Sensor device for taking an impression of a body part

Description

[0001] The invention relates to a device for capturing an impression of a body part. It finds application in the field of fingerprint recognition. Contexte de l'invention

[0002] The use of fingerprints, for example, a single finger, a plurality of fingers, or a palm print, makes it possible to secure access to buildings or machines. Such technology eliminates the need for access codes or cards that can be lent, stolen, or forged. Using this technology enhances security because the probability of two people having two identical fingerprints is virtually zero.

[0003] A fingerprint capture device captures an image of a fingerprint. For identification purposes, this fingerprint is compared with a set of reference fingerprints contained in a database. For authentication purposes, this fingerprint is compared with a single fingerprint. The comparison determines whether the captured fingerprint belongs to a person listed in the database or whether the person is who they claim to be.

[0004] There Fig. 5 schematically describes a fingerprint capture device working in total reflection and capable of operating in light or dark background.

[0005] The device 50 described in Fig. 5comprises a prism 500, two light sources 501A and 501B, and an optical system 502 such as for example a CCD (Charge-Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) sensor and one or more lenses.

[0006] The light source 501A generates a light beam that passes through a first face 500A of the prism 500 to a second face 500C of the prism 500 where the part of the body (here a finger D) bearing the fingerprint is positioned. The fingerprint is made up of valleys and ridges. The light beam generated by the light source 501A forms an incident angle α A with the normal to the face 500C. The angle α A is greater than a critical angle θ c and less than a limiting angle θ l .

[0007] The face 500C separates a first medium corresponding to the prism 500 from a second medium corresponding to air or to the finger D. The critical angle θ c (resp. the limit angle θ l ) is defined as the angle beyond which total reflection occurs when a beam reaches the 500C face when the second medium is air (resp. when the second medium is finger D).

[0008] When the finger D is placed on the face 500C, the light beam generated by the source 501A undergoes total reflection when, at the position struck by the light beam on the face 500C, the finger forms a valley, ie there is air between the 500C face and the finger D. When, at the position struck by the light beam on the 500C face, the finger forms a crest, ie the skin of the finger is in direct contact with the 500C face, there is no total reflection. The total reflection is then said to be frustrated and the light beam is diffused in the finger D.

[0009] After reflection on the face 500C, the light beam passes through the fourth face 500D and reaches the optical system 502. The optical system 502 then forms an image of the fingerprint with high contrast between the valleys and the ridges. The valleys correspond to beams totally reflected by the face 500C and therefore appear very bright in the image. The ridges correspond to diffused light beams, partly absorbed in the finger D and which have emerged from the finger to reach the optical system 502. The ridges therefore appear darker in the image.

[0010] The optical system 502 therefore receives both light beams reflected by the face 500C and diffused in the finger D. The device formed by the source 501A, the prism 500 and the optical system 502 is a device using the principle of total reflection with bright field. A similar device can be found in the American patent US3200701.

[0011] The light source 501B generates a light beam which passes through a third face 500B of the prism 500 to the second face 500C where the finger D is positioned. The light beam generated by the light source 501B forms an incident angle α B with the normal to the face 500C less than the critical angle θ c (here, the incident angle α B is zero degrees). The light beam generated by the source 501B is therefore not totally reflected by the face 500B.

[0012] The optical system 502 receives the light beam generated by the source 501B after diffusion by the finger D. The optical system 502 is configured so as to receive light beams after diffusion in the finger D forming an angle between the critical angle θ c and the limiting angle θ lwith the normal to the face 500C. The optical system 502 therefore only receives light beams resulting from diffusion in the finger D but no light beams resulting from reflection on the upper face 500C. Here again, the optical system 502 forms an image of the fingerprint with high contrast between the valleys and the ridges. The ridges correspond to diffused light beams, partly absorbed in the finger D and which have emerged from the finger at the ridges in contact with the face 500B to reach the optical system 502. No light beam diffused in the finger D and emerging from the finger D at the valleys can reach the optical system 502 because they cannot pass through the layer of air and then propagate in the prism 500 while forming an angle relative to the normal to the face 500B greater than the critical angle θ c. The ridges therefore appear brighter in the fingerprint image than the valleys. The device formed by the source 501B, the prism 500 and the optical system 502 is a device using the principle of total reflection with a dark field. A similar device can be found in the French patent FR2757974.

[0013] The critical angle θ c is given by the following formula: θ c = arcsin n 0 n 1 n 1 being the refractive index of the prism and n 0 being the refractive index of air or the finger. For a refractive index of air equal to "1" and a refractive index of the prism equal to "1.5", we obtain a critical angle θ c = 41.8 degrees. The refractive index of the skin is, in the visible range, between "1.41" and "1.47". Considering the minimum value of "1.41", we therefore obtain a limiting angle θ l of "70" degrees. Considering the maximum value, we obtain an angle θ l max of “76” degrees.

[0014] Devices for capturing a biometric print working in total reflection with a bright field have the known disadvantage of being sensitive to defects that could be present on the face on which the finger rests (here the 500C face): scratches, dirt, etc. In fact, these defects locally modify the reflection properties of the 500C face (which is then no longer a total reflection) by creating a little absorption or diffusion.

[0015] A fingerprint capture device described in patent application US20170017824 is known, compatible with portable equipment because it is more compact.

