Patent Publication Number: US-9904836-B2

Title: Reducing edge effects within segmented acoustic imaging systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a nonprovisional patent application of, and claims the benefit under 35 U.S.C. § 119 (e) to, Provisional Patent Application No. 62/057,884, filed Sep. 30, 2014 and titled “Systems and Methods for Acoustic Fingerprint Imaging,” the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD 
     This disclosure relates generally to security features for electronic devices, and more particularly, to systems and methods for operating segmented acoustic imaging systems. 
     BACKGROUND 
     Many electronic devices include security features to prevent unauthorized access. For example, an electronic device can include a biometric sensor configured to establish a user&#39;s identity by determining whether obtained biometric data matches known biometric data of an authorized user. A fingerprint imaging system is one example of a biometric sensor. 
     In many cases, the performance of a biometric sensor may be affected by the precision with which biometric data can be detected. Although increased precision may lead to improved security for the electronic device, it may also result in a physical reduction in the size of one or more sensing components of the sensor. For example, a high-precision fingerprint imaging system may require smaller imaging sensors than a low-precision imaging system. 
     The quality of a signal obtained from physically smaller sensing components is often negatively affected by the components&#39; smaller size. For example, small imaging sensors may detect less image information than large imaging sensors, generating lower-amplitude signals that may be more sensitive to interference. In other cases, small sensing components may be more susceptible to signal distortion from environmental conditions (e.g., temperature, pressure, humidity, and so on) than larger sensing components. 
     To account for the lower signal quality that may be associated with smaller sensing components, many biometric sensors may require advanced signal processing capability to mitigate effects of noise and/or interference, which may undesirably increase power consumption and/or processing delays of the system. In addition, smaller sensing components that are more densely arranged may often require more intricate and/or complex circuitry for operating each individual sensing component. As a result, high-precision biometric sensors may be difficult to include within small form factor electronic devices having limited power and processing resources. 
     SUMMARY 
     Embodiments described herein take the form of a fingerprint imaging system including a substrate having a surface (e.g., interrogation surface) that receives a subject for imaging (e.g., pad of a finger). The fingerprint imaging system can direct an acoustic plane wave toward the interrogation surface and thereafter monitor for and obtain an acoustic reflection (e.g., echo) from the interrogation surface. By analyzing the acoustic reflection for variations an image of the subject can be captured. 
     In many embodiments, the imaging system includes a plurality of transducers in acoustic communication with the substrate. The plurality of transducers can be distributed in an array along a surface of the substrate opposite the interrogation surface. Each transducer can be configured to generate an acoustic output (e.g., pulse, wave, and so on) into the substrate in response to an electronic activation pulse. In one embodiment, an electronic activation pulse can be short in duration (in certain embodiments, five to ten nanoseconds, although the duration may be longer or shorter in other embodiments). 
     The imaging system can also include a controller configured to provide electronic activation pulses, substantially simultaneously, to each transducer such that the acoustic output produced by each respective transducer arrives substantially simultaneously at the interrogation surface. In this manner, the plurality of transducers can produce a plane wave directed toward the interrogation surface. 
     Some embodiments described herein may take the form of an interrogation controller in communication with a plurality of acoustic transducers coupled to the bottom surface of a substrate. The interrogation controller can be configured to perform interrogation of a top surface of the substrate. For example, the interrogation controller can select a first subarray of adjacent transducers associated with a first subarea of the bottom surface. The interrogation controller can also select a second subarray of adjacent transducers associated with a second subarea positioned on the perimeter of first subarea. Next, the interrogation controller can activate the first and second subarray of transducers simultaneously. 
     Some embodiments described herein may take the form of method of interrogating a subarea of a surface of a substrate including at least the operations of selecting a first subarray of transducers associated with the subarea, obtaining electrical signals from each of the transducers of the first subarray, selecting a second subarray of transducers associated with the subarea and obtaining electrical signals from each of the transducers of the second subarray. 
     Some embodiments described herein may relate to, include, or take the form of a method of operating an acoustic imaging system including at least the operations of selecting a first set of transducers from a plurality of transducers, selecting a second set of transducers from the plurality of transducers (disjoint with the first set) and adjacent to at least a portion of a perimeter of the first set. The method may also include the operation of coupling each transducer of the first and second set to a plurality of drive lines, providing an electronic activation pulse to each transducer associated with the first and second set of transducers via the drive lines, coupling each transducer of the first set to a plurality of read lines, each read line configured to obtain an electronic signal from at least one transducer to which the read line may be coupled, and obtain electrical signals from each transducer of the first set. In this manner, certain embodiments can include different numbers of drive and read lines. 
     Some embodiments described herein may relate to, include, or take the form of a method of operating an acoustic imaging system with a plurality of transducers in acoustic communication with a bottom surface of a substrate, each configured to produce an acoustic output into the substrate toward a top surface of the substrate in response to an electronic activation pulse, the method including at least the operations of selecting a first set of transducers, selecting a second set of transducers (disjoint with the first set) adjacent to at least a portion of a perimeter of the first set, and providing an electronic activation pulse to each transducer associated with the first and second set of transducers such that an acoustic output produced by each respective transducer arrives at the top surface substantially simultaneously. 
     Some embodiments described herein may relate to, include, or take the form of an acoustic imaging system including at least a substrate having a top surface for receiving a subject for imaging, and a bottom surface opposite the top surface, a plurality of transducers in acoustic communication with the bottom surface, each transducer configured to produce an acoustic output into the substrate toward the top surface in response to an electronic activation pulse, the plurality of transducers including at least a first set with at least one transducer, and a second set (disjoint with the first set) positioned proximate a perimeter of the first set, and a controller configured to provide an electronic activation pulse substantially simultaneously to each transducer associated with the first and second sets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made to representative embodiments illustrated in the accompanying figures. It should be understood that the following descriptions are not intended to limit the disclosure to one preferred embodiment. To the contrary, each is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the described embodiments as defined by the appended claims. 
         FIG. 1  depicts an example electronic device that can include an acoustic fingerprint imaging system. 
         FIG. 2A  depicts a simplified block diagram of an acoustic fingerprint imaging system. 
         FIG. 2B  depicts a side cross-section simplified block diagram of another acoustic fingerprint imaging system. 
         FIG. 2C  depicts the acoustic fingerprint imaging system of  FIG. 2A  emitting an acoustic pulse toward an imaging surface. 
         FIG. 2D  depicts the acoustic fingerprint imaging system of  FIG. 2A  receiving an acoustic reflection from an imaging surface. 
         FIG. 2E  depicts the acoustic fingerprint imaging system of  FIG. 2A  emitting a planar acoustic pulse toward an imaging surface. 
         FIG. 2F  depicts the acoustic fingerprint imaging system of  FIG. 2A  receiving a planar acoustic reflection from an imaging surface. 
         FIG. 3A  depicts a top view an acoustic transducer array configured for interrogation of an imaging surface. 
         FIG. 3B  depicts a top view of one state of the acoustic transducer array of  FIG. 3A . 
         FIG. 4  depicts example operations of a method of interrogating a subarea of an imaging surface with an acoustic transducer array. 
         FIG. 5  depicts example operations of a method of performing interrogation of an imaging surface. 
         FIG. 6  depicts example operations of another method of interrogating a subarea of an imaging surface with an acoustic transducer array. 
         FIG. 7  depicts example operations of another method of interrogating a subarea of an imaging surface with an acoustic transducer array. 
         FIG. 8  depicts example operations of a method of assembling a composite image from a series of subimages. 
         FIG. 9  depicts example operations of another method of interrogating a subarea of an imaging surface with an acoustic transducer array coupled to a plurality of drive lines and a plurality of read lines. 
     
