Patent Publication Number: US-2020279093-A1

Title: System and method of capturing and producing biometric-matching quality fingerprints and other types of dactylographic images with a mobile device

Description:
RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Application No. 61/590,836, filed Jan. 26, 2012, titled “System and Method of Capturing and Producing Biometric-Matching Quality Fingerprints through a Mobile Device”, and to U.S. Provisional Application No. 61/682,346, filed Aug. 13, 2012, titled “System and Method of Capturing and Producing Biometric-Matching Quality Fingerprints through a Mobile Device”, the entireties of which applications are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to dactylographic systems. More specifically, the invention relates to systems and methods of capturing and producing biometric-matching quality dactylographic images with a mobile device. 
     BACKGROUND 
     Fingerprints are well known for their use in personal biometric identification and verification applications. Fingerprinting can often be encumbered by having to bring the person to be fingerprinted to a fingerprinting station. This process can be inconvenient and time consuming. Fingerprinting “in the field” is thought to be a potential alternative by which to perform the task in real time, thereby avoiding the inconvenience and the consequent time delay. 
     SUMMARY 
     In one aspect, the invention features an optical module comprising a housing with a lighting mechanism, a side with an aperture formed therein, and a window framing a transparent surface used to contact a skin surface of a person. The lighting mechanism is adapted to provide light that illuminates the the skin surface of the person placed upon the transparent surface. The optical module further comprises a prism having a first side facing the lighting mechanism, a second side facing the window that frames the transparent surface used to contact the skin surface of the person, and a third side through which a dactylographic image of the skin surface of the person exits the prism. A plurality of light reflecting surfaces is within the housing, spatially separated from the prism. The plurality of light reflecting surfaces is disposed within the housing to reflect the dactylographic image of the skin surface of the person exiting the prism towards the aperture in the side of the housing. 
     In another aspect, the invention features a method for acquiring a dactylographic image comprising: acquiring, by a camera of a mobile device, a dactylographic image of a skin surface of a person, and adjusting, by the mobile device, the acquired dactylographic image to produce a dactylographic image of a quality suitable for biometric matching. 
     In still another aspect, the invention features a mobile device comprising a camera, memory storing program code that, when executed, processes images of skin surfaces of people acquired by the camera, and a processor configured to execute program code stored in the memory in response to the camera acquiring a dactylographic image of a skin surface of a person. The processor, in response to executing the program code, adjusts the acquired dactylographic image of the skin surface of a person to produce a dactylographic image of a quality suitable for biometric matching. 
     In yet another aspect, the invention features a dactylographic image-capture system comprising a mobile device with a native camera having a lens, memory storing program code that, when executed, processes dactylographic images of skin surfaces of people acquired by the camera, and a processor configured to execute program code stored in memory in response to the camera acquiring a dactylographic image of a skin surface of a person, the processor, in response to executing the program code, adjusting the acquired dactylographic image of the skin surface of the person to produce a dactylographic image of a quality suitable for biometric matching. 
     The dactylographic image-capture system further comprises an optical module that is coupled to the mobile device. The optical module comprises a housing with a lighting mechanism, a side with an aperture formed therein, and a window framing a transparent surface used to contact the skin surface of the person. The lighting mechanism is adapted to provide light to illuminate the skin surface of the person placed upon the transparent surface. The optical module further comprises a prism having a first side facing the lighting mechanism, a second side facing the window that frames the transparent surface used to contact the skin surface of the person, and a third side through which a dactylographic image of the skin surface of the person exits the prism. The optical module further comprises a plurality of light reflecting surfaces within the housing spatially separated from the prism. The one or more light reflecting surfaces are disposed within the housing to reflect the dactylographic image of the skin surface exiting the prism towards the aperture in the side of the housing. The lens of the native camera of the mobile device aligns with the aperture in the side of the housing to receive the dactylographic image of the skin surface passing therethrough. 
     In yet another aspect, the invention features an optical module comprising a housing with a diffuser window adapted to diffuse ambient light, a frame portion with an aperture, and a second window framing a transparent surface used to contact a skin surface of a person. The frame portion is adapted to couple to a mobile device with a built-in camera such that the aperture of the frame portion aligns with a lens of the camera of the mobile device. A prism is disposed in the housing with a first side of the prism at the second window and a second side of the prism facing the diffuser window. The diffused ambient light entering the housing through the diffuser window illuminates the skin surface of the person that is in contact with the transparent surface. A lens is disposed within the housing to focus a dactylographic image of the skin surface that exits the prism and propagates towards the aperture in the frame portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a block diagram of an embodiment of a dactylographic image acquisition system including an optical module coupled to a mobile device. 
         FIG. 2  is a flow chart of an embodiment of a process for automatic and real-time capture, processing, and use of a high-quality dactylographic image in an example biometric matching application. 
         FIG. 3  is a side view of an embodiment of the optical module. 
         FIG. 4  is an isometric view of the embodiment of the optical module of  FIG. 3  coupled to a mobile device. 
         FIG. 5  is a side view of another embodiment of the optical module without the housing and various internal components. 
         FIG. 6  is a side view of the embodiment of the optical module of  FIG. 5  with its housing. 
         FIG. 7  is a diagram illustrating paths taken generally by light passing through the optical modules of  FIG. 3  and  FIG. 5 . 
         FIG. 8  is an exploded view of the optical module of  FIG. 5 . 
