FR3116620A1 - Waveguide for image acquisition system - Google Patents

Abstract

Waveguide for image acquisition system The present description relates to a waveguide comprising at least one microstructure (27) defining, with respect to a first plane (P1) parallel to a first face of the waveguide , a protuberance in the direction of a second face of the waveguide, the protuberance having the shape of a cylinder (29) inclined with respect to said first plane (P1) and intersected by the latter. Figure for the abstract: Fig. 2

Description

Waveguide for image acquisition system

The present description generally relates to waveguide layers and, more particularly, to waveguide layers integrated into image acquisition systems, for example integrated into biometric acquisition systems.

The waveguide layers and more particularly the waveguide layers integrated into image acquisition systems are generally composed of several media of different refractive index. These different media can be layers, portions of layers or structures of different sizes and shapes.

There is a need to improve waveguides.

One embodiment overcomes all or part of the drawbacks of known waveguides.

One embodiment provides a waveguide comprising at least one microstructure defining, with respect to a first plane parallel to a first face of the waveguide, a protrusion in the direction of a second face of the waveguide, the protrusion having the shape of a cylinder inclined with respect to said first plane and intersected by the latter.

According to one embodiment, the microstructure comprises a non-planar face, defined by the curved part of the portion of the cylinder emerging from the first plane.

According to one embodiment, the axis of the cylinder makes, with the first plane, an angle comprised between 20° and 60°, preferably equal to around 45°, more preferably equal to 45°.

According to one embodiment, the axis of the cylinder is inscribed in a second plane perpendicular to the first plane.

According to one embodiment, the microstructure comprises a first planar face in a third plane perpendicular to the second plane.

According to one embodiment, the microstructure comprises a second planar face in the first plane.

According to one embodiment, the first flat face and the second flat face of the microstructure intersect with an angle comprised between 45° and 135°, preferably less than or equal to around 90°, more preferably equal to around 90°.

According to one embodiment, the inclination of the microstructure is in the direction of the waves conveyed by the guide.

According to one embodiment, the waveguide comprises at least a first set of microstructures whose axes of the cylinders are parallel and of the same orientation.

According to one embodiment, the waveguide comprises at least a second set of microstructures whose axes of the cylinders are parallel and of the same orientation, the first face of each microstructure of the first set being common to the first face of each microstructure of the second set.

According to one embodiment, the waveguide comprises, in a direction parallel to the axis of the microstructures of the first set, a first part without microstructure followed by a second part comprising microstructures.

According to one embodiment, the waveguide comprises, in a direction parallel to the axis of the microstructures of the second set, a third part without microstructure followed by the second part comprising microstructures.

According to one embodiment, the density of microstructures in the second part is all the lower as one approaches the first part or the third part.

One embodiment provides an image acquisition system comprising:
a device comprising an image sensor, the upper face of which is parallel to the second face of the waveguide layer, an angular filter and the waveguide as described;
a light source; And
a processing unit.

According to one embodiment, the light source is one or more light-emitting diodes.

According to one embodiment, the light source is located in the waveguide layer.

According to one embodiment, the system comprises a passive liquid crystal screen, illuminated by the waveguide layer, the liquid crystal screen being located between the waveguide layer and an object to be imaged.

According to one embodiment, the light source is an organic light-emitting diode located between the waveguide layer and an object to be imaged.

These characteristics and advantages, as well as others, will be set out in detail in the following description of particular embodiments given on a non-limiting basis in relation to the attached figures, among which:

there is a sectional, partial and schematic view of an example of an image acquisition system;

there is a perspective view, partial and schematic, of an embodiment of a microstructure of the image acquisition system illustrated in ;

there is a sectional view, partial and schematic, in a plane perpendicular to the plane of the section of the , of an embodiment of the microstructure illustrated in ;

there is a partial and schematic sectional view, in a plane perpendicular to the planes of the sections of FIGS. 1 and 2, of an embodiment of the microstructure illustrated in ;

there represents sectional views A, B and C, partial and schematic, in the plane of the section of the , of alternative embodiments of the microstructure illustrated in ;

there is a top view, partial and schematic, of an embodiment of the image acquisition device illustrated in ;

there is a partial schematic top view of another embodiment of the image acquisition device illustrated in ;

there is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of a microstructure of the image acquisition device illustrated in ;

there is seen in section, partial and schematic, in a plane perpendicular to the section plane of the , of an embodiment of another microstructure of the image acquisition device illustrated in ;

there represents by a top view, partial and schematic, yet another embodiment, of the image acquisition device illustrated in ;

there is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of a microstructure of the image acquisition device illustrated in ;

there is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of another microstructure of the image acquisition device illustrated in ;

there is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of yet another microstructure of the image acquisition device illustrated in ;

there is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of yet another microstructure of the image acquisition device illustrated in ; And

there represents, by a perspective view A and a sectional view B, partial and schematic, another embodiment of microstructures of the image acquisition device illustrated in .