[0016] There Fig. 6schematically describes such a compact fingerprint capture device. This device 6 comprises a first transparent blade 60 comprising a face 600 on which the part of the body (here the finger D) bearing the fingerprint can rest. Below the transparent blade 60 is a light source 61 formed from a thin layer of organic LED (Organic Light-Emitting Diode (OLED)). Hereinafter we use the term LED to designate indifferently light-emitting diodes and organic light-emitting diodes. Below the light source 61 is a collimation layer 62 in the form of a more or less opaque blade comprising holes (pinholes in Anglo-Saxon terminology) generally of circular shape distributed regularly on the blade. Two holes 65A and 65B passing through the thin LED layer 61 and the collimation layer 62 are shown.The collimation layer 62 is followed by a second transparent plate 63, of thickness less than the thickness of the glass plate 60. Below the transparent plate 63 is a sensor 64 sensitive to the light emitted by the light source 61 such as a CCD or CMOS sensor. In the device described in relation to the . Fig. 6, the shape of the holes (i.e. height, diameter) is of paramount importance. Indeed, in this device, only light beams having an angle of incidence on the face 600 close to “0 degrees” relative to the normal to the face 600 (such as the beam 66B) can pass through the collimation layer. Light beams having a greater angle of incidence (such as the beam 66A) must be blocked by the collimation layer 62. Indeed, it is essential in this device that the images formed opposite each hole are disjointed on the sensor 64, otherwise the overlapping areas would appear blurred. However, by preventing light beams having a significant angle of incidence from reaching the sensor 64, it is prevented from taking advantage of total reflection, in particular the light beams after diffusion in the finger D forming an angle between the critical angle θ c and the limiting angle θ lwith the face normal 600, which prevents obtaining a high contrast image between the valleys and ridges of a fingerprint.

[0017] It is desirable to overcome these disadvantages of the state of the art. In particular, it is desirable to provide a fingerprint capture device that is both compact and operates in total reflection. It is desirable that this device be little or not sensitive to possible defects in the surface on which the body part to be imaged rests. EXPOSE DE L'INVENTION

[0018] The invention is defined by a device according to claim 1 and by a method according to claim 15. SHORT DESCRIPTION OF THE DRAWINGS

[0019] The above-mentioned and other features of the invention will become more clearly apparent from the following description of an exemplary embodiment, said description being made in relation to the accompanying drawings, among which: there Fig. 1 schematically illustrates equipment comprising a device for capturing an impression of a body part according to the invention; the Fig. 2 schematically illustrates a first embodiment of a device for capturing an impression of a body part according to the invention; the Fig. 3 schematically illustrates a front view of a sub-part of a light-sensitive sensor suitable for the first embodiment of the device for capturing an impression of a body part; the Fig. 4A schematically illustrates a second embodiment of the device for capturing an impression of a body part according to the invention; Fig. 4Bschematically illustrates an operation of the second embodiment of the device for capturing an impression of a body part according to the invention; Fig. 4C schematically illustrates a sub-part seen from the front of a first example of a sensor adapted to the second embodiment of the device for capturing an impression of a body part according to the invention; Fig. 4D schematically illustrates a sub-part seen from the front of a second example of a sensor adapted to the second embodiment of the device for capturing an impression of a body part according to the invention; Fig. 4E schematically illustrates a sub-part seen from the front of a third example of a sensor adapted to the second embodiment of the device for capturing an impression of a body part according to the invention; Fig. 5schematically describes a device for capturing a fingerprint of the prior art working in total reflection and capable of operating in a light background or a dark background; the Fig. 6 schematically illustrates a compact device for capturing an impression of a body part of the prior art; the Figs. 7A and 7B describe in detail a particular implementation of the first embodiment of the device for capturing an impression of a body part in which a light source is integrated on a lower face of a first blade on which a body part can rest; the Fig. 8 schematically illustrates a fraud detection method using the device according to the invention; and, the Fig. 9 schematically illustrates an example of hardware architecture of a processing module implementing the fraud detection method. DESCRIPTION DETAILS OF DIFFERENT MODES OF REALIZATION

[0020] The following description details more particularly embodiments of the present invention in a smartphone context. The invention can be applied to other equipment that may include a device for capturing an impression of a body part such as a computer, a tablet, etc. Furthermore, the invention is described in a context where the body part is a finger. However, it applies to other body parts such as several fingers, a palm of the hand, etc.

[0021] There Fig. 1 schematically illustrates equipment comprising a device for capturing an impression of a body part according to the invention.

[0022] The equipment 1 is here a smartphone comprising a screen 10, a processing module 11 and a device for capturing an impression of a body part 12. We subsequently call the device for capturing an impression of a body part dispositif biométrique. The processing module 11, which we describe below in relation to the Fig. 9 , can implement several functionalities of the equipment 1 including in particular processing of data from the biometric device 12. The biometric device 12 is for example used by an owner of the equipment 1 to authenticate himself with the equipment 1 and thus be able to use it.

[0023] In a particular implementation, the device for capturing an impression of a body part 12 is integrated into the screen 10.

[0024] There Fig. 2 schematically illustrates a first embodiment of the biometric device 12 according to the invention.

[0025] The biometric device 12 comprises a plurality of optically coupled elements comprising: a first transparent blade 120 thick E 120 comprising an upper face 1200 on which the body part (here the finger D) bearing the fingerprint to be imaged can rest; a light source 121 adapted to illuminate the finger D composed here of LEDs. Four LEDs 121A, 121B, 121C and 121D are shown in the Fig. 2 ; a light-opaque screen 122 located below the first transparent blade 120; a second transparent blade 123 of thickness E 123 located below the opaque screen 122; a sensor 124 comprising light-sensitive photoreceptors located below the second transparent blade 123.

[0026] Transparent blades 120 and 123 have a refractive index greater than a predefined minimum refractive index n mingreater than the refractive index of air.

[0027] In one embodiment, this minimum refractive index is greater than “1.3”.

[0028] In another embodiment, this minimum refractive index is greater than the refractive index of the finger D, more particularly greater than 1.47. It is known in fact that when the refractive index of at least the second blade 123 is greater than the refractive index of the finger, the image of the finger is of finite extent.

[0029] In one embodiment, the two transparent plates 120 and 123 have a different refractive index.

[0030] In the remainder of the description, for simplicity, we assume that the two transparent plates 120 and 123 have an identical refractive index, for example equal to “1.5”. Optically coupled means that a ray going from the upper face of the first plate to the lower face of the second plate does not pass through any medium with an index lower than n min This can be done in the case of two glass slides, for example, by gluing the two slides together with a glue of sufficient refractive index.

[0031] The faces of the transparent blades 120 and 123, the opaque screen 122 and the sensor 124 are parallel. Here, the opaque screen 122 and the sensor 124 are considered as blades of thickness less than the thickness of the two transparent blades 120 and 123.

