Patent Description:
Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.

In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance and user acceptance.

Fingerprint sensors can sometimes get activated prematurely, before the finger has made proper contact with the fingerprint sensor, or unintentionally, by a finger or other body part making contact with the fingerprint sensor by mistake, unnecessarily using up power and processing resources. It is preferable that a fingerprint sensor is only activated when a finger makes proper contact with it.

<CIT> (<NUM>-<NUM>-<NUM>), discloses a system for determining if a finger is in stable contact with the sensor.

<CIT> (<NUM>-<NUM>-<NUM>), discloses a fingerprint reading system where overlapping fingerprints are used.

<CIT> discloses a method using electric field sensors for determining whether a sufficient part of the fingerprint sensor is covered by a finger, and whether the finger is in stable contact. It is determined whether a threshold number of subarrays from at least three of five regions of the fingerprint sensor have acquired finger stability data indicative of a finger. Then it is determined whether the finger is stable based upon whether the threshold number of sub-arrays indicates stability over successive data acquisitions.

The invention is defined by the set of appended claims. It is an objective of the present invention to provide an improved way of determining whether a fingerprint topology of a finger is in stable contact with a surface covering a sensor area of a fingerprint sensor.

It has now been realised that it may be desirable to further ensure that the fingerprint topography of a finger, typically the ridges thereof, is in contact with the detection surface of the fingerprint sensor over the whole sensor area, and that said contact is stable over time, before triggering e.g. an authentication operation to verify that the fingerprint topography of the finger, as sensed by the fingerprint sensor, corresponds with a stored representation of an enrolled fingerprint, indicating that the user to whom the finger belongs is authorised to e.g. interact in some preprogramed ways with an electronic device comprising the fingerprint sensor.

According to an aspect of the present invention, there is provided a method of determining that a finger is in stable contact with a surface covering a sensor area of a fingerprint sensor. The method comprises, on the surface of the fingerprint sensor, receiving the finger having a fingerprint topography. The method also comprises, by means of the fingerprint sensor, acquiring a time-sequence of images of the fingerprint of the received finger, said time-sequence comprising at least a first image taken at a first time point and a second image taken at a second time point which is after the first time point. The method also comprises, for each image of the time-sequence, dividing an image area of the image, corresponding to the sensor area of the fingerprint sensor, into a plurality of image regions, said regions partly overlapping each other and covering the whole image area. The method also comprises, based on image analysis of each of the plurality of image regions of each image of the time-sequence, determining that the finger is in stable contact with the surface covering the sensor area.

According to another aspect of the present invention, there is provided a computer program product comprising computer-executable components for causing a fingerprint sensing system to perform the method of any preceding claim when the computer-executable components are run on processing circuitry comprised in the fingerprint sensing system.

According to another aspect of the present invention, there is provided a fingerprint sensing system comprising a fingerprint sensor, processing circuitry, and data storage storing instructions executable by said processing circuitry whereby said fingerprint sensing system is operative to, on the surface of the fingerprint sensor, receive a finger having a fingerprint topography. The fingerprint sensing system is also operative to, by means of the fingerprint sensor, acquire a time-sequence of images of the fingerprint of the received finger, said time-sequence comprising at least a first image taken at a first time point and a second image taken at a second time point which is after the first time point. The fingerprint sensing system is also operative to, for each image of the time-sequence, divide an image area of the image, corresponding to a sensor area of the fingerprint sensor, into a plurality of image regions, said regions partly overlapping each other and covering the whole image area. The fingerprint sensing system is also operative to, based on image analysis of each of the plurality of image regions of each image of the time-sequence, determine that the finger is in stable contact with the surface covering the sensor area.

According to another aspect of the present invention, there is provided an electronic device comprising an embodiment of the fingerprint sensing system of the present disclosure, and a device control unit configured to interact with the fingerprint sensing system.

