BIOMETRIC IMAGING DEVICE COMPRISING COLLIMATING STRUCTURES AND METHOD OF IMAGING IN THE BIOMETRIC IMAGING DEVICE

Abstract

An optical biometric imaging device comprising: an image sensor comprising a plurality of photodetector pixels; a plurality of collimating structures arranged above the image sensor, the collimating structures being configured to limit the incident angle of light reaching the image sensor to be lower than a first predetermined incident angle Θ.sub.1; wherein at least a subset of the collimating structures comprises a light-blocking element configured to block light having an incident angle lower than a second predetermined incident angle Θ.sub.2, the second incident angle being lower than the first incident angle, wherein the light-blocking element is configured to allow light having an incident angle between the first and second predetermined incident angles to pass and wherein the light-blocking element is configured to block light within a first predetermined wavelength range.

Claims

1. An optical biometric imaging device comprising: an image sensor comprising a plurality of photodetector pixels; a plurality of collimating structures arranged above the image sensor, the collimating structures being configured to limit the incident angle of light reaching the image sensor to be lower than a first predetermined incident angle Θ.sub.1; wherein at least a subset of the collimating structures comprises a light-blocking element configured to block light having an incident angle lower than a second predetermined incident angle Θ.sub.2, the second incident angle being lower than the first incident angle, wherein the light-blocking element is configured to allow light having an incident angle between the first and second predetermined incident angles to pass, and wherein the light-blocking element is configured to block light within a first predetermined wavelength range.

2. The biometric imaging device according to claim 1, wherein the light-blocking elements are configured such that biometric images formed based on pixels receiving light from collimating structures having a light-blocking element has a first contrast type and biometric images formed based on pixels receiving light from collimating structures without a light-blocking element have a second contrast type different from the first contrast type.

3. The biometric imaging device according to claim 1, wherein every second collimating structure in the array of collimating structures comprises a light-blocking element.

4. The biometric imaging device according to claim 1, wherein a first subset of collimating structures comprises light-blocking elements configured to block light within a first predetermined wavelength range, and a second subset of collimating structures comprises light-blocking elements configured to block light within a second predetermined wavelength range different from the first wavelength range.

5. The biometric imaging device according to claim 1, wherein the collimating structure comprises: a first aperture layer arranged above the image sensor; a first transparent layer arranged above the first aperture layer; and an array of microlenses arranged above the first transparent layer.

6. The biometric imaging device according to claim 5, further comprising: a second aperture layer arranged on the first transparent layer; and a second transparent layer arranged on the second aperture layer, wherein the array of microlenses is arranged on the second transparent layer.

7. The biometric imaging device according to claim 6, wherein the light-blocking element is located in an aperture of the first or second aperture layer.

8. The biometric imaging device according to claim 6, wherein the light-blocking element is arranged in a central portion of the aperture of the first or second aperture layer.

9. The biometric imaging device according to claim 6, wherein the light-blocking element is arranged between the first aperture layer and the second aperture layer.

10. The biometric imaging device according to claim 1, wherein at least a subset of the collimating structures comprises an angle-limiting element configured to limit the angle of incident light reaching the image sensor to be lower than a third predetermined incident angle Θ.sub.3, the third predetermined incident angle being lower than the first incident angle.

11. The biometric imaging device according to claim 10, wherein the subset of collimating structures comprising an angle-limiting element is non-overlapping with the subset of collimating structures comprising a light-blocking element.

12. The biometric imaging device according to claim 10, wherein the angle-limiting element is configured to block light within a first predetermined wavelength range.

13. A display arrangement comprising: a display panel having an array of light emitting elements; and a biometric imaging device according to claim 1 arranged underneath the display panel.

14. A display arrangement comprising: a display panel having an array of light emitting elements; and a biometric imaging device according to claim 1 arranged underneath the display panel, wherein the biometric imaging device is configured to capture an image when the display panel is controlled to emit light only within the first wavelength range.

15. Method of imaging in a biometric imaging device comprising: an image sensor comprising a plurality of photodetector pixels and a plurality of collimating structures arranged above the image sensor, the collimating structures being configured to limit the incident angle of light reaching the image sensor to a first predetermined incident angle, wherein at least a subset of the collimating structures comprises a light-blocking element configured to block light having an incident angle lower than a second predetermined incident angle, the second incident angle being lower than the first incident angle, and wherein the light-blocking element is configured to allow light having an incident angle between the first and second incident angles to pass, the method comprising: forming a first image using pixels located under the subset of collimating structures comprising a light-blocking element; forming a second image using pixels located under the subset of collimating structures not comprising a light-blocking element; and forming a combined image based on the first and second images.

