IL137175A - Fingerprinting device - Google Patents

Abstract

The invention concerns a device for taking prints of skin tissues comprising an optical sensor (10) with an object surface (12) for receiving tissues whereof the print has to be taken and, on a fraction of the object surface, a thin transparent electrode coated with an electroluminescent dielectric layer (34). Means provide difference of potential between the tissues pressed on the sensitive layer and the transparent electrode and other means (22) form an image of the tissues pressed on the object surface, including said fraction. The imaging means provide an electronic treatment of the image which enables identification or verification by comparing with one or several stored prints by verifying the continuity of relief on the fraction and the remainder of the object surface.

Description

Fingerprinting device Sagem SA C. 126315 A DEVICE FOR IMAGING PRINTS The present invention relates to devices for imaging prints of skin tissues having a pattern of alternating linear ridges and grooves. It has a particularly important, but not exclusive, use consisting in providing a representation of a fingerprint or a full hand palm print, for instance for identification or verification purposes .

Numerous devices are alread known that are capable of forming a recordable image of the pattern of ridges on the end phalange of a finger or more . None of such devices is completely safe against fraud. The devices which use an optical sensor providing an image of a finger located on an object surface of a total reflection prism can hardly discriminate a print of a living finger directly applied on the object surface from the print of a foreign finger coated with a molding of the print to be simulated, consisting of a flexible thin film, transparent enough for the optical sensor to see the color of the skin and for the sensor not to be able to discriminate by colorimetry. Addition of means for photoplastismography, for verifying the existence of heart beats, renders the device more complex and lengthens the verification time.

There is also known (US-A-4 336 998) an electrical device comprising a transparent electrically insulating substrate, a thi transparent electrode, and a sensing layer for receiving finger pressure. The sensing layer is arranged for being deformed under the ridges.

Thereafter, the shape of the imprint is then reinforced by the action of an electric field prior to taking the image.

That method is not very accurate, has a low 2 137175/2 discriminating power and is slow. An electrical source delivering a high voltage is required.

There is also known (SU-A-94 2684) an electroluminescent cell suitable for amplifying the brightness of a print, comprising a glass substrate, a transparent electrode, a luminescent layer and a layer of semiconductor material. The human finger operates as a second electrode.

There is also known (JP 02126381) a surface irregularity patent identification sensor used for identification of prints of skin tissues. The identification is based on electrical features.

The present invention is particularly for providing a device for imaging prints of skin tissues which fulfils practical requirements better than those previously known. A particular object is to provide a device having an enhanced capability to discriminate between the print of a living finger and that of a fake finger or a dead finger.

To this end, the invention provides a device comprising: an optical sensor having an object face for receiving the object whose print is to be taken, on part of the object face, a transparent thin electrode covered with an electroluminescent dielectric layer, means for applying a voltage difference between the tissues applied on the sensitive layer and the transparent layer, means for imaging the tissues pressed onto the object face, including said part, and electronic means for processing the image delivered by the imaging means , enabling identification or verification by comparison with one or a plurality of stored prints while verifying continuity of the ridges on said part and on the balance of the object face.

The optical sensor may have one among many known constructions, including a total reflection prism having a surface which constitutes the object face, a light source for lighting the object face through the prism and an image grabbing camera. The sensor is for instance of the type 3 137175/2 described in document EP-A-0 851 381.

The same camera may be used for simultaneously forming images of said part and of the balance of the object face. The layer and the power supply may for instance have the' construction described thereinafter or in US 6,002,786.

Typically, said part will have a position and an area such that the "core" of the print which includes the most significant minutiae or graphemes is adjacent to the sensitive layer. In general, the sensitive layer will have an area which is between one fourth and one half of the area of the full processed image.

The unit consisting of the thin transparent electrode, the electroluminescent dielectric layer, the means for generating a voltage difference and the means for image grabbing may constitute a device used independently and a constitute complete print imaging device. Such a device, comprising a transparent electrically insulating substrate carrying a thin transparent electrode with an overlaying sensitive layer is characterized in that the sensitive electroluminescent layer is constituted by an electroluminescent dielectric material or a dielectric material in' which particles of electroluminescent material, constituting as many luminophores , are dispersed and in that the device further comprises means for establishing a voltage difference between the tissue that is to be pressed against the sensitive layer and the thin transparent electrode.

