FR3046276A1 - METHOD FOR OBTAINING A FUSED IMAGE OF A PAPILLARY FOOTPRINT - Google Patents

Abstract

The invention relates to a method for obtaining a fused image of a papillary impression. The method is implemented in a papillary impression sensor (110) having, superimposed beneath a contact surface (106), a matrix optical sensor (112), and illumination means (111) formed by a plurality of lighting devices parallel to each other. The method comprises the following steps: illumination of the papillary impression, the lighting devices being switched on and off so as to successively form, on the contact surface (106), different illumination figures each consisting of at least one light strip extending over the entire width (L1) of a detection surface of the matrix optical sensor, and acquisition by the matrix optical sensor of a series of raw images, each associated with one of the illumination figures ; and constructing the merged image by combining pixels of the raw images, each pixel series forming a line parallel to the light bands. The invention makes it possible to obtain high quality images.

Description

METHOD FOR OBTAINING A FUSED IMAGE OF A PAPILLARY FOOTPRINT

DESCRIPTION

TECHNICAL FIELD The invention relates to the field of processes for forming an image related to the particular folds of the skin, in particular a fingerprint, but also a palmar, plantar, or phalangeal imprint. These various imprints are collectively referred to as papillary impressions.

STATE OF THE PRIOR ART

Various prior art papillary impression sensors, in particular optical type sensors, in which the acquisition of an image of the impression merely implements a lighting of the impression and then an acquisition of image using a photosensitive component.

These optical type impression sensors are generally in the form of a stack comprising a lower light-emitting layer, called a backlighting layer, a photodetector array, and optionally a transparent protective layer on which the user must position his finger.

The photo-detector matrix, also called matrix optical sensor, is formed of a plurality of photo-detectors arranged in a grid. The grid preferably has a square mesh with N rows and M columns. Each photo-detector corresponds to a pixel of the matrix optical sensor.

In use, the photodetector array receives light emitted from the light emitting layer and backscattered onto the finger. It converts the light signal thus received into an image of the fingerprint present on this finger. This image is then used directly to determine certain characteristics of the finger, in particular the pattern formed by the fingerprint.

An object of the present invention is to provide a method for providing an image of the papillary impression having improved quality compared to that obtained by the methods known in the prior art. In particular, an objective is to provide an image of the footprint reproducing with greater fidelity the drawing formed by the imaged impression.

STATEMENT OF THE INVENTION

This objective is achieved with a method for obtaining a fused image of a papillary impression, the impression being in direct physical contact with a contact surface of a papillary impression sensor comprising, superposed beneath the contact surface, a sensor matrix optic, and lighting means formed by a plurality of lighting devices parallel to each other.

The method according to the invention comprises the following steps: illumination of the papillary impression by the lighting means, the lighting devices being switched on and off so as to successively form, on the contact surface, different illumination figures each consisting of at least one light band extending over the entire width of a detection surface of the matrix optical sensor, and acquisition by the matrix optical sensor of a series of raw images of the papillary impression each associated with one of the figures of illumination; constructing the merged image by combining series of pixels of the raw images, each series of pixels forming a line parallel to the light bands.

The series of pixels form lines of any extent, which do not necessarily extend from one edge to the other of the matrix optical sensor.

Thanks to the method according to the invention, one acquires several images of the same imprint, which simply differ by the lighting conditions.

We then have several images of the same footprint, in which each location on the footprint is illuminated in different ways according to the image considered.

The different lighting conditions correspond to different illumination figures, that is to say different combinations of the lighting or extinction of each of the lighting devices forming the lighting means. In other words, we illuminate a papillary impression with different structured lighting.

Preferably, each light band is parallel to the pixel lines of the matrix optical sensor, and the construction of the merged image implements a combination of pixel lines directly from the raw images, or formed by pixels from different raw images. and associated with the same pixel line of the matrix optical sensor.

The construction of the merged image may comprise a selection of pixels of the raw images, each associated with the same range of distances between the corresponding pixel of the matrix optical sensor and the nearest luminous band.

The method may then comprise the construction of several merged images, each associated with one of several distance ranges.

Regions of interest associated with different regions of the detection surface of the matrix optical sensor may be defined, and the construction of the merged image may include, for each region of interest, a selection of pixels of the raw images associated with a region of interest. same range of distances between the corresponding pixel of the matrix optical sensor and the nearest light band, the plurality of regions of interest not being associated with the same range of distances.

Preferably, the light bands associated with each of the illumination figures are offset from each other in a regular pitch equal to the pixel pitch of the matrix optical sensor. The illumination of the papillary impression is advantageously implemented by illumination means consisting of organic light-emitting diodes arranged above the matrix optical sensor.

Preferably, each illumination figure may consist of a single light band, and the various illumination figures may correspond to successive offsets of the light band, so that each of the lighting devices is switched on at the same time. less once. The fused image is advantageously constructed from lines of the raw images located at the same distance from the light band.

Preferably, each illumination figure consists of several light bands distributed in a periodic arrangement, and the various illumination figures correspond to successive offsets of the light bands, so that each of the lighting devices is switched on at the same time. less once.

Two adjacent light bands of the same figure can then be separated by at least 3 mm.

A scanning frequency associated with these successive offsets can be synchronized with a scanning frequency of the integration of the rows of pixels of the matrix optical sensor.

Advantageously, the acquisition of a raw image does not include the reading of all the pixels of the matrix optical sensor, only the lines of pixels located near the edge of a light strip being read, that is, say spaced from this edge by a distance less than a predetermined threshold.

