Patent Publication Number: US-10318791-B2

Title: Anti-spoofing sensing for rejecting fake fingerprint patterns in under-screen optical sensor module for on-screen fingerprint sensing

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent document claims the benefits and priority of U.S. Provisional Patent Application No. 62/534,171 entitled “ANTI-SPOOFING SENSING FOR REJECTING FAKE FINGERPRINT PATTERNS IN UNDER-SCREEN OPTICAL SENSOR MODULE FOR ON-SCREEN FINGERPRINT SENSING” and filed Jul. 18, 2017 by Shenzhen Goodix Technology Co., Ltd. The entire content of the before-mentioned patent application is incorporated by reference as part of the disclosure of this patent document. 
    
    
     TECHNICAL FIELD 
     This patent document relates to optical sensor modules capable of performing one or more sensing operations such as fingerprints or other parameter measurements based on optical sensing in an electronic device such as a mobile device or a wearable device or a larger system. 
     BACKGROUND 
     Various sensors can be implemented in electronic devices or systems to provide certain desired functions. 
     A sensor that enables user authentication is one example of such sensors for devices including portable or mobile computing devices (e.g., laptops, tablets, smartphones), gaming systems, various databases, information systems or larger computer-controlled systems can employ user authentication mechanisms to protect personal data and prevent unauthorized access. User authentication on an electronic device can be carried out through one or multiple forms of biometric identifiers, which can be used alone or in addition to conventional password authentication methods. A popular form of biometric identifiers is a person&#39;s fingerprint pattern. A fingerprint sensor can be built into the electronic device to read a user&#39;s fingerprint pattern so that the device can only be unlocked by an authorized user of the device through authentication of the authorized user&#39;s fingerprint pattern. Another example of sensors for electronic devices or systems is a biomedical sensor, e.g., a heartbeat sensor in wearable devices like wrist band devices or watches. In general, different sensors can be provided in electronic devices to achieve different sensing operations and functions. 
     Fingerprints can be used to authenticate users for accessing electronic devices, computer-controlled systems, electronic databases or information systems, either used as a stand-alone authentication method or in combination with one or more other authentication methods such as a password authentication method. For example, electronic devices including portable or mobile computing devices, such as laptops, tablets, smartphones, and gaming systems can employ user authentication mechanisms to protect personal data and prevent unauthorized access. In another example, a computer or a computer-controlled device or system for an organization or enterprise should be secured to allow only authorized personnel to access in order to protect the information or the use of the device or system for the organization or enterprise. The information stored in portable devices and computer-controlled databases, devices or systems, may be personal in nature, such as personal contacts or phonebook, personal photos, personal health information or other personal information, or confidential information for proprietary use by an organization or enterprise, such as business financial information, employee data, trade secrets and other proprietary information. If the security of the access to the electronic device or system is compromised, these data may be accessed by others, causing loss of privacy of individuals or loss of valuable confidential information. Beyond security of information, securing access to computers and computer-controlled devices or systems also allow safeguard the use of devices or systems that are controlled by computers or computer processors such as computer-controlled automobiles and other systems such as ATMs. 
     Secured access to a device (such as a mobile device) or a system (such as an electronic database and a computer-controlled system) can be achieved in different ways. For example, one common way for implementing a secured access is by using user passwords. A password, however, may be easily to be spread or obtained and this nature of passwords can reduce the level of the security. Moreover, a user needs to remember a password to use electronic devices or systems, and, if the user forgets the password, the user needs to undertake certain password recovery procedures to get authenticated or otherwise regain the access to the device and such processes may be burdensome to users and have various practical limitations and inconveniences. The personal fingerprint identification can be utilized to achieve the user authentication for enhancing the data security while mitigating certain undesired effects associated with passwords. 
     Electronic devices or systems, including portable or mobile computing devices, may employ user authentication mechanisms to protect personal or other confidential data and prevent unauthorized access. User authentication on an electronic device or system may be carried out through one or multiple forms of biometric identifiers, which can be used alone or in addition to conventional password authentication methods. One form of biometric identifiers is a person&#39;s fingerprint pattern. A fingerprint sensor can be built into an electronic device or an information system to read a user&#39;s fingerprint pattern so that the device can only be unlocked by an authorized user of the device through authentication of the authorized user&#39;s fingerprint pattern. 
     SUMMARY 
     Optical sensor modules can be placed under display screens such as, among others, liquid crystal display (LCD) screens or organic light emitting diode (OLED) display screens, to provide optical sensing functions including optical fingerprint sensing. In some implementations, optical sensing is provided for determining whether an object in contact is from a live person. 
     In one aspect, an electronic device capable of detecting a fingerprint by optical sensing is provided to include a screen that provides touch sensing operations and includes a display panel structure to display images; a top transparent layer formed over the device screen as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user; an optical sensor module located below the display panel structure to receive probe light that passes through the screen to detect a fingerprint; and a capacitive sensor array coupled to the screen to measure capacitance signals as a capacitive anti-spoofing sensor for rejecting a fake fingerprint pattern. In some implementations, the display panel structure is a LCD display structure. In other implementations, the display panel structure is an OLED display structure. 
     In another aspect, a method is provided for operating an electronic device that includes an optical sensor module for optically detecting a fingerprint and a capacitive fingerprint senor module for capacitively detecting the fingerprint. This method includes operating a capacitive fingerprint sensor module having an array of capacitive sensors to obtain capacitive sensor signals representing an input fingerprint pattern of an object; processing the obtained capacitive sensor signals to obtain a capacitive sensor signal parameter that includes a capacitive signal strength, an image constructed from the obtained capacitive sensor signals or an image contrast of the image constructed from the obtained capacitive sensor signals; applying the capacitive sensor signal parameter to determine whether the object is a finger of a live user; operating an optical fingerprint sensor module to optically sense whether the object is a finger of a live user and to obtain an optical image of fingerprint of the object; and combining measurements by both the optical fingerprint sensor module and the capacitive fingerprint sensor module to authenticate whether a measured fingerprint is from a finger of a liver user and whether the fingerprint is from an authorized user. 
     In another aspect, an electronic device capable of detecting a fingerprint by optical sensing is provided to include a screen that provides touch sensing operations and includes a display panel structure to display images; a top transparent layer formed over the device screen as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user; and an optical sensor module located below the display panel structure to receive probe light that passes through the screen to detect a fingerprint and to measure signals at different optical wavelengths rejecting a fake fingerprint pattern. 
     In another aspect, the disclosed technology can be used to construct an electronic device capable of detecting a fingerprint by optical sensing to include a liquid crystal display (LCD) screen that provides touch sensing operations and includes a LCD display panel structure to display images; a top transparent layer formed over the device screen as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user; and an optical sensor module located below the display panel structure to receive probe light that passes through the LCD screen to detect a fingerprint, wherein the optical sensor module includes an optical collimator array of optical collimators that receives the probe light and an optical sensor array of optical sensors to receive the probe light from the optical collimator array. 
     In another aspect, the disclosed technology can be used to construct an electronic device capable of detecting a fingerprint by optical sensing that includes (1) a liquid crystal display (LCD) screen that provides touch sensing operations and includes a LCD display panel structure to display images; a LCD backlighting light module coupled to the LCD screen to produce backlighting light to the LCD screen for display images; (2) a top transparent layer formed over the device screen as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user; (3) an optical sensor module located below the LCD display panel structure to receive probe light that is reflected from the top transparent layer and passes through the LCD screen to detect a fingerprint; (4) one or more probe light sources, separate from the LCD backlighting light module, located under the LCD display panel structure, to produce the probe light that passes through he LCD display panel structure to illuminate a designated fingerprint sensing area on the a top transparent layer to be visibly different from a surrounding area of the top transparent layer for a user to place a finger for optical fingerprint sensing; and (5) a device control module coupled to the optical sensor module to process an output of the optical sensor module to determine whether a detected fingerprint by the optical sensor module matches a fingerprint an authorized user, in addition to detecting fingerprints, also detect a biometric parameter different form a fingerprint by optical sensing to indicate whether a touch at the top transparent layer associated with a detected fingerprint is from a live person. 
     In yet another aspect, the disclosed technology can be used to construct an electronic device capable of detecting a fingerprint by optical sensing to include a liquid crystal display (LCD) screen that provides touch sensing operations and includes a LCD display panel structure to display images; a LCD backlighting light module coupled to the LCD screen to produce backlighting light to the LCD screen to display images; a top transparent layer formed over the LCD screen as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user; and an optical sensor module located below the LCD panel structure to receive light returned from the top transparent layer to detect a fingerprint. The optical sensor module includes a transparent block in contact with the display panel substrate to receive the light from the display panel structure, an optical sensor array that receives the light and an optical imaging module that images the received light in the transparent block onto the optical sensor array. One or more probe light sources, separate from the LCD backlighting light module, are located under the LCD display panel structure, to produce the probe light that passes through he LCD display panel structure to illuminate a designated fingerprint sensing area on the top transparent layer to be visibly different from a surrounding area of the top transparent layer for a user to place a finger for optical fingerprint sensing. 
     The drawings, the description and the claims below provide a more detailed description of the above and other aspects, their implementations and features of the disclosed technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example of a system with a fingerprint sensing module which can be implemented to include an optical fingerprint sensor disclosed in this document. 
         FIGS. 2A and 2B  illustrate one exemplary implementation of an electronic device  200  having a touch sensing display screen assembly and an optical sensor module positioned underneath the touch sensing display screen assembly. 
         FIGS. 3A and 3B  illustrate an example of a device that implements the optical sensor module in  FIGS. 2A and 2B . 
         FIGS. 4A and 4B  show an example of one implementation of an optical sensor module under the display screen assembly for implementing the design in  FIGS. 2A and 2B . 
         FIGS. 5A, 5B and 5C  illustrate signal generation for the returned light from the sensing zone on the top sensing surface under two different optical conditions to facilitate the understanding of the operation of the under-screen optical sensor module. 
         FIGS. 6A-6C, 7, 8A-8B, 9, and 10A-10B  show example designs of under-screen optical sensor modules. 
         FIG. 11  shows imaging of the fingerprint sensing area on the top transparent layer via an imaging module under different tiling conditions where an imaging device images the fingerprint sensing area onto an optical sensor array and the imaging device may be optically transmissive or optically reflective. 
         FIG. 12  shows an example of an operation of the fingerprint sensor for reducing or eliminating undesired contributions from the background light in fingerprint sensing. 
         FIG. 13  shows a process for operating an under-screen optical sensor module for capturing a fingerprint pattern. 
         FIGS. 14, 15  and  FIG. 16  show an example of an operation process for determining whether an object in contact with the LCD display screen is part of a finger of a live person by illuminating the finger with light in two different light colors. 
         FIGS. 17A-17B, 18 and 19A-19C  show optical collimator designs for optical fingerprint sensing suitable for implementing the disclosed under-screen optical sensor module technology. 
         FIGS. 20, 21, 22A, and 22B  illustrate examples of various designs for fingerprint sensing using an under-screen optical sensor module using an array of optical collimators or pinholes for directing signal light carrying fingerprint information to the optical sensor array. 
         FIGS. 23 and 24  show examples of under-screen optical sensor modules with optical collimators. 
         FIG. 25  shows an example an optical collimator array with optical filtering to reduce background light that reaches the photodetector array in the under-screen optical sensor module. 
         FIGS. 26A, 26B, 27 and 28  show examples of optical collimator designs for the optical sensing under the LCD display screen. 
         FIGS. 29, 30 and 31  illustrate improved optical imaging resolution based on a pinhole camera effect in designing the optical sensor module. 
         FIG. 32  shows an example of an under-LCD optical sensor module using an optical pinhole array for optical sensing. 
         FIGS. 33A and 33B  show an example of an optical fingerprint senor under a LCD display panel having an optical deflection or diffraction device or layer. 
         FIGS. 34A, 34B and 34C  show examples of LCD diffuser designs for improved under-LCD optical sensing. 
         FIGS. 35A and 35B  show examples of LCD reflector designs for improved under-LCD optical sensing. 
         FIG. 36  shows an example of a LCD light source design for improved under-LCD optical sensing. 
         FIGS. 37A-37D  show examples of enhancement features for improved under-LCD optical sensing. 
         FIG. 38  shows an example of a LCD waveguide design for improved under-LCD optical sensing. 
         FIGS. 39A-39C  show examples of LCD backlighting light source and illumination light source for improved under-LCD optical sensing. 
         FIG. 40  shows two different fingerprint patterns of the same finger under different press forces: the lightly pressed fingerprint and the heavily pressed fingerprint. 
         FIGS. 41A and 41B  show examples of under-OLED optical sensor module structures for achieving the above optical filtering and the reducing Moiré patterns. 
         FIG. 42  shows an example of an under-screen optical sensor module based on a pinhole-lens assembly. 
         FIGS. 43, 44 and 45  show examples of capacitive anti-spoofing sensing designs for rejecting fake fingerprint models. 
         FIGS. 46A-46B  show examples of optical anti-spoofing sensing based on different optical reflection/scattering by living tissues in a live finger at different optical wavelengths. 
         FIGS. 47 and 48  show two examples for operating an anti-spoofing capacitive sensor in connection with optical fingerprint sensing based on the disclosed technology. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices or systems may be equipped with fingerprint authentication mechanisms to improve the security for accessing the devices. Such electronic devices or system may include, portable or mobile computing devices, e.g., smartphones, tablet computers, wrist-worn devices and other wearable or portable devices, larger electronic devices or systems, e.g., personal computers in portable forms or desktop forms, ATMs, various terminals to various electronic systems, databases, or information systems for commercial or governmental uses, motorized transportation systems including automobiles, boats, trains, aircraft and others. 
     Fingerprint sensing is useful in mobile applications and other applications that use or require secure access. For example, fingerprint sensing can be used to provide secure access to a mobile device and secure financial transactions including online purchases. It is desirable to include robust and reliable fingerprint sensing suitable for mobile devices and other applications. In mobile, portable or wearable devices, it is desirable for fingerprint sensors to minimize or eliminate the footprint for fingerprint sensing given the limited space on those devices, especially considering the demands for a maximum display area on a given device. 
     The fingerprint sensing technology disclosed in this patent document provides optical sensing of fingerprints and identification of an authorized user based on optical fingerprint sensing for granting or denying an access to a device (such as a mobile device) or a system (such as an ATM, a database or a vehicle). Specific examples for optical sensing by using an under-display optical sensor module for optical fingerprint sensing and other optical sensing are disclosed, including implementations for OLED and LCD displays. 
     In applications with fingerprint sensing security features, it is desirable to include robust and reliable fingerprint sensor features for identifying proper authorized users. Notably, various anti-spoofing sensing mechanisms may be implemented based on the disclosed optical fingerprint sensing techniques to increase the security of a device or a system. For example, an anti-spoofing sensor disclosed herein can be implemented, in some implementations, to determine not only whether an input object with a fingerprint pattern is from a living person but also whether the living person uses a fabricated fake fingerprint layer on the person&#39;s finger in an attempt to invade the fingerprint sensing security of the system. Specific examples of anti-spoofing sensing mechanisms are provided in this patent document, such as the designs in connection with  FIGS. 14, 15, 40, 43, 44, 45, 46A-46B, 47 and 48 . 
     In some implementations of the disclosed optical sensing techniques, the light produced by a display screen for displaying images necessarily passes through the top surface of the display screen in order to be viewed by a user. A finger can touch the top surface and thus interacts with the light at the top surface to cause the reflected or scattered light at the surface area of the touch to carry spatial image information of the finger to return to the display panel underneath the top surface. In touch sensing display devices, the top surface is the touch sensing interface with the user and this interaction between the light for displaying images and the user finger or hand constantly occurs but such information-carrying light returning back to the display panel is largely wasted and is not used in most touch sensing devices. In various mobile or portable devices with touch sensing displays and fingerprint sensing functions, a fingerprint sensor tends to be a separate device from the display screen, either placed on the same surface of the display screen at a location outside the display screen area such as in some models of Apple iPhones and Samsung smartphones, or placed on the backside of a smartphone, such as some models of smart phones by Huawei, Lenovo, Xiaomi or Google, to avoid taking up valuable space for placing a large display screen on the front side. Those fingerprint sensors are separate devices from the display screens and thus need to be compact to save space for display and other functions while still providing reliable and fast fingerprint sensing with a spatial image resolution above a certain acceptable level. However, the need to be compact and small and the need to provide a high spatial image resolution in capturing a fingerprint pattern are in direct conflict with each other in many fingerprint sensors because a high spatial image resolution in capturing a fingerprint pattern in based on various suitable fingerprint sensing technologies (e.g., capacitive touch sensing or optical imaging) requires a large sensor area with a large number of sensing pixels. 
     The sensor technology and examples of implementations of the sensor technology described in this patent document provide an optical sensor module that uses, at least in part, the light from a display screen as the illumination probe light to illuminate a fingerprint sensing area on the touch sensing surface of the display screen to perform one or more sensing operations based on optical sensing of such light. A suitable display screen for implementing the disclosed optical sensor technology can be based on various display technologies or configurations, including, a display screen having light emitting display pixels without using backlight where each individual pixel generates light for forming a display image on the screen such as liquid crystal display (LCD) screens, organic light emitting diode (OLED) display screens, or electroluminescent display screens. 
     In the disclosed examples for integrating optical sensing to LCD based on the disclosed optical sensor technology, the under LCD optical sensor can be used to detect a portion of the light that is used for displaying images in a LCD screen where such a portion of the light for the display screen may be the scattered light, reflected light or some stray light. For example, in some implementations, the image light of the LCD screen based on backlighting may be reflected or scattered back into the LCD display screen as returned light when encountering an object such as a user finger or palm, or a user pointer device like a stylus. Such returned light can be captured for performing one or more optical sensing operations using the disclosed optical sensor technology. Due to the use of the light from LCD screen for optical sensing, an optical sensor module based on the disclosed optical sensor technology is specially designed to be integrated to the LCD display screen in a way that maintains the display operations and functions of the LCD display screen without interference while providing optical sensing operations and functions to enhance overall functionality, device integration and user experience of an electronic device or system such as a smart phone, a tablet, or a mobile/wearable device. 
     In addition, in various implementations of the disclosed optical sensing technology, one or more designated probe light sources may be provided to produce additional illumination probe light for the optical sensing operations by the under LCD screen optical sensing module. In such applications, the light from the backlighting of the LCD screen and the probe light from the one or more designated probe light sources collectively form the illumination light for optical sensing operations. 
     Regarding the additional optical sensing functions beyond fingerprint detection, the optical sensing may be used to measure other parameters. For example, the disclosed optical sensor technology can measure a pattern of a palm of a person given the large touch area available over the entire LCD display screen (in contrast, some designated fingerprint sensors such as the fingerprint senor in the home button of Apple&#39;s iPhone/iPad devices have a rather small and designated off-screen fingerprint sensing area that is highly limited in the sensing area size that may not be suitable for sensing large patterns). For yet another example, the disclosed optical sensor technology can be used not only to use optical sensing to capture and detect a pattern of a finger or palm that is associated with a person, but also to use optical sensing or other sensing mechanisms to detect whether the captured or detected pattern of a fingerprint or palm is from a live person&#39;s hand by a “live finger” detection mechanism, which may be based on, for example, the different optical absorption behaviors of the blood at different optical wavelengths, the fact that a live person&#39;s finger tends to be moving or stretching due to the person&#39;s natural movement or motion (either intended or unintended) or pulsing when the blood flows through the person&#39;s body in connection with the heartbeat. In one implementation, the optical sensor module can detect a change in the returned light from a finger or palm due to the heartbeat/blood flow change and thus to detect whether there is a live heartbeat in the object presented as a finger or palm. The user authentication can be based on the combination of the both the optical sensing of the fingerprint/palm pattern and the positive determination of the presence of a live person to enhance the access control. For yet another example, the optical sensor module may include a sensing function for measuring a glucose level or a degree of oxygen saturation based on optical sensing in the returned light from a finger or palm. As yet another example, as a person touches the LCD display screen, a change in the touching force can be reflected in one or more ways, including fingerprint pattern deforming, a change in the contacting area between the finger and the screen surface, fingerprint ridge widening, or a change in the blood flow dynamics. Those and other changes can be measured by optical sensing based on the disclosed optical sensor technology and can be used to calculate the touch force. This touch force sensing can be used to add more functions to the optical sensor module beyond the fingerprint sensing. 
     With respect to useful operations or control features in connection with the touch sensing aspect of the LCD display screen, the disclosed optical sensor technology can provide triggering functions or additional functions based on one or more sensing results from the optical sensor module to perform certain operations in connection with the touch sensing control over the LCD display screen. For example, the optical property of a finger skin (e.g., the index of refraction) tends to be different from other artificial objects. Based on this, the optical sensor module may be designed to selectively receive and detect returned light that is caused by a finger in touch with the surface of the LCD display screen while returned light caused by other objects would not be detected by the optical sensor module. This object-selective optical detection can be used to provide useful user controls by touch sensing, such as waking up the smartphone or device only by a touch via a person&#39;s finger or palm while touches by other objects would not cause the device to wake up for energy efficient operations and to prolong the battery use. This operation can be implemented by a control based on the output of the optical sensor module to control the waking up circuitry operation of the LCD display screen which, the LCD pixels are put in a “sleep” mode by being turned off (and the LCD backlighting is also turned off) while one or more illumination light sources (e.g., LEDs) for the under-LCD panel optical sensor module are turned on in a flash mode to intermittently emit flash light to the screen surface for sensing any touch by a person&#39;s finger or palm. Under this design, the optical sensor module operates the one or more illumination light sources to produce the “sleep” mode wake-up sensing light flashes so that the optical sensor module can detect returned light of such wake-up sensing light caused by the finger touch on the LCD display screen and, upon a positive detection, the LCD backlighting and the LCD display screen are turned on or “woken up”. In some implementations, the wake-up sensing light can be in the infrared invisible spectral range so a user will not experience any visual of a flash light. The LCD display screen operation can be controlled to provide an improved fingerprint sensing by eliminating background light for optical sensing of the fingerprint. In one implementation, for example, each display scan frame generates a frame of fingerprint signals. If, two frames of fingerprint signals with the display are generated in one frame when the LCD display screen is turned on and in the other frame when the LCD display screen is turned off, the subtraction between those two frames of signals can be used to reduce the ambient background light influence. By operating the fingerprint sensing frame rate is at one half of the display frame rate in some implementations, the background light noise in fingerprint sensing can be reduced. 
