FR3135343A1 - Fingerprint acquisition device and method of controlling such a device - Google Patents

Abstract

Fingerprint acquisition device and method of controlling such a device The present description relates to a fingerprint acquisition device comprising an image sensor, comprising a matrix of organic photodetectors, and a processing circuit, the device being configured to implement the following steps: a) capture (201 , 203, 205), by means of the image sensor, a first image of at least one finger of a user with first acquisition parameters; b) determine (209), by means of the processing circuit, an indicator relating to the first image; and c) after step a) and before the end of step b), initiate the capture (251, 253, 255), by means of the image sensor, of a second image of said at least one finger of the user with second acquisition parameters. Figure for abstract: Fig. 3

Description

Fingerprint acquisition device and method of controlling such a device

The present description generally concerns the field of electronic devices, and relates more particularly to an electronic device configured to acquire fingerprints, or fingerprint sensor. The present description further relates to a method of controlling such a device.

Various applications can benefit from a fingerprint sensor. Such a sensor can for example be integrated into a mobile phone, a touchscreen tablet, a watch or a connected bracelet, etc.

An object of one embodiment is to overcome all or part of the disadvantages of known fingerprint sensors and their control methods. One embodiment aims more particularly to reduce the duration of identification or authentication of a user by means of a fingerprint sensor.

For this, one embodiment provides a fingerprint acquisition device comprising an image sensor, comprising a matrix of organic photodetectors, and a processing circuit, the device being configured to implement the following steps:
a) capture, by means of the image sensor, a first image of at least one finger of a user with first acquisition parameters;
b) determine, by means of the processing circuit, an indicator relating to the first image; And
c) after step a) and before the end of step b), initiate the capture, by means of the image sensor, of a second image of said at least one finger of the user with second parameters d 'acquisition.

According to one embodiment, the indicator relating to the first image is a quality indicator of the first image.

According to one embodiment, the device is further configured to continue and complete the capture of the second image if the quality indicator of the first image is outside a range of values for which the first image is considered usable by the processing circuit.

According to one embodiment, the device is further configured to, after the end of capturing the second image, extract, by means of the processing circuit, characteristic data from the second image.

According to one embodiment, the device is further configured to interrupt the capture of the second image if the quality indicator of the first image is included in the range of values for which the first image is considered usable by the detection circuit. treatment.

According to one embodiment, the device is further configured to, after the end of capturing the first image, extract, by means of the processing circuit, characteristic data from the first image.

According to one embodiment, the quality indicator is a function of a contrast evaluation of the first image.

According to one embodiment, the indicator relating to the first image is a score of correspondence of the first image to one or more reference images.

According to one embodiment, the device is further configured to continue and complete the capture of the second image if the correspondence score of the first image is outside a range of values for which the first image is considered to correspond to the first image. 'one of the reference images.

According to one embodiment, the first and second images respectively have first and second exposure durations, the second exposure duration being greater than the first exposure duration.

According to one embodiment, the first and second images also present respectively first and second integration gains, the second integration gain being greater than the first integration gain.

According to one embodiment, the matrix of organic photodetectors comprises at least 200 lines of pixels.

One embodiment provides a mobile telephone comprising a device as described.

One embodiment provides a connected watch or bracelet comprising a device as described.

One embodiment provides a method of acquiring fingerprints by a device comprising an image sensor, comprising a matrix of organic photodetectors, and a processing circuit, the method comprising the following steps:
a) capture, by means of the image sensor, a first image of at least one finger of a user with first acquisition parameters;
b) measure, by means of the processing circuit, an indicator relating to the first image; And
c) after step a) and before the end of step b), initiate the capture, by means of the image sensor, of a second image of said at least one finger of the user with second parameters d 'acquisition.

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

there is an electrical diagram partially illustrating an example of a fingerprint acquisition device of the type to which the described embodiments apply, by way of example;

there is a chronogram illustrating, schematically and partially, successive stages of an example of a method of controlling the device of the ;

there is a chronogram illustrating, schematically and partially, successive steps of an example of a method of controlling the device of the according to one embodiment; And

there is a chronogram illustrating, schematically and partially, successive steps of an example of a method of controlling the device of the according to one embodiment.