Preferably, the illumination means are configured to emit light radiation centered on different wavelengths, and the steps of illumination of the papillary impression and construction of the fused image are implemented for each of said different lengths. wave.

The construction of the merged image may include a substitution of a group of pixels from a first raw image with a group of pixels from a second raw image.

The substitution can be performed to replace one or more overexposed or underexposed pixels. The invention also relates to a method for papillary impression capture, comprising in particular the steps of: placing the papillary impression in direct physical contact with a contact surface of a papillary impression sensor; and obtaining a fused image of the papillary impression, said obtaining comprising the steps of the method according to the invention to obtain a fused image. The invention also covers a system for implementing a method according to the invention, comprising: a papillary impression sensor comprising, superimposed, a contact surface for applying the imprint to be imaged, a matrix optical sensor, and lighting means formed by a plurality of lighting devices parallel to each other; control means, configured to turn on and off the lighting devices so as to successively form the different illumination figures; and computing means, configured to receive the raw images acquired by the array optical sensor and each associated with one of the illumination figures, and to construct the merged image by combining series of pixels of the raw images, each series pixels forming a line parallel to the light bands.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given purely by way of indication and in no way limiting, with reference to the appended drawings in which: FIG. 1A schematically illustrates a system according to the invention for implement a method according to the invention; Fig. 1B schematically illustrates the illumination means of the system of Fig. 1A; Figure IC schematically illustrates the steps of the method according to the invention; FIG. 2 illustrates a first embodiment of a papillary impression sensor, especially adapted to the implementation of a method according to the invention; FIGS. 3A and 3B illustrate a second embodiment of a papillary impression sensor, especially adapted to the implementation of a method according to the invention; FIG. 4 illustrates steps of illumination and acquisition of a series of raw images, of a first method of carrying out the method according to the invention; Figure 5 illustrates a variant of the steps illustrated in Figure 4; FIG. 6 illustrates a particular example of construction of a merged image according to the invention; Figure 7 illustrates a variant of the embodiment illustrated in Figure 6; FIGS. 8A and 8B schematically illustrate two variants of lighting means according to the invention; and Figure 9 schematically illustrates a variant of the lighting devices as illustrated in Figure 1B.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Figures IA to IC schematically illustrate a method according to the invention and a system 100 specially adapted to the implementation of this method.

In order to facilitate the understanding of the invention, the system 100 is first described to obtain a fused image of a papillary impression.

In the following, we will consider, by way of example and without limitation, that it is a fingerprint.

The system 100 comprises a fingerprint sensor 110 constituted by: a contact surface 106, on which, in use, the user places his finger 300 so that the skin, in other words the tissues, or at least the skin of the ridges of the papillary impression, itself in direct physical contact with said surface 106; a matrix optical sensor 112 as described in the introduction, the photodetectors being here for example PiN diodes (for "P-Type Intrinsic N-Type"), organic photodetectors (so-called OPDs), phototransistors, or any other element photosensitive; and lighting means 111.

The matrix optical sensor 112 and the lighting means 111 are superimposed under the contact surface 106.

If necessary, the element among the illumination means 111 and the matrix optical sensor 112 situated above the other allows a sufficient quantity of light to pass through in order to carry out the imaging function. This corresponds for example to a transmission coefficient of at least 10% to the central wavelength of emission of the lighting means.

The matrix optical sensor comprises photodetectors distributed in rows and columns. The extent of the lines defines the width L1 of the detection surface 125 of the matrix optical sensor. The extent of the columns defines the length L2 of the detection surface 125 of the matrix optical sensor. The width L1 is aligned with the axis (OY). The length L2 is aligned with the axis (OX). The largest side of the matrix optical sensor can be indifferently the length or the width. The detection surface is the surface on which the photodetectors extend, parallel to the (XOY) plane.

The lighting means 111 are configured to emit a light signal 121 in the direction of a fingerprint located on the finger 300 (which is in direct physical contact with the matrix optical sensor, on the contact surface 106). This signal is backscattered by the finger and returns to the fingerprint sensor 110 in the form of a backscattered signal 122, received by the matrix optical sensor 112.

The lighting means 111 consist of a plurality of lighting devices 1110. The fingerprint sensor 110 comprises, for example, more than ten lighting devices 1110, preferably several tens.

Each lighting device 1110 extends in one piece over more than one third of the width L1 of said detection surface.

In addition, they extend together in one or two series of patterns parallel to each other, distributed along the length L2 of the detection surface.

The lighting devices 1110 are therefore coplanar, distributed here on a surface of dimensions greater than or equal to those of the detection surface 125 of the matrix optical sensor.

In Fig. 1B, the lighting devices 1110 extend together in a single series of patterns parallel to each other.

They each extend substantially over the entire width L1 of the detection surface 125 of the matrix optical sensor, for example at least 90% of this width. In FIG. 1B, they extend even beyond this detection surface, which limits edge effects on the images acquired. In other words, they extend below (or above) said detection surface, protruding from both sides of the latter in a plane (XOY).

Here, the lighting devices all have the same extent along the axis (OY). Preferably, they also have the same dimensions according to (OX) and (OZ).

They extend in patterns parallel to each other, here in parallel bands between them.

Alternatively, each pattern has a trapezoidal shape, or in particular zigzag sinusoidal, or any other shape elongated according to (OY). In all these variants, the lighting devices provide lighting in parallel strips.

The patterns extend here parallel to each other and parallel to the axis (OY), that is to say parallel to the rows of pixels of the matrix optical sensor.

In a variant, the patterns may extend parallel to each other and slightly inclined relative to the axis (OY) and to the pixel lines of the matrix optical sensor. This inclination, less than 8 °, in some cases makes it possible to improve the resolution of the images obtained.