         FIG. 9  is a top down view of the optical module of  FIG. 5 . 
         FIG. 10  is a side view of the optical module of  FIG. 5 . 
         FIG. 11  is an isometric front view of the optical module of  FIG. 5 . 
         FIG. 12  is a top view of the optical module of  FIG. 5  coupled to a mobile device. 
         FIG. 13  is a bottom view of the optical module of the  FIG. 5  coupled to the mobile device. 
         FIG. 14  is a top view of the optical module of  FIG. 5  coupled to a mobile device from the perspective of the individual administering the fingerprinting. 
         FIG. 15  is a bottom view of the optical module of the  FIG. 5  coupled to the mobile device from the perspective of the individual being fingerprinted. 
         FIG. 16A  is a diagram illustrating a top view of another embodiment of an optical module with a transparent surface. 
         FIG. 16B  is a side view of the optical module of  FIG. 16A , wherein the end with the transparent surface has a sloped side. 
         FIG. 16C  is the optical module of  FIG. 16A  from the sloped end with the transparent surface. 
         FIG. 16D  is a bottom view of the optical module of  FIG. 16A , with a diffuser window at the sloped end of the optical module. 
         FIG. 16E  is a diagram showing a mobile device, having a native camera, coupled to one side of the optical module of  16 A. 
         FIG. 17A  is a diagram of a top view of another embodiment of an optical module housing having a prism at one end, an interior lens, and a mobile device attachment mechanism at the opposite end. 
         FIG. 17B  is a diagram of a side view of the optical module housing with an interior mirror for directing ambient light into the prism. 
         FIG. 17C  is a diagram of an end view of the optical module housing with a diffuser window adjacent to a window that frames a transparent surface used for making contact with a skin surface of the user. 
         FIG. 18A  is a diagram of a top view of another embodiment of an optical module. 
         FIG. 18B  is a diagram of a side view of the housing of the optical module of  FIG. 18A . 
         FIG. 18C  is a diagram of the housing viewed from the sloped end having a window that frames a transparent surface used for making contact with a skin surface of the user. 
         FIG. 18D  is a diagram of the housing viewed from the end opposite the sloped end, the housing having an interior mirror and lens appearing in phantom. 
         FIG. 18E  is a diagram of a bottom view of the housing, with a diffuser window at the sloped end of the housing. 
         FIG. 18F  is a diagram of a mobile device, having a native camera, coupled to the bottom side of the housing. 
     
    
    
     DETAILED DESCRIPTION 
     The optical modules, software system, and processes described herein facilitate convenient, “in the field” fingerprint-based enrollment, identification, and verification operations using a mobile device. The mobile device is configured with a software system that is used in conjunction with an optical module to capture and process fingerprint images and to produce therefrom high-quality fingerprint images suitable for biometric matching. Because the capture of fingerprint images can occur in a minimally controlled lighting environment, the software system running on the mobile device applies certain image processing techniques to improve the quality of the image and minimize false match and non-false match rates. To compensate for the uncontrolled, non-homogeneous lighting of the fingerprint, one or more fingerprint images may be acquired, adaptively enhanced, and merged to create a single illumination-normalized fingerprint image. Various optical modules described herein used with a mobile device are lightweight (in terms of ounces) and small, typically measuring less than four inches in length, approximately one inch in height, and approximately one inch in width. 
     Although described herein predominantly with use in the acquisition and processing of fingerprint images, the described embodiments of optical modules and software system can be adapted to acquire various types of dactylographic images which include, but are not limited to, one or more, full or partial fingerprints, thumbprints, palm prints, or footprints (e.g., plantar prints). For purposes of readability, references herein made to a dactylographic image encompass each of these types of dactylographic images. Also for purposes of readability, those principles described in this detailed description in connection with fingerprints shall be understood to apply also to the other types of dactylographic images. 
       FIG. 1  shows a functional block diagram of an embodiment of a dactylographic image acquisition system  10  comprised of an optical module  12  coupled to a mobile device  14  having one or more built-in (i.e., native) cameras  16 . The optical module  12  can be designed for detachable attachment to any of a variety of mobile devices having one or more cameras. Examples of mobile devices for which the optical module  12  can be customized, as described herein, include smart phones (e.g., the Apple iPhone™, RIM Blackberry™, Samsung Galaxy™ phones, and Google Android™ phones), tablets (e.g., the Apple iPad™), and portable media players (e.g., Apple iPod Touch™). Means for attachment include a mobile device frame, fastened or mounted to the optical module  12 , into which the mobile device  14  can snap or slide and be securely held in place during the photographing of the fingerprint as described herein. 
     The optical module  12  operates as a finger-to-camera interface and includes a lighting mechanism  18  for illuminating the skin surface (e.g., fingertip) of the user during the fingerprint capture process. In one embodiment, the optical module  12  is a passive device, meaning the optical module  12  needs no electrical power for its operation, wherein the lighting mechanism  18  comprises a diffuser window that allows diffused ambient light to enter the optical module  12 . In another embodiment, the lighting mechanism  18  requires electrical power, for example, one or more LEDs. In this embodiment, the optical module  12  has a power source  20 , for example, one or more solar cells or one or more batteries. 