    
    
     The use of the same or similar reference numerals in different drawings indicates similar, related, or identical items where appropriate. 
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalties of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Embodiments described herein relate to methods and systems for operating acoustic fingerprint imaging systems. Although many embodiments are described herein with reference to fingerprint imaging systems for use with small form factor electronic devices, it should be appreciated that some embodiments can take other acoustic imaging and sensing forms and additionally or separately may be included within electronic devices or systems having different form factors. Accordingly, it should be appreciated that the various embodiments described herein, as well as the functionality, operation, components, and capabilities thereof may be combined with other elements as necessary, and so any physical, functional, or operational discussion of an element or feature is not intended to be limited solely to a particular embodiment to the exclusion of others. 
     Additionally, although many embodiments are described herein with reference to acoustic imaging systems that generate plane waves when in an integration or interrogation mode, it should be appreciated that some embodiments can utilize other signals and/or waveforms (e.g., chirp wave(s), square wave(s), sinusoidal wave(s), and so on). 
     Conventional acoustic imaging systems can include a substrate with two parallel surfaces such as a top surface and a bottom surface. An array of acoustic transducers can be arranged in a pattern and positioned near the bottom surface. To capture an image of an object engaging the top surface, the conventional acoustic imaging system can cause the entire array to propagate a plane wave toward the object by activating every transducer simultaneously. When the plane wave reaches the top surface of the substrate, a portion of the plane wave may be reflected back towards the array of transducers. The reflection can be collected and an image of the top surface of the substrate can be approximated. Such systems are herein referred to as “global” acoustic imaging systems or “conventional” acoustic imaging systems. 
     In many cases, however, an image obtained from a global acoustic imaging system may be of insufficient quality, contrast, or accuracy for use with a fingerprint imaging system. For example, a global acoustic imaging system typically requires a substrate having a high degree of acoustic impedance uniformity. As one example, a substrate having non-uniform acoustic impedance may not be capable of propagating a plane wave. As another example, a portion of a substrate having higher acoustic impedance may propagate an acoustic wave faster than a portion having lower acoustic impedance. Accordingly, images derived by global acoustic imaging systems are often distorted, inaccurate, or imprecise. 
     Furthermore, global acoustic imaging systems typically require a large number of complex electrical circuits in order to generate a plane wave. For example, some acoustic transducers may require more than 100 volts to generate an acoustic pulse, which may necessitate a large number of drive circuits capable to handle and deliver 100 volts to each transducer within an array simultaneously. Similarly, global imaging systems typically require a large number of complex electrical circuits to obtain electrical signals from each transducer simultaneously. 
     Accordingly, many embodiments described herein relate to acoustic imaging systems that can be used to efficiently obtain a high-quality image of a fingerprint. An imaging system in accordance with embodiments described herein can include a substrate with two parallel surfaces such as a top surface and a bottom surface. An array of acoustic transducers can be arranged in a pattern (e.g., grid, matrix, and so on) and positioned along the bottom surface. In many cases, the array of acoustic transducers can be formed to take a particular shape, such as a square or rectangular shape. To capture an image of an object engaging the top surface (e.g., fingertip, stylus tip, and so on), the imaging system can cause a selected subarray of the array to propagate a plane wave normal to the bottom surface and/or the top surface and toward the object and to monitor for any acoustic signals reflected therefrom. In many cases, the time period between the excitation of the plane wave and the time any reflections related thereto are received can be based, at least in part on the acoustic impedance (and/or an average acoustic impedance) of the material selected for the substrate. Accordingly, various methods of operation of acoustic imaging system embodiments described herein, in addition to material selections, layout, and or structure of said acoustic imaging systems, can be selected (at least in part) on the acoustic properties of the substrate. 
     When the plane wave reaches the top surface of the substrate, a portion of the plane wave may be reflected back towards the subarray of transducers as a result of the acoustic boundary between the top surface and the object engaging it (e.g., due to acoustic impedance mismatch). For example, a ridge of a finger typically reflects an acoustic plane wave differently than a valley of a finger. 
     When the acoustic reflection returns to the subarray of transducers, the subarray can be used to capture the reflection as electrical signals. For example, when a transducer receives a portion of the acoustic reflection affected by a ridge of a fingerprint, that transducer may produce an electrical signal that is different than the electrical signal produced by a transducer receiving a reflection affected by a valley of a fingerprint. 
     By analyzing the electrical signals, the imaging system derives a portion of an image (e.g., subimage) of the object engaging the top surface of the substrate. For example, each electrical signal can correspond to one pixel of the subimage. In one embodiment, a pixel corresponding to an electrical signal affected by a ridge of a fingerprint may be lighter than a pixel corresponding to an electrical signal affected by a valley of a fingerprint. 
     Once a subimage is obtained, a second subarray of acoustic transducers can be selected and a second subimage can be obtained therefrom. The imaging system can continue until each transducer in the entire array of transducers has been selected at least once. Thereafter all obtained subimages can be assembled together to approximate an image of the object engaging the top surface of the substrate. 