[0032] The light source 121 generates light beams passing through the upper blade 120 having an incident angle relative to the normal to the upper face 1200 of a maximum value less than the critical angle θ c .

[0033] Each LED of the light source 121 generates a light beam above the opaque screen 122 towards the upper face 1200. Each LED is configured such that each light ray emanating from this LED has an incident angle relative to the normal to the upper face 1200 of a maximum value less than the critical angle θ c. In this way, no light ray emanating from an LED of the light source 121 undergoes total reflection on the upper face 1200. With such a configuration of the light source 121, the entire surface of the finger D facing the face 1200 may not be illuminated. However, since a finger is a volume diffusing medium, the entire finger D facing the upper face 1200 will return light if the distance between two illuminated parts is less than a predefined distance dpcharacteristic of a depth of penetration of the light in the finger D varying from approximately one millimeter for blue light to a few centimeters for infrared light. Therefore, in order for the entire finger D opposite the upper face 1200 to be illuminated, the LEDs of the light source are therefore configured in such a way that each LED and the LED(s) which are the closest neighbors of said LED generate on the finger D parts illuminated directly by the LEDs distant by a distance δ less than the distance dp , the distance δ being the minimum distance between two boundaries of illuminated parts. The sensor 124 therefore receives light beams resulting from diffusion by the finger D of the light rays produced by the light source 121. This type of light source makes it possible to obtain A digital camera capture device working in total reflection in the dark.An advantage of dark-field total reflection fingerprint capture devices is that they are less sensitive to topside defects 1200.

[0034] The opaque screen 122 is a thin layer that can be made for example by printing or by depositing an opaque coating on the blade 123. The opaque screen 122 is however not completely opaque since it is composed of a network of holes. Each ray of light directed towards the sensor 124 which reaches the opaque screen 122 at a hole passes through the opaque screen 122 and reaches the sensor 124. Unlike the collimation layer of the Fig. 6 , each hole therefore allows a light ray coming from finger D directed towards sensor 124 at the entrance of said hole, to reach sensor 124.

[0035] In a particular implementation, the opaque screen 122 is a thin layer which can be produced by printing or by depositing an absorbent coating on the upper face of the transparent blade 123 or on the lower face of the transparent blade 120 such as a metal deposit. Each hole of the opaque screen 122 is filled with a material having a refractive index greater than the predefined minimum refractive index. n min .

[0036] The sensor 124 is for example a CCD sensor or a CMOS sensor composed of a matrix of photoreceptors (such as the photoreceptor 1241) sensitive to the wavelength of the light beams emitted by the light source 121. The sensor is optically coupled to the blade 123. The sensor 124 receives light passing through the holes in the opaque screen 122 and generates information from the received light which is used by the processing module 11 to produce a fingerprint image. The fingerprint image thus produced is composed of a matrix of pixels, each pixel coming from one or more photoreceptors. To obtain a good contrast between the peaks and valleys of the fingerprints, only the light rays coming from the finger D having an angle of incidence relative to the normal to the upper face 1200 between the critical angle θ c and the limiting angle θ lmust be considered. Light rays coming from finger D after scattering in finger D have any angle of incidence. Light rays coming from finger D and having angles of incidence less than the critical angle θ c could therefore reach the sensor 124 if they succeed in crossing one of the holes in the opaque screen 122.

[0037] In order to prevent the consideration of light rays having an angle of incidence less than the critical angle θ c in the fingerprint images generated by the processing module 11, the sensor 124 does not include light-sensitive photoreceptors at each position of the sensor that can be struck by a light ray from the finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ c. Each light-sensitive photoreceptor of the sensor (124) is therefore positioned outside each position of the sensor (124) which can be struck by a light ray from the finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ c In other words, each photoreceptor is placed outside a set of zones corresponding to a union of the projections of the holes on the sensor (124) along all directions having an angle relative to said normal less than the critical angle. θ c . In this way, only information from photoreceptors located at positions that can be struck by light rays from finger D after diffusion in finger D having an angle of incidence relative to the normal to the upper face 1200 between the critical angle θ c and the limiting angle θ lare used by the processing module 11 to form fingerprint images.

[0038] In a particular implementation, the sensor 124 does not include photoreceptors at each position that can be struck by a light ray coming from the finger D after diffusion in the finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ c The processing module 11 cannot therefore receive information corresponding to light rays coming from the finger D after diffusion in the finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ c .

[0039] In a particular implementation, the processing module 11 does not consider any information coming from a photoreceptor located at a position that can be struck by a light ray coming from the finger D after diffusion in the finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ c to generate a fingerprint image. The processing module 11 therefore uses exclusively information from photoreceptors struck by light rays from the finger D after diffusion in the finger D having an angle of incidence relative to the normal to the upper face 1200 between the critical angle θ c and the limiting angle θ l . The photoreceptors generating information which is not considered by the processing module 11 are therefore made insensitive to light. in hindsight by means of software processing.

[0040] In a particular implementation, each photoreceptor of the sensor 124 corresponding to a position which can be struck by a light ray coming from the finger D after diffusion in the finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ c is masked by an opaque metal layer, for example aluminum. The photoreceptors located below the opaque metal layer therefore become insensitive to light and therefore cannot provide information corresponding to light rays coming from finger D after diffusion in finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ cto the processing module 11. It is known that a photoreceptor of a CCD or CMOS sensor struck by a beam of light risks disturbing the photoreceptors in its vicinity, in particular when these photoreceptors are highly saturated (for example when the sensor 124 is directed towards the sun). An advantage of this particular implementation compared to the two previous particular implementations is that, the masking of the photoreceptors which can be struck by a light ray coming from the finger D after diffusion in the finger D having an angle of incidence relative to the normal to the upper face 1200 less than the critical angle θ c prevents these photoreceptors from disturbing their neighbors.