     In addition to the one or more cameras, the mobile device  14  comprises hardware  22 , including memory  24 , a processor  26 , a network interface  28 , and a software system  30  that runs on the mobile device  14  to process captured fingerprint images and to produce therefrom high-quality fingerprints suitable for biometric matching. Without the processing performed by the software system  30 , the fingerprints captured by the camera of the mobile device would be unsuitable for biometric matching. For one, the native camera system of the mobile device  14  is not designed to produce monochromatic close-up images of fingerprints (for example) at 500 ppi to 1000 ppi resolution, whereas the software system  30  is designed towards this end. 
     The memory  24  stores the program code associated with the software system  30  and the processor  26  executes the program code to accomplish the fingerprint capture process, as described herein. The network interface  28  enables the mobile device  14  to communicate with a computer system  32 , for purposes of downloading data from the computer system  32  or uploading data to the computer system  32  as described herein. Such communication can be over a wired or wireless communication links. Example embodiments of the wireless communication link include, but are not limited to, satellite communication links and radio frequency (RF) links. 
     In brief, the software system  30  includes an illumination normalization algorithm  34  to correct for the non-homogeneous lighting produced by the natural environment in which a snapshot of the fingerprint is taken, and an image warping and scaling algorithm  36  to compensate for distortions due to perspective, magnification, and camera barrel distortion. More specifically, the image-warping and scaling algorithm  36  includes a perspective adjustment module  38 , which compensates for the perspective distortion caused by the angled prism  88  (e.g.,  FIG. 3 ) of the unpowered optical module  12  used to acquire the fingerprint, a scaling and resolution adjustment module  40 , which compensates for the magnification performed by the lens  92  (e.g.,  FIG. 3 ), and a barrel distortion adjustment module  42  to compensate for barrel distortion and magnification introduced by the camera  16  in the mobile device  14 . Other software modules of the software system  30  can include a biometric template extractor  44 , a code reader  46 , and a biometric matcher  48 . 
     A calibration process determines parameters used by the image-warping and scaling algorithm  36  during the image processing. Such parameters depend upon the design of the optical module  12  (e.g., the distance of a mirror from the lens) and on the type of mobile device (e.g., the camera lens location on the mobile device, focal length, magnification and barrel distortion). 
     During the calibration process, the camera  16  of the mobile device  14  captures an image from a predetermined calibration object (using the particular optical module designed for that model of mobile device). This image captured by the camera of the mobile device is subsequently compared with a calibrated image (preloaded on the mobile device); certain points in the captured image are compared with corresponding points in the calibrated image. From this comparison, the software system  30  generates and stores a configuration file of parameter values in the memory  24  of the mobile device  14 , to be used in the processing of subsequently captured fingerprint images. 
     The calibration process can also compensate for image reproduction variance caused by mobile device manufacturing inconsistencies, by making use of data from a statistical analysis of the mobile device manufacturing error to determine an optimal image acquisition process. 
     The calibration process can be run once for each type of mobile device (e.g., at manufacture, one configuration file for each brand of Android™ smart phone, another configuration file for iPhones™, etc.). The necessary calibration configuration file can then be automatically downloaded from a web service during setup (to ensure an up-to-date version), or supplied with the mobile device, to be loaded onto the mobile device during setup. Alternatively, users can calibrate their own mobile devices individually, prior to first use of the mobile device for capturing a fingerprint image. 
     The result from the image acquisition is one or more processed monochromatic fingerprint images at a 500 ppi or 1000 ppi resolution (depending on the biometric matcher requirements). Each resulting fingerprint image is thus suitable for biometric matching applications. 
     The dactylographic image capture system  10  for capturing and producing high-quality fingerprint images can be used in conjunction with other biometric applications. Such applications, in addition to collecting “live” fingerprints “in the field”, can also collect other optional data, including biographic information, acquire biometric samples, including face images and iris images. A camera of the mobile device (whether the same camera used to capture the fingerprints or, preferably a second camera facing the opposite side of the mobile device) can acquire these other biometric samples or read in biometric data from QR codes or bar codes or some other graphical representation of biometric data. 
     These other biometric applications include, but are not limited to: 
     1) Biometric verification, which is a process by which a single set (derived from a single person) of one (unimodal) or more (multimodal) live biometric samples is compared to a single “reference” set of one or more biometric samples in order to ascertain their similarity and thus whether they belong to the same person;
 
2) Biometric identification, which is a process by which a single “probe” set of one or more live biometric samples is compared to a “gallery” of multiple sets of one or more biometric samples, in order to ascertain which set in the gallery is most likely to belong to the same individual as that of the probe; and
 
3) Biometric enrollment, which is a process by which biometric data such as fingerprints, facial images, or iris images are collected using the mobile device, along with biographic data such as name and contact information. This data is then either stored locally or transmitted by a network connection to a database containing a biometric gallery.
 
     The biometric applications can involve any one or more of the following matching modes. These modes are differentiated from each other by where the reference biometric sample or biometric sample gallery resides and where the biometric comparison occurs. 
     1) Match-to-device. In this mode, the reference biometric sample or gallery of biometric samples resides on the mobile device  14 . The biometric comparison is performed on the mobile device  14  and the user is informed of the comparison result through a software application user interface running on the mobile device  14 .
 
2) Match-to-code. In this mode, the reference biometric sample is encoded in a QR (Quick Response) code or bar code that is either printed on a surface or displayed on a screen. A camera on the mobile device is used to take a photograph of the code and then the mobile device decodes the biometric sample data from the photograph. A comparison is then made between the live biometric sample data and the data stored in the code. The comparison result is reported to the user through a software application user interface running on the mobile device  14 .