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

For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been represented and are detailed. In particular, the various applications of the fingerprint sensors described have not been detailed, the embodiments described being compatible with all or most of the applications likely to benefit from a fingerprint sensor, in particular applications in which the fingerprint sensor is integrated into a mobile phone.

Unless otherwise specified, when we refer to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when we refer to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be linked through one or more other elements.

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

Unless otherwise specified, the expressions "approximately", "approximately", "substantially", and "of the order of" mean to the nearest 10%, preferably to the nearest 5%.

There is an electrical diagram partially illustrating an example of a fingerprint acquisition device 100, or fingerprint sensor, of the type to which the described embodiments apply, by way of example.

In the example shown, the device 100 comprises an image sensor 101 (SENSOR) comprising a plurality of PIX pixels. The PIX pixels are, in this example, arranged in matrix 103 according to rows and columns. The lines are for example substantially perpendicular to the columns. In the orientation of the , the rows and columns correspond respectively to horizontal rows and vertical rows of adjacent PIX pixels. The PIX pixels of the matrix 103 of the image sensor 101 all have, for example, lateral dimensions and a shape, for example substantially square, which are approximately identical, except for manufacturing dispersions. The matrix 103 includes for example at least 300 lines of PIX pixels. As an example, matrix 103 has approximately 400 lines, approximately 1000 lines, or approximately 2000 lines of PIX pixels. The matrix 103 of PIX pixels has, for example, a surface area of at least 4 cm ² .

In the example illustrated in , each PIX pixel of the matrix 103 comprises a photodetector 105, for example a photosensitive diode, or photodiode, of which an anode is connected to a node 107 for applying a polarization potential VBIAS and of which a cathode is connected to a node 109 for storing charges photogenerated by the photodetector 105. In this example, each PIX pixel of the matrix 103 has a “1T” type architecture. More precisely, each PIX pixel of the matrix 103 comprises a transfer transistor 111, for example a field effect transistor, for example a thin-film transistor ("Thin-Film Transistor" - TFT, in English), one of which has a terminal conduction terminal (for example the drain) is connected to the charge storage node 109 and another conduction terminal (for example the source) of which is connected to a charge collection node 113. In the example shown, the nodes 107 of the PIX pixels forming part of the same column of the matrix 103 are interconnected by a conductive track 115 brought to the polarization potential VBIAS. Furthermore, in this example, the nodes 113 of the PIX pixels forming part of the same column of the matrix 103 are interconnected by a conductive track 117.

For example, the photodetectors 105 are organic photodetectors, that is to say photodetectors comprising at least one photoconversion layer, or active layer, in at least one organic material.

In the example illustrated in , the PIX pixels of the matrix 103 are connected to a control circuit 119 and to a reading circuit 121.

The control circuit 119 of the image sensor 101 is for example of the “Gate On Array” (GOA) type. More precisely, in this example, the control circuit 119 comprises, for each line of PIX pixels of the matrix 103, a switch 123 connected to a conductive track 125 interconnecting control terminals (the gates, in this example) of the transistors of transfer 111 of the PIX pixels of the line. In the example shown, each switch 123 is of the single pole double throw type ("Single Pole Double Throw" - SPDT, in English) and more precisely comprises an input terminal IN connected to the conductive track 125, a first output terminal OUT1 connected to a node 127 for applying a potential VON, for example a high potential, and a second output terminal OUT2 connected to a node 129 for applying a potential VOFF, for example a low potential, for example lower at 0 V, for example of the order of -10 V. More generally, the values of the VON and VOFF potentials are for example respectively chosen so that the transistors 111 of the PIX pixels of the same line are in the on state when the input terminal IN of the switch 123 of the line is connected to the first output terminal OUT1, brought to the potential VON, and in the blocked state when the input terminal IN of the switch 123 of the line is connected to the second output terminal OUT2, brought to potential VOFF. The control circuit 119 has, for example, an operation similar to that of a shift register.

In the example illustrated in , the reading circuit 121 of the image sensor 101 comprises, for each column of PIX pixels of the matrix 103, an integration circuit 131 of the photogenerated charges, for example photogenerated electrons, by the photodetectors 105 of the PIX pixels of the column. In this example, each integration circuit 131 comprises an input terminal 133, connected to one end of one of the conductive tracks 117, and an output terminal 135. The integration circuits 131 are for example, as illustrated in , located at the foot of the column of the matrix 103 of PIX pixels.