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

For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been represented and are detailed. In particular, only the embodiments and the modes of implementation of the waveguide layers have been detailed, these being compatible with the usual image acquisition systems.

Unless otherwise specified, when reference is made to two elements connected together, this means directly connected without intermediate elements other than conductors, and when reference is made to two elements connected (in English "coupled") between them, this means that these two elements can be connected or be linked through one or more other elements.

In the following description, when referring to absolute position qualifiers, such as "front", "rear", "up", "down", "left", "right", etc., or relative, such as the terms "above", "below", "upper", "lower", etc., or to qualifiers of orientation, such as the terms "horizontal", "vertical", etc., it reference is made unless otherwise specified to the orientation of the figures.

Unless specified otherwise, the expressions “about”, “approximately”, “substantially”, and “of the order of” mean to within 10%, preferably within 5%.

For the purposes of the present description, the refractive index of a medium is defined as being the refractive index of the material constituting the medium for the range of wavelengths of the radiation picked up by the image sensor. The refractive index is considered to be substantially constant over the range of wavelengths of the useful radiation, for example equal to the average of the index of refraction over the range of wavelengths of the radiation picked up by the image sensor .

There is a sectional, partial and schematic view of an example of an image acquisition system 10.

The system 10 comprises a device 11 comprising, from bottom to top in the orientation of the figure:
a single organic image sensor 13; And
a layer 17, called the waveguide, covering the upper face of the image sensor 13 and defined by an upper face 17s and a lower face 17i.

The device 11 also preferably comprises an optical filter 15, for example an angular filter, between the image sensor 13 and the waveguide layer 17.

In the present description, the embodiments of FIGS. 1 to 14 are represented in space according to a direct orthogonal XYZ frame, the Y axis of the XYZ frame being orthogonal to the upper face of the sensor 13 and oriented from the sensor 13 towards the layer 17.

The system 10 further comprises a processing unit 18 (PU), connected to the device 11. The processing unit 18 preferably comprises means for processing the signals supplied by the device 11, not shown in . The processing unit 18 further comprises, for example, a microprocessor. The device 11 and the processing unit 18 are preferably connected by a link 19. The device 11 and the processing unit 18 are, for example, integrated in the same circuit.

The device 11 comprises a light source 21 adapted to emit radiation 23.

According to one embodiment, the source 21 is laterally coupled to the layer 17 and is located out of plumb, in the direction Y, of the stack of the sensor 13, of the angular filter 15 and of the layer 17. The source 21 is, for example, composed of one or more light-emitting diodes (LED, Light-Emitting Diode). Preferably, the source 21 is composed of several LEDs organized in "bars" along one or the side edges of the layer 17.

According to one embodiment, the device 11 comprises an organic light-emitting diode (OLED, Organic Light-Emitting Diode) screen, not shown, located, for example, between the layer 17 and an object 25 to be imaged.

According to one embodiment, the device 11 comprises a liquid crystal display (LCD, liquid crystal display) not shown located between the layer 17 and the object 25 to be imaged, preferably without a lighting system.

According to the embodiment illustrated in , the device 11 picks up the image response of the object 25, partially represented, preferably a hand or one or more fingers. The image processing unit 18 is adapted to extract information relating to fingerprints and/or to a network of finger veins 25 imaged by the sensor 13.

The radiation 23 corresponds to light radiation in the visible and/or infrared, that is to say radiation whose component wavelength or wavelengths are between 400 nm and 1700 nm.

According to one embodiment, the source 21 is positioned on the periphery of the layer 17. For example, the source 19 is located to the left of the layer 17, in the orientation of the .

According to a variant, not shown, there are several sources 21 located indifferently with respect to each other. The sources 21 are positioned, for example, on the same side of the layer 17, one next to the other or one behind the other or so that the radiations 23 are orthogonal.

Layer 17, called the waveguide layer, comprises a structure of two or three media with different refractive indices.

A waveguide layer is structurally adapted to allow the confinement and propagation of electromagnetic waves. The media are, for example, arranged in the form of a stack of three sub-layers, a central layer sandwiched between an upper sheath and a lower sheath, the refractive indices of the materials making up the sheaths being lower than the index refraction of the material making up the central layer, the lower sheath being located on the side of the angular filter 15. By way of example, the sheaths have a refractive index of the order of 1.33 and the central layer has an index of refraction of the order of 1.59 at 530 nm. According to one embodiment, microstructures are formed by nanoimprinting in a layer of resin located between the central layer and the lower sheath. Alternatively, the microstructures are formed by stamping or by photolithography. The resin layer is, for example, composed of a resin having a refractive index substantially equal to the refractive index of the material constituting the central layer and this for the wavelengths considered.

The central layer is for example made of poly(methyl methacrylate) (PMMA), polycarbonate (PC), cycloolefin polymer (COP) or poly(ethylene terephthalate) (PET). By way of example, the central layer is made of a polycarbonate known under the trade name Makrolon LED2245.

