Display arrangement and method for operating a display arrangement

Abstract

The invention concerns a display arrangement comprising a pixel array with a plurality of pixels (10), at least some of the plurality of pixels each comprising a first and a second pLED for emitting light in the red spectrum (100r, 101r), a third pLED for light in the blue spectrum (100b) and a fourth pLED for light in the green spectrum (100g). The first and the second pLED are separated by one of the third and fourth LED. The display arrangement is configured to operate the at least some of the plurality of pixels in an emission mode, in which at least one of the first and second pLED, the third pLED and fourth pLED are biased in forward direction; and configured to operate a first subset of at least some of the plurality of pixels in a sensing mode, in which one of the third and fourth pLEDs of the first subset are biased in forward direction; and at least one of the first and second pLEDs of the first subset are biased in reverse direction or zero biased.

Description

[0001] DISPLAY ARRANGEMENT AND METHOD FOR OPERATING A DISPLAY ARRANGEMENT The present application claims priority of German patent application DE 102023124 917.7 dated September 14, 2023, the disclosure of which is incorporated herein by reference in its entirety. The present invention concerns a display arrangement having a plurality of pixels implemented with µLEDs and a method for operating a display arrangement. The present invention also concerns a fingerprint scan or detection application for such display. BACKGROUND Modern mobiles provide a high-resolution, dynamic, and intuitive display technology, which not only presents information with exceptional clarity, but also facilitates user interaction. Advanced mobile displays today provide unparalleled resolution in the range of more than a hundred dots per inch as well as high color accuracy, presenting images and text with crisp sharpness and vivid detail. They offer increased brightness, adaptive refresh rates, and enhanced power efficiency, all contributing to varying environments, like shading, different sunlight reflections, and the alike. The displays themselves are using µLEDs, in which each pixel comprises at least three subpixels. Each subpixel comprises at least one µLED responsible for emitting light in one of the green, blue and red portion of the visible spectrum, also referred to as green µLED, blue µLED and red µLED. Those µLEDs are often configured together with a second, redundant, µLED in order to avoid the need of repairing or replace damaged or non-functional µLEDs. The expression “µLEDs” refers to an optoelectronic device that comprises a size with less than 50 µm in diameter, and particularly between 2 µm and 25 µm, although these specifications may vary. µLEDs are implemented on different material systems depending on the desired emission wavelength. For light in the blue and green portion of the visible spectrum, the µLEDs are usually based on nitride, while for yellow / red and infrared, phosphide or arsenide material systems are used. However, the capabilities of these displays extend far beyond the mere presentation of visual information. More particularly, it has been proposed to use the display as a sensor device, e.g. as a fingerprint sensor. Further applications lie in the sensor reading of vital sensory information, as well as scanner of pictures or other objects being placed in front of the displays. Current technologies, mainly on fingerprint sensors are primarily based on either capacitive or ultrasonic technology, can identify and authenticate the unique patterns in a user's fingerprint. Capacitive fingerprint sensors, the more common type, function by measuring the differences in capacitance caused by the ridges and valleys of a fingerprint. Ultrasonic sensors, on the other hand, send an ultrasonic pulse against the finger to map the details of the fingerprint. Both methods provide secure, fast, and user-friendly ways to unlock devices and authenticate identity, thus adding an additional layer of functionality to the display. It has also been proposed to implement photodetectors into the display, enabling the display arrangement to scan the fingerprint of the user. One drawback lies in the fact that the sensors cannot be used in a µLED display. The µLED display comprises usually a lot of layers with metal trays to route the driver signal towards the respective emitter. This is (in contrast to OLED displays) essentially a non-transparent layer stack that renders all behind display technologies not usable. Consequently, it may be quite cumbersome to correctly identify the fingerprint or more generally fulfil the desired functionality. Current fingerprint applications require a resolution of more than 300 dpi with a limit of 500 dpi and more. Likewise, scanners and the like offer high resolutions as well, in the range of 300 dpi and more. It is therefore an object of the present application to improve the current display arrangement implementing dual use functionality that can compete with the current state-of-the-art requirements. SUMMARY OF THE INVENTION This and other objects are addressed by the subject matter of the independent claims. Features and further aspects of the proposed principles are outlined in the dependent claims. The inventors propose a new bi-directions display arrangement, in which optoelectronic devices forming subpixels of a plurality of pixels, said subpixels implemented as µLEDs are provided with a dual-use functionality. By doing so, pixels and their respective subpixels of the display arrangement are operated in two different modes. The first mode, referred to as emission mode, is characterized by biasing optoelectronic devices in the forward direction, such that they emit light. However, it has been found that µLEDs and optoelectronic devices can also be operated as photodetectors, when being operated with a reverse bias or a zero bias with its generated photo current being read out. Consequently, when biased in reverse direction, these µLEDs, subpixels and pixels are operated in detection or sensor mode. The expression “reverse bias” or also “bias in reverse direction”, for the purpose of this application shall also include zero bias and just reading out any photo current. Hence pixels, subpixels or µLEDs switched into zero bias or reverse bias are used as detectors. The inventors now propose to implement the pixels with the individual subpixels and its µLEDs of a display arrangement in a certain way and to operate pixels of a pixel arrangement in those two consecutive modes. The proposed approach enables an entire new set of different application. For example, one may use the µLEDs for green light emission of some pixels in emission mode, while operating µLEDs for red light in other pixels in sensing mode, thereby implementing a fingerprint or vital monitoring detector. Utilizing a RGBR scheme for the respective pixels, one can effectively double the current display resolution for such fingerprint and vital signs monitoring applications. Various pixels (or its respective subpixels) can be addressed and set to one of emission and sensing mode, thereby allowing certain emission and detection pattern to be generated. These patterns can provide a compensation in case of dirt or dust on the display or the object to be scanned, i.e. a fingerprint, an iris scanner, an object arranged directly on or above the display and the like. Such a display arrangement may comprise in some aspects a pixel array having a plurality of pixels arranged in rows and columns. At least some of the plurality of pixels each comprise an adjusted RGBR configuration. Such a configuration is characterized by two µLEDs configured to emit light in the red spectrum; a third µLED configured to emit light in the blue spectrum; and a fourth µLED configured to emit light in the green spectrum. The first and the second µLED are separated by one of the third and fourth µLED. In accordance with the proposed principle, the display arrangement is configured to operate the at least some of the plurality of pixels in an emission mode, in which at least one of the first and second µLED, the third µLED and fourth µLED are biased in forward direction. The display arrangement is further configured to operate a first subset of at least some of the plurality of pixels in a sensing mode, in which one of the third and fourth µLEDs of the first subset are biased in forward direction or unbiased; and at least one of the first and second µLEDs of at least some pixels of the first subset are biased in reverse direction. The expression unbiased means that those µLEDs are neither emitting light nor are they acting as photodetectors, i.e. no photo current or signal is read out of such LEDs. One could also say, that unbiased µLEDs are effectively switched off. Consequently, the third and fourth µLEDs of the first subset may emit green light, for example. The display according to the proposed principle implements an in- displayfingerprint sensor by using reverse biased µLEDs. The in-display sensor requires only one additional red µLED per pixel to reach a sufficient resolution of more than 400ppi, and effectively doubles the display resolution. This allows for minimal additional costs in a standard display manufacturing process. Additionally, structured illumination is used to increase the contrast in fingerprint detection. Both red µLEDs in a pixel are used for sensing, and the green µLED of either the same pixel or neighboring pixels are used for illumination. A structured illumination scheme is used to account and compensate in- plane cross talk from the illuminating green µLED to the sensing red µLEDs and to increase the fingerprint contrast. The display arrangement may comprise a control device coupled to the pixel array for operating the at least some of the plurality of pixels. Likewise, the µLEDs can be attached to a back plane, said back plane including control circuitry as well as current and voltage sources to supply each pixel and subpixel individually. The display arrangement can be configured to execute certain illumination patterns, that is operated certain pixels in sensing mode while also executing corresponding adjusted sensing patterns. This will provide a high degree of freedom with options to compensate for stray light or irregularities in the user’s skin during the scanning process. One aspect concerns the approach to use one subpixel in a pixel for emission, e.g. the µLED configured to emit green light, while utilizing the first and second µLEDs in said pixel as well as adjacent or neighboring in an adjusted sensing mode, in which those µLEDs are operated in reverse bias direction acting as detectors for green light reflected at a user’s finger. Consequently, the display arrangement may be configured to operate, - while operating the first subset in the sensing mode- a second subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and second µLEDs of the second subset are biased in reverse direction and the third and fourth µLEDs of the second subset are substantially not biased in forward direction or biased less than the threshold voltage. In some aspects, the display arrangement is configured to operate a second subset of at least some of the plurality of pixels in the sensing mode, in which one of the third and fourth µLEDs of the second subset are biased in forward direction; and at least one of the first and second µLEDs of the second subset are biased in reverse direction; and wherein -when operating the second subset of at least some of the plurality of pixels in the sensing mode-, operating first subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and second µLEDs of the first subset are biased in reverse direction and the third and fourth µLEDs of the first subset are substantially not biased in forward direction or biased less than the threshold voltage. The display arrangement may be configured to operate pixels of the first subset to form a regular pattern, particularly a chess pattern. The display arrangement can also be configured to operate pixels of the second subset to form a regular pattern, particularly a chess pattern. Apart from a chess pattern, other patterns are possible, in particular patterns, in which the number of pixels operated in sensing mode is