     An optical sensor module based on the disclosed optical sensor technology can be coupled to the backside of the LCD display screen without requiring creation of a designated area on the surface side of the LCD display screen that would occupy a valuable device surface real estate in some electronic devices such as a smartphone, a tablet or a wearable device. This aspect of the disclosed technology can be used to provide certain advantages or benefits in both device designs and product integration or manufacturing. 
     In some implementations, an optical sensor module based on the disclosed optical sensor technology can be configured as a non-invasive module that can be easily integrated to a display screen without requiring changing the design of the LCD display screen for providing a desired optical sensing function such as fingerprint sensing. In this regard, an optical sensor module based on the disclosed optical sensor technology can be independent from the design of a particular LCD display screen design due to the nature of the optical sensor module: the optical sensing of such an optical sensor module is by detecting the light that is emitted by the one or more illumination light sources of the optical sensor module and is returned from the top surface of the display area, and the disclosed optical sensor module is coupled to the backside of the LCD display screen as a under-screen optical sensor module for receiving the returned light from the top surface of the display area and thus does not require a special sensing port or sensing area that is separate from the display screen area. Accordingly, such an under-screen optical sensor module can be used to combine with a LCD display screen to provide optical fingerprint sensing and other sensor functions on an LCD display screen without using a specially designed LCD display screen with hardware especially designed for providing such optical sensing. This aspect of the disclosed optical sensor technology enables a wide range of LCD display screens in smartphones, tablets or other electronic devices with enhanced functions from the optical sensing of the disclosed optical sensor technology. 
     For example, for an existing phone assembly design that does not provide a separate fingerprint sensor as in certain Apple iPhones or Samsung Galaxy smartphones, such an existing phone assembly design can integrate the under-screen optical sensor module as disclosed herein without changing the touch sensing-display screen assembly to provide an added on-screen fingerprint sensing function. Because the disclosed optical sensing does not require a separate designated sensing area or port as in the case of certain Apple iPhones/Samsung Galaxy phones with a front fingerprint senor outside the display screen area, or some smartphones with a designated rear fingerprint sensor on the backside like in some models by Huawei, Xiaomi, Google or Lenovo, the integration of the on-screen fingerprint sensing disclosed herein does not require a substantial change to the existing phone assembly design or the touch sensing display module that has both the touch sensing layers and the display layers. Based on the disclosed optical sensing technology in this document, no external sensing port and no extern hardware button are needed on the exterior of a device are needed for adding the disclosed optical sensor module for fingerprint sensing. The added optical sensor module and the related circuitry are under the display screen inside the phone housing and the fingerprint sensing can be conveniently performed on the same touch sensing surface for the touch screen. 
     For another example, due to the above described nature of the optical sensor module for fingerprint sensing, a smartphone that integrates such an optical sensor module can be updated with improved designs, functions and integration mechanism without affecting or burdening the design or manufacturing of the LCD display screens to provide desired flexibility to device manufacturing and improvements/upgrades in product cycles while maintaining the availability of newer versions of optical sensing functions to smartphones, tablets or other electronic devices using LCD display screens. Specifically, the touch sensing layers or the LCD display layers may be updated in the next product release without adding any significant hardware change for the fingerprint sensing feature using the disclosed under-screen optical sensor module. Also, improved on-screen optical sensing for fingerprint sensing or other optical sensing functions by such an optical sensor module can be added to a new product release by using a new version of the under-screen optical sensor module without requiring significant changes to the phone assembly designs, including adding additional optical sensing functions. 
     The above and other features of the disclosed optical sensor technology can be implemented to provide a new generation of electronic devices with improved fingerprint sensing and other sensing functions, especially for smartphones, tablets and other electronic devices with LCD display screens to provide various touch sensing operations and functions and to enhance the user experience in such devices. The features for optical sensor modules disclosed in this patent document may be applicable to various display panels based on different technologies including both LCD and OLED displays. The specific examples below are directed to LCD display panels and optical sensor modules placed under LCD display panels. 
     In implementations of the disclosed technical features, additional sensing functions or sensing modules, such as a biomedical sensor, e.g., a heartbeat sensor in wearable devices like wrist band devices or watches, may be provided. In general, different sensors can be provided in electronic devices or systems to achieve different sensing operations and functions. 
     The disclosed technology can be implemented to provide devices, systems, and techniques that perform optical sensing of human fingerprints and authentication for authenticating an access attempt to a locked computer-controlled device such as a mobile device or a computer-controlled system, that is equipped with a fingerprint detection module. The disclosed technology can be used for securing access to various electronic devices and systems, including portable or mobile computing devices such as laptops, tablets, smartphones, and gaming devices, and other electronic devices or systems such as electronic databases, automobiles, bank ATMs, etc. 
       FIG. 1  is a block diagram of an example of a system  180  with a fingerprint sensing module  180  including a fingerprint sensor  181  which can be implemented to include an optical fingerprint sensor based on the optical sensing of fingerprints as disclosed in this document. The system  180  includes a fingerprint sensor control circuit  184 , and a digital processor  186  which may include one or more processors for processing fingerprint patterns and determining whether an input fingerprint pattern is one for an authorized user. The fingerprint sensing system  180  uses the fingerprint sensor  181  to obtain a fingerprint and compares the obtained fingerprint to a stored fingerprint to enable or disable functionality in a device or system  188  that is secured by the fingerprint sensing system  180 . In operation, the access to the device  188  is controlled by the fingerprint processing processor  186  based on whether the captured user fingerprint is from an authorized user. As illustrated, the fingerprint sensor  181  may include multiple fingerprint sensing pixels such as pixels  182 A- 182 E that collectively represent at least a portion of a fingerprint. For example, the fingerprint sensing system  180  may be implemented at an ATM as the system  188  to determine the fingerprint of a customer requesting to access funds or other transactions. Based on a comparison of the customer&#39;s fingerprint obtained from the fingerprint sensor  181  to one or more stored fingerprints, the fingerprint sensing system  180  may, upon a positive identification, cause the ATM system  188  to grant the requested access to the user account, or, upon a negative identification, may deny the access. For another example, the device or system  188  may be a smartphone or a portable device and the fingerprint sensing system  180  is a module integrated to the device  188 . For another example, the device or system  188  may be a gate or secured entrance to a facility or home that uses the fingerprint sensor  181  to grant or deny entrance. For yet another example, the device or system  188  may be an automobile or other vehicle that uses the fingerprint sensor  181  to link to the start of the engine and to identify whether a person is authorized to operate the automobile or vehicle. 
     As a specific example,  FIGS. 2A and 2B  illustrate one exemplary implementation of an electronic device  200  having a touch sensing display screen assembly and an optical sensor module positioned underneath the touch sensing display screen assembly. In this particular example, the display technology can be implemented by a LCD display screen with backlight for optically illuminating the LCD pixels or another display screen having light emitting display pixels without using backlight (e.g., an OLED display screen). The electronic device  200  can be a portable device such as a smartphone or a tablet and can be the device  188  as shown in  FIG. 1 . 
       FIG. 2A  shows the front side of the device  200  which may resemble some features in some existing smartphones or tablets. The device screen is on the front side of the device  200  occupying either entirety, a majority or a significant portion of the front side space and the fingerprint sensing function is provided on the device screen, e.g., one or more sensing areas for receiving a finger on the device screen. As an example,  FIG. 2A  shows a fingerprint sensing zone in the device screen for a finger to touch which may be illuminated as a visibly identifiable zone or area for a user to place a finger for fingerprint sensing. Such a fingerprint sensing zone can function like the rest of the device screen for displaying images. As illustrated, the device housing of the device  200  may have, in various implementations, side facets that support side control buttons that are common in various smartphones on the market today. Also, one or more optional sensors may be provided on the front side of the device  200  outside the device screen as illustrated by one example on the left upper corner of the device housing in  FIG. 2A . 
       FIG. 2B  shows an example of the structural construction of the modules in the device  200  relevant to the optical fingerprint sensing disclosed in this document. The device screen assembly shown in  FIG. 2B  includes, e.g., the touch sensing screen module with touch sensing layers on the top, and a display screen module with display layers located underneath the touch sensing screen module. An optical sensor module is coupled to, and located underneath, the display screen assembly module to receive and capture the returned light from the top surface of the touch sensing screen module and to guide and image the returned light onto an optical sensor array of optical sensing pixels or photodetectors which convert the optical image in the returned light into pixel signals for further processing. Underneath the optical sensor module is the device electronics structure containing certain electronic circuits for the optical sensor module and other parts in the device  200 . The device electronics may be arranged inside the device housing and may include a part that is under the optical sensor module as shown in  FIG. 2B . 
     In implementations, the top surface of the device screen assembly can be a surface of an optically transparent layer serving as a user touch sensing surface to provide multiple functions, such as (1) a display output surface through which the light carrying the display images passes through to reach a viewer&#39;s eyes, (2) a touch sensing interface to receive a user&#39;s touches for the touch sensing operations by the touch sensing screen module, and (3) an optical interface for on-screen fingerprint sensing (and possibly one or more other optical sensing functions). This optically transparent layer can be a rigid layer such as a glass or crystal layer or a flexible layer. 
     One example of a display screen is an LCD display having LCD layers and a thin film transistor (TFT) structure or substrate. A LCD display panel is a multi-layer liquid crystal display (LCD) module that includes LCD display backlighting light sources (e.g., LED lights) emitting LCD illumination light for LCD pixels, a light waveguide layer to guide the backlighting light, and LCD structure layers which can include, e.g., a layer of liquid crystal (LC) cells, LCD electrodes, transparent conductive ITO layer, an optical polarizer layer, a color filter layer, and a touch sensing layer. The LCD module also includes a backlighting diffuser underneath the LCD structure layers and above the light waveguide layer to spatially spread the backlighting light for illuminating the LCD display pixels, and an optical reflector film layer underneath the light waveguide layer to recycle backlighting light towards the LCD structure layers for improved light use efficiency and the display brightness. 
     Referring to  FIG. 2B , the optical sensor module in this example is placed under the LCD display panel to capture the returned light from the top touch sensing surface and to acquire high resolution images of fingerprint patterns when user&#39;s finger is in touch with a sensing area on the top surface. In other implementations, the disclosed under-screen optical sensor module for fingerprint sensing may be implemented on a device without the touch sensing feature. In addition, a suitable display panel may be in various screen designs different from OLED displays. 
       FIGS. 3A and 3B  illustrate an example of a device that implements the optical sensor module in  FIGS. 2A and 2B .  FIG. 3A  shows a cross sectional view of a portion of the device containing the under-screen optical sensor module.  FIG. 3B  shows, on the left, a view of the front side of the device with the touch sensing display indicating a fingerprint sensing area on the lower part of the display screen, and on the right, a perspective view of a part of the device containing the optical sensor module that is under the device display screen assembly.  FIG. 3B  also shows an example of the layout of the flexible tape with circuit elements. 
     In the design examples in  FIGS. 2A, 2B, 3A and 3B , the optical fingerprint sensor design is different from some other fingerprint sensor designs using a separate fingerprint sensor structure from the display screen with a physical demarcation between the display screen and the fingerprint sensor (e.g., a button like structure in an opening of the top glass cover in some mobile phone designs) on the surface of the mobile device. In the illustrated designs here, the optical fingerprint sensor for detecting fingerprint sensing and other optical signals are located under the top cover glass or layer (e.g.,  FIG. 3A ) so that the top surface of the cover glass serves as the top surface of the mobile device as a contiguous and uniform glass surface across both the display screen layers and the optical detector sensor that are vertically stacked and vertically overlap. This design for integrating optical fingerprint sensing and the touch sensitive display screen under a common and uniform surface provides benefits, including improved device integration, enhanced device packaging, enhanced device resistance to exterior elements, failure and wear and tear, and enhanced user experience over the ownership period of the device. 
     Referring back to  FIGS. 2A and 2B , the illustrated under-screen optical sensor module for on-screen fingerprint sensing may be implemented in various configurations. 
     In one implementation, a device based on the above design can be structured to include a device screen a that provides touch sensing operations and includes a LCD display panel structure for forming a display image, a top transparent layer formed over the device screen as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user, and an optical sensor module located below the display panel structure to receive light that returns from the top transparent layer to detect a fingerprint. 
     This device and other devices disclosed in this document can be further configured to include various features. 
     For example, a device electronic control module can be included in the device to grant a user&#39;s access to the device if a detected fingerprint matches a fingerprint an authorized user. In addition, the optical sensor module is configured to, in addition to detecting fingerprints, also detect a biometric parameter different form a fingerprint by optical sensing to indicate whether a touch at the top transparent layer associated with a detected fingerprint is from a live person, and the device electronic control module is configured to grant a user&#39;s access to the device if both (1) a detected fingerprint matches a fingerprint an authorized user and (2) the detected biometric parameter indicates the detected fingerprint is from a live person. The biometric parameter can include, e.g., whether the finger contains a blood flow, or a heartbeat of a person. 
     For example, the device can include a device electronic control module coupled to the display panel structure to supply power to the light emitting display pixels and to control image display by the display panel structure, and, in a fingerprint sensing operation, the device electronic control module operates to turn off the light emitting display pixels in one frame to and turn on the light emitting display pixels in a next frame to allow the optical sensor array to capture two fingerprint images with and without the illumination by the light emitting display pixels to reduce background light in fingerprint sensing. 
     For another example, a device electronic control module may be coupled to the display panel structure to supply power to the LCD display panel and to turn off power to the backlighting of the LCD display panel in a sleep mode, and the device electronic control module may be configured to wake up the display panel structure from the sleep mode when the optical sensor module detects the presence of a person&#39;s skin at the designated fingerprint sensing region of the top transparent layer. More specifically, in some implementations, the device electronic control module can be configured to operate one or more illumination light sources in the optical sensor module to intermittently emit light, while turning off power to the LCD display panel (in the sleep mode), to direct the intermittently emitted illumination light to the designated fingerprint sensing region of the top transparent layer for monitoring whether there is a person&#39;s skin in contact with the designated fingerprint sensing region for waking up the device from the sleep mode. 
     For another example, the device can include a device electronic control module coupled to the optical sensor module to receive information on multiple detected fingerprints obtained from sensing a touch of a finger and the device electronic control module is operated to measure a change in the multiple detected fingerprints and determines a touch force that causes the measured change. For instance, the change may include a change in the fingerprint image due to the touch force, a change in the touch area due to the touch force, or a change in spacing of fingerprint ridges. 
     For another example, the top transparent layer can include a designated fingerprint sensing region for a user to touch with a finger for fingerprint sensing and the optical sensor module below the display panel structure can include a transparent block in contact with the display panel substrate to receive light that is emitted from the display panel structure and returned from the top transparent layer, an optical sensor array that receives the light and an optical imaging module that images the received light in the transparent block onto the optical sensor array. The optical sensor module can be positioned relative to the designated fingerprint sensing region and structured to selectively receive returned light via total internal reflection at the top surface of the top transparent layer when in contact with a person&#39;s skin while not receiving the returned light from the designated fingerprint sensing region in absence of a contact by a person&#39;s skin. 
     For yet another example, the optical sensor module can be structured to include an optical wedge located below the display panel structure to modify a total reflection condition on a bottom surface of the display panel structure that interfaces with the optical wedge to permit extraction of light out of the display panel structure through the bottom surface, an optical sensor array that receives the light from the optical wedge extracted from the display panel structure, and an optical imaging module located between the optical wedge and the optical sensor array to image the light from the optical wedge onto the optical sensor array. 
     Specific examples of under-screen optical sensor modules for on-screen fingerprint sensing are provided below. 
       FIG. 4A  and  FIG. 4B  show an example of one implementation of an optical sensor module under the display screen assembly for implementing the design in  FIGS. 2A and 2B . The device in  FIGS. 4A-4B  includes a display assembly  423  with a top transparent layer  431  formed over the device screen assembly  423  as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user. This top transparent layer  431  can be a cover glass or a crystal material in some implementations. The device screen assembly  423  can include a LCD display module  433  under the top transparent layer  431 . The LCD display layers allow partial optical transmission so light from the top surface can partially transmit through the LCD display layers to reach the under-LCD optical sensor module. For example, LCD display layers include electrodes and wiring structure optically acting as an array of holes and light scattering objects. A device circuit module  435  may be provided under the LCD display panel to control operations of the device and perform functions for the user to operate the device. 
     The optical sensor module  702  in this particular implementation example is placed under LCD display module  433 . One or more illumination light sources  703  are provided for the optical sensor module  702  and can be controlled to emit light to at least partially pass through the LCD display module  433  to illuminate the fingerprint sensing zone  615  on the top transparent layer  431  within the device screen area for a user to place a finger therein for fingerprint identification. The illumination light from the one or more illumination light sources  703  can be directed to the fingerprint sensing area  615  on the top surface as if such illumination light is from a fingerprint illumination light zone  613 . As illustrated, a finger  445  is placed in the illuminated fingerprint sensing zone  615  as the effective sensing zone for fingerprint sensing. A portion of the reflected or scattered light in the zone  615  is directed into the optical sensor module underneath the LCD display module  433  and a photodetector sensing array inside the optical sensor module receives such light and captures the fingerprint pattern information carried by the received light. 
     In this design of using one or more illumination light sources  703  to provide the illumination light for optical fingerprint sensing, each illumination light source  703  maybe controlled in some implementations to turn on intermittently with a relatively low cycle to reduce the power used for the optical sensing operations. The fingerprint sensing operation can be implemented in a 2-step process in some implementations: first, the one or more illumination light sources  703  are turned on in a flashing mode without turning on the LCD display panel to use the flashing light to sense whether a finger touches the sensing zone  615  and, once a touch in the zone  615  is detected, the optical sensing module is operated to perform the fingerprint sensing based on optical sensing and the LCD display panel may be turned on. 
     In the example in  FIG. 4B , the under-screen optical sensor module includes a transparent block  701  that is coupled to the display panel to receive the returned light from the top surface of the device assembly, and an optical imaging block  702  that performs the optical imaging and imaging capturing. Light from the illumination light source  703 , after reaching the cover top surface, e.g., the cover top surface at the sensing area  615  where a user finger touches, is reflected or scattered back from the cover top surface. When fingerprint ridges in close contact of the cover top surface in the sensing area  615 , the light reflection under the fingerprint ridges is different, due to the presence of the skin or tissue of the finger in contact at that location, from the light reflection at another location under the fingerprint valley, where the skin or tissue of the finger is absent. This difference in light reflection conditions at the locations of the ridges and valleys in the touched finger area on the cover top surface forms an image representing an image or spatial distribution of the ridges and valleys of the touched section of the finger. The reflection light is directed back towards the LCD display module  433 , and, after passing through the small holes of the LCD display module  433 , reaches the interface with the low index optically transparent block  701  of the optical sensor module. The low index optically transparent block  701  is constructed to have a refractive index less than a refractive index of the LCD display panel so that the returned light can be extracted out of the LCD display panel into the optically transparent block  701 . Once the returned light is received inside the optically transparent block  701 , such received light enters the optical imaging unit as part of the imaging sensing block  702  and is imaged onto the photodetector sensing array or optical sensing array inside the block  702 . The light reflection differences between fingerprint ridges and valleys create the contrast of the fingerprint image. As shown in  FIG. 4B , a control circuit  704  (e.g., a microcontroller or MCU) is coupled to the imaging sensing block  702  and to other circuitry such as the device main processor  705  on a main circuit board. 
     In this particular example, the optical light path design is such the light ray enters the cover top surface within the total reflect angles on the top surface between the substrate and air interface will get collected most effectively by the imaging optics and imaging sensor array in the block  702 . In this design the image of the fingerprint ridge/valley area exhibits a maximum contrast. Such an imaging system may have undesired optical distortions that would adversely affect the fingerprint sensing. Accordingly, the acquired image may be further corrected by a distortion correction during the imaging reconstruction in processing the output signals of the optical sensor array in the block  702  based on the optical distortion profile along the light paths of the returned light at the optical sensor array. The distortion correction coefficients can be generated by images captured at each photodetector pixel by scanning a test image pattern one line pixel at a time, through the whole sensing area in both X direction lines and Y direction lines. This correction process can also use images from tuning each individual pixel on one at a time, and scanning through the whole image area of the photodetector array. This correction coefficients only need to be generated one time after assembly of the sensor. 
     The background light from environment (e.g., sun light or room light) may enter the image sensor through the LCD panel top surface, through holes in the LCDD display assembly  433 . Such background light can create a background baseline in the interested images from fingers and is undesirable. Different methods can be used to reduce this baseline intensity. One example is to tune on and off the illumination light source  703  at a certain frequency f and the image sensor accordingly acquires the received images at the same frequency by phase synchronizing the light source driving pulse and image sensor frame. Under this operation, only one of the image phases contain light from the light source. By subtracting even and odd frames, it is possible to obtain an image which most consists of light emitted from the modulated illumination light source. Based on this design, each display scan frame generates a frame of fingerprint signals. If two sequential frames of signals by turning on the illumination light in one frame and off in the other frame are subtracted, the ambient background light influence can be minimized or substantially eliminated. In implementations, the fingerprint sensing frame rate can be one half of the display frame rate. 
     A portion of the light from the illumination light source  703  may also go through the cover top surface and enter the finger tissues. This part of light power is scattered around and a part of this scattered light may be eventually collected by the imaging sensor array in the optical sensor module. The light intensity of this scattered light depends on the finger&#39;s skin color, the blood concentration in the finger tissue and this information carried by this scattered light on the finger is useful for fingerprint sensing and can be detected as part of the fingerprint sensing operation. For example, by integrating the intensity of a region of user&#39;s finger image, it is possible to observe the blood concentration increase/decrease depends on the phase of the user&#39;s heart-beat. This signature can be used to determine the user&#39;s heart beat rate, to determine if the user&#39;s finger is a live finger, or to provide a spoof device with a fabricated fingerprint pattern. 