Although this was not detailed in for the sake of simplification, each integration circuit 131 comprises for example an operational amplifier whose inverting and non-inverting inputs are connected respectively to terminal 133 and to a node for applying a reference potential VREF, for example greater than 0 V, and an output of which is connected to terminal 135. Each integration circuit 131 for example further comprises a capacitive component, for example a capacitor, and a switch associated in parallel between the input and output terminals 133 135 of the integration circuit 131.

In the example shown, the output terminals 135 of the integration circuits 131 are connected to a correlated double sampling circuit 137 (CDS). In this example, the correlated double sampling circuit 137 is connected to an analog-digital conversion circuit 139 (ADC). For example, the correlated double sampling circuits 137 and analog-digital conversion circuits 139 are integrated into the read circuit 121.

We illustrated in an exemplary embodiment in which the reading circuit 121 of the image sensor 101 comprises a single correlated double sampling circuit 137 and a single analog-digital conversion circuit 139, the circuits 137 and 139 being common to all columns of pixels PIX of the matrix 103. This example is however not limiting, the image sensor 101 being able to comprise several correlated double sampling circuits 137 and/or several analog-digital conversion circuits 139, for example a circuit 137 and/or a circuit 139 per column, or per subset of columns, of PIX pixels of the matrix 103.

We represented in an example of image sensor 101 in which the matrix 103 of PIX pixels is connected to a single control circuit 119 and to a single reading circuit 121, the control circuits 119 and reading circuits 121 being for example integrated in the same integrated circuit chip. This example is however not limiting, the image sensor 101 being able to comprise several control circuits 119 connected or connected respectively to different conductive tracks 125, and/or several reading circuits 121 connected or connected respectively to conductive tracks 117 different. In other words, the control circuit 119 can be divided into several control sub-circuits, for example identical or similar, respectively connected to different subsets of conductive tracks 125, and/or the reading circuit 121 can be divided into several reading sub-circuits, for example identical or similar, respectively connected to different subsets of conductive tracks 117. For example, the different control and reading sub-circuits are integrated in the same integrated circuit chip.

We illustrated in an example of so-called passive architecture, in which each PIX pixel of matrix 103 comprises a single transistor ("1T" pixel). As a variant, we could provide a so-called active architecture, in which each pixel of the matrix comprises for example three transistors ("3T" pixel). In this case, the integration of the charges is for example carried out inside the pixel which has the advantage of simplifying the reading circuit 121, the reading being carried out for example in voltage.

In the example shown, the fingerprint sensor 100 further comprises a processing circuit 151 (CONTROLLER), for example integrated in an integrated circuit chip distinct from that of which the sensor 101 is a part. The circuit 151 is for example a microcontroller or microprocessor for executing program code instructions of an image processing algorithm. For example, circuit 151 corresponds to the main processor of a mobile telephone integrating the fingerprint sensor 100. Circuit 151 executes more precisely, in the example illustrated, an algorithm 153 (IMAGE PROCESSOR), or block software, comprising image processing program code instructions and another algorithm 155 (IMAGE MATCHER) comprising fingerprint verification or identification program code instructions. In this example, the image processing algorithm 153 receives, as input, output data from the analog-digital conversion circuit 139 of the image sensor 101, and the fingerprint recognition algorithm 155 receives, as input, output data from the image processing algorithm 153.

By way of example, the algorithm 153 is configured to carry out processing of digital images from the sensor 101, each comprising for example at least one fingerprint of at least one finger of a user, prior to the processing of these images by the algorithm 155. The algorithm 153 can for example implement various pre-processing and/or improvement operations on the digital images from the sensor 101, for example conversion operations into gray levels or images. monochrome, noise reduction, sharpening, filtering, etc. As an alternative, Algorithm 153 can be omitted.