A network of microstructures 27 is, for example, adapted to extract, from the layer 17, the waves of the radiation 23 emitted by the source 21, in the direction of the object 25 to be imaged. The geometry and the structure of the microstructures 27 will be detailed below in relation to the .

According to one embodiment, layer 17 has a thickness of between 100 μm and 1 mm, the thickness of layer 17 is preferably between 150 μm and 500 μm. According to one embodiment, the central layer has a thickness of between 100 μm and 1 mm, the thickness of the central layer is preferably between 200 μm and 500 μm and more preferably of the order of 250 μm. The sheaths preferably have thicknesses comprised between 1 μm and 100 μm, the thickness of the claddings are preferably comprised between 10 μm and 50 μm and more preferentially of the order of 25 μm.

According to one embodiment, not shown in , the layer 17 is covered, in the stack of the image acquisition device 11, by a protective layer. The protective layer makes it possible, in particular, to prevent the layer 17 from being scratched when the device 11 is used by a user.

There is a perspective view, partial and schematic, of an embodiment of a microstructure 27 of the image acquisition system 10 illustrated in .

According to the embodiments illustrated in FIGS. 2 to 15, the microstructures 27 of the layer 17 have substantially all the same dimensions and the same geometry within manufacturing tolerances, however in practice, the microstructures 27 may have different dimensions.

Each microstructure 27 defines, with respect to a first plane P1 parallel to the lower face 17i ( ) of the waveguide layer 17, a protrusion in the direction of the upper face 17s ( ) of the waveguide layer 17. The protrusion has the shape of a cylinder 29 inclined with respect to the plane P1 and cut by the plane P1.

The axis 31 of the cylinder 29 forms, with the plane P1, an angle φ comprised between 20° and 60°, preferably equal to about 45°, more preferably equal to 45°. The axis 31 of the cylinder 29 is inscribed in a second plane P2, perpendicular to the first plane P1 and preferably cutting the microstructure 27 associated with the cylinder 29 into two equal and symmetrical parts. Each microstructure 27 is further delimited by a third plane P3 perpendicular to the second plane P2 and, preferably, perpendicular to the plane P1.

Each microstructure 27 illustrated in FIGS. 2 to 5 thus comprises:
a first planar face F1 in the third plane P3;
a second flat face F2 in the first plane P1; And
a non-planar face F3 defined by the curved part of the portion of the cylinder 29 emerging from the first plane P1.

In other words, the non-planar face F3 connects the curved sides of two planar faces F2, F1. The face F3 describes all the space between the two flat faces F1 and F2. The non-planar face F3 has, when flattened, the shape of a pointed oval.

There is a sectional view, partial and schematic, in a plane perpendicular to the plane of the section of the , of an embodiment of the microstructure 27 illustrated in .

There is a partial and schematic sectional view, in a plane perpendicular to the planes of the sections of FIGS. 1 and 2, of an embodiment of the microstructure 27 illustrated in .

More specifically, the is a sectional view of the microstructure 27, along the section plane P1 of the and the is a sectional view of the microstructure 27, along the section plane P3 of the .

Each of the faces F1, F2 has the shape of part of an ellipse. According to the embodiment illustrated in at 4, the faces F1, F2 intersect over a distance equal to d1. The distance d1 is preferably less than or equal to half the size of one side of a pixel.

For example, the distance d1 is between 1 μm and 25 μm for pixels measuring 50 μm by 50 μm, the distance d1 is preferably of the order of 4 μm.

There represents sectional views A, B and C, partial and schematic, in the plane of the section of the , of alternative embodiments of the microstructure 27 illustrated in .

More specifically, the three views of the correspond to cross-sectional views of the microstructure 27 along the section plane P2 illustrated in .

The section of the microstructure 27 according to the section plane P2 of view A of the is a triangle whose two sides of the faces F1 and F2 have respectively dimensions d2 and d3, preferably equal. The dimensions d2 and d3 are, for example, between 1 μm and 25 μm, preferably between 2 μm and 12 μm.

Preferably, the first flat face F1 and the second flat face F2 of each microstructure 27 intersect with an angle θ of between 45° and 135°, preferably less than or equal to around 90°, more preferably equal to around 90° . For example, in view A, the angle θ is around 90°, in view B, the angle θ is between 90° and 135° and in view C, the angle θ is between between 45° and 90°.

According to the preferred embodiment, the angles α, φ, formed respectively between the face F3 and the face F1 and between the face F3 and the face F2, are, in the plane P2 of the , for example, between 20° and 60° and are preferably equal to about 45°, more preferably equal to 45°.

By way of example, the section of a microstructure 27 along the section plane P2 of the is a right triangle, preferably isosceles.

By way of example, the dimension d2 can be equal to half the dimension d3, the angle θ equal to approximately 90°, the angle φ equal to approximately 30° and the angle α equal to approximately 60°.