[0041] To prevent the incidence zones from overlapping, the holes of the opaque screen 122 are arranged in such a way that the distance L between a hole and the hole(s) which are its closest neighbors, taken from center to center, is greater than the diameter of a projection of the finger D on the sensor 124 seen by a hole when the finger D is placed on the upper face 1200. If d T is the diameter of the hole, the diameter of the projection of the finger D on the sensor 124 seen through a hole d AP is given by: d AP = d T + 2 . E 123 . tan θ l and therefore: L > d AP

[0042] It is noted that in a general case where the refractive indices of the blade 120 and the blade 123 are different, the diameter d AP is given by: d AP = d T + 2 . γ . E 120 . tan θ l where y represents a magnification of the device 12: γ = − E 123 × n 120 E 120 × n 123 Or n 120 is the refractive index of the 120 plate and n 123 is the refractive index of the 123 plate.

[0043] In a particular implementation, the holes of the opaque screen 122 are spaced apart by a distance L > d AP and, provided that the constraint on the distance L is respected, placed in any manner on the opaque screen 122.

[0044] In a particular implementation, the holes of the opaque screen 122 are spaced apart by a distance L > d AP and placed regularly, for example in the form of a rectangular matrix or a hexagonal mesh, on the opaque screen 122.

[0045] In the Fig. 2 , the photoreceptors of the sensor 124 shown in white (such as the photoreceptor 1241) are light-sensitive photoreceptors. The photoreceptors of the sensor 124 shown in black (such as the photoreceptor 1242) are non-light-sensitive photoreceptors.

[0046] There Fig. 3schematically illustrates a front view of a sub-part of the sensor 124 adapted for the first embodiment of the biometric device 12.

[0047] We are considering here the case where the holes of the opaque screen 122 are spaced apart by a distance L > d AP and placed regularly in the form of a rectangular matrix of holes.

[0048] The 124 sensor consists of a matrix of square photoreceptors generally from “1” to “10” µm aside.

[0049] Superimposed on the sensor 124 is represented a series of incidence zones distributed regularly on the sensor 124. Each incidence zone comprises a central disc such as the disc 1244 and a peripheral ring such as the ring 1243, the central disc and the peripheral ring of an incidence zone being concentric. Each incidence zone corresponds to one of the holes of the opaque screen 122 and represents a projection of the finger D on the sensor 124 seen by a hole when the finger D is placed on the upper face 1200. For example, the incidence zone comprising the central disc 1244 and the peripheral ring 1243 corresponds to the hole 122A. The outside diameter of each peripheral ring therefore corresponds to the diameter d APof a projection of the finger D on the sensor 124 seen through a hole when the finger D is placed on the upper face 1200. The holes of the opaque screen 122 taking the form of a rectangular matrix of holes, the incidence zones follow this form on the sensor 124. When the holes of the opaque screen 122 are circular, the center of the incidence zone corresponding to a hole and the center of said hole are merged. The part located in a peripheral ring (for example the peripheral ring 1243) corresponds to an area receiving light rays having passed through the opaque screen 122 through a hole (here the hole 122A) and having an incident angle with the normal to the upper face 1200 between the critical angle θ c and the limiting angle θ lThe part located inside the central disc (for example the central disc 1244) corresponds to an area receiving light rays having passed through the opaque screen 122 through a hole (here the hole 122A) and having an incident angle with the normal to the upper face 1200 less than the critical angle θ c. Each part of the sensor 124 located inside a central disk therefore corresponds to a part from which we do not wish to recover information. The photoreceptors located in each of these parts must therefore be insensitive to light. Each part of the sensor 124 located in a peripheral ring therefore corresponds to a part from which we wish to recover information. The photoreceptors located in each of these parts must therefore be sensitive to light. The photoreceptors located outside a peripheral ring receive little, if any, light from the finger if the refractive index of the finger placed on the upper face 1200 is lower than the refractive index of the transparent plates 120 and 123.

[0050] It is noted that each point of the finger D placed on the upper face 1200 is imaged at least once on the sensor 124. The biometric device 12 having a known geometry, it is possible to determine which photoreceptor of the sensor 124 images a point of the finger D. It then becomes possible to reconstruct an image of the print of the finger D using known techniques.

[0051] In a particular implementation, the distance L between each hole makes it possible to image at least twice each point of the finger D opposite the upper face 1200.

[0052] In a particular implementation, when the same point of the finger D is imaged by several photoreceptors of the sensor 124, the processing module 11 takes into account the information from each photoreceptor having imaged this point to generate a representation of this point in the image of the print.

[0053] In a particular implementation, the transparent blades 120 and 123 are square glass blades of “4.4 »mm sideways and, as we saw above, refractive index n 1 = 1.5. The 124 sensor is square in shape of “3.9 »mm side comprising square photoreceptors of “4” µm aside.

[0054] In a particular implementation, the transparent blade 123 has a thickness E 123 three to ten times less than the thickness E 120 of the transparent blade 120. For example, the thickness E 123 = 60 µm and thickness E 120 = 300 µm allow, when the two blades have the same refractive index, to obtain a magnification of -1 < / 5 (iean image of an object on the sensor 124 is five times smaller than the actual object placed on the upper face 1200 and conversely an area on the sensor 124 corresponds to an area 5 times larger on the upper face 1200). In this particular implementation, the transparent blade 123 is glued to the sensor 124 or produced by a series of deposits on the sensor 124.

[0055] In another implementation, the thickness E 123 = 150 µm and thickness E 120 = 750 µm , producing the same magnification of -1< / 5 .

[0056] Fingerprint imaging standards recommend finger image resolutions greater than 500 or 1000 dots per inch (dpi). At a magnification of -1 < / 5 , if a finger image sampled at more than 500 dpi (resp. 1000 dpi) is desired, pixels smaller than 10 µm (resp. less than 5 µm) are required.

[0057] In a particular implementation, the holes in the opaque screen 122 have a diameter of “7”. µm and form a regular matrix of 10 × 10 holes in which the holes are spaced at a distance L=400 µm from center to center of each other as shown in Fig. 3 . With a critical angle θ c = 41.8 degrees, a limiting angle θ l = 70 degrees, a hole diameter of "7" µm and a blade thickness 123 of “60” µm , each central disc has a diameter of approximately "114" µm and each peripheral ring has an outer diameter of about "337" µm .

