Patent Publication Number: US-2015071507-A1

Title: Reconstructing a Biometric Image

Description:
TECHNICAL FIELD 
     The present invention relates to electronic devices, and more particularly to a biometric sensing device included in, or connected to an electronic device. 
     BACKGROUND 
     Biometric sensing devices are increasingly used in conjunction with computer or network security applications, financial applications, surveillance applications, and system access control applications. For example, a common approach to fingerprint identification involves capturing a sample fingerprint image and storing the image and/or the unique characteristics of the fingerprint image. The characteristics of the sample fingerprint may be compared to a reference fingerprint image and/or the unique characteristics of the reference fingerprint image to identify or verify the identity of a person. 
     Typically, the biometric sensing device is connected to an analog processing channel that includes an analog-to-digital converter (ADC). The resolution of an ADC can be defined as the number of discrete digital values the ADC can produce over the range of analog input signals. For example, an ADC with a resolution of 8 bits can convert an analog input signal into one of 256 digital values. Typically, the biometric image is processed further after being output from the analog processing channel. In some embodiments, the resolution of the ADC can limit the quality or effectiveness of subsequent processing operations because information can be lost or restricted when the analog biometric signals are converted into a relatively small number of discrete digital values. 
     SUMMARY 
     In one aspect, a biometric image can be divided logically into image blocks, with each image block including pieces of a biometric image that were obtained from respective sensing elements in a biometric sensing device. Each image block can be processed individually by a processing channel operatively connected to the biometric sensing device. A particular gain of at least one variable amplifier and/or a particular offset value at least one variable offset circuit in the processing channel can be transmitted to a processing device along with the respective image block that was processed by the processing channel. The processing device can reconstruct the signal levels (e.g., voltage levels) on the respective sensing elements associated with the image block based on the particular gain and/or offset values. Each reconstructed image block can be combined to form a reconstructed biometric image. Additional processing operations such as deblurring and/or feature extraction can be performed on the reconstructed biometric image. 
     In another aspect, a processing channel can be operatively connected to a biometric sensing device that includes sensing elements. The processing channel can include at least one variable gain amplifier and/or at least one variable offset circuit operatively connected to an analog-to-digital converter. A processing device can be operatively connected to an output of the analog-to-digital converter. The processing device can receive one or more image blocks of the biometric image along with a particular gain of at least one variable amplifier in the processing channel and/or a particular offset value of at least one variable offset circuit in the processing channel and reconstruct the voltage levels of respective sensing elements associated with the image block(s) based on one or more particular gains and/or one or more particular offset values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures. 
         FIG. 1  is a perspective view of an example electronic device that can include a biometric sensing device; 
         FIG. 2  illustrates an enlarged and simplified cross-section view of a portion of a fingerprint sensing device taken along line  2 - 2  in  FIG. 1 ; 
         FIG. 3  depicts a portion of an example fingerprint image that may be captured by the fingerprint sensing device shown in  FIG. 2 ; 
         FIG. 4  illustrates a conceptual drawing of a fingerprint image logically divided into image blocks; 
         FIG. 5  depicts a capacitive sensing element connected to a processing channel; and 
         FIG. 6  is a flowchart of a method for operating a fingerprint sensing device. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments described herein can include a biometric sensing device operatively connected to a processing channel. The processing channel can include one or more variable gain amplifiers and/or one or more variable offset circuits. The voltage levels of respective sensing elements that are associated with a section of a biometric image can be reconstructed using a digitized section of the biometric image and a particular gain and/or a particular offset value used in the processing channel to process the digitized section of the biometric image. The reconstructed sections of the biometric image can be combined to form a reconstructed biometric image. Additional processing operations such as deblurring and/or feature extraction can be performed on the reconstructed biometric image. 
     In some embodiments, a variable gain or variable offset value can be set to a default preset value. The variable gain or offset values set to default values may not be transmitted to a processing device along with the section of the biometric image. The processing device can use the default values when values for respective gain and/or offset values are not received by the processing device. Similarly, one or more amplifiers in the processing channel may be fixed gain amplifiers, and/or one or more offset circuits can produced fixed offset values. The fixed gain and/or fixed offset values may or may not be transmitted to the processing device with the section of the biometric image. 
     Embodiments are described herein in conjunction with a fingerprint sensing device. Other embodiments, however, are not limited to a fingerprint sensing device. Any suitable type of biometric sensing device can be used to detect or acquire biometric data in other embodiments. For example, a person&#39;s fingerprint, eye, DNA, vein patterns, typing speed or patterns, gait, voice, face, and heart or brain signals are examples of a physical characteristic or a behavioral trait that can be detected or imaged by a biometric sensing device. A biometric sensing device can employ capacitance, ultrasonic, optical, resistive, thermal, or other sensing technologies to detect or image a biometric attribute. The term “biometric attribute” is meant to encompass a physical or behavioral trait that can be detected by a biometric sensing device. 