16. Method of imaging in a biometric imaging device arranged under a display panel comprising a plurality of light emitting elements, the biometric imaging device comprising: an image sensor having a plurality of photodetector pixels and a plurality of collimating structures arranged above the image sensor, the collimating structure being configured to limit the incident angle of light reaching the image sensor to a first predetermined incident angle, wherein at least a subset of the collimating structures comprises a light-blocking element configured to block light having an incident angle lower than a second predetermined incident angle, the second incident angle being lower than the first incident angle, the light-blocking element being configured to allow light having an incident angle between the first and second incident angles to pass, and wherein the light-blocking element is configured to block light within a first predetermined wavelength range, the method comprising: controlling the display panel to emit light only within the first wavelength range; forming a first image based on pixels receiving light from the subset of collimating structures comprising a light-blocking element; controlling the display panel to emit light outside of the first wavelength range; forming a second image based on pixels receiving light from all collimating structures; and forming a combined image based on the first and second images.

17. Method of imaging in a biometric imaging device arranged under a display panel comprising a plurality of light emitting elements, the biometric imaging device comprising: an image sensor comprising a plurality of pixels forming a photodetector pixel array and an array of collimating structures arranged above the image sensor, the collimating structure being configured to limit the incident angle of light reaching the image sensor, wherein a first subset of the collimating structures comprises a light-blocking element configured to block light having an having an incident angle lower than a second predetermined incident angle, the second incident angle being lower than the first incident angle, the light-blocking element of the first subset of collimating structures being configured to allow light having an incident angle between the first and second incident angles to pass, and wherein the light-blocking element of the first subset of collimating structures is configured to block light within a first predetermined wavelength range, and a second subset of the collimating structures comprises a light-blocking element configured to block light having an having an incident angle lower than the second predetermined incident angle, the light-blocking element of the second subset of collimating structures being configured to allow light having an incident angle between the first and second incident angles to pass, and wherein the light-blocking elements of the second subset of collimating structures are configured to block light within a second predetermined wavelength range different from the first wavelength range, the method comprising: controlling the display panel to emit light only within the first wavelength range; forming a first image based on pixels located under the first subset of collimating structures; forming a second image based on pixels located under all collimating structures; controlling the display panel to emit light only within the second wavelength range; forming a third image based on pixels located under the second subset of collimating structures; forming a first combined image based on first and third images; and forming a second combined image based on first combined image and the second image.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

[0040] FIG. 1 schematically illustrates a biometric imaging device according to an embodiment of the invention;

[0041] FIGS. 2A-B schematically illustrates details of a biometric imaging device according to an embodiment of the invention;

[0042] FIGS. 3A-E schematically illustrates details of a biometric imaging device according to embodiments of the invention;

[0043] FIG. 4A-C schematically illustrate features of a biometric image acquired using a device and method according to embodiments of the invention;

[0044] FIG. 5 is a flow chart outlining general details of a method according to an embodiment of the invention;

[0045] FIG. 6 schematically illustrate selected steps of a method according to an embodiment of the invention;

[0046] FIG. 7 is a flow chart outlining general details of a method according to an embodiment of the invention; and

[0047] FIG. 8 is a flow chart outlining general details of a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0048] In the present detailed description, various embodiments of the biometric imaging system according to the present invention are mainly described with reference to a fingerprint imaging sensor suitable for use under a display panel of a consumer device such as a smartphone, tablet computer and the like.

[0049] FIG. 1 schematically illustrates a portion of an optical biometric imaging device 100 according to an embodiment of the invention. In particular, FIG. 1 illustrates a cross section of a portion of the biometric imaging device 100, and it should be understood that the imaging device 100 extends further to form an imaging device of suitable size.

[0050] The optical biometric imaging device 100 comprises an image sensor 102 comprising a plurality of photodetector pixels 104. A plurality of collimating structures 106 are arranged above the image sensor 102, the collimating structures 106 being configured to limit the incident angle of light reaching the image sensor to be lower than a first predetermined incident angle Θ.sub.1, with further reference to FIGS. 2A-B illustrating a portion of a collimating structure 106 comprising a light-blocking element 108, the light-blocking element being configured to block light within a first predetermined wavelength range.

