example of a projection screen, in normal transmission in video projector mode off (left) and the same projection screen illuminated by the video projector (right).

[0062] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0063] In Figure 1, a cross-sectional view of a material 110 according to the present disclosure is schematically shown. The material 110 is formed from a layer 10 having two main faces 1, 2 and side faces 3, 4. As described below, an arrangement of microcavities 5 is formed inside the transparent layer 10 between the two main faces 1, 2. The main faces extend over a suitable surface depending on the application, for example to form a projection screen or to form an optical diffuser for a window material. The side faces 3, 4 extend along the thickness of the material. The side faces 3, 4 are also called edges of the material or glazing. In other words, the thickness of the material corresponds to the distance between the two main faces 1, 2. The thickness of the material may be uniform. Alternatively, the thickness of the material varies depending on the position. Layer 10 is usually planar, i.e. say that the two main faces 1, 2 are planar and parallel. Alternatively, the two main faces 1, 2 are planar and form an angle. Alternatively, the two main faces 1, 2 extend along non-planar but parallel surfaces. For example, in an application to a curved screen, one of the main faces 1, 2 is concave and the other main face is convex, the two main faces 1, 2 being locally parallel to each other. Alternatively, the two main faces 1, 2 extend along non-planar and non-parallel surfaces.

[0064] The layer 10 that goes into the manufacture of the material can be a layer of mineral glass or transparent polymer. The layer is preferably made of mineral glass. The mineral glass layer can be soda-lime (silico)glass, aluminosilicate or borosilicate. The layer can be a thermally toughened glass. The layer can be transparent polymer which includes polymethyl methacrylate (PMMA), polycarbonate (PC), polyurethane (PU) sheets.

[0065] The material 110 comprises a layer 10 and microcavities 5 (described below) formed within the layer 10. More specifically, the microcavities 5 are formed and arranged in the layer 10 during the manufacture of the material 110.

[0066] Alternatively, the material 110 comprises several layers assembled to form a stack, one of the layers comprising microcavities. For example, the microcavities are formed in a layer forming part of a laminated glass assembly comprising an interlayer sheet between two glass sheets. In this case, the microcavities are preferably formed in one of the glass sheets, generally before the assembly of the laminated glass or possibly after the assembly. Alternatively, the microcavities are formed in an interlayer sheet of the already laminated glazing preferably. The lamination interlayer may be manufactured from a material such as, for example, polyvinyl butyral (PVB), an ethylene vinyl acetate copolymer (EVA) or thermoplastic polyurethane (TPU).

[0067] The present disclosure also relates to a glazing comprising the material. The glazing may be single glazing, laminated glazing or multiple glazing, such as double glazing or triple glazing.

[0068] In Figure 1, an orthonormal XYZ reference frame is shown. For clarity of the description, without being in any way limiting, the two main faces 1, 2 of the layer or sheet 10 extend for example parallel to an XY plane.

[0069] Main face 1 has a normal 11 to its surface and, respectively, main face 2 has a normal 12 to its surface (not shown in Figure 1).

[0070] According to the present disclosure, the material 110 comprises microcavities 5 included in the transparent layer 10 between the two main faces 1, 2, microcavities 5 being empty or filled with a gas, with a refractive index close to 1 less than n2. Advantageously, the microcavities 5 are entirely included in the layer 10 of the material 110. The microcavities 5 are located in a single layer of the material 110, as schematically illustrated in FIG. 1. For example, the microcavities 5 are at a distance of between 200 micrometers and several millimeters from each of the main faces 1, 2 of the layer 10. Advantageously, the microcavities 5 are closed. The method for manufacturing the microcavities 5 does not modify the surface of the main faces 1, 2 which remain flat.The distance between the microcavities 5 and the main faces 1, 2 and the fact that the microcavities are closed makes it possible to maintain the mechanical strength of the material 110, unlike a surface structuring or texturing likely to weaken the material.

[0071] The microcavities 5 have an elongated shape along a longitudinal direction. The microcavities 5 generally have a symmetrical shape of revolution around their longitudinal direction, for example ellipsoidal or cylindrical with a circular section. We denote L the largest dimension of a microcavity 5 along its longitudinal direction and T the small dimension along a direction transverse to the longitudinal direction. The small dimension T is also called the diameter of the microcavity. The small dimension T is measured along a section of the microcavity, for example at mid-height of the largest dimension L.

[0072] In certain exemplary embodiments, each microcavity 5 comprises a solid sheath 15 surrounding the core (vacuum, gas). The sheath 15 has a refractive index different from the refractive index of the layer. The sheath 15 has a thickness of between 5 and 50 μm, for example approximately 10 μm. The presence of a sheath around the core of the microcavities can be measured, for example by optical microscopy, as illustrated for example in Figure 4. Alternatively, a thermal annealing process is applied after the laser treatment, so as to homogenize the properties of the layer (sheet) around the microcavities. In this case, no cladding with a refractive index different from that of the layer around the core of the microcavities 5 is detected. In the case of microcavities comprising a cladding, the largest dimension L of a microcavity is the largest external dimension of the cladding and the small dimension T is the external dimension or diameter of the cladding perpendicular to the largest dimension.

[0073] In addition, the microcavities 5 are arranged in the layer according to a particular arrangement. The longitudinal direction of the microcavities 5 included in the layer 10 is parallel to a first alignment axis 6. In other words, the microcavities 5 are parallel to each other. The first alignment axis 6 is generally parallel to a normal 11, 12 to one of the two main faces 1, 2. According to one embodiment, illustrated in FIGS. 4, 7 or 8, the microcavities 5 are arranged in parallel lines separated by a separation distance such as Sx, Sy,. In other words, the lines of microcavities 5 are arranged regularly with a periodic pitch denoted P along an axis X. The pitch P from one line to the other along the axis X is equal to the sum of the separation distance S x and the small dimension T of the microcavities 5. In a given incidence plane, for example the XZ plane of figure 1, we note a ratio RI = L / (PT) = h / S x, defined as the ratio of the largest dimension to the separation distance S x between lines of microcavities. According to the present disclosure, the ratio (RI = h / S x ) is greater than 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 0.9, 1.0, 1.1, and preferably between 0.4 and 0.8.