significantly smaller than pixels operating in adjusted sensing mode. For example, in a certain area of the display arrangement only every third or fourth pixel is controlled to operate in sensing mode for the green light, while the subpixel of said pixels and other pixels in said area are controlled to operate in adjusted sensing mode. In this regard, it may be suitable to have the display arrangement configured to operate first and second subsets alternatively in the sensing mode and the adjusted sensing mode. While the display arrangement may embed a control device in some aspects, the control device can also be a separated device being coupled to the pixel array for operating at least some of the plurality of pixels. In some aspects, the respective µLEDs are evenly distributed across the display arrangement area or a portion thereof. In some other aspects, the µLEDs are at least equally distributed across a pixel area. Consequently, they may comprise equidistant distances from each other. In some aspects, they are arranged along a virtual line. In some other aspects, µLEDs of each pixel are located on the edges of a virtual rectangle. Likewise, in some aspects, the µLEDs are arranged in a diamond like shape. As some application can make use of and benefit from the µLEDs configured to emit light in the red portion of the visible spectrum, these µLEDs should be placed spaced apart. Consequently, those µLEDs are separated by at least one of the green and the blue µLED. This distance may effectively increase the resolution for such µLEDs when being operated in sensing mode, suitable for example in fingerprint or vital signs monitoring applications. In some aspects, the display arrangement or the control unit is configured to obtain information about the location of the user’s finger or any other object being placed on the display arrangement and select pixels of the first and second subsets accordingly. Some aspects concern the utilization of µLEDs as light emitters and photodetectors. In some aspects, the first and second µLED comprise an external quantum efficiency when reverse biased of larger than 0.15 for light of a wavelength between 500 nm and 535 nm and between 575 nm and 620 nm; and particularly larger than 0.2 for light of a wavelength between 500 nm and 535 nm and larger than 0.14 for light of a wavelength between 480 nm and 620 nm. While such numbers do not seem to be overly large, they effectively enable µLEDs configured to emit light in the red spectrum to be used as photodetectors for light in the green and red portions of the visible spectrum. Hence, good results can be achieved when such µLEDs are used as photodetectors. In some aspects, it is suitable to ensure, that at least one of the first and second µLED (particular the one not configured to be operated in emission mode) only detects the reflected light from the user’s finger. Hence, in some aspects, the display arrangement further comprises a color filter having a large transmittance in the green spectrum arranged over one of the first and second µLED, said one of the first and second µLED biased in reverse direction in the sensing mode or the adjusted sensing mode. Some aspects concern the color filter covering the surface of one of the first and second µLEDs. These color filters support the sensing mode by blocking undesired portions of the visible spectrum. The color filter may comprise a transmittance of larger than 60% for light of a wavelength between 512 nm and 565 nm, and particularly larger than 70 % for light of a wavelength between 520 nm and 560 nm and particularly larger than 75 % for light of a wavelength of 540 nm. This color filter effectively blocks light in the red and blue portions of the visible spectrum and is transparent to light in the green portion. When being arranged on the emission surface of µLED configured to emit light in the red portion of the visible spectrum, the µLED can be used as a detector for green light when operated reversely biased. The transmittance of said color filter is less than 30% for light of a wavelength above 600 nm and below 475 nm, and particularly less than 20% for light of a wavelength less than 450 nm. Different embodiments for arranging the color filter above the respective µLEDs are possible. In some aspects, the color filters are arranged on a light emission surface of the respective first or second µLEDs. In some other aspects, the display arrangement further comprises a transparent cover that is arranged above the main emission surfaces of the plurality of pixels arranged in rows and columns. The transparent cover comprises a plurality of color filters located opposite emission areas of the first or the second µLEDs. In some aspects, an area of at the color filters is larger than an emission area of the µLED associated with the color filter. This will prevent stray light from reaching the µLED’s surface when it is operated in sensing mode. In this regard, the µLED’s can be embedded in recesses of a back plane surface. A transparent cover can be arranged on top of the recesses, thereby forming an even surface. The back plane may comprise respective contact areas in the recesses to contact the µLEDs arranged therein. In some aspects, the display arrangement further comprises a glass cover arranged above light emission surfaces of the respective µLEDs. This may ensure an even surface. The color filter can be attached to the glass cover. The display arrangement may further comprise a contrast enhancement layer arranged on the glass cover facing away from the light emission surfaces. This will improve the contrast during sensing and capturing the structure of a fingerprint. Some other aspects concern a method for operating a display arrangement comprising a plurality of pixels. Each pixel comprises two µLEDs configured to emit light in the red spectrum, Furthermore, the pixels also comprise a third µLED configured to emit light in the blue portion of the spectrum and a fourth µLED configured to emit light in the green portion of the visible spectrum. The method comprises the steps of operating during a first time period the at least some of the plurality of pixels in an emission