The above features, as well as others, will be better understood on reading the following description of particular embodiments of the invention given as non-limiting examples .

The description refers to the accompanying drawing, wherein : • Figure 1 is a schematic illustration of the arrangement of the ridges of a finger print ; • Figure 2 is a schematic representation of an hybrid device according to an aspect of the invention ; · Figure 3 is a simplified flow sheet indicating the steps taken for identifying a print ; • Figure 4 is a cross-section illustrating a possible arrangement of the device which only uses an electroluminescent layer ; · Figure 5 is a representation of a modified embodiment.

The skin on the end portion of a finger is not smooth, but exhibits an alternation of "ridges" separated by hollows or grooves. The ridges are irregular and exhibit bifurcations and endings . On figure 1 , some endings and bifurcations are indicated by circles. Numerous sensors are i existence which provide an image of the ridges, as lines thicker tha those illustrated in figure 1, or as gray values . The conventional methods for verification include image filtering, detection of the minutiae or graphems (ends or bifurcations) , comparison with a previously enrolled and stored fingerprint, stored for instance on a portable support, such as a smart card,_ or in a data base (when a person among a plurality should be identified) .

Referring to figure 2, a device according to an embodiment of the invention includes elements of an optical detection device which is sold under the trademark "MORPHOKIT" by SAGEM SA, Security and morpho systems, Nanterre, France. That device comprises a transparent block 10 having an object face 12 on which the individual will press one or more fingers 14. The block 10 constitutes a total reflection prism. A face opposed to the object face 137175/2 carries a light source 16 which may typically be a two- dimensional matrix of light emitting diodes for lighting the object face. One of the lateral surfaces 18 is coated with a layer which absorbs light. The light reflected by the object face 12 except from the locations which receive ridges 20 is collected by a camera 22, schematized as lens 24 and a 2 D sensor, such as a matrix sensor 26. A remote camera may also be used.

The device further comprises a central control and computation unit. hich controls the measurement sequence and processes the signal from camera 22. That unit controls lighting by source 16 and has a circuit' 32 for reading out the 2D sensor 26 and preprocessing the signal.

The device of figure 2 implements the invention. For that, part cf the object face 12 is covered with a sensitive layer 34. That layer consists of electroluminescent dielectric material or of a dielectric material wherein particles of electroluminescent material, constituting lumincphores , are dispersed. The device further has means 36 for applying a voltage between the tissue to be pressed onto the sensitive layer 34 and a . thin transparent layer 36 located under the sensitive layer. The two terminals of means 36 are connected to an electrode 40 on which the finger will be pressed and to the thin transparent electrode 38, by ' a connection (not shown) . The dielectric material preferably has a high dielectric constant and a transmission coefficient which is compatible with the wave length generated by the luminophores . The transparent electrode is for instance constituted by a thin layer of Injh or indium-tin oxide.

Several possible constructions of that portion of the device are described hereunder with reference to Figs. 4 and 5.

Preferably the sensitive layer 34 has a location and a size such that only the end portion 42 of the analyzed print is on it. The core 47 of the print, which constitutes the more significant portion is then in the remaining zone 44 of the 137175/2 processed image.

The device- advantageously further includes a reflection responsive sensor 46 connected to an analyzer 48 which makes it possible to authentify the tissue applied on the sensor as being live tissue, by comparing the reflected color spectrum with reference spectra. The sensor analyzer may be as described in French Patent Application No. 97 03482.