If it is only checked that the fingerprint topography is only in stable contact with a few separate parts of the detection surface of the sensor area, nothing is known about whether the finger is also in stable contact with the detection surface of the sensor area between said parts. By dividing the image area, corresponding to the sensor area, into image regions which together cover the whole image area, the risk of not detecting non-contacted parts of the sensor area detection surface is reduced. However, a non-contacted part of the sensor area which extends over two adjacent, but not overlapping, image regions of the corresponding image area may still not trigger detection of the non-contacted part since the non-contacted part is divided between two or more regions, each only being affected to a relatively small degree. The solution of the present invention is to use overlapping regions, whereby there is increased probability that at least one of the regions is affected to such a degree that detection of the non-contacted part is made by image analysis. By analysing a time-sequence of a plurality of images, it is possible to determine whether the contact made by the fingerprint topography on the detection surface is stable over time.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects.

<FIG> shows an electronic device <NUM>, here in the form of mobile phone, e.g. smartphone, comprising a display <NUM> of a display stack <NUM>, e.g. comprising touch functionality (i.e. a touch display <NUM>) and a fingerprint sensor <NUM>. The fingerprint sensor <NUM> comprises fingerprint sensor circuitry, e.g. for outputting a grey-scale image or the like where different intensities in the image indicate the contact between a detection surface of the fingerprint sensor <NUM> and a finger <NUM> placed there on, e.g. as part of fingerprint authentication or navigation using the fingerprint sensor.

The fingerprint sensor <NUM> may operate according to any sensing technology. For instance, the finger print sensor may be a capacitive, optical or ultrasonic sensor. Herein, a capacitive fingerprint sensor, which may be preferred for some applications, is discussed as an example. The fingerprint sensor may comprise a two-dimensional array of fingerprint sensing elements, each corresponding to a pixel of the image outputted by the fingerprint sensor, the pixel e.g. being represented by a grey-scale value. The fingerprint sensor may be located at a side of the display stack <NUM>, outside of the display area of the display <NUM>, as shown in <FIG>. The outputted image is in the form of a two-dimensional or one-dimensional pixel array, e.g. of grey-scale values. Each image pixel may provide an image intensity, be it of a grey-scale value or other value. For example, for a capacitive fingerprint sensor, a high pixel intensity (e.g. white in grey-scale) implies low capacitive coupling and thus a large sensed distance between the detection surface and the fingerprint topography. A high pixel intensity may result because the finger does not cover the part of the detection surface where the sensing element corresponding to the pixel is located. Conversely, a low pixel intensity (e.g. black in grey-scale) implies high capacitive coupling and thus a small sensed distance between the detection surface and the fingerprint topography. A high pixel intensity may result because the corresponding sensing element is located at a ridge of the fingerprint topography. An intermediate pixel intensity may indicate that the sensing element is covered by the finger topology but is located at a valley of the fingerprint topography.

Referring to the block diagram in <FIG>, the electronic device <NUM> in <FIG> comprises the display stack <NUM> comprising a touch sensor <NUM> and the display <NUM>. The electronic device also comprises a fingerprint sensing system <NUM> comprising the fingerprint sensor <NUM>, fingerprint image acquisition circuitry <NUM> and image processing circuitry <NUM>. Further, the electronic device <NUM> comprises a data storage <NUM>, e.g. in the form of a memory, which maybe shared between different components of the electronic device, such as the fingerprint sensing system <NUM>. The data storage <NUM> holds software <NUM>, in the form of computer-executable components corresponding to instructions, e.g. for the fingerprint sensing system <NUM>. Thus, the data storage <NUM> may functionally at least partly be comprised in the fingerprint sensing system <NUM>.

Accordingly (see also <FIG>), embodiments of the fingerprint sensing system <NUM> comprises a fingerprint sensor <NUM>, processing circuitry <NUM>, and data storage <NUM> storing instructions <NUM> executable by said processing circuitry whereby said fingerprint sensing system is operative to, on the surface of the fingerprint sensor, receive a finger <NUM> having a fingerprint topography. The fingerprint sensing system is also operative to, by means of the fingerprint sensor, acquire a time-sequence <NUM> of images n. m of the fingerprint of the received finger, said time-sequence comprising at least a first image n taken at a first time point t1 and a second image n+<NUM> taken at a second time point t2 which is after the first time point. The fingerprint sensing system is also operative to, for each image of the time-sequence, divide an image area <NUM> of the image, corresponding to a sensor area <NUM> of the fingerprint sensor, into a plurality of image regions r, said regions partly overlapping each other and covering the whole image area. The fingerprint sensing system is also operative to, based on image analysis of each of the plurality of image regions of each image of the time-sequence, determine that the finger is in stable contact with the surface covering the sensor area.