There is a top view, partial and schematic, of an embodiment of the image acquisition device 11 illustrated in .

More specifically, the illustrates an example of evolution of the density of microstructures 27 in the waveguide layer 17 of a device 111.

The device 111 illustrated in comprises two sources 21, on the same side of the layer 17 and, each emitting a radiation 23 however in practice the number of sources 21 can be different from 2.

According to one embodiment, not shown, the sources 21 can be located in the waveguide layer 17.

According to the present embodiment, the layer 17 is, in top view, rectangular. In practice, layer 17 can have an oval, triangular or polygonal shape. Those skilled in the art will know how to adapt the present description to the shape of the layer 17.

In top view, the device 111, and more precisely the layer 17, comprises two parts:
a first part 33, optional, not comprising microstructures 27, located on the source side 21; And
a second part 35, comprising microstructures 27, and in which the density of microstructures 27 is, for example, not homogeneous over its entire surface.

In other words, the layer 17 is divided into two parts, the first part 33 being the part of the layer 17 located close to the sources 21, to the left of the layer 17 in , and the second part 35 being the part of the layer 17 located opposite the sources 21, to the right of the layer 17 in . The first part 33 extends from the edge of the layer 17, source side 21, into the layer 17 over a distance L1. The second part extends from the edge of the layer 17, opposite the sources 21, over a distance L2. Layer 17 has a width l.

An advantage of this embodiment is that the first part 33 of the layer 17 makes it possible to ensure that the entry of the second part 35 of the layer 17, that is to say the junction between the first part 33 and the second part 35 is illuminated by the radiation 23 coming from the layers 21 in a homogeneous manner. In other words, this embodiment allows the cones of revolution or portions of cone of revolution in which the radiation 23 is propagated to intersect in the first part 33 of the layer 17, and outside the second part 35 of the layer 17. The distance L1 thus depends on the distance which separates the layer 17 and the sources 21. The distance L1 is therefore smaller in a device 111 where the sources 21 are located in the layer 17, than in a device 111 where the sources 21 are located outside the layer 17.

In , layer 17 appears darker when the density of microstructures 27 is higher and layer 17 appears lighter when the density of microstructures 27 is lower. Microstructures 27 extend from the entrance to the second part 35 of the layer 17, to the exit of the second part 35 of the layer 17 (that is to say opposite the entrance to the second part 35 of layer 17). The density of the microstructures 27, which corresponds to the ratio between the total surface occupied by the microstructures 27 and the surface of the waveguide 17 in the XZ plane, is preferably much greater near the exit of the layer 17 only close to the entrance to layer 17. The evolution of the density of microstructures 27 within layer 17 is adapted so that the object 25 to be imaged is illuminated by homogeneous radiation in the XZ plane. The evolution of the density of the microstructures 27 within the layer 17 is, for example, linear. By way of example, the evolution of the density of the microstructures 27 within the layer 17 has a logarithmic, power or exponential trend. By way of example, the density changes between 0.1 and 1, preferably between 0.2 and 0.8.

The waveguide layer 17 has, for example, dimensions adapted so that the dimension of the image sensor 13 ( ) meets Federal Bureau of Investigation (FBI) certifications.

According to one embodiment, the waveguide layer 17 has dimensions adapted so that the image sensor 13 is adapted to capture the image of a finger, that is to say that it respects the category FAP 10 certification. Distance L2 is greater than or equal to 16.5 mm. In order to comply with the FBI FAP 10 certification dimensions, the layer 17 has a width l greater than 12.7.

The number of sources 21 is, for example, of the order of 5 or 6.

According to one embodiment, the waveguide has dimensions adapted so that the image sensor 13 is adapted to capture the image of a hand, that is to say that it respects the certification category FAP 60. The distance L1 is, for example, between 5 mm and 15 mm and the distance L2 is greater than or equal to 81.28 mm. In order to comply with the FBI FAP 60 certification dimensions, the layer 17 has a width l at least equal to 76.2 mm. The number of sources 21 can be of the order of 26 but can be different from 26.

According to one embodiment, the waveguide has dimensions adapted so that the image sensor 13 respects the certification category FAP 10, FAP 30, FAP 60 or FAP 45.

According to another embodiment, the waveguide has a shape and dimensions adapted so that the shape and dimension of the image sensor 13 are adapted to the system or to the object 25 to be imaged.

According to the embodiment illustrated in , the microstructures 27 have all the axes 31 of the cylinders 29 ( ), by which they are defined, parallel and of the same orientation.

According to the embodiment illustrated in , the microstructures 27 are oriented in the layer 17 so that the radiation 23 encounters a microstructure 27 via its face F3. In other words, the inclination of the microstructures is in the direction of the radiation 23 conveyed by the layer 17. The microstructures 27 are oriented so that the face F2 is parallel to the plane formed by the upper face 17s ( ) of the layer 17 and that the face F2 looks at the upper face of the angular filter 23 ( ). The face F1 faces the outlet of the second part 35 of the layer 17. The angle formed by the faces F3 and F1 then points in the direction of the upper face 17s ( ) of the layer 17 and, more particularly in the direction of the finger 25.