[0074] According to another embodiment, illustrated in FIG. 5, the microcavities 5 are arranged in parallel lines separated by a separation distance S x and in parallel columns separated by a separation distance S y In other words, the lines of microcavities 5 are arranged with a periodic pitch denoted P along an X axis and the columns of microcavities 5 are arranged with a periodic pitch denoted Q along a Y axis. The pitch P from one line to another along the X axis is equal to the sum of the separation distance S xand the small dimension T of the microcavities 5 and the step Q from one column to another along the Y axis is equal to the sum of the separation distance S y and the small dimension T of the microcavities 5. We note a ratio R2 = L / S y , defined as the ratio of the largest dimension to the distance separation S y between columns. According to the present disclosure, the ratio (RI = L / S x ) and the ratio (R2 = L / Sy) are greater than 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 0.9, 1.0, 1.1, and preferably between 0.4 and 0.8.

[0075] As described in more detail below and without being bound by any theory, the ratio RI, respectively R2, determines a cut-off angle in a plane of incidence parallel to the plane XZ, respectively YZ, when RI, respectively R2, is greater than a predetermined threshold: where n2 represents the refractive index of the layer, ni the refractive index of air and x a proportion of incident light scattered on the microcavities for an angle of incidence greater than or equal to the cut-off angle in the plane of incidence XZ, respectively YZ.

[0076] For example, the ratio (RI = L / S x) is greater than or equal to 1.145x for a glass sheet having a refractive index of approximately 1.5 arranged in air with a refractive index equal to 1.0. The percentage of scattered light, denoted x, is here calculated relative to the total transmission, with 0 < x < 100%. In an exemplary embodiment, the percentage of scattered light for a light beam having an angle of incidence greater than or equal to a cut-off angle of 34° is advantageously greater than 30% for microcavity lines having a pitch P of 100 pm, a largest dimension L of approximately 55-60 pm, a small dimension T of approximately 7 pm and a cladding with a diameter of approximately 26 pm. Process

[0077] The microcavities 5 are generated in a layer 10, as described above, by a laser process. The laser process operates in a relatively narrow operating parameter range as illustrated in connection with Figures 2 and 3. For example, a solid-state laser, fiber laser or laser diode type laser is used. In one example, a Yb:YAG / Nd:YAG solid-state laser or a Yb:YVO4 laser is used.

[0078] In a first step 60, a femtosecond pulsed laser beam is generated. The laser pulses are emitted at an inter-pulse repetition frequency or repetition frequency, denoted Frep. The repetition frequency Frep is for example 1.5 MHz. According to a variant, the femtosecond pulses are emitted in bursts, with a repetition frequency between pulse packets lower than the inter-pulse repetition frequency. Each pulse packet may comprise from 1 to 20 pulses, preferably between 1 and 5 pulses. The laser beam has an energy per pulse E. More particularly, as illustrated in Figure 3, we place ourselves in a particular laser-matter interaction domain so as to generate microcavities 5 inside the layer.

[0079] The method comprises an optional step 61 of spatial beam shaping to adjust the shape of the laser beam, generally Gaussian, for example to form a Bessel beam (focused on a focal line) or to focus the beam (on a focal point), for example using a standard lens or an f-theta lens generally used in scanner systems. In a step 62, the femtosecond pulsed laser beam is applied to a transparent layer to be treated (thus becoming selectively scattering). The laser beam is focused inside the layer 10, between the two main faces. The laser beam is generally applied to a solid layer, for example through face 1 of the layer 10. The laser beam is generally applied at normal incidence to face 1 of the layer 10. In a step 63, a scan of the laser beam is carried out relative to an area of ​​the layer to be treated.Alternatively, in step 63, the layer is moved relative to the laser beam which is fixed to produce the scanning of the laser beam over the area to be treated. The scanning can be carried out step by step, so as to apply the laser beam at determined points of the layer, the points being regularly spaced from one another. Alternatively, the scanning is carried out during the laser shots, for example to generate microcavities along lines regularly spaced from one another. Steps 60, if applicable 61, 62 and 63 are iterated until the entire area of ​​the layer to be treated is covered. A material 110 is thus obtained comprising microcavities 5 arranged inside the layer 10.

[0080] In some embodiments, each microcavity5 produced is surrounded by an envelope. The solid envelope has a structure and / or an optical index of refraction different from the microcavity and / or the optical index of refraction of the transparent layer, respectively. This envelope is supposedly induced by compressive stresses in the layer during the laser process. This envelope makes it possible to increase the scattering under oblique illumination as described below. However, this envelope also reduces the transparency of the illuminated material under normal incidence. These stresses compression can be relaxed via a thermal annealing step which causes the envelope to disappear. Optionally, after finishing treating the layer, the process includes a heat treatment step 65 or thermal annealing. This heat treatment makes it possible to increase the transparency of the material at a given angle of incidence, while reducing scattering under oblique incidence.

[0081] Generally speaking, the application of a femtosecond pulsed laser beam to a transparent layer is likely to result in different laser-material interaction processes depending on the layer and the laser operating parameters. Laser operating parameters are understood here to mean all the laser parameters including in particular the energy per pulse, the duration of the laser pulses, the number of pulses, the inter-pulse repetition frequency, the wavelength of the laser beam, its spatial shape, the parameter M 2of a Gaussian beam, the fixed-frequency or packet pulse regime, the scanning speed of the laser beam over the layer, the orientation and focusing diameter and the Rayleigh length of the laser beam.