The flow sheet identifying operation of a device may be that schematized on figure 3. Camera 22 delivers the collected images to a module .50 for analyzing the optical image and a module 52 for analyzing the image obtained by electroluminescence. Module 52 may be associated to a sensor 54 for determining, by a method such as heart beat detection, that a living finger is applied. The output of the sensor constitutes a signal for authorizing operation of module 52. The latter delivers to a selection module, which is hardware or software in nature, elements for determining whether the object pressed onto the sensor is truly an actual finger. Each module 50 and 52 compares the received image with a gray scale for determining the pattern of ridges and then filters the image for delivering, on its output, a binary image which renders detection of the minutiae easier. The selection module 54 transfers the image, if the verification is satisfactory, to a reconstruction device 56 which connects the image processed by module 50 and the image transferred from the selection module 54. Last, a decision module 58 carries out two steps: it checks --continuity between the two images ; - if successful, it compares the distribution and nature of the significant points of the image with either a single reference, delivered for instance by a chip card inserted into the device, or a plurality of references stored in a memory 60 and then it provides a result.

The optional color sensor 46 authorizes reconstruction of the image by module 56 only if there is a successful comparison between the reflected spectrum and one or more references spectra, or triggers a fraud alarm.

The sequence in the method may differ from that illustrated in figure 3. The device may also be complemented for automatically adjust the voltage applied to the thin electrode for balancing brightness of the images of zones 42 and 44." A more complete description of a plurality of possible constructions of an electroluminescent device apt to be incorporated to the device of figure 2 or to be used separately will now be given. In that electroluminescent device, the luminophores may be powders similar to those used for coating electroluminescent flat screens or for constituting light emitting diodes, emitting in the infrared, visible or even ultra-violet spectrum, dispersed as microcrystals in the dielectric material. It is possible to use copper doped zinc sulfide. It is preferable to use dielectric material having a high dielectric constant, of at least 10, and having a high transmission coefficient for the wave length emitted by the luminophores . Epoxy resins , which have an acceptable degree of transmission when in thin layer, can be used if their dielectric coefficient is higher than 10. For a higher dielectric constant, it is possible to lower the voltage from which the luminophore starts emission (threshold voltage), for a given layer thickness. It will then be possible to reduce the thickness of the sensitive layer which has a lesser risk of destruction, with the advantage that light transmission is increased. It is also possible to use thin layers of electroluminescent materials. It is also possible to use some conjugated conducting polymers having an intrinsic electroluminescence.

The transparent electrode will often consist of a thin indium-tin oxide, known as ITO, which is frequently used for manufacturing flat sens . That technology is well mastered and enables satisfactory bonding of varnishes and resins .

The sensor for delivering a signal representing the optical image generated by the sensitive layer may be of any one of numerous types. It is possible to use a camera having a matrix of photosensitive sites and an optical system for forming an image of the back face of the substrate on the matrix (e.g. CCD or CMOS camera) ; it is also possible to use other recording media such as a photographic film. Often the matrix of photosensitive sites, selected to be sensitive to the radiation from the luminophores , can be applied directly to the rear face or can be separated therefrom by a light amplifier. A color filter can be placed between the rear face and the sensor to reduce the sensitivity of the device to ambient light.

The device shown diagrammatically in Figure 4 , which is not to scale for reasons of clarity, comprises a transparent electrically insulating substrate 110 covered by a thin transparent electrode 112 which in turn supports a sensitive layer 14 for receiving pressure from an organ (finger 116 or entire hand) whose print is to be taken.

The substrate. 110 may be constituted by various substances that are transparent in the wavelength range used. In the visible range, glass will generally be used. The thin transparent electrode may be identical in structure to backing electrodes used for liquid crystal displays . In that case it is constituted by indium-tin oxide and it is a few hundreds of nanometers thick.

The sensitive layer 114 may have various structures.

In a first ' embodiment, it is constituted by a dielectric material in which particles of electroluminescent material are dispersed to constitute luminophores . As a general rule, the layer used will be less than 50 |Jm thick and will be made of a dielectric material having a dielectric coefficient greater than 20, which makes it possible to adopt a low threshold voltage, below 40 volts, which is effective and compatible with present safety standards. The material must also adhere satisfactory to the thin electrode 112.

.With a sensitive layer of this constitution, the method of taking a print requires a voltage applied between the finger 116 and the electrode .112. A convenient, but not exclusive, way of doing this consists i depositing a ring of conductive material 118 on a peripheral portion of the sensitive layer 114. When a finge is pressed on the sensitive layer, it also presses on the ring 118 which may be constituted by a etal coating.