The lighting devices are distributed here regularly according to (OX), their ends being aligned on an axis parallel to (OX).

In particular, they are regularly distributed along the length L2 of the detection surface 125 of the matrix optical sensor, with a repetition pitch PI equal to the pixel pitch of the matrix optical sensor along the axis (OX), for example 25 μm or 50 pm.

As a variant, the repetition step PI of the lighting devices is constant, and distinct from the pixel pitch P2 of the matrix optical sensor according to (OX). In particular, the repetition step PI may be an integer multiple of P2. For example, P2 is of the order of 50 μm, or even 25 μm, and PI is between 10 and 20 times greater, for example of the order of 500 μm, preferably less than one millimeter. Each lighting device can then extend above or below several rows of pixels of the matrix optical sensor.

In operation, not all lighting devices are lit simultaneously, and only the pixels of the matrix optical sensor that are not located directly beneath a lit lighting device can be processed.

The lighting devices 1110 are connected to control means 130, configured to turn on and off each of the lighting devices 1110 independently of each other.

The control means 130 are configured in particular to successively form different illumination figures, each consisting of at least one light band which extends over the entire width L1 of the detection surface 125 of the matrix optical sensor.

An illumination figure corresponds to the spatial distribution of the illumination provided by the illumination means, at the level of the contact surface 106. Here, each illumination figure extends above the matrix optical sensor, on the same width L1 as the detection surface thereof.

It is preferably a binary image in which a high level corresponds to a lighting device turned on and a low level corresponds to an off lighting device.

A light band corresponds to a lit lighting device, or to several adjacent illuminated lighting devices, surrounded on both sides by extinguished lighting devices.

A light band extends along (OX) on less than one-tenth of the width L2 of the detection surface 125.

Here, each lighting device extends over the entire width L1 of the detection surface of the matrix optical sensor. Thus, each light band corresponds to a single illuminated lighting device (possibly several adjacent devices according to (OX) to form a wide band).

The illumination figures according to the invention are thus made particularly easy, in particular without the need for matrix lighting means having, for example, more than 3 lines and more than 3 columns. In other words, the lighting means according to the invention thus make it possible to offer structured lighting, without requiring bulky connection or control devices.

The matrix optical sensor 112 is connected to calculation means 140, adapted to receive the images, called raw images, acquired directly by the matrix optical sensor, and to construct a fused image from them.

The raw image is an image obtained directly at the output of the matrix optical sensor. For each illumination figure formed by the illumination means 111, a raw image is acquired using the matrix optical sensor 112.

A merged image is an image constructed by combining pixels from a plurality of said raw images. The merged image is a two-dimensional view of the footprint.

The light bands composing the illumination figure extend in lines.

Here, these lines are parallel to the pixel lines of the matrix optical sensor.

Thus, the construction of the merged image comprises a combination of pixel lines of the raw images.

In this case, a very simple merged image can be constructed by combining pixel lines of the raw images. The merged image is said to be high quality because it reproduces with a better fidelity the drawing formed by the imprint image thanks to a suitable selection of pixels of the raw images.

If the lighting devices extend in patterns parallel to each other and slightly inclined relative to the pixel lines of the matrix optical sensor, the light bands are inclined in the same way relative to the pixel lines of the matrix optical sensor. The construction of the merged image then comprises a combination of series of pixels of the raw images, each series of pixels extending along a line parallel to the light bands.

In all the following, and to simplify the presentation, it is considered that the light bands (and lighting devices) extend parallel to the pixel lines of the matrix optical sensor (and raw images acquired by this sensor).

A pixel line of a raw image has properties that vary as a function of the distance along the axis (OX) between the corresponding pixel line of the array optical sensor and the edge of the nearest light strip (that is, between said pixel line and the edge of the nearest lit illumination device 1110). These properties relate, for example, to the contrast, the effect of soiling above the matrix optical sensor, the under-illumination or saturation of the sensor pixels, the signal-to-noise ratio, etc. By appropriately selecting certain lines of pixels on the raw images, we build a merged image reproducing the design of the impression with optimal fidelity. In the following, various particular combinations of the pixel lines of the raw images are described.

Figure IC schematically illustrates the steps of a method according to the invention, implemented using the system 100.

In a first step 17li, using a lighting means 111, a first illumination figure defined by a first combination of illuminated lighting devices is formed, the other lighting devices being extinguished. The matrix optical sensor 112 acquires, in step 172i, a first raw image associated with said first illumination figure.

These steps are repeated with a second illumination figure, to acquire a second raw image (steps 1712 and 1722).

In total, these two elementary steps 171, and 172, are implemented N times, until the steps 171n and 172n, with N an integer greater than or equal to 2, 3 or more. Each time, the figure of illumination is different. All of these elementary steps form a first step 170 of illumination of the imprint by different illumination figures and acquisition of a series of raw images.

The several acquisitions are made during a very short period of time, during which the finger is immobile or almost (for example a maximum displacement of the finger less than 50 micrometers). If necessary, it can be identified that there has been no movement, and if necessary reject a movement zone, or even all the raw images acquired from a predetermined time.

By using the raw images, the merged image is constructed in step 180, as detailed above. For this construction, all the raw images acquired are preferably used.

FIG. 2 illustrates in detail a first embodiment of a papillary impression sensor 210, adapted to the implementation of a method according to the invention.

The lighting means 211 are formed by organic light-emitting diodes called OLED each forming a lighting device 2110 according to the invention.

The OLEDs are arranged above the optical array sensor 212. They have, for example, strip shapes, in particular strips parallel to each other and parallel to the pixel lines of the matrix optical sensor.