[0082] In particular, the application of a femtosecond laser to a transparent layer is likely to produce different effects on the layer, as illustrated in Figure 3. Figure 3 represents different laser-matter interaction domains between a femtosecond laser beam focused inside a clear (silico)soda-lime glass sheet (e.g. Planiclear from Saint-Gobain) having a thickness of 4 mm, as a function of two selected laser parameters: the energy E per pulse (on the abscissa) and the repetition frequency Frep (on the ordinate) between 250 kHz and 2 MHz. In this example, the other laser parameters are assumed to be constant. In this example, a Yb:YAG, Satsuma HP laser is used2 from the company Amplitude Systèmes, in single-pulse mode, which generates pulses with a duration of approximately 300 fs, at a wavelength of 1030 nm, at a fixed frequency with a beam scanning speed on the sheet of 5 mm / s. The laser beam is focused with a Gaussian profile of 28pm in diameter (diameter at 1 / e 2 ) with a quality factor M 2 around 1.1. The laser beam is essentially directed along the normal to the surface of the sheet. In Figure 3, no effect is observed in the sheet in a domain 50 located below a first energy threshold per pulse, here about 2 pj / pulse, this first threshold being almost independent of the repetition frequency Frep. Beyond the first energy threshold per pulse and below a second energy threshold per pulse, the creation of colored centers in the sheet is visually observed in a 51 domain. For example, at Frep = 500 kHz, the second threshold energy per pulse is about 4.5 pJ / pulse. The second threshold energy per pulse gradually decreases with increasing Frep repetition frequency up to 2 MHz, where the second threshold energy per pulse is very close to the first threshold energy per pulse. In a 52 domain, located above the second threshold energy per pulse and below a third threshold energy per pulse, in a 52 domain, a change in refractive index is observed in the sheet. The third threshold energy per pulse decreases sharply with increasing Frep repetition frequency up to 2 MHz, where the third threshold energy per pulse is very close to the second threshold energy per pulse.This refractive index modification domain 52 covers a wide range of operating parameters, the energy per pulse being able to be between about 5 pJ / pulse and 20 pJ / pulse at the Frep repetition frequency of 250 kHz, or between about 3 pJ / pulse and 9 pJ / pulse at the Frep repetition frequency of 666 kHz, or between about 2 pJ / pulse and 6 pJ / pulse at the Frep repetition frequency of 1 MHz and about 2.5 pJ / pulse at the Frep repetition frequency of 2 MHz. There is also a domain 54 with high energy per pulse and high Frep repetition frequency, in which an ablation of the layer (sheet), i.e. a removal of material, is observed.However, the present disclosure highlights a narrow domain 53 of parameters, located between the domain 52 of modification of the refractive index of the layer (sheet), and the domain 54 of ablation of the layer (sheet), in which the creation of microcavities inside the layer (sheet) is observed, without removal of material. This domain 53 of generation of microcavities is located between the third threshold of energy per pulse and a fourth threshold of energy per pulse. The domain 54 of ablation of the layer (sheet), is located beyond the fourth threshold of energy per pulse. The fourth threshold of energy per pulse decreases sharply as a function of the increasing repetition frequency Frep up to 2 MHz, where the fourth threshold of energy per pulse is approximately 4 pJ / pulse.Domain 53 thus covers a narrow range of operating parameters, the energy per pulse being about 22 pJ / pulse at the Frep repetition frequency of 250 kHz, or between about 13.5 pJ / pulse and 14.5 pJ / pulse at the Frep repetition frequency of 500 kHz, or between about 6 pJ / pulse and 7.5 pJ / pulse at the Frep repetition frequency of 1 MHz and between about 2.5 pJ / pulse and 4 pJ / pulse at the. Frep repetition frequency of 2 MHz. However, it is observed that this domain 53 widens when the Frep repetition frequency increases, which makes it possible to define a domain wide enough to ensure the stability of the process, for example for a Frep repetition frequency greater than or equal to 1 MHz.

[0083] Figure 4 illustrates a top view (left) and a sectional view (right) of a material 110 following femtosecond laser treatment according to the present disclosure. The material 110 is here formed from a sheet of clear (silico)soda-lime glass (Planiclear from the company Saint-Gobain) with main faces 1, 2 that are planar and parallel. The thickness of the sheet 10 between the main faces 1, 2 is approximately 4 mm. In this example, a laser of the same type as above is used. The laser beam is focused through the main face 1 halfway between the two faces 1, 2. Scanning is carried out by means of an f-theta lens along lines parallel to the Y axis spaced at a pitch P of 100 pm with a scanning speed of approximately 5 mm / s.The laser operates in pulse burst mode, with two pulses per burst and a burst repetition frequency of 2 MHz, with an energy per pulse burst of 21% of 10 pJ distributed between the different pulses of the burst. Advantageously, the lines are arranged regularly, that is to say that the pitch P from one line to another along the X axis is constant on the treated surface. This produces a material 110 comprising lines forming a one-dimensional periodic structure. In the top view, microcavities 5 are observed aligned along the scanning lines of the laser beam. Each microcavity 5 is surrounded by a cladding of index 15 visible under an optical microscope. In the sectional view, it is observed that the microcavities 5 are elongated parallel to a first alignment axis 6.The microcavities 5 with their sheath 15 have here, on average, a large dimension L of between approximately 300 pm and 600 pm and a small dimension T of approximately 25 pm to 50 pm. We denote S. x the average separation distance, here along the X axis, between two adjacent lines of microcavities 5. The step P from one line to the other along the X axis is equal to the sum of the average separation distance S x and the small dimension D of the microcavities 5.

[0084] The longitudinal end of the microcavities 5 is at a distance of about 200 pm to 300 pm from the main surface 1. The other longitudinal end of the microcavities 5 is at a distance of about 1 mm from the main surface 2. The microcavities 5 do not form any hollows or bumps on the main faces 1 and 2 of the sheet which are flat.

[0085] Figure 4' illustrates an embodiment in which the first alignment axis 6 is inclined by an angle BETA1, respectively BETA2, relative to the normal 11, respectively 12, to the main face 1, respectively to the main face 2. The angle BETA1, respectively BETA2, is between 0 deg. and almost 90 degrees, for example approximately 30 degrees or even 83 degrees for a high speed. The angle of inclination of the first alignment axis 6 varies depending on the scanning speed of the laser beam. More precisely, the angle of inclination of the first alignment axis 6 increases depending on the scanning speed of the laser beam. This angle of inclination can be measured non-destructively, preferably using an optical microscope.