     These embodiments, relating to acoustic imaging systems that are configured to stitch multiple segments or subimages together to form a single image, are referred to herein as “segmented” acoustic imaging systems. 
     Still further embodiments described herein relate to acoustic imaging systems that progressively obtain electrical signals from individual sections (e.g., row, column, line, and so on) of a subarray to form a subimage (hereinafter “progressive” acoustic imaging systems). To capture an image, the progressive system can cause the acoustic transducers (or a subarray of the acoustic transducers) to direct a series of plane waves toward the top surface of the substrate. 
     Thereafter, the progressive system can sequentially capture a portion of an image of an acoustic reflection and obtain any electrical signals from the transducers associated with subsections of the subarray. For example, if a subarray can be divided into ten rows, the imaging system may cause the entire subarray to propagate ten plane waves in succession toward the top surface of the substrate. Once the ten plane waves are reflected, each of the ten rows can be sequentially coupled to circuits for obtaining electrical signals. The electrical signals can thereafter be correlated to pixels of the image or subimage. 
     Still further embodiments described herein can implement both segmented acoustic imaging and progressive acoustic imaging. 
       FIG. 1  depicts an example electronic device that can include an acoustic fingerprint imaging system such as a segmented imaging system and/or a progressive imaging system. In the present example, the electronic device  100  can be implemented as a portable electronic device such as a cellular phone. The electronic device  100  can include a housing  102  and a display  104 . The display  104  can be disposed below a cover glass to protect the display  104  from damage. In many cases, the cover glass can be formed from an optically transparent and mechanically rigid material such as glass, sapphire, polycarbonate, and the like. 
     In many examples, the display  104  may include one or more of a variety of display elements. For example, the display  104  may include a liquid crystal display (LCD), a thin film transistor display (TFT), an organic light emitting diode display (OLED), organic electroluminescence (OEL) display, or other type of display. The display  104  may be used to present visual information to the user and may be operated in accordance with one or more display modes or the software applications being executed on the electronic device  100 . In many cases, the display  104  can include or operate in conjunction with one or more touch input devices. For example, the display  104  may be configured to receive touch, gesture, and/or force input. 
     The electronic device  100  can also include one or more input elements such as a button  106 . The button  106  may be a physical button such as a push button or switch. In other examples, the button  106  can be a touch input device that does not physically depress such as a capacitive button. In other cases, the button  106  can be a virtual button shown on the display  104 . 
     In many embodiments, an acoustic fingerprint imaging system (not shown) can be positioned below a portion of the cover glass that may regularly receive a user&#39;s touch input. For example, the acoustic fingerprint imaging system can be positioned below the button  106 . In this manner, each time the user  108  presses the button  106 , the acoustic fingerprint imaging system can be activated. Once an image of fingerprint of the user  108  is obtained by the acoustic fingerprint imaging system, the obtained image can be compared to a database of known fingerprint images to determine if the obtained fingerprint image matches a known fingerprint image. 
     In some embodiments, the acoustic fingerprint imaging system can be positioned below the display  104 . For example the acoustic fingerprint imaging system can be positioned below the display element associated with the display  104 . In other examples, the acoustic fingerprint imaging system can be at least partially transparent and can be disposed above the display element associated with the display  104 . In this manner, an image of a fingerprint may be obtained by touching any portion of the display  104 . In other examples, the acoustic fingerprint imaging system can be at least partially disposed below a portion of the display (e.g., bottom third of the display, edge of the display, top of the display, middle of the display and so on), so that an image of a fingerprint can be obtained by receiving a user touch at a specific area of the display. 
     In other examples, the acoustic fingerprint imaging system can be positioned within the sidewall of the housing  102 . In this manner, when a user grips the electronic device  100 , an image can be obtained of one or more fingerprints or palm prints of the user. In still further examples, more than one acoustic fingerprint imaging system can be included within the electronic device  100 . For example, a first acoustic imaging system can be included below (or within) the button  106 , and a second acoustic imaging system can be included below (or within) the display  104 . 
       FIG. 2A  depicts a simplified block diagram of an acoustic fingerprint imaging system that can be used with the electronic device  100  of  FIG. 1 . The acoustic fingerprint imaging system  200  can include one or more acoustic transducers  202 . The acoustic transducers  202  can contract or expand rapidly in response to an electrical stimulus such as a voltage or current (e.g., electroacoustic transducer). For example, the acoustic transducers  202  can be formed, in certain embodiments, from a piezoelectric material. In other examples, the acoustic transducers  202  may be formed as a component configured to rapidly displace in response to an electrical stimulus such as a voltage or current (e.g., voice coil). 
     In many embodiments, the acoustic transducers  202  can be configured for both emitting and detecting acoustic signals. In other words, an acoustic transducer of the one or more acoustic transducers  202  can be used to both transmit an acoustic pulse in response to an electrical stimulus and, in addition, can generate an electrical signal in response to an acoustic input. In many cases, an acoustic transducer of the one or more acoustic transducers  202  can take a particular shape. For example, in some embodiments an acoustic transducer can take a substantially rectilinear shape such as a square or rectangle. In some embodiments, an acoustic transducer can take a circular shape. 