The OLEDs, and the photodetectors 2121 of the array optical sensor 212, are formed on a single substrate 201. In the example shown, the OLEDs are formed on a planarization layer 202 covering the photodetectors 2121.

Each OLED 2110 is defined by the intersection between a lower electrode 2111, a stack 2112 of at least one layer of organic semiconductor material, and an upper electrode 2113. In the example shown, the lower electrodes are each specific to an OLED, while a single stack of organic semiconductor material and a single upper electrode extend above all photo-detectors 2121.

Many variants can be implemented, for example with a stack of organic semiconductor material specific to each OLED.

In each embodiment of a fingerprint sensor according to the invention, the distance between the contact surface and the matrix optical sensor is less than 25 μm. Here, the matrix optical sensor being disposed under the OLEDs, this implies a small thickness of the OLEDs, advantageously less than 25 μm and even 20 μm.

This embodiment is particularly advantageous, especially since it offers a wide variety of dimensions and shapes of lighting devices. For example, trapezoid-shaped or zigzag-shaped OLEDs, which extend along (OY) and each provide strip illumination, may be realized.

This embodiment makes it possible to produce lighting devices of very small dimension along the axis (OX), for example so as to extend only between two rows of photo-detectors of the matrix optical sensor, without covering the photosensitive zones. of the last. For example, the (OX) dimension of each OLED is less than 50 μm, or 25 μm, or even less than 10 μm.

It also makes it possible to promote the amount of light arriving in the tissues, and to minimize the diffusion of light before reaching the papillary impression.

This embodiment is also particularly compact, because it does not require the presence of OLED driving transistors located under the detection surface. These can simply be at the edge of the detection surface.

Finally, there is little light that can go directly from the OLEDs to the pixels of the matrix optical sensor. And even if there were, it would be enough not to exploit the pixels located directly under the OLED lit.

FIGS. 3A and 3B schematically illustrate a second embodiment of a papillary impression sensor 310, adapted to the implementation of a method according to the invention. Figure 3A illustrates a sectional view of such a fingerprint sensor, while Figure 3B shows a detail in perspective view.

The lighting means 311 are formed by lighting devices 3110 each comprising a light guide 3112, and arranged under the matrix optical sensor 312.

In particular, each lighting device comprises a light source 3111 disposed at the entrance of the light guide 3112. Each light source consists of one or more light emitting diode (s) (LED). The use of LEDs makes it possible to choose a lighting spectrum. Each light source may consist of several types of LEDs that differ in their emission wavelength. Inside the light guide, the light propagates along the axis (OY) by successive reflections on internal faces of the light guide, in particular the reflective lateral faces 3113. This propagation is illustrated in FIG. 3B by a series arrows. The side faces 3113 are parallel to the plane (YOZ).

A reflector 3114 extends under the light guides 3112, so that light emerges from the light guides to the matrix optical sensor.

Correction films can be added to limit the opening angle along the axis (OX).

It is not necessary to add a film diffusing between the light guides 3112 and the optical array sensor 312, because one does not want a uniform illumination in the plane (OXY), but simply uniform according to (OY).

The array optical sensor 312 comprises a photodetector array 3121 which extends over a substrate 301 disposed above the light guides.

Many other embodiments of the fingerprint sensor can be implemented without departing from the scope of the present invention. For example, OLED strips can be placed under a matrix optical sensor.

Illumination and acquisition steps of a series of raw images according to the invention are then described.

These steps perform a scanning of the position of a light band.

Here, all the lighting devices extend over the entire width L1 defined above.

In Figure 4, there is shown in white a light strip 401, corresponding to the illumination provided by the lighting means, among which a lighting device is turned on, the other lighting devices in black are extinguished.

The position of the single illuminated lighting device is progressively shifted, so that each lighting device is lit once and extinguished otherwise. Thus, the position of the light band is gradually shifted.

Each composition of a lighting device turned on, the other lighting devices being extinguished, forms an illumination figure.

For each illumination figure, a raw image is acquired.

Preferably, each lighting device corresponds to a line of pixels of the matrix optical sensor, so that the light bands are shifted in a regular pitch, equal to the pixel pitch according to (OX) of the matrix optical sensor.

A first illumination figure 450i is thus formed, corresponding to a first illuminated lighting device situated at the edge of the series of lighting devices. We acquire a first raw image IM_1.

Then, a second illumination figure 4502 is formed, the previously lit lighting device being extinguished, and its neighbor switched on. We acquire a second raw image IM_2.

Next, a third illumination figure 4503 is formed, the previously lit illumination device being turned off, and its neighbor turned on. We acquire a third raw image IM_3.

One proceeds in the same way until the acquisition of the raw image IM_N, each lighting device being thus lit once, extinguished otherwise.

Figure 5 illustrates a variant, which will be described only for its differences with respect to Figure 4.

Here, each illumination figure consists of several light bands 501 distributed in a periodic arrangement.

The plurality of light bands extend together on less than one third of the detection surface 125. In addition, they are not adjacent, but separated by black bands corresponding to extinct lighting devices.

The scanning gradually shifts the position of the light bands on, so that after the scanning, each lighting device has been lit once.

It can be seen that this variant makes it possible to reduce a total number of illumination figures to be formed and raw images to be acquired.

In particular, only four images IM_1, IM_2, IM_3, IM_4, related to the illumination figures 550i, 5502, 5503, 5504, allow to scan all the lighting devices.

This reduces the total acquisition time.