mode, in which at least one of the first and second µLED, the third µLED and fourth µLED are biased in a forward direction. This mode of operation corresponds to normal display operation. However, the method also comprises operating during a second time period a first subset of at least some of the plurality of pixels in a sensing mode, in which one of the third and fourth µLEDs of the first subset are one of unbiased or biased in forward direction; and at least one of the first and second µLEDs of the first subset are biased in reverse direction. Consequently, the method proposes two different modes of operation. In a first operation mode, at least some of the µLEDs of several pixels of the plurality of pixels are biased in a forward direction, thus emitting light in their respective portions of the visible spectrum. In a sensing mode, at least some of the µLEDs are operated in reverse direction, acting as photodetectors. In some aspects, forward biasing during sensing mode is sequentially performed, with only some µLEDs, particularly of green color, being activated at a certain time. At the same time, some of the first and second µLEDs are operated in sensing mode, thus only capturing reflected light of said color. Biasing only some µLEDs of a certain color and detecting by adjacent µLEDs at different distances, the reflected light portion thereof, can improve the signal-to-noise ratio. In applications like fingerprint sensing or vital monitoring, the µLEDs configured for emission of light in the green portion of the spectrum are most suitable for emission, as such light has good penetration and scattering properties at human tissue (e.g. the finger and fingerprint of a user). In some aspects, the method comprises, - while operating the first subset in the sensing mode-, the step of operating a second subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and second µLEDs of the second subset are biased in reverse direction and the third and fourth µLEDs of the second subset are substantially not biased in forward direction or biased less than the threshold voltage. This enables to detect reflected light at different distances from the µLED acting as a light source for the light to be reflected by a user’s finger. The distance between the µLEDs acting as photodetectors may vary and can range from the µLEDs in the same pixels to µLEDs in adjacent pixels and even further away. For example, in some instances pixels (and their respective µLEDs) operated in adjusted sensing mode as photodetectors are at least two pixels away from a pixel, whose µLED is operated in forward direction. Capturing the reflected signal at various distances not only allows to increase the overall resolution for such applications, but also enables in vital signs monitoring or capturing vital signs, improving robustness against faulty signals or fraud fingerprint structures. Consequently, in some instances, the method comprises the step of operating a second subset of at least some of the plurality of pixels in the sensing mode, in which one of the third and fourth µLEDs of the second subset are biased in forward direction; and at least one of the first and second µLEDs of the second subset are biased in reverse direction; and when operating the second subset of at least some of the plurality of pixels in the sensing mode-, operating first subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and second µLEDs of the first subset are biased in reverse direction and the third and fourth µLEDs of the first subset are substantially not biased in forward direction or biased less than the threshold voltage. The first and second subsets can vary and change over time, resulting in a “moving” scan of a certain area of the display. Hence, a user’s finger is scanned various time, in some instances in different directions or with different illumination patterns. In some aspects, the first and second subsets can be operated alternatively in the sensing mode and the adjusted sensing mode. Furthermore, the overall number of pixels being operated in the sensing mode is usually smaller than the number of pixels being operated in the adjusted sensing mode. In some instances, the method operates the first subset in sensing mode such to form a regular pattern, in particular a chess pattern. Likewise, the pixels of the second subset can be operated to form a regular pattern, particularly a chess pattern. In some aspects, the subset operated in sensing mode lies on one or more parallel virtual lines. In some other aspects, the subset operated in sensing mode from a star like structure with a couple of virtual rays facing outwards. In some aspects, the first subsets are positioned such that each pixel of the first subset is surrounded by at least two pixels of another subset. In some other aspects, the display arrangement is configured to and operates the first subset such that a portion of the subset forms a square of NxN pixels operated in sensing mode surrounded by a plurality of pixels operated in adjusted sensing mode. The proposed display as well as the proposed method require an RGBR configuration with two µLEDs configured to emit light in the red portion of the visible spectrum. Depending on the implementation, pixels may contain a redundant µLEDs for the red, green or blue. Those redundant µLEDs usually not used for light emission can be re-purposed for the sensing applications mentioned herein, even is the sensor part (the ones being biased in reverse direction) has a lower yield or reduce resolution. Such µLEDs have a sufficiently high quantum efficiency for capturing photons in the green spectrum. However, the µLEDs can also detect photons in the red spectrum as well as in the blue portion of the spectrum. To improve the signal-to-noise ratio and reduce undesired portions of the spectrum, it is proposed, in some aspects, to arrange a color filter covering an emission surface of one of the first and secondμLEDs . Said color filter comprises a large transmittance in the green spectrum arranged over one of the first and second μLED, said one of the first and second μLEDbiased in reverse direction in the sensing mode or the adjusted sensing mode.