In the example shown, the algorithm 155 receives for example the images processed by the algorithm 153 and analyzes them, for example to extract characteristic data, for example minutiae corresponding to locations and directions of ends and /or bifurcations of friction ridges of the user's fingerprint(s). By way of example, the algorithm 155 is further configured to compare the characteristic data coming from the images of the sensor 101 to reference characteristic data, for example reference minutiae corresponding to one or more fingerprints of one or more fingers of an owner of an electronic device, for example a mobile telephone, equipped with the fingerprint sensor 100. The reference minutiae are for example stored, in encrypted form, in a memory not shown of the fingerprint sensor 100 during a prior personalization step ("enrollment", in English) during which the owner of the mobile phone is invited to place his finger, for example several times and in different positions, directly above the fingerprint sensor fingerprints 100. This corresponds for example to a comparison of fingerprints called “by comparison of minutiae cards”. As a variant, one could plan to directly compare the orientations of ridges and valleys of the fingerprints. In this case, this corresponds to a so-called “pattern-based recognition” comparison of fingerprints.

The image processing 153 and fingerprint recognition 155 algorithms have been symbolized in by separate blocks. However, these algorithms can in practice be part of the same set of computer program code instructions executable by the processing circuit 151. In addition, the assignment of the operations implemented by the algorithms 153 and 155 can be different from that described above.

There is a chronogram illustrating, schematically and partially, successive steps of an example of a method of controlling the device 100 of the . There illustrates more precisely, as a function of time t, a sequence of steps respectively implemented by the image sensor 101 (SENSOR, on the left in ) and by the processing circuit 151 (CONTROLLER, on the right in ) of the fingerprint sensor 100.

Between an instant t0 and an instant t1, after the instant t0, the image sensor 101 implements a reset step 201 (RESET FRAME), during which photogenerated charges, previously accumulated at the storage nodes 109 or at the terminals of the capacitive component of each integration circuit 131, are evacuated. During the reset step 201, the switches of each integration circuit 131 are closed in order to connect the input terminals 133 of the integration circuits 131 to their output terminals 135.

Furthermore, during step 201, the switches 123 of the control circuit 119 are controlled sequentially, one after the other, to connect their input terminal IN to their first output terminal OUT1, the potential VON thus being applied, via the conductive tracks 125, to the gates of the transfer transistors 111 of the same line of PIX pixels so that the transistors 111 are in the on state. We thus transfer successively, for example line after line, the charges previously accumulated at the storage nodes 109 of the PIX pixels of the different lines of the matrix 103 to the integration circuits 131 of the reading circuit 121.

More precisely, for example, we first control the switch 123 connected to the conductive track 125 of one of the PIX pixel lines of the matrix 103, for example the upper line in the orientation of the , in order to apply the VON potential to the gates of the transfer transistors 111 of this line. This has the effect of transferring, or emptying, the photogenerated charges from the storage nodes 109 of the PIX pixels of the line to the integration circuits 131, each node 109 being connected to one of the circuits 131 during this operation. The photogenerated charges coming from the PIX pixels of the line are then evacuated via the output terminal 135 of the circuits 131. Once the charges have been evacuated, we control, for example, the switch 123 connected to the line of PIX pixels from which we have just transferred the loads in order to apply the VOFF potential to the gates of the transfer transistors 111 of this line, in order to isolate the storage nodes 109 of the PIX pixels of this line with respect to the integration circuits 131. We then repeat the following operations -above on another line of PIX pixels, for example a line adjacent to the line whose charges have just been transferred. All the PIX pixel lines of the matrix 103 are thus scanned sequentially, for example from top to bottom in the orientation of the .

For example, the reset step 201 begins for example at a moment when at least one finger, for example a single finger, of a user is placed on the image sensor 101, directly above the 103 matrix of PIX pixels.

Between time t1 and a time t2, subsequent to time t1, the image sensor 101 implements a first exposure step 203 (EXPOSURE), during which charges photogenerated by the photodetectors 105 of the PIX pixels of the matrix 103 are accumulated at the storage nodes 109. The first exposure step 203 corresponds for example to a situation in which the photodetectors 105 of the PIX pixels of the matrix 103 are exposed to light and where it is desired to capture a first image of a fingerprint corresponding to the user's finger placed on the image sensor 101. The transfer transistors 111 are controlled in the off state during the first exposure step 203.