[0086] The scanner settings are adjusted so that the distance S xbetween two adjacent lines of microcavities is less than or equal to the product of the large dimension L and a factor depending on the refractive index of the layer and the targeted diffusion level, . The ratio L / S x determines a cut-off angle in a plane of incidence parallel to the XZ plane, as detailed below.

[0087] In Figures 4 and 4', we observe a very high regularity of the microcavities 5 concerning not only their dimensions L and T, but also their position in the material between the two main faces 1, 2 and also the orientation of their longitudinal axis. This high regularity is allowed by the laser process. Adjusting the laser scanning parameters makes it possible to adjust the spatial distribution of the microcavities 5. Adjusting the number of pulses, the repetition frequency Frep, the packet repetition frequency, the scanning speed and the focusing plane make it possible to control the regularity of the microcavities 5. It is thus possible to generate microcavities 5 having in an XY plane a random spatial distribution, or in lines, in circles (to not favor any direction) or in points on a two-dimensional network, following a rectangular mesh grid for example.

[0088] As indicated previously, the scanning direction and speed also make it possible to adjust the orientation and inclination of the first alignment axis 6 of the microcavities 5. It is thus possible to adjust a determined inclination angle of the first alignment axis 6 of the microcavities 5 relative to the normal 11, 12 to the main face 1, 2 of the material 110.

[0089] Figure 5 illustrates another particular example of spatial distribution of microcavities 5 in a material following a two-dimensional network.

[0090] In Figure 5, the laser beam is scanned point by point in rows and columns, with the laser beam interrupted between each point. The laser beam is static here during each irradiation. The laser operates here with a burst energy of 21% of 10 pJ, two pulses per burst, with a burst repetition frequency of 2 MHz. The microcavities 5 appear spontaneously at the points arranged in a regular mesh pattern in rows and columns, here with a pitch P of 100 pm and a pitch Q of 100 pm. This gives a distribution of the microcavities 5 according to a periodic lattice of square mesh with sides of 100 pm. In this example, the lines are spaced by a distance S x of about 90 pm and the columns are spaced at a distance S yof approximately 90 pm. In addition, a very high regularity of the transverse dimensions of the microcavities 5 is observed over the entire laser-treated area. In this example, the laser beam being static during the irradiation of each point, the microcavities 5 are aligned in a direction perpendicular to the surface of the main faces 1, 2. Alternatively, the lines are arranged periodically with a pitch P greater than or equal to 30 pm, for example 50 pm or 200 pm, and the columns are arranged periodically with a pitch Q greater than or equal to 30 pm, for example 50 pm or 200 pm. Preferably, the pitch P is equal to the pitch Q, so as to allow homogeneous diffusion of the light in both directions X and Y. However, in certain applications, the pitch P is different from the pitch Q.

[0091] Advantageously, the average distance S y, here along the Y axis, between two adjacent microcavities of the same column is less than or equal to the product of the large dimension L and a factor depending on the refractive index of the layer and the targeted diffusion level. This ratio determines a cut-off angle in an incidence plane parallel to the YZ plane, as detailed below. By choosing a pitch Q different from the pitch P, a different cut-off angle can be obtained depending on the incidence plane.

[0092] The laser-treated area advantageously extends over the entire surface of the material. In other embodiments, the treated area extends over a limited portion of the surface of the material.

[0093] We will now describe the optical properties and operation of the material 110 in connection with FIGS. 1, 6 and the various examples of spatial distribution of the microcavities 5. The material 110 has both angularly selective diffusion and transmission properties. More specifically, the material 110 is transparent when illuminated at a limited angle of incidence relative to the first alignment axis 6 of the microcavities 5 and the material 110 is diffusing when illuminated at oblique incidence relative to this determined angle of incidence.

[0094] In the example of Figures 1 and 6, the first alignment axis 6 of the microcavities 5 is parallel to the normal 11 to the main face 1. For example, the microcavities 5 are arranged in lines parallel to a second alignment direction oriented along the Y axis, as illustrated in Figures 4 or 5. Alternatively, the microcavities are arranged in concentric circles in the XY plane. The XZ incidence plane of Figure 1 here comprises the X axis parallel to the main face 1 and the Z axis parallel to the normal 11 to the main face 1.

[0095] We will describe the transmission and diffusion properties of the material 110, depending on the distribution and orientation of the microcavities. In general, transmission and diffusion are angularly selective and depend in particular on the two-dimensional distribution, the density of the microcavities, their small dimension D and their large dimension h, the separation distances between adjacent (S x , S y ), of the h / S ratio x , respectively h / S y , and the angle of incidence of the external illumination light beam. Of course, the total amount of light transmitted (with or without diffusion) also depends on the absorption of the material and its reflectivity.

[0096] Consider an external light beam 20 incident on the main face 1 in the XZ incidence plane of Figure 1. The external light beam 20 transmitted through the main face 1 forms a refracted light beam 21 in the material 110, then exits through the opposite main face 2, in the form of a transmitted light beam 22. When the external light beam 20 is parallel to the normal 11 to the main face 1, the refracted light beam 21 is also parallel to the normal 11. In the example of Figure 1, the refracted light beam 21 is then parallel to the first alignment axis 6 of the microcavities 5. The transverse dimension T of the microcavities 5 is less than 130 pm, and preferably between 0.5 pm and 30 pm. In addition, the density of the microcavities 5 is relatively low in the XY plane. It follows that the light beam transmitted 22 through the material parallel to the normal 11 to the main face 1 undergoes practically no or little diffusion on the microcavities 5. In the case where the first alignment axis 6 is inclined with respect to the normal 11 to the main face 1 in the YZ plane, the refracted beam 21 inside the material parallel to the first alignment axis 6 is also transmitted without diffusion. More generally, in the XZ plane of incidence, as long as the angle between the refracted light beam 21 and the first alignment axis 6 of the microcavities remains less than an internal cut-off angle, noted 0 r , the refracted light beam 21 is little diffused by the microcavities 5. Consequently, the transmitted beam 22 has no or very little diffusion and the product is translucent.