[0058] In a particular implementation, the lower face of the opaque screen 122 and the edges of the holes are made absorbent by applying known techniques (application of a layer of black chrome, a layer of ink, texturing of the lower face, etc.) in order to minimize reflections between the opaque screen 122 and the sensor 124. In a particular implementation, the holes of the opaque screen 122 are not circular, but have a ring shape or a more complex shape. This makes it possible, with the coded aperture principle (described in the internet page https: / / en.wikipedia.org / wiki / Coded_aperture ), to increase the surface area of the hole while improving the accuracy of the image obtained after software processing (deconvolution). This makes it possible to simultaneously improve a signal-to-noise ratio and an accuracy of the fingerprint image.

[0059] In a particular implementation, the holes in the opaque screen 122 are coded holes as defined in the document "Image and Depth from a Conventional Camera with a Coded Aperture; A. Levin, R. Fergus, F. Durand, W.T. Freeman, ACM Tr. On Graphics, Vol. 26, No. 3, Article 70, July 2007".

[0060] In a particular implementation, the LEDs, which are shown in the Fig. 2 apart from blades 120 and 123 for clarity, are either integrated on a lower face of blade 120 (ie the face of the blade 120 which is in contact with the opaque screen 122), or integrated on the upper face of the blade 123 (ie the face of the blade 123 comprising the opaque screen 122).

[0061] THE Figs. 7A and 7B describe in detail a particular implementation of the first embodiment in which the LEDs are integrated on the underside of the blade 120.

[0062] There Fig. 7Adescribes, in detail, an LED adapted to the first embodiment.

[0063] In the case of the Fig. 7A , the LED is an OLED. Here we take the example of the 121A LED.

[0064] The LED 121A comprises in a first layer a first transparent electrode 70 produced for example by deposition and etching on the lower face of the blade 120 of an alloy of indium oxide doped with tin (“Indium tin oxide (ITO)” in English terminology).

[0065] In a second layer located immediately below the first, the LED 121A comprises a stack of layers to form the OLED 71 produced for example by deposition and etching or by printing. The layers forming the OLED 71 are almost transparent. Note that the second layer may comprise another type of LED.

[0066] In a third layer immediately below the second, the LED 121A includes a second electrode 72. The electrode 72 includes a metal layer that acts as a reflective mirror to collect the light that is directed downwards. (ie towards the sensor 124) and thus increase the light output upwards. In addition, this avoids dazzling the sensor 124. In one embodiment, the second electrode 72 comprises a second layer located below the metal layer, the second layer being non-conductive and absorbent. We will subsequently show in relation to the Fig. 7B that the second electrode 72 also serves as an opaque screen 122.

[0067] As shown by the dotted arrow, the light produced by the LED 121A is directed upwards, i.e. towards the transparent blade 120.

[0068] The different layers constituting the 121A LED have a thickness of the order of a micrometer.

[0069] There Fig. 7B details an example of assembly of the blade 120, the light source 121, the opaque screen 122, the blade 123 and the sensor 124 in the biometric device 12.

[0070] We have not represented in the Fig. 7B that a sub-part of the biometric device 12.

[0071] In the Fig. 7B, we find the blade 120 which includes the LED 121A and the LED 121B, which is identical to the LED 121A. For each LED we find the first electrode 70, the OLED stack 71 and the second electrode 72. The second electrode 72 extends on the lower face of the blade 120, but includes a hole. The second electrode 72 is joined between each LED making up the light source 121. The hole shown in the second electrode 72 corresponds to a hole, here the hole 122A, of the opaque screen 122. More generally, the metal layer forming the second electrode includes holes located between the LEDs and the opaque screen 122 is implemented by said metal layer. In one embodiment, these holes have a diameter of more than “100” µm.

[0072] In one embodiment, the blade 123 is glued on its upper face to the opaque screen 122 and on its lower face to the sensor 124.

[0073] In one embodiment, the first electrodes are also connected together, allowing all OLEDs to be controlled together.

[0074] In one embodiment, the sensor 124 is produced on a glass slide 125 or on a silicon slide.

[0075] In one particular implementation, the LEDs are gallium nitride (GaN) LEDs or OLEDs.

[0076] In one particular implementation, the LEDs could be replaced by a device for generating a laser beam.

[0077] In one particular implementation, each LED generates a light beam with a wavelength greater than "600 » nm, for better penetration into the finger and thus better diffusion. In a particular implementation, the wavelength of the light beams emitted by the LED is less than "980 »nm in order to be able to use light-sensitive silicon sensors.

[0078] In a particular implementation, converging lenses are placed in the holes of the opaque screen 122. These lenses ideally have a focal length at most equal to the distance between the lens and the sensor (here this distance is equal to the thickness E 123 ). The diameter of the lenses can be larger than the diameter of the holes in order to collect more light. This particular implementation allows the use of holes and lenses with a diameter larger than the diameter of the holes used so far in previous implementations and therefore allows more light to be collected.

[0079] In a particular implementation, a filter may be added at the holes to filter out light rays having wavelengths that do not correspond to the wavelengths generated by the LEDs. In this way, a risk of the fingerprint images being interfered with by light rays external to the biometric device 12 is reduced.

[0080] There Fig. 4A schematically illustrates a second embodiment of the biometric device 12 according to the invention.

[0081] In this embodiment, we find the blade 120, the opaque screen 122, the blade 123 and the sensor 124.

[0082] In this embodiment, the light source is no longer located at the opaque screen 122. LEDs are inserted at the sensor 124, ie under the opaque screen 122. At least a portion of the holes of the opaque screen 122 has an LED opposite.

[0083] In order for the entire finger D to diffuse light, in all particular implementations relating to the embodiment of the Fig. 4A , the holes of the opaque screen 122 having an LED opposite are arranged in such a way that each LED and the LED(s) which are the closest neighbors of said LED generate parts directly illuminated by the LEDs distant from a distance δ less than the distance dp .

[0084] Furthermore, in order to avoid overlaps between the incidence zones, in all specific implementations relating to the method of carrying out the Fig. 4A, the holes of the opaque screen 122 used to image the finger D are arranged in such a way that the minimum distance L between a hole and the hole(s) which are its closest neighbors, taken from center to center, is greater than the diameter of the image of the finger D projected onto the sensor 124 seen by a hole when the finger D is placed on the upper face 1200, that is to say L > d AP .