     The one or more illumination light sources  703  in  FIG. 4B  can be designed to emit light of different colors or wavelengths and the optical sensor module can capture returned light from a person&#39;s finger at the different colors or wavelengths. By recording the corresponding measured intensity of the returned light at the different colors or wavelengths, information associated with the user&#39;s skin color can be determined. As an example, when a user registers a finger for fingerprint authentication operation, the optical fingerprint sensor also measures intensity of the scatter light from finger at color A, and B, as intensity Ia, Ib. The ratio of Ia/Ib could be recorded to compare with later measurement when user&#39;s finger is placed on the sensing area to measure fingerprint. This method can help reject the spoof device which may not match user&#39;s skin color. 
     The one or more illumination light sources  703  can be controlled by the same electronics  704  (e.g., MCU) for controlling the image sensor array in the block  702 . The one or more illumination light sources  703  can be pulsed for a short time, at a low duty cycle, to emit light intermittently and to provide pulse light for image sensing. The image sensor array can be operated to monitor the light pattern at the same pulse duty cycle. If there is a human finger touching the sensing area  615  on the screen, the image that is captured at the imaging sensing array in the block  702  can be used to detect the touching event. The control electronics or MCU  704  connected to the image sensor array in the block  702  can be operated to determine if the touch is by a human finger touch. If it is confirmed that it is a human finger touch event, the MCU  704  can be operated to wake up the smartphone system, turn on the illumination light source  703  for performing the optical fingerprint sensing), and use the normal mode to acquire a full fingerprint image. The image sensor array in the block  702  will send the acquired fingerprint image to the smartphone main processor  705  which can be operated to match the captured fingerprint image to the registered fingerprint database. If there is a match, the smartphone will unlock the phone, and start the normal operation. If the captured image is not matched, the smartphone will feedback to user that the authentication is failed. User may try again, or input passcode. 
     In the example in  FIGS. 4A and 4B , the under-screen optical sensor module uses the optically transparent block  701  and the imaging sensing block  702  with the photodetector sensing array to optically image the fingerprint pattern of a touching finger in contact with the top surface of the display screen onto the photodetector sensing array. The optical imaging axis or detection axis  625  from the sensing zone  615  to the photodetector array in the block  702  is illustrated in  FIG. 4B . The optically transparent block  701  and the front end of the imaging sensing block  702  before the photodetector sensing array form a bulk imaging module to achieve proper imaging for the optical fingerprint sensing. Due to the optical distortions in this imaging process, a distortion correction can be used, as explained above, to achieve the desired imaging operation. 
     In the optical sensing by the under-screen optical sensor module in  FIGS. 4A and 4B  and other designs disclosed herein, the optical signal from the sensing zone  615  on the top transparent layer  431  to the under-screen optical sensor module include different light components.  FIGS. 5A, 5B and 5C  illustrate signal generation for the returned light from the sensing zone  615  under different optical conditions to facilitate the understanding of the operation of the under-screen optical sensor module. 
       FIG. 5A  shows an example of how illumination light from the illumination light source  703  propagates through the OLED display module  433 , after transmitting through the top transparent layer  431 , and generates different returned light signals including light signals that carry fingerprint pattern information to the under-screen optical sensor module. For simplicity, two illumination rays  80  and  82  at two different locations are directed to the top transparent layer  431  without experiencing total reflection at the interfaces of the top transparent layer  431 . Specifically, the illumination light rays  80  and  82  are perpendicular or nearly perpendicular to the top layer  431 . A finger  60  is in contact with the sensing zone  615  on the e top transparent layer  431 . As illustrated, the illumination light beam  80  reaches to a finger ridge in contact with the top transparent layer  431  after transmitting through the top transparent layer  431  to generate the light beam  183  in the finger tissue and another light beam  181  back towards the LCD display module  433 . The illumination light beam  82  reaches to a finger valley located above the top transparent layer  431  after transmitting through the top transparent layer  431  to generate the reflected light beam  185  from the interface with the top transparent layer  431  back towards the LCD display module  433 , a second light beam  189  that enters the finger tissue and a third light beam  187  reflected by the finger valley. 
     In the example in  FIG. 5A , it is assumed that the finger skin&#39;s equivalent index of refraction is about 1.44 at 550 nm and the cover glass index of refraction is about 1.51 for the top transparent layer  431 . The finger ridge-cover glass interface reflects part of the beam  80  as reflected light  181  to bottom layers  524  below the LCD display module  433 . The reflectance can be low, e.g., about 0.1% in some LCD panels. The majority of the light beam  80  becomes the beam  183  that transmits into the finger tissue  60  which causes scattering of the light  183  to produce the returned scattered light  191  towards the LCD display module  433  and the bottom layers  524 . The scattering of the transmitted light beam  189  from the LCD pixel  73  in the finger tissue also contributes to the returned scattered light  191 . 
     The beam  82  at the finger skin valley location  63  is reflected by the cover glass surface (e.g., about 3.5% as the reflected light  185  towards bottom layers  524 , and the finger valley surface reflects about 3.3% of the incident light power (light  187 ) to bottom layers  524 . The total reflection may be about 6.8%. The majority light  189  is transmitted into the finger tissues  60 . Part of the light power in the transmitted light  189  in the figure tissue is scattered by the tissue to contribute to the scattered light  191  towards and into the bottom layers  524 . 
     Therefore, the light reflections from various interface or surfaces at finger valleys and finger ridges of a touching finger are different and the reflection ratio difference carries the fingerprint map information and can be measured to extract the fingerprint pattern of the portion that is in contact with the top transparent layer  431  and is illuminated the OLED light. 
       FIGS. 5B and 5C  illustrate optical paths of two additional types of illumination light rays at the top surface under different conditions and at different positions relative to valleys or ridges of a finger, including under a total reflection condition at the interface with the top transparent layer  431 . The illustrated illumination light rays generate different returned light signals including light signals that carry fingerprint pattern information to the under-screen optical sensor module. It is assumed that the cover glass  431  and the LCD display module  433  are glued together without any air gap in between so that illumination light with a large incident angle to the cover glass  431  will be totally reflected at the cover glass-air interface.  FIGS. 5A ,  5 B and  5 C illustrate examples of three different groups divergent light beams: (1) central beams  82  with small incident angles to the cover glass  431  without the total reflection ( FIG. 5A ), (2) high contrast beams  201 ,  202 ,  211 ,  212  that are totally reflected at the cover glass  431  when nothing touches the cover glass surface and can be coupled into finger tissues when a finger touches the cover glass  431  ( FIGS. 5B and 5C ), and (3) escaping beams having very large incident angles that are totally reflected at the cover glass  431  even at a location where the finger issue is in contact. 
     For the central light beams  82 , the cover glass surface reflects about 0.1%˜3.5% to light beam  185  that is transmitted into bottom layers  524 , the finger skin reflects about 0.1%˜3.3% to light beam  187  that is also transmitted into bottom layers  524 . The reflection difference is dependent on whether the light beams  82  meet with finger skin ridge  61  or valley  63 . The rest light beam  189  is coupled into the finger tissues  60 . 
     For high contrast light beams  201  and  202 , the cover glass surface reflects nearly 100% to light beams  205  and  206  respectively if nothing touches the cover glass surface. When the finger skin ridges touch the cover glass surface and at light beams  201  and  202  positions, most of the light power is coupled into the finger tissues  60  by light beams  203  and  204 . 
     For high contrast light beams  211  and  212 , the cover glass surface reflects nearly 100% to light beams  213  and  214  respectively if nothing touches the cover glass surface. When the finger touches the cover glass surface and the finger skin valleys happen to be at light beams  211  and  212  positions, no light power is coupled into finger tissues  60 . 
     As illustrated in  FIG. 5A , light beams that are coupled into finger tissues  60  will experience random scattering by the figure tissues to form low-contrast light  191  and part of such low-contrast light  191  will pass through the LCD display module  433  to reach to the optical sensor module. 
     Therefore, in high contrast light beams illuminated area, finger skin ridges and valleys cause different optical reflections and the reflection difference pattern carries the fingerprint pattern information. The high contrast fingerprint signals can be achieved by comparing the difference. 
     The disclosed under-screen optical sensing technology can be in various configurations to optically capture fingerprints based on the design in  FIGS. 2A and 2B . 
     For example, the specific implementation in  FIG. 4B  based on optical imaging by using a bulk imaging module in the optical sensing module can be implemented in various configurations.  FIGS. 6A-6C, 7, 8A-8B, 9, 10A-10B, 11 and 12  illustrate examples of various implementations and additional features and operations of the under-screen optical sensor module designs for optical fingerprint sensing. 
       FIG. 6A ,  FIG. 6B  and  FIG. 6C  show an example of an under-screen optical sensor module based on optical imaging via a lens for capturing a fingerprint from a finger  445  pressing on the display cover glass  423 .  FIG. 6C  is an enlarged view of the optical sensor module part shown in  FIG. 6B . The under-screen optical sensor module as shown in  FIG. 6B  is placed under the LCD display module  433  includes an optically transparent spacer  617  that is engaged to the bottom surface of the LCD display module  433  to receive the returned light from the sensing zone  615  on the top surface of the top transparent layer  431 , an imaging lens  621  that is located between and spacer  617  and the photodetector array  623  to image the received returned light from the sensing zone  615  onto the photodetector array  623 . Like the imaging system in the example in  FIG. 4B , this imaging system in  FIG. 6B  for the optical sensor module can experience image distortions and a suitable optical correction calibration can be used to reduce such distortions, e.g., the distortion correction methods described for the system in  FIG. 4B . 
     Similar to the assumptions in  FIGS. 5A, 5B and 5C , it is assumed that the finger skin&#39;s equivalent index of refraction to be about 1.44 at 550 nm and a bare cover glass index of refraction to be about 1.51 for the cover glass  423 . When the OLED display module  433  is glued onto the cover glass  431  without any air gap, the total inner reflection happens in large angles at or larger than the critical incident angle for the interface. The total reflection incident angle is about 41.8° if nothing is in contact with the cover glass top surface, and the total reflection angle is about 73.7° if the finger skin touches the cover glass top surface. The corresponding total reflection angle difference is about 31.9°. 
     In this design, the micro lens  621  and the photodiode array  623  define a viewing angle θ for capturing the image of the a contact finger in the sensing zone  615 . This viewing angle can be aligned properly by controlling the physical parameters or configurations in order to detect a desired part of the cover glass surface in the sensing zone  615 . For example, the viewing angle may be aligned to detect the total inner reflection of the LCD display assembly. Specifically, the viewing angle θ is aligned to sense the effective sensing zone  615  on the cover glass surface. The effective sensing cover glass surface  615  may be viewed as a mirror so that the photodetector array effectively detects an image of the fingerprint illumination light zone  613  in the LCD display that is projected by the sensing cover glass surface  615  onto the photodetector array. The photodiode/photodetector array  623  can receive the image of the zone  613  that is reflected by the sensing cover glass surface  615 . When a finger touches the sensing zone  615 , some of the light can be coupled into the fingerprint&#39;s ridges and this will cause the photodetector array to receive light from the location of the ridges to appear as a darker image of the fingerprint. Because the geometrics of the optical detection path are known, the fingerprint image distortion caused in the optical path in the optical sensor module can be corrected. 
     Consider, as a specific example, that the distance H in  FIG. 6B  from the detection module central axis to the cover glass top surface is 2 mm. This design can directly cover 5 mm of an effective sensing zone  615  with a width Wc on the cover glass. Adjusting the spacer  617  thickness can adjust the detector position parameter H, and the effective sensing zone width Wc can be optimized. Because H includes the thickness of the cover glass  431  and the display module  433 , the application design should take these layers into account. The spacer  617 , the micro lens  621 , and the photodiode array  623  can be integrated under the color coating  619  on the bottom surface of the top transparent layer  431 . 
       FIG. 7  shows an example of further design considerations of the optical imaging design for the optical sensor module shown in  FIGS. 6A-6C  by using a special spacer  618  to replace the spacer  617  in  FIGS. 6B-6C  to increase the size of the sensing area  615 . The spacer  618  is designed with a width Ws and thickness is Hs to have a low refraction index (RI) ns, and is placed under the LCD display module  433 , e.g., being attached (e.g., glued) to the bottom surface the LCD display module  433 . The end facet of the spacer  618  is an angled or slanted facet that interfaces with the micro lens  621 . This relative position of the spacer and the lens is different from  FIGS. 6B-6C  where the lens is placed underneath the spacer  617 . The micro lens  621  and a photodiode array  623  are assembled into the optical detection module with a detection angle width θ. The detection axis  625  is bent due to optical refraction at the interface between the spacer  618  and display module  433  and at the interface between the cover glass  431  and the air. The local incident angle ϕ 1  and ϕ 2  are decided by the refractive indices RIs, ns, nc, and na of the materials for the components. 
     If nc is greater than ns, ϕ 1  is greater than ϕ 2 . Thus, the refraction enlarges the sensing width Wc. For example, assuming the finger skin&#39;s equivalent RI is about 1.44 at 550 nm and the cover glass index RI is about 1.51, the total reflection incident angle is estimated to be about 41.8° if nothing touches the cover glass top surface, and the total reflection angle is about 73.7° if the finger skin touches the cover glass top surface. The corresponding total reflection angle difference is about 31.9°. If the spacer  618  is made of same material of the cover glass, and the distance from the detection module center to the cover glass top surface is 2 mm, if detection angle width is θ=31.9°, the effective sensing area width Wc is about 5 mm. The corresponding central axis&#39;s local incident angle is ϕ 1 =ϕ 2 =57.75°. If the material for the special spacer  618  has a refractive index ns about 1.4, and Hs is 1.2 mm and the detection module is tilted at ϕ 1 =70°. The effective sensing area width is increased to be greater than 6.5 mm. Under those parameters, the detection angle width in the cover glass is reduced to 19°. Therefore, the imaging system for the optical sensor module can be designed to desirably enlarge the size of the sensing area  615  on the top transparent layer  431 . 
     When the RI of the special spacer  618  is designed to be sufficiently low (e.g., to use MgF 2 , CaF 2 , or even air to form the spacer), the width Wc of the effective sensing area  615  is no longer limited by the thickness of the cover glass  431  and the display module  433 . This property provides desired design flexibility. In principle, if the detection module has a sufficient resolution, the effective sensing area may even be increased to cover the entire display screen. 
     Since the disclosed optical sensor technology can be used to provide a large sensing area for capturing a pattern, the disclosed under-screen optical sensor modules may be used to capture and detect not only a pattern of a finger but a larger size patter such a person&#39;s palm that is associated with a person for user authentication. 
       FIGS. 8A-8B  show an example of further design considerations of the optical imaging design for the optical sensor module shown in  FIG. 7  by setting the detection angle θ′ of the photodetector array relative in the display screen surface and the distance L between the lens  621  and the spacer  618 .  FIG. 8A  shows a cross-sectional view along the direction perpendicular to the display screen surface and  FIG. 8B  shows a view of the device from either the bottom or top of the displace screen. A filling material  618   c  can be used to fill the space between the lens  621  and the photodetector array  623 . For example, the filling material  618   c  can be same material of the special spacer  618  or another different material. In some designs, the filling material  618   c  may the air space. 
       FIG. 9  shows another example of an under-screen optical sensor module based on the design in  FIG. 7  where one or more illumination light sources  61  are provided to illuminate the top surface sensing zone  615  for optical fingerprint sensing. The illumination light sources  614  may be of an expanded type, or be a collimated type so that all the points within the effective sensing zone  615  is illuminated. The illumination light sources  614  may be a single element light source or an array of light sources. 
       FIGS. 10A-10B  show an example of an under-screen optical sensor module that uses an optical coupler  628  shaped as a thin wedge to improve the optical detection at the optical sensor array  623 .  FIG. 10A  shows a cross section of the device structure with an under-screen optical sensor module for fingerprint sensing and  FIG. 10B  shows a top view of the device screen. The optical wedge  628  (with a refractive index ns) is located below the display panel structure to modify a total reflection condition on a bottom surface of the display panel structure that interfaces with the optical wedge  628  to permit extraction of light out of the display panel structure through the bottom surface. The optical sensor array  623  receives the light from the optical wedge  628  extracted from the display panel structure and the optical imaging module  621  is located between the optical wedge  628  and the optical sensor array  623  to image the light from the optical wedge  628  onto the optical sensor array  623 . In the illustrated example, the optical wedge  628  includes a slanted optical wedge surface facing the optical imaging module and the optical sensing array  623 . Also, as shown, there is a free space between the optical wedge  628  and the optical imaging module  621 . 
     If the light is totally reflected at the sensing surface of the cover glass  431 , the reflectance is 100%, of the highest efficiency. However, the light will also be totally reflected at the LCD bottom surface  433   b  if it is parallel to the cover glass surfaces. The wedge coupler  628  is used to modify the local surface angle so that the light can be coupled out for the detection at the optical sensor array  623 . The micro holes in the LCD display module  433  provide the desired light propagation path for light to transmit through the LCD display module  433  for the under-screen optical sensing. The actual light transmission efficiency may gradually be reduced if the light transmission angle becomes too large or when the TFT layer becomes too thick. When the angle is close to the total reflection angle, namely about 41.8° when the cover glass refractive index is 1.5, the fingerprint image looks good. Accordingly, the wedge angle of the wedge coupler  628  may be adjusted to be of a couple of degrees so that the detection efficiency can be increased or optimized. If the cover glass&#39; refractive index is selected to be higher, the total reflection angle becomes smaller. For example, if the cover glass is made of Sapphire which refractive index is about 1.76, the total reflection angle is about 34.62°. The detection light transmission efficiency in the display is also improved. Therefore, this design of using a thin wedge to set the detection angle to be higher than the total reflection angle, and/or to use high refractive index cover glass material to improve the detection efficiency. 
     In the under-screen optical sensor module designs in  FIGS. 6A-10B , the sensing area  615  on the top transparent surface is not vertical or perpendicular to the detection axis  625  of the optical sensor module so that the image plane of the sensing area is also not vertical or perpendicular to the detection axis  625 . Accordingly, the plane of the photodetector array  523  can be tilted relative the detection axis  625  to achieve high quality imaging at the photodetector array  623 . 
       FIG. 11  shows three example configurations for this tiling.  FIG. 11  (1) shows the sensing area  615   a  is tilted and is not perpendicular the detection axis  625 . In a specified case shown in (2), the sensing area  615   b  is aligned to be on the detection axis  625 , its image plane will also be located on the detection axis  625 . In practice, the lens  621  can be partially cut off so as to simplify the package. In various implementations, the micro lens  621  can also be of transmission type or reflection type. For example, a specified approach is illustrated in (3). The sensing area  615   c  is imaged by an imaging mirror  621   a . A photodiode array  623   b  is aligned to detect the signals. 
     In the above designs where the lens  621  is used, the lens  621  can be designed to have an effective aperture that is larger than the aperture of the holes in the LCD display layers that allow transmission of light through the LCD display module for optical fingerprint sensing. This design can reduce the undesired influence of the wiring structures and other scattering objects in the LCD display module. 
       FIG. 12  shows an example of an operation of the fingerprint sensor for reducing or eliminating undesired contributions from the background light in fingerprint sensing. The optical sensor array can be used to capture various frames and the captured frames can be used to perform differential and averaging operations among multiple frames to reduce the influence of the background light. For example, in frame A, the illumination light source for optical fingerprint sensing is turned on to illuminate the finger touching area, in frame B the illumination is changed or turned off. Subtraction of the signals of frame B from the signals of frame A can be used in the image processing to reduce the undesired background light influence. 
     The undesired background light in the fingerprint sensing may also be reduced by providing proper optical filtering in the light path. One or more optical filters may be used to reject the environment light wavelengths, such as near IR and partial of the red light etc. In some implementation, such optical filter coatings may be made on the surfaces of the optical parts, including the display bottom surface, prism surfaces, sensor surface etc. For example, human fingers absorb most of the energy of the wavelengths under ˜580 nm, if one or more optical filters or optical filtering coatings can be designed to reject light in wavelengths from 580 nm to infrared, undesired contributions to the optical detection in fingerprint sensing from the environment light may be greatly reduced. 
       FIG. 13  shows an example of an operation process for correcting the image distortion in the optical sensor module. At step  1301 , the one or more illumination light sources are controlled and operated to emit light in a specific region, and the light emission of such pixels is modulated by a frequency F. Ate step  1302 , an imaging sensor under the display panel is operated to capture the image at frame rate at same frequency F. In the optical fingerprint sensing operation, a finger is placed on top of the display panel cover substrate and the presence of the finger modulates the light reflection intensity of the display panel cover substrate top surface. The imaging sensor under the display captures the fingerprint modulated reflection light pattern. At step  1303 , the demodulation of the signals from image sensors is synchronized with the frequency F, and the background subtraction is performed. The resultant image has a reduced background light effect and includes images from pixel emitting lights. At step  1304 , the capture image is processed and calibrated to correct image system distortions. At step  1305 , the corrected image is used as a human fingerprint image for user authentication. 
     The same optical sensors used for capturing the fingerprint of a user can be used also to capture the scattered light from the illuminated finger as shown by the back scattered light  191  in  FIG. 5A . The detector signals from the back scattered light  191  in  FIG. 5A  in a region of interest can be integrated to produce an intensity signal. The intensity variation of this intensity signal is evaluated to determine the heart rate of the user. 
     The above fingerprint sensor may be hacked by malicious individuals who can obtain the authorized user&#39;s fingerprint, and copy the stolen fingerprint pattern on a carrier object that resembles a human finger. Such unauthorized fingerprint patterns may be used on the fingerprint sensor to unlock the targeted device. Hence, a fingerprint pattern, although a unique biometric identifier, may not be by itself a completely reliable or secure identification. The under-screen optical sensor module can also be used as an optical anti-spoofing sensor for sensing whether an input object with fingerprint patterns is a finger from a living person and for determining whether a fingerprint input is a fingerprint spoofing attack. This optical anti-spoofing sensing function can be provided without using a separate optical sensor. The optical anti-spoofing can provide high-speed responses without compromising the overall response speed of the fingerprint sensing operation. 