[0097] The presence of the small transverse microcavities aligned in a direction normal to the surface has the effect that the material 110 is translucent under normal incidence or when the angle of incidence of the light beam on one of the main faces of the layer is less than a cut-off angle determined in the plane of incidence. The translucency can be evaluated by means of a haze meter, which measures the transmission intensity T , the haze value and the clarity value of the material. For example, a HazeGard Plus haze meter from BYK-Gardner is used to carry out measurements according to the ASTM D1003 standard. More precisely, a measurement of the light transmitted off the propagation axis is carried out in different angular domains.The haze meter measurements in the table below were performed at normal incidence for different samples, in which microcavity lines are formed, at a scanning speed of 5 mm / s and with a separation distance between microcavity lines of 100 pm, 150 pm and 200 pm respectively. The microcavities have a largest dimension h of about 60 pm ± 5 pm, a center diameter of about 7 pm ± 5 pm and a cladding diameter with index D of about 27 ± 1 pm. The microcavities are here fabricated in a layer with an optical index of refraction n2 of about 1.5. [Table 1]

[0098] As the separation distance between microcavity lines increases, blur decreases and clarity increases.

[0099] Furthermore, the presence of microcavities elongated in a direction normal to the surface has the effect of observing an external cut-off angle or cut-off angle, noted 9c, beyond which the material 110 has low translucency. The microcavities are modeled in the form of cylinders of circular section (see figure 6), having a height equal to the largest dimension L and a diameter equal to the smallest dimension T, the separation distance between microcavities being noted S x = P -T, where P is the pitch between lines of microcavities. We denote by x the percentage of light diffused on the microcavities, for an incident light beam in the XZ plane of incidence forming an angle of incidence on face 1 greater than or equal to the cut-off angle. The percentage x is calculated relative to the total light transmitted, in the plane of the microcavities. We calculate the cut-off angle 6 C according to the following formula: 0 c = arcsin

[0100] In other words, for a light beam 20 having an angle of incidence less than the cut-off angle in the plane of incidence XZ, the percentage of light diffused by diffuse transmission is less than x and for a light beam 40 having an angle of incidence greater than or equal to the cut-off angle in the plane of incidence XZ, the percentage of light diffused by diffuse transmission is greater than or equal to x.

[0101] The internal cut-off angle 9r corresponds to the external cut-off angle 9c via refraction through face 1: ni * sin 9c = n2 * sin 9r. The internal cut-off angle 9r is defined by the following relation: sin 9r = s / (s 2 +h 2 ).

[0102] The value of the percentage x of scattered light is predetermined for a beam having an angle of incidence greater than or equal to the cut-off angle. Approximately x 10% and even 30% of scattered light is estimated to be the maximum threshold for good translucency, allowing clear vision through the area of ​​the material comprising the microcavities. Conversely, above 30% of scattered light, the opacity is increased and does not allow clear vision through the area of ​​the material comprising the microcavities. The determined value of the percentage x is thus determined to be greater than or equal to 10 and preferably greater than or equal to 30%. In the examples above, for a determined value of the percentage x of 30%, the cut-off angle 0c is respectively 34 degrees for an interline pitch P of 100 pm, 49 degrees for an interline pitch P of 150 pm, and, respectively, 75 degrees for an interline pitch P of 200 pm.The incident light illuminating the material above the cut-off angle is at least partly scattered on the microcavities. In other words, for an incidence angle of 90°, about 70% of the incident light sees the microcavities with a pitch P equal to 100 pm, respectively, about 45% of the incident light sees the microcavities with a pitch P equal to 150 pm, and about 35% of the incident light sees the microcavities with a pitch P equal to 200 pm.

[0103] Therefore, in the above examples, the material exhibits high visual transparency for an incident light beam with an angle of incidence less than the cut-off angle in the plane of incidence considered and the material exhibits low visual transparency for an incident light beam with an angle of incidence greater than the cut-off angle in the plane of incidence considered.

[0104] The cut-off angle in the XZ plane of incidence is determined in particular by the ratio h / Si between the largest dimension h and the average separation distance S x between lines along the X direction. The external light beam 20 is transmitted with a scattering percentage lower than the determined value x when it forms an angle smaller than this cut-off angle in a determined XZ incidence plane. Above the cut-off angle, the light beam 20 is transmitted partly by direct transmission or regular transmission, without beam deflection, and partly by diffuse transmission with angular beam deflection bright. The transmitted beam that is deflected in all directions ("wide-angle scattering") contributes to image blur. The transmitted beam that is slightly deflected, generally less than 2.5 degrees, ("narrow-angle scattering") contributes to image clarity.

[0105] Let us now consider an external light beam 40 incident on the main face 1 in the XZ incidence plane of figure 1 or 6 at an oblique incidence angle GAMMA relative to the normal 11 to the main face 1, here parallel to the first alignment axis 6. The GAMMA angle is here greater than the external cut-off angle 0 C In the absence of microcavities 5, the light beam 40 propagates by refraction through the main face 1, in the material, then again by refraction through the main face 2 without undergoing diffusion.

[0106] As illustrated in Figure 1 or 6, the microcavities 5, possibly with index 15 sheath, are configured and arranged to diffuse at least a portion of the oblique light beam 40 propagating in the material after refraction through the main face 1. For simplicity, in Figure 1, the angle of refraction of the light beam 40 through the main face 1 has not been shown. The oblique light beam 40 incident on the microcavities 5 is diffused in different directions and forms a diffused internal beam 35. A portion of the diffused internal beam which forms an angle less than the total internal reflection angle with the normal to the main face 1 is extracted from the material and forms a diffused external beam 41 coming from the main face 1.Similarly, another part of the scattered internal beam which forms an angle smaller than the total internal reflection angle with the normal to the main face 2 is extracted from the material and forms a scattered external beam 42 coming from the main face 2. On the other hand, the parts of the scattered internal beam which form an angle greater than the total internal reflection angle with the normal to the main face 1, respectively 2, remain trapped and are likely to be scattered again on other microcavities 5. This gives a scattered external beam 41, respectively 42, emitted via the main face 1, respectively the main face 2. When the angle of incidence of the light beam 40 is greater than or equal to the cut-off angle 9c, the percentage of scattered light 42 by diffuse transmission is greater than or equal to the determined scattering value x.