[0085] Each LED can be produced by deposition on the sensor 124. In this case, each LED is integrated into the surface of the sensor 124.

[0086] In a particular implementation, each LED is integrated into the sensor 124.

[0087] In a particular implementation, each LED generates a beam of light directed towards the holes having a maximum angle of incidence θ maxrelative to a normal to the upper face 1200 making it possible to prevent these LEDs from illuminating light-sensitive photoreceptors after reflection on the opaque screen 122. In a particular implementation, θ max = 23 degrees.

[0088] There Fig. 4C schematically illustrates a front view of a sub-part of a first example of sensor 124 adapted to the second embodiment of the biometric device 12.

[0089] In the case of the Fig. 4C , the holes of the opaque screen 122 form a rectangular matrix of holes.

[0090] In the Figure 4C , LEDs were inserted at positions corresponding to the center of each central disc. We find LEDs 121A and 121B represented in Fig. 4A, and LEDs 121C, 121D, 121E and 121F. There is therefore an LED at each position of the sensor 124 receiving light rays having passed through the opaque screen 122 through a hole and having an incident angle with the normal to the upper face 1200 less than the critical angle θ c . In this particular implementation, there is therefore an LED positioned at the sensor 124 opposite each hole in the opaque screen 122. Therefore, in the same way that the holes form a matrix of holes on the opaque screen 122, the LEDs form a matrix of LEDs on the sensor 124. As in the first embodiment of the biometric device 12, the photoreceptors located in a central disk are non-sensitive to light.

[0091] With circular holes, each LED illuminates a disc opposite it on the finger D when the latter is placed on the upper face 1200. When, for example, the transparent blade 120 and the transparent blade 123 respectively have a thickness of E 123 = 60 µm And E 120 = 300 µm , the holes have a diameter of “7” µm , and each LED is circular with a diameter of “10” µm , each LED illuminates a disc of approximately "92" µm . With an opaque screen comprising regularly distributed holes with a distance of “400” µm between the centers of the holes, the entire surface of the finger D placed on the upper face 1200 is not illuminated by the LEDs. But, as we saw above, a finger being a diffusing medium, the entire surface of the finger opposite the upper face 1200 will reflect light.

[0092] In this configuration, if an incidence zone is projected onto the upper face 1200 through the hole in the opaque screen 122 corresponding to said incidence zone, a projection of the central disc with a diameter approximately equal to “544” is obtained. µm and a projection of the peripheral ring with an outer diameter approximately equal to “1656” µm. With holes regularly distributed with a distance of "400" µm between the centers of the holes, the projections of the peripheral rings overlap.

[0093] There Fig. 4B schematically illustrates an operation of the second embodiment of the biometric device 12.

[0094] In the Fig. 4B , we take up the device of the Fig. 4C .

[0095] In the Fig. 4B , we have represented the projections of 8 incidence zones on the upper face 1200. A point P, also represented in the Fig. 4A, appears in the projection of three different peripheral rings. This point P is therefore imaged three times on the sensor 124: a first time at a point P1 following a ray R P 1, a second time at a point P2 following a ray R P 2 and a third time at a point not shown following a ray R P 3. Imaging each point on finger D multiple times results in better image quality.

[0096] In the second embodiment of the biometric device 12, it is difficult to prevent an LED from disturbing the photoreceptors in its vicinity. To limit these disturbances, in a particular implementation, every other LED for each row and every other LED for each column of the LED matrix are lit to enable the capture of a first intermediate image and conversely only the LEDs not lit for the first intermediate image are lit to capture a second intermediate image. Each photoreceptor located in a predefined neighborhood of a lit LED is not used to form an intermediate image. Each photoreceptor used to form an image is therefore located outside a predefined neighborhood of each lit LED.For example, in this particular implementation, only the photoreceptors located in the peripheral rings corresponding to unlit LEDs are used to form the first and second intermediate images. The first and second intermediate images are then combined by the processing module 11 to form the image of the fingerprint D. In this way, the number of photoreceptors that are close to a lit LED is reduced.

[0097] More generally, in another particular implementation, the LEDs are grouped into a plurality of LED sets. The LED sets are lit successively and an intermediate image is acquired each time a LED set is lit. When a LED set is lit, the other LED sets are switched off. Only the photoreceptors located in the peripheral rings corresponding to switched off LEDs are used to form the intermediate images. The intermediate images obtained are then combined by the processing module 11 to form the image of the fingerprint D.

[0098] In a particular implementation, at least one set of LEDs is illuminated to form an intermediate image. The intermediate images obtained are then combined by the processing module 11 to form the image of the fingerprint D.

[0099] In another particular implementation to limit disturbances, only every other hole per row of the hole matrix and every other hole per column of the hole matrix of the opaque screen 122 are associated with an LED. In this particular implementation, only one image is generated. Only the photoreceptors located in the peripheral rings not comprising LEDs are used to form said image. In a variant of this particular implementation, the holes can be specialized. Thus, the holes used to bring the light towards the finger D can be larger than the holes used to image the finger D on the sensor 124. In a particular implementation, the holes used for imaging have a diameter of "7" µm whereas the holes used to bring the light towards finger D have a diameter of "50" µm , the LED can be adapted to the size of the hole and have the same diameter.

[0100] More generally, in another particular implementation, at least one hole in the opaque screen is associated with an LED.

[0101] In another implementation to limit disturbances, the LEDs are not located in the center of each central disc but outside the peripheral rings.

[0102] There Fig. 4D schematically illustrates a front view of a sub-part of a second example of sensor 124 adapted to the second embodiment of the biometric device 12 in which the LEDs are positioned outside the peripheral rings.