The ridges of the finger 116 can be considered as being elementary capacitors associated with the electrode 112 and the sensitive layer 114.

When the luminophores are of a type that respond to a current flow, the set of "elementary capacitors" is fed by a voltage other than DC. As shown in Figure 1, the voltage source is constituted b a transformer 120 whose primary is connected to a power supply. A cold terminal of the secondary is connected to the ring 118 and is grounded. The other end (hot end) of the secondary is connected to a conductive frame 122 in communication with the electrode 12.

The frame may be constituted merely b a metal coating, and it may be covered by a strip 124 that is also of metal in the zone where the connection is made with the secondary winding of the transformer 120.

The voltage applied between the electrode 112 and the finger 116 depends on the nature of the current and on its frequency. For a sine-shaped alternating current, it is possible to adopt a peak-to-peak voltage of 20 V to 100 V. A 5 frequency in the range 1 kHz to 50 kHz gives good results. Regarding safety, the acceptable voltage is at a higher frequency.

It is also possible to feed current in the form of unipolar pulses, or of sine-shaped, unipolar, or rectangular 0 pulse trains .

Instead of indium-tin oxide In.C /SnO;, it is possible to consider using indium oxide that is pure or that contains zinc.

Such a device provides a high definition image generated = by radiation from the lu inophcres situated between the electrode and th points between ridges on the finger are pressed against the sensitive layer and the electrode. The image is sensed by a sensor 126 which may be a TV camera connected to a computer and enabling the image of the .finger Q print to be displayed on the computer screen. Instead of using a camera, it is possible to use a solid state sensor applied directly against the bottom face of the substrate 110. A color filter 128 having a transmission peak corresponding to the wavelength of the electroluminescent 5 material is advantageously interposed between the sensitive layer and the sensor. However that is not essential.

To increase the light efficiency of the device and to limit the contribution from ambient light, the sensitive layer 114 is advantageously covered by film 130 of light 0 reflecting or diffusing dielectric material and/or by an opaque film 132.. The reflecting film can be constituted, in particular, by barium titanate having a thickness of 5 to μιη. The opaque film 132 may be constituted, in particular, by black manganese oxide having a thickness of 5 |Jm to 20 fim.

Instead of using' two distinct protective films, one diffusing and the other opaque, it is possible to make a single layer whose composition is varied as the layer is grown.

A device according to the invention may be made as follows .

A transparent electrode 112 is deposited up to a thickness of about 300 nm on a glass substrate that is a few millimeters thick. Deposition can be performed, in particular, by spraying and physical vapor deposition (PVD) or chemical vapor deposition (CVD) .

The thickness of the substrate 110 is selected as a function of its area. For a device that is to take fingerprints, a thickness of 5 mm is generally satisfactory. Other layers may be deposited on the electrode 12 : the . sensitive layer (for instance doped Zn s or Zn Se lacquer) - where appropriate, a reflecting film of TiBaC ; also optionally, an opaque protective layer of manganese oxide ; and the ring 118 for contact with a finger.

To take a print of a finger, the finger is pressed flat o the sensitive layer or the protective film, as shown in Figure 4, and an alternating voltage is applied. The sensor is used in conventional manner to provide a representation of the image to a computer processor system 134.

The device makes it possible to distinguish the finger of a living person from a molding, thereby countering fraud consisting in pressing such a molding on the device in a system for giving access to protected premises. A molding has electrical characteristics that are not the same as those of a human finger.

It should also be observed that the device makes it possible to verify that a living finger has been applied. The pulse gives rise to cyclic variation in the image, with the bright pattern or points changing in intensity at the pulse rate. The computer system 34 can be designed to verify the- existence of such variation.

In another embodiment (Fig. 5) , the device may be designed to be energized by a DC voltage. Then the substrate 110 again carries a thin transparent electrode 112 that is a few hundreds of nanometers thick. This electrode, generally made of ITO, can be constituted by spraying and physical vapor deposition (FVD) , or by chemical vapor deposition (CVD) , i.e. using one of the methods commonly used in thin film technology.