     Directional terminology, such as “top”, “bottom”, “front”, “back”, “leading”, “trailing”, etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments described herein can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration only and is in no way limiting. When used in conjunction with layers of a display or device, the directional terminology is intended to be construed broadly, and therefore should not be interpreted to preclude the presence of one or more intervening layers or other intervening features or elements. Thus, a given layer that is described as being formed, positioned, disposed on or over another layer, or that is described as being formed, positioned, disposed below or under another layer may be separated from the latter layer by one or more additional layers or elements. 
     Referring now to  FIG. 1 , there is shown a perspective view of one example of an electronic device that can include a biometric sensing device. In the illustrated embodiment, the electronic device  100  is implemented as a smart telephone. Other embodiments can implement the electronic device differently, such as, for example, as a laptop or desktop computer, a tablet computing device, a display, an input device, and other types of electronic devices that include, or are connected to a biometric sensing device. 
     The electronic device  100  includes an enclosure  102  at least partially surrounding a display  104  and one or more buttons  106  or input devices. The enclosure  102  can form an outer surface or partial outer surface and protective case for the internal components of the electronic device  100 , and may at least partially surround the display  104 . The enclosure  102  can be formed of one or more components operably connected together, such as a front piece and a back piece. Alternatively, the enclosure  102  can be formed of a single piece operably connected to the display  104 . 
     The display  104  can be implemented with any suitable technology, including, but not limited to, a multi-touch sensing touchscreen that uses liquid crystal display (LCD) technology, light emitting diode (LED) technology, organic light-emitting display (OLED) technology, organic electroluminescence (OEL) technology, or another type of display technology. The button  106  can take the form of a home button, which may be a mechanical button, a soft button (e.g., a button that does not physically move but still accepts inputs), an icon or image on a display, and so on. Further, in some embodiments, the button  106  can be integrated as part of a cover glass of the electronic device. 
     Embodiments of an electronic device can include a biometric sensing device in the display  104 , the home button  106 , the enclosure  102 , and/or as a separate electronic device that is connected to another electronic device. As one example, a fingerprint sensing device can be included in the button  106 . The fingerprint sensing device can be implemented with any suitable sensing technology, including, but not limited to, capacitive, resistive, ultrasound, piezo-electric, and thermal sensing technology. Embodiments described herein include a capacitive fingerprint sensing device. The fingerprint sensing device can capture images one or more fingers or a portion of one or more fingers in some embodiments. As used herein, the term “image” or “fingerprint image” includes an image and other types of data that can be captured by a fingerprint sensing device. By way of example only, a fingerprint sensing device can produce a data structure that defines the features in a fingerprint. 
     Additionally, as discussed earlier, other embodiments can include any suitable type of biometric sensing device. The terms “image” and “biometric image” are meant to encompass an image and other types of data that can be captured by a biometric sensing device. 
       FIG. 2  illustrates an enlarged and simplified cross-section view of a portion of a fingerprint sensing device taken along line  2 - 2  in  FIG. 1 . The capacitive fingerprint sensing device  200  can capture an image the fingerprint of at least a portion of the finger  202 . In the illustrated embodiment, the first layer  210  is included in a stack of layers. By way of example only, first layer  210  can be a dielectric layer such as an exterior surface of a button or other input device (e.g., home button  106  in  FIG. 1 ), an exterior surface of a trackpad, and/or a cover glass of a display (e.g., display  104  in  FIG. 1 ). Disposed under the first layer  210  is a dielectric layer  220 . By way of example only, the dielectric layer  220  can be a color layer that can be used to reduce the visibility of the electrodes and other circuitry of the capacitive sensing device. 
     A fingerprint is generally formed from ridges  204  and valleys  206  arranged in a unique pattern. When the finger  202  touches an input region  208  of a first layer  210 , the capacitance values (represented by capacitors  212 ) between the finger  202  and one or more electrodes  214  changes, and the variations in the measured capacitance values can be used to capture the fingerprint image. In the illustrated embodiment, a capacitive sensing element  222  is effectively formed by each electrode  214  in combination with a respectively overlying portion of the finger surface. 
     The skin on the finger  202  includes a dead skin layer  216  disposed over a live skin layer  218 . The capacitive fingerprint sensing device typically images the dead skin layer  216  to obtain an image of the fingerprint. However, if a portion of the dead skin layer  216  is damaged or missing, the capacitive fingerprint sensing device can obtain an image of the fingerprint by imaging the live skin layer  218  by itself, or by imaging both the remaining dead skin layer  216  and the exposed live skin layer  218 . 