     In many examples, the acoustic transducers  202  can be arranged in a pattern. For example, in some embodiments the acoustic transducers  202  can be arranged in an evenly spaced line such as illustrated in  FIG. 2A . In some embodiments, the acoustic transducers  202  can be arranged in a matrix or grid, as shown in  FIG. 2B . In some examples, the matrix of the acoustic transducers  202  can be square or otherwise rectangular. In other examples, the matrix of the acoustic transducers  202  can take other shapes, such as a circular pattern (not shown). 
     Although many embodiments described herein distribute the acoustic transducers  202  in a uniform pattern (e.g., matrix, square, line, circle, and so on), such uniformity is not necessarily required. 
     The acoustic transducers  202  can be coupled to a controller  204 . The controller  204  can be configured to provide electrical energy to each acoustic transducer of the one or more acoustic transducers  202  independently. For example, the controller  204  can provide a first voltage to a first transducer and a second voltage to a second transducer. In addition, the controller  204  can control the duration and magnitude of the electrical energy applied to each independent acoustic transducer of the one or more acoustic transducers  202 . 
     In many examples, the controller  204  can operate in one or more modes. In certain embodiments, the controller  204  can have an interrogation mode. In some embodiments and implementations, the interrogation mode can be referred to as an integration mode or a drive mode. Accordingly, as used herein, terms and phrases such as “integration mode”, “drive mode”, and “interrogation mode” may be understood to each refer to the same operational mode of an acoustic fingerprint imaging system. When in the interrogation mode, the controller  204  can be configured to provide electrical energy to one or more of the acoustic transducers  202  and in response, the acoustic transducers  202  can produce an acoustic output. In many embodiments, the electrical energy provided by the controller  204  can be an abbreviated electrical pulse. In response to the abbreviated electrical pulse, the one or more acoustic transducers  202  can produce a punctuated acoustic output. 
     After producing the punctuated acoustic pulse, the controller  204  can transition into an imaging mode. In an imaging mode the controller  204  can couple to one or more acoustic transducers  202  and monitor for electrical signals therefrom that may be generated by the acoustic transducers  202  in response to receiving an acoustic signal (e.g., acoustic reflection). The controller  204  can, thereafter, analyze the obtained electrical signals and can approximate and/or derive an image therefrom. 
     In many embodiments, the acoustic fingerprint imaging system  200  can be disposed within a housing of an electronic device. For example,  FIG. 2C  depicts the acoustic fingerprint imaging system of  FIG. 2A  positioned below one or more layers of a substrate  208 . As illustrated, the substrate  208  can have a bottom surface  210  and a top surface  212 . The bottom surface  210  can be oriented to face the acoustic transducers  202 , and the top surface  212  can be oriented to receive a user&#39;s finger  206 , illustrated in simplified cross-section showing ridges  206   a  and valleys  206   b  of the user&#39;s fingerprint. Although the bottom surface  210  and top surface  212  are shown as parallel, such a configuration may not be required in all embodiments. 
     As noted with respect to  FIG. 1 , the substrate  208  can be a portion of a display, a portion of an input device (e.g., button, switch, and so on), or a portion of the housing of the electronic device. Although illustrated as two separate layers of material, many embodiments can implement the substrate  208  as a single layer of material, or more than two layers of material. The substrate  208  can include active components (e.g., circuits, circuit traces, batteries, and so on) or passive components (e.g., glass sheet, metal sheet, and so on) or a combination thereof. 
     For example, if for a selected embodiment, the substrate  208  is a portion of the housing of an electronic device, the substrate  208  can be formed from one or more layers of metal, glass, ceramic, plastic, and so on. In some embodiments, if the substrate  208  is a portion of a cover glass disposed above a display, the substrate  208  can be an optically transparent material such as glass, sapphire, plastic, and so on. In other cases, the substrate  208  can include layers formed from more than one material. For example, if the substrate is a portion of a display, the substrate  208  can include a filter layer, a polarizing layer, a backlight layer, and so on. 
     The acoustic transducers  202  can be positioned below the substrate  208  so as to be in acoustic communication with the bottom surface  210 . In many examples, the acoustic transducers  202  are adhered to (or formed onto or into) the bottom surface  210 . In this manner, when an acoustic transducer of the one or more acoustic transducers  202  generates an acoustic output  214  in response to an activation signal from the controller  204  (when in the interrogation mode), the acoustic output  214  can propagate into the substrate  208  from the bottom surface  210  toward the top surface  212 . 
     Once the acoustic output  214  reaches the top surface  212 , the controller  204  can transition into the imaging mode, such as depicted in  FIG. 2D . When in the imaging mode, the controller  204  can be configured to receive and analyze an electrical signal from one or more of the acoustic transducers  202  that corresponds to a portion of an acoustic reflection of the acoustic output  214  (see, e.g.,  FIG. 2C ). 
     For example, in many embodiments, the acoustic reflection from a particular location along the top surface  212  may depend upon whether that location is below a ridge  206   a  or a valley  206   b . More particularly, the acoustic boundary between the substrate  208  and a ridge  206   a  (having an acoustic impedance of soft tissue) may cause a measurably smaller-amplitude acoustic reflection than the acoustic boundary between the substrate  208  and a valley  206   b  (having an acoustic impedance of air). 
     For example, the percent P diff  of amplitude of an acoustic reflection as a result of an acoustic boundary between two materials M 1  and M 2 , having an acoustic impedance Z 1  and Z 2  respectively may be modeled as: 
     