 
3) Match-to-chip. In this mode, the reference biometric sample is encoded on a chip, such as is found on a smart card used as a credential. The mobile device  14  uses near field communication (NFC) to communicate with the chip on the smart card and retrieve the reference biometric sample. The mobile device  14  then performs a comparison between the live sample data and the reference biometric sample. The comparison result is reported to the user through a software application user interface running on the mobile device  14 .
 
4) Match-to-server. In this mode, the reference sample or gallery of samples resides on an external computer system (e.g.,  32 ), preferably within physical proximity of the mobile device  14 . The mobile device  14  communicates with the computer system  32  over a network connection established between the mobile device  14  and the computer system  32  using, for example, a wireless technology such as GSM, wifi, or Bluetooth. After establishing a network connection with the computer system  32 , the mobile device  14  retrieves either a reference sample or gallery and performs biometric comparison for verification or identification at the mobile device  14 . Alternatively, the mobile device  14  submits the live probe sample to the computer system  32 , where a biometric reference sample or gallery resides, and where the comparison to the reference or gallery is performed. In this instance, the computer system  32  can return the comparison result to the mobile device  14 , which is then reported to the user through a software application user interface running on the mobile device  14 .
 
       FIG. 2  shows an embodiment of a process  50  for automatic and real-time capture and generation of a high-quality fingerprint image and use of the fingerprint image in an example biometric matching application. In the description of the process  50 , reference is made to elements of  FIG. 1 . At step  52 , the mobile device  14  is calibrated (which, for example, can occur during manufacturing or by the user). The camera  16  of the mobile device  14  acquires (step  54 ) an image of fingerprint from a finger submitted to the optical module  12 . The software system  30  of the mobile device  14  processes (step  56 ) the acquired fingerprint image using parameters determined during the calibration process, to produce a high-quality fingerprint image suitable for biometric matching. 
     In one embodiment, at step  54 , the camera acquires multiple images of the fingerprint, each image being acquired with one or more different focal properties (e.g., focal length). Small changes in the focal length, for example, bring different features of the fingerprint into focus. Each acquired image corresponds to a “slice” of the fingerprint. An image stacking technique then combines the slices in a manner that selects the features in focus, and produces (step  56 ) a composite image of the fingerprint. 
     The high-quality fingerprint image can be subsequently used in enrollment, verification, and/or identification processes. For example, in a verification application, the high-quality fingerprint image is encoded (at step  58 ) into a minutiae-based biometric template, and, from the biometric template, into a revocable biotoken. The software system  30  applies a biometric template extraction algorithm  44  to extract a biometric template from each acquired live fingerprint image. Such data corresponding to the high-quality fingerprint image is herein referred to as “live biometric sample data.” A camera  16  of the mobile device  14  reads in (step  60 ) an image of a QR code or a bar code or graphical representation (the camera can read the code whether in print form or displayed on a device screen). Software  46  executing on the mobile device  14  decodes (step  62 ) the contents of the QR code, bar code, or graphical representation and extracts (step  64 ) biometric data (e.g., a biometric template) from the decoded contents. Biometric matching algorithms  48  then perform (step  66 ) a comparison between the biometric data extracted from the code and the live biometric sample data (in either the biometric template or revocable biotoken form) in order to verify or authenticate the individual with respect to the QR code, bar code, or graphical representation. 
       FIG. 3  shows a side view of one embodiment of the optical module  12  that can be used in the acquisition of fingerprints as described herein. The optical module  12  includes a housing  80  with a lighting mechanism  18 , also referred to as a diffuser window  18 , and a window  82  that frames a transparent surface  70 , which, in one embodiment, is one side of a prism. The diffuser window  18  diffuses the ambient light entering the housing  80 . In a preferred embodiment, the diffuser window  18  is made of a rigid translucent material, such as glass or plastic. A rigid translucent diffuser window  18  has been found to sufficiently homogenize the ambient light. (Non-homogenous lighting can reduce the quality of captured fingerprint images, because non-uniform lighting can produce false or mask true biometric features, rendering such resulting images problematic for purposes of biometric matching). Another embodiment of the diffuser window  18  comprises a piece of translucent tape. 
     Instead of the diffuser window  18 , other different embodiments can use sources of light other than ambient light, for example, one or more LEDs powered, for example, by one or more solar cells or a small battery. In addition, other embodiments can be adapted to capture fingerprints of multiple fingers simultaneously (e.g., a four-finger slap), using, for example, multiple separate transparent finger surfaces or an enlarged single finger surface. Still other embodiments can have an single transparent surface enlarged to contact skin surfaces for acquiring dactylographic images of palm prints or footprints. The side of the housing  80  is shown to be transparent for purposes of revealing the various components within the housing; the transparency of the side is not a requirement for the proper operation of the optical module  12 . 
     The housing  80  encloses a first mirror (i.e., light reflecting surface)  84 , a prism  88 , a second mirror (i.e., light reflecting surface)  90 , a lens  92 , and a third mirror  94 . The first mirror  84  is angled within the housing  80  to direct ambient light entering the housing  80  through the diffuser window  18  towards one side  88   a  of the prism  88 . This ambient light serves to illuminate the fingertip of the finger placed on the transparent surface  70  (e.g., the exposed prism surface) framed by the window  82 . Light carrying the image of the fingerprint leaves the prism  88  through side  88   b . The second mirror  90  is angled within the housing  80  to reflect the image of the fingerprint towards the lens  92 . The lens  92  is disposed between the second and third mirrors  90 ,  94 , and focuses the image of the fingerprint from the second mirror  90  onto the third mirror  94 . The third mirror  94  is angled within the housing  80  to reflect the image of the fingerprint towards an aperture (e.g., reference numeral  100  in  FIG. 5 ). The attachment mechanism  96  holds the mobile device  14  in place against the optical module  12  when capturing fingerprints. 