[0051] Moreover, at least a subset of the collimating structures 106 comprises a light-blocking element 108 configured to block light having an incident angle lower than a second predetermined incident angle, Θ.sub.2, where the second incident angle Θ.sub.2 is lower than the first incident angle Θ.sub.1, i.e. Θ.sub.2<Θ.sub.1. The light-blocking element 108 is further configured to allow light having an incident angle in the range between the first and second predetermined incident angles Θ.sub.1, Θ.sub.2 to pass. The incident angles Θ.sub.1 and Θ.sub.2 are further illustrated in FIGS. 2A-B where it can be seen that light having an incident angle lower than Θ.sub.1 and higher than Θ.sub.2 is allowed to pass through the collimating structure to reach the image sensor 102. In the present context, incident angles are illustrated in relation to the object, FIG. 2A, and in relation to the plane of the first aperture 110 layer, FIG. 2B, which can be assumed to be the same as the plane of the image sensor 102. This means that light which has reached the collimating structure with an incident angle between Θ.sub.1 and Θ.sub.2 is allowed to pass through the collimating structure to reach the image sensor 102.

[0052] In a non-limiting example, the first incident angle Θ.sub.1 may be approximately 20° and the second incident angle Θ.sub.2 may be approximately 10°, thereby meaning that light having an incident angle in the range of 10° to 20° reaches the image sensor. The light blocking element 108 is thus here configured to block light having an incident angle between 10° and 0°.

[0053] In the example embodiment illustrated in FIG. 1, the collimating structure 106 comprises: a first aperture layer 110 arranged on the image sensor 102; a first transparent layer 112 arranged on the first aperture layer 110; a second aperture layer 114 arranged on the first transparent layer 112; a second transparent layer 116 arranged on the second aperture layer 114, and an array of microlenses 118 arranged on the second transparent layer 116. In some embodiments, the collimating structure 106 may be implemented as only the array of microlenses 118, the first transparent layer 112 and the first aperture layer 110. Moreover, the light-blocking elements 108 may be made from the same material and formed in the same process step as the first aperture layer 110.

[0054] The microlenses 118 are aligned with corresponding apertures of the first and second aperture layers 110, 114 so that light reaching the image sensor 102 pass through one microlens 118 and further through corresponding apertures of the first and second aperture layers 110, 114. Here, the apertures of the second aperture layer 114 are preferably configured to prevent optical crosstalk, i.e. such that the light from a neighboring collimating structure only reaches its corresponding vertically aligned pixel 104. The size of the pixel is in the illustrated example configured so that light which has passed through a collimating structure reaches one corresponding pixel.

[0055] The material of the first transparent layer 112 may be configured to block light within the infrared wavelength range and is in such embodiments referred to as an IR-cut layer. The first transparent layer 112 may for example be configured to block light having a wavelength higher than approximately 600 nm, thereby ensuring that infrared light or light in the visible range near infrared does not reach the image sensor 102. An IR-cut layer may also be provided as a separate layer in the material stack.

[0056] The biometric imaging device 100 may also comprise additional intermediate layers not described herein, such as adhesive layers, as long as the layers are sufficiently transparent to allow light to travel from the microlens to the image sensor without excessive losses.

[0057] As further illustrated in FIG. 1, the biometric imaging device comprises an opaque mask layer 120 which comprises openings 122 at the locations of the respective microlenses 118. The opaque mask layer 120 may be arranged on the second transparent layer 116 before or after the microlenses 118 has been arranged on the second transparent layer 116. In either case, the openings 122 of the opaque mask layer 120 have a size which is equal to or smaller than the size of the microlens 118. The opaque mask layer 120 also allows for a sparse arrangement of microlenses 118 in the microlens array such that there is a distance between adjacent microlenses 118.

[0058] Moreover, the first aperture layer 110 may be formed from the topmost metal layer in a CMOS chip in which the image sensor 102 is formed. Thereby, the image sensor 102 and the first aperture layer 110 can be formed in the same manufacturing process. The first aperture layer 114 may thus be a metal layer. The first aperture layer 114 may also be a metal oxide (black chrome) or a black polymer (photoresist) layer which is non-transparent in the visible range. The first and second transparent layers 112, 116 may be spin-coated polymer layers. In applications where the first and second transparent layers 112, 116 are spin-coated, it simplifies the manufacturing process if also the second aperture layer 114 is spin coated, and it may then be formed by e.g. a black photoresist. It would also be possible to form either one or both of the first and second aperture layer(s) on a separate sheet of transparent material, such as glass, and to subsequently arrange the aperture layer(s) and glass sheet onto the image sensor 102.

[0059] FIG. 3A is a schematic illustration of a biometric imaging device 100 arranged under a display panel 300 and FIG. 3B illustrates a collimating structure 106 according to an embodiment of the invention. A finger 302 is in contact with an outer surface 304 of the display panel 300, such as a cover glass, where fingerprint ridges 306 and valleys 308 can be seen. In the illustrated implementation, there is an air gap 312 between the bottom surface of the display panel 300 and the microlenses 118 of the biometric imaging device 100.