In some embodiments, the fingerprint sensor <NUM> is a capacitive, ultrasonic or optical fingerprint sensor, e.g. a capacitive fingerprint sensor.

In some embodiments, the fingerprint sensor <NUM> is covered by a glass layer, e.g. by means of a cover glass or a glass coating, e.g. protecting the sensing elements and providing the detection surface of the fingerprint sensor.

The data storage <NUM> may be regarded as a computer program product <NUM> comprising computer-executable components <NUM> for causing the fingerprint sensing system <NUM> to perform an embodiment of the method of the present disclosure when the computer-executable components are run on processing circuitry <NUM> comprised in the fingerprint sensing system. Additionally, any mobile or external data storage means, such as a disc, memory stick or server may be regarded as such a computer program product.

The electronic device also comprises a device control unit <NUM> configured to control the electronic device <NUM> and to interact with the fingerprint sensing system <NUM>. The electronic device also comprises a battery <NUM> for providing electrical energy to the various components of the electronic device <NUM>. Although not shown in <FIG>, the electronic device may comprise further components depending on application. For instance, the electronic device <NUM> may comprise circuitry for wireless communication, circuitry for voice communication, a keyboard etc..

The electronic device <NUM> may be any electrical device or user equipment (UE), mobile or stationary, e.g. enabled to communicate over a radio channel in a communication network, for instance but not limited to e.g. mobile phone, tablet computer, laptop computer or desktop computer.

The electronic device <NUM> may thus comprise an embodiment of the fingerprint sensing system <NUM> discussed herein, and a device control unit <NUM> configured to interact with the fingerprint sensing system.

In some embodiments, the device control unit <NUM> is configured to interact with the fingerprint sensing system <NUM> to authenticate a user based on a fingerprint representation and perform at least one action in response to said authentication.

As a finger <NUM> contacts a detection surface of the fingerprint sensor <NUM>, the fingerprint sensor is activated to by means of the fingerprint image acquisition circuitry <NUM> acquire an image n, or a time-sequence <NUM> of images n to m (herein also denoted n. m) as illustrated in <FIG>. Such a time-sequence <NUM> may comprise at least a first image n, taken at a first time point t1, and a second image n+<NUM>, taken at a second time point t2 which is in time domain after the first time point t1. Embodiments of the present invention may be applied to any at least two, e.g. the first and the second, of the images n. m, e.g. n, n+<NUM>, n+<NUM>, m-<NUM>, m-<NUM> or m as shown in <FIG>. Typically, a plurality of serial images are analysed to determine that the fingerprint contact is stable over time.

<FIG> illustrates how an image area <NUM> corresponds to a sensor area <NUM> of the detection surface of the fingerprint sensor <NUM>. The image area <NUM> may be the whole or a part of the images n. m, and may correspond to the whole or a part of the detection surface of the fingerprint detector <NUM>. In the example of <FIG>, the sensor area <NUM> is a sub-area of the detection surface, why the image area <NUM> comprises only pixels from a subgroup of the sensing elements of the fingerprint sensor, typically those sensing elements positioned right underneath the sensor area <NUM>.

<FIG> shows an example of an image area <NUM> which has been schematically divided into a plurality of image regions r. To not clutter the figure, only a few of the image regions which the image area is divided into are shown. Coordinates of each image region r is given in the upper left corner of each image region. In the example of the figure, each image region is square, i.e. of 8x8 pixels, but any pixel ratio or number of pixels of each region r is possible. It may be convenient that all regions r are of the same size, but using different size regions may also be desirable in some embodiments. In the figure, the image area is divided into a total of <NUM> (<NUM>×<NUM>) image regions, with coordinates of <NUM> to <NUM> in each dimension, but any number of regions may be used, e.g. <NUM> (<NUM>×<NUM>) or <NUM> (<NUM>×<NUM>) or more regions. The regions r are overlapping. Each region r overlaps with neighbouring regions in both dimensions of a two-dimensional image area <NUM>, e.g. by at least <NUM>, <NUM>, <NUM> or <NUM>%. In the example of <FIG>, each image region r overlaps by <NUM>% with each of its closest neighbour images, i.e. having integer coordinates in any one of the two dimensions which is higher or lower by one.