       FIG. 14  shows exemplary optical extinction coefficients of materials being monitored in blood where the optical absorptions are different between the visible spectral range e.g., red light at 660 nm and the infrared range, e.g., IR light at 940 nm. By using probe light to illuminate a finger at a first visible wavelength (Color A) and a second different wavelength such as an IR wavelength (Color B), the differences in the optical absorption of the input object can be captured determine whether the touched object is a finger from a live person. The one or more illumination light sources for providing the illumination for optical sensing can be used to emit light of different colors to emit probe or illumination light at least two different optical wavelengths to use the different optical absorption behaviors of the blood for live finger detection. When a person&#39; heart beats, the pulse pressure pumps the blood to flow in the arteries, so the extinction ratio of the materials being monitored in the blood changes with the pulse. The received signal carries the pulse signals. These properties of the blood can be used to detect whether the monitored material is a live-fingerprint or a fake fingerprint. 
       FIG. 15  shows a comparison between optical signal behaviors in the reflected light from a nonliving material (e.g., a fake finger) and a live finger. The optical fingerprint sensor can also operate as a heartbeat sensor to monitor a living organism. When two or more wavelengths of the probe light are detected, the extinction ratio difference can be used to quickly determine whether the monitored material is a living organism, such as live fingerprint. In the example shown in  FIG. 15 , probe light at different wavelengths were used, one at a visible wavelength and another at IR wavelength as illustrated in  FIG. 14 . 
     When a nonliving material touches the top cover glass above the fingerprint sensor module, the received signal reveals strength levels that are correlated to the surface pattern of the nonliving material and the received signal does not contain signal components associated with a finger of a living person. However, when a finger of a living person touches the top cover glass, the received signal reveals signal characteristics associated with a living person, including obviously different strength levels because the extinction ratios are different for different wavelengths. This method does not take long time to determine whether the touching material is a part of a living person. In  FIG. 15 , the pulse-shaped signal reflects multiple touches instead of blood pulse. Similar multiple touches with a nonliving material does not show the difference caused by a living finger. 
     This optical sensing of different optical absorption behaviors of the blood at different optical wavelengths can be performed in a short period for live finger detection and can be faster than optical detection of a person&#39;s heart beat using the same optical sensor. 
     In LCD displays, the LCD backlighting illumination light is white light and thus contains light at both the visible and IR spectral ranges for performing the above live finger detection at the optical sensor module. The LCD color filters in the LCD display module can be used to allow the optical sensor module to obtain measurements in  FIGS. 14 and 15 . In addition, the designated light sources  436  for producing the illumination light for optical sensing can be operated to emit probe light at the selected visible wavelength and IR wavelength at different times and the reflected probe light at the two different wavelengths is captured by the optical detector array  621  to determine whether touched object is a live finger based on the above operations shown in  FIGS. 14 and 15 . Notably, although the reflected probe light at the selected visible wavelength and IR wavelength at different times may reflect different optical absorption properties of the blood, the fingerprint image is always captured by both the probe light the selected visible wavelength and the probe light at the IR wavelength at different times. Therefore, the fingerprint sensing can be made at both the visible wavelength and IR wavelength. 
       FIG. 16  shows an example of an operation process for determining whether an object in contact with the LCD display screen is part of a finger of a live person by operating the one or more illumination light sources for optical sensing to illuminate the finger with light in two different light colors. 
     For yet another example, the disclosed optical sensor technology can be used to detect whether the captured or detected pattern of a fingerprint or palm is from a live person&#39;s hand by a “live finger” detection mechanism by other mechanisms other than the above described different optical absorptions of blood at different optical wavelengths. For example, a live person&#39;s finger tends to be moving or stretching due to the person&#39;s natural movement or motion (either intended or unintended) or pulsing when the blood flows through the person&#39;s body in connection with the heartbeat. In one implementation, the optical sensor module can detect a change in the returned light from a finger or palm due to the heartbeat/blood flow change and thus to detect whether there is a live heartbeat in the object presented as a finger or palm. The user authentication can be based on the combination of the both the optical sensing of the fingerprint/palm pattern and the positive determination of the presence of a live person to enhance the access control. For yet another example, as a person touches the LCD display screen, a change in the touching force can be reflected in one or more ways, including fingerprint pattern deforming, a change in the contacting area between the finger and the screen surface, fingerprint ridge widening, or a change in the blood flow dynamics. Those and other changes can be measured by optical sensing based on the disclosed optical sensor technology and can be used to calculate the touch force. This touch force sensing can be used to add more functions to the optical sensor module beyond the fingerprint sensing. 
     In the above examples where the fingerprint pattern is captured on the optical sensor array via an imaging module as in  FIG. 4B  and  FIG. 6B , optical distortions tend to degrade the image sensing fidelity. Such optical distortions can be corrected in various ways. For example, a known pattern can be used to generate an optical image at the optical sensor array and the image coordinates in the know pattern can be correlated to the generated optical image with distortions at the optical sensor array for calibrating the imaging sensing signals output by the optical sensor array for fingerprint sensing. The fingerprint sensing module calibrates the output coordinates referencing on the image of the standard pattern. 
     In light of the disclosure in this patent document, various implementations can be made for the optical sensor module as disclosed. 
     For example, a display panel can be constructed in which each pixel emitting lights, and can be controlled individually; the display panel includes an at least partially transparent substrate; and a cover substrate, which is substantially transparent. An optical sensor module is placed under the display panel to sense the images form on the top of the display panel surface. The optical sensor module can be used to sense the images form from light emitting from display panel pixels. The optical sensor module can include a transparent block with refractive index lower than the display panel substrate, and an imaging sensor block with an imaging sensor array and an optical imaging lens. In some implementations, the low refractive index block has refractive index in the range of 1.35 to 1.46 or 1 to 1.35. 
     For another example, a method can be provided for fingerprint sensing, where light emitting from a display panel is reflected off the cover substrate, a finger placed on top of the cover substrate interacts with the light to modulate the light reflection pattern by the fingerprint. An imaging sensing module under the display panel is used to sense the reflected light pattern image and reconstruct fingerprint image. In one implementation, the emitting light from the display panel is modulated in time domain, and the imaging sensor is synchronized with the modulation of the emitting pixels, where a demodulation process will reject most of the background light (light not from pixels being targeted). 
     Various design considerations for the disclosed under-screen optical sensor module for optical fingerprint sensing are further described in Attachment 3 entitled “MULTIFUNCTION FINGERPRINT SENSOR AND PACKAGING” (41 pages in text and 26 sheets of drawings) as part of U.S. Provisional Patent Application No. 62/289,328 and U.S. Provisional Patent Application No. 62/330,833 and in the International Patent Application No. PCT/US2016/038445 entitled “MULTIFUNCTION FINGERPRINT SENSOR HAVING OPTICAL SENSING CAPABILITY” filed at U.S. Patent and Trademark Office on Jun. 20, 2016 (claiming priority from U.S. Provisional Patent Application No. 62/181,718, filed on Jun. 18, 2015 and published under No. WO 2016/205832 on Dec. 22, 2016) and MULTIFUNCTION FINGERPRINT SENSOR HAVING OPTICAL SENSING AGAINST FINGERPRINT SPOOFING filed at Chinese State Intellectual Property Office on Nov. 2, 2016 (claiming priority from U.S. Provisional Patent Application No. 62/249,832, filed on Nov. 2, 2015 and published under No. WO 2017/076292). The entire disclosures of the above mentioned patent applications are incorporated by reference as part of the disclosure of this patent document. 
     In various implementations of the under-screen optical sensor module technology for fingerprint sensing disclosed herein, the optical imaging of the illuminated touched portion of a finger to the optical sensor array in the under-screen optical sensor module can be achieved without using an imagine module such as a lens by imaging the returned light from the touched portion of the finger under optical illumination. One technical challenge for optical fingerprint sensing without an imaging module is how to control the spreading of the returned light that may spatially scramble returned light from different locations on the touched portion of the finger at the optical sensor array so that the spatial information of different locations may be lost when such returned light reaches the optical sensor array. This challenge can be addressed by using optical collimators or an array of pinholes to replace the optical imaging module in the under-screen optical sensor module for detecting a fingerprint by optical sensing. A device for implementing such optical fingerprint sending can include a device screen that provides touch sensing operations and includes a display panel structure having light emitting display pixels, each pixel operable to emit light for forming a portion of a display image; a top transparent layer formed over the device screen as an interface for being touched by a user for the touch sensing operations and for transmitting the light from the display structure to display images to a user; and an optical sensor module located below the display panel structure to receive light that is emitted by at least a portion of the light emitting display pixels of the display structure and is returned from the top transparent layer to detect a fingerprint, the optical sensor module including an optical sensor array that receives the returned light and an array of optical collimators or pinholes located in a path of the returned light to the optical sensor array. The array of optical collimators is used to collect the returned light from the display panel structure and to separate light from different locations in the top transparent layer while directing the collected returned light to the optical sensor array. 
     The imaging by using collimators relies on using different collimators at different locations to spatially separate light from different regions of a fingerprint to different optical detectors in the optical detector array. The thickness or length of each collimator along the collimator can be designed to control the narrow field of optical view of each collimator, e.g., the light from only a small area on the illuminated finger is captured by each collimator and is projected onto a few adjacent optical detectors in the optical detector array. As an example, the thickness or length of each collimator along the collimator can be designed to be large, e.g., a few hundred microns, so that the field of optical view of each collimator may allow the collimator to deliver imaging light to a small area on the optical detector array, e.g., one optical detector or a few adjacent optical detectors in the optical detector array (e.g., an area of tens of microns on each side on the optical detector array in some cases). 
     The following sections explain how an array of optical collimators or pinholes can be used for under-screen optical fingerprint sensing by the examples for using optical collimators in optical fingerprint sensing in hybrid sensing pixels each having a capacitive sensor for capturing fingerprint information and an optical sensor for capturing fingerprint information. 
       FIGS. 17A and 17B  show two examples of hybrid sensing pixel designs that combine capacitive sensing and optical sensing within the same sensing pixel. 
       FIG. 17A  shows an example of a fingerprint sensor device  2100  that incorporates a capacitive sensor in addition to an optical sensor for each sensor pixel of an array of sensor pixels in capturing fingerprint information. By combining both capacitive sensors and optical sensors, fingerprint images obtained with the optical sensors can be used to better resolve the 3D fingerprint structure obtained with the capacitive sensors. For illustrative purposes, the structure shown in  FIG. 17A  represents one sensor pixel in an array of sensor pixels and each sensor pixel includes an optical sensor  2102  and a capacitive sensor  2114  that are disposed next to each other within the same pixel. 
     The optical sensor  2102  includes a photodetector  2108  and a collimator  2106  disposed over the photodetector  2108  to narrow or focus reflected light  2124  from finger  2102  toward the photodetector  2108 . One or more light sources, such as LEDs (not shown) can be disposed around the collimator  2106  to emit light, which is reflected off the finger as reflected light  2124  and is directed or focused toward the corresponding photodetector  2108  to capture a part of the fingerprint image of the finger  2102 . The collimator  2106  can be implemented using an optical fiber bundle or one or more metal layer(s) with holes or openings. This use of multiple optical collimators above the optical detector array may be used as a lensless optical design for capturing the fingerprint image with a desired spatial resolution for reliable optical fingerprints sensing.  FIG. 17A  shows the collimator  2106  implemented using one or more metal layers  2110  with holes or openings  2112 . The collimator  2106  in the layer between the top structure or layer  2104  and the photodetectors  2108  in  FIG. 17A  includes multiple individual optical collimators formed by optical fibers or by holes or openings in one or more layers (e.g., silicon or metal) and each of such individual optical collimators receives light ray  2124  in a direction along the longitudinal direction of each optical collimator or within a small angle range that can be captured by the top opening of each opening or hole and by the tubular structure as shown so that light rays incident in large angles from the longitudinal direction of each optical collimator are rejected by each collimator from reaching the optical photodiode on the other end of the optical collimator. 
     In the capacitive sensing part of each sensing pixel, the capacitive sensor  2114  includes a capacitive sensor plate  2116  that is electromagnetically coupled to a portion of a finger that is either nearby or in contact with the sensing pixel to perform the capacitive sensing. More specifically, the capacitive sensor plate  2116  and the finger  2102  interact as two plates of one or more capacitive elements  2122  when the finger  2102  is in contact with or substantially near the optional cover  2104  or a cover on a mobile device that implements the fingerprint sensor device  2100 . The number of capacitive sensor plates  2116  can vary based on the design of the capacitive sensor  2114 . The capacitive sensor plate  2116  can be implemented using one or more metal layers. The capacitive sensor plate  2116  is communicatively coupled to capacitive sensor circuitry  2120  so that the capacitive sensor circuitry  2120  can process the signals from the capacitive sensor plate  2116  to obtain data representing the 3D fingerprint structure. A routing or shielding material can be disposed between the capacitive sensor plate  2116  and the capacitive sensor circuitry to electrically shield the metal plate  2116 . The capacitive sensor circuitry  2120  can be communicatively coupled to both the capacitive sensor plate  2116  and the photodetector  2108  to process both the signal from the capacitive sensor plate  2116  and the signal from the photodetector  2108 . In  FIG. 17A , the capacitive sensor and the optical sensor within each hybrid sensing pixel are adjacent to and displaced from each other without being spatially overlapped. 
     In implementations, the optical sensing features in the hybrid sensor design in  FIG. 17A  such as the optical collimator design can be used in an under-screen optical sensor module. Therefore, the optical sensing with the optical collimator feature in  FIG. 17A  may be implemented in a mobile device or an electronic device is capable of detecting a fingerprint by optical sensing to include a display screen structure; a top transparent layer formed over the display screen structure as an interface for being touched by a user and for transmitting the light from the display screen structure to display images to a user; and an optical sensor module located below the display screen structure to receive light that is returned from the top transparent layer to detect a fingerprint. The optical sensor module includes an optical sensor array of photodetectors that receive the returned light and an array of optical collimators to collect the returned light from the top transparent layer via the display screen structure and to separate light from different locations in the top transparent layer while directing the collected returned light through the optical collimators to the photodetectors of the optical sensor array. 
       FIG. 17B  illustrates another example of a fingerprint sensor device  2130  that structurally integrates an optical sensor and a capacitive sensor in each hybrid sensor pixel in a spatially overlap configuration in an array of sensor pixels to reduce the footprint of each hybrid sensing pixel. The fingerprint sensor device  2130  includes a semiconductor substrate  2131 , such as silicon. Over the substrate  2131 , multiple sensing elements or sensing pixels  2139  are disposed. Each sensing element or sensing pixel  2139  includes active electronics circuitry area  2132  including CMOS switches, amplifier, resistors and capacitors for processing sensor signals. Each sensing pixel or sensing element  2139  includes a photodetector  2133  disposed or embedded in the active electronics circuitry area  2132 . A capacitive sensor plate or a top electrode  2134  of the capacitive sensor for capacitive sensing is disposed over a photodetector  2133  and includes a hole or opening  2138  on the sensor plate  2134  to function also as a collimator of light for directing light onto the photodetector  2133 . A via  2135  filled with conductive material is disposed to electrically connect the top electrode  2134  to the active circuit elements  2132 . By adjusting the opening or the hole and the distance of the top electrode  2134  with the photodetector  2133 , the light collecting angle  2137  of the photodetector (e.g., photodiode)  2133  can be adjusted. The fingerprint sensor device  2130  is covered by a protective cover  2136 , which includes hard materials, such as sapphire, glass etc. Photodetector  2133  light collection angle  2137  can be designed to preserve the spatial resolution of the image collected by the photodiode arrays. A light source  2140 , such as an LED, is placed under the cover, on the side of fingerprint sensor device  2130  to emit light, which is reflected off the finger and directed toward the photodetector  2133  to capture the fingerprint image. When a finger touches or comes substantially near the protective cover, the finger and the sensing top electrode  2134  in combination form a capacitive coupling (e.g., capacitor  2142 ) between the human body and sensing top electrode  2134 . The fingerprint sensor device  2130  that includes both optical and capacitive sensors can acquire images of both a light reflection image of fingerprint and also a capacitive coupling image. The sensing top electrode  2134  serves dual purpose: 1) for capacitive sensing, and 2) as a collimator (by fabricating one or more holes on the sensing top electrode  2134 ) to direct, narrow or focus reflected light from the finger toward the photodetector  2133 . Reusing the sensing top electrode  2134  eliminates the need for additional metal layer or optical fiber bundle, and thus reduces each pixel size and accordingly the overall size of the fingerprint sensor device  2130 . 
     In  FIG. 17B , the optical sensing design uses the holes or openings  2138  formed between the top layer  2136  and the bottom array of photodetectors  2133  as optical collimators to select only light rays within certain angles  2137  to preserve the spatial resolution of the image collected by the photodetectors  2133  in the photodetector array as illustrated. Similar to the fiber or other tubular shaped optical collimators in  FIG. 17A , the holes or openings  2138  formed between the top layer  2136  and the bottom array of photodetectors  2133  constitute optical collimators to collect the returned light from the top transparent layer via the display screen structure and to separate light from different locations in the top transparent layer while directing the collected returned light through the optical collimators to the photodetectors  2133 . 
       FIG. 18  is a top-down view of an exemplary hybrid fingerprint sensor device  2200  incorporating both an optical sensor and a capacitive sensor in each hybrid sensing pixel. The fingerprint sensor device  2200  is implemented as a CMOS silicon chip  2221  that includes an array of hybrid (incorporating both an optical sensor and a capacitive sensor) sensing elements or pixels  2222 . Alternatively, the optical layout in  FIG. 18  can also be for all optical sensing designs disclosed in this document where the openings or holes  2223  represent the optical collimators in  FIG. 17A or 17B . The size or dimension of the sensing elements can be in the range of 25 μm to 250 μm, for example. The hybrid sensor device  2200  can include an array of support circuitry including amplifiers, ADCs, and buffer memory in a side region  2224 . In addition, the hybrid sensor device  2200  can include an area for wire bonding or bump bonding  2225 . A top layer  2226  of the hybrid sensor element  2222  can include a metal electrode for capacitive sensing. One or more openings or holes  2223  can be fabricated on each top metal electrode  23  to structurally serve as a collimator for directing light in a vertical direction to shine on a photodetector under the top electrode. Thus, the top layer  2226  structure can serve dual purposes of optical and capacitive sensing. A sensor device processor can be provided to process the pixel output signals from hybrid sensing pixels to extract the fingerprint information. 
     In addition to sharing the same structure for capacitive sensing and for focusing light in the vertical direction as a collimator, one instance of sensor signal detection circuitry can be shared between the optical and capacitive sensors to detect the sensor signals from both a photodetector and a capacitive sensor plate. 
       FIG. 19A  illustrates a circuit diagram for an exemplary hybrid fingerprint sensing element or pixel  2300  having both capacitive sensing and optical sensing functions for fingerprints. The exemplary sensor pixel  2300  includes sensor signal detection circuitry  2316  to selectively switch between detecting or acquiring sensor signals from a sensing top electrode (e.g., a top metal layer)  2308  based on capacitive sensing and a photodetector (e.g., a photodiode)  2314  based on optical sensing to acquire both a reflective optical image from the photodetector  2314  and a capacitive coupled image from the capacitive sensor electrode  2308  from a finger. In some implementations, the two images from the two sensing mechanisms in each hybrid sensing pixel can be serially processed by the sensor signal detection circuitry. In the illustrated example, switches  2310  and  2312  have first terminals that are electrically coupled to the sensing top electrode  2308  and the photodetector  2314 , respectively, and second terminals that are coupled to a common input terminal of the sensor signal detection circuitry  2316  to provide corresponding optical detector signal from the photodetector  2314  and the corresponding capacitive sensing signal from the sensing top electrode  2308  to the sensor signal detection circuitry  2316 . When the switch  2310  is turned off (CAP_EN=0) and the switch  2312  is turned on (Optical_en=1), the sensor signal detection circuitry  2316  acquires the optical detector signal representing the optical image of the scanned fingerprint received at the particular hybrid sensing pixel. The sensor signal detection circuitry  2316  can acquire the capacitive sensing signal representing the capacitive image of the scanned fingerprint when switch  2310  CAP_EN=1 and Optical_en=0. After both the optical and capacitive images are acquired, both images can be processed in downstream circuitry separately and in combination to identify the fingerprint characteristics. 
     With the two modality of imaging by the above hybrid sensing pixels, the performance of the fingerprint identification can be enhanced by making use of the two types of the images in different ways. This enhanced fingerprint identification can be achieved by the sensor device processor, such as sensor device processor  2321 , for processing the pixel output signals from the hybrid sensing pixels to extract the fingerprint information. For example, the capacitive image can provide a 3D image on the depth of the ridges and valleys of the fingerprint features. Complementing the 3D capacitive image, the optical image can provide a high resolution 2D information on the fingerprint characteristics. The optical 2D image having a higher spatial resolution can be used to recover the capacitive sensing image resolution because both images information on the same ridges of the fingerprint. In some implementations where the capacitive sensing method may be more sensitive and accurate on identifying the valleys of the fingerprint than the optical sensing method, the spatial resolution of images acquired using the capacitive sensing method can degrade based on the thickness of the cover. This aspect of the capacitive sensing can be supplemented by the optical sensing. In operation, the sensor response may be fixed and the point spread function of the capacitive sensor may be fixed for all sensor positions. The higher resolution optical sensing can be used as a resolution recovery method and can be applied on the capacitive sensing image to enhance the 3D image. A partial high resolution image from optical sensing can be available to help with the recovering method. Thus, the 3D capacitive image can be enhanced to provide more information on the valleys and ridges by interpolating or recovering based on the high resolution 2D image. 
     The enhanced 3D image can provide an improved fingerprint recognition and matching. In another example, the optical and capacitive images can be stored together to provide two comparisons each time a fingerprint recognition or matching is performed. The use of two types of images for comparison enhances the accuracy and security of the fingerprint sensing system. 