[0103] In the example of the Fig. 4D, the holes of the opaque screen 122 associated with an LED form a first rectangular matrix and the holes of the opaque screen not associated with an LED form a second rectangular matrix of holes nested in the first rectangular matrix of holes. Each hole associated with an LED is positioned at an equal distance from the center of the four holes closest to its neighborhood. On the sensor 124, each LED (1210 to 1221) is therefore positioned at an equal distance from the center of the four peripheral rings closest to its neighborhood. In this particular implementation, each LED therefore faces a hole in the opaque screen 122 making it possible to bring the light towards the finger D and each incidence zone faces a hole in the opaque screen making it possible to image the finger on the sensor 124.As in the previous particular implementation, the holes used for imaging and the holes used to bring the light towards the finger D may have a different diameter. In this particular implementation, to avoid any disturbance of the photoreceptors located in the vicinity of the LEDs, the photoreceptors located outside a peripheral ring are made insensitive to light.

[0104] Furthermore, as in the previous particular implementation described in connection with the Fig. 4B , to limit disturbances to the photoreceptors by LEDs that are too close, the LEDs can be grouped into a plurality of sets of LEDs lit successively to generate intermediate images which are then combined to form a fingerprint image.

[0105] There Fig. 4Eschematically illustrates a sub-part seen from the front of a third example of a sensor adapted to the second embodiment of the device for capturing an impression of a body part according to the invention.

[0106] In the example, the holes of the opaque screen 122 are spaced apart by a distance L > d APand, provided that the constraint on the distance L is respected, placed in any way on the opaque screen 122. Each LED is associated with a hole, that is to say that each LED faces a hole. Some holes are used exclusively to bring the lights of the LEDs towards the finger D placed on the face 1200: these are the holes associated with the LEDs 121B, 121D and 121F. Some holes are used both to bring the lights of the LEDs towards the finger D placed on the face 1200 and to image the finger D on the sensor 124: these are the holes associated with the LEDs 121A, 121C and 121E. Some holes are used exclusively to image the finger D on the sensor 124: these are the holes associated with the incidence zones 124A, 124B and 124C.

[0107] Furthermore, as in the previous particular implementation described in connection with the Fig. 4B, to limit disturbances to the photoreceptors by LEDs that are too close, the LEDs can be grouped into a plurality of sets of LEDs lit successively to generate intermediate images which are combined to form a fingerprint image.

[0108] It is noted that the first and second embodiments of the biometric device 12 have been presented separately but that certain particular implementations of each embodiment can be easily adapted to the other embodiment. In particular, the opaque screen 122 of the Fig. 4E could also be used in the first embodiment.

[0109] Furthermore, when the biometric device 12 is integrated into the screen 10, the upper face 1200 is integrated into the glass of the screen 10. It is then considered that the face 1200 is the sub-part of the glass of the screen 10 which is opposite the opaque screen 122, the blade 123 and the sensor 124.

[0110] In one embodiment, the processing module 11 implements a fraud detection method taking advantage of the characteristics of the biometric device 12.

[0111] The outer diameter of the peripheral ring of each incidence zone depends only on the limiting angle θ l and the thickness of the transparent blade 123. The limiting angle θ l depends on the refractive index of the skin of finger D. As we have seen above, it is generally accepted that the refractive index of the skin, depending on the wavelength of the light source, is between "1.41" and "1.47". The limiting angle θ l is then included between a minimum value θ l min here equal to “70°” and a maximum value θ l max here equal to “76°”, which allows us to deduce a range of values in which the external diameter of a peripheral ring must be included.

[0112] There Fig. 8schematically illustrates a fraud detection method using the biometric device 12.

[0113] In a step 81, the processing module 11 determines whether the sensor 124 is struck by light rays having an incident angle relative to the normal to the upper face 1200 greater than the maximum angle θ l max or if in at least one predetermined area of the sensor 124, said sensor 124 is exclusively struck by light rays having an incident angle relative to the normal to the upper face 1200 less than the minimum angle θ l min . To do this, in one embodiment, the processing module determines whether the actual external diameter of at least one peripheral ring is greater than a maximum diameter. d AP max such as : d AP max = d T + 2 . γ . E 120 . tan θ l max or less than a minimum diameter d AP max such as : d AP min = d T + 2 . γ . E 120 . tan θ l min

[0114] The processing module 11 can determine whether the actual external diameter of at least one peripheral ring is greater than the maximum diameter d AP max by determining whether at least one photoreceptor located outside the peripheral rings of rays d AP max generates information indicating that it is struck by a light beam.

[0115] The processing module 11 can determine whether the actual external diameter of at least one peripheral ring is less than the minimum diameter d AP min by determining whether any photoreceptors located outside the peripheral rings of rays d AP min generates information indicating that it is struck by a light beam.

[0116] In a step 82, the processing module 11 detects that there is fraud if the sensor 124 is struck by light rays having an incident angle relative to the normal to the upper face 1200 greater than the maximum angle θ l max or exclusively less than the minimum angle θ l min The processing module 11 then deduces that the finger D facing the face 1200 comprises at least one part which is not covered with skin.

[0117] There Fig. 9 schematically illustrates an example of hardware architecture of the processing module 11.

[0118] According to the hardware architecture example shown in Fig. 9 , the processing module 11 then comprises, connected by a communication bus 110: a processor or CPU (“Central Processing Unit” in English) 111; a RAM (“Random Access Memory” in English) 112; a ROM (“Read Only Memory” in English) 113; a storage unit such as a hard disk or a storage media reader, such as an SD (“Secure Digital” in English) card reader 114; at least one communication interface 115 allowing the processing module 11 to communicate with the biometric device 12.

[0119] The processor 111 is capable of executing instructions loaded into the RAM 112 from the ROM 113, from an external memory (not shown), from a storage medium (such as an SD card), or from a communication network. When the analysis module 11 is powered on, the processor 111 is capable of reading instructions from the RAM 112 and executing them. These instructions form a computer program causing the processor 111 to implement the method described in relation to the Fig. 8 .

[0120] The process described in relation to the Fig. 8can be implemented in software form by executing a set of instructions by a programmable machine, for example a DSP ("Digital Signal Processor" in English), a microcontroller or a GPU (graphics processor, "Graphics Processing Unit" in English terminology), or be implemented in hardware form by a machine or a dedicated component, for example an FPGA ("Field-Programmable Gate Array" in English) or an ASIC ("Application-Specific Integrated Circuit" in English).