An advantage with overlapping regions r is illustrated by means of <FIG>, in which three of the regions r of <FIG> are shown. A high-intensity part <NUM> of the image area <NUM>, corresponding to a non-contacted part of the surface of the sensor area <NUM>, is divided between the two non-overlapping regions r having the coordinates <NUM>,<NUM> and <NUM>,<NUM>, respectively. Each of these two non-overlapping regions may only to a lesser degree comprise the high-intensity part <NUM>, why an average intensity value of the region may not be affected by the high-intensity part to such a degree that detection of the high-intensity part, and thus the non-contacted part, is triggered. However, by using a third region r, having the coordinates <NUM>,<NUM> in this example, which overlaps both of the <NUM>,<NUM> and <NUM>,<NUM> regions, here by <NUM>% each, this third region may to a larger degree comprise the high-intensity part and be affected by the high-intensity part to such a degree that detection of the high-intensity part, and thus the non-contacted part, is triggered.

<FIG> shows a time-sequence <NUM> of grey-scale <NUM>×<NUM> pixel images n to n+<NUM> from a capacitive fingerprint sensor <NUM> after activation. The sequence <NUM> of <FIG> illustrates how a finger <NUM> is only partly in stable contact with the detection surface, in this case with the middle part of the detection surface, in the first images while being in stable contact over the whole sensor area in the last images. The sequence of <FIG> appears to show a finger making contact first in the middle of the sensor area and then then also at its outer edges as the finger is more firmly pressed down against the detection surface.

<FIG> is a flow chart of an embodiment of the method of the present invention. On a detection surface of the fingerprint sensor <NUM>, a finger having a fingerprint topography is received S1. Then, by means of the fingerprint sensor <NUM>, a time-sequence <NUM> of images n. m of the fingerprint of the received finger is acquired S2. The time-sequence comprises at least a first image n taken at a first time point t1 and a second image n+<NUM> taken at a second time point t2 which is after the first time point. Then, for each image of the time-sequence <NUM>, an image area <NUM> of the image, corresponding to the sensor area <NUM> of the fingerprint sensor <NUM>, is divided S3 into a plurality of image regions r. The regions r are partly overlapping each other and together cover the whole image area <NUM>. Based on image analysis of each of the plurality of image regions r of each image n. m of the time-sequence <NUM>, it is then determined S4 that the finger <NUM>, i.e. the ridges of the fingerprint topography, is in stable physical contact with the surface covering the sensor area, preferably over the whole sensor area <NUM>.

In some embodiments, after the determining S4 that the finger <NUM> is in stable contact with the detection surface, an authentication operation is performed S5, including acquiring a fingerprint image of the received S1 finger and comparing said acquired fingerprint image with a stored representation of an enrolled fingerprint. As mentioned above, an authentication operation may be performed to verify that the fingerprint topography of the finger <NUM>, as sensed by the fingerprint sensor <NUM>, corresponds with a stored representation of an enrolled fingerprint, e.g. in the data storage <NUM>, indicating that the user to whom the finger <NUM> belongs is authorised to e.g. interact in some preprogramed ways with the electronic device <NUM> comprising the fingerprint sensor.

Herein, embodiments of the present invention are exemplified by a time-sequence <NUM> comprising first and second images n and n+<NUM>, comparing said first and second images to determine whether the finger contact is stable. However, the time-sequence <NUM> may comprise any number of images, e.g. within the range of <NUM>-<NUM> images, and comparison may be performed between any of said images, e.g. between a first image n and a last image m of the sequence <NUM>, or between a second image n+<NUM> and third image n+<NUM> e.g. if it has already been determined that the finger was not in stable contact between the first and second images n and n+<NUM>.