Between time t2 and a time t3, after time t2, the image sensor 101 implements an acquisition step 205 (ACQUISITION FRAME). Unlike the reset step 201, during which the photogenerated charges previously accumulated at the storage nodes 109 of the PIX pixels of the matrix 103 are evacuated without being integrated, it is planned, during the acquisition step 205, to integrate the photogenerated charges coming from the PIX pixels of the matrix 103. More precisely, the charges photogenerated by the photodetectors 105 of the PIX pixels are for example transferred from the storage nodes 109 to the read circuit 121 via the transistors 111 and conductive tracks 117 connected to the input terminals 133 of the integration circuits 131. When the switch of each integration circuit 131 is in the open state, an integrator type assembly is obtained in which the photogenerated charges are accumulate at the terminals of the capacitive component. The operational amplifier then applies at output, on terminal 135, a potential depending on a quantity of photogenerated charges accumulated at the terminals of the capacitive component. This potential is for example then transmitted to the correlated double sampling circuit 137, then to the analog-digital conversion circuit 139. For example, we thus obtain, at the output of the analog-digital conversion circuit 139, data based on the quantity of charges integrated across the terminals of the capacitive component of the integration circuit 131 for each PIX pixel of the sensor 101. This data makes it possible, for example, to form the first image of the user's fingerprint.

Between time t3 and a time t4, after time t3, the image sensor 101 and the processing circuit 151 implement an image transfer step 207 (IMAGE TRANSFER). More precisely, during this step, the data representative of the first image previously acquired by the sensor 101, coming from the analog-digital conversion circuit 139, are transmitted from the sensor 101 to the processing circuit 151.

For example, a duration of between 50 and 200 ms, for example equal to approximately 100 ms, separates times t0 and t4.

Between time t4 and a time t5, subsequent to time t4, the processing circuit 151 implements an analysis step 209 (IMAGE PROCESSING) of the first image acquired by the sensor 101. During the step 209, the image processing algorithm 153 is for example configured to carry out pre-processing of the first image comprising the fingerprint of the user whose finger was located above the matrix 103 of PIX pixels during of the exposure phase 203. Furthermore, during step 209, the algorithm 155 is for example configured to analyze the first image, after processing by the algorithm 153, in order to determine an indicator relating to the first image .

For example, the algorithm 155 determines whether the first image has sufficient quality to enable the user's finger to be verified or identified, for example by checking whether the acquired fingerprint can be used. More precisely, the algorithm 155 is for example configured to calculate or measure a quality indicator of the first image processed by the algorithm 153 and to compare this quality indicator to at least one threshold. For example, in a case where the quality indicator is included in a range of values between two thresholds, the image is considered usable and the minutiae or orientations of the peaks and valleys, depending on the type of recognition used, are extracted from the first image by algorithm 155. On the other hand, in a case where the quality indicator is for example outside the range of values, the image is considered not usable. The quality indicator corresponds for example to a contrast evaluation, that is to say a measurement of the difference in brightness between the brightest point and the darkest point, of the first image processed by the algorithm 153 .

In the example illustrated in , the quality indicator of the first image is not included in the range of values for which the image is considered usable by the algorithm 155. This can for example come from the fact that the first image has too much exposure. important, for example in the case of an acquisition in conditions of high light, for example under a solar simulator, or on the contrary insufficient, for example in the case of an acquisition in conditions of low light or in the presence of a dry finger. In this example, at time t5, the processing circuit 151 transmits to the image sensor 101 a command to capture a second image (arrow 211, REQUEST NEW IMAGE) with acquisition parameters different from those of the first image.

Alternatively, algorithm 155 may directly attempt to verify or identify the user's finger. More precisely, we can for example provide that the indicator relating to the first image, analyzed by the algorithm 155, is a score representative of a correspondence of the first image, acquired by the sensor 101, to one or more fingerprints of reference, for example the fingerprints of an authorized user of the device integrating the fingerprint sensor 100. In this case, if the score is not included in a range of values for which the fingerprint acquired by the fingerprint sensor images 101 is considered to correspond to one of the reference fingerprints, the processing circuit 151 transmits to the image sensor 101, at time t5, the command to capture the second image with acquisition parameters different from those of the first image, for example a longer exposure time.

For example, a duration of between 20 and 200 ms, for example equal to approximately 50 ms, separates times t4 and t5.