[0107] Therefore, in the presence of microcavities 5, the material 110 is visually transparent to an external light beam in the XZ plane of incidence when the angle of incidence of the light beam on the main face 1 or 2 is less than a cut-off angle with respect to the first alignment axis 6 and the same material is diffusing when the angle of incidence of the light beam on the main face 1 or 2 is greater than a cut-off angle. The cut-off angle in the XZ plane depends on the spatial distribution of the microcavities, in particular on the average separation distance between the microcavities in the plane of incidence and on the largest dimension of the microcavities.

[0108] In a plane YZ transverse to the plane of Figure 1, the response of the material for light transmission and scattering depends on the spatial distribution and orientation of the microcavities 5.

[0109] For example, the microcavities 5 are arranged in lines parallel to the Y axis, the distribution of the microcavities 5 along the Y axis being ordered or not. In this case, a light beam propagating by transmission in the YZ plane between the lines of the microcavities 5 undergoes practically no scattering.

[0110] In a first example, the microcavities arranged along lines parallel to the Y axis have their first alignment axis 6 parallel to the normal 11. In this case, a light beam 20 in the plane of incidence YZ, at a normal angle of incidence, is not practically diffused by the microcavities, because it only sees their small dimension, and therefore forms a transmitted beam 22 without diffusion.

[0111] In another example, the microcavities arranged along lines parallel to the Y axis have their first alignment axis 6 inclined by an angle BETA1 with respect to the normal 11. In this case, the transmission and scattering properties of the material are asymmetric with respect to the normal 11. A light beam incident in the plane of incidence YZ, refracted inside the material parallel to the first alignment axis 6, is also transmitted without scattering. More generally, there is then another cut-off angle between the refracted light beam and the first alignment axis in the YZ plane, this other cut-off angle being a function of the ratio Q=L / S y between the large dimension L and the distance S ybetween adjacent alignment microcavities in the YZ plane. When the material 110 is illuminated in the YZ plane at an incidence lower than this other cut-off angle, the material 110 is transparent and low-scattering, with a haze value lower than 30%. When the material 110 is illuminated in the YZ plane at an incidence higher than this other cut-off angle, cut, the material 110 is diffusing and not transparent. When the distance S x is different from the distance S y , the cut-off angle varies depending on the plane of incidence.

[0112] According to another example, the microcavities 5 are arranged in concentric circles, the circles being spaced apart by a distance S x . Inside each circle, the microcavities 5 are preferably spaced by a distance less than or equal to S x. In this case, the material is transparent for an external light beam included in a cone of revolution of circular section having as its apex angle the cut-off angle and the material is diffusing for an external light beam outside this cone. The axis of the cone depends on the orientation of the first axis of alignment of the microcavities.

[0113] The material thus exhibits angularly selective transmission and scattering properties. The material properties are easily adjustable depending on the laser parameters. Indeed, the transmission, scattering and cut-off angle depend in particular on the laser repetition frequency, the energy per pulse, and the laser scanning speed. Consider a distribution of microcavities following a one-dimensional periodic lattice, the microcavities being arranged in lines spaced from each other by a distance S xconstant, as illustrated in Figures 1, 4 and 5. In this case, the layer has a lateral cut-off angle for an oblique light beam in the plane of incidence perpendicular to the lines, the lateral cut-off angle being a function of the distance S x and the large dimension L. On the other hand, the material does not have any cut-off angle for an oblique light beam in a plane parallel to the lines. Such a material finds advantageous applications for an angularly selective optical diffusing filter.

[0114] The distribution of microcavities 5 along lines finds applications in particular in the manufacture of a privacy filter or an anti-reflection filter. For this purpose, the laser-processed material is arranged on or in front of a display screen, the lines of microcavities being arranged vertically. As described in more detail above, the large dimension of the microcavities being L, the adjustment of the distance S x between lines of microcavities 5 to a fraction of L, allows to adjust the lateral cut-off angle of the privacy filter or the anti-reflection filter. For example, to have more than 30% of transmitted light scattered, the cut-off angle is respectively 34 degrees for a line spacing P of 100 pm, 49 degrees for a line spacing P of 150 pm, and, respectively, 75 degrees for a line spacing P of 200 pm, for microcavities having a larger dimension h of approximately 60 pm ± 5 pm, a small dimension D of approximately 7 pm ± 5 pm and a cladding diameter of approximately 27 ± 1 pm in a sheet of clear soda-lime glass (Planiclear type marketed by Saint-Gobain Glass).

[0115] Figure 7 illustrates the application to a filter integrated into a material 110 comprising an array of microcavity lines as described above with the following parameters in this example: a pitch P of 120 pm, a scanning speed of 10 mm / s, a burst energy of 25% of 10 pJ, two pulses per burst, with a burst repetition frequency of 2 MHz. The microcavity lines are here aligned parallel to the vertical sides. The material 110 is placed in front of a back panel 9. When the back panel 9 is viewed through the material at normal incidence (0 deg.), the material is transparent and allows the letters displayed on the back panel to be clearly read. On the other hand, as soon as the lateral angle of incidence is greater than approximately 10 deg., the transmitted beam is strongly scattered. The scattering increases with the oblique angle of incidence on the screen of ±10 deg., ±45 deg., or ±80 deg. The scattering here is symmetrical about a vertical plane.However, the material remains clear above the lateral cut-off angle of transmission without scattering, which is here less than 10 degrees. Unlike prior art micro-shutter filters, the material 110 is not opaque or black above the lateral cut-off angle but only scattering. Under normal incidence, the material of Figure 7 offers high visual transparency which allows clear vision by transmission: the contours of the background image appear sharp with high contrast. On the other hand, for an angle of incidence greater than the cut-off angle, the material of Figure 7 has reduced visual transparency which does not allow clear vision by transmission: the contours of the background image appear blurred with degraded contrast.