There is a partial and schematic top view of another embodiment of the image acquisition device 11 illustrated in .

More specifically, the illustrates an example of evolution of the density in microstructures 27 in the waveguide layer 17 of a device 113. The device 113 illustrated in is similar to device 111 shown in with the difference that the device 113 comprises light sources 21 on two opposite sides of the layer 17 among the four sides of the layer 17.

The device 113 illustrated in comprises two sources 21, on two of the sides of the layer 17, each emitting a radiation 23, however, in practice the number of sources 21 can be, per side, different from two.

Unlike the first part 33 of the layer 17 of the device 111 illustrated in , the first part 33 of the layer 17 of the device 113 illustrated in is divided into two portions each located on an edge of the layer 17 facing the sources 21. The second part 35 is located at the center of the layer 17. In other words, the second part 35 is, according to the embodiment illustrated in sandwiched between the two portions of the first part 33 of the layer 17, the portions of the first part 33 being located opposite the sources 21.

The second part 35 has, for example, dimensions adapted in order to comply with the FBI certifications, for example, FAP 10, FAP 20, FAP 30, FAP 45 or FAP 60. By way of example, and in order to comply with the FBI certification FAP 10, the dimension of the second part 35 along the Z axis is greater than or equal to 12.7 mm and the dimension of the second part 35 along the X axis is greater than or equal to 16.5 mm or vice versa. By way of example, and in order to comply with the FBI FAP 60 certification, the dimension of the second part 35 along the Z axis is greater than or equal to 81.28 mm and the dimension of the second part 35 along the X axis is greater than or equal to 76.2 mm or vice versa.

Each of the two portions of the first part 33 extends, for example, from the edge of the layer 17, into the layer 17, over a constant distance, along the Z axis, substantially equal to the distance L1 mentioned in relation to the . By way of example, the distance L1 is preferably between 5 mm and 15 mm.

According to the embodiment illustrated in , part 35 is divided into two separate portions, for example substantially identical and symmetrical along the XY plane passing through the center of layer 17.

Each of the portions of the second part 35 accommodates a network of microstructures 27. A portion 351 of the second part 35 thus comprises a network of microstructures 271 and a portion 352 of the second part 35 comprises a network of microstructures 272.

The density of microstructures in each portion of the second part 35 is such that the density of microstructures is low in a zone of the portion in the vicinity of the sources 21 and is increasingly high the further away from the sources 21. For each of the two portions 351, 352, the density is maximum in microstructures 271, 272 at the level of the XY plane passing through the center of the layer 17.

The orientation of the microstructures according to their locations in the layer 17 and more particularly in the second part 35 of the layer 17 will be described below. More specifically, the orientation of the microstructures 271 present in the portion 351 of the second part 35 is described in relation to the . The orientation of the microstructures 272 present in the portion 352 of the second part 35 is described in relation to the .

There is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of a microstructure of the image acquisition device illustrated in .

More specifically, the illustrates an example of orientation of the microstructures 271 of the layer 17 illustrated in and in layer 17, along section plane AA of the .

According to the embodiment illustrated in FIGS. 7 and 8, the microstructures 271 are oriented in the layer 17, more precisely in the portion 351, so that the radiation 23, coming from the sources 21 adjacent to the portion 351, encounters each microstructure 271 by its face F3.

The microstructures 271 are oriented so that the face F2 is parallel to the plane formed by the upper face, 17s of the layer 17 and that the face F2 faces the upper face of the angular filter 23 ( ). The face F1 is oriented so that it is orthogonal to the Z axis and facing the source or sources 21 located on the opposite side of the layer 17. The angle formed by the faces F3 and F2 then points in the opposite direction of the Z axis and the angle formed by the faces F3 and F1 point in the direction of the upper face 17s of the layer 17 and, more particularly in the direction of the finger 25.

There is seen in section, partial and schematic, in a plane perpendicular to the section plane of the , of an embodiment of another microstructure 27 of the image acquisition device illustrated in .

More specifically, the illustrates an example of orientation of the microstructures 272 of the layer 17 illustrated in and in the layer 17, according to the section plane BB of the .

According to the embodiment illustrated in FIGS. 7 and 9, the microstructures 272 are oriented in the layer 17, more precisely in the portion 352, so that the radiation 23, coming from the sources 21 adjacent to the portion 352, encounters each microstructure 272 by its face F3.

The microstructures 272 are oriented so that the face F2 is parallel to the plane formed by the upper face 17s of the layer 17 and that the face F2 faces the upper face of the angular filter 23 ( ). The face F1 is oriented so that it is orthogonal to the Z axis and facing the source or sources 21 located on the opposite side of the layer 17. The angle formed by the faces F3 and F2 then points in the direction of the Z axis and the angle formed by the faces F3 and F1 points in the direction of the upper face 17s of the layer 17 and, more particularly in the direction of the finger 25.