       
         
           
             
               
                 
                   
                     P 
                     diff 
                   
                   = 
                   
                     
                       
                         
                           Z 
                           1 
                         
                         
                           Z 
                           2 
                         
                       
                       - 
                       1 
                     
                     
                       
                         
                           Z 
                           1 
                         
                         
                           Z 
                           2 
                         
                       
                       + 
                       1 
                     
                   
                 
               
               
                 
                   Equation 
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                   1 
                 
               
             
           
         
       
     
     In this example, if the acoustic impedances Z 1  and Z 2  of the two materials are substantially equal, the amplitude of any acoustic reflection is minimal or zero. On the other hand, if one of the two acoustic impedances Z 1  or Z 2  is larger than the other, P diff  approaches ±1.0. In many examples, the direction of the wave (e.g., from high impedance to low impedance medium or the reverse) can determine whether the amplitude will be inverted upon reflection. 
     As noted above, the amplitude of a reflection from a ridge-substrate acoustic boundary may be a smaller than the amplitude of a reflection from a valley-substrate acoustic boundary. In other words, the amplitude of an acoustic reflection  216  from an area of the top surface  212  that is below a ridge  206   a  may be less than the amplitude of an acoustic reflection  218  from an area of the top surface  212  that is below a valley  206   b . Accordingly, the controller  204 , when in an imaging mode, can monitor the amplitude of an acoustic reflection to derive, determine, assemble, or create, an image of the ridges and valleys of a user&#39;s fingerprint. As illustrated, the differences in the thickness of the lines depicting the acoustic reflection  218  and the acoustic reflection  216  is intended to indicate a difference in amplitude between the acoustic reflection affected by a ridge and an acoustic reflection affected by a valley of a fingerprint. 
     In many cases, the quality of the approximated image may be directly affected by attenuation of the acoustic reflection. For example, an acoustic output  214  may not propagate through the substrate  208  uniformly and as a result, the acoustic reflection(s)  216 ,  218  and the image approximated therefrom can be distorted. 
     To mitigate the effects of reflection attenuation that may result from activation of a single acoustic transducer of the one or more acoustic transducers  202  such as shown in  FIG. 2C , the acoustic fingerprint imaging system  200  can be configured to cause (via the controller  204 ) the acoustic transducers  202  to propagate a plane wave acoustic output  220  into the substrate  208  such as shown in  FIG. 2E . The plane wave can thereafter be reflected from the top surface  212  as a planar acoustic reflection  222 , such as shown in  FIG. 2F . As may be appreciated, a plane wave (across certain distances) can propagate along a vector without substantially propagating in other directions. In this manner, the amplitude of the plane wave acoustic output  220  when it reaches the top surface  212  (see e.g.,  FIG. 2E ) may be greater than the amplitude of the acoustic output  214  that propagates spherically (see, e.g.  FIG. 2C ). 
     Accordingly, many embodiments can propagate plane waves to obtain a high-quality image of a fingerprint. For example, as noted above, certain embodiments include an array of acoustic transducers arranged in a pattern (e.g., grid, matrix, and so on) and positioned along the bottom surface of the substrate. To capture an image of an object engaging the top surface (e.g., fingertip, stylus tip, and so on), the imaging system can cause a selected subarray of the array to propagate a plane wave normal to the bottom surface and toward the object in order to monitor for any acoustic signals reflected therefrom. 
     For example,  FIG. 3A  depicts a top view of an acoustic transducer array  300  configured for interrogation of a top surface of a substrate. As illustrated, the acoustic transducer array  300  can be formed from a grid of four hundred individual transducers arranged in a 20×20 grid. As with some embodiments described herein the acoustic transducer array  300  can be coupled to the bottom surface of the substrate. 
     Although illustrated as a grid, the acoustic transducer array  300  can take other shapes. For example, the acoustic transducer array  300  can be arranged as a rectangle in certain embodiments. In some embodiments, the acoustic transducer array  300  can be arranged as a circle. In further embodiments, other arrangements are possible. 
     Although illustrated with each transducer immediately adjacent neighboring transducers, such a configuration may not be required in all embodiments. For example, certain embodiments can dispose individual transducers of the array slightly separated from one another. 
     In still further embodiments, a greater or fewer number of transducers can be used. In some embodiments, the transducers of the acoustic transducer array  300  may not necessarily be distributed evenly. For example, in certain embodiments, certain transducers may be spaced a greater distance from that transducers nearest neighboring transducer than other transducers of the acoustic transducer array  300 . 
     As noted above, many embodiments described herein can implement segmented acoustic imaging. For example, the acoustic transducer array  300  can be coupled to and at least partially controlled by a controller, such as the controller  302 . To capture an image with the acoustic transducer array  300 , the controller  302  can cause a selected subarray  304  of the acoustic transducer array  300  to propagate a plane wave normal to the bottom surface and toward the object in order to monitor for any acoustic signals reflected therefrom. As illustrated, the plane wave generated by the selected subarray  304  may be directed out of the page. 
     As illustrated, the selected subarray  304  can include twenty five individual transducers arranged in five rows of five columns, although this configuration or number of transducers is not required for each embodiment. For example, different embodiments can take another shape (e.g., rectangle, square, oval, hexagon, and so on), and/or may include a different number of acoustic transducers. 