[0121] It is noted that the processing module 11 could just as well have been included in the biometric device 12.

Claims

1. Device for capturing a print of a body part comprising a plurality of optically coupled elements comprising: • a first transparent plate (120) comprising an upper face (1200) on which the part of the body bearing the print to be imaged can rest; • a light source (121); • a light-impermeable screen (122) located under the first transparent plate (120); • a second transparent plate (123) located under the opaque screen (122); • a sensor (124) comprising light-sensitive photoreceptors located under the second transparent plate (123) providing information enabling to generate an image of the print; the faces of the transparent plates, the opaque screen and the sensor being parallel, characterised in that : the light source generates light beams passing through at least the first transparent plate in the direction of the upper face (1200) having an incident angle with respect to a normal to the said upper face (1200) of a value less than a critical angle given by the refractive indices of the first plate and of the air and beyond which critical angle a light ray is totally reflected by said upper face in the presence of air above said upper face; the opaque screen (122) comprises an array of holes such that each light ray directed towards the sensor (124) which reaches the opaque screen (122) at a hole passes through the opaque screen (122) and reaches the sensor (124); each light-sensitive photoreceptor of the sensor (124) is positioned at positions of the sensor (124) that can be hit by a light ray emanating from the body part after diffusion in the body part having an angle of incidence with respect to said normal lying between the critical angle and the limit angle, the limit angle being given by the refractive indices of the first plate and of the skin and beyond which limit angle a light ray is totally reflected by said upper face in the presence of a ridge of the skin above said upper face; each point of the body part in contact with the upper face (1200) is imaged by at least one photoreceptor of said sensor.

2. Device according to claim 1, characterised in that at least some of the holes, known as imaging holes, serve to image the body part, and in that a distance between an imaging hole and the imaging hole or holes which are its nearest neighbours, taken from centre to centre, is greater than the diameter of an image of the body part seen through a hole when the body part is placed on the upper face.

3. Device according to claim 1 or 2, characterised in that the light source is composed of a plurality of light-emitting diodes, known as LEDs, the LEDs are configured in such a way that each LED producing light and the LED or LEDs producing light which are the closest to said LED generate, on the body part, parts illuminated directly by said LEDs at a distance less than a predefined distance characteristic of a depth of penetration of the light into the body part dependent on a wavelength of the light produced by each LED.

4. Device according to claim 3, characterised in that the light source is integrated on a lower face of the first plate or integrated on an upper face of the second plate and generates a light beam above the opaque screen.

5. Device according to claim 3, characterized in that the light source is integrated into the surface of the sensor or integrated into the sensor and generates a light beam below the opaque screen passing through the opaque screen via the holes towards the upper face and each LED of the light source is positioned opposite a hole in the opaque screen, at least part of the holes being associated with an LED.

6. Device according to claim 5, characterised in that the LEDs are grouped into sets of LEDs, each set of LEDs being arranged to be switched-on independently of the other sets of LEDs, at least one of the sets of LEDs being switched-on to acquire an intermediate image, each acquired intermediate image being used to form the image of the print.

7. Device according to claim 6, characterised in that each photoreceptor used to form an image is located outside a predefined vicinity of each switched-on LED.

8. Device according to claim 5 or 6, characterised in that the holes of the opaque screen form a rectangular array of holes, one out of two holes per row of the rectangular array of holes and one out of two holes per column of the rectangular array of holes are associated with one LED wherein the holes of the opaque screen form first and second nested rectangular arrays of holes, the holes of the first rectangular array of holes are associated with one LED and the holes of the second rectangular array of holes are not associated with any LED, each hole associated with one LED of the first rectangular array of holes being positioned equidistant from the centre of the four holes of the second array of holes in its nearest vicinity.

9. Device according to claim 8, characterised in that the holes which are associated with one LED have a larger diameter than the holes which are not associated with any LED.

10. Device according to claim 6 or 7, characterised in that the holes in the opaque screen form a rectangular array of holes, each hole being associated with one LED, the device is adapted so that one out of LEDs for each row and one out of two LEDs for each column of the array of LEDs are switched-on to enable a capture of a first intermediate image and only those LEDs not switched-on for the first intermediate image are switched-on to capture a second intermediate image, an image of the print being formed from the first and second intermediate images.

11. Device according to any one of the preceding claims, characterised in that each photoreceptor of the sensor corresponding to a position which can be hit by a light ray emanating from the body part having an angle of incidence with respect to the normal to the upper surface less than the critical angle is masked by an opaque metal layer.

12. Device according to any one of the preceding claims, characterised in that the first plate has a thickness three to ten times greater than a thickness of the second plate.

13. Device according to claim 4, characterized in that the light source is integrated on the underside of the first plate and in that each light-emitting diode comprises, in a first layer, a first transparent electrode (70), in a second layer located immediately below the first one, a stack of layers forming a LED (71) and, in a third layer common to each LED of the light source, a metal layer forming a second electrode (72), each layer being achieved by coating and etching on the underside of the first plate, the screen being implemented by said metal layer and comprising the holes.

14. Equipment comprising a device according to any one of claims 1 to 13.

15. Method for detecting fraud during use of a device for capturing a print of a body part according to any one of claims 1 to 13, characterised in that the method comprises: determining (81) whether the sensor (124) is hit by light rays having an incident angle with respect to the normal to the upper face (1200) greater than a maximum value that can be taken by a limit angle (θl) depending on the refraction indices of the first plate (120) and the body part and beyond which a light ray is totally reflected by said upper face when said light ray hits the upper face at a point of contact between the upper face and a finger or, if in at least one predetermined area of the sensor, the sensor (124) is hit exclusively by light rays having an incident angle to the normal to the upper face (1200) lower than a minimum value which can be taken by the limit angle; and, detecting (82) that there is a fraud when the sensor (124) is hit by light rays having an incident angle with respect to the normal to the upper face (1200) greater than said maximum value or, if in at least one predetermined zone of the sensor, the sensor (124) is hit exclusively by light rays having an incident angle with respect to the normal to the upper face (1200) less than said minimum value.