Two light bands of the same illumination figure are distant by a length D according to (OX), of at least 3 mm, preferably between 3 and 10 mm, or even between 3 and 5 mm. This distance is measured between the edges closest to each other of these two light bands.

Thus, an extension according to (OX) of the elementary pattern of the illumination figure, is preferably less than or equal to 10 mm, for example between 3 mm and 10 mm, or even between 3 mm and 5 mm (neglecting the size the light band itself, which may be less than 25 μm).

When the lighting devices are sufficiently close to the contact surface, for example less than 20 μm, the distances between two light bands correspond to distances between illuminated lighting devices.

This variant exploits the fact that there is a maximum diffusion length of the visible light in a human tissue, for example 1.5 mm.

Thus, a light band only influences the response of the nearest pixels, for example pixels located less than 1.5 mm on either side of its edges.

It is therefore possible to simultaneously light several light bands, and to study for each of them the response of the pixels of the matrix optical sensor that will be affected by it.

One can also consider that if one wants then to constitute a merged image with pixels located at a distance Kl of a luminous band (or several merged images with a maximum distance Kl), it is necessary that the distance between two bands of light is large in front of Kl (at least 3 times, preferably more than 4 times higher).

According to a variant not shown, instead of lighting a single lighting device, adjacent lighting devices according to (OX) are switched on simultaneously. Thus, the impression receives a more intense illumination. This variant can be applied to one or other of the embodiments of FIG. 4 or 5. The pitch of the offset always corresponds to the distribution step of the lighting devices according to (OX).

It is also possible to implement variants in which the distribution pitch of the lighting devices, according to (OX), is a multiple of the pixel pitch according to (ΟΧ), so that the light bands are shifted at a regular pitch, equal to a multiple of the pixel pitch according to (OX) of the matrix optical sensor.

In order to further reduce a total acquisition time of the raw images, it is possible, in addition or in a variant, to limit the reading of the matrix optical sensor to only the pixels situated near an illuminated lighting device.

For example, only pixels of the matrix optical sensor located near an illuminated lighting device, for example less than 1.5 mm (OX) on either side of the edge of a light-emitting device, are read. light on.

It is furthermore possible to limit the reading of the pixels of the matrix sensor to pixels not situated directly under an illuminated lighting device, in particular when the matrix optical sensor is placed under the lighting means.

Various examples of construction of a merged image from several raw images are then described.

FIG. 6 shows illumination figures 6501 to 6504 of the type of those described with reference to FIG. 4.

To simplify the presentation, it is assumed that each light band and lighting device, corresponds to a pixel line of the matrix optical sensor, and therefore a line of pixels in each raw image.

Here, the merged image is constructed by selecting the pixel lines of the raw images located at a distance K1 from a light band.

If necessary, for each luminous band, two lines of pixels are selected on the corresponding raw image located on either side of the light band.

For example, there is a pixel pitch of the matrix optical sensor of 50 μm, and a distance K1 = 150 μm, or 200 μm, or 250 μm.

Kl is typically measured between the middle of a light band, according to (OX), and the middle of a pixel line, according to (OX).

In FIG. 6, in order to simplify the presentation, the example of raw images is taken to only four lines of pixels and K1 corresponds to the pixel pitch according to (OX).

When acquiring the raw image IM_1, the light band is located above the line 1 of pixels of the matrix optical sensor. Line 2 is selected on the raw image IM_1.

When acquiring the raw image IM_2, the light band is located above the pixel line 2 of the matrix optical sensor. Lines 1 and 3 are selected on the raw image IM_2.

Similarly, lines 2 and 4 on IM_3 are selected, and line 3 on IM_4.

We see that for lines 2 and 3, we have two rows of pixels.

Whenever we have several lines of pixels on the raw images, associated with the same pixel line of the matrix optical sensor, we can select one of the lines, or combine them (for example the average or other linear combination , to limit the effect of the temporal noise of the matrix optical sensor).

The merged image IM_Q is then formed from the selected pixel lines (and if necessary combined, as detailed above). The fused image is called image at the distance K1.

Depending on the value of K1, the characteristics of the merged image vary. For example, when Kl increases, the signal decreases because the tissues absorb more and more light.

According to a first embodiment, an optimal distance Klopt is selected, then a single merged image is constructed at this optimal distance Klopt.

Note that for a value of Kl too low, the pixels of the pixel line of the matrix optical sensor are saturated. In addition, there is image of soiling on the contact surface of the fingerprint sensor. For a value of Kl too high, the diffusion effects problematically reduce the amount of light returning to the optical array sensor.

The choice of the optimal distance Klopt is a compromise, which allows to obtain a good signal-to-noise ratio, and / or a good dynamic.

The value of the optimal distance Klopt can depend on the state of the tissues (bad contact, dry / wet tissues ...), and the pigmentation of the tissues.

As a variant, a series of merged images each linked to a difference value of K1 is constructed.

Depending on the value of K1, it is possible to obtain different information related to the variation of the distance traveled in the tissues.

One can then select the merged image having the best performance, for example in terms of contrast, or noise.

These multiple merged images can also be processed so as to obtain an image of optimal quality, called "super fused" image, which combines several merged images. Indeed, it may be advantageous for a final image to have half of its area corresponding to an image at 2 steps from the illumination pattern (first value of K1), and the other half to 3 steps (second value of K1). ). Thus, different regions of interest are defined on the matrix optical sensor. For each of them, an optimal value of Klopt is selected, the value of Klopt may vary from one region of interest to another. Then, we build an image from the images of the different regions of interest, each at the corresponding Klopt distance.

It is also possible to deduce from the several merged images the noise due to the external lighting and then to subtract this noise from one of the merged images. The effect of outdoor lighting is recognized because it does not vary with the distance to a light band.