In some embodiments, the determining S4 that the finger is in stable contact is based on an average change in intensity imean,diff in the image area <NUM> between the first and second images n and n+<NUM>, and on a change in intensity ir,mean,diff between the first and second images for each of the image regions r. The average intensity value imean of the image area <NUM> may e.g. be an average grey-scale value of all pixels in the image area, or of groups of pixels in the image area. Thus, imean,diff may e.g. be the imean of image n (in,mean) subtracted by the imean of image n+<NUM> (in+<NUM>,mean). The average intensity value ir,mean of each region r may similarly be of grey-scale values of all pixels in the image region, or of groups of pixels in the image region. Thus, ir,mean,diff may e.g. be the ir,mean of image n (in,r,mean) subtracted by the ir,mean of image n+<NUM> (in+<NUM>,r,mean).

Additionally or alternatively, in some embodiments, the determining S4 that the finger <NUM> is in stable contact is based on a maximum intensity value in+<NUM>,max in the image area <NUM> of the second image n+<NUM>, and on a maximum value ir,mean,diff,max from changes in intensity ir,mean,diff between the first and second images n and n+<NUM> for each of the image regions r. The maximum intensity value in+<NUM>,max in the image area <NUM> may then e.g. be the highest grey-scale value of any pixel, or group of pixels, in the image area <NUM> of the second image n+<NUM>. As above, the change in intensity ir,mean,diff between the first and second images n and n+<NUM> for each of the regions r may be calculated as ir,mean of image n (in,r,mean) subtracted by the ir,mean of image n+<NUM> (in+<NUM>,r,mean). Then, the maximum value ir,mean,diff,max may be determined as e.g. the highest grey-scale value exhibited by the ir,mean,diff of any one of the regions r.

<FIG> is a flow chart illustrating an example of how it can be determined S4 that the finger is in stable contact with the detection surface covering the sensor area <NUM>.

The image maximum intensity value in+<NUM>,max in the image area <NUM> of the second image n+<NUM> is determined S41.

For each of the first and second images n and n+<NUM>, the image average intensity value in,mean and in+<NUM>,mean of the image area <NUM> is determined S42. Then, for each of the first and second images n and n+<NUM>, the change imean,diff of the image average intensity value in,mean and in+<NUM>,mean between the first and second images is calculated S43, e.g. as the imean of image n (in,mean) subtracted by the imean of image n+<NUM> (in+<NUM>,mean) as discussed above.

For each of the regions r, of each of the first and second images n and n+<NUM>, the region average intensity value in,r,mean and in+<NUM>,r,mean is determined S44.

Then, for each of the regions r, of each of the first and second images n and n+<NUM>, the change ir,mean,diff in the region average intensity value in,r,mean and in+<NUM>,r,mean between the first and second images n and n+<NUM> is calculated S45, e.g. as the ir,mean of image n (in,r,mean) subtracted by the ir,mean of image n+<NUM> (in+<NUM>,r,mean) as discussed above.

Further, the maximum region average change value ir,mean,diff,max from the calculated changes in region average intensity value ir,mean,diff is determined S46, e.g. as the highest grey-scale value exhibited by the ir,mean,diff of any one of the regions r as discussed above.

Then, a difference between the maximum region average change value ir,mean,diff,max and the change (imean,diff) of the image average intensity value is calculated S47. A ridge contact score (RCS) is calculated S48 as the ratio between the calculated S47 difference and the image maximum intensity value in+<NUM>,max. Thus, the <MAT>. In alternative embodiments, the RCS may instead be calculated in relative terms as <MAT>.

It may then be determined S49 that the finger <NUM> is in stable contact with the detection surface covering the sensor area <NUM> when the RCS is below a predetermined threshold tRCS. The RCS may for example be within the range of from <NUM> to <NUM> when the contact is stable, and the tRCS could be set within the range of from <NUM> to <NUM>.