The microstructures 271 and the microstructures 272 are symmetrical with respect to the XZ plane passing through the center of the layer 17.

There represents by a top view, partial and schematic, yet another embodiment, of the image acquisition device illustrated in .

More specifically, the illustrates an example of evolution of the density in microstructures 27 in the waveguide layer 17 of a device 115. The device 115 illustrated in is similar to device 111 shown in with the difference that the device 115 comprises light sources 21 on each of the four sides of the layer 17.

The device 115 illustrated in thus comprises at least one source 21, on each of the four sides of the layer 17, each emitting radiation 23, however, in practice the number of sources 21 may be, per side, different from one.

Unlike the first part 33 of the layer 17 of the device 111 illustrated in , the first part 33 of the layer 17 of the device 115 illustrated in forms a frame, for example rectangular or square, around the second part 35. In other words, the second part 35 of the layer 17 of the device 115 illustrated in is, for example, rectangular and located in the center of the layer 17 and is surrounded by the first part 33.

The second part 35 has, for example, dimensions adapted in order to comply with the FBI certifications, for example FAP 10, FAP 20, FAP 30, FAP 45 or FAP 60. By way of example, and in order to comply with the FBI FAP certification 10, the dimension of the second part 35 along the Z axis is greater than or equal to 12.7 mm and the dimension of the second part 35 along the X axis is greater than or equal to 16.5 mm or vice versa. By way of example, and in order to comply with the FBI FAP 60 certification, the dimension of the second part 35 along the Z axis is greater than or equal to 81.28 mm and the dimension of the second part 35 along the X axis is greater than or equal to 76.2 mm or vice versa.

The first part 33 has, for example, a constant frame width substantially equal to the distance L1 of the first part 33 mentioned in relation to the . By way of example, the distance L1 is preferably between 5 mm and 15 mm.

According to the embodiment illustrated in , part 35 is divided into four equivalent triangular portions whose bases each correspond to one side of part 35 and whose vertices coincide at the center of layer 17.

Analogously to the device 113 illustrated in , each of the portions of the second part 35 accommodates a network of microstructures. Thus, a portion 353 of the second part 35 comprises a network of microstructures 273, a portion 355 of the second part 35 comprises a network of microstructures 275, a portion 357 of the second part 35 comprises a network of microstructures 277, a portion 359 of the second part 35 comprises a network of microstructures 279.

Again analogously to the device 113 illustrated in , the density of microstructures in each portion of the second part 35 is such that the density of microstructures is low in the part of the portion near the sources 21 and is increasingly high as it approaches the center of the layer 17 .

The orientation of the microstructures according to their locations in the layer 17 and more particularly in the second part 35 of the layer 17 will be described below. More specifically, the orientation of the microstructures 273 present in the portion 353 of the second part 35 is described in relation to the . The orientation of the microstructures 275 present in the portion 355 of the second part 35 is described in relation to the . The orientation of the microstructures 277 present in the portion 357 of the second part 35 is described in relation to the . The orientation of the microstructures 279, present in the portion 359 of the second part 35, is described in relation to the . For each of the portions 353, 355, 357, 359 the density is maximum in microstructures 273, 275, 277, 279 at the center of the layer 17.

There is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of a microstructure 273 of the image acquisition device illustrated in .

More specifically, the illustrates an example of orientation of the microstructures 273 of the layer 17 illustrated in and in layer 17, along section plane AA of the .

According to the embodiment illustrated in FIGS. 10 and 11, the microstructures 273 are oriented in the layer 17, more precisely in the portion 353, so that the radiation 23, originating from the sources 21 adjacent to the portion 353, encounters each microstructure 273 by its face F3.

The microstructures 273 are oriented so that the face F2 is parallel to the plane formed by the upper face of the layer 17 and that the face F2 faces the upper face of the angular filter 23 ( ). The face F1 is oriented so that it is orthogonal to the axis X and facing the source or sources 21 located on the opposite side of the layer 17. The angle formed by the faces F3 and F2 then points in the direction of the axis X and the angle formed by the faces F3 and F1 points in the direction of the upper face of the layer 17 and, more particularly in the direction of the finger 25.

There is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of another microstructure 275 of the image acquisition device illustrated in .

More specifically, the illustrates an example of orientation of the microstructures 275 of the layer 17 illustrated in and in the layer 17, according to the section plane BB of the .

According to the embodiment illustrated in FIGS. 10 and 12, the microstructures 275 are oriented in the layer 17, more precisely in the portion 355, so that the radiation 23, coming from the sources 21 adjacent to the portion 355, encounters each microstructure 275 by its face F3.