[0116] Such a material finds applications as an angularly selective diffusion material, equivalent to an integrated Venetian blind. In this case, the material is installed so that the lines of microcavities are arranged horizontally. When the sun is low, for example in winter, or at sunrise and sunset, the angle of incidence of sunlight on the material is low and the sunlight is not diffused. The sunlight is then transmitted without diffusion through the material into the interior of a room. On the other hand, when the sun is higher in the sky, the angle of incidence of the light on the material is greater than the cut-off angle. The sunlight is then diffused, better distributed within the room, which helps avoid glare. Unlike Venetian blinds, the microcavities do not absorb light and do not darken the room.

[0117] In another example, the microcavities 5 are arranged in a two-dimensional periodic network comprising lines parallel to the Y axis and columns parallel to the X axis. We denote by S x , respectively S y, the distance between rows, respectively columns. It is assumed that the alignment axis 6 of the microcavities 5 is parallel to the normal 11 to the main face 1. In this case, analogously to the description of the diffusion in the XZ plane of incidence, the external light beam under oblique incidence undergoes diffusion under certain angular conditions. The external light beam incident in the transverse plane YZ to the plane of Figure 1 on the main face 1 is diffused on the columns of microcavities when the angle of incidence of the refracted beam inside the material forms an angle greater than a cut-off angle with respect to the alignment axis 6 of the microcavities 5. The cut-off angle in the YZ plane is determined in particular by the distance S ybetween columns and the length L of the microcavities 5. In the case of microcavities arranged in a two-dimensional network, the screen is then diffusing as soon as the angle of incidence of the light beam is greater than a cut-off angle relative to the normal 11 to the main face, regardless of the plane of incidence. However, the cut-off angle of the diffusion depends on the plane of incidence, for example when the distance S x is different from the distance S y . As highlighted above, the screen nevertheless remains transparent under normal or quasi-normal incidence, more precisely, under an angle of incidence less than the cut-off angle, whatever the plane of incidence. The material comprising elongated and ordered two-dimensional microcavities finds applications for a projection screen of a video projection system as illustrated in connection with figures 8 to 10.

[0118] Figure 8 is a schematic view of a screen 100 for a video projection system. The screen 100 comprises a material 110 including microcavities 5 as described above. Advantageously, the microcavities 5 are here arranged in a two-dimensional periodic network. The screen is here transparent for a light beam perpendicular to the main face 1 and diffusing in all directions when it is illuminated at an angle of incidence greater than a cut-off angle regardless of the plane of incidence. A video projector 14 emits a light beam 40 so as to project an image onto the main face 1 of the screen 100. Advantageously, the video projector 14 has a distance of projection between 0 and 80 cm (projector-screen distance). The video projector 14 is arranged so that the light beam 40 is incident on the main face 1 of the screen at an angle of incidence greater than the cut-off angle of the material in the plane of incidence considered. The light beam 40 is diffused on the microcavities 5 so as to form a back-scattered beam 41 via the main face 1 and another beam diffused 42 forwards via the main face 2. An observer 15 placed facing the screen 100 can thus view the image projected by the video projector 14 by looking at the main face 1. The advantage of this projection screen 100 is that it is double-sided. Indeed, placed facing the other main face 2 of the screen, another observer can view the same image from the same video projector 14, via the other diffused beam 42.Such a display screen allows double-sided display using only one video projector 14, therefore at lower cost. The diffused beams 41, 42 are visible under a wide range of angles of incidence, for example from a normal incidence up to approximately 45 degrees. Such a display device makes it possible to broadcast messages to a large audience which can be placed on both sides of the screen. In addition, when the video projector 14 is switched off, the screen 100 becomes transparent again under normal incidence and therefore does not obstruct the view of an observer placed in front of the main face 1 or 2. Unlike conventional opaque screens, the transparent and diffusing screen can be placed in the center of a room and not necessarily against a wall. This diffusing screen makes it possible to see a projected image even in the presence of daylight.

[0119] Figure 9 illustrates an example of a video projection screen. In this example, the distribution of microcavities is in horizontal lines with a pitch P of approximately 120 pm. The laser process parameters are as follows: point-by-point scanning, energy 45% of 10 pj, single-pulse mode with a repetition frequency of 2 MHz, laser beam exposure time of 40 ps at each point. On the left, the video projector is off, the screen is observed in normal transmission. The background placed in the background appears clearly and allows reading. The direct or regular transmission under normal incidence is here approximately 90%. On the right of Figure 9, the video projector is on and illuminates the same projection screen at an angle of incidence oblique incidence of approximately 45 degrees, greater than the cut-off angle. The video projector projects an image onto the screen which is seen clearly by diffusion on the screen towards the front or towards the rear.As highlighted above, the projected image is viewed from both sides of the screen.

[0120] Figure 10 illustrates another example of a video projection screen. In this example, the distribution of microcavities is in horizontal lines with a pitch P of approximately 120 pm. The laser process parameters are as follows: line-by-line scanning at a speed of 5 mm / s, energy 25% of 10 pJ, single-pulse mode with a repetition frequency of 2 MHz. On the left, the video projector is off, the screen is observed in normal transmission. The direct transmission under normal incidence is here approximately 90%. The background arranged in the background appears clear and with a slight blur. On the right of Figure 10, the video projector is on and illuminates the same projection screen at an angle of incidence oblique incidence of approximately 45 deg., greater than the cut-off angle. The video projector projects the same image onto the screen which is seen even more clearly by diffusion on the screen towards the front or towards the rear. Likewise, the projected image is seen from both sides of the screen.

[0121] Of course, various other modifications may be made to the invention within the scope of the appended claims.

