The invention will be better understood on reading the following detailed description of certain preferred embodiments given purely as non-limiting examples. In the description, reference is made to the accompanying drawings, in which:
FIG. 1A is a diagrammatic view of an optical device for biometric sensing by contact to which the invention applies;
FIG. 1B is an image of a fingerprint detected by the sensor means of the FIG. 1A device under normal operating conditions;
FIG. 2A is a diagrammatic view of the FIG. 1A device under abnormal operating conditions;
FIG. 2B shows a latent image of the kind that can be detected by the sensor means under the abnormal operating conditions shown in FIG. 2A;
FIG. 3A is a diagram of the FIG. 1A optical device arranged in accordance with the invention and shown under the operating conditions of FIG. 2A;
FIG. 3B is a diagram of the pattern applied on the face of the optical element of FIG. 2A in accordance with the invention;
FIGS. 4 and 5 are views of images picked up by the sensor means in various operating conditions of a device arranged in accordance with the invention;
FIGS. 6A to 6C show several variant embodiments of dispositions of the invention in the device of FIG. 1A; and
FIG. 7 is a simplified block diagram of a biometric sensor installation arranged in accordance with the invention.
FIG. 1A is a highly diagrammatic view showing the main structure of an optical device for biometric sensing by contact, which device is given overall reference 1. The device 1 as shown comprises an optical element 2 (in this case constituted by way of example by a prism of trapezoidal section) having one face 3 (in this case the large face of the prism) constituted at least in part by a surface for apposition of a body part from which a biometric characteristic is to be sensed.
By way of concrete example, since this is the most common although not exclusive example, the device 1 is assumed to be for making an image of fingerprints 5 of a finger placed on the face 3 of the prism.
The device 1 also includes a radiation source 6, generally a visible light source, which is associated with the optical element 2 in such a manner as to illuminate the above-mentioned face 3. In general, since this is the simplest solution to implement, the radiation source 6 illuminates the face 3 through the optical element 2, as shown in FIG. 1A. In the example shown in FIG. 1A, the radiation source 6 is situated facing the small face of the prism opposite from its large face 3, and the radiation is directed substantially perpendicularly to the face 3 such that the finger 4 is illuminated from in front. In the presence of a finger, the projecting zones of the finger absorb radiation and then re-emit it in all directions, in particular towards the sensor, whereas the sensor picks up no radiation from the furrows of fingerprints or when there is no finger.
A portion of the radiation diffused by the face 3 leaves the optical element through a side face 7 and is focused by focusing means 8 on sensor means 9, which means then deliver electrical signals to a processor unit 10.
Under the lighting conditions shown in FIG. 1A, the sensor means 9 receive a “positive” image of fingerprints 5 (the ridges are pale, the furrows are dark) as shown in FIG. 1B, when a finger 4 is placed on the face 3. In contrast, when no finger is placed on the face 3, the sensor 9 is adjusted not to be excited by the residual light reflected by the face 3.
It can be observed at this point that the provisions specific to the invention as explained below can also be applied when the lighting is of the “shadow” type, i.e. when the radiation source is disposed facing the other sloping face 11 of the prism 2 so as to illuminate the face 3 with grazing light. The image received by the sensor 9 is then a “negative” image (dark ridges, pale furrows).
The apposition of a finger on the face 3 can leave traces of grease, represented coarsely at 12 on the prism 2 of FIG. 2A. Under certain circumstances, it can also happen that the face 3 receives external radiation 13 (parasitic radiation) that is relatively intense (sunlight, halogen lamp, . . . ), that is inclined relative to the face 3, and that is directed approximately perpendicularly to the outlet sloping face 7 of the optical element 2. This parasitic radiation shows up the traces of grease 12, and because of its angle of inclination, reaches the sensor means 9.
If the traces of grease 12 are significant, the sensor means 9 then receive a fingerprint image as shown in FIG. 2B (a false image or a “latent” image) which, because of its relatively complete appearance and its sufficient contrast, is processed like a real image obtained from a finger present on the face 3.
Such a situation is unacceptable because of the errors to which it leads and to the risks of fraud to which it can give rise.
As shown in FIG. 3A, the invention makes provision for the face 3 of the optical element 2 to be coated in a thin layer 4 defining a pattern (which pattern could be arbitrary and has no incidence on implementing the provisions of the invention). FIG. 3B shows a portion of the face 3 as fitted in this way. The layer 14 is made of a material which is such that when facing ambient air and the material from which the optical element 2 is made, the layer 14 is substantially transparent in the presence of the radiation emitted by the radiation source 6 when a finger 4 is placed on the face 3, while being substantially less transparent in the presence of parasitic radiation 13 in the absence of a finger placed on said face 3.