The microstructures 275 are oriented so that the face F2 is parallel to the plane formed by the upper face of the layer 17 and that the face F2 faces the upper face of the angular filter 23 ( ). The face F1 is oriented so that it is orthogonal to the Z axis and facing the source or sources 21 located on the opposite side of the layer 17. The angle formed by the faces F3 and F2 then points in the direction of the axis Z and the angle formed by the faces F3 and F1 point towards the upper face of the layer 17 and, more particularly towards the finger 25.

There is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of yet another microstructure 277 of the image acquisition device illustrated in .

More specifically, the illustrates an example of orientation of the microstructures 277 of the layer 17 illustrated in and in layer 17, along section plane CC of the .

According to the embodiment illustrated in FIGS. 10 and 13, the microstructures 277 are oriented in the layer 17, more precisely in the portion 357, so that the radiation 23, coming from the sources 21 adjacent to the portion 357, encounters each microstructure 277 by its face F3.

The microstructures 277 are oriented so that the face F2 is parallel to the plane formed by the upper face of the layer 17 and that the face F2 faces the upper face of the angular filter 23 ( ). The face F1 is oriented so that it is orthogonal to the axis X and facing the source or sources 21 located on the opposite side of the layer 17. The angle formed by the faces F3 and F2 then points in the opposite direction of the axis X and the angle formed by the faces F3 and F1 points in the direction of the upper face of the layer 17 and, more particularly in the direction of the finger 25.

The microstructures 277 and the microstructures 273 are symmetrical with respect to the XZ plane passing through the center of the layer 17.

There is a sectional view, partial and schematic, in a plane perpendicular to the sectional plane of the , of an embodiment of yet another microstructure 279 of the image acquisition device illustrated in .

More specifically, the illustrates an example of orientation of the microstructures 279 of the layer 17 illustrated in and in the layer 17, according to the section plane DD of the .

According to the embodiment illustrated in FIGS. 10 and 14, the microstructures 279 are oriented in the layer 17, more precisely in the portion 359, so that the radiation 23, coming from the sources 21 adjacent to the portion 359, encounters each microstructure 279 by its face F3.

The microstructures 279 are oriented so that the face F2 is parallel to the plane formed by the upper face of the layer 17 and that the face F2 faces the upper face of the angular filter 23 ( ). The face F1 is oriented so that it is orthogonal to the Z axis and facing the source or sources 21 located on the opposite side of the layer 17. The angle formed by the faces F3 and F2 then points in the opposite direction of the Z axis and the angle formed by the faces F3 and F1 points in the direction of the upper face of the layer 17 and, more particularly in the direction of the finger 25.

The microstructures 279 and the microstructures 275 are symmetrical with respect to the XY plane passing through the center of the layer 17.

There represents, by a perspective view A and a sectional view B, partial and schematic, another embodiment of microstructures 27' of the image acquisition device illustrated in .

More particularly, view A represents a perspective view of a microstructure 27' as it is present in layer 17. View B of is a sectional view along plane P of view A.

The 27' microstructures illustrated in are comparable to the microstructures 27 illustrated in since a microstructure 27' illustrated in corresponds to the assembly of two microstructures 27 illustrated in by their faces F1.

In other words, layer 17 in comprises two sets of microstructures 27 as illustrated in whose axes of the cylinders intersect forming an angle substantially equal to 180 -2xφ. The axes of the cylinders are preferably orthogonal in the same plane and the orientation is, in the two assemblies, opposite. The first face F1 (defined on the microstructure 27' as being the face F6) of each microstructure 27 of the first set being common to the first face F1 of each microstructure 27 of the second set.

Each microstructure 27' then corresponds to the assembly of two microstructures 27 by their first faces F1.

Each microstructure 27' comprises, according to the embodiment illustrated in two non-flat faces F5 and F5' connecting the curved sides of two flat surfaces F6, F4. Face F6 has the shape of part of an ellipse and face F4 has the shape of an entire ellipse. The two plane faces F6, F4 are orthogonal and secant so that the right edge of face F6 coincides with the axis of symmetry of face F4. The faces F5 and F5' describe all the space between the two faces F4 and F6. The non-planar faces F5 and F5' have, when flattened, the same shape and the shape of a pointed oval.

According to the embodiment illustrated in , the faces F6, F4 intersect over a distance equal to d1.

According to the preferred embodiment, the section of a microstructure 27' along the section plane P of view A of the is a right triangle whose height is equal to d2 and whose dimension of the hypotenuse corresponds to twice the dimension d3. In other words, the section of a microstructure 27' along the section plane P of view A is a right triangle, preferably isosceles.

According to another embodiment, the microstructure 27' is a non-rectangular and/or non-isosceles triangle whose height is equal to d2.

According to the embodiment illustrated in , the angles formed between face F5 and face F4 and between face F5' and face F4 are, in plane P of view A, equal to φ ( ) and of the order of 45°.