     Referring now to  FIG. 3 , there is shown a portion of an example fingerprint image. In  FIG. 3 , the ridges  204  are represented with dashed lines. The valleys  206  are located in the areas between the ridges  204 . Typically, the capacitance measured between a ridge  204  and an electrode  214  varies from the capacitance measured between a valley  206  and an electrode  214 . The measured capacitance between a ridge and an electrode can be greater than the measured capacitance between a valley and an electrode because the ridge is closer to the electrode. The differences in the measured capacitances can be used to distinguish between ridges and valleys to produce a fingerprint image. 
     In some embodiments, an entire fingerprint image can be processed at one time, while other embodiments can divide a fingerprint image logically into blocks, and one or more blocks are then processed at a time.  FIG. 4  illustrates a conceptual drawing of a fingerprint image logically divided into image blocks. A fingerprint image  400  can include image blocks  402 , each of which represents a section of the fingerprint image. Each image block  402  can include pieces of the fingerprint image  404  that have been captured by respective capacitive sensing elements (e.g., capacitive sensing elements  222  in  FIG. 2 ). By way of example only, the fingerprint image  400  can include eighty-eight by eighty-eight image blocks, and each image block  602  can include eight-by-eight fingerprint image pieces. Thus, in one embodiment, a fingerprint image can include eleven image blocks, with each block representing a non-overlapping portion of the fingerprint image. 
     In some embodiments, each image block  402  is processed individually when producing a fingerprint image.  FIG. 5  illustrates one example of a simplified portion of a processing channel suitable for use with a capacitive fingerprint sensing device. Other embodiments can construct a processing channel differently, with fewer, additional, or different components. By way of example only, fewer amplifiers or only one offset circuit can be used. 
     The example processing channel  500  includes four amplifiers  502 ,  504 ,  506 ,  508  connected in series. An analog-to-digital converter (ADC)  510  is connected to the output of the fourth amplifier  508 . A first summing circuit  512  is connected to an output of the first amplifier  502  and an input of the second amplifier  504 . A second summing circuit  514  is connected to an output of the third amplifier  506  and an input of the fourth amplifier  508 . 
     In the illustrated embodiment, an analog fingerprint signal received from each capacitive sensing element in an image block  402  is input into the first amplifier  502  on signal line  516 . The first summing circuit  512  combines the signal output from the first amplifier  502  with a first offset signal produced by a first offset circuit  518 . The analog signal output from the first summing circuit  512  is then input into the second amplifier  504 . The signal output from the second amplifier is input into the third amplifier  506 . The second summing circuit  514  combines the analog signal output from the third amplifier  506  with a second offset signal produced by a second offset circuit  520 . The signal output from the second summing circuit  514  is input into the fourth amplifier  508 . The signal output from the fourth amplifier  508  is then input into the ADC  510 . 
     Any suitable type of amplifier can be used for each amplifier  502 ,  504 ,  506 ,  508 . By way of example only, the first amplifier  502  can be a variable gain differential amplifier, the second amplifier  504  a variable gain AC amplifier, the third amplifier  506  a variable gain correlated double sampling (CDS) amplifier, and the fourth amplifier  508  a programmable gain amplifier. 
     Any suitable type of offset circuit can be used in the processing channel  500 . By way of example only, the first and second offset circuits can be integrating digital-to-analog converters. 
     As will be described in more detail later, an operating parameter of at least one amplifier and/or at least one offset circuit is transmitted to a processing device  522  along with the image block processed by the processing channel  500 . The operating parameter can be a gain of an amplifier or an offset value of an offset signal produced by an offset circuit. The processing device  522  can reconstruct the signal levels (e.g., voltage levels) on respective capacitive sensing elements associated with the pieces of fingerprint image in the image block using the operating parameter of at least one amplifier and/or at least one offset circuit. Each reconstructed image block can be combined to form a reconstructed fingerprint image. Additional processing operations such as deblurring and/or feature extraction can be performed on the reconstructed fingerprint image. 
     The processing device  522  can be adapted to adjust the gain of one or more amplifiers in the processing channel  500 , to adjust the offset signal produced by one or more offset circuits, and/or to adjust the reference voltage of the ADC  510 . The processing device  522  can be implemented with one or more processors, such as, for example, a microprocessor, an application-specific integrated circuit (ASIC), or a field programmable gate array (FPGA), either individually or in various combinations. The processing device  522  can be implemented on the same chip or integrated circuit as the processing channel  500  or the processing device  522  can be separated from the processing channel  500 . For example, the processing device  522  can be a processor of the electronic device. 