According to a variant not shown, the pitch of the light bands is a multiple of the pixel pitch, according to (OX).

In this case, lines of pixels of the raw images located in a range of distances [K; K '] of a light band, between the distance K and the distance K' of this light band. The width of this distance range preferably corresponds to the pitch of the light bands according to (OX).

Distance ranges are also discussed when the light bands are not parallel to the pixel lines. In these cases, when the light bands are locally slightly inclined relative to the axis OY, for simplification, we can use the projected distance on the axis OX instead of the actual distance between the pixel and the band. The integration of the pixels of the matrix optical sensor is advantageously line by line.

It is possible to synchronize a scanning frequency associated with the successive offsets of the position of a light band, and a scanning frequency of the integration of the rows of pixels of the matrix optical sensor.

It is thus possible to directly acquire an image associated with a predetermined distance K i or a predetermined distance range.

In this case, the control means of the lighting devices are synchronized with control means of the matrix optical sensor.

For example, during the lighting period of a light band, the line of pixels located at the distance K1, or, as the case may be, the lines of pixels situated in the range of distances [K; K '] of this light band.

In this case, a raw image is an image at the distance K1, and a merged image is a combination of images associated with different values of K1.

Figure 7 illustrates a variant of the steps illustrated in Figure 6.

According to this embodiment, instead of reconstructing merged images for one or more values of the distance K1 between a light band and a line of pixels of the matrix optical sensor, a fused image is reconstructed in which for each line of pixels is selected the optimum distance K2 between a light band and a pixel line of the matrix optical sensor.

In other words, it is considered that the distance having the best compromise as defined above may vary depending on the location of the pixel line on the matrix optical sensor.

In Figure 7, there is shown by way of example four illumination figures 750i, 7502, 7503, and 754. Each illuminance figure has a single light band.

As in FIG. 6, each light band corresponds to a line of pixels li of the matrix optical sensor, and raw images.

We are interested in the line lt.

The distance K2 varies between 5 * P and 2 * P.

In the illumination figure 750i, the line lt is located at a distance K2i = 5 * P from the light band, where P is the pixel pitch according to (OX).

In the illumination figure 7502, the line Zj is located at a distance K22 = 4 * P from the light band.

In the illumination figure 7503, the line Ζέ is located at a distance K23 = 3 * P from the light band.

In the illumination figure 7504, the line Zj is located at a distance K24 = 2 * P from the light band.

In a step 751, one of the respective lines Z 1 is selected from the raw images corresponding to the illumination figures 750 1 to 7504. The criterion is, for example, the optimum contrast.

The selected line is noted. Step 751 is implemented for each line lj. The merged image is formed by each of the lines thus selected.

In particular, it is possible to form the merged image, for example by replacing in a first raw image, the line Zj (respectively lj) by the line Lt (respectively Lj) selected in step 751. Here again, the pitch of the bands luminous can be a multiple of the pixel pitch, according to (OX), and then series of lines of pixels in a range of distances of a light band are selected.

In each embodiment detailed with reference to FIGS. 6 and 7, the construction of the merged image may furthermore comprise a substitution of a group of pixels originating from a first raw image, by a group of pixels originating from a other raw image.

For example, we can give a realistic value to saturated pixels, and increase the signal-to-noise ratio of dark pixels.

If necessary, the difference in illumination between these two groups of pixels is compensated.

Such a substitution may also be relevant to eliminate artifacts that are visible only in certain images (stripe, trace ..). Again, you have to balance the levels to make the images comparable.

According to an advantageous variant, the method according to the invention is implemented for different wavelengths.

One can then combine merged images each associated with a wavelength, to obtain a merged multi-spectral image.

For this, the lighting means emit simultaneously or in sequence at several wavelengths.

In particular, each lighting device may comprise several types of LEDs or several types of OLEDs emitting monochromatic radiation, and which differ in their emission wavelength. In the embodiment of FIGS. 3A and 3B, the light source 3111 consists for example of a red LED, a blue LED and a green LED.

Alternatively, the lighting devices are of different types, which differ in their emission wavelength.

According to another variant, each lighting device can be multi-spectral, for example OLED whose emission wavelength depends on a bias voltage.

FIGS. 8A and 8B particularly illustrate lighting means 811A comprising several types of lighting devices 8110A emitting monochromatic radiation with a spectral width of less than 80 nm. They are distributed in a periodic pattern comprising lighting devices of different types, which differ by a central wavelength of emission.

In particular, a mean repetition step of a lighting device configured to emit radiation at a first central wavelength is less than a mean repetition step of a lighting device configured to emit radiation at a second central wavelength, greater than the first wavelength. In FIG. 8A, the lighting devices comprise at least one first type of lighting device, suitable for emitting light radiation with a central wavelength of less than 590 nm, and a second type of lighting device, adapted to to emit light radiation with a central wavelength greater than 600 nm.

In particular, a first type of lighting device, noted B, is configured to emit radiation in the blue, at a central wavelength between 400 and 495 nm. A second type of lighting device, denoted R, is configured to emit red radiation, at a central wavelength of between 620 and 750 nm.

The lighting devices are of the same size, and regularly spaced from each other. They together form a periodic arrangement, with a multi-spectral elementary pattern 8119A comprising two blue-emitting lighting devices and a red-emitting illumination device. Thus, the average repetition pitch of a blue emitting light is less than the average repetition pitch of a red emitting light.

One can thus characterize at best a papillary impression, by studying a radiation backscattered by this sample, and for several wavelengths located preferably in the visible one. Indeed, tissues absorb more or less depending on the wavelength. Here, the two wavelengths chosen are located substantially at both ends of the visible spectrum.