       FIG. 4  shows an example in which the mobile device  14  (with a back-facing camera, and optionally, a front-facing camera) is coupled to the optical module  12  for capturing a fingerprint. The attachment mechanism  96  has a slot  98  adapted to closely receive and hold one end of the mobile phone  14  such that the lens of the back-facing camera of the mobile device aligns with the housing aperture (e.g., reference numeral  100  in  FIG. 5 ) when the mobile phone  14  is coupled to the optical module  12 . Rather than directly couple to the mobile device, the attachment mechanism  96  may be designed to attach to a casing that frames and holds the mobile device. Designing the attachment mechanism for attachment to a casing can make the optical module  14  flexible for use with different mobile device models. 
     Further, another embodiment can have the attachment mechanism  96  movably attached to the mobile device casing so as to remove the optical module  12  from the field of view of the camera aperture on the mobile device  14 , for example, on a rail extending left-to-right along the upper back region of the mobile device. Moved in one direction along this rail, the mobile device slides in place over the aperture; moved in the opposite direction, the mobile device slides away from and out of the way of the optical module  12 . This movable attachment can facilitate joining the optical module  12  to and removing the optical module  12  from the mobile device  14 . 
     Another embodiment can have the attachment mechanism  96  movably attached to the mobile device casing so as to remove the optical module  12  from the field of view of the camera aperture on the mobile device  14 , for example, on a pivotable arm that allows the optical module to twist around an axis that is perpendicular to the back of the mobile device  14 . Twisted on this axis, the mobile device turns into place over the aperture; moved in the opposite direction, the mobile device is turned out of the way of the optical module  12 . This movable attachment mechanism can facilitate joining the optical module  12  to and removing the optical module  12  from the mobile device  14 . 
     Yet another embodiment can have the attachment mechanism  96  movably attached to the mobile device casing so as to remove the optical module  12  from the field of view of the camera aperture on the mobile device  14 , for example, on an axle that allows the optical module twist around an axis that is parallel to the back of the mobile device  14 . Twisted on this axis, the mobile device is brought down into place over the aperture; moved in the opposite direction, the mobile device is flipped above the mobile device  14  and out of the way of the optical module  12 . This movable attachment mechanism can facilitate joining the optical module  12  to and removing the optical module  12  from the mobile device  14 . 
     Provided the front-facing camera can attain sufficient resolution, other embodiments of the optical module  12  can be adapted to use the front-facing camera instead of the back-facing camera for capturing fingerprints. Alternatively, the front-facing camera can be used simultaneously with the back-facing camera; that is, the front-facing camera can take a photograph of the user&#39;s face as an additional biometric, while the back-facing camera captures the user&#39;s fingerprint. 
       FIG. 5  shows a side view of another embodiment of the optical module  12  that can be used in the acquisition of fingerprints. The housing  80 , the first, second and third mirrors  84 ,  90 , and  94 , and the lens  92  have been omitted from the drawing to simplify the illustration of the propagation of the fingerprint image through the optical module  12 ; their corresponding reference numerals being used here to indicate their general locations within the optical module  12 . A planar section  102  of the housing frame has an aperture  100  formed therein. The image of the fingerprint of a fingertip pressed on the transparent surface  70  exits through this aperture  100 , as illustrated by light path  104 . 
       FIG. 6  shows a side view of the embodiment of the optical module  12  of  FIG. 5  with its housing  80 . The housing  80  is here shown as transparent for purposes of displaying the internal components of the optical module  12 . Generally, the housing  80  is opaque, with the translucent diffuser window  18  attached thereto. The housing  80  also has a window  108  that frames the transparent surface  70  of the prism  88 , and a pair of brackets  110 . Each bracket  110  has an opening  112 . The brackets  110  are part of an attachment mechanism for coupling the optical module  12  to a casing made for the mobile device  14 . 
       FIG. 7  shows schematically the paths generally taken by light passing through the optical modules of  FIG. 3  and  FIG. 5 . Ambient light, represented by arrow  120 , enters the housing  80  through the diffuser window  18  and reflects off the first mirror  84  into the prism  88  and onto the interior side of the transparent surface  70 , where it illuminates a fingertip placed on the exterior side of the transparent surface  70 . Light carrying the image of the fingerprint, represented by the arrow  122 , bends through the prism  88  towards the second mirror  90 , which reflects the fingerprint image through the lens  92 . The lens  92  focuses the fingerprint image onto the third mirror  94 , which reflects the fingerprint image into the aperture  100  (formed in the planar portion  102  of the housing frame). 
       FIG. 8  shows an exploded view of the embodiment of optical module  12  of  FIG. 5 , including two housing halves  80 - 1 ,  80 - 2  that form the housing  80  when joined together, and fasteners  130  for joining the halves  80 - 1 ,  80 - 2  together. The diffuser window  18  has tabs  128  that couple to the housing halves. The joined halves form an opening that aligns with the aperture  100  of the planar portion  102  of the housing frame. This opening closely receives a transparent exit window  132  made of glass or hardened clear plastic. 
