BACKGROUND

Fingerprint sensing is becoming an increasingly common tool for authorizing (a) access to electronic devices, such as smartphones, (b) access to confidential electronic records, and (c) electronic transactions such as financial transactions carried out over the Internet. Fingerprint sensing satisfies market demand for an authentication mechanism that eliminates the need to enter (and remember) a passcode, and also eliminates the need to keep track of multiple different passcodes associated with different devices and/or accounts. Fingerprint sensing is a well-established form a biometric identification and optical fingerprint sensors have been used for years, e.g., by law enforcement agencies.

Several different types of fingerprint sensors have been designed for use in smartphones and other mobile devices. Each of these types of fingerprint sensors images a finger to obtain the fingerprint. A camera-based fingerprint scanner captures an image of a fingerprint using an imaging lens and an image sensor. A collimator-based fingerprint scanner senses a fingerprint with a lens array aligned over an image sensor with the addition of a collimator above the image sensor. The collimator helps map light from a specific region of the fingerprint sample to be detected by a corresponding region of the image sensor by forming channels the light must propagate through to reach the image sensor.

One issue with fingerprint image sensing is the possibility of generating fake, or spoof, fingerprints that trick the fingerprint sensor in order to gain access to a device. Spoof fingerprints can be fabricated in a range of ways including printing an image of the fingerprint on paper and forming molded objects that reproduce the desired fingerprint pattern. If created with sufficiently high resolution and fidelity to the real fingerprint, these spoofing methods can trick fingerprint sensors and allow unauthorized access to devices and accounts. Fingerprint sensors based on image sensors struggle to distinguish real fingerprints from spoof fingerprints, thereby making fingerprint sensors a less reliable authentication method.

An optical fingerprint sensor with spoof detection using polarization includes a plurality of lenses Lm=0, L1, . . . LJ-1with a lens pitch along a horizontal direction that is orthogonal to a lens axis of lens L0, each lens having a width; an image sensor including a pixel array that includes a plurality of first photodiodes Im=0, I1, . . . IJ-1with a first photodiode pitch that is equal to the lens pitch in the horizontal direction, a line between a center of a light-sensitive surface of each first photodiode Imand an optical center of each lens Lmforms an optical axis Omof a plurality of optical axes Om=0, O1, . . . OJ-1; at least one apertured baffle-layer positioned between the image sensor and the plurality of lenses, each of the at least one apertured baffle-layer being located at a respective height zkabove the pixel array and each having a respective plurality of aperture stops Am=0, A1, . . . AJ-1, each aperture stop Ambeing center-aligned with the optical axis Om; a plurality of second photodiodes Sm=0, S1, . . . SK-1that is included in the pixel array, the plurality of second photodiodes is intercalated with the plurality of first photodiodes such that a second photodiode Smis between first photodiode Imand first photodiode Im+1in the horizontal direction, wherein each second photodiode is configured to detect electromagnetic energy having passed through lens Lmand at least one aperture stop Ax≠mnot aligned with Lmalong optical axis Om; and at least one polarizing element positioned to only polarize electromagnetic energy impinging the plurality of second photodiodes.

A method for detecting spoof fingerprints detected using an optical fingerprint sensor and polarization includes controlling a display of an electronic device to output a pattern of light to illuminate a fingerprint sample touching the display near an optical fingerprint sensor including a pixel array and positioned beneath the display; blocking smaller-angle light from impinging a plurality of anti-spoof photodiodes of the pixel array, the smaller-angle light being electromagnetic energy with incident angle less than five degrees from an optical axis of each anti-spoof photodiode of the plurality of anti-spoof photodiode; filtering larger-angle light incident on the plurality of anti-spoof photodiodes to at least one polarization direction, the larger-angle light being electromagnetic energy with incident angle greater than five degrees from the optical axis; detecting the larger-angle light at the at least one polarization direction using the plurality of anti-spoof photodiodes; correlating the larger-angle light at the at least one polarization direction with the pattern of light; determining the fingerprint spoofing based at least in part on the correlation of the larger-angle light at the at least one polarization direction and the pattern of light; and wherein the plurality of anti-spoof photodiodes is interleaved with a plurality of imaging photodiodes such that each anti-spoof photodiode of the plurality of anti-spoof photodiodes is between adjacent imaging photodiodes of the plurality of imaging photodiodes.

In an embodiment, a method for detecting spoof fingerprints detected using an optical fingerprint sensor and polarization includes controlling a display of an electronic device to output a pattern of light to illuminate a fingerprint sample touching the display near an optical fingerprint sensor including a pixel array and positioned beneath the display; blocking smaller-angle light from impinging a plurality of anti-spoof photodiodes of the pixel array, the smaller-angle light being electromagnetic energy with incident angle less than five degrees from an optical axis of each anti-spoof photodiode of the plurality of anti-spoof photodiode; filtering larger-angle light incident on the plurality of anti-spoof photodiodes to at least one polarization direction, the larger-angle light being electromagnetic energy with incident angle greater than five degrees from the optical axis; detecting the larger-angle light at the at least one polarization direction using the plurality of anti-spoof photodiodes; correlating the larger-angle light at the at least one polarization direction with the pattern of light; determining the fingerprint spoofing based at least in part on the correlation of the larger-angle light at the at least one polarization direction and the pattern of light; and wherein the plurality of anti-spoof photodiodes is interleaved with a plurality of imaging photodiodes such that each anti-spoof photodiode of the plurality of anti-spoof photodiodes is between adjacent imaging photodiodes of the plurality of imaging photodiodes.