Such microstructures are, for example, used in the device 113 illustrated in , whose layer 17 is illuminated by two rows of sources 21 facing each other. The density of the microstructures 27' in the layer 17 can be homogeneous over the entire surface of the layer, the density can follow a curve divided into two linear and symmetrical portions with respect to the center of the layer 17 (for example according to the XY plane) or another distribution making it possible to obtain homogeneous illumination over the whole of the object 25 to be imaged.

An advantage of the embodiments and modes of implementation described is that they make it possible to extract more light in the direction of the finger 25 rather than in the direction of the image sensor 13.

Another advantage of the embodiments and modes of implementation described is that they make it possible to limit the degradation of the image, the sensor thus picks up an image of good quality, even for a high density of microstructures.

Yet another advantage of the embodiments and modes of implementation described is that they make it possible to extract the light coming from the sources 21 with an angle of 90°, that is to say that they make it possible to illuminate the finger mainly perpendicular to the surface of the waveguide layer 17.

Various embodiments and variants have been described. The person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to the person skilled in the art.

Finally, the practical implementation of the embodiments and variants described is within the abilities of those skilled in the art based on the functional indications given above.

Claims (18)

Waveguide (17) comprising at least one microstructure (27; 271, 272; 273, 275, 277, 279; 27') defining, with respect to a first plane (P1) parallel to a first face (17i) of the waveguide, a protuberance in the direction of a second face (17s) of the waveguide, the protuberance having the shape of a cylinder (29) inclined with respect to said first plane (P1) and intersected by the latter. Waveguide according to Claim 1, in which the axis (31) of the cylinder (29) makes, with the first plane (P1), an angle (φ) comprised between 20° and 60°, preferably equal to about 45 °, more preferably equal to 45°. Waveguide according to Claim 1 or 2, in which the axis (31) of the cylinder (29) lies in a second plane (P2) perpendicular to the first plane (P1). Waveguide according to Claim 3, in which the microstructure (27; 271, 272; 273, 275, 277, 279) comprises a first planar face (F1) in a third plane (P3) perpendicular to the second plane (P2) . Waveguide according to any one of Claims 1 to 4, in which the microstructure (27; 271, 272; 273, 275, 277, 279) comprises a second planar face (F2) in the first plane (P1). Waveguide according to claims 4 and 5, in which the first flat face (F1) and the second flat face (F2) of the microstructure (27; 271, 272; 273, 275, 277, 279) are secant with a angle (θ) between 45° and 135°, preferably less than or equal to approximately 90°, more preferably equal to approximately 90°. Waveguide according to any one of Claims 1 to 6, in which the inclination of the microstructure (27; 271, 272; 273, 275, 277, 279) is in the direction of the waves (23) conveyed by the guide (17). Waveguide according to any one of Claims 1 to 7, comprising at least a first set of microstructures (27; 271, 272; 273, 275, 277, 279) whose axes (31) of the cylinders (29) are parallel and of the same orientation. Waveguide according to Claims 6 and 8, comprising at least a second set of microstructures (27; 271, 272; 273, 275, 277, 279) whose axes (31) of the cylinders (31) are parallel and likewise orientation, the first face (F1) of each microstructure (27; 271, 272; 273, 275, 277, 279) of the first set being common to the first face (F1) of each microstructure of the second set. Waveguide according to claim 9, in which the axes of the cylinders of the microstructures (27; 271, 272; 273, 275, 277, 279) of the first set are perpendicular to the axes of the cylinders of the microstructures of the second set. Waveguide according to any one of Claims 8 to 10, comprising, in a direction parallel to the axis of the microstructures (27; 271, 272; 273, 275, 277, 279; 27') of the first set, a first part (33) without microstructure followed by a second part (35; 351, 352; 353, 355, 357, 359) comprising microstructures. Waveguide according to Claim 11, comprising, in a direction parallel to the axis of the microstructures (27; 271, 272; 273, 275, 277, 279; 27') of the second set, a third part (33) without microstructure followed by the second part (35; 351, 352; 353, 355, 357, 359) comprising microstructures. Waveguide according to any one of Claims 9 to 12, in which the density of microstructures (27; 271, 272; 273, 275, 277, 279; 27') in the second part (35; 351, 352; 353, 355, 357, 359) is all the weaker as one approaches the first part (33) or the third part (33). Image acquisition system (10) comprising:
a device (11) comprising an image sensor (13), the upper face of which is parallel to the second face (17s) of the waveguide layer (17), an angular filter (15) and the waveguide wave according to any one of claims 1 to 13;
a light source (21); And
a processing unit (18). A system according to claim 14, wherein the light source is light emitting diode(s). A system according to claim 14 or 15, wherein the light source is located in the waveguide layer (17). Image acquisition system according to claim 14, comprising a passive liquid crystal display, illuminated by the waveguide layer (17), the liquid crystal display being located between the waveguide layer (17) and an object to be imaged (25). A system according to claim 14, wherein the light source is an organic light emitting diode located between the waveguide layer (17) and an object to be imaged (25).