     The mirror  84  is a diffuse mirror, and mirrors  90 ,  94  are fold mirrors. In one embodiment, the mirrors  84 ,  90  are rectangular in shape; the mirror  94  is disc-shaped and fits in an opening (not visible) in a sloped panel  134  of the housing frame, this opening being sized to closely receive the mirror  94 . This embodiment of attachment mechanism  96  includes a spring latch  136 , a torsion spring  138 , and a latch axle  140 . When the attachment mechanism  96  is assembled, the latch axle  140  passes through openings in the spring latch  136 , the torsion spring  138 , and the brackets  110  of the housing frame. 
       FIG. 9  shows a top view,  FIG. 10 , a side view, and  FIG. 11 , an isometric front view of the optical module of  FIG. 5 . The isometric view in  FIG. 11  shows the spring-loaded latch  136 , which is absent in  FIG. 9  and  FIG. 10  (i.e., the brackets  110  are unattached).  FIG. 12  shows a top view of the optical module  12  coupled to a mobile device  14 . With the optical module  12  coupled to a mobile device  14 , the diffuser window  18  and mirror  84  do not impede view of the display of the mobile device  14 .  FIG. 13  shows a bottom view of the optical module  12  coupled to the mobile device  14 , wherein the optical module slides into rails of the mobile device casing  150  and locks in place with the spring-loaded latch  136 . 
       FIG. 14  shows a top view of the optical module of  FIG. 5  coupled to a mobile device  14  from the perspective of an individual administering the fingerprinting, with the mobile device  14  being proximal and the optical module  12  being distal with respect to the administrator. In this orientation, the display of the mobile device  14  is visible to the administrator, who can view the captured fingerprint image and any biometric comparison results in real time. 
       FIG. 15  is a bottom view of the optical module of the  FIG. 5  coupled to the mobile device from the perspective of the individual being fingerprinted, with the optical module  12  being proximal and the mobile device  14  being distal with respect to the individual being fingerprinted. In this orientation, the transparent surface  70  faces the one being fingerprinted. 
     It is to be understood that the principles described herein extend to various other shapes of optical modules, with one, two, or more light reflecting surfaces for defining the path taken by the light passing through the optical module housing. For example,  FIG. 16A  shows a top view of another embodiment of an optical module  12  used in the acquisition of fingerprints as described herein. Like the optical modules described in connection with  FIG. 3  and  FIG. 5 , the optical module  12  is designed for detachable attachment to any of a variety of mobile devices having one or more native cameras. In this embodiment, the optical module  12  is a passive device, using ambient lighting through a diffuser window  178  ( FIG. 16D ) to illuminate a skin surface placed on the transparent surface  170  during the dactylographic image capture process. This transparent surface  170  can be an exposed side of the prism  188  ( FIG. 16B ) within the housing  180 , or a pane (e.g., glass) that abuts that side of the prism  188 . Alternatively, illumination of the display of the mobile device  14  can be modulated to provide a uniform illumination for the image acquisition. Shown in phantom in  FIG. 16A , to signify components within the housing  180 , the optical module  12  comprises a prism lens  160  and a plurality of light reflecting surfaces or mirrors  162 - 1 ,  162 - 2 . The lens  160  may be an integral part of the construction of the prism  188 . 
       FIG. 16B  shows a side view of the optical module  12  of  FIG. 16A . From this side view perspective, the housing  180  is generally L-shaped with a window  182  at the sloped end of the housing. A right-angle prism  188  resides at the sloped end, with the hypotenuse side  164   a  of the prism  188  being exposed within the window  182  to serve as the transparent surface  170  or, if the window  182  has a pane, abutting a transparent surface of the pane. The lens  160  extends from another side  164   b  of the prism  188 . A third side  164   c  of the prism  188  abuts the bottom the housing  180 . 
     The mirrors  162 - 1 ,  162 - 2  are arranged to reverse the direction of the light as illustrated by arrow  168 ; the mirror  162 - 1  being directly opposite the lens  160  and fixed at a 45-degree angle within the housing  180  in order to reflect light coming through the lens  160 , which carries the image of a fingerprint, towards the mirror  162 - 2 . The mirror  162 - 2  is at a 90-degree angle with respect to the mirror  162 - 1  in order to complete the 180-degree reflection. The particular placement of the mirrors  162 - 1 ,  162 - 2  is configurable within each housing design and determined by the relative physical location of the camera lens of the mobile device  14 . The mirrors  162 - 1 ,  162 - 2  are disposed at preconfigured distances from the lens  160 , such distances from the lens  160  varying for different embodiments of the optical module  12 , depending upon the model (type) of mobile device  14  with which the optical module  12  is designed for use. The fingerprint image exits the optical module  12  through an aperture  166  ( FIG. 16C ) in a side  172  of the housing  180 . When positioned to capture fingerprint images, a mobile device  14  abuts this side  172  of the housing  180 , with the camera lens of the mobile device aligned with the aperture  166 . An attachment mechanism holds the mobile device  14  securely in place during the photographing of the fingerprint. 
       FIG. 16C  shows the housing  180  from the sloped end with the transparent surface  170 . This end view shows the aperture  166  below the transparent surface  170  (although, they are on different planes of the optical module  12 ). The mirror  162 - 2  reflects light out through this aperture  166 . 