[0002] Some further aspects concern the subsequent process steps to obtain a fingerprint. In some aspects, the method further comprises the steps of detecting a photo current of the first and second μLEDs of the first and / or second subsets and deriving fingerprint information therefrom. In order to improve the application against faulty signals or against manipulation during fingerprint detection, the method may also comprise the step of deriving a PPG signal from the detected photocurrent and comparing the PPG signal with a threshold. In response to the comparison, the fingerprint information may be derived from the detected photocurrent.

[0003] The proposed display arrangement and method is usable as an in-display fingerprint sensor and an in-display PPG sensor, the latter being a stand-alone application or in combination to detect possible fraudulent attempts during fingerprint sensing.

[0004] SHORT DESCRIPTION OF THE DRAWINGS

[0005] Further aspects and embodiments in accordance with the proposed principle will become apparent in relation to the various embodiments and examples described in detail in connection with the accompanying drawings, in which

[0006] Figure 1 shows a first embodiment of a display arrangement in accordance with some aspects of the proposed principle;

[0007] Figure 2 illustrates a second embodiment of a display arrangement in accordance with some aspects of the proposed principle;

[0008] Figure 3 shows a third embodiment of a display arrangement in accordance with some aspects of the proposed principle; Figure 4 illustrates a wavelength efficiency diagram for light in the green spectrum to outline some aspects of the proposed principle;

[0009] Figure 5 shows a wavelength-efficiency diagram for light in the blue spectrum to outline some aspects of the proposed principle;

[0010] Figure 6 illustrates a wavelength-efficiency diagram to outline some aspects of the proposed principle;

[0011] Figure 7 illustrates a first embodiment of a pixel in accordance with some aspects;

[0012] Figure 8 shows a second embodiment of a pixel in accordance with some aspects;

[0013] Figures 9A to 9C show embodiments of a pixel in display or sensing mode in accordance with some aspects;

[0014] Figure 10 illustrates another embodiment of a pixel in a display and sensing mode in accordance with some aspects;

[0015] Figure 11 shows a wavelength quantum efficiency diagram forμLEDs covered by a color filter in accordance with some aspects;

[0016] Figure 12 illustrates a first embodiment of a plurality of pixels including an arrangement of their respectiveμLEDs in accordance with some aspects;