Between time t5 and a time t6, subsequent to time t5, the image sensor 101 implements another reset step 251 (RESET FRAME), during which photogenerated charges, previously accumulated at the storage nodes 109 or at the terminals of the capacitive component of each integration circuit 131, are evacuated. The reset step 251 is for example identical or similar to the reset step 201 described above.

Between time t6 and a time t7, subsequent to time t6, the image sensor 101 implements a second exposure step 253 (EXPOSURE), during which charges photogenerated by the photodetectors 105 of the PIX pixels of the matrix 103 are accumulated at the storage nodes 109. The second exposure step 253 is for example similar to the first exposure step 203 but differs from step 203 in that it has for example a different duration, for example larger example as illustrated in . In the example shown, the second exposure step 253 has more precisely a duration approximately two or three times greater than that of the first exposure step 203. In this example, the second exposure step 253 is carried out in the presence of the same finger of the user as the first exposure step 203.

Between time t7 and a time t8, after time t7, the image sensor 101 implements another acquisition step 255 (ACQUISITION FRAME) during which the photogenerated charges coming from the PIX pixels of the matrix 103 are integrated by the integration circuits 131. The acquisition step 255 is for example identical or similar to the acquisition step 205 previously described. For example, during the acquisition step 255 of the second image, a different integration gain can be provided, for example greater than that applied during the acquisition step 205 of the first image. .

Between time t8 and a time t9, subsequent to time t8, the image sensor 101 and the processing circuit 151 implement an image transfer step 257 (IMAGE TRANSFER), for example identical or similar in the image transfer step 207 described above.

For example, the moments t5 and t9 are separated by a duration of between 100 and 200 ms, for example equal to approximately 200 ms in the case illustrated in where the exposure time of the second image is doubled compared to that of the first image.

Between time t9 and a time t10, subsequent to time t4, the processing circuit 151 implements an analysis step 259 (IMAGE PROCESSING AND MATCHING) of the second image acquired by the sensor 101. In the example illustrated in , the quality indicator of the second image is included in the range of values for which the algorithm 155 estimates that an image coming from the sensor 101 is usable, in the case where the indicator relating to the second image is an indicator image quality. For example, characteristic data of the second image, for example minutiae or orientations of the crests and valleys, depending on the type of recognition used, are then for example extracted by the algorithm 155 then compared to minutiae or orientations of reference peaks and valleys previously recorded in the device 100, for example in the circuit 151. For example, the minutiae or the reference orientations are stored in a secure memory, for example in the form of encrypted data , of the processing circuit 151. At the end of this comparison, the algorithm 155 determines for example whether the fingerprint contained in the second image corresponds to a fingerprint of an authorized user of the electronic device equipped with the fingerprint sensor. fingerprints 100.

As a variant, between times t9 and t10, the score representative of the correspondence of the second image to one or more reference fingerprints is included in the range of values for which the fingerprint acquired by the image sensor 101 is considered to correspond to one of the reference fingerprints, in the case where the quality indicator of the second image is a score of correspondence of the second image to one or more reference fingerprints.

For example, a duration of between 25 and 250 ms separates moments t9 and t10.

We illustrated in an example in which the acquisition steps 205, 255 and transfer 207, 257 of the same image are implemented successively. In this example, the data corresponding to the image acquired by the sensor 101 are for example first stored in a memory, for example a memory buffer ("buffer", in English), of the reading circuit 121, progressively. as the PIX pixel lines of the matrix 103 are scanned and the charges accumulated at the nodes 109 are integrated by the circuits 131. Then, once all the PIX pixel lines of the matrix 103 have been scanned and all the photogenerated charges have been integrated, the data are for example transferred from the memory of the reading circuit 121 to a memory of the processing circuit 151. As a variant, it could be envisaged that the acquisition steps 205, 255 and transfer 207, 257 are, for each of the first and second images, implemented simultaneously. For this, we could for example provide an "on the fly" transfer of data coming from the integration of the charges by the circuits 131, the data of each line being in this case transferred from the image sensor 101 to the circuit of processing 151 before the data for the next line is produced. This would make it possible, for example, to omit the memories of circuits 121 and 151, or to provide memories with lower storage capacities.