Claims

Claims

1. Diffusing material (110), the material (110) comprising a transparent and diffusing layer having two main faces (1, 2), characterized in that the layer (10) comprises at least one zone comprising microcavities (5), included between the two main faces, the microcavities (5) having an elongated shape having a larger dimension in a longitudinal direction and a smaller dimension transverse to the larger dimension, the larger dimension being greater than the smaller dimension, the larger dimension being greater than 50 micrometers, the smaller dimension being between 0.5 micrometers and 130 micrometers, the longitudinal direction of the microcavities (5) being parallel to a normal (11, 12) to one of the main faces (1, 2) of the layer or inclined relative to said normal, the microcavities (5) having, in a plane of incidence, including the normal,on said area a spatial distribution determined by a separation distance between microcavities, a ratio between the largest dimension and the separation distance of the microcavities being greater than a predetermined threshold, the layer (10) having in the plane of incidence a cut-off angle which is a function of said ratio, the layer (10) having a percentage of light diffused by diffuse transmission less than a determined value of diffusion x for a light beam (20) incident on said main face (1, 2) with a first angle of incidence less than the cut-off angle determined in said plane of incidence and the layer (10) having a percentage of light diffused by diffuse transmission greater than or equal to the determined value of diffusion x for a light beam (40) incident on said main face (1, 2) with a second angle of incidence greater than or equal to the cut-off angle in said plane of incidence,the determined diffusion value x being greater than or equal to 10% and preferably greater than or equal to 30%.,

2. Material (110) according to claim 1 in which the determined threshold of the ratio is equal to the product of the determined value of diffusion x and a function of the refractive index n2 of the layer (10).

3. Material (110) according to one of claims 1 or 2 in which the threshold is the refractive index of the layer (10) and neither the refractive index of air, and even said threshold is greater than or equal to 0-3 F nj - 1. i

4. Material (110) according to one of claims 1 to 3 wherein the predetermined threshold is greater than or equal to 0.

1.

5. Material (110) according to one of claims 1 to 4 in which the microcavities have an angular distribution with a standard deviation of angles of less than 10 degrees and / or with an angle value at the peak of the distribution of at most 50 degrees.

6. Material (110) according to one of claims 1 to 5 in which the microcavities (5) are closed.

7. Material (110) according to one of claims 1 to 6 in which the spatial distribution of the microcavities is a network of one-dimensional microcavities, in particular along a first direction, or a network of two-dimensional microcavities, in particular along a first direction and a second direction normal to the first direction, or the spatial distribution of the microcavities is a network of concentric circles.

8. Material (110) according to the preceding claim in which the one-dimensional network comprises one or more lines of microcavities parallel to the main face (1, 2) of the layer.

9. Material (110) according to claim 7 wherein the two-dimensional network comprises lines of microcavities and columns of microcavities parallel to the main face, lines perpendicular to the columns.

10. Material (110) according to one of claims 1 to 9 in which the microcavities (5) have a separation distance of between 20 micrometers and 5 mm in a first direction parallel to the main face (1, 2) of the layer and the microcavities (5) optionally have another separation distance of between 20 micrometers and 5 mm in a second direction parallel to the main face (1, 2) of the layer and in particular perpendicular to the first direction.

11. Material (110) according to one of claims 1 to 10 in which the microcavities (5) have an aspect ratio between the largest dimension and the smallest dimension of at least 1.5 or 5 and even of at least 20, 50 or 100.

12. Material (110) according to one of claims 1 to 11 in which the cut-off angle 6 C in a plane of incidence is determined by the ratio between the largest dimension L and the separation distance S between microcavities in the plane of incidence, the refractive index r?2 of the layer (10), the refractive index ni of air and the determined value of diffusion x according to the relation: 0 c = arcsin

13. Material (110) according to one of claims 1 to 12 in which the transparent layer comprises a sheet of mineral glass, in particular based on soda-lime glass, aluminosilicate or borosilicate, in particular thermally toughened glass, or the transparent layer comprises a sheet of transparent polymer in particular based on polymethyl methacrylate, polycarbonate, polyurethane, polyesters, in particular an interlayer lamination sheet based on polyvinyl butyral, a vinyl acetate-ethylene copolymer, thermoplastic polyurethane.

14. Material (110) according to one of claims 1 to 13 in which the transparent layer, in particular a sheet of mineral or polymer glass, is part of a monolithic, laminated or multiple glazing.

15. Material (110) according to one of claims 1 to 14 in which the microcavities comprise a solid sheath.

16. Material (110) according to one of claims 1 to 15 in which the transparent layer, in particular a sheet of mineral or polymer glass, is part of a building glazing, in particular a facade glazing, in particular a window, or partition.

17. Display screen (100) for video projector comprising a material (110) according to one of claims 1 to 15.

18. A method of manufacturing a diffusing material according to any one of claims 1 to 16 comprising a diffusing layer, the method comprising a following laser treatment step: - applying a laser beam to a transparent layer having two main faces (1, 2), the laser beam being incident on one of the two main faces (1, 2), the laser beam being focused in the layer between the two main faces (1, 2), the laser beam scanning at least one area of ​​the layer with a determined scanning speed or point by point, the laser beam comprising laser pulses of duration less than or equal to 100 picoseconds, the laser beam having an energy per pulse and a repetition frequency adapted to generate microcavities (5) between the two main faces (1, 2), the microcavities (5) having an elongated shape having a largest dimension in a longitudinal direction and a small dimension transverse to the largest dimension, the largest dimension being greater than the small dimension, the largest dimension being greater than 50 micrometers, the small dimension being between 0.5 micrometers and 130 micrometers, the longitudinal direction of the microcavities (5) being parallel or inclined relative to a normal (11, 12) to one of the two main faces (1, 2),the scanning of the laser beam being adapted to generate in a plane of incidence on said zone a spatial distribution of the microcavities (5) determined by a separation distance (Si, S2) between adjacent microcavities, and a ratio between the largest dimension and the separation distance of the microcavities being greater than a predetermined threshold.,

19. Method according to claim 18 in which the energy per pulse is between 1 microjoule and 100 microjoules and even between 2 microjoules and 25 microjoules and / or the repetition frequency is less than or equal to 10 or 2 megahertz, for example between 250 kHz and 2 MHz.

20. Method according to one of claims 18 to 19 comprising a step of spatially shaping the beam to adjust the shape of the laser beam, in particular to form a Bessel beam or to focus the beam.

21. Method according to one of claims 18 to 20 comprising a heat treatment step (65) following the laser treatment step, in particular of at least 500°C for a layer which is a sheet of mineral glass, in particular monolithic glazing.

22. Method according to one of claims 18 to 20 characterized in that the layer is an interlayer of lamination, the application of the laser beam is in said layer forming part of a laminated glazing.

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