[0017] Figures 13 to 15 illustrate further embodiments of a plurality of pixels including an arrangement of their respectiveμLEDs in accordance with some aspects;

[0018] Figure 16 shows a wavelength-transmittance diagram for different color filters in accordance with some aspects of the proposed principle;

[0019] Figure 17 shows a cut-view of some pixels with a glass cover to indicate some aspects of the proposed principle; Figure 18 shows a cut-view of some pixels with a glass cover to indicate some aspects of the proposed principle; Figure 19 illustrates aspects of a method for operating a display arrangement in accordance with the proposed principle; Figure 20 shows a pattern periodicity-contrast diagram illustrating some aspects of a method for operating a display arrangement in accordance with the proposed principle. DETAILED DESCRIPTION The following embodiments and examples disclose various aspects and their combinations according to the proposed principle. The embodiments and examples are not always to scale. Likewise, different elements can be displayed enlarged or reduced in size to emphasize individual aspects. It goes without saying that the individual aspects of the embodiments and examples shown in the figures can be combined with each other without further ado, without this contradicting the principle according to the invention. Some aspects show a regular structure or form. It should be noted that in practice slight differences and deviations from the ideal form may occur without, however, contradicting the inventive idea. In addition, the individual figures and aspects are not necessarily shown in the correct size, nor do the proportions between individual elements have to be essentially correct. Some aspects are highlighted by showing them enlarged. However, terms such as "above", “over”, "below", "under" "larger", "smaller" and the like are correctly represented with regard to the elements in the figures. Therefore it is possible to deduce such relations between the elements based on the figures. Figure 1 illustrates a display arrangement in accordance with some aspects of the proposed principle. The display arrangement 1 comprises an array of pixels 10, which are arranged in rows and columns on a respective substrate or back plane 12. The pixels 10 each comprise a plurality of µLEDs capable of generating light in various portions of the visible spectrum. For the purpose of simplicity, a μLED or optoelectronic device configured to emit light in the red portion of the visible spectrum is referred to as a red μLED. Likewise, a green μLED is an optoelectronic device configured to emit light in the green portion of the visible spectrum and a blue μLEDis configured to emit blue light in operation thereof.

[0020] In this particular case, each pixel 10 comprisesμLEDs in an RGBR configuration in accordance with the proposed principle. That is, two μLEDs are configured to emit red light, and one μLEDeach is configured to emit blue light as well as green light, respectively. The individual μLEDs can either be integrated and embedded into back plane 12 using monolithic integration, or be attached to back plane 12 using certain bonding techniques. Back plane 12 comprises a plurality of integrated circuits, which are configured to control the respectiveμLEDs in each of the pixels 10 individually to act as emitter and partially as detectors in accordance with the proposed principle.

[0021] The individual pixels andμLEDs are connected to a control circuit 13 via the back plane 12. Control circuit 13 can also be integrated into back plane 12. Control circuit 13 is configured to supply the necessary current signals to individually address theμLEDs as well as the voltage or current signals thereto. To this extent, the embodiment according to Figure 1 to conventional display arrangements, in which a plurality of pixels is arranged in rows and columns, each pixel comprising a plurality of pixels. As illustrated, the various subpixels in the pixels 10 are arranged at certain location, forming a pattern. These positions can depend on the various manufacturing techniques, but also on the individual functionality, as explained further below and with respect to some further figures below.

[0022] At least some of the pixels 10, their subpixels and respectiveμLEDs in the present display arrangement are used in accordance with the proposed principle as photodetectors arrangement for various purposes. In some aspects, as explained further below, the pixels can be used to capture portions of the visible spectrum with various wavelength sensitivity in order to scan an object arranged directly in front of the display arrangement.

[0023] In another application, various pixels are used to illuminate an object, while other pixels in the vicinity are utilized as photodetectors to detect and capture the light portions of the emitted light reflected at an object opposite the various pixels. Apart from scanning the object, thereby creating a picture, other use cases include a fingerprint sensor, in which the reflected portions correspond to the structures of a user's finger being arranged on the display surface. The display arrangement according to the proposed principle can also be used for PPG or vital sign monitoring. In this regard, the expression PPG stands for photoplethysmogram, which is an optically obtained plethysmogram that can be used to detect blood volume changes in the microvascular bed of tissue.Because blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, and other circulatory conditions. PPG measurements can be combined with the fingerprint sensor technique in order to ensure that any fingerprint is taken from a lively person and not from a picture or any artificial model.