       FIG. 16D  shows a bottom view of the housing  180 , with a diffuser window  178  at the sloped end of the housing  180 , for purposes of diffusing the incoming ambient light. In a preferred embodiment, the diffuser window  178  is made of a rigid translucent material, such as glass or plastic. In another embodiment, the diffuser window  178  is a piece of translucent tape placed over an opening in the housing at the third side  166   c  ( FIG. 16B ) of the prism  188 . Other embodiments of the diffuser window  178  can be fully transparent. 
       FIG. 16E  shows an end view (opposite the sloped end) of a mobile device  14  with a native camera coupled to one side of the housing  180 . A back-facing camera lens  190  of the mobile device  14  aligns with the aperture  166  opposite the mirror  162 - 2 , which is within the housing  180 . In this position, the front-side display of the mobile device  14  faces in the general direction of the diffuser window  178  and can thus be a source of light during image acquisition. Control of the illumination of the display during image acquisition may produce a more uniform illumination than ambient light alone. 
     To acquire a fingerprint, a subject presses his or her finger against the transparent surface framed by the window  182  at the sloped end of the housing. Ambient light enters the third side  164   c  of the prism  188  through the diffuser window  178  and illuminates the fingertip. The lens  160  of the prism  188  focuses the image of the fingerprint onto the mirror  162 - 1 , the mirror  162 - 1  reflects the light onto the mirror  162 - 2 , and the mirror  162 - 2  reflects the fingerprint image into the camera lens  190 . The camera system native to the mobile device  14  captures and saves the fingerprint image. The software system  30  ( FIG. 1 ) running on the mobile device  14  can subsequently process the fingerprint image to produce a high-quality image suitable for biometric matching. 
       FIG. 17A  shows a top view of another example of a different design for a housing  200  of an optical module  12 . At one end of the housing  200  is a right-angle prism  202 . At the end opposite the prism  202  is an attachment mechanism  204  for holding a mobile device  14 . Disposed within the housing  200 , between the prism  202  and the attachment mechanism  204 , is a lens  206 . 
       FIG. 17B  shows a side view of the housing  200 . A first side  208   a  of the prism  202  abuts a window  210  with a pane that provides a surface for the placement of one or more fingers. Alternately, the window  210  can be open (i.e., a frame without a pane), and the first side  208   a  of the prism  202  serves as a transparent surface for placement of the finger(s). The housing  200  further includes an interior light reflecting surface (e.g., a diffuse mirror)  212  for reflecting light into another side  208   b  of the prism  202 . The attachment mechanism  204  holds a mobile device  14  such that the lens  214  of a front-facing camera of the mobile device  14  faces the prism  202 . (The mobile device  14  may be held in a casing designed specifically to fit closely in and couple to the attachment mechanism  204 ). The camera lens  214  aligns with an aperture (not shown) in the housing. 
     Ambient light enters the housing  200  through a translucent or transparent diffuser window  216  (see also  FIG. 17C ), generally following path  218 , reflecting off the light reflecting surface  212 , passing into the prism  202  through side  208   b , and illuminating the one or more fingers placed on the transparent surface framed by window  210 . The image of the fingerprint(s) exits the prism  202  through its third side  208   c  and passes through the lens  206 . The lens  206  focuses the fingerprint image at the aperture, behind which is disposed the lens  214  of the front-facing camera of the mobile device  14 . In accordance with the principles described herein, the camera of the mobile device takes a snapshot of the fingerprint, and software running on the mobile device processes the image to produce a matching quality image. 
       FIG. 17C  shows an end view of the housing  200  with the diffuser window  216  adjacent to (or, in the figure, below) the transparent surface (i.e., window  210  or prism surface  208   a ). 
       FIG. 18A  shows a top view of still another example of a different design for a housing  220  of an optical module  12 . The housing  220  includes a transparent surface  222  (e.g., glass) at one end of the housing. Within the housing  220  is a mirror  224  at a preconfigured distance from a lens  226 . This distance varies for different embodiments of the optical module  12 , depending upon the model (type) of mobile device with which the optical module is designed for use. The particular placement of the mirror  224  can be configured appropriately within the housing to suit the type of mobile device and its relative physical location of the camera lens. 
       FIG. 18B  shows a side view of the housing  220 , wherein the end with the transparent surface  222  is sloped in shape. The housing  220  encloses a prism  228  disposed at the sloped end. One side  230   a  of the prism  228  can serve as the transparent surface  222 . (Alternatively, the transparent surface  222  can be a pane that abuts this side  230   a  of the prism  228 ). The lens  226  is disposed at another side  230   b  of the prism  228  (the lens  226  may be an integral part of the construction of the prism  228 ). The mirror  224  is fixed at a 45-degree angle within the housing  220  in order to reflect light coming through the lens  226  to one side of the housing, as illustrated by arrow  232 . A third side  230   c  of the prism  228  abuts the bottom the housing  220 . Where the third side  230   c  abuts, the housing  220  can have an open window so that the third side  230   c  is directly exposed to ambient light. This open window is covered by a diffuser  234  ( FIG. 18E ). 
       FIG. 18C  shows the housing  220  from the sloped end having the transparent surface  222 ;  FIG. 18D  shows the housing  220  from the end opposite the sloped end, with the mirror  224  and lens  226  appearing in phantom. 
