U.S. patent application number 09/878966 was filed with the patent office on 2001-10-04 for fingerprint-reading system.
This patent application is currently assigned to Thomson-CSF. Invention is credited to Mainguet, Jean-Francois.
Application Number | 20010026636 09/878966 |
Document ID | / |
Family ID | 9493060 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010026636 |
Kind Code |
A1 |
Mainguet, Jean-Francois |
October 4, 2001 |
Fingerprint-reading system
Abstract
A fingerprint-reading system comprises a fingerprint sensor
having an active surface sensitive to the pressure and temperature
of a finger. The surface area of this sensor is far smaller than
the surface area of the fingerprint to be read. The reading is done
when the sensor and the finger are in contact and in a relative
motion of sliding of the sensor and the finger with respect to each
other. The system has means to reconstitute a complete image of the
fingerprint from the partial images given by the sensor during this
motion. Application inter alia to devices for the authentication of
individuals.
Inventors: |
Mainguet, Jean-Francois;
(Grenoble, FR) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
Thomson-CSF
Paris
FR
|
Family ID: |
9493060 |
Appl. No.: |
09/878966 |
Filed: |
June 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09878966 |
Jun 13, 2001 |
|
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|
08870002 |
Jun 5, 1997 |
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Current U.S.
Class: |
382/124 |
Current CPC
Class: |
G06V 40/1335 20220101;
G06V 40/1306 20220101 |
Class at
Publication: |
382/124 |
International
Class: |
G06K 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 1996 |
FR |
96-07419 |
Claims
What is claimed is:
1. A fingerprint-reading system comprising means for reading a
fingerprint when the finger and a sensor belonging to the reading
means are in contact and in a relative motion of sliding of the
sensor and the finger with respect to each other and means to
reconstitute an image of the fingerprint from partial images
obtained during this motion.
2. A fingerprint-reading system according to claim 1, wherein the
sensor is fixed to a frame, the relative motion of the finger with
respect to the sensor being done by the sliding of the finger on
the sensor.
3. A fingerprint-reading system according to claim 1, wherein means
are provided to shift the sensor with respect to a surface on which
a finger may be placed, the relative motion of the finger with
respect to the sensor resulting from the sliding of the sensor with
respect to the finger.
4. A fingerprint-reading system according to any of the claims 1 to
3, wherein the sensor is an integrated circuit comprising a matrix
of sensitive elements integrated into a semiconductor substrate
into which there is integrated a multiplexer enabling the
individual measurement of a signal generated in the active layer of
the sensor during the relative shift of the finger and the sensor
with respect to each other.
5. A fingerprint-reading system according to any of the claims 1 to
5, wherein the sensor has an active layer sensitive to pressure
and/or to temperature.
6. A fingerprint-reading system according to claim 5, wherein the
active layer of the integrated circuit is a
pyroelectric/piezoelectric layer enabling the sensing of a matrix
pattern of pressure and/or temperature created by the
fingerprint.
7. A fingerprint-reading system according to any of the claims 1 to
4, wherein the sensitive elements of the sensor are constituted by
capacitive elements enabling the sensing of the matrix pattern of
capacitance created by the lines of the finger.
8. A fingerprint-reading system according to any of the claims 4 to
7, wherein the sensitive element of the sensor is rectangular.
9. A fingerprint-reading system according to any of the claims 1 to
8, wherein the surface area of the sensor is smaller than the
surface area of the fingerprint and delivers only partial images of
the complete fingerprint.
10. A fingerprint-reading system according to any of the claims 1
to 9, wherein the sensor takes the form of a small bar with a
length far smaller than its width.
11. A fingerprint-reading system according to claim 10, wherein the
width of the small bar is substantially equal to that of a
finger.
12. A fingerprint-reading system according to one of the claims 10
and 11, wherein the sensor has an active surface whose width ranges
from about 1 centimeter to 2.5 centimeters and whose length is
smaller than 5 millimeters.
13. A fingerprint-reading system according to any of the claims 1
to 12, wherein the sensor comprises only one row of sensitive
elements.
14. A fingerprint-reading system according to any of the claims 1
to 13 comprising, for the reconstruction of an image of a
fingerprint, an electronic circuit with a microprocessor, a
read-only memory programmed with an algorithm enabling the
reconstruction of the complete image of the fingerprint and the
identification of the individual, and a read-only memory.