[0024] With respect to the functionality of the embodiment of Figure 1 in accordance with the proposed principle, each pixel 10 comprises in total four different subpixels. Two of these pixels are configured to emit light in the red spectrum, while the third pixel is configured to emit light in the green spectrum and the fourth pixel is configured to emit light in the blue spectrum. In accordance with the proposed principle, a surface of theμLEDs configured to emit light in the red spectrum is covered by a red color filter. Likewise, the second μLED configured to emit light in the red spectrum is covered by a green color filter. The red color filter is transparent for light substantially within the red spectrum that is above 650 nm, while the color filter covering the secondμLEDs is transparent for light within the green spectrum. The respective transmission curves for the color filters are explained in detail a further below. The functionality of such pixel 10 is now as follows. In the emission mode of the display arrangement, the control circuit controls theμLEDs to emit light depending on the desired color and brightness. To this extent, the emission mode corresponds to a conventional operation of a display arrangement. The red color filter does not tamper the emission of light due to its very high transmittance of above 92% in said emission range. The second μLEDfor red light is switched off in this mode. The control circuit 13 controls the mixing of light with the respective portions and controls the on-off ratio of theμLEDs . By controlling the respective pixels 10 and the subpixels within each pixel, various pictures, signs and other and other structures can be displayed on the display arrangement.

[0025] In a second mode of operation, variousμLEDs are reversely biased, thereby acting as photodetectors. More particularly, at least some of the firsμtLEDs configured to emit red light with the red color filter placed upon the emission surface are also switched in a reverse bias mode to act as a photodetector. Due to the red color filter, these μLEDs only detect light within the red spectrum. Finally, some of the secondμLEDs covered by the green color filter are switched into reverse bias mode to act as a photodetector of green light, as thoseμLEDs comprise a quantum efficiency large enough to capture green light. Because of the green color filter, light within the blue spectrum as well as within the red spectrum is absorbed or reflected and thereby does not contribute significantly to the photocurrent. The thirdμLEDs configured to emit green light are partially switched on in the sensing mode to emit green light. The light illuminates an object placed on the cover of the display arrangement, like a user's finger. A portion of the light is reflected towards the otherμLEDs and captured by the μLEDs. Due to the red color filter and the μLED for blue light being relatively insensitive to such portion of the spectrum, the signals of thoseμLEDs can be used to compensate for possible noise. The second μLEDcovered with the green color filter captures the reflected light. Due to small variations in the finger's surface, the portion of light varies, enabling to scan the fingerprint of a user. In the above embodiment, the red color filter may effectively block the green light, thus reducing the overall solution of the fingerprint scanner. In some alternatives, the red color filter is left out. Thus, the first μLEDnow captures light within the green and red potions of the spectrum. If the red portion is small compared to the green portion of the reflected light, such an application effectively doubles the overall resolution for a fingerprint scanner. In other words, a display with RGBR configuration with one μLEDcovered by a green color filter is dual used, as a display and as a fingerprint. Moreover, in some instances, the first μLED can be used in reverse bias direction as a detector with the green portion being removed as noise later in postprocessing using the photocurrent from the second μLED.

[0026] Figures 2 and 3 illustrate different modes of operation, wherein the user's finger to be scanned is placed directly upon the top surface of the display arrangement.

[0027] The display arrangement in Figures 2 and 3 both comprise a respective array of a plurality of pixels 10 being arranged in rows and columns. To this extent, the display arrangement is similar to the embodiment of Figure 1 with the exception that no color filter is arranged above the respectiveμLEDs . The display is in an RGBR configuration. Furthermore, the display arrangement in Figures 2 and 3 are operated in slightly different modes in which, a portion of the respective pixels and greenμLEDs are used for emission to illuminate the finger to be scanned, while other portions of pixels are operated in an adjusted sensing mode.