     As noted with respect to embodiments described above, when the plane wave from the selected subarray  304  reaches a top surface of the substrate through which the plane wave propagates, a portion of it may be reflected back towards the selected subarray  304  which can, in turn, be used to capture the reflection as one or more electrical signals. For example, as illustrated, the controller  302  can receive twenty five independent electrical signals. By analyzing the electrical signals, the controller  302  can derive a subimage of the object engaging the top surface of the substrate. For example, in some embodiments each electrical signal can correspond to one pixel of the subimage. As one example, some embodiments can generate a twenty-five pixel square image from the electrical signals received from the selected subarray  304 . 
     Once a subimage is obtained from the selected subarray  304 , a second subarray  306  of acoustic transducers can be selected and a second subimage can be obtained therefrom. As illustrated, the second subarray  306  can be selected to be adjacent to the selected subarray  304 , although this may not be required for all embodiments. For example, different subarrays can partially or completely overlap with one or more other subarrays. In some embodiments, subarrays can take the shape of a square or rectangle although this configuration is not required. In other cases, certain embodiments can select subarrays having a circular, oval, or hexagonal shape. In some embodiments, other shapes can be used. In still further examples, different subarrays can have different shapes. 
     Thereafter, the controller  302  can continue until each transducer of the acoustic transducer array  300  has been selected and imaged at least once. Thereafter all obtained subimages can be assembled together to approximate an image of the object engaging the top surface of the substrate. 
     As noted above, implementing a segmented plane wave technique for acoustic imaging with the acoustic transducer array  300  can provide several advantages over global imaging systems. For example, a segmented technique for acoustic imaging (such as partially depicted in and described with respect to  FIG. 3A ) can be used with a wider variety of substrates than global imaging system. For many materials, local uniformity of acoustic impedance may be greater than global uniformity. As one example, the acoustic impedance of a portion of a substrate may be sufficiently uniform to effectively propagate a plane wave despite the fact that the entire substrate may not itself be sufficiently uniform. Accordingly, a segmented acoustic imaging system may be capable to generate higher-quality plane wave through subareas via of a non-uniform substrate than a global imaging system can generate through the entire substrate. Correspondingly, the quality of the acoustic reflections from segmented acoustic imaging systems may be greater than those from global acoustic imaging systems and, consequently, the quality of images derived by segmented acoustic imaging systems may be greater than those approximated from global acoustic imaging systems. 
     In other cases, a segmented technique for acoustic imaging may require fewer high voltage drive circuit components and fewer circuits configured to collect the electrical signals from the several transducers (e.g., “sense” circuitry). For example, in the embodiment illustrated in  FIG. 3A , a global imaging system may require four hundred independent drive circuits and four hundred sense circuits. A segmented imaging system, on the other hand, may only require twenty five or less independent drive circuits and twenty five or less sense circuits. In many cases, one or more multiplexers can couple the drive and sense circuitry to individual subarrays. In some embodiments, other means of connecting drive and sense circuitry to selected subarrays can be used. In these examples, a segmented imaging system may provide substantial power efficiency advantages over a conventional acoustic imaging system. 
     However, despite the several advantages of segmented imaging techniques, in some embodiments, the quality of the plane wave emitted from a subarray may not be uniform across the entire area of the plane. More particularly, in addition to propagating from the bottom surface of the substrate to the top surface of the substrate, the perimeter of a plane wave may propagate laterally through the substrate to some extent, reducing the amplitude of any acoustic reflection thereof. In other words, in many embodiments, electrical signals generated by perimeter acoustic transducers  306   a  nearby the boundaries of a plane wave may be of low quality. In many embodiments, the partial lateral propagation of a plane wave and related effects are generally referred to as “edge effects.” 
     Accordingly, to mitigate edge effects many imaging systems described herein can propagate a plane wave that is larger than subarray that is configured to receive any acoustic reflection. For example,  FIG. 3B  depicts the acoustic transducer array  300  of  FIG. 3A , showing perimeter subarrays  308  activated at the same time the selected subarray  304  is activated. In this embodiment, the perimeter subarrays  308  can be activated simultaneously with the selected subarray  304 . As illustrated four perimeter subarrays  308 , each including twenty transducers arranged in a 5×4 grid are positioned about the four edges of the selected subarray  304 . In this manner, the entire configuration forms the shape of an addition sign. 
     In this configuration, a singular plane wave that is larger than the area of the selected subarray  304  can be propagated toward the top surface of the substrate. 
     Although the perimeter subarrays  308  participate with the selected subarray  304  to generate a plane wave, the acoustic reflection(s) may be received by the selected subarray  304  only. As one example, the controller  302  can obtain electrical signals generated by the selected subarray  304 , ignoring any electrical signals received within the perimeter subarrays  308 . In this manner, the electrical signals used by the controller  302  to derive and/or approximate a subimage associated with the selected subarray  304  may be less susceptible to edge effects. 
     One may appreciate that the illustrated configuration of the perimeter subarrays  308  is not the only possible configuration and that other perimeter subarrays are possible within the scope of the disclosure provided herein. For example, in certain embodiments, the perimeter subarrays  308  may not necessarily extend the same depth as the measurement subarray. For example, in some embodiments the perimeter subarray can extend only a single transducer. In another embodiment, the perimeter subarray can take a shape different from that of the measurement subarray. For example, in the illustrated embodiment, the selected subarray  304  is a square. In some cases, the perimeter subarrays can be rectangular, triangular, or may take a ring configuration. In some embodiments, other configurations are possible. 
       FIG. 4  depicts example operations of a method of interrogating a segment of an imaging surface with an acoustic transducer array. For example, the method depicted in  FIG. 4  can be performed, in some embodiments, by the controller  302  of  FIGS. 3A-3B . The method may begin at operation  400  in which an interrogation area can be selected. At operation  402 , a transducer subarray that is associated with the interrogation area can be selected. Thereafter, at operation  404 , a selected number of transducers that are associated with the perimeter of the selected transducer subarray can be selected. As noted above, these perimeter transducers may take any suitable shape and/or may extend beyond the edges of the selected transducer group for different amounts for different embodiments. Lastly, at operation  406 , all selected transducers can be activated substantially simultaneously. In this manner, a plane wave can be produced. 