In addition, and since the fabrics transmit the light better in the red than in the blue, the so-called R lighting devices, emitting in the red, may be further spaced from each other than the so-called B lighting devices, emitting in the blue.

FIG. 8B illustrates a variant in which the lighting means 811B comprise: a first type of lighting device, denoted B, adapted to emit a light radiation of central wavelength between 400 and 495 nm (blue), a second type of lighting device, denoted G, adapted to emit light of central wavelength between 495 and 590 nm, preferably between 495 and 570 nm, more particularly between 500 and 570 nm (green), and a third type of lighting device, denoted R, adapted to emit a light radiation of central wavelength greater than 600 nm, for example between 620 and 750 nm (red).

The lighting devices 8110B together form a periodic arrangement, with a multi-spectral elementary pattern 8119B comprising two lighting devices of the first type, a lighting device of the second type and a lighting device of the third type. In particular, each lighting device emitting in the blue which is flanked by two lighting devices is framed by a lighting device emitting in the red and a lighting device emitting in the green. Thus, the repetition pitch of blue-emitting lighting devices is even lower than that of other types of lighting devices (one in two emits in blue, one in four emits in red, one in four emit in the green).

We can imagine all kinds of variants, including variants including lighting devices emitting outside the visible spectrum, for example in the near infrared or the near ultraviolet (between 350 and 440 nm).

An extension of the multi-spectral elementary patterns along the axis (OX) is preferably less than or equal to 5 mm, for example between 3 mm and 5 mm.

In operation, it is possible to turn on simultaneously only lighting devices emitting at the same wavelength. In other words, in each illumination figure, all the light bands emit radiation at the same wavelength.

Preferably, for each wavelength, a separate scanning of the lighting of the lighting devices adapted to emit at this wavelength is carried out.

Similarly, the method according to the invention can be implemented for different light intensities of the light bands.

One can then combine merged images each associated with an intensity. The intensity of a light band can be controlled by controlling a current injected into the lighting devices.

FIG. 9 illustrates a variant of the fingerprint sensor according to the invention, in which the lighting devices are divided into two series.

A first set of 1110A lighting devices extends on the left side of the fingerprint sensor.

It consists of lighting devices which each extend in one piece over approximately half of the detection surface 125 of the matrix optical sensor. They are distributed along the length L2 of the detection surface 125.

They extend in particular above said detection surface, from the middle of the latter in the direction of the width L1, to the edge of this detection surface and even beyond.

A second set of 1110B lighting devices extends on the right side of the fingerprint sensor.

The second series of lighting devices 1110B is symmetrical to the first series 1110A relative to a plane parallel to (XOZ) passing through the middle of the detection surface 125, in the direction of its width L1.

Each lighting device 1110A of the first series and its symmetrical 1110B in the second series, are spaced apart by a distance that does not exceed one third of the width L1 of the detection surface 125, preferably only a few micrometers.

In this case, a light band corresponds to the lighting of at least one lighting device of the first series, and its symmetrical / their symmetrical in the second series. Here again, strip-structured lighting is very easily realized, being able to position the control units of each illumination device at the edge of the detection surface 125.

Many other variants can be implemented without departing from the scope of the present invention.

For example, the surface of the matrix optical sensor can be divided into several regions of interest, and each of these regions can be processed separately. In particular, a merged image of a region of interest can be constructed by combinations of pixel lines of a raw image of a region of interest. In each region of interest, an optimal distance Klopt can be selected, the value of Klopt being able to vary from one region of interest to another.

Frontiers of regions of interest may vary depending on the imaged footprint, especially since different users will not necessarily place the finger at the same location on the contact surface.

The definition of the region of interest can be obtained from the contours present in a first merged image, the region of interest then being used to construct the so-called "super fused" image described above, formed by combining several merged images.

In each of the embodiments detailed above, whole lines of pixels have been selected. Alternatively, fragments of each line can be selected, with different fragments being combinable to produce a final line.

It is also possible to create a fused image from a selection of pixels, and not from lines of pixels, in particular to select only pixels receiving at least a portion of the image formed by the imprint.

The method according to the invention can also be implemented to obtain an image of any document, in particular a passport.

It can also make it possible to determine whether the material constituting the impression is a living human tissue or not.

For this, we use several merged images associated with several distances K1. Decreasing lighting profiles are obtained as a function of the distance to the nearest lit lighting device.

These lighting profiles are characteristic of the material constituting the impression.

This material can therefore be characterized by comparison with a predictive model or with profiles obtained experimentally for various known materials.

It is thus possible to simultaneously characterize as many rows of pixels of the matrix optical sensor as there is lighting device. If necessary, a profile is calculated for each pixel of a line of the matrix optical sensor. A false cavity detection is thus performed on the entire surface of the matrix optical sensor.

This analysis can be done on a selection of pixels of the merged images, associated with the valleys, respectively valleys of the footprint.

It could also be determined whether the material constituting the impression is a living human tissue or not using a single merged image, associated with a given value of K1. This is done using a database of responses to such illumination, various known samples. Next, we look for the closest response to the merged image obtained experimentally.

Fraud can also be detected from two merged images, associated with a high value of K1 for one, for example K1 = 200 μm, and a low K1 value for the other, for example K1 = 50 μm. The comparison between these two images makes it possible to recognize a plane imitation of a print.