       FIG. 18E  shows a bottom view of the housing  220 , with a diffuser window  234  at the sloped end of the housing  220 . The diffuser window  234  can be made of a translucent tape placed over an open window in the bottom of the housing and covering the exposed side  230   c  of the prism  228 . The housing  220  also has an elongated aperture  236  behind which the mirror  224  is visible. 
       FIG. 18F  shows a mobile device  14  with a camera coupled to the bottom side of the housing  220 . The lens  238  of the mobile device&#39;s camera aligns opposite the mirror  224  within the housing  220  to receive the image of the fingerprint. To acquire a fingerprint, a subject presses his or her fingertip against the transparent surface  222  at the sloped end of the housing  220 . Ambient light enters the third side  230   c  of the prism  228  through the diffuser window  234  and illuminates the fingertip. The lens  226  of the prism  228  directs the image of the fingerprint onto the mirror  224 , and the mirror  224  reflects the fingerprint image into the camera lens  238 . The arrangement is analogous to a simple periscope design, utilizing a lens and mirror to project the image onto a plane other than that of the camera lens  238 . The camera system, native to the mobile device  14 , captures and saves the fingerprint image. In another embodiment, a powered light source (e.g., a flash or display screen of the camera, an LED) is used to illuminate the fingertip pressed on the transparent surface  222  at the sloped end of the housing. 
     As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method, and computer program product. Thus, aspects of the present invention may be embodied entirely in hardware, entirely in software (including, but not limited to, firmware, program code, resident software, microcode), or in a combination of hardware and software. All such embodiments may generally be referred to herein as a circuit, a module, or a system. In addition, aspects of the present invention may be in the form of a computer program product embodied in one or more computer readable media having computer readable program code embodied thereon. 
     The computer readable medium may be a computer readable storage medium, examples of which include, but are not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination thereof. As used herein, a computer readable storage medium may be any tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, device, computer, computing system, computer system, or any programmable machine or device that inputs, processes, and outputs instructions, commands, or data. A non-exhaustive list of specific examples of a computer readable storage medium include an electrical connection having one or more wires, a portable computer diskette, a floppy disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), a USB flash drive, an non-volatile RAM (NVRAM or NOVRAM), an erasable programmable read-only memory (EPROM or Flash memory), a flash memory card, an electrically erasable programmable read-only memory (EEPROM), an optical fiber, a portable compact disc read-only memory (CD-ROM), a DVD-ROM, an optical storage device, a magnetic storage device, or any suitable combination thereof. 
     Program code may be embodied as computer-readable instructions stored on or in a computer readable storage medium as, for example, source code, object code, interpretive code, executable code, or combinations thereof. Any standard or proprietary, programming or interpretive language can be used to produce the computer-executable instructions. Examples of such languages include C, C++, Pascal, JAVA, BASIC, Smalltalk, Visual Basic, and Visual C++. 
     Transmission of program code embodied on a computer readable medium can occur using any appropriate medium including, but not limited to, wireless, wired, optical fiber cable, radio frequency (RF), or any suitable combination thereof. 
     The program code may execute entirely on a user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on a remote computer or server. Any such remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     In addition, the described methods can be implemented on an image processing device, fingerprint processing device, or the like, or on a separate programmed general purpose computer having image processing capabilities. Additionally, the methods of this invention can be implemented on a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit element(s), an ASIC or other integrated circuit, a digital signal processor, a hard-wired electronic or logic circuit such as discrete element circuit, a programmable logic device such as PLD, PLA, FPGA, PAL, or the like. In general, any device capable of implementing a state machine that is in turn capable of implementing the proposed methods herein can be used to implement the image processing system according to this invention. 
     Furthermore, the disclosed methods may be readily implemented in software using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer or workstation platforms. Alternatively, the disclosed system may be implemented partially or fully in hardware using standard logic circuits or a VLSI design. Whether software or hardware is used to implement the systems in accordance with this invention is dependent on the speed and/or efficiency requirements of the system, the particular function, and the particular software or hardware systems or microprocessor or microcomputer systems being utilized. The methods illustrated herein however can be readily implemented in hardware and/or software using any known or later developed systems or structures, devices and/or software by those of ordinary skill in the applicable art from the functional description provided herein and with a general basic knowledge of the computer and image processing arts. 
     Moreover, the disclosed methods may be readily implemented in software executed on programmed general purpose computer, a special purpose computer, a microprocessor, or the like. In these instances, the systems and methods of this invention can be implemented as program embedded on personal computer such as JAVA® or CGI script, as a resource residing on a server or graphics workstation, as a routine embedded in a dedicated fingerprint processing system, as a plug-in, or the like. The system can also be implemented by physically incorporating the system and method into a software and/or hardware system, such as the hardware and software systems of an image processor. 
     It is, therefore, apparent that there has been provided, in accordance with the present invention, methods for capturing and processing fingerprint images to produce high-quality fingerprints suitable for biometric matching applications. While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skill in the applicable arts. For example, an optical module can be adapted to capture fingerprints without the use of mirrors; for example, an elongated tube or rectangular housing with the prism at one end of the housing and the mobile phone with the camera at the opposite end of the housing. Accordingly, it is intended to embrace all such alternatives, modifications, equivalents, and variations that are within the spirit and scope of this invention. 
     Relative terms used herein, such as top, bottom, front, back, side, left, right, above, below, upper, and lower, refer to how features of the apparatus appear in the figures, and serve to facilitate the description of the invention, and are not meant to be interpreted as limitations.