15. A fingerprint-reading system according to any of the claims 1
to 14, comprising partial image processing means provided by the
sensor, making it possible in particular to provide contours of
fingerprint ridge lines, the image reconstitution means setting up,
on the basis of these contours, a total fingerprint image in the
form of contours.
16. A fingerprint-reading system according to claims 15, wherein
the partial image-processing means take account of the results
preceding a new search for optimal superimposition between two
successive images to predict the most probable position of
overlapping for the next image by the fact that there is a very
high probability that the relative shift of the finger with respect
to the sensor will be substantially constant.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to systems for the reading of
fingerprints used especially in devices for authenticating
individuals.
[0003] 2. Description of the Prior Art
[0004] The many systems used to authenticate individuals, based on
fingerprint analysis, comprise at least one sensor used to obtain
an image of the fingerprint of the individual to be identified. In
present systems, the finger is placed on the sensor whose reading
surface must necessarily have a size of the order of magnitude of
the size of the finger. The sensor is associated with a system of
analysis used to compare the image of the fingerprint that it gives
with an image of a reference fingerprint stored in an adequate
medium, for example a chip card.
[0005] In most cases, the sensors give an analog type of
information element and the system of analysis makes use of an
operation for the digital processing of the image of the
fingerprint which must be digitized at output of the sensor by
means of an analog-digital converter. In certain embodiments, the
sensor delivers the digitized image directly.
[0006] Fingerprint reading systems are often based on the use of
optical devices such as a video camera picking up the image of the
finger but a simple photograph of the same finger can be used to
obtain the same image at output of the camera and thus defraud the
system. To overcome this drawback, certain systems use prisms or
microprisms in order to ascertain that it is really a genuine
finger and not a photograph that is being placed before the sensor,
the light being reflected only at the places where the lines of the
fingerprint do not touch the prism. A photograph then becomes
inoperative. However, the optical systems cannot be used to
ascertain that the finger that has been placed before the sensor is
truly a live finger and is not for example a mold. The optical
systems have other drawbacks such as for example their great volume
and high production cost.
[0007] Other means have been proposed to make devices for the
authentication of individuals by fingerprints, making use of the
batch-processing possibilities of the semiconductor industry, which
are therefore potentially less costly and provide advantages of the
integration of the sensor and of all or a part of the
data-processing sequence of the authentication device, especially
the operations of image digitizing at output of the sensor, the
storage of the reference image and authentication. The
fingerprint-reading sensor has a matrix of sensitive elements
organized in rows and columns, giving an electric signal that
differs depending on whether a ridge of the fingerprint line
touches or does not touch a sensitive element of the sensor.
[0008] Patents have been filed on various means of reading
fingerprints:
[0009] the U.S. Pat. No. 4,353,056 describes a principle of reading
based on the capacitance of the sensitive elements of the
sensor.
[0010] Other systems comprise sensors having components sensitive
to pressure, temperature or else to pressure and temperature
converting the spatial information of pressure and/or temperature
into an electric signal that is then collected by a
semiconductor-based multiplexer which may for example be a CCD
matrix. The U.S. Pat. No. 4,394,773 describes a principle of this
kind.
[0011] The sensors based on the piezoelectric and/or pyroelectric
effects are the most valuable for they are sensitive to pressure
and/or to heat exerted on their sensitive elements. This feature
makes it possible to ascertain, during the reading of fingerprints,
that the finger is truly part of a living individual through the
inherent heat that it releases. It is also possible to detect the
variations due to the flow of blood in the finger, inducing a
variation of heat and/or pressure, thus providing for greater
reliability in the authentication of the fingerprint.
[0012] These types of sensors, which can be directly integrated
into a semiconductor substrate, have drawbacks that hamper their
entry into the market. The surface area of the sensor necessarily
has the order of magnitude of the size of a finger, namely about
several square centimeters to about ten square centimeters when it
is desired to have the entire first phalanx of the finger which in
this case has to be rolled on the sensor so as to have the entire
fingerprint on the sensor. This reduces the number of individuals
that can be authenticated by means of a silicon wafer. The
efficiency of manufacture of silicon wafers diminishes in
proportion to their surface area and thus considerably increases
the cost of manufacture.
[0013] The electric signal given by the sensors integrated into a
semiconductor substrate is fleeting and a specific system is
necessary to maintain it in time for the electric charges are
induced by variations of the physical effects (temperature,
pressure, etc.) on the sensor. As a consequence, the signal at its
output tends to disappear when the physical effects are balanced.