     The sensor signal detection circuitry  2316  can be implemented in various ways using a number different circuitry designs. In one example, integrator sensing circuitry  2318  can be implemented to store the electric charges caused by ridges and valleys touching or being substantially near the cover of the fingerprint sensor device of the cover of the mobile device. The inclusion of the integrator circuitry  2318  enhances the signal-to-noise ratio (SNR). The integrator sensing circuitry includes an operational amplifier  2322  to amplify a sensor signal, such as a capacitance related or optical related signal (e.g., voltage signal), detected by the sensing top electrode  2308  or the photodetector  2314  of the exemplary sensor pixel  2300 . The sensing top electrode  2308  that include a conductive material, such as one of a variety of metals is electrically connected to a negative or inverting terminal  2328  of the amplifier  2322  through the switch  2310 . The sensing top electrode  2108  and a local surface of the finger  2302  function as opposing plates of a capacitor Cf  2302 . The capacitance of the capacitor Cf  2302  varies based on a distance ‘d’ between the local surface of the finger and the sensing top electrode  2308 , the distance between the two plates of the capacitor Cf  2302 . The capacitance of capacitor Cf  2302  is inversely proportional to the distance ‘d’ between the two plates of the capacitor Cf  2302 . The capacitance of capacitor Cf  2302  is larger when the sensing top electrode  2308  is opposite a ridge of the finger than when opposite a valley of the finger. 
     In addition, various parasitic or other capacitors can be formed between different conductive elements in the exemplary sensor pixel  2300 . For example, a parasitic capacitor CP  2304  can form between the sensing top electrode  2308  and a device ground terminal  2305 . Device ground is coupled to earth ground closely. Another capacitor Cr  2324  can form between an output conductor of the amplifier  2322  and the negative or inverting terminal  2328  of the amplifier  2322  and functions as a feedback capacitor to the amplifier  2322 . Also, a switch  2326  can be coupled between the output of the amplifier  2322  and the negative or inverting terminal  2328  of the amplifier  2322  to reset the integrator circuitry  2318 . 
     The positive terminal of the amplifier  2322  is electrically connected to an excitation signal Vref. The excitation signal Vref can be directly provided to the positive terminal of a dedicated amplifier in each sensor pixel. By providing the excitation signal Vref directly to the positive terminal of the amplifier  2322 , the exemplary sensor pixel  2100  becomes an active sensor pixel. In addition, providing the excitation signal Vref directly to the positive terminal of the amplifier  2322  eliminates the need to include an excitation electrode, common to all sensor pixels, which reduces a conductive (e.g., metal) layer from the semiconductor structure of the sensor chip. In some implementations, an optional excitation electrode  2306  can be implemented to enhance the SNR based on the design of the sensor pixel. In addition, by providing the excitation signal Vref  2330  directly to the amplifier  2322 , the excitation signal Vref  2322  is not applied directly to the finger to avoid potentially irritating or injuring the finger. Moreover, when the excitation electrode for applying the excitation signal directly to the finger is not used, all components of the fingerprint sensor device can be integrated into a single packaged device, and the entire fingerprint sensor device can be disposed under the protective cover glass. With the entire fingerprint sensor device disposed under the protective cover glass, the fingerprint sensor device is protected from the finger and other external elements that can potentially damage the fingerprint sensor. 
     In  FIG. 19A , the output signal (optical and capacitive) of the sensor signal detection circuitry  2316  (e.g., Vpo of the amplifiers  2322 ) in the sensor pixels  2300  is electrically coupled to a switch  2320  to selectively output the output signal Vpo from the sensor pixel  2300  to a signal processing circuitry including a filter. The switch  2320  can be implemented using a transistor or other switching mechanisms and electrically coupled to a controller to control the switching of the switch  2320 . By controlling the switches  2320 ,  2310  and  2312 , the sensor pixels in an array of sensor pixels can be selectively switched between acquiring the optical signals and the capacitive signals. In one implementation, the optical or capacitive signal can be acquired for each line, row or column of sensor pixels in the array and then switched to acquire the other type of signal for the line, row or column. The switching between the optical and capacitive signal acquisition can be performed line-by-line. In another implementation, one type of signal (capacitive or optical) can be acquired for all sensor pixels or elements in the array and then switched to acquire the other type of signal for all of the sensor pixels or elements. Thus, the switching between acquisition of different signal types can occur for the entire array. Other variations of switching between acquisition of the two types of sensor signals can be implemented. 
       FIG. 19B  illustrates a circuit diagram for another exemplary hybrid fingerprint sensing element or pixel  2340 . The hybrid fingerprint sensing element or pixel  2340  is substantially the same as the hybrid fingerprint sensing element or pixel  2300  with respect to the components having the same reference number. For descriptions of the common components having the same reference number, refer to the description of  FIG. 19A . 
     The hybrid fingerprint sensing element or pixel  2340  implements the sensing top electrode  2308  to include a hole or opening  2342  that functions as a collimator to focus or narrow the reflected light  2344  toward the photodetector  2314  (e.g., photodiode). The photodetector  2314  can be positioned or disposed below the collimator implemented using the sensing top electrode  2308  to capture the reflected light  2344  focused by the collimator  2308 . 
     In some implementations, separate instances of sensor signal detection circuitry can be included for the optical and capacitive sensors to detect in parallel the sensor signals from both a photodetector and a capacitive sensor plate. 
       FIG. 19C  illustrates a circuit diagram of an exemplary hybrid fingerprint sensing element or pixel  2350  for performing parallel detection of sensor signals from the photodetector and the capacitive sensor plate. The hybrid fingerprint sensing element or pixel  2350  is substantially the same as the hybrid fingerprint sensing element or pixel  2340  with respect to the components having the same reference number. For descriptions of the common components having the same reference number, refer to the description of  FIG. 19A . 
     To perform sensor signal detection from both the capacitive plate and the photodetector in parallel, the hybrid fingerprint sensing element or pixel  2350  includes separate sensor signal detection circuitry  2316  and  2317  communicatively coupled to the sensing top electrode  2308  and the photodetector  2324  respectively. Sensor signal detection circuitry  2317  can be implemented to be substantially similar to sensor signal detection circuitry  2316 . In some implementations, switches  2310  and  2312  can be disposed to have first terminals that are electrically coupled to the sensing top electrode  2308  and the photodetector  2314 , respectively, and second terminals that are coupled to respective sensor signal detection circuitry  2316  and  2317  to provide the optical detector signal from the photodetector  2314  and the capacitive sensing signal from the sensing top electrode  2308  to the sensor signal detection circuitry  2316  and  2317  respectively When the switches  2310  and  2312  are turned on and off together, the sensor signal detection circuitry  2316  and  2317  can perform sensor signal detection from the capacitive plate  2308  and the photodetector  2314  in parallel. When the switches  2310  and  2312  are turned on and off out of phase with each other, the sensor signal detection circuitry  2316  and  2317  can perform sensor signal detection from the capacitive plate  2308  and the photodetector  2314  in series. In addition, the sensor device processor  2321  can be communicatively coupled to the sensor signal detection circuitry  2316  and  2317  either directly or indirectly through switches  2320 A and  2320 B to process the detected sensor signals from the capacitive plate  2308  and the photodetector  2314  in parallel or in series. 
     In another aspect of the disclosed technology, the optical sensor described with respect to  FIGS. 17A, 17B, 18, 19A and 19B  can be used to measure human heart beat by measuring the reflected light intensity change with time caused by blood flow variations in fingers due to the heart beat and pumping actions of the heart. This information is contained in the received light that is reflected, scattered or diffused by the finger and is carried by the optical detector signal. Thus, the optical sensor can serve multiple functions including acquiring an optical image of the fingerprint and to measure human heart beat. In implementations, a sensor device processor is used to process one or more optical detector signals to extract the heart beat information. This sensor device processor may be the same sensor device processor that processes the pixel output signals from optical sensing pixels or hybrid sensing pixels to extract the fingerprint information. 
     The following sections describe examples of various designs for fingerprint sensing using an under-screen optical sensor module using an array of optical collimators or pinholes for directing signal light carrying fingerprint information to the optical sensor array. Such optical collimators or pinholes are placed between the LCD display screen and the optical sensor array in the under-screen optical sensor module to couple desired returned light from the display panel while filtering out background light in the optical detection by the optical sensor array. Implementation of such optical collimators or pinholes can simplify the optical designs of the optical detection by the optical sensor array, e.g., without using complex optical imaging designs in other designs disclosed in this patent document, such as the imaging designs in  FIGS. 6B, 7, 10A, and 11 . In addition, implementation of such optical collimators or pinholes can simplify the optical alignment of the overall optical layout to the optical sensor array and improve reliability and performance of the optical detection by the optical sensor array. Furthermore, such optical collimators or pinholes can significantly simplify the fabrication and reduce the overall cost of the under-screen optical sensor module. 
       FIG. 20  shows an example of a smartphone having a liquid crystal display (LCD) display and an under-screen optical sensor module that includes an optical collimator array for collecting and directing light to an optical detector array for optical fingerprint sensing. The LCD-based touch sensing display system  423  implements an optical sensing module with a photo detector array  621  under the LCD display system  423 . 
     The touch sensing display system  423  is placed under a top cover glass  431  which serves as a user interface surface for various user interfacing operations, including, e.g., touch sensing operations by the user, displaying images to the user, and an optical sensing interface to receive a finger for optical fingerprint sensing and other optical sensing operations where probe light is directed from inside the device to the top cover glass  431  to illuminate the finger. The display system  423  is a multi-layer LCD module  433  that includes LCD display backlighting light sources  434  (e.g., LED lights) that provide the white backlighting for the LCD module  433 , a light waveguide layer  433   c  coupled to the LCD display backlighting light sources  434  to receive and guide the backlighting light, LCD structure layers  433   a  (including, e.g., a layer of liquid crystal (LC) cells, LCD electrodes, transparent conductive ITO layer, an optical polarizer layer, a color filter layer, and a touch sensing layer), a backlighting diffuser  433   b  placed underneath the LCD structure layers  433   a  and above the light waveguide layer  433   c  to spatially spread the backlighting light for illuminating the LCD display pixels in the LCD structure layers  433   a , and an optical reflector film layer  433   d  underneath the light waveguide layer  433   c  to recycle backlighting light towards the LCD structure layers  433   a  for improved light use efficiency and the display brightness. When the LCD cells in the sensing window are turned on, most of the LCD structure layers  433   a  (include liquid crystal cells, electrodes, transparent ITO, polarizer, color filter, touch sensing layer etc.) become partially transparent although the micro structure may extinct partial of the probe light energy. The light diffuser  433   b , the light waveguide  433   c , the reflector film  433   d , and the LCD module frame are treated to hold the fingerprint sensor and provide transparent or partially transparent sensing light path so that a portion of the reflected light from the top surface of the cover glass  431  can reach a photo detector array  621  with an under-LCD-screen optical sensor module for fingerprint sensing and other optical sensing operations. As illustrated, this optical sensor module under the LCD screen includes various fingerprint sensor parts, e.g., an optical collimator array  617  for collimating and directing reflected probe light to the photo detector array  621 , and an optical sensor circuit module  623  that receives and conditions the detector output signals from the photo detector array  621 . The optical collimator array  617  can include optical collimators and may be a waveguide based image transmitter, an optical fiber array, a micro lens array, or a pinhole array. The optical collimators operate to limit the numeral aperture (NA) of the sampling image and to form corresponding image elements. Each optical collimator unit gets a part of the image of the touched portion of a target finger on the top glass cover  431 . The transmitted light beams of all the collimators collectively form a full image of the target at the photo detector array  621 . The photodiode array  621  may be a CMOS sensor of CMOS sensing pixels, a CCD sensor array or a suitable optical sensor array that is sensitive to light. 
     The example illustrates includes an electronics module  435  for the LCD display and touch sensing operations, one or more other sensors  425  such as an optical sensor for monitoring the light level of the surroundings, optional side buttons  427  and  429  for controls of certain smartphone operations. 
     In the example in  FIG. 20 , the light sources in the illustrated example include the display back lighting light sources  434  and the extra designated probe light sources  436 . The light beams  442   a  from extra designated probe light sources  436  and the light beams  442   b  from the display light sources  434  can be used as the sensor probe light for illuminating a finger in contact with the top glass cover  431  to generate the desired reflected probe light carrying the fingerprint pattern and other information to the optical sensor module. 
     When the LCD cells in the sensing window are turned on, most of the LCD structure layers  433   a  (include liquid crystal cells, electrodes, transparent ITO, polarizer, color filter, touch sensing layer etc.) become partially transparent although the micro structure may extinct partial of the probe light energy. The light diffuser  433   b , the light waveguide  433   c , the reflector film  433   d , and the LCD module frame are treated to hold the fingerprint sensor and provide transparent or partially transparent sensing light path. 
       FIG. 21  below further illustrates the operation of the under LCD screen optical sensor module in the above example in  FIG. 20 . On the top cover glass  431 , a fingerprint sensing area or window is an area on the top surface of the top cover glass  431  that is right above or near the underlying optical sensor module. Since the optical sensor module is underneath the LCD structure, the sensing window is part of the contiguous top surface of the top cover glass  431  and is also part of the display area for the LCD display. Accordingly, there may be no visible physical demarcation on the top surface to indicate this sensing window. This sensing window may be indicated to a user via other means to assist the user to place a finger within the sensing window for fingerprint sensing and other optical sensing operations. For example, the extra designated probe light sources  436  may be used to illuminate the sensing window so that the area for the sensing window is distinctly different from the surrounding areas on the top cover glass and is readily visible to the user. This can be done when the LCD panel is turned off or when the LCD panel is turned on. 
     As shown in  FIG. 21 , a user presses a finger on the sensing window and the probe light beam  82 P illuminates the finger. The finger and the cover glass  431  reflect the probe light as a reflected signal light beam  82 R. Various scattering interfaces  433 S in the LCD module  433  diffuses the reflected signal light beam  82 R to form diffused light beam  82 D. Individual collimator units in the collimator array  617  select light component  82 S and guide the selected light component  82 S into corresponding photosensing detectors of the photodetector array  621 . The photosensing detectors, e.g., photodiodes or CMOS sensing detectors, generate corresponding sensor signals that contain information on the fingerprint pattern. A portion of the source light may enter the fingerprint sensor module without first going through the finger sensing area on the top surface of the LCD panel. This part of light contributes background noise and can be eliminated by calibration. Each collimator unit of the collimator array  617  only selects the light be transmitted along its permitted direction at a relatively low optical loss to corresponding photo detectors in a part of the photodetector array  621 . Accordingly, each collimator unit in the collimator array  617  and its corresponding photo detectors in the photodetector array  621  operate together to define the effective detecting optical numeral aperture NA. This NA directly defines the spatial resolution of the image produced by the optical sensor module. 
     Based on the disclosed under LCD screen optical sensing designs, a person&#39;s finger, either in direct touch with the LCD display screen or in a near proximity about the LCD display screen, can produce the returned light back into the LCD display screen while carrying information of a portion of the finger illuminated by the light output by the LCD display screen. Such information may include, e.g., the spatial pattern and locations of the ridges and valleys of the illuminated portion of the finger. Accordingly, the optical sensor module can be integrated to capture at least a portion of such returned light to detect the spatial pattern and locations of the ridges and valleys of the illuminated portion of the finger by optical imaging and optical detection operations. The detected spatial pattern and locations of the ridges and valleys of the illuminated portion of the finger can then be processed to construct a fingerprint pattern and to perform fingerprint identification, e.g., comparing with a stored authorized user fingerprint pattern to determine whether the detected fingerprint is a match as part of a user authentication and device access process. This optical sensing based fingerprint detection by using the disclosed optical sensor technology uses the LCD display screens as an optical sensing platform and can be used to replace existing capacitive fingerprint sensors or other fingerprint sensors that are basically self-contained sensors as “add-on” components without using light from display screens or using the display screens for fingerprint sensing for mobile phones, tablets and other electronic devices. 
     Notably, an optical sensor module based on the disclosed optical sensor technology can be coupled to the backside of the LCD display screen without requiring a designated area on the display surface side of the LCD display screen that would occupy a valuable device surface real estate in some electronic devices such as a smartphone, a tablet or a wearable device. Such an optical sensor module can be placed under the LCD display screen that vertically overlaps with the display screen area, and, from the user&#39;s perspective, the optical sensor module is hidden behind the display screen area. In addition, because the optical sensing of such an optical sensor module is by detecting the light from the LCD display screen and is returned from the top surface of the display area, the disclosed optical sensor module does not require a special sensing port or sensing area that is separate from the display screen area. Accordingly, different from fingerprint sensors in other designs, including, e.g., Apple&#39;s iPhone/iPad devices or Samsung Galaxy smartphone models where the fingerprint sensor is located at a particular fingerprint sensor area or port (e.g., the home button) on the same surface of the display screen but located in a designated non-displaying zone that is outside the display screen area, the optical sensor module based on the disclosed optical sensor technology can be implemented in ways that would allow fingerprint sensing to be performed at any location on the LCD display screen by using unique optical sensing designs by routing the returned light from the finger into an optical sensor and by providing proper optical imaging mechanism to achieve high resolution optical imaging sensing. In this regard, the disclosed optical sensor technology provides a unique on-screen fingerprint sensing configuration by using the same top touch sensing surface that displays images and provides the touch sensing operations without a separate fingerprint sensing area or port outside the display screen area. 
     In addition to fingerprint detection by optical sensing, the optical sensing may be used to measure other parameters. For example, the disclosed optical sensor technology can measure a pattern of a palm of a person given the large touch area available over the entire LCD display screen (in contrast, some designated fingerprint sensors such as the fingerprint senor in the home button of Apple&#39;s iPhone/iPad devices have a rather small and designated off-screen fingerprint sensing area that is highly limited in the sensing area size not suitable for sensing large patterns). For yet another example, the disclosed optical sensor technology can be used not only to use optical sensing to capture and detect a pattern of a finger or palm that is associated with a person, but also to use optical sensing or other sensing mechanisms to detect whether the captured or detected pattern of a fingerprint or palm is from a live person&#39;s hand by a “live finger” detection mechanism based on the fact that a live person&#39;s finger tends to be moving or stretching due to the person&#39;s natural movement or motion (either intended or unintended) or pulsing when the blood flows through the person&#39;s body in connection with the heartbeat. In one implementation, the optical sensor module can detect a change in the returned light from a finger or palm due to the heartbeat/blood flow change and thus to detect whether there is a live heartbeat in the object presented as a finger or palm. The user authentication can be based on the combination of the both the optical sensing of the fingerprint/palm pattern and the positive determination of the presence of a live person to enhance the access control. For yet another example, the optical sensor module may include a sensing function for measuring a glucose level or a degree of oxygen saturation based on optical sensing in the returned light from a finger or palm. As yet another example, as a person touches the LCD display screen, a change in the touching force can be reflected in one or more ways, including fingerprint pattern deforming, a change in the contacting area between the finger and the screen surface, fingerprint ridge widening, or a change in the blood flow dynamics. Such changes can be measured by optical sensing based on the disclosed optical sensor technology and can be used to calculate the touch force. This touch force sensing adds more functions to the optical sensor module beyond the fingerprint sensing. 
     With respect to useful operation or control features in connection with the touch sensing aspect of the LCD display screen, the disclosed optical sensor technology can provide triggering functions or additional functions based on one or more sensing results from the optical sensor module to perform certain operations in connection with the touch sensing control over the LCD display screen. For example, the optical property of a finger skin (e.g., the index of refraction) tends to be different from other artificial objects. Based on this, the optical sensor module may be designed to selectively receive and detect returned light that is caused by a finger in touch with the surface of the LCD display screen while returned light caused by other objects would not be detected by the optical sensor module. This object-selective optical detection can be used to provide useful user controls by touch sensing, such as waking up the smartphone or device only by a touch via a person&#39;s finger or palm while touches by other objects would not cause the device to wake up for energy efficient operations and to prolong the battery use. This operation can be implemented by a control based on the output of the optical sensor module to control the waking up circuitry operation of the LCD display screen which, for example, may include designed extra light sources for optical sensing and the designed extra light sources may turned on in a flash mode to intermittently emit flash light to the screen surface for sensing any touch by a person&#39;s finger or palm while the LCD display screen can be placed in a sleep mode to save power. In some implementations, the wake-up sensing light can be in the infrared invisible spectral range so a user will not experience any visual of a flash light. 
     An optical sensor module based on the disclosed optical sensor technology can be coupled to the backside of the LCD display screen without requiring creation of a designated area on the surface side of the LCD display screen that would occupy a valuable device surface real estate in some electronic devices such as a smartphone, a tablet or a wearable device. This aspect of the disclosed technology can be used to provide certain advantages or benefits in both device designs and product integration or manufacturing. 
     In some implementations, an optical sensor module based on the disclosed optical sensor technology can be configured as a non-invasive module that can be easily integrated to a LCD display screen without requiring changing the design of the LCD display screen for providing a desired optical sensing function such as fingerprint sensing. In this regard, an optical sensor module based on the disclosed optical sensor technology can be independent from the design of a particular LCD display screen design due to the nature of the optical sensor module: the optical sensing of such an optical sensor module is by detecting the light from the LCD display screen and is returned from the top surface of the display area, and the disclosed optical sensor module is coupled to the backside of the LCD display screen for receiving the returned light from the top surface of the display area and thus does not require a special sensing port or sensing area that is separate from the display screen area. Accordingly, such an optical sensor module can be used to combine with LCD display screens to provide optical fingerprint sensing and other sensor functions on a LCD display screen without using a specially designed LCD display screen with hardware especially designed for providing such optical sensing. This aspect of the disclosed optical sensor technology enables a wide range of LCD display screens to be used in smartphones, tablets or other electronic devices with enhanced functions from the optical sensing of the disclosed optical sensor technology. 
     For example, for an existing phone assembly design that does not provide a separate fingerprint sensor as in the Apple iPhones or Samsung models, such an existing phone assembly design can integrate the under-screen optical sensor module as disclosed herein without changing the touch sensing-display screen assembly to provide an added on-screen fingerprint sensing function. Because the disclosed optical sensing does not require a separate designated sensing area or port as in the case of the Apple iPhones/Samsung phones with a front fingerprint senor outside the display screen area, or some smartphones with a designated rear fingerprint sensor on the backside like in some models by Huawei, Xiaomi, Google or Lenovo, the integration of the on-screen fingerprint sensing disclosed herein does not require a substantial change to the existing phone assembly design or the touch sensing display module that has both the touch sensing layers and the display layers. Simply put, no external sensing port and no extern hardware button are needed on the exterior of a device are needed for adding the disclosed optical sensor module for fingerprint sensing. The added optical sensor module and the related circuitry are under the display screen inside the phone housing and the fingerprint sensing is conveniently performed on the same touch sensing surface for the touch screen. 