The layer 114 is generally thinner than in the case of Figure 4 and has a high content of electroluminescent material. In particular, it is possible to use a layer 114 made of crystals of a material such as zinc sulfide doped with copper and manganese and _ using a varnish having a high dielectric coefficient as a binder.

The films carried by the sensitive layer 114 may have the same constitution as in Figure 4. Given the small thickness of the sensitive layer, a DC voltage of about 10 V generally gives satisfactory results. When this voltage is applied, the luminophores lying in volumes defined by the ridges of a finger pressed on the device generate light under the action of the electric field caused by the voltage applied between the ridges and the electrode 112. The light diffused by the sensitive layer 114 can be picked up through the substrate 10 by a sensor such as a camera connected to a computer for displaying the print and making use of the print by image processing software.

In yet another embodiment, the sensitive layer is composite and made up of a thin central zone of luminophore material, e.g. copper-doped zinc sulfide, covered on both faces by a fine dielectric film. Then the glass substrate still carries an electrode, e.g. of ITO, having a thickness that may be about 300 nm. The electrode carries the composite sensitive layer made up of a first dielectric film that is a few hundreds of nanometers thick, a thin electroluminescent layer that is, for example, approximately twice as thick as the film, and a second dielectric film having the same constitution as the first dielectric layer. A protective layer, e.g. of manganese oxide, may be deposited on the sensitive layer. The films constituting the sensitive layer may again be made by spraying and physical vapor deposition or by chemical vapor deposition. - 14- 137175/2

Claims (11)

1. A device for imaging prints of skin tissues, comprising: - an optical sensor having an object face for receiving tissue whose print is to be taken, - means for imaging the tissues pressed onto the object face, - a transparent thin electrode covered with an electroluminescent dielectric layer, disposed only on part of the object face, - electronic means for processing the image delivered by the image means, enabling identification or verification by comparison with one or a plurality of stored prints, said means for imaging the tissues being providing for the image of tissues pressed on the whole object face, including the part covered, and the said electronic means for processing the image being providing for verifying in the image the continuity of the ridge of tissues on said part and on the balance of the object face, and - means for establishing a voltage difference between the tissues to be applied on to the dielectric layer and the transparent electrode.

2. A device according to claim 1, characterized in that the optical sensor comprises a total reflection prism, a surface of which constitutes the object face, a light source lighting the object face through the prism and an image grabbing camera.

3. A device according to claim 2, characterized in that the same camera simultaneously forms images of said part and of the balance of the object face.

4. A device according to anyone of the preceding claims, characterized in that said part has a position and an area such that the core of a finger print is adjacent to the sensitive layer.

5. A device according to any one of the preceding claims, characterized in that the image means give to the image of the part of the object face an area which is between one fourth and one half of the area of the full processed image.

6. A device according to any one of the preceding claims, characterized in that a processing electronic circuit comprise a module for analyzing the optic image and 126315-1U6-01 - 15 - 137175/2 a module for analyzing the image resulting from electroluminescence, said analysis module delivering binary images to a module for reconstructing the images processed by said analysis modules by connecting them.

7. A device according to claim 6, characterized in that it further comprise a sensor for verifying a living tissue is involved and for delivering an authorization signal for operation of the module which analyses the image obtained by electroluminescence on part of the object face.

8. A device according to anyone of the preceding claims, characterized in that the electroluminescent layer consists of a dielectric material in which particles electroluminescent matter are dispersed and constitute as many luminophores.

9. A device according to claim 8, characterized in that the luminophores consist of electroluminescent particles dispersed as micro-crystals in the dielectric material and generating IR, visible or UV light.

10. A device according to claims 8 or 9, characterized in that the dielectric material has a dielectric coefficient of at least 10 and a transmission coefficient which is high for the wave length emitted by the luminophores.

11. A device according to any one of claims 1-10, characterized in that the means for applying a voltage difference comprise a ring or bar of electrically conducting material on a peripheral portion of the electroluminescent layer arranged for contact with the tissue. For the Applicants REINHOLD COHN AND PARTNERS 126315-1U6-01