The time constants of disappearance of the signal are in the range
of some milliseconds to some seconds in favorable cases.
[0014] The practical result of this is that a series of images is
produced starting from the time when the finger is placed on the
sensor. The quality of contrast of these images is not stable and
they tend to fade away. This complicates the task of the
recognition system for it is then necessary to analyze all the
images that are being constantly produced by the sensor in order to
find the one most appropriate for authentication.
[0015] Systems with excitation external to the sensor have been
proposed, for example the sending of an energy beam in the form of
microwaves, but they complicate the system and increase its volume
and cost.
[0016] It is possible to overcome the effects of the disappearance
of the image of the fingerprint by means of an electronic memory.
However this complicates the designing of the sensor and increases
its cost of manufacture for it requires technology that enables
memory storage. It is very difficult to build a system that is
sufficiently precise, reliable and inexpensive, and capable of
deciding which is the best image among those produced by the
sensor.
SUMMARY OF THE INVENTION
[0017] The present invention proposes to overcome the drawbacks of
the prior art by proposing a fingerprint-reading system comprising
means for reading a fingerprint when the finger and a sensor
belonging to the reading means are in contact and in a relative
motion of sliding of the sensor and the finger with respect to each
other and means to reconstitute an image of the fingerprint from
partial images obtained during this motion.
[0018] A sliding of the finger on a sensor fixed to a frame or the
sliding of a mobile sensor on a finger that is held stationary or
more generally the sliding of the finger and of the sensor with
respect to each other stabilizes the quality of the image given by
the sensor. Indeed, when there is a sliding of the finger on the
sensor, the physical variations at each sensitive element of the
sensor are permanent for the lines of the fingerprint touch it
successively with a speed that is in the same range as or faster
than the time constant characteristic of the sensitive layer of the
sensor. The sensor, under these conditions, provides a sequence of
images with a constant quality of contrast.
[0019] Another aspect of this invention lies in the fact that,
inasmuch as a relative sliding of the finger on the sensor is done,
it is possible to reduce the size of the sensor to dimensions
smaller than the size of the finger. For example, assuming that the
finger shifts on the sensor in the direction of its length, the
length of the sensor may be reduced and will no longer cover more
than a small surface area of the fingerprint. In this case, the
electric signals given by the sensor during a relative sliding of
the finger on the sensor correspond to a succession of partial
images of the fingerprint and inasmuch as the relative speed of
shift of the finger with respect to the sensor does not exceed a
certain maximum value, an image given by the sensor at a given
instant will at least partially overlap the following one. The
complete image of the fingerprint could be reconstituted by a
specific processing system.
[0020] The reduction of the size of the sensor and hence its
surface area will have the consequence of providing a major
reduction of its cost of manufacture.
[0021] The invention proposes a sensor belonging to the
fingerprint-reading means wherein the surface area of the sensor is
smaller than the surface area of the fingerprint and delivers only
partial images of the complete fingerprint. The reconstruction of
the complete image of the fingerprint is obtained by the
superimposition of successive images given by the sensor during its
relative shift with respect to the finger.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Other features of the invention shall appear from the
detailed description of the following embodiments, this description
being made with reference to the appended drawings, of which:
[0023] FIG. 1 shows a general view of the fingerprint sensor;
[0024] FIG. 2 shows the use of the fingerprint sensor;
[0025] FIG. 3 shows a schematic sectional view of the constitution
of the sensor;
[0026] FIG. 4 shows a block diagram of an exemplary embodiment of a
fingerprint reading system according to the invention;
[0027] FIG. 5 shows five relative positions of the sensor of the
finger at the time of the reading of the fingerprint;
[0028] FIGS. 6 and 7 show two consecutive images at output of the
sensor;
[0029] FIGS. 8, 9 and 10 show tests of the overlapping of two
successive images at the output of the sensor;
[0030] FIGS. 11 and 12 show two steps of the reconstitution of the
complete image of a fingerprint.
MORE DETAILED DESCRIPTION
[0031] FIG. 1 shows a general view of an exemplary embodiment of a
fingerprint sensor according to the invention. The fingerprint
sensor 10 is an integrated circuit having the shape of a small bar
with a width that substantially equal to that of a finger 11. For
example its width is 1 or 2 centimeters. However its length is far
smaller than its width. For example its length is some millimeters.