In some embodiments, the sensor area <NUM> covers the whole detection surface of the fingerprint sensor <NUM>, i.e. the sensor area <NUM> is not a sub-area of the detections surface, whereby essentially all the sensing elements of the fingerprint sensor <NUM> are used to form pixels in the image area <NUM>. In some other embodiments, the sensor area <NUM> covers only a part of the fingerprint sensor <NUM>. It may e.g. be enough that the sensor area <NUM> covers only a part, such as <NUM>, <NUM> or <NUM>%, of the detection surface of the fingerprint sensor <NUM>, e.g. depending on the size and/or resolution of the fingerprint sensor <NUM>, in order to be enabled for an action, such as authentication as discussed herein.

The determining S4 that the finger <NUM> is in stable contact with the detection surface of the sensor area <NUM> may comprise determining that the finger is in contact with the surface of the fingerprint sensor <NUM> over the whole sensor area. Although some sensor technologies, e.g. capacitive, may allow for fingerprint sensing also when the fingerprint topography hovers just above the detection surface, it may be convenient, e.g. for obtaining a stable image n, that the fingerprint topography, typically the ridges thereof, is in direct physical contact with the detection surface. The detection surface may be provided by e.g. a cover glass or glass coating of the fingerprint sensor <NUM>, protecting the fingerprint sensing elements.

As also discussed with reference to <FIG>, in some embodiments of the present invention, an overlap between at least two, e.g. called a first region and a second region, of the plurality of image regions r is at least <NUM>%, such as at least <NUM>, <NUM> or <NUM>%. Preferably, each of the plurality of regions r has an overlap with all its immediate neighbours in both dimensions.

The image regions r may have any shape or size, but in some embodiments each of the plurality of image regions r has size of at least 8x8 pixels of the acquired image n. In some embodiments, each of the plurality of image regions r corresponds to a sensor region, being a sub-area of the sensor area <NUM>, having a size of at least <NUM> times <NUM> of the sensor area.

As previously mentioned, the acquired images n. m, of an acquired time-sequence <NUM> of images, may be grey-scale images, typically two-dimensional but made into one-dimensional vectors for analysis.

In some embodiments, the time-sequence <NUM> of images n. m is acquired S2 at a rate of at least <NUM> images per seconds, such as at least <NUM> images per second, in order to quickly trigger authentication or other action once the finger contact is stable against the detection surface.

Claim 1:
A method of determining that a finger (<NUM>) is in stable contact with a surface covering a sensor area (<NUM>) of a fingerprint sensor (<NUM>), the method comprising:
on the surface of the fingerprint sensor, receiving (S1) the finger having a fingerprint topography;
by means of the fingerprint sensor, acquiring (S2) a time-sequence (<NUM>) of images (n..m) of the fingerprint of the received finger, said time-sequence comprising at least a first image (n) taken at a first time point (t1) and a second image (n+<NUM>) taken at a second time point (t2) which is after the first time point, each of the images being in the form of a two-dimensional or one-dimensional array of pixels, each pixel having an intensity;
for each image of the time-sequence, dividing (S3) an image area (<NUM>) of the image, corresponding to the sensor area (<NUM>) of the fingerprint sensor, into a plurality of image regions (r), said regions partly overlapping each other and covering the whole image area; and
based on image analysis of each of the plurality of image regions of each image of the time-sequence, determining (S4) that the finger is in stable contact with the surface covering the sensor area;
wherein the determining (S4) that the finger is in stable contact comprises, in at least one of the time-sequence (<NUM>) of images, detecting a non-contacted part of the surface of the sensor area (<NUM>) by determining that an average intensity value of any one of the regions (r) is affected by a high-intensity part (<NUM>) of the image area (<NUM>), corresponding to the non-contacted part, to such a degree that detection of the high-intensity part, and thus the non-contacted part, is triggered, wherein said one of the regions (r) overlaps with neighbouring regions by at least <NUM>% in both dimensions of the two-dimensional image area (<NUM>), said one of the regions overlapping both of two regions which do not overlap each other, wherein for each of the two non-overlapping regions an average intensity value of the region is not affected by the high-intensity part (<NUM>) to such a degree that detection of the high-intensity part, and thus the non-contacted part, is triggered.