     For another example, due to the above described nature of the optical sensor module for fingerprint sensing, a smartphone that integrates such an optical sensor module can be updated with improved designs, functions and integration mechanism without affecting or burdening the design or manufacturing of the LCD display screens to provide desired flexibility to device manufacturing and improvements/upgrades in product cycles while maintain the availability of newer versions of optical sensing functions to smartphones, tablets or other electronic devices using LCD display screens. Specifically, the touch sensing layers or the LCD display layers may be updated in the next product release without adding any significant hardware change for the fingerprint sensing feature using the disclosed under-screen optical sensor module. Also, improved on-screen optical sensing for fingerprint sensing or other optical sensing functions by such an optical sensor module can be added to a new product release by using a new version of the under-screen optical sensor module without requiring significant changes to the phone assembly designs, including adding additional optical sensing functions. 
     The above and other features of the disclosed optical sensor technology can be implemented to provide a new generation of electronic devices with improved fingerprint sensing and other sensing functions, especially for smartphones, tablets and other electronic devices with LCD display screens to provide various touch sensing operations and functions and to enhance the user experience in such devices. 
     The optical sensor technology disclosed herein uses the light for displaying images in a display screen that is returned from the top surface of the device display assembly for fingerprint sensing and other sensing operations. The returned light carries information of an object in touch with the top surface (e.g., a finger) and the capturing and detecting this returned light constitute part of the design considerations in implementing a particular optical sensor module located underneath the display screen. Because the top surface of the touch screen assembly is used as a fingerprint sensing area, the optical image of this touched area should be captured by an optical imaging sensor array inside the optical sensor module with a high image fidelity to the original fingerprint for robust fingerprint sensing. The optical sensor module can be designed to achieve this desired optical imaging by properly configuring optical elements for capturing and detecting the returned light. 
       FIGS. 22A-22B  show an exemplary implementation of the optical collimator design in  FIGS. 20 and 21 . The optical collimator array  2001  in this example includes an array of optical collimators  903  and an optical absorption material  905  filled between the optical collimators  903  to absorb light to reduce cross talk between different optical collimators. Each collimator  903  of the collimator array  2001  may be channels that are extended or elongated along a direction perpendicular to the display panel and lets the light be transmitted along its axis with a low loss. The collimator array  2001  is designed to reduce optical crosstalk between different optical collimators and to maintain a desired spatial resolution in the optical sensing. In some implementations, one optical collimator may correspond to only one photodetector in the photodetector array  2002 . In other implementations, one optical collimator may correspond to two or more photodetectors in the photodetector array  2002 . As illustrated in  FIG. 22B , the axis of each collimator unit may be perpendicular to the display screen surface in some designs and may be slanted with respect to the display surface. In operation, only the light that propagates along a collimator axis carries the image information. For example, the proper incident light  82 P is reflected to form light  82 R. Light  82 R is then diffracted by the small holes of the TFT and expanded to light  82 D. The light portion  82 S is transmitted into the photodiode array  2002 . The light portion  82 E away from the axis is absorbed by the filling material. The reflectance on the cover glass surface  431  carries the fingerprint information. Light rays  901  at an angle with respect to the collimator unit axis and thus may be blocked. A part of the reflected light, such as  901 E, transmits into a corresponding optical collimator to reach the photodetector array  2002 . 
     The optical collimator array can be made by different techniques, including, e.g., etching holes through a flat substrate, forming a light waveguide array, forming a micro lens array matching with optical filters, using coreless optical fiber bundle, or printing collimators on a transparent sheet. The desired features for such a collimator array include: (1) sufficient transmission contrast between the light component that propagates along the axis and the component that propagates off the axis so that the collimators ensures the desired spatial resolution in the optical sensing of the fingerprint pattern at the photodetector array; (2) the permitted transmission numerical aperture be sufficiently small to realize a desired high spatial resolution for the optical sensing. 
     Various optical collimator array designs may be used. Each optical collimator in the optical collimator array is structured to perform spatial filtering by transmitting light in directions along or close to an axis of the optical collimator while blocking light in other directions and to have a small optical transmission numerical aperture to achieve a high spatial resolution by the array of optical collimators. The small optical transmission numerical aperture also reduces the amount of the background light that enters the optical sensor array. The collimator element aperture and the pitch (i.e., the distance between two nearby collimator elements) can be designed to achieve a desired spatial resolution for the optical fingerprint sensing. 
       FIG. 23  shows an example of a collimator design that is part of a CMOS structure by using aligned holes in two different metal layers in the CMOS structure. Each optical collimator in the array is an elongated channel along a direction that is perpendicular to the display panel in this particular example. 
       FIG. 24  shows an example of an optical fingerprint sensor module under a LCD display structure that incorporates an optical sensor array and an integrated collimator array for each optical sensor pixel in capturing fingerprint information. The optical sensor array includes an array of photodetectors and a collimator array is disposed over the photodetector array to include optically transparent vias as optical collimators and optically opaque metal structures between the vias as shown. Illumination light is directed to illuminate the touched portion of a finger and the light reflected off the finger is directed through the optical collimator array to reach the photodetector array which captures a part of the fingerprint image of the finger. The optical collimator array can be implemented using one or more metal layer(s) with holes or openings integrated via the CMOS process. 
     Such optical collimators in the under-screen optical sensor module can be structured to provide direct point to point imaging. For example, the dimensions of the optical collimator array and individual collimators can be designed to closely match the dimensions of the photodetector array and the dimensions of individual photodetectors, respectively, to achieve one to one imaging between optical collimators and photodetectors. The entire image carried by the light received by the optical sensor module can be captured by the photodetector array at individual photodetectors simultaneously without stitching. 
     The spatial filtering operation of the optical collimator array can advantageously reduce the amount of the background light that enters the photodetector array in the optical sensor module. In addition, one or more optical filters may be provided in the optical sensor module to filter out the background light and to reduce the amount of the background light at the photodetector array for improved optical sensing of the returned light from the fingerprint sensing area due to the illumination by emitted light from the OLED pixels. For example, the one or more optical filters can be configured, for example, as bandpass filters to allow transmission of the illumination light generated for optical sensing while blocking other light components such as the IR light in the sunlight. This optical filtering can be an effective in reducing the background light caused by sunlight when using the device outdoors. The one or more optical filters can be implemented as, for example, optical filter coatings formed on one or more interfaces along the optical path to the photodetector array in the optical sensor module or one or more discrete optical filters. 
       FIG. 25  shows an example an optical collimator array with optical filtering to reduce background light that reaches the photodetector array in the under-screen optical sensor module. This example uses an array of optical waveguides as the optical collimators and one or more optical filter films are coupled to the optical waveguide array to reduce undesired background light from reaching the photodetector array coupled to the optical waveguide array, e.g. the IR light from the sunlight while transmitting desired light in a predetermined spectral band for the probe light that is used to illuminate the finger. The optical waveguide can include a waveguide core with or without an outside waveguide cladding. The optical waveguide may also be formed by a coreless fiber bundle with different fibers where each unit collimator is a piece of fiber without a fiber core structure. When the coreless fibers are made into bundle, the filling material between the fibers may include a light absorbing material so as to increase the absorption of stray light that is not guided by the coreless fibers. The final collimator may be assembled with multiple layers of sub-collimator arrays. 
     The following sections provide examples of various optical collimator designs and their fabrication. 
       FIGS. 26A and 26B  show examples of fabricating collimators by etching. In  FIG. 26A , a layer of a suitable material for forming optical collimators in the collimator array is formed on or supported by a support substrate which is optically transparent. An etching mask is formed over the layer and has a pattern for etching the underlying layer to form the optical collimators. A suitable etching process is performed to form the optical collimators. The support substrate may be bound with the collimator array and may be formed from various optical transparent materials including, e.g., silicon oxide. 
       FIG. 26B  shows an example of an optical collimator array that is assembled by stacking multiple layers of sub-collimator arrays via an inter-layer connector material which may be an adhesive, a glass, or a suitable optically transparent material. In some implementations, different layers of sub-collimator arrays may be stacked over one another without the inter-layer connector material. This stacking allows fabrication of optical collimators with desired lengths or depths along the collimator axis to achieve desired optical numerical apertures. The holes of the collimators geometrically limit the viewing angle. The transmitting numeral aperture is decided by the thickness of the collimator and the hole aperture. The holes may be filled with an optically transparent material in some applications and may be void in some designs. 
     In implementations, the support substrate may be coated with one or more optical filter films to reduce or eliminate background light such as the IR light from the sunlight while transmitting desired light in a predetermined spectral band for the probe light that is used to illuminate the finger. 
       FIG. 27  shows an array of optical spatial filters coupled with micro lens array where each microlens is located with respect to a corresponding through hole of an optical spatial filter so that each unit collimator includes a micro lens and a micro spatial filter, such as a micro hole. Each micro lens is structured and positioned to focus received light to the corresponding micro spatial filter without imaging the received light. The micro hole limits the effective receiving numerical aperture. The spatial filter may be printed on an optically transparent substrate, or etched on a piece of silicon wafer. The micro lens array may be etched by MEMS processing or chemical processing. The micro lens may also be made of a gradient refractive index material, e.g., cutting a piece of gradient refractive index glass fiber to a quarter pitch of length. The focal length of the micro lenses and the diameter of the spatial filter hole can be used to control the transmitting numerical aperture of each unit. Like in other designs, the collimator board may be coated with filter films to reduce or eliminate the light band not used in the sensor such as the IR light from the sunlight. 
       FIG. 28  shows an example of an integrated CMOS photo detection array sensor, with built-in collimation of light. The collimator is built by combing an array of aligned holes  705  in different metal layers  704  and oxide layers  702 , 703  which are interleaved between metal layers to provide separation. These holes can be aligned with photo sensitive elements  701  in the optical sensor array. Optical fingerprint imager is implemented with this integrated CMOS photo detection array sensor with built-in collimation of light under the LCD display module  710  and cover glass. The fingerprint of the user&#39;s finger in touch with the sensor window area of the cover glass can be imaged by detection of the light reflected off the fingerprint valley and ridges. The light from a fingerprint ridge area would be reduced, because the light is absorbed in fingerprint tissue at the ridge area while the light from the fingerprint valley area stronger by comparison. This difference in the light levels between the ridges and valleys of a fingerprint produces a fingerprint pattern at the optical sensor array. 
     In the above optical sensor module designs based on collimators, the thickness or length of each collimator along the collimator can be designed to be large to deliver imaging light to a small area on the optical detector array or to be small to deliver imaging light to a large area on the optical detector array. When the thickness or length of each collimator along the collimator in a collimator array decreases to a certain point, e.g., tens of microns, the field of the optical view of each collimator may be relatively large to cover a patch of adjacent optical detectors on the optical detector array, e.g., an area of 1 mm by 1 mm. In some device designs, optical fingerprint sensing can be achieved by using an array of pinholes with each pinhole having a sufficiently large field of optical view to cover a patch of adjacent optical detectors in the optical detector array to achieve a high image resolution at the optical detector array in sensing a fingerprint. In comparison with a collimator design, a pinhole array can have a thinner dimension and a smaller number of pinholes to achieve a desired high imaging resolution without an imaging lens. Also, different from the imaging via optical collimators, imaging with the array of pinholes uses each pinhole as a pinhole camera to capture the image and the image reconstruction process based on the pinhole camera operation is different that by using the optical collimator array: each pinhole establishes a sub-image zone and the sub image zones by different pinholes in the array of pinholes are stitched together to construct the whole image. The image resolution by the optical sensor module with a pinhole array is related to the sensitive element size of the detector array and thus the sensing resolution can be adjusted or optimized by adjusting the detector dimensions. 
     A pinhole array can be relatively simple to fabricate based on various semiconductor patterning techniques or processes or other fabrication methods at relatively low costs. A pinhole array can also provide spatial filtering operation to advantageously reduce the amount of the background light that enters the photodetector array in the optical sensor module. Similar to designing the optical sensor modules with optical collimators, one or more optical filters may be provided in the optical sensor module with a pinhole array to filter out the background light and to reduce the amount of the background light at the photodetector array for improved optical sensing of the returned light from the fingerprint sensing area due to the illumination by the illumination light generated for optical sensing. For example, the one or more optical filters can be configured, for example, as bandpass filters to allow transmission of the illumination light for optical sensing while blocking other light components such as the IR light in the sunlight. This optical filtering can be an effective in reducing the background light caused by sunlight when using the device outdoors. The one or more optical filters can be implemented as, for example, optical filter coatings formed on one or more interfaces along the optical path to the photodetector array in the optical sensor module or one or more discrete optical filters. 
     In an optical sensor module based on optical collimators, the optical imaging resolution at the optical sensor array can be improved by configuring the optical collimators in a way to provide a pinhole camera effect.  FIG. 29  shows an example of such a design. 
     In  FIG. 29 , a collimator unit  618  of an array of such optical collimators guides the light from the corresponding detection area unit to the photo detector array  621 . The aperture of the collimator unit forms a small field of view (FOV)  618   b . If the detector in the photo detector array  621  does not capture the details in each unit FOV, the imaging resolution is decided by the FOV of each collimator unit. To improve the detection resolution, the FOV of each collimator unit needs to be reduced. However, when a gap  618   a  is provided between each photo detector in the photo detector array  621  and the corresponding collimator  618 , the small aperture of the collimator unit acts as a pinhole. This pinhole camera effect provides a higher imaging resolution in the image of each unit of FOV. When there are multiple detector elements in a unit FOV, such as shown in the insert  621   a , the images details in the unit FOV can be recognized. This means that the detection resolution is improved. In implementations, such a gap can be provided in various ways, including, e.g., adding optical filter films  618   a  between the collimators  618  and the optical sensor array  621 . 
     With the help of the pinhole camera effect, the fill factor of the collimator board, may be optimized. For example, to detect an area of 10 mm×10 mm in size, if each unit FOV covers an area of 1 mm×1 mm, a 10×10 collimator array can be used. If in each unit FOV the detector can get 20×20 definition image, the overall detection resolution is 200×200, or 50 micron, or 500 psi. This method can be applied for all types of collimator approaches. 
       FIG. 30  shows another example for using the pinhole camera effect to improve the optical imaging resolution. In this example, the optical sensor module includes several layers: a spacer  917 , a pinhole array  617  which may be an optical collimator array with a sufficiently small thickness, a protection material  919 , a photo detector array  621 , and a circuit board  623 . The object optical distance is decided by the total material thickness from sensing surface to the pinhole plane, including the optical thickness of the display module  433 , the thickness of the spacer  917 , any filter coating thickness, any air gap thickness, and any glue material thickness. The image optical distance is decided by the total material thickness from the pinhole plane to the photo detector array, including the protection material thickness, any filter coating thickness, any air gaps thickness, any glue material thickness. The image magnification is decided by the image optical distance comparing with the object optical distance. The detection mode can be optimized by setting a proper magnification. For example, the magnification may be set to be less than 1, such as, 0.7, or 0.5 etc. In some device designs, the spacer and the pinhole array layer may be combined into a single component. In other designs, the pinhole array and the protection layer may be combined to a single component so as to pre-define the center coordinates of each pinhole. 
       FIG. 31  shows an example of the optical imaging based on the pinhole camera effect. On the object side, the whole detection zone  921  on the LCD display panel is divided into multiple sub-detection zones  923 . A pinhole array  920  is provided for imaging the detection zone  921 . Each pinhole unit in the pinhole array  920  is responsible for a small field of view (FOV)  925 . Each small FOV  925  covers a sub-detection zone  923 . As shown in  FIG. 31 , each small FOV of one pinhole can overlap with small FOVs of its neighboring pinholes. On the image side, each sub-detection zone  923  in the optical sensor array captures an image  933 . Also shown in  FIG. 31 , each small FOV  925  of a pinhole has a corresponding image zone  935 . The magnification of this system can be optimized so that the images of each sub-detection zone can be separately distinguished. In other words, the images of the small FOVs do not overlap each other. In this detection mode, the central coordinates of each pinhole are pre-defined and the image spot coordinates of each LCD display pixel can be pre-calibrated and this pre-calibration may be used to generate a calibration table for the calibration during the sensor operation. In this design, the image of the pinhole camera is inversed and the signal processing can recover the whole image based on the calibration table. 
     In the above illustrated examples for optical collimators, the direction of the optical collimators for directing light from a finger on the top of the display screen into the optical sensor array for fingerprint sensing may be either perpendicular to the top touch surface of LCD display screen to collect returned probe light from the finger for fingerprint sensing, a majority of which is in a light direction perpendicular to the top touch surface. In practice, when a touched finger is dry, the image contrast in the detected images in the optical sensor array by sensing such returned probe light that is largely perpendicular to the top touch surface is lower than the same image obtained from returned probe light that is at an angle with respect to the perpendicular direction of the top touch surface. This is in part because optical sensing of angled returned light spatially filters out the strong returned light from the top touch surface that is mostly perpendicular to the top touch surface. In consideration of this aspect of the optical sensing of the returned probe light from the top touch surface, the optical collimators may be oriented so that the axis of each collimator unit may be slanted with respect to the top touch surface as shown in the example in  FIG. 22B . 
     In fabrication, however, it is more complex and costly to fabricate slanted collimators. One way to use perpendicular optical collimators as shown in  FIGS. 20 and 21  while still achieving a higher contrast in the optical sensing by selectively detecting angled returned light from the top touch surface is to provide an optical deflection or diffraction device or layer between the perpendicular optical collimators and the returned light from the top touch surface prior to entering the perpendicular optical collimators. This optical deflection or diffraction device or layer can be, in some implementations, between the OLED display panel and the perpendicular optical collimators to select only returned probe light that is at some slanted angle to enter the perpendicular optical collimators for optical detection by the optical detector array on the other end of the perpendicular optical collimators while blocking or reducing the amount of the returned probe light from the top touch surface that is perpendicular to the top touch surface from entering the optical collimators. This optical deflection or diffraction device or layer may be implemented in various forms, including, e.g., an array of prisms, an optical layer with a diffraction pattern, or other devices located between the optical collimators and the display panel to select angled probe light returned from the display panel to enter the optical collimators while reducing an amount of the returned probe light that is perpendicular to the display panel and enters the optical collimators. 
       FIG. 32  shows an example of an optical sensor module using an optical pinhole array for optical sensing. As illustrated, a pinhole array  920   a  is formed between the LCD display module  433  and the optical photo detector array  621  to image the sensing area where finger  60  is pressed on onto the optical photo detector array  621 . 
     The thickness T of the pinhole layers  920   a  dictates the field of view (FOV) angles. Together with the distances from the sensing surface to the pinhole array  920   a  and from the image plane to the pinhole array  920   a , the sensing area FOVs and imaging area FOVi are defined. The image magnification is given by Di/Ds, where Di is the thickness of the optical transparent layer  919   a  between the pinhole array  920   a  and the optical sensor array  621  and Ds is the thickness of the combined stack by the spacer  917 , the LCD display module  433  and the top cover glass layer  431 . The device parameters such as the pinhole layer thickness T, Ds, and Di can be optimized for a desired combination of FOV and image magnification. For example, the optical sensor module can be configured with desired parameters to render the neighboring FOVs of corresponding neighboring pinholes in the array  920   a  to properly overlap if so desired. Similarly, the neighboring FOVi can also be adjusted to be overlapped or fully separated as discrete FOVi. In an optical sensor module designed to cause neighboring FOVs to overlap each other, some of the spots on the sensing surface can have multiple image spots. This signature can be used to enhance the detection. 
     In the example in  FIG. 32 , optical filter films for reducing the background light may be formed or coated on the spacer  917 , on the pinhole layers  920   a , on the protection  919   a , or on the display surfaces. As illustrated in the figure, when background light  937  is projected onto the finger tissues  60 , short wavelength component are mostly absorbed, partial long wavelength (such as red light or infrared light) light is transmitted towards the detector  621 . The optical filter films can be used to reject those long wavelength components to improve the detection of returned light signals that carry the finger information. 
       FIGS. 33A and 33B  show an example of an optical fingerprint senor under a LCD display panel having an optical deflection or diffraction device or layer. 
     As shown in  FIG. 33A , each collimator  2001  in the collimator array may be an extended channel along an axis vertical or perpendicular to the display surfaces. A viewing angle adaptor optical layer  2210  is used to adjust the viewing angle of the returned probe light from the display panel and is located between the optical collimators  2001  and the LCD display panel to select angled probe light returned from the display panel to enter the optical collimators  2001  while reducing an amount of the returned probe light that is perpendicular to the display panel and enters the optical collimators  2001 . 
       FIG. 33B  shows more details of the viewing angle adaptor optical layer  3210  and the major probe light paths. For example, the viewing angle adaptor optical layer  3210  may be implemented as a diffraction pattern layer such as a prism structure  3210   a . Only the returned probe light  82   a  and  82   b  from the finger with proper incident angles out of the display panel can be bent to transmit through the collimator  2001 . In comparison, the returned probe light that is perpendicular to the display panel is directed by the viewing angle adaptor optical layer  2210  to be away from the original direction that is perpendicular to the display panel and thus becomes off-axis incident light to the optical collimator  2001 . This reduces the amount of the returned probe light that is perpendicular to the display panel and that can enter the optical collimator  2001 . 
     When the viewing angle is adjusted properly, the receiving light from different locations  63   a  and  63   b  of the fingerprint valley carries the fingerprint information. For example, under same illumination, light  82   a  may be stronger than light  82   b  because of the viewing angel and the fingerprint profiles of the fingertip skin. This design allows the optical sensor module to obtain some level of fingerprint shade. This arrangement improves the detection when the finger is dry. 
     In designing optical sensor modules under LCD display modules, various technical features or properties of LCD display modules should be considered and factored into the overall optical sensor module designs to improve the optical sensing operation. The following sections described several design examples. 
     One common component in various LCD display modules is a light diffuser which may a sheet that diffuses the incident light to different directions to achieve a large viewing angle and the spatial uniformity of the display. The presence of this LCD diffuser layer, however, can degrade the optical detection by the under-LCD optical sensor module. 