Claims (16)

A method for obtaining a fused image (IM_Q) of a papillary impression, the fingerprint being in direct physical contact with a contact surface (106) of a papillary impression sensor (110; 210; 310) including superimposed beneath the contact surface, a matrix optical sensor (112; 212; 312), and illumination means (111; 211; 311; 811A; 811B) formed by a plurality of lighting devices (1110; 2110; 3110; 8110A; 8110B; 1110A, 1110B) parallel to each other, characterized in that it comprises the following steps: - illumination of the papillary impression by the lighting means, the lighting devices (1110; 2110; 3110 8110A; 8110B; 1110A, 1110B) being switched on and off so as to successively form, on the contact surface (106), different illumination figures (450i, 450¾, 4500, 450n, 550i, 550¾ 5503, 5504, 650i, 650¾ 6503, 6504; 750¾ 750¾ 7503, 7504) each consisting of at least one light strip (4 Ù1; 501) extending over the entire width (Ll) of a detection surface of the matrix optical sensor, and acquisition by the matrix optical sensor of a series of raw images (IM_1, IM_2, IM_3, IM_4, IM_N ) the papillary impression each associated with one of the illumination figures; Constructing (180) the merged image (IM_Q) by combining series of pixels of the raw images, each pixel series forming a line parallel to the light bands. Method according to claim 1, characterized in that each light band (401; 501) is parallel to the pixel lines of the matrix optical sensor (112; 212; 312), and that the construction of the fused image implement a combination of pixel lines (^) directly from the raw images, or formed by pixels from different raw images and associated with the same pixel line of the matrix optical sensor. Method according to claim 1 or 2, characterized in that the construction of the merged image (IM_Q) comprises a selection of pixels of the raw images (IM JL; IM_2; IM_3; IM4), each associated with the same range of distances between the corresponding pixel of the matrix optical sensor and the nearest light band. 4. Method according to claim 3, characterized in that it comprises the construction of several merged images (IM_Q), each associated with one of several ranges of distances. Method according to claim 1 or 2, characterized in that regions of interest associated with different regions of the detection surface of the matrix optical sensor are defined, and in that the construction of the merged image comprises for each region of interest, a selection of pixels of the raw images (IM_1; IM_2; IM_3; IM4) associated with the same range of distances between the corresponding pixel of the matrix optical sensor and the nearest light band; interest not being associated with the same range of distances. 6. Method according to claim 1, characterized in that the light bands (401; 650i, 6502, 6503, 6504; 750i, 7502, 7503, 7504) are shifted from each other at a regular pitch equal to the pixel pitch of the matrix optical sensor. 7. Method according to any one of claims 1 to 6, characterized in that the illumination of the papillary impression is implemented by lighting means consisting of organic light-emitting diodes (2110), arranged above the optical array sensor (212). 8. Method according to any one of claims 1 to 7, characterized in that each illumination figure (450a, 4502, 450s, 450N) consists of a single light band (401), and in that the different Illuminance figures correspond to successive offsets of the light band, so that each of the lighting devices is lit at least once. 9. Method according to claim 8, characterized in that the merged image is constructed from lines of the raw images located at the same distance from the light band. 10. Method according to any one of claims 1 to 7, characterized in that each illumination figure (550i, 5502, 553, 5504) consists of several light bands (501) distributed in a periodic arrangement, and in that that the different illumination figures correspond to successive offsets of the light bands, so that each of the lighting devices is lit at least once. 11. The method of claim 10, characterized in that two adjacent light bands (501) of the same illumination figure (550i, 5502, 5503, 5504) are at least 3 mm apart. 12. Method according to any one of claims 8 to 11, characterized in that a scanning frequency associated with these successive offsets is synchronized with a scanning frequency of the integration of the pixel lines of the matrix optical sensor (112; 212; 312). 13. Method according to claim 12, characterized in that the acquisition of a raw image does not include the reading of all the pixels of the optical array sensor (112; 212; 312), only the rows of pixels located at near the edge of a light band (401; 501) being read, that is to say spaced from this edge by a distance less than a predetermined threshold. 14. Method according to any one of claims 1 to 13, characterized in that the lighting means (811A; 811B) are configured to emit light radiation centered on different wavelengths, and in that the steps of illumination of the papillary impression and construction of the fused image are implemented for each of said different wavelengths. 15. Method according to any one of claims 1 to 14, characterized in that the construction of the merged image (IM_Q) comprises a substitution of a group of pixels from a first raw image (IM_1; IM_2; IM_3 IM_4) by a group of pixels from a second raw image. Method according to claim 15, characterized in that the construction of the merged image (IM_Q) comprises a substitution of a group of pixels from a first raw image (IM_1; IM_2; IM_3; IM_4) by a group pixels from a second raw image, to replace one or more overexposed or underexposed pixels. 17. System (100) for implementing a method according to any one of claims 1 to 16, characterized in that it comprises: - a papillary impression sensor (110; 210; 310) comprising, superimposed, a contact surface (106) for applying the imprint to be imaged therein, a matrix optical sensor (112; 212; 312), and illumination means (111; 211; 311; 811A; 811B) formed by a plurality of devices lights (1110; 2110; 3110; 8110A; 8110B; 1110A, 1110B) parallel to each other; - control means (130), configured to turn on and off the lighting devices so as to successively form the different illumination figures (450¾ 4502, 4503, 450n; 550i, 5502, 550a, 5504; 650i, 650¾ 6503; 6504; 7501, 7502, 7503, 7504); and> calculation means (140), configured to receive the raw images (IM_1, IM_2, IM_3, IM_4, IM_N) acquired by the matrix optical sensor and each associated with one of the illumination figures, and to construct the merged image (IM_Q) by combining series of pixels of the raw images, each series of pixels forming a line parallel to the light bands.