In other words, the material of the layer 14 possesses a radiation transmission coefficient that is slightly different from that of the material constituting the optical element 2 (in practice slightly less than, i.e. the layer 14 is slightly less transparent than the optical element 2).
Under such conditions, the pattern formed by the layer 14 is invisible in the presence of radiation directed perpendicularly to the face 3, and under normal conditions of operation of the device (lighting perpendicular to the face 3 and no parasitic radiation), the pattern formed by the layer 14 remains invisible for the sensor 9 which receives only the image of the finger as shown in FIG. 1B.
In contrast, the pattern of the layer 14 becomes apparent in the presence of parasitic light. The image that the sensor 9 can receive when no finger is placed on the face 3 and when the face 3 is clean (no traces of grease) is analogous to the image shown in FIG. 3B: the sensor receives only the image 17 of the pattern of the layer 14. FIG. 4 shows the image picked up by the sensor in the presence of a finger placed on the face 3: the sensor picks up the image 15 of the fingerprint clearly without the pattern of the layer 14 being visible through the image (the image 17 of the pattern might still be visible, but only around the image 15 if the field of view is larger than the finger, as shown in FIG. 4). Finally, in the presence of traces 12 only on the face 3 (no finger placed thereagainst), the sensor 9 picks up the latent image 16 corresponding to the traces superposed on the image 17 of the pattern of the layer 14 that is visible through the latent image, as shown in FIG. 5.
It is then easy, given that the shape of the pattern of the layer 14 can be stored in memory, to discriminate amongst the images received by the sensor 9 between those that correspond to FIG. 4 and those that correspond to FIG. 5, by looking for the presence or the absence of the image 17 of the pattern of the layer 14 in the image of the fingerprint. These discriminator means can, in particular, be included in the processor means 10.
The layer 14 may be made of any material that is suitable for giving rise to the looked-for result. In particular, this layer can be constituted by indium-tin-oxide (ITO) commonly used in objects and suitable for being deposited as a very thin layer. Other materials, such as transparent polymers, could also be used.
The layer 14 may be deposited directly on the face 3 of the optical element 2, through a mask, in order to form the required pattern as shown in FIG. 6A. The layer 14 is then discontinuous.
It is also possible, as shown in FIG. 6B, to envisage forming a continuous layer 14 of substantially constant thickness, presenting transparency analogous to that of the optical element 2, with the exception of localized positions 18 that are distributed and configured to present the predetermined pattern.
Another advantageous solution consists in making the predetermined pattern on a separate substrate that can itself be placed on the face 3. The layer 14 in FIG. 6B could be made in this way; the selective deposition of the layer 14 envisaged in FIG. 6A could be implemented on a continuous transparent film 19, itself applied to the face 3, as shown in FIG. 6C.
A most useful advantage of the provisions in accordance with the invention lies in the fact that it is possible not only to fit the invention to new equipment, but also to retrofit it to equipment already in operation, and in a manner that is relatively simple and inexpensive.
Another advantage of the provisions of the invention lies in the fact that these provisions are independent of the structure of the device (direct or shadow lighting) and of the way the optical element is made (out of glass, synthetic material) providing the index of the layer 14 matches that of the optical element 2.
Another advantage of the provisions of the invention lies in the fact that they do not alter the structure of the sensor device nor its operation, apart from the discriminator means that need go be implemented in order to eliminate latent images.
These discriminator means are given overall reference 20 in FIG. 7 and can advantageously be integrated as an initial stage in the above-mentioned processor unit 10. They may comprise memory means 21 for keeping in memory data that defines the pattern formed by the layer 14, comparator means 22 suitable for comparing the data stored in 21 with the data delivered by the sensor 9, selector means 23 for delivering a first signal (a blocking signal) if the comparator means identify a pattern image superposed on a fingerprint image (as shown in FIG. 5), or a second signal (an unblocking or confirmation signal) if the comparator means identify a fingerprint image on its own (as shown in FIG. 4). The output from the selector means 23 is connected to one input of a gate 24 whose other input receives the signal output by the sensor 9. The gate 24 is closed under the control of the first signal and it delivers no signal (of a latent image), while it is opened under the control of the second signal, in which case the signal coming from the sensor is delivered to the processor unit 10 (a real image). The comparator means 22 and the selector means 23 thus define detector means suitable for controlling discrimination means implemented in the form of the gate 24.
Patent Application
20080062402