The fingerprint sensor 10 partially covers the fingerprint to be
read. The sensor is contained in a support 12 comprising external
connection pins 13.
[0032] In one embodiment, the integrated circuit consists of an
active layer of pyroelectric/piezoelectric material placed between
an upper electrode and a matrix array of lower electrodes. The
lower electrodes lie on a semiconductor substrate in which there is
formed an integrated electronic circuit capable of processing the
electric charges generated by the pyroelectric/piezoelectric layer
on each of the electrodes of the array. This integrated electronic
circuit is connected to external connection pins which can transmit
electric signals, all of which represent an image of a pattern of
pressure exerted on the active layer. The constitution of the lower
electrodes in the form of a matrix array enables the making of an
array of individual pyroelectric/piezoelectric sensitive elements
even if the pyroelectric/piezoelectric layer is continuous. The
matrix array of sensitive elements is organized in rows and
columns.
[0033] The sensitive elements of the sensor are generally
square-shaped. The sensitivity of the sensitive elements is
proportional to their surface area. It is possible to increase the
sensitivity of the sensitive elements by increasing their surface
area. This can be done for example, while keeping the same width of
sensitive element, by increasing its length in the direction of
relative shift of the finger with respect to the sensor. For
example, in the case of a relative shift of the finger with respect
to the sensor in the direction of the columns of the matrix of
sensitive elements, it is possible in practice to double their
sensitivity by making rectangular sensitive elements whose length
in the direction of the columns is twice their width in the
direction of the rows of the matrix of sensitive elements. This has
the advantage of increasing the quality of definition and contrast
of the images given by the sensor.
[0034] FIG. 2 shows the finger 11 when it is pressed on the active
surface of the integrated circuit at a given point in time during
its relative shift on the sensor 10. A pressure pattern is
generated in the pyroelectric and piezoelectric layer and this
pattern is detected by the matrix array. The detection is done in
the form of a measurement of variation of charges generated in the
different pyroelectric/piezoelectri- c elements of the array. These
variations of charges are obtained on the lower electrodes of the
array. The electric signals given by the sensor correspond to an
image of the patterns of pressure and temperature applied to the
active surface of the sensor at a given instant. If these signals
were to be used to display this image at a given instant, an image
would be observed, representing a part of the imprint of the finger
pressing on the sensor at a given point in time during its relative
shift on the sensor.
[0035] In another embodiment of the invention, the sensitive
elements of the matrix of the sensor are formed by capacitive
elements used to pick up the matrix pattern of capacitance created
by the ridges and hollows of the finger sliding on the surface of
the sensor. The matrix pattern of the capacitance is converted by
the sensor into electric signals which, as in the case of the
previous embodiment, correspond to a part of the finger at a given
point in time in its relative shift on the sensor.
[0036] In order to reduce the cost of the system, it would be
possible to use a sensor comprising only one row of sensitive
elements and carry out a relative shift of the finger in a
direction substantially perpendicular to the row of sensitive
elements. However, in this case, it would be necessary to have
precise knowledge of the speed of relative shift of the finger with
respect to the sensor at all times during the shift in order to
obtain an undistorted reconstitution of the complete image of the
fingerprint. One way to reconstitute the image without distortion
would be to lay down the relative speed of shift of the sensor with
respect to the finger, for example by using a sensor drawn by a
servo-controlled motor, with the finger being held stationary.
[0037] In a very low-cost fingerprint authentication system
according to the invention, it would be possible to use a sensor
with only one row of sensitive elements, and without any knowledge
by the system or any imposition by the system of the speed of
relative shift of the finger on the sensor. Indeed, although the
fingerprint cannot be reconstituted in its exact shape, it could be
authenticated by means of an appropriate image-processing
algorithm.
[0038] In order to overcome these constraints, the sensor must have
several rows of sensitive elements used for the reconstitution, by
the reading system, of the complete image of the fingerprint.
Preferably, the number of rows of the sensor will be as small as
possible in order to obtain a sensor with a very small surface area
and therefore at low cost.
[0039] The minimum number of rows needed for the sensor depends
on:
[0040] the size of the sensitive elements of the sensor
(pixels),
[0041] the relative speed of the finger with respect to the
sensor,
[0042] the number of images per second that can be delivered by the
sensor for it is absolutely necessary to have sufficient
overlapping between two successive images,
[0043] the efficiency of the algorithm for the processing of the
partial images coming from the sensor, enabling full reconstitution
of the image of the fingerprint.