       FIGS. 34A and 34B  show a LCD light diffuser layer  433   b  located between the LCD waveguide layer  433   c  and other LCD layers  433   a . In some LCD assemblies, the cover glass layer  431  may be separated by a distance from the underlying diffuser sheet  433   b  (e.g., several millimeters in some LCD devices), and the optical collimator array  617  is separated from the diffuser sheet  433   b  by the light waveguide board  433   c  (which may be sub mini-meters thick). Under this structure, the strong diffusion in the diffuser sheet  433   b  can significantly reduce the signal contrast in the signal light that passes through the LCD display module  433  to reach the optical detector array  621 . The light diffusion at the LCD diffuser layer  433   b , although desirable for display operations, degrades the fingerprint detection performance. 
     This undesired effect of the LCD diffuser layer  433   b  may be mitigated by using different techniques. Two examples are illustrated in  FIGS. 34A and 34B . 
       FIG. 34A  shows one example in which holes  951   a  can be made in the corresponding area or all over the diffuser sheet  433   b  in the LCD display module in the section of the diffuser sheet  433   b  above the optical sensor module to improve the transmission of the returned light from the top cover glass  431  to the optical detector array  621 . The hole sizes, shapes and distribution can be selected based on the specific design needs. For example, the hole size may be larger than the probe light wave lengths so as to avoid strong diffraction. For example, a collimator unit aperture may be about 40 microns in diameter and the diffuser sheet hole size may be 5 micron, 10 micron, 30 micron, 40 micron, or 100 micron and so on. The inclusion of the holes  951   a  in the LCD diffuser layer  433   b  in this design is to establish a light path for each of the collimator unit. Each collimator unit aperture may have one or multiple holes in the diffuser sheet to provide a desired light path from the top cover glass  431  to the optical detector array  621 . If the collimator unit apertures are discrete with a relatively large pitch distance (for example 1 mm or so), the holes in the diffuser sheet may be drilled with the same pitch distance. The non-uniformity in the detection can be calibrated. 
       FIG. 34B  shows another example where the diffuser sheet can be structured to include low diffusion optical transparent spots  951   b  where the light diffusion is weak in the region above the optical sensor module to improve the transmission of the light to the optical sensor module. The transparent spot sizes, shapes and distribution e can be selected based on the specific design needs. For example, the hole size may be larger than the probe light wave lengths so as to avoid strong diffraction, and the spot distribution be such that each collimator unit has one or more transparent light paths to allow efficient reception of the returned light from the top cover glass  431  through the LCD display layers. If the collimator unit apertures are discrete with large pitch distance (for example 1 mm or so), the transparent spots in the diffuser sheet may be made with the same pitch distance. If the diffuser sheet is made of a rough surface material that diffracts or diffuses light, a selected material can be selectively applied to the rough surface to provide some transparent material to reduce the original optical diffusion of the rough surface. Examples for suitable materials include epoxy, wax, or oil and can effectively modify the diffusion. 
     For a given LCD diffuser layer, a long wavelength light source may be selected to generate the probe or illumination light so that the diffuser scattering for such light is weak so that more light can pass through the diffuser layer to reach the optical sensor module. 
     For another example, referring to  FIGS. 35A and 35B , various LCD display modules include an optical reflector layer or film  433   d  in LCD below the LCD waveguide layer  433   c  to reflect the unused light back to the LCD layers for enhancing the display brightness. However, the presence of this optical reflector film  433   d  can block most of the light from reaching the optical sensor module under the LCD and thus can adversely affect the optical fingerprint sensing. This optical reflector layer can be modified in a way that maintains the desired optical reflection under the LCD waveguide layer in most locations while allowing for desired optical transmission at the location of the under-LCD optical sensor module. In some implementations, the collimator module  617  for the optical sensor under the LCD can be fixed to touch the reflector film  433   d.    
       FIG. 34C  shows another example for providing transparent light paths for guiding light from one or more illumination light sources  436  to improve the fingerprint sensing of the detection module without significant diffusion by the diffusion layer. For example, holes  969  may be selectively formed in the light diffuser film  433   b  to improve light transmission to the under-LCD optical fingerprint sensor. To avoid the influence of the display performance, the light path holes may be tilted to maintain some level of light diffusion function in the area of the holes  969 . In addition, such holes  969  may be designed to be small, e.g., 0.3 mm or less, to further enhance diffusion of the backlighting while still providing improved optical imaging at the under-LCD optical fingerprint sensor. In implementations, the light path holes may be empty with air, may be filled with a transparent material. 
     In some designs, the light path holes  969  may not be limited at a certain area but may be distributed all over the light diffuser film  433   b , e.g., the holes  969  may be evenly distributed in the entire film  433   b . This design eliminates the undesired spatial non-uniform illumination created by the selected holes  969  in certain area but not in other areas. In some designs, the light path holes  969  may be distributed in a spatial gradient pattern so that any change in the LCD illumination caused by the holes  969  would be gradual and less visible. 
       FIG. 35A  shows one example for modifying the optical reflector layer by including or forming light-transmitting holes in the region of the optical sensor module location in the optical reflector film to allow optical reflection for LCD display in most parts of the optical reflector film while providing the optical collimator array  617  with transparent light paths for receiving light reflected from the finger on top of the LCD. The hole sizes, shapes and distribution can be configured to meet the needs of optical sensing. For example, the hole size may be larger than the probe light wave lengths so as to avoid strong diffraction. For example, the collimator unit aperture may be around 40 microns in diameter and the diffuser sheet hole size may be 5 micron, 10 micron, 30 micron, 40 micron, or 100 micron and so on. Each collimator unit aperture may have one or multiple holes in the optical reflector layer to provide desired light paths for optical sensing. The non-uniformity in the detection can be calibrated. If the collimator unit apertures are discrete with large pitch distance (for example 1 mm or so), the holes in the reflector film may be drilled with the same pitch distance. 
       FIG. 35B  shows another example for modifying the optical reflector layer in the LCD in which the optical reflectance of the optical reflector film may be modified to allow for some degree of optical transmission for optical sensing by the underlying optical sensor. Various commercial LCD reflector films use flexible plastic material as substrate and the optical transmittance of such plastic materials may be sufficient for transmitting sufficient light to the optical sensor module for fingerprint sensing. 
     In the above designs for the LCD diffuser layer and LCD reflector layer, the holes may be formed in a region where the one or more illumination light sources are located to allow sufficient transmission of the illumination light to pass through the LCD display module layers to reach the top cover glass for illuminating a finger for the optical sensing operation. 
     In the above designs, the optical sensor module is located underneath the LCD display module and thus is under the LCD waveguide layer which is designed to guide the backlighting light from the backlighting light source to the LCD display area. As shown in  FIG. 36 , the backlighting light from the display light sources  434  (e.g., LEDs) is guided by the waveguide  433   c  and is diffused by the LCD diffuser layer to leave the waveguide  433   c  to provide the needed backlighting for the LCD. The light may be uniformly leaked from one side surface of the waveguide  433   c  and is then diffused by the diffuser sheet  433   b . In some LCDs, about half of the diffused light  957  may propagate towards the collimator  617  and becomes strong background light in the optical sensing detection. 
     One or more extra light sources  436  can be provided in connection with the optical sensor module: to illuminate the finger and to provide the light carrying the fingerprint pattern information to the optical sensor module underneath the LCD. Due to the location of the illumination light sources  436  (e.g., below the reflector film  433   d  next to or adjacent to the optical sensor), the light guide function of the waveguide  433   c  is not effective to the light from the illumination light sources  436  so that the light from the  436  can be more efficiently reach the top surface of the LCD panel for illuminating a finger. 
     In addition, the illumination light sources  436  can be designed to emit illumination at one or more optical wavelengths different from the LCD display illumination light wavelengths from the LCD display backlighting light sources  434 . The illumination light sources  436  can be used for both fingerprint sensing and other sensing functions. 
     The above design for selecting the illumination light at one or more optical wavelengths that are different from the optical wavelength of the backlighting light for the LCD display may be used to reduce power consumption. Using the display backlighting light sources for the fingerprint detection requires the display backlighting light sources to be turned on for performing optical fingerprint sensing. This design consumes more power when compared to the above design where the illumination light for optical sensing is different from the backlighting light in optical wavelength in part to allow for optical sensing operation without turning on the LCD backlighting light. The above design for selecting the illumination light at one or more optical wavelengths that are different from the optical wavelength of the backlighting light for the LCD display enables flexible selection of the illumination light sources to gain additional advantages. For example, infrared light can be used as the illumination sources  436  so that the LCD diffuser layer becomes more transparent to the IR illumination light for desired higher transmission of the IR illumination light. For another example, the illumination light sources can be selected to provide multiple wavelengths for other functions such as anti-spoof liveness sensing, heartbeat sensing etc. 
     In designing an optical sensor module under LCD, the locations and spatial distribution of the illumination light sources  436  can be used to adjust the observing angle so as to optimize the sensing quality. 
     In placing an optical sensor module under a LCD module, additional optical designs may be used to enhance the delivery of the backlighting light from the waveguide layer into the LCD layers while maintaining sufficient delivery of the illumination light for optical sensing to the optical sensor module. 
       FIGS. 37A-37D  show examples of an enhancement structure that includes two or more layers of back light enhancement films such as  433   px  and  433   py  as part of the LCD layer structure shown as  433   a . The backlight enhancement films  433   px  and  433   py  are formed on top of the light diffuser layer  433   b.    
     In the example in  FIG. 37A , each of the enhancement films  433   px  and  433   py  includes a polarized prism structure. The prism groove directions of the two enhancement films  433   px  and  433   py  are substantially perpendicular to each other to collectively form a pair of enhancement films to improve delivery of the illumination light to the LCD panel. However, this function of the enhancement films may adversely affect optical imaging of the under-LCD optical fingerprint sensor module  621 U if not properly configured. 
     As shown in the examples in  FIGS. 37B and 37C , the extra light source illumination direction and the detector viewing direction can be specifically configured not to be along the prism groove directions of the enhancement films  433   px  and  433   py  to reduce the adverse imaging impact of the enhancement films for optical fingerprint sensing. This design is to get a clear image without punch holes in the enhancement films. The viewing angle ϕ 1  and the illumination angle ϕ 2  should be adjusted according to the design of the enhancement films. 
     The example in  FIG. 37D  shows a particular design of the collimator unit  617 U and a photodetector array unit  621 U. The collimator unit  617 U is used to provide an imaging function that is realized by a micro lens, pinholes or a combination of a micro lens and pinholes. A larger sensing area at the photodetector array unit  621 U can be realized by optimizing the single detection unit design or by using multiple detection units. 
       FIG. 38  shows an example of a light waveguide layer in the LCD module to include a partially transparent section in the detection light paths to allow for improve optical transmission of the illumination light for optical sensing to pass through the waveguide layer. 
       FIGS. 39A-39C  show examples for designing an illumination light source for the optical sensing in an optical sensor module under the LCD display module. In a LCD display module, the optical reflector layer enhances the LCD display brightness by recycling unused backlighting light back to the LCD layers. In this regard, a defect in the optical reflectivity along the optical reflector film, e.g., a mechanical defect in the reflector film, can cause a visible change in the brightness of the LCD display and thus is undesirable.  FIGS. 39A-39C  illustrate a design feature for reducing the adverse effect of defects in the reflector layer or film. 
     As shown in  FIG. 39A , micro holes  973  can be provided in the reflector layer  433   d  at the location of the illumination light sources  436  for the visible light component in the illumination light. This visible light component is used to provide illumination in a limited area of the display to show essential text or sign information without turning on the display back light. 
     As shown in  FIG. 39B , another solution is to select the illumination light source wavelengths to be out of the reflector film&#39;s working band which is generally in the visible band. The illumination light sources  436  for optical sensing may be outside the reflection spectral range of the reflector film, e.g., short wavelength range below 400 nm (e.g., 380 nm) or long wavelength range beyond the visible red range (e.g., 780 nm, 900 nm, 940 nm, etc.) so that the illumination light can pass through the reflector film or layer without the need to form holes in the reflector film. 
       FIG. 39C  shows yet another design solution that designs the reflector film to include a narrow band transmission window  975  for transmitting the illumination light for optical detection. For example, this narrow transparent or transmission window in the reflector film may be between 525 nm and 535 nm. 
     Portable devices such as mobile phones or other devices or systems based on the optical sensing disclosed in this document can be configured to provide additional operation features. 
     For example, the LCD display panel can be controlled to provide a local flash mode to illuminate the fingerprint sensing area by operating selected LCD display pixels underneath the sensing area. This can be provided in an optical sensor module under the LCD display panel, e.g.,  FIGS. 4A and 4B  based on an optical imaging design or  FIG. 21  based on optical imaging via an optical collimator array. In the event of acquiring a fingerprint image, the LCD display pixels in the sensing window area and the illumination light sources can be turned on momentarily to produce high intensity illumination for optical sensing of a fingerprint, and, at the same time, the photo detection sensor array  621  is turned on to capture the fingerprint image in sync with the turning on of the illumination light. The time to turn on the illumination light can be relatively short but the emission intensity can be set to be high. For this reason, this mode for optical fingerprint sensing is a flash mode that enable the photo detector sensor array  621  to detect a larger amount of light to improve the image sensing performance. 
     The optical sensors for sensing optical fingerprints disclosed above can be used to capture high quality images of fingerprints to enable discrimination of small changes in captured fingerprints that are captured at different times. Notably, when a person presses a finger on the device, the contact with the top touch surface over the display screen may subject to changes due to changes in the pressing force. When the finger touches the sensing zone on the cover glass, changes in the touching force may cause several detectable changes at the optical sensor array: (1) fingerprint deforming, (2) a change in the contacting area, (3) fingerprint ridge widening, and (4) a change in the blood flow dynamics at the pressed area. Those changes can be optically captured and can be used to calculate the corresponding changes in the touch force. The touch force sensing adds more functions to the fingerprint sensing. 
     Referring to  FIG. 40 , the contact profile area increases with an increase in the press force, meanwhile the ridge-print expands with the increase in the press force. Conversely, the contact profile area decreases with a decrease in the press force, meanwhile the ridge-print contracts or shrinks with the decrease in the press force.  FIG. 40  shows two different fingerprint patterns of the same finger under different press forces: the lightly pressed fingerprint  2301  and the heavily pressed fingerprint  3303 . The returned probe light from a selected integration zone  3305  of the fingerprint on the touch surface can be captured by a portion of the optical sensors on the optical sensor array that correspond to the selected integration zone  3305  on the touch surface. The detected signals from those optical sensors are analyzed to extract useful information as further explained below. 
     When a finger touches the sensor surface, the finger tissues absorb the light power thus the receiving power integrated over the photo diode array is reduced. Especially in the case of a total inner reflection mode that does not sense the low refractive index materials (water, sweat etc.), the sensor can be used to detect whether a finger touches the sensor or something else touches the sensor accidentally by analyzing the receiving power change trend. Based on this sensing process, the sensor can decide whether a touch is a real fingerprint touch and thus can detect whether to wake up the mobile device based on whether the touch is a real finger press. Because the detection is based on integration power detection, the light source for optical fingerprint sensing at a power saving mode. 
     In the detailed fingerprint map, when the press force increases, the fingerprint ridges expand, and more light is absorbed at the touch interface by the expanded fingerprint ridges. Therefore, within a relatively small observing zone  3305 , the integrated received light power change reflects the changes in the press force. Based on this, the press force can be detected. 
     Accordingly, by analyzing the integrated received probe light power change within a small zone, it is possible to monitor time-domain evolution of the fingerprint ridge pattern deformation. This information on the time-domain evolution of the fingerprint ridge pattern deformation can then be used to determine the time-domain evolution of the press force on the finger. In applications, the time-domain evolution of the press force by the finger of a person can be used to determine the dynamics of the user&#39;s interaction by the touch of the finger, including determining whether a person is pressing down on the touch surface or removing a pressed finger away from the touch surface. Those user interaction dynamics can be used to trigger certain operations of the mobile device or operations of certain apps on the mobile device. For example, the time-domain evolution of the press force by the finger of a person can be used to determine whether a touch by a person is an intended touch to operate the mobile device or an unintended touch by accident and, based on such determination, the mobile device control system can determine whether or not to wake up the mobile device in a sleep mode. 
     In addition, under different press forces, a finger of a living person in contact with the touch surface can exhibit different characteristics in the optical extinction ratio obtained at two different probe light wavelengths as explained with respect  FIGS. 14 and 15 . Referring back to  FIG. 40 , the lightly pressed fingerprint  3301  may not significantly restrict the flow of the blood into the pressed portion of the finger and thus produces an optical extinction ratio obtained at two different probe light wavelengths that indicates a living person tissue. When the person presses the finger hard to produce the heavily pressed fingerprint  3303 , the blood flow to the pressed finger portion may be severely reduced and, accordingly, the corresponding optical extinction ratio obtained at two different probe light wavelengths would be different from that of the lightly pressed fingerprint  3301 . Therefore, the optical extinction ratios obtained at two different probe light wavelengths vary under different press forces and different blood flow conditions. Such variation is different from the optical extinction ratios obtained at two different probe light wavelengths from pressing with different forces of a fake fingerprint pattern of a man-made material. 
     Therefore, the optical extinction ratios obtained at two different probe light wavelengths can also be used to determine whether a touch is by a user&#39;s finger or something else. This determination can also be used to determine whether to wake up the mobile device in a sleep mode. 
     For yet another example, the disclosed optical sensor technology can be used to monitor the natural motions that a live person&#39;s finger tends to behave due to the person&#39;s natural movement or motion (either intended or unintended) or pulsing when the blood flows through the person&#39;s body in connection with the heartbeat. The wake-up operation or user authentication can be based on the combination of the both the optical sensing of the fingerprint pattern and the positive determination of the presence of a live person to enhance the access control. For yet another example, the optical sensor module may include a sensing function for measuring a glucose level or a degree of oxygen saturation based on optical sensing in the returned light from a finger or palm. As yet another example, as a person touches the display screen, a change in the touching force can be reflected in one or more ways, including fingerprint pattern deforming, a change in the contacting area between the finger and the screen surface, fingerprint ridge widening, or a change in the blood flow dynamics. Those and other changes can be measured by optical sensing based on the disclosed optical sensor technology and can be used to calculate the touch force. This touch force sensing can be used to add more functions to the optical sensor module beyond the fingerprint sensing. 
     Multi-Layer Optical Design for Under-Screen Optical Fingerprint Sensors 
     In implementing the above disclosed under-screen optical sensors, one significant technical issue is the strong background light from the natural day light from the sun. In the examples of disclosed optical sensors under LCD panels such as the one shown in  FIG. 30 , an optical filtering mechanism can be built into the under-LCD optical sensor stack to block or reduce the strong background light from the natural day light from the sun that enters the optical sensor array  621 . This blocking or reduction of the background light can significantly improve the optical detection for optical fingerprint sensing. 
     Accordingly, one or more optical filter layers may be integrated into the under-LCD optical sensor stack above the optical sensor array  621  to block the undesired background day light from the sun while allowing the illumination light for the optical fingerprint sensing to pass through to reach the optical sensor array  621 . For example, the illumination light may be in the visible range, e.g., from 400 nm to 650 nm, in some implementations and the one or more optical filters between the LCD panel and the optical sensor array  621  can be optically transmissive to light between 400 nm and 650 nm while blocking light with optical wavelengths longer than 650 nm, including the strong IR light in the day light. In practice, some commercial optical filters have transmission bands that may not desirable for this particular application for under screen optical sensors disclosed in this document. For example, some commercial multi-layer bandpass filters may block light above 600 nm but would have transmission peaks in the spectral range above 600 nm, e.g., optical transmission bands between 630 nm and 900 nm. Strong background light in the day light within such optical transmission bands can pass through to reach the optical sensor array  621  and adversely affect the optical detection for optical fingerprint sensing. Those undesired optical transmission bands in such optical filters can be eliminated or reduced by combining two or more different optical filters together with different spectral ranges so that undesired optical transmission bands in one filter can be in the optical blocking spectral range in another optical filter in a way that the combination of two or more such filters can collectively eliminate or reduce the undesired optical transmission bands between 630 nm to 900 nm. Specifically, for example, two optical filters can be combined by using one filter to reject light from 610 nm through 1100 nm while transmitting visible light below 610 nm in wavelength and another filter to reject light in a shifted spectral range from 700 nm through 1100 nm while transmitting visible light under 700 nm in wavelength. This combination of two or more optical filters can be used to produce desired rejection of the background light at optical wavelengths longer than the upper transmission wavelength. Such optical filters may be coated over the spacer  917 , collimator  617 , and/or protection material  919  shown in  FIG. 30 . 
     In some implementations, when using two or more optical filters as disclosed above, an optical absorbing material can be filled between the two filters to exhibit proper absorption for the rejected light band so that the bouncing light between the two optical filters can be absorbed. For example, one filter may be coated on spacer  917 , and the other filter be coated on protection material  919 , while the collimator  617  can be made optically absorbing to absorb the rejected light band by the two filters. As a specific example, a piece of blue glass that has high absorption from 610 nm to 1100 nm can be used as base of the filters. In this case the two filters are coated on up and down surfaces of the blue glass, and this component can be used as the spacer or the protection material. 
     Referring back to the example for an under-LCD optical fingerprint sensor again, the different layers in the optical stack for the optical sensor module contain various patterns. For example, the LCD panel has periodic LCD pixels above the optical sensor module; the optical collimator array layer  617  has periodic collimators; and the optical sensor array  621  contains period optical detector units. Those and other patterned layers in the device in  FIG. 30  may produce an undesired optical interference pattern known as a Moiré pattern with Moiré fringes due to the spatial overlapping of those layers. Notably, for the moiré interference pattern to appear, the two overlapping layers of patterns must not be completely identical in that they must be displaced, rotated, etc., or have different but similar pitch patterns. The presence such a Moiré pattern severely degrades the optical fingerprint image to be detected at the optical sensor array  621  and may completely render the optical sensor module to malfunction. 