[0060] The light bundles 310a-e schematically illustrate light reaching the collimating structure with different incident angles, where the first light bundle 310a illustrates orthogonal light and where the fifth light bundle 310e illustrates light with the highest incident angle at the image sensor. In FIG. 3B, a collimating structure 106 comprising a light-blocking element 108 is illustrated. The light blocking element 108 is a circular structure arranged in the center of a circular aperture of the first aperture layer 110 such that an annulus-shaped opening 314 is formed, as is further illustrated in the top view of FIG. 3D. The first to fourth light bundles 310a-d illustrate light having an incident angle which is lower than the first predetermined incident angle Θ.sub.1 thereby being allowed to pass through the collimating structure 106, while the fifth light bundle 310e represents light having an incident angle higher than Θ.sub.1 which is thereby blocked by the collimating structure 106. Furthermore, the first and second light bundles 310a-b illustrate light having an incident angle which is lower than the second predetermined incident angle Θ.sub.2, resulting in that such light is blocked by the light-blocking element 108 and prevented from reaching the photodetector pixel 104. Accordingly, in the illustrated collimating structure 106, only light having an incident angle between the first and second predetermined incident angles Θ.sub.1 and Θ.sub.2 reach a pixel 104 the image sensor 102.

[0061] The collimating structures 106 and the light-blocking elements 108 are preferably configured such that biometric images formed based on pixels receiving light from collimating structures having a light-blocking element 108 has a first contrast type and biometric images formed based on pixels receiving light from collimating structures 106 without a light-blocking element have a second contrast type different from the first contrast type.

[0062] The acceptance angle of the collimating structure 106 can be further controlled by introducing a horizontal offset between the microlens 118 and the corresponding aperture in the first aperture layer 110.

[0063] FIG. 3C illustrates a collimating structure 106 comprising an angle-limiting element 320, and a top view of the angle-limiting element 320 is illustrated in FIG. 3E. The angle-limiting element 320 is configured to limit the angle of incident light reaching the image sensor so that it is lower than a third predetermined incident angle Θ.sub.3, with the third predetermined incident angle Θ.sub.3 being lower than the first incident angle Θ.sub.1, Θ.sub.3<Θ.sub.1. Moreover, Θ.sub.3 may be equal to Θ.sub.2. In practice, the angle-limiting element 320 could be implemented by a collimating structure having a lower acceptance angle, at least for a selected wavelength. Thereby, the angle-limiting element 320 could be part of the collimating structure 106.

[0064] In FIG. 3C, light having the lowest incident angles, i.e. 310a-b, reach the image sensor while light with higher incident angles, i.e. 310c-d, is blocked by the angle-limiting element. The angle limiting element 320 may have the same size as the previously described annular opening 314 such that the opening 316 of the collimating structure with the angle-limiting element 320 has the same size as the light-blocking element 108. Thereby, only light with incident angles being blocked by the light-blocking element 108 reaches the image sensor 102 for collimating structures with an angle-limiting element 320, i.e. Θ.sub.3=Θ.sub.2.

[0065] By means of the collimating structures 106 comprising light-blocking elements 108 and angle-limiting elements 320, it is possible to form complimentary images with different contrast types as will be described in further detail in the following.

[0066] The specific configuration of the collimating structures, light-blocking elements and angle-limiting elements depends in part on properties of the display panel and of the image sensor. The collimating structures are advantageously configured to suit a specific implementation such that images with the two different contrasts can be captured.

[0067] FIGS. 4A-C schematically illustrate the two contrast types and a combination of the two contrast types. FIG. 4A schematically illustrates an image 400 of a finger captured using collimating structures having a light-blocking element such that orthogonal and near-orthogonal light is prevented from reaching the image sensor. The first contrast type, based on light reaching the pixel from the wider angles, will then be “camera-like” where the valleys 402 are relatively darker and the ridges 404 are relatively brighter in the image 400.

[0068] FIG. 4B schematically illustrates an image 406 of a finger captured using collimating structures having an angle-limiting element such that only orthogonal and near-orthogonal light reaches the image sensor. The second contrast type, based on light reaching the pixel from the narrower angles, will then be of “index-matched-type” where the ridges 408 are relatively darker and the valleys 410 are relatively lighter in the image 406. In some implementations, the second contrast type may arise due to boundary mismatch causing differences in reflectivity.

[0069] In FIG. 4C, an image consisting of a combination of the two contrast types is schematically illustrated.