[0044] There should be overlapping by at least one row between two
successive images given by the sensor but, in practice, overlapping
by about five to six rows appears to be necessary in order to
overcome certain defects of the sensor and make the system more
tolerant to losses of image quality, given that the average
distance between two consecutive lines of the fingerprint is about
120 micrometers. The sensor must have a number of rows sufficient
to enable the reconstitution, without excessive difficulty, of the
complete image of the fingerprint. The number of rows may be
established as follows:
[0045] Let it be assumed that the distance between two consecutive
sensitive elements is about 50 micrometers and that the width of
the active zone of the sensor is 2.5 centimeters. Each row of the
sensor will have 500 sensitive elements. Taking a sensor with 40
rows (namely a sensor length of 2 millimeters), the total number of
sensitive elements to be read will be 20,000. Should the reading
speed be limited to one million sensitive elements per second, the
sensor will give 50 images per second. If we take a value of
overlapping on the length of the images equal to 10 sensitive
elements, namely 10 rows, then the maximum shift of the finger
between two consecutive images should not exceed 30 sensitive
elements between two images, giving 1500 micrometers in 20
milliseconds, or 7.5 centimeters per second. This is a reasonable
speed for the relative shift of the finger with respect to the
sensor.
[0046] The reduction of the number of rows of the sensor gives more
images per second for one and the same speed of reading of
sensitive elements per second, but the maximum distance that can be
travelled by the finger on the sensor is reduced proportionately.
It is necessary rather to increase the frequency of reading of the
sensitive elements to enable the acceptance of greater speeds of
relative shift of the finger on the sensor.
[0047] The dimensions of the active surface of the sensor will
preferably range from 1 centimeter to 2.5 centimeters for the width
and will be less than 5 millimeters for the length.
[0048] It is possible, although this would make the electronic
processing more complex, to consider the use of a sensor with a
width far smaller than that of a finger provided that the finger is
made to pass several times over the sensor (or the sensor is made
to pass several times over the finger) to cover the entire desired
surface of the fingerprint to be read. This makes it possible to
have a small-sized sensor, hence one that is less costly to
make.
[0049] Systems for authenticating individuals by their fingerprints
in practice always comprise a system for the digital processing of
images in order to authenticate the individuals. The simplest
embodiment consists in incorporating the image reconstruction
algorithm into the system comprising the authentication
algorithm.
[0050] One possible approach lies in the integration, on the same
substrate, of the sensor of the analog-digital converter which
digitizes the image and sends the resultant data to a
microprocessor comprising a read-only memory containing the
reconstruction algorithm and a random-access memory containing the
image reconstructed at the end of processing. This image will then
be processed in a device of the system performing the
identification.
[0051] These various approaches proposed are not exhaustive and
other solutions of integration are possible depending on the
possibilities provided by semiconductor technologies.
[0052] FIG. 3 gives a schematic view of an exemplary integrated
circuit constituting the fingerprint sensor according to the
invention.
[0053] The integrated circuit is formed by a semiconductor
substrate 20 which in principle is a silicon substrate. In this
substrate there are formed circuits 22 for the reading and
processing of electric charges. These circuits are for example CCD
(charge-coupled devices working by charge transfer) circuits or
C-MOS circuits. They are made according to the standard
technologies for the manufacture of integrated circuits made of
silicon. The circuits are formed in an array as a function of the
matrix pattern of piezoelectric elements which will be formed
subsequently.
[0054] All the signal reading and processing circuits are covered,
in principle, with a planarization layer 24 which, for example, is
a polyimide layer with a thickness of some micrometers, deposited
by spin-coating.
[0055] The planarization layer 24 is etched periodically, as a
function of the pattern of piezoelectric elements that will be
formed, to make apertures 26 by which the individual piezoelectric
elements can each be connected to a respective charge-reading
circuit of the silicon substrate.
[0056] An array of lower electrodes 28 is formed on the
planarization layer. Each electrode comes into contact, through a
respective aperture 26, with a charge-reading circuit of the
silicon substrate.
[0057] An active piezoelectric layer 30 is deposited on the
substrate thus covered with an array of electrodes. This layer is
preferably a layer of pyroelectric polymer material and it may be
continuous. This layer is relatively flexible (made of a plastic
polymer material). It is covered with a continuous upper electrode
32. Thus, there is defined an array of piezoelectric elements each
formed by a lower electrode 28, the portion of piezoelectric layer
30 located just above it and the portion of upper electrode 32 that
covers it. The electric charges generated by a pressure exerted
locally on this element are read by the corresponding read circuit,
electrically connected to the corresponding lower electrode through
an aperture 26.