     Many embodiments described above relate to segmented acoustic imaging. However, in certain cases, imaging of an entire subarray may be difficult for certain implementations. Accordingly, still further embodiments described herein relate to acoustic imaging systems that progressively obtain electrical signals from individual sections (e.g., row, column, line, and so on) of a subarray to form a subimage. To capture an image, the progressive system can cause the acoustic transducers (or a subarray of the acoustic transducers) to direct a series of plane waves toward the top surface of the substrate. Thereafter, the same acoustic transducers (or subarray or subset of the same acoustic transducers) can monitor for a series reflections, spaced in time at approximately the same interval as the series of plane waves directed toward the top surface of the substrate. 
     For example, in certain embodiments, a single plane wave can be generated by a progressive imaging system by exiting a number of transducers of a subarray for a selected number of nanoseconds (e.g., 1-10 ns). As noted above, the progressive imaging system causes the acoustic transducers of these embodiments to generate nanosecond-scale pulses in rapid succession. For example, in some embodiments, a single plane wave can be defined by a 5 ns excitation of a subarray of transducers. In this example, a series of plane waves may be propagated by exciting the subarray for 5 ns, disabling the subarray for a certain period (e.g., 10-20 ns), and exciting the subarray for another 5 ns. 
     In some embodiments, one or more transducers can be driven by other waveforms having different periods, frequencies, peak amplitudes, and so on. In one example, three to five periods of a sine wave (for example, with a frequency of twenty megahertz) can be used to propagate three to five plane waves, respectively, toward the top surface of the substrate. 
     Thereafter, the progressive system can sequentially capture a portion of an image of an acoustic reflection and obtain any electrical signals from the transducers associated with subsections of the subarray. For example, if a subarray can be divided into ten rows, the imaging system may cause the entire subarray to propagate ten plane waves in succession toward the top surface of the substrate. Once the ten plane waves are reflected, each of the ten rows can be sequentially coupled to circuits for obtaining electrical signals. The electrical signals can thereafter be correlated to pixels of the image or subimage. 
     In an alternative and non-limiting phrasing, a subarray including ten rows can be excited at the same time and, thereafter, a first row of the subarray can be activated to receive the reflected waves. Next the entire subarray can be excited for a second time and then a second row of the subarray can receive the reflected waves. This operation can be repeated ten times until all ten rows of the subarray have received the reflected waves. In other cases, excitations of the entire subarray can be sequential as described above. For example, the subarray can be exited ten times in a row, with a short pause between each excitation. Thereafter, the respective corresponding reflected waves can also be separated by the same short pause. In these embodiments, the ten rows can be activated to receive the reflected waves one at a time, separated by the short pause. In other cases, sequential excitation can occur in groups. For example, the subarray may be excited five time in a row, after which the first five rows may be activated to receive five reflected waves, after which the subarray may be activated another five times in a row, after which the last five rows may be activated to receive five reflected ways. In other embodiments, different sweep patterns can be used. For example, in some cases a subarray may be divided into rows whereas in another embodiment, the subarray may be divided into columns. In one embodiment, a subarray can be activated by rows to receive reflected waves in a first sweep mode and can be activated by columns to receive reflected waves in a second sweep mode. In other embodiments, other sweep modes can be used (e.g., diagonal, serpentine, quartered, perimeters of different radius, and so on). In some examples, a first sweep mode can be immediately followed by a second sweep mode. In other embodiments, instead of deactivating or activating particular transducers when reading signals resulting from reflected waveforms, only the signals obtained from selected transducers can be retained, whereas other signals obtained from other transducers can be rejected and/or discarded. 
       FIG. 5  depicts example operations of a method of performing segmented imaging of a surface by using both progressive and segmented acoustic imaging techniques. In some embodiments, method depicted in  FIG. 5  can be performed by the controller  302  of  FIGS. 3A-3B . The method can begin at operation  500  at which a subarray of transducers can be selected based on a particular selected interrogation area. Next at operation  502 , the selected transducers in  500  can be driven at the same time. Next at operation  504 , a row of the interrogation area can be selected. Next at operation  506 , electrical signals output from the selected row of transducers can be read. Thereafter, the selected transducers in  500  can be driven again and, thus the next row of the particular interrogation area can be selected and read at operation  508 . Once all rows are selected and read, the next interrogation area can be selected at  510  and the method can repeat if additional rows remain by returning to operation  502 . One may appreciate that for certain embodiments, columns may be read in lieu of rows. In some embodiments neither rows nor columns may be read; for example, a different subdivision of the subarray can be accomplished. 
       FIG. 6  depicts example operations of another method of performing segmented imaging of a surface with an acoustic transducer array. In some embodiments, the method depicted in  FIG. 6  can be performed by the controller  302  of  FIGS. 3A-3B . The method can begin at operation  600  during which a transducer array is selected. Once selected, the transducer array can be excited and/or activated to produce a series of pulses as plane waves at operation  602 . Thereafter, at operation  604 , the rows of the transducer array can be read in sequence, matching the sequence of acoustic reflections associated with the series of plane waves of operation  604 . In other words, if five plane waves were generated at operation  602  from a transducer array divisible into five rows, the first of five rows can be read at a time approximately equal to the round trip time of the first of the five plane waves, the second row can be read at a time corresponding to the round-trip time of the second of the five plane waves, and so on. In some examples, adjacent rows can be read in a sequence (e.g., row one, row two, row three, and so on). In other examples, non-adjacent rows can be read in a sequence (e.g., row one, row five, row two, row four, and so on). In other examples, other patterns can be used. For example, a single row may be read for more than one time in a sequence (e.g., row one, row one, row two, row two, and so on). Thereafter, multiple readings of a single row can be statistically combined, for example by averaging. 