     In some embodiments, the processing device  522  is a secure processor that is generally used to manipulate secure data. For example, the secure processor can decrypt an encrypted fingerprint image and match the decrypted fingerprint image with a reference fingerprint image. The secure processor  522  may have access to a key or other security parameter usable to decrypt data received from the fingerprint sensing device. For example, the secure processor and the fingerprint sensing device may share a factory provisioned key, enabling the secure processor to decrypt data received from the fingerprint sensing device. A separate application processor (not shown) that can be included in the fingerprint sensing device and/or in an electronic device that includes or is connected to the fingerprint sensing device may not have access to the key or other security parameter, and may be unable to decrypt data received from the fingerprint sensing device. In this manner, the application processor may be prevented from ever accessing decrypted fingerprint image(s) from the fingerprint sensing device, which may improve the security of the fingerprint image(s), for example, making a decrypted fingerprint image inaccessible or less accessible to other programs which may be running on the application processor. 
     A storage device  524  can be used to store the settings for the gains, offset signals, and/or reference voltage of the ADC that correspond to respective image blocks. The storage device  524  can store one or more reference fingerprint images. The storage device can be used to store one or more image blocks or a fingerprint image that is used to calibrate or test the processing channel. The storage device can store each image block and/or each reconstructed image block of the fingerprint image. The storage device  524  can be implemented with one or more suitable types of memory, such as, for example, dynamic random access memory, flash memory, and EEPROM, either individually or in various combinations. The storage device  524  can be implemented on the same chip or integrated circuit as the processing channel  500 , or the storage device  524  can be separated from the processing channel  500 . 
     Referring now to  FIG. 6 , there is shown a flowchart of a method for operating a fingerprint sensing device. Initially, an image block of a fingerprint image is captured using at least one variable operating parameter of a processing channel (block  600 ). The at least one variable operating parameter can be a gain of at least one variable amplifier in the processing channel and/or an offset value of at least one variable offset circuit in the processing channel. The image block (e.g., image block  402  in  FIG. 4 ) of the fingerprint image can have any given size and/or dimension in the fingerprint image. In some embodiments, an analog-to-digital converter included in the processing channel can convert the voltage levels associated with the image block into 8 bit digital values. 
     A determination is then made at block  602  as to whether one or more variable operating parameters used to capture the image block is set at a default value. In some embodiments, a variable operating parameter can be set at a preset value. The preset value can be modified based on a desired image characteristic or quality of the captured image block. For example, a variable gain and/or offset value can be modified to increase or decrease the contrast of an image block. Additionally or alternatively, one or more amplifiers may be fixed gain amplifiers and one or more offset circuits may have a fixed offset value. The operating parameters that have a fixed value and the operating parameters that are at a preset value may be known by a processing device and therefore not transmitted to the processing device. Only the operating parameters that are at non-default values may be transmitted to the processing device and used to reconstruct the voltage levels associated with an image block. 
     If the at least one operating parameter is not set at a fixed or default value, the process passes to block  604  where the image block and the variable operating parameters not set to a default value are transmitted to a processing device (e.g., secure processing device  522  in  FIG. 5 ). Other embodiments can omit block  602  and transmit the default, fixed, and/or non-default values of the operating parameters to the processing device. 
     Next, as shown in block  606 , the signal levels (e.g., voltage levels) of the respective capacitive sensing devices associated with the fingerprint pieces in the image block are reconstructed using the at least one operating parameter. By way of example only, an eight bit image block can be reconstructed into a 16 bit or 24 bit image block. The reconstructed image block can be stored in a storage device, such as in storage device  524  in  FIG. 5 . 
     A determination is then made at block  608  as to whether another image block is to be captured. If so, the method returns to block  600  and repeats until all of the image blocks have been captured and transmitted (along with the operating parameter(s)) to the processing device. When all of the image blocks have been captured at block  610 , the process passes to block  610  where the reconstructed image blocks are combined to produce a reconstructed fingerprint image. The reconstructed fingerprint image can then be processed at block  612 . By way of example only, a deblurring operation and/or a feature extraction operation can be performed on the reconstructed fingerprint image. 
     Various embodiments have been described in detail with particular reference to certain features thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the disclosure. For example, a fingerprint sensing device can include a different type of sensing elements. Additionally or alternatively, a processing channel can include any number of variable gain amplifiers and/or variable offset circuits. Other types of biometric sensing devices can be used in other embodiments. 
     Even though specific embodiments have been described herein, it should be noted that the application is not limited to these embodiments. In particular, any features described with respect to one embodiment may also be used in other embodiments, where compatible. Likewise, the features of the different embodiments may be exchanged, where compatible