SUMMARY

Collimator-based fingerprint sensors narrow the acceptable paths that electromagnetic energy may travel between the fingerprint sample and the image sensor. In some instances, they do so by forming channels or cones that transmit electromagnetic energy but block incoming electromagnetic energy with angles and positions not within this intended path. This allows electromagnetic energy from a specific region of the fingerprint sample to reach only the region of the image sensor intended to image that region of the fingerprint and reduces electromagnetic energy from any other region of the fingerprint sample from reaching that region of the image sensor. This can be accomplished in several ways including an array of apertured baffle-layers positioned between a lens array and the image sensor. Each apertured baffle-layer has apertures aligned above each pixel of the image sensor, with the width of each aperture configured to transmit only a narrow cone of acceptable angles of incoming electromagnetic energy. This design has been used to help improve image quality and performance of under-screen fingerprint sensors. The term “light” and the term “electromagnetic energy” are used interchangeable herewithin. Electromagnetic energy herein refers to electromagnetic energy with wavelength between 0.4 and 2 microns.

Techniques exist to overcome fingerprint sensors and gain unauthorized access to devices and information including creating spoof fingerprints. Spoof fingerprints include printed images of fingerprints and molded objects that reproduce the three-dimensional structure of the target fingerprint sample. Current techniques to detect spoof fingerprints largely rely on reviewing real and spoof fingerprints and using algorithms to detect the differences in the images generated of each by the fingerprint sensor. As spoof fingerprints are made with higher resolution and fidelity, the images they produce on the fingerprint sensor become challenging to distinguish from real fingerprint samples. Using only the image generated limits spoof detection and leaves fingerprint sensors vulnerable to high-quality, high-resolution spoof fingerprint attacks.

When printed or molded spoof fingerprint samples are used on fingerprint sensors, the material properties of the sample play a role in how electromagnetic energy interacts between the sample and the fingerprint sensor. This can be exploited to increase spoof detection. Within the glass the fingerprint sensor, electromagnetic energy is scattered from the sample with a range of outgoing angles. This distribution of scattered electromagnetic energy depends on the material being sampled. Human fingers, printed images, and molded objects may produce the same image on the image sensor but they produce different angular distributions of reflected and scattered electromagnetic energy. A fingerprint sensor capable of detecting this angular distribution has superior ability to detect spoofs than one which only records an image of the sample.

Detecting the angular distribution of electromagnetic energy is possible by exploiting the physical design of an apertured baffle-layer collimator that may be used in an optical fingerprint sensor. By design, electromagnetic energy with a small angle of incidence, which is to say with a small angular divergence from the surface normal of the image sensor, is transmitted through the collimator to a pixel of the image sensor corresponding to a physical location on the fingerprint sample from which the electromagnetic energy originated. These pixels of the image sensor are called imaging pixels, herein. Generally, an imaging pixel is aligned with at least one aperture and a given lens along a line roughly parallel to the surface normal of the image sensor. Electromagnetic energy traveling toward the fingerprint sensor with sufficiently large angle of incidence may traverse through a given lens and then through apertures not aligned with that lens, after which it may be incident on image sensor between adjacent imaging pixels. This light is generally not useful for imaging the fingerprint sample and is thus disregarded. In the embodiments described herein, electromagnetic energy with large angle of incidence, so called large-angle light, is detected and used to measure the veracity of a fingerprint sample. By detecting large-angle light, the fingerprint sensor records additional information about the sample that can be used to confirm whether a sample is a real finger or a spoof finger.

When employing algorithms to detect spoof fingerprints, the additional information provided by detecting large-angle light may also be inputted into the algorithms for enhanced spoof-detection accuracy. This is especially valuable with the use of machine learning algorithms, which automatically compare differences between training sets. Variations in angular distributions that are caused by material properties of the sample will be available to machine learning algorithms, which will be able to identify spoof fingerprints based on combination of image details and angular distribution.

In an embodiment, An optical fingerprint sensor with spoof detection using polarization includes a plurality of lenses Lm=0, L1, . . . LJ−1 with a lens pitch along a horizontal direction that is orthogonal to a lens axis of lens L0, each lens having a width; an image sensor including a pixel array that includes a plurality of first photodiodes Im=0, I1, . . . IJ−1 with a first photodiode pitch that is equal to the lens pitch in the horizontal direction, a line between a center of a light-sensitive surface of each first photodiode Im and an optical center of each lens Lm forms an optical axis Om of a plurality of optical axes Om=0, O1, . . . OJ−1; at least one apertured baffle-layer positioned between the image sensor and the plurality of lenses, each of the at least one apertured baffle-layer being located at a respective height zk above the pixel array and each having a respective plurality of aperture stops Am=0, A1, . . . AJ−1, each aperture stop Am being center-aligned with the optical axis Om; a plurality of second photodiodes Sm=0, S1, . . . SK−1 that is included in the pixel array, the plurality of second photodiodes is intercalated with the plurality of first photodiodes such that a second photodiode Sm is between first photodiode Im and first photodiode Im+1 in the horizontal direction, wherein each second photodiode is configured to detect electromagnetic energy having passed through lens Lm and at least one aperture stop Ax≠m not aligned with Lm along optical axis Om; and at least one polarizing element positioned to only polarize electromagnetic energy impinging the plurality of second photodiodes.