[0058] A protection layer 34, for example a polyimide layer with a
thickness of about 10 micrometers, is deposited above the upper
electrode 32. This protection layer must be both rigid enough and
flexible enough to transmit, vertically and without modification,
the pattern of pressures that is exerted on it (the finger being
pressed directly on this layer).
[0059] The electronic circuits of the substrate 20 are connected to
the exterior by means of pads of contacts (not shown), located on
the surface of the circuit.
[0060] The material of the pyroelectric/piezoelectric layer may for
example be a polyvinylidene fluoride (PVDF), polyvinylidene
fluoride--trifluoroethylene (PVDF-TrFE), polyvinylidene
cyanide--vinyl acetate (PVDCN-VAc) or polyvinylidene
cyanide--vinylidene fluoride (PVDCN-VDF). Other sensitive layers
are possible, especially those producing electric charges as a
function of a physical parameter.
[0061] In the case of the above-mentioned copolymers, the main
effect used is that of the generation of the electric charges
induced by the variation in temperature and/or pressure of the
copolymer. This variation in temperature and/or pressure is induced
by the contact of the ridges of the lines of the fingerprint with
the surface of the sensor, generally constituted by a thin
protective layer some tens of micrometers thick, preventing
excessive lateral heat dissipation, deposited on an array of
electrodes connected to the multiplexing circuit.
[0062] Hereinafter, a description shall be given of an exemplary
embodiment of a system according to the invention comprising a
sensor with a surface area far smaller than the surface area of the
fingerprint to be read, and having a length (the number of matrix
rows of the sensor) far smaller than its width (the length of the
rows of the sensor), the width of the sensor in this example being
at least equal to the width of the finger for which the
corresponding fingerprint is to be read.
[0063] FIG. 4 shows a block diagram of a system comprising a sensor
50 on a semiconductor substrate, having an analog/digital converter
51 integrated into the same substrate and giving digitized partial
images of the fingerprint 52, for example of a finger 53, at
successive points in time during a relative shift of the finger 53
on the sensor 50. The digitized partial images are presented to the
processing inputs 55 of a microprocessor 60 comprising a
random-access memory 61 and a read-only memory 63 containing a
processing algorithm that enables the reconstruction of the
complete image of the fingerprint 52 of the finger 53 and the
authentication of this fingerprint.
[0064] A description shall be given of the system represented by
the block diagram of FIG. 4.
[0065] Let us consider the finger 53 and its fingerprint 52, shown
in FIG. 5. The finger 53 slides on the sensor perpendicularly to
the rows of the matrix of sensitive elements of the sensor, in the
direction V. The different positions at the instants t0, t1, t2, .
. . , tn of the active window of the sensor during its relative
shift with respect to the finger 53 are shown in dashes. The sensor
generates the successive images I0, I1, I2, . . . , In at the
respective instants t0, t1, 15 t2, . . . , tn and the speed of
relative shift of the finger on the sensor is such that at least
one image partially overlaps the next one. For example I0 partially
overlaps I1, I1 partially overlaps I2 and so on and so forth.
[0066] To give a clearer view of the relative motion of the finger
53 with respect to the sensor 50 in FIG. 5, the finger 53 is shown
as being stationary and the sensor 50 is shown as being mobile with
respect to the finger. The working of the system would be the same
in the case of a mobile finger and a stationary sensor or more
generally a mobile finger sliding on a mobile sensor. The parameter
to be considered is the relative motion of the finger and of the
sensor with respect to each other, in a direction substantially
perpendicular to the width of the sensor.
[0067] Let the initial instant t0 be taken as the instant of
reading of the first partial image I0 of the fingerprint 52. FIG. 6
shows the first partial image I0 of the fingerprint 52 given by the
sensor at the instant t0 and FIG. 7 shows the second partial image
I0 of this fingerprint 52 given by the sensor at the instant
following t1.