Although this was not illustrated in , we could foresee a case in which the quality indicator of the first image would be included in the range of values for which an image coming from the sensor 101 is considered as usable by the algorithm 155 or a case in which the correspondence score from the first image to one of the reference fingerprints would be included in the range of values for which an image coming from the sensor 101 is considered to correspond to a reference image. For example, the second image would then not be captured, that is to say that the operations of steps 251, 253, 255 and 257 would be omitted, and that algorithms 153 and 155 would implement, at the in place of step 209, a step identical or similar to step 259.

There is a chronogram illustrating, schematically and partially, successive steps of an example of a method of controlling the device 100 of the according to one embodiment. The process of includes elements in common with the process of . These common elements will not be described again below.

The process of differs from the process of in that, in the process of , the capture of the second image by the image sensor 101 (SENSOR) is initiated after the capture of the first image and before the end of the step of analyzing the first image 209 (IMAGE PROCESSING) by the circuit of treatment 151 (CONTROLLER). In the example shown, the method of differs more precisely from the process of in that, in the process of , the capture of the second image is initiated after the end of the image transfer step 207 (IMAGE TRANSFER), for example from the end of step 207, at time t4. As a variant, depending on the architecture of the read circuit 121, it could be planned to start the reset step 251 during the transfer step 207.

In this example, the reset step 251 (RESET FRAME) prior to capturing the second image is implemented by the sensor 101 between time t4 and a time t5', after time t4. Furthermore, in the example shown, the step of exposing the second image 253 (EXPOSURE FRAME) is implemented between time t5' and time t7 of the start of the step of acquiring the image. second image 255 (ACQUISITION FRAME).

In the example illustrated in , step 251 and part of step 253 are implemented during the analysis step 209 (IMAGE PROCESSING) of the first image acquired by the sensor 101. In this example, step 209 ends at a time t6', after time t5' and before time t7. In the example illustrated in , the indicator relating to the first image is for example outside the range of values for which the circuit 151 estimates that an image coming from the sensor 101 is usable, in the case where the indicator relating to the first image is an indicator image quality, or outside the range of values for which the first image is considered to correspond to a reference image, in the case where the indicator relating to the first image is a score of correspondence to one or more images reference. The command to capture the second image (arrow 211, REQUEST NEW IMAGE) is then transmitted by the processing circuit 151 at time t6', that is to say during step 253 of exposure of the second picture.

An advantage of the process presented in relation to the is due to the fact that the capture of the second image is carried out in parallel with the analysis of the first image. This allows, as in the case illustrated in where the first image is considered non-usable, to reduce the duration separating the start of the acquisition of the first image (time t0) from the end of the analysis of the second image (time t10), for example by the order from 20 to 200 ms, compared to the method previously explained in relation to the . We thus reduce the duration of identification or authentication of the user, for example when we are in the presence of environmental conditions unfavorable to the analysis of the first image by the circuit 151. This advantageously makes it possible to improve the user experience of the electronic device equipped with the fingerprint sensor 100.

We illustrated in an example in which the command 211 for acquiring the second image occurs during the exposure step 253 associated with this image. This example is however not limiting, the command 211 being able for example to reach the image sensor 101 during any step 251, 253 or 255 of the capture phase of the second image, or even after the acquisition of the second image.

In the case where the indicator relating to the first image is a score of correspondence to one or more reference images, the method illustrated in advantageously makes it possible to confirm, by the acquisition and analysis of the second image, that the finger placed in front of the sensor 100 is not a finger of an authorized user of the device integrating the sensor 100.

There is a chronogram illustrating, schematically and partially, successive steps of an example of a method of controlling the device 100 of the according to one embodiment. The process of includes elements in common with the process of . These common elements will not be described again below.

The process of differs from the process of in that, in the process of , the first image is considered as usable or as corresponding to one of the reference images. More precisely, in the process of , the indicator relating to the first image is for example included in the range of values for which the circuit 151 estimates that an image coming from the sensor 101 is usable, in the case where the indicator relating to the first image is an indicator image quality, or in the range of values for which the first image is considered to correspond to a reference image, in the case where the indicator relating to the first image is a score of correspondence to one or more images of reference.

In this example, the processing circuit 151 implements, between time t4 and a time t7" after time t4, an analysis step 409 (IMAGE PROCESSING AND MATCHING) of the first image acquired by the sensor 101. The operations carried out during step 409 are for example identical or similar to those implemented during step 259 previously described in relation to the .