[0070] FIG. 5 is a flow chart outlining the general steps of a method according to the invention. The method will be described with further reference to FIG. 6 schematically illustrating steps of the method. The method comprises forming 500 a first image 602 using pixels located under the subset 606 of collimating structures comprising a light-blocking element; forming 502 a second image 604 using pixels located under the subset 608 of collimating structures not comprising a light-blocking element; and forming 504 a combined image 630 based on the first and second images 602, 604.

[0071] In FIG. 6, a biometric imaging device 100 is illustrated which comprises an array of collimating structures where a subset 606 of the collimating structures comprises a light-blocking element 108 as described earlier. The remaining collimating structures 608 does not comprise a light-blocking element, such that a checkerboard pattern is formed. A single image capture is performed to capture an image 600 after which the image processing is split into two separate image processing streams 610, 612. Each of the two image processing streams 610, 612 will process an image having a resolution which is approximately half of the full resolution of the image sensor. The first image processing stream 610 deals with the pixels located under collimating structures with the light-blocking element. In the illustrated example, the pixels 614 of the captured image 600 corresponding to one subset 606 of collimating structures is up-sampled to form an interpolated image, and after analysis of the interpolated image, a fingerprint image 602 showing only the first contrast type based on light with an incident angle between Θ.sub.1 and Θ.sub.2 is formed.

[0072] In the second image processing stream 612, the pixels located under collimating structures without a light-blocking element are handled. Here, the pixels 618 of the captured image 600 corresponding to the complementary subset 608 is up-sampled to form an interpolated image, and after analysis of the interpolated image, a fingerprint image 604 showing both contrast types based on light is having an incident angle lower than Θ.sub.1 is formed. Based on the two fingerprint images 602, 604 from the corresponding two image processing streams, 610, 612, a combined fingerprint image 630 can be formed.

[0073] The described method can be implemented in a similar manner in an imaging device comprising collimating structures with light blocking elements, and the combination of collimating structures with or without light blocking and/or angle limiting elements can be configured so that the properties of the imaging device is suitable for a given application.

[0074] FIG. 7 is a flow chart outlining the general steps of an embodiment of the invention for an image sensor similar to the image sensor described with reference to FIG. 6, but with the difference that the light-blocking element is configured to block light within a first predetermined wavelength range. The light blocking element may thus act as a band-stop filter, or as a band-pass filter in which case the first wavelength range will be defined by two separate ranges. A subset of the light-blocking elements may also be opaque so that all visible light is blocked.

[0075] The method comprises controlling 700 the display panel to emit light only within the first wavelength range, forming 702 a first image based on pixels receiving light from the subset of collimating structures comprising a light-blocking element; controlling 704 the display panel to emit light outside of the first wavelength range; forming 706 a second image based on pixels receiving light from all collimating structures; and forming 708 a combined image based on the first and second images.

[0076] The first image will then exhibit a single contrast type based on non-orthogonal light, and it will have approximately half of the full resolution of the image sensor. The second image will show both contrasts and have the full resolution of the image sensor. Accordingly, by using a light-blocking element also acting as an optical filter, it is possible to form one image with full resolution.

[0077] FIG. 8 is a flow chart outlining the general steps of an embodiment of the invention for an image sensor where all of the collimating structures comprise light-blocking elements, and wherein the light-blocking element of the first subset of collimating structures is configured to block light within a first predetermined wavelength range, and wherein the light-blocking elements of the second subset of collimating structures are configured to block light within a second predetermined wavelength range different from the first wavelength range. The first and second subsets may for example form a checkerboard pattern.

[0078] The method comprises controlling 800 the display panel to emit light only within the first wavelength range; forming 802 a first image based on pixels located under the first subset of collimating structures; forming 804 a second image based on pixels located under all collimating structures; controlling 806 the display panel to emit light only within the second wavelength range; forming 808 a third image based on pixels located under the second subset of collimating structures; forming 810 a first combined image based on first and third images; and forming 812 a second combined image based on first combined image and the second image.

[0079] The first image will have a single contrast type based on non-orthogonal light for the first subset of collimating structures, the second image will have dual contrast and full resolution and the third image will have single contrast based on non-orthogonal light for the second subset of collimating structures. Thereby, by combining the first and third images, a full resolution image for the non-orthogonal light can be achieved, and biometric analysis can be performed based on two full resolution images, one with only one contrast type and one with dual contrast.

[0080] The above described embodiments can be used and/or complemented with collimating structures comprising angle-limiting elements, which also may act as wavelength filters, thereby making the possible combinations endless. For a specific implementation, it will thus be possible to find a suitable balance between device complexity and biometric accuracy.

[0081] Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that parts of the device may be omitted, interchanged or arranged in various ways, the device yet being able to perform the functionality of the present invention.

[0082] Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.