     In the example in  FIG. 30 , the spacer layer  917  is placed between the LCD panel and the optical collimator array layer  617  and another protection layer  919  is placed between the optical collimator array layer  617  and the optical sensor array  621 . The layers  917  and  919  can be made sufficiently thick to destroy the conditions for forming the Moiré pattern between the between the LCD panel and the optical collimator array layer  617 , for forming the Moiré pattern between the optical collimator array layer  617  and the optical sensor array  621  or for forming the Moiré pattern by the LCD panel, the optical collimator array layer  617  and the optical sensor array  621 . For example, adding optical bandpass filter layers to the layers  917  and  919  may be used to gain the sufficient thickness for reducing the Moiré patterns. 
     In addition, the Moiré pattern formed by the optical interference between the optical collimator array layer  617  and the optical sensor array  621  may the dominate source of undesired optical interference. As such, the optical thickness between the optical collimator array layer  617  and the optical sensor array  621  can be increased to a sufficiently large value to reduce the undesired optical interference. This can be done by filling the collimators with an optical transparent material with a sufficiently large refractive index to increase the optical thickness or to increasing the optical thickness of the protection layer  919 . Filing the collimators with a transparent material can facilitate packaging the optical sensor since the filled optical collimators are no longer hollow structures. 
     The above optical designs of using multiple layers for optical filtering to reduce the background light and for reducing the Moiré patterns can be applied to under-OLED display panel optical sensor modules. 
     Some examples of various optical fingerprint sensor designs placed under OLED panels are disclosed in U.S. patent application Ser. No. 15/616,856 entitled “OPTICAL COLLIMATORS FOR UNDER-SCREEN OPTICAL SENSOR MODULE FOR ON-SCREEN FINGERPRINT SENSING” and filed on Jun. 7, 2017 by Applicant Shenzhen Goodix Technology Co., Ltd. This application is published as U.S. Patent Application Publication No. US 2017/0270342 A1 and is incorporated by reference as part of this patent document. 
     As a specific example,  FIG. 41A  shows an under-OLED sensor module structure for achieving the above optical filtering and the reducing Moiré patterns. This structure includes four layers. From top to bottom, these layers are: spacer  917 , collimator  617 , protection material  919 , and photo detector array  621 . Between the four layers adhesive or filling materials  600   f  are applied. The key layers are packaged with housing  600   b , which blocks all light bands from side or bottom. Circuit board or flexible printed circuit (FPC)  623  stretches out from the housing. 
     The band pass filter coatings are made on the surfaces of the spacer  917  or the protection material  919 . In some implementations, two optical filters may be applied where one filter rejects 610 nm through 1100 nm, the other one rejects 700 nm through 1100 nm. To do so, the rejected band of both filters are removed more thoroughly from the signal band. 
     The material between the two optical filters features proper absorption for the rejected light band so that the bouncing light between the two filters can be extinct. For example, one filter may be coated on spacer  917 , and the other filter be coated on protection material  919 , while the collimator  617  absorbs the rejected light band. For example, a piece of blue glass that has high absorption from 610 nm to 1100 nm can be used as base of the filters. In this case the two filters are coated on up and down surfaces of the blue glass, and this component can be used as the spacer or the protection material. 
     One function of the spacer  917  and the protection material  919  is to eliminate the Morie fringes. The thickness of the two components can be adjusted to reduce or eliminate Morie fringes. With this structure, the inner wall of the collimator units may not necessarily be blackened. In implementations, the thickness of the protection material  919  can be adjusted to optimize the detection performances, such as the resolution etc. 
       FIG. 41B  further shows that the collimator  617   k  is a pinhole array that can be made from etching a wafer. After etching pinholes in the wafer, the pinholes are filled with a transparent material so that the collimator array is a solid material without hollow structures. This can increase the optical thickness between the collimator array and the optical sensor array to reduce the undesired Moiré interference pattern. Under this design, the spacer  917  between the OLED panel and the collimator array can be made very thin or even removed. The filling material  600   f  can also be very thin. 
     In various implementations of the under-screen optical sensor module technology for fingerprint sensing disclosed herein, an imagine module having at least one imaging lens is used to achieve the optical imaging of the illuminated touched portion of a finger onto the optical sensor array in the under-screen optical sensor module. The lensing effect of the imaging module is in part for controlling the spatial spreading of the returned light that may spatially scramble returned light from different locations on the touched portion of the finger at the optical sensor array so that the spatial information on the returned light corresponding to the fingerprint pattern on a finger can be preserved by the imaging lens with a desired spatial imaging resolution when the imaging lens directs the returned light to reach the optical sensor array. The spatial imaging resolution of an imaging module having a single imagine lens or an assembly of two or more imaging lenses is proportional to the numerical aperture of the imaging module. Accordingly, a high-resolution imaging lens requires a large numerical aperture and thus a lens with a large diameter. This aspect of a lens-based imaging module inevitably requires a bulking lens system to produce a high-resolution imaging system. In addition, a given imaging lens has a limited field of view which increases as the focal length decreases and decreases as the focal length increases. 
     In many fingerprint sensing applications such as optical fingerprint sensors implemented under a display screen in a mobile device, it is desirable to have a compact imaging system with a high spatial imaging resolution and a large field of view. In view of the trade-offs in various imaging features of a lens-based imaging system discussed above, a compact optical imaging system for optical fingerprint sensing is provided below by combining a lens-based imaging system to achieve a high spatial imaging resolution via the lens and a reduced size in the captured image at the optical detector array to reduce the size the optical detector array via the same lens. The pinhole is placed in front of the lens to produce a field of view in optical imaging by effectuating a pinhole camera while without requiring a large diameter lens. A conventional pinhole camera can include a small aperture for optical imaging and can produce a large field of view while suffering a limited image brightness due to the small aperture and a low spatial imaging resolution. A combination of an imaging lens and a pinhole camera, when properly designed, can benefit from the high spatial imaging resolution of the imaging lens and the large field of view of the pinhole camera. 
       FIG. 42  shows one example of an optical sensor module  620  placed under an OLED display screen where a pinhole and a lens used to form the optical imaging system for the optical sensor module  620 . In this example, the optical sensing module  620  is a compact module by using a micro lens  621   e  with a small diameter that can be about the same size of the pinhole so slightly larger than the pinhole. The micro lens  621   e  is engaged to a pinhole structure  621   g  that is optically opaque and may be a layer of a blackened or metal material formed on a surface of a pinhole substrate  621   f  of an optically transparent material with an opening as the pinhole  643 . The micro lens  621   e  is placed on the lower side of the pinhole substrate  621   f . In operation, the optical layers above the pinhole  643  in the pinhole structure  621   g  are structured to produce a large optical field of view in collecting the returned light from the OLED display panel and to transmit the collected light towards the optical sensor array  623   e . The optical detectors in the optical sensor array  623   e  respond to the received optical pattern to produce detector signals and a detector circuitry module  623   f  is coupled to the optical sensor array  623   e  to receive and process the detectors signals. detector circuitry module  623   f  may include, in some implementations, a flexible printed circuit (PFC). The micro lens  621   e  receives the transmitted light from the pinhole and to focus the received light onto the optical sensor array  623   e  for optical imaging at an enhanced spatial imaging resolution at the optical sensor array  623   e  when compared to a lower spatial imaging resolution of the pinhole in projecting light onto the optical sensor array  623   e  without the micro lens  621   e . In this design, the low resolution of the pinhole is compensated by using the micro lens  621   e  and the limited field of view of the micro lens  621   e  is compensated by the large field of view of the pinhole  643 . 
     In the illustrated example of using the pinhole-lens assembly for optical imaging in  FIG. 25 , the object plane of the pinhole-lens assembly is near the top effective sensing zone  615  on the top surface of the transparent layer  431  such as a cover glass for the touch sensing OLED display panel and the imaging plane of the pinhole-lens assembly is the receiving surface of the optical detectors of the optical sensor array  623   e . In addition to the pinhole substrate  621   f , an optically transparent spacer  618   e  with a refractive index lower than that of the pinhole substrate  621   f  is provided between the pinhole substrate  621   f  and the OLED display panel. This use of a lower index material above the pinhole substrate  621   f  is part of the optical design to achieve a large field of view for receiving light from the OLED display panel. In some implementations, the lower-index spacer  618   e  may be an air gap. This design provides an optical interface of two different optical materials between lower-index spacer  618   e  and the higher-index pinhole substrate  621   f  and the optical refraction at this interface converts a large field of view (FOV) (e.g., around 140 degree in some cases) of incident light from the OLED display panel in the lower-index spacer  618   e  into a smaller FOV in the higher-index pinhole substrate  621   f . Accordingly, the output light rays produced by the pinhole-lens assembly have a relatively small FOV. 
     This design of reducing the FOV is advantageous in several aspects. First, the optical input FOV in the lower-index spacer  618   e  of the optical sensor module  620  is a large FOV. Second, the actual FOV handled at by the pinhole-lens assembly located below the higher-index pinhole substrate  621   f  is a reduced FOV with respect to the optical input FOV so that light rays with large incident angles are limited by this reduced FOV. This is beneficial because image distortions caused by light rays at large incident angles at the pinhole-lens assembly are reduced by this reduced FOV. In addition, this reduced FOV at the pinhole-lens assembly reduces the undesired pinhole shading effect that would distort the brightness distribution of the image at the optical sensor array. 
     Different from a convention pinhole camera with uses a pinhole with a diameter around 40 microns in some pinhole camera designs, the pinhole  643  is designed to have a diameter much larger than the typical size of a pinhole in a pinhole camera, e.g., greater than 100 microns, or 200 microns (e.g., 250 microns) in some designs. In this combination of the lens and the pinhole, the use of the high-index material for the pinhole substrate  612   f  just above the pinhole  643  and the use of the lower-index layer  618   e  above the pinhole substrate  612   f  allows the pinhole  643  to have a diameter much larger than the typical size of a pinhole in a pinhole camera while still achieving a large FOV. For example, in some implementations, the diameter of the pinhole  643  may be about the same as or similar to the radius of curvature of the curve surface of the lens  621   e  when structured as a half ball lens with a flat surface facing the pinhole  643  and a partial spherical surface that directs the light from the pinhole  643  towards the photodetector array  621   e.    
     Additional design features can also be implemented to improve the overall optical performance and the compactness of the optical imaging system based on the pinhole-lens assembly. For example, as illustrated in  FIG. 42 , additional optical layers can be placed between the lens-pinhole assembly and the photodiode array  623   e . In this example, an optically transparent spacer  621   h  and a protection material  623   g  are provided in the light path from the pinhole-lens assembly to the optical sensor array  623   e . In some implementations, the spacer  621   h  may be a low-index layer such as an air gap, and the protection material  623   g  may be a layer covering the top of the optical detectors of the optical sensor array  623   e  and having a refractive index higher than that of the spacer  621   h . The layers  621   h  and  623   g  can be structured to reduce or eliminate the imaging distortion at the optical sensor array  623   e . When light is refracted at media interfaces, the nonlinearity in the directions of refracted rays exists and creates image distortions at the optical sensor array  623   e . Such distortions become more pronounced when the incident angles are large. To reduce such distortions, the optical thickness ratio of spacer  621   h  and  623   g  can be selected in light of the optical structure of the pinhole-lens assembly and the optical objective field of the pinhole-lens assembly (e.g., the optical layers from the top sensing surface of the top glass layer  431  to the pinhole substrate  621   f ). 
     Optical distortions occur at each interface of different optical materials along the optical path of light from the top of the OLED display panel to the optical sensor array  623   e . One design technique for reducing such optical distortions is to provide optically matching structures on lower side of the pinhole-lens assembly (i.e., the optical layers on the imaging side of the pinhole-lens assembly) to corresponding to optical structures on the upper side of the pinhole-lens assembly (i.e., the optical layers on the object side of the pinhole-lens assembly) so that an optical distortion incurred at one interface along the optical path from the OLED panel to the pinhole-lens assembly in the object side of the pinhole-lens assembly is countered or offset by optical refraction at a matching interface along the optical path from the pinhole-lens assembly to the optical sensor array  623   e  in the imaging side of the pinhole-lens assembly. The optical matching layers in the imaging side of the pinhole-lens assembly are designed by taking into account of the optical power of the lens in the pinhole-lens assembly. 
     In implementations, the above pinhole-lens assembly may also be used with an optical sensing module placed under an LCD display panel. 
     In the above disclosed implementations for under-display optical sensing, the anti-spoofing feature can be implemented by detecting certain features associated with a live person based on optical sensing techniques as disclosed in the examples in  FIGS. 14, 15 and 40 . In addition, other anti-spoofing sensing features may be incorporated with the disclosed under-display optical sensing and four additional examples are disclosed below. Those and other anti-spoofing features may be implemented with the disclosed under-display optical sensing in various ways, including as a stand-alone anti-spoofing mechanism or as in a combination of two or more different anti-spoofing mechanisms to operate collectively. 
       FIG. 43  shows one example of an anti-spoofing sensor based on a capacitive sensor array that is integrated to a device with an under-display optical sensor module for optical fingerprint sensing. The capacitor “C” shown in  FIG. 43  may be an array of capacitor sensors in implementations, e.g., the capacitive touch sensing array in a touch sensing display which is used to serve two different sensing operations of (1) capacitive touch sensing and (2) capacitive fingerprint sensing. In implementation, the processing circuitry coupled to the capacitive touch sensing array can be operated to process capacitor signals generated locally at different capacitors and use the local capacitor signals to construct a spatial pattern representing a fingerprint pattern. This capacitive fingerprint sensing operation, although using the same capacitors that are also operated for capacitive touch sensing, is separate from the capacitive touch sensing operation and can be performed in serial or parallel with respect to the processing for capacitive touch sensing. The under-display optical sensor module is placed under the display panel. When a live finger presses on the display, the capacitive touch sensing array responds to the press or touch to produce capacitor signals from capacitive touch sensors. The capacitor signals are captured and the capacitor signal strengths of the captured signals can be measured. The produced capacitor signals can be used as a trigger to activate the under-display optical fingerprint sensor module for optical sensing of a fingerprint (and other optical sensing operations such as the optical sensing operations in  FIGS. 14, 15 and 40 ). In addition, the produced capacitor signals can be also used as an anti-spoofing sensor to evaluate whether the touch material or object is a real live finger. For example, the device can be designed to compare the capacitor signal strengths with the registered capacitor signal strength data (e.g., the registered data represents a stored fingerprint pattern of a registered user) to determine whether the touched object or material is from the finger of the registered user. If the detected capacitor signals are different from the stored data, the access is denied without activating and operating the optical sensing module. 
     In some implementations of the capacitive anti-spoofing sensing, the local touch sensing sensitivity may be temporarily reduced when the fingerprint is examined, this capacitor sensor may also be used to reject a fake fingerprint pattern. The capacitive anti-spoofing sensing may be designed to be effective in rejecting thick fake fingerprints. 
       FIG. 44  shows another example of an anti-spoofing sensor based on a capacitive sensor array that is integrated to a device with an under-display optical sensor module for optical fingerprint sensing based on differences in the image contrasts of real fingerprints and fake fingerprint patterns captured by a capacitive sensing array. This technique is based on the recognition that various fake fingerprint patterns are printed on a piece of paper or a film featuring a high image contrast that is higher than the typical image contrast from a fingerprint pattern of a real finger. The image contrast of such a printed fake fingerprint pattern (see the right-hand-side image in  FIG. 44  as an example) is mostly different from the real fingerprint signatures (see the left-hand-side image in  FIG. 44  as an example). The capacitive signal patterns of real fingerprints of a user can be recorded by the capacitive sensor array when the user performs the fingerprint registration for the device or system. The registered fingerprint image contrast is normally low. Accordingly, the comparison in the image contrast can be used to reject the printed fake fingerprint models. 
     An optical anti-spoofing sensing mechanism based on the under-display optical sensor module may be additionally used for rejecting a fake fingerprint. For example, the optical reflection from an input object can be used as a criterion to judge whether the touching input object is a real live finger with fingerprint. When the live finger is used for a user fingerprint registration, optical reflectance signatures of the real finger are captured by the under-display optical sensor module. In general, a copied or otherwise fake fingerprint model can hardly have same optical reflectance signatures of the real finger. Such difference can be determined and be used to reject the fake fingerprint model by comparing the integrated signal strength. This optical anti-spoofing sensing may be designed to work well in rejecting printed fake fingerprint models. 
       FIG. 45  shows yet another example of an anti-spoofing sensor based on a capacitive sensor array that is integrated to a device with an under-display optical sensor module for optical fingerprint sensing based on sensing a presence of a compounded fingerprint image from two sets of fingerprints. Some fake fingerprint models use transparent materials such as silicone rubber, projector films and others to carry fabricated fingerprints. These materials are normally nonconductive and must be sufficiently thin to generate sufficient touch capacitance for capacitive sensing. This use of thin transparent material for carrying a fake fingerprint model allows the under-display optical fingerprint sensor to detect both the pattern of the fake fingerprint model and the live finger&#39;s fingerprint behind the fake fingerprint model so that both the fake fingerprint pattern and the real fingerprint pattern of a finger can be captured in one image. This signature is a compound image of two different sets of fingerprint patterns and thus can be detected and rejected. 
     The above disclosed anti-spoofing sensing based on capacitive sensors can be in form of an electrode array as an array of capacitors for performing the anti-spoofing sensing operations. In some implementations, such anti-spoofing sensing based on capacitive sensors can utilize the capacitive touch sensing array in a touch sensing display without adding additional electrodes that are designated for anti-spoofing sensing. In other implementations, an electrode array can be formed as a designated array of capacitors for performing the anti-spoofing sensing operations which is separated from the capacitive touch sensing array of electrodes in the touch sensing display. 
     Fingerprint spoofing can be designed to be sufficiently sophisticated to create fake fingerprint models that have the proper capacitor signal strength, have similar image contrast and optical reflectance features, or may even avoid appearance of compounded fingerprint image. Optical anti-spoofing sensing based on optical spectral measurements by using probe light at different optical wavelengths can be used to reject such sophisticated fake fingerprint models.  FIGS. 46A-46B  show an example of an optical anti-spoofing sensor by using probe light at different optical wavelengths. 
     As shown in  FIG. 46A , a finger liveness detection zone  3  is designed within the fingerprint detection window  2  in display  1  to include optical probe light sources  4 ,  5  and  6  at different optical emission wavelengths (e.g., red (R), green (G) and blue (B) light sources) and the under-screen optical sensor module may be used to collect and detect scattered light from the user finger. For example, this zone  3  can be designed to be close to the bottom with 0.2 mm in width in some designs. When the liveness detection function is triggered, probe light sources  4 , 5 , 6  are operated to emit probe light at different optical wavelengths (e.g., red, green and blue light) and the under screen optical sensor module is turned on to perform optical sensing. The living tissues within a live finger exhibit certain optical reflection or scattering of such probe light at the different optical wavelengths and the optical reflection/scattering signals from the finger at those different optical wavelengths collectively constitute a live finger signature that is difficult to duplicate by using a fake fingerprint model. For example, living tissues may reflect or scatter red light stronger than scattering or reflecting green or blue light. This spectrum signature forms the basis for rejecting a fake fingerprint model. 
       FIG. 46B  illustrates the sensing operation. The sensor module has fingerprint detection zone  10  and finger liveness detection zone  3   a . One or more photodetectors (PD) are placed within zone  3   a  to receive the reflected/scattered RGB probe light in red (R), green (G) and blue (B) colors. By analyzing the spectrum signatures at the different optical wavelengths and by comparing the captured signals from stored known user spectrum signatures, the device can determine whether the touching material is a live finger because the light reflected/scattered from a live finger is dependent on light wavelengths, while the light reflected/scattered from fake fingerprint materials is not so dependent on light wavelengths. This difference produces different detected signal strengths at different optical wavelengths which can be used to establish the criterion for rejecting a fake fingerprint model. 
     In operation, the RGB probe light sources can be turned on and off to check the change of the PD received signals in corresponding positions to determine if a touched object is a live finger. In the examples in  FIGS. 46A-46B , the light from zone  2  is largely responsible for producing background light and thus does not carry the optical signature at different optical wavelengths for the liveness detection. In implementations, the RGB light sources  4 , 5 , 6  may be fully, partially, or not overlapped with the liveness detection zone  3   a . The two light sources  4  and  5  may emit red and green light, or emit green and blue light, in some implementations. 
     Also shown in the examples in  FIGS. 46A-46B , one or more extra light sources  9  can be provided to improve the liveness detection quality. The extra light sources can be visible (e.g., red or green) or near infrared (e.g., at 800 nm or 940 nm). 
       FIGS. 47 and 48  show two examples for operating an anti-spoofing capacitive sensor in connection with optical fingerprint sensing based on the disclosed technology. The two illustrated operation processes demonstrate different detection algorithms based on the disclosed fingerprint sensing techniques and device designs. 
       FIG. 47  shows an example of a fingerprint detection process for using both the capacitive sensor array as an anti-spoofing device and the under-screen optical detector array to enable an optical sensing operation. First, the capacitive sensor array is turned on, without turning on the optical sensing module, to perform an initial capacitive sensing and to evaluate the capacitor signal strength. If the capacitor signal strength is weak below a predetermined signal strength threshold, it is then determined that the evaluation has failed and, accordingly, the access may be denied. Next, if the captured capacitor signals meet the capacitor signal strength threshold, the device continues on to process the detected capacitor signals to construct a spatial image by the spatial distribution of the capacitor signals. This constructed capacitive spatial image is used to determine the image contrast which is compared to the image contrast of a previously obtained capacitive fingerprint image of the user. If the image contrast passes the test and there is a match, an optical sensing operation for determining the liveness of the finger can be performed by using the under-screen optical sensing module. If it is determined that the object is a live finger, then the request for access is granted. If it is determined that the object is not a live finger, then the request for access is denied. 
       FIG. 48  shows another example of a fingerprint detection process for using both the capacitive sensor array as an anti-spoofing device and the under-screen optical detector array to enable an optical sensing operation. Different from the process in  FIG. 47 , the process in  FIG. 48  provides a parallel sensing operation for determining the liveness of a finger while the other operations of the capacitive sensing are performed in parallel to shorten the overall processing time of the process in  FIG. 47 . 
     While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments. 
     Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.