[0068] The images I0, I1, I2, . . . , In are transmitted to the
processing inputs 53 of the microprocessor 60 and stored in the
random-access memory 61. The algorithm located in the read-only
memory 63 performs operations for the processing of the images
stored in the random-access memory 61. These operations consist in
successively trying out all the possible cases of overlapping
between the images I0 and I1 and in assigning a correlation
coefficient to each trial. The best correlation coefficient will
inform the system of the optimum position of overlapping of the two
images I0 and I1, and the operation will be recommenced with the
next image I2 given by the sensor 50 to the microprocessor 60 and
so on and so forth until the fingerprint is completely
reconstituted.
[0069] Various strategies of correlation may be used in order to
reconstitute the complete image of the fingerprint from the
successive partial images of this very same fingerprint. For
example, one correlation strategy consists in comparing the levels
of all the sensitive elements of each of the first two successive
images I0 and I1 for each possible case of overlapping of two
images.
[0070] FIG. 8 shows a first trial performed by the processing
algorithm of the system in a first position P1 of superimposition
of the two images I0 and I1 on a zone Z0 common to the two images.
The processing system compares the levels of sensitive elements of
each image I0 and I1 located at the same points of the common zone
Z0. If the number of sensitive elements having substantially
identical levels is smaller than a predetermined value, the system
modifies the position of superimposition of the two images into a
following position P2 (shown in FIG. 9) corresponding to a new zone
of superimposition Z1 of the images I0 and I1. The system carries
out a new comparison of the levels of the sensitive elements of the
two images I0 and I1 in the zone Z1. It continues to operate in
this fashion for the following positions P3, . . . , Pn of the two
images (shown in FIG. 10) until the number of sensitive elements
with substantially identical levels located at the same points in a
common overlapping zone Zn of the two images I0 and I1 is greater
than a predetermined value corresponding to a probable identity of
the zones Zn of the respective images I0 and I1 in the position
Pn.
[0071] A resultant image Ir1, shown in FIG. 10, of the two images
I0 and I1 could be an image coming from a weighting between the two
images I0 and I1 in their optimum overlapping position Pn, enabling
an improvement in the quality of the image resulting from
superimposition. The image Ir1 is kept in the random-access memory
of the microprocessor for the rest of the processing operation.
[0072] The next image I2 shown in FIG. 11 at the instant t2 at
output of the sensor 50 is transmitted to the microprocessor 60.
This image I2 is, in turn, compared to the resultant image Ir1 in
the same way as here above enabling the obtaining of an image Ir2,
shown in FIG. 11, resulting from the superimposition of I0, I1 and
I2 in their optimum overlapping position. The process is repeated
in the same way until the complete image Irn of the fingerprint 52,
as shown in FIG. 12, is obtained.
[0073] The processing algorithm of the system could take account of
the results preceding a new search for optimal superimposition
between two successive images to predict the most probable position
of overlapping for the next image by the fact that there is a very
high probability that the relative shift of the finger with respect
to the sensor will be substantially constant. This considerably
accelerates the speed of processing and reconstruction of the
complete image Irn of the fingerprint by avoiding unnecessary
computations.
[0074] The exemplary reconstruction of the complete image is not
exhaustive and other strategies of reconstruction of the complete
fingerprint may be envisaged.
[0075] In particular, here above, it has been assumed for
simplicity's sake that the image of the fingerprint is
reconstituted dot by dot out of partial images that are also
obtained dot by dot. However, in view of the fact that these images
must subsequently be used for identification and that this
identification will generally be done by shape-recognition
algorithms that may use processing operations for the extraction of
contours, operations for vectorizing these contours etc., it is
also possible to envisage a case where the image reconstitution is
directly done in the form of sets of contour lines or vectors
representing these contours. The useful image of a fingerprint is
indeed a set of contours corresponding to the ridges of the lines
of this fingerprint. For authentication, the sets of contours
detected are compared with sets of pre-recorded contours
corresponding to an individual whose identity is to be
authenticated. The sets of contours could then be stored in the
form of tables of vectors describing these contours.
[0076] It is then possible to carry out a contour extraction
processing operation and/or a vectorization processing operation
directly on a partial image and then perform correlations on the
contours of successive vectors of partial images to assemble the
partial images together and establish a complete image directly in
the form of sets of contours or sets of vectors.
[0077] This solution makes it possible to avoid a dot-by-dot
reconstitution of an image when this image would in any case have
to be converted into a set of contours.
[0078] In other embodiments, the width of the sensor may be smaller
than the width of the finger, thus further reducing its surface
area. It would be enough then to scan the entire fingerprint at
appropriate speed, with the system performing the reconstitution of
the complete image.
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