     One may appreciate that for certain embodiments, columns may be read in lieu of rows. In some embodiments neither rows nor columns may be read; for example, a different subdivision of the array can be accomplished. In one example, quadrants of an array can be read in a sequence. In another example, columns and rows can be read in a sequence (e.g., column one, row one, column two, row two, and so on). In other examples, perimeters of the array can be read sequentially with increasing or decreasing radius. 
     In further examples, more than one series of drive pulses (e.g., activation pulse) may be required or desired to assemble an image or subimage. For example, the reflections resulting from a first series of drive pulses can be read by a first grouping of rows, and the reflections resulting from a second series of drive pulses can be read by a second grouping of rows. 
       FIG. 7  depicts example operations of another method of performing segmented imaging of a subarea of an imaging surface with an acoustic transducer array. In some embodiments, the method depicted in  FIG. 7  can be performed, in some embodiments, by the controller  302  of  FIGS. 3A-3B . The method can begin at operation  700  during which a subarray of transducers can be selected. Thereafter at  702 , all transducers of the subarray can be activated and/or excited to produce a plane wave. Next at operation  704 , electrical signals can be obtained from each of the transducers. Finally at operation  706 , electrical signals corresponding to transducers along the perimeter of the subarray can be rejected. In some cases, signals can be rejected from only selected perimeter transducers. 
       FIG. 8  depicts example operations of a method of assembling a composite image from a series of subimages. In some embodiments, the method depicted in  FIG. 8  can be performed, in some embodiments, by the controller  302  of  FIGS. 3A-3B . The method can begin at operation  800  during which an image segment (or a plurality of image segments) is received. Next, at operation  802 , each of the received image segments can be assembled into a single composite image. 
       FIG. 9  depicts example operations of another method of performing segmented imaging of a subarea of an imaging surface with an acoustic transducer array coupled to a plurality of drive lines and a plurality of read lines. The method can begin at operation  900  at which a first and second set of transducers can be selected from a group or plurality of transducers. In many examples, the first set is disjoint with the second set. In other words, the sets do not share any common element. At operation  902 , the method can continue by coupling the first and second set of transducers to one or more drive lines. Next, at operation  904 , the method can continue by providing an electronic activation signal to the first and second set of transducers via the drive lines. Next, at operation  906 , the first set of transducers can be coupled to one or more read lines which, thereafter at operation  908 , can be used to obtain electrical signal from each of the first set of transducers. In many examples, the number of drive lines and the number of read lines may be different, although this is not necessarily required for all embodiments. 
     Many embodiments of the foregoing disclosure may include or may be described in relation to various methods of operation, use, manufacture, and so on. Notably, the operations of methods presented herein are meant only to be exemplary and, accordingly, are not necessarily exhaustive. For example an alternate operation order, or fewer or additional steps may be required or desired for particular embodiments. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not meant to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. In particular, any features described with respect to one embodiment may also be used in some embodiments, where compatible. Likewise, the features of the different embodiments may be exchanged, substituted, or omitted where compatible and appropriate. 
     Additionally, the present disclosure recognizes that personal information data, including biometric data and fingerprint data, in the present technology, can be used to the benefit of users. For example, the use of biometric authentication data can be used for convenient access to device features without the use of passwords. In other examples, user biometric data is collected for providing users with feedback about their health or fitness levels. Further, other uses for personal information data, including biometric data that benefit the user are also contemplated by the present disclosure. 
     The present disclosure further contemplates that the entities responsible for the collection, analysis, disclosure, transfer, storage, or other use of such personal information data will comply with well-established privacy policies and/or privacy practices. In particular, such entities should implement and consistently use privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining personal information data private and secure, including the use of data encryption and security methods that meets or exceeds industry or government standards. 
     For example, personal information from users should be collected for legitimate and reasonable uses of the entity and not shared or sold outside of those legitimate uses. Further, such collection should occur only after receiving the informed consent of the users. Additionally, such entities would take any needed steps for safeguarding and securing access to such personal information data and ensuring that others with access to the personal information data adhere to their privacy policies and procedures. Further, such entities can subject themselves to evaluation by third parties to certify their adherence to widely accepted privacy policies and practices. 
     Despite the foregoing, the present disclosure also contemplates embodiments in which users selectively block the use of, or access to, personal information data, including biometric data. That is, the present disclosure contemplates that hardware and/or software elements can be provided to prevent or block access to such personal information data. For example, in the case of biometric authentication methods, the present technology can be configured to allow users to optionally bypass biometric authentication steps by providing secure information such as passwords, personal identification numbers (PINS), touch gestures, or other authentication methods, alone or in combination, known to those of skill in the art. In another example, users can select to remove, disable, or restrict access to certain health-related applications collecting users&#39; personal health or fitness data.