The second image is for example not useful for verifying or identifying the user's fingerprint, the acquisition of the second image is interrupted (arrow 411, STOP) for example at a time t6", subsequent to the instant t5' of start of step 253 (EXPOSURE) of exposure of the second image and prior to instant t7" of end of step 409. The second image is then not transferred to the processing circuit

We illustrated in an example in which the interruption 411 of the acquisition of the second image occurs during the exposure step 253 associated with this image. This example is however not limiting, the interruption 411 being able for example to occur during any step 251, 253 or 255 of the capture phase of the second image.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants could be combined, and other variants will become apparent to those skilled in the art. In particular, although application examples have been detailed above in which the first and second images each include a single fingerprint of the same finger of a user, the person skilled in the art is capable of adapting The described embodiments relate to applications in which each image includes fingerprints from multiple fingers.

Furthermore, although examples of application have been described in relation to Figures 2 to 4 in which the exposure duration of the second image is greater than the first image, the indications in the present description allow the person skilled in the art to adapt the embodiments described to applications in which the exposure duration of the second image is less than that of the first image and/or the integration gain of the second image is less than the gain of The first picture.

Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art based on the functional indications given above. In particular, the choice of the indicator relating to the first and second images, the exposure times of the first and second images, the range of values of the indicator relating to the first and second images, etc. are within the reach of those skilled in the art based on the information in this description.

Claims (15)

Device (100) for acquiring fingerprints comprising an image sensor (101), comprising a matrix (103) of organic photodetectors (105), and a processing circuit (151), the device being configured to implement performs the following steps:
a) capturing (201, 203, 205), by means of the image sensor (101), a first image of at least one finger of a user with first acquisition parameters;
b) determine (209), by means of the processing circuit (151), an indicator relating to the first image; And
c) after step a) and before the end of step b), initiate the capture (251, 253, 255), by means of the image sensor (101), of a second image of said at least one user's finger with second acquisition parameters. Device (100) according to claim 1, in which the indicator relating to the first image is a quality indicator of the first image. A device (100) according to claim 2, further configured to continue and terminate capture (251, 253, 255) of the second image if the quality indicator of the first image is outside a range of values for which the first image is considered usable by the processing circuit (151). Device (100) according to claim 3, further configured to, after the end of capturing the second image (251, 253, 255), extract, by means of the processing circuit (151) characteristic data from the second image . Device (100) according to any one of claims 2 to 4, further configured to interrupt the capture (251, 253, 255) of the second image if the quality indicator of the first image is within the range of values for which the first image is considered usable by the processing circuit (151). Device (100) according to claim 5, further configured to, after the end of capturing the first image (201, 203, 205), extract, by means of the processing circuit (151) characteristic data from the first image . Device (100) according to any one of claims 2 to 6, in which the quality indicator is a function of a contrast evaluation of the first image. Device (100) according to claim 1, in which the indicator relating to the first image is a score of correspondence of the first image to one or more reference images. The device (100) of claim 8 further configured to continue and terminate capture (251, 253, 255) of the second image if the match score of the first image is outside a range of values for which the first image is considered to correspond to one of the reference images. Device (100) according to any one of claims 1 to 9, wherein the first and second images respectively have first and second exposure times, the second exposure time being greater than the first exposure time. Device (100) according to any one of claims 1 to 10, wherein the first and second images further respectively have first and second integration gains, the second integration gain being greater than the first integration gain. Device (100) according to any one of claims 1 to 11, wherein the matrix (103) of organic photodetectors (105) comprises at least 200 lines of pixels (PIX). Mobile telephone comprising a device (100) according to any one of claims 1 to 12. Connected watch or bracelet comprising a device (100) according to any one of claims 1 to 12. Method for acquiring fingerprints by a device (100) comprising an image sensor (101), comprising a matrix (103) of organic photodetectors (105), and a processing circuit (151), the method comprising the following steps :
a) capturing (201, 203, 205), by means of the image sensor (101), a first image of at least one finger of a user with first acquisition parameters;
b) measure (209), by means of the processing circuit (151), an indicator relating to the first image; And
c) after step a) and before the end of step b), initiate the capture (251, 253, 255), by means of the image sensor (101), of a second image of said at least one user's finger with second acquisition parameters.