[0028] More particularly, Figure 2 illustrates a respective embodiment used for parallel illumination and sensing mode. In this configuration, the display arrangement comprises a plurality of pixels, in which a first portion Pl of pixels are operated in adjusted sensing mode while a second portion P2 of pixels are operated in sensing modes. During sensing, the individual green subpixels of the second portion P2 of pixels are operated in sensing mode. In sensing mode, the green μLED emits green light, while the blue μLED is switched off and the μLED suitable to emit red light are switched into reverse bias direction. The emitted green light is reflected from the object being located directly above the display arrangement and is detected by the red subpixels of first portion Pl of pixels. These subpixels of portion Pl are operated in reverse bias direction, acting as respective photodetectors.

[0029] Depending on the application, the shape size and location of the different portions Pl and P2 of pixels can vary over time, such that the object is scanned sequentially. For example, the embodiment in Figure 2 represents a configuration of the pixel portions Pl and P2 during a first time period. In a subsequent time period, the respective portions Pl and P2 are alternated such that the subpixels in pixel portions P2 are switched to adjusted sensing mode (that is the red μLEDs are biased in a reversed bias operation while the green and blue μLED are not emitting any light, thus operation acting as photodetectors), while a portion Pl' of the pixel is switched into sensing mode now emitting respective light.

[0030] Such switch between sensing and adjusted sensing mode for the various pixel portions generates several benefits. For once, a more equal illumination of the finger to be scanned is achieved. Furthermore, depending on the respective application, irregularities like undesired reflections, dirt, skin irregularities and the like can be recognized and subsequently corrected. In the present embodiment, the overall number of pixels being operated in sensing mode is smaller and approximately a quarter of the number of pixels being used for adjusted sensing mode. This ratio is dependent on the application, but also caused by the smaller quantum efficiency for capturing photons. Furthermore, it is possible to detect stray light or average the captured light to obtain a good scan of the object arranged above the display. The pattern can change and "move across the display" similar to conventional scanners.

[0031] Figure 3 illustrates another possibility of such a configuration. In this embodiment, a first portion Pl of pixels is set into adjusted sensing mode. Each direct adjacent pixel of such pixel portion Pl is either switched off or also set into adjusted sensing mode. The pixel portions P2 are switched into sensing mode. Consequently, for each row and each column the pixel portions Pl and P2 are alternating with one pixel in the between. Similar to the previous embodiment, the portions of pixels in sensing mode as well as the pixel portions being switched into adjusted sensing mode can alternate as well, whereas pixel portions Pl are subsequently set into sensing mode, while pixel portions P2 are switched into the adjusted sensing mode. Likewise, the pixel portions in the rows and columns can move one pixel to the left / right or up / down, such that the whole area of the display arrangement is completely scanned. Various other options and possible sensing and an adjusted sensing patterns are possible for the display arrangement in accordance with the proposed principle to provide the necessary application.

[0032] In the respective embodiments illustrated in Figure 1 to Figure 3, each green subpixel of the respective pixels operated in sensing mode is switched on to provide certain illumination. However, this is not a requirement. Rather, the individual subpixels that are the subpixels configured to emit green light can be activated and deactivated sequentially in each pixel operated in the sensing mode. In other words, for scanning the finger, the subpixels are sequentially activated, such that the finger to be scanned is illuminated with substantially one colored green light in a "moving fashion" for example from left to right, from top to bottom or from a central point to the outside (or vice versa). Likewise, the subpixels used for adjusted sensing mode can be switched on and off sequentially. This may be beneficial due to an increased signal-to-noise ratio, whereas the other photodetectors can be used to detect a potential noise portion during the overall illumination of the finger to be scanned.

[0033] As stated before, the proposed principle lies on the dual use of the respective optoelectronic devices andμLEDs , both for light emission and light detection. Consequently, theμLEDs are either switched into a forward bias mode, in which they are configured to emit light, or into a reverse biased mode, in which they act as photodetectors. Depending on the material system, the quantum efficiency for emission as well as detection may vary. Figure 4 to Figure 6 illustrate the various external quantum efficiencies for the respective operating modes for the µLEDs based on gallium nitride used for blue and green emission, as well as the phosphide or arsenide material system used for red emission. Figure 4 illustrates a wavelength-quantum efficiency diagram for a µLED configured to emit green light with the wavelengths between 500 nm and 550 nm. This is given by straight curve K1. The base material system for such a µLEDs is usually based on gallium nitride. However, as illustrated, for µLEDs based on phosphide or arsenide systems switched in reverse bias mode, the dotted line K2 represents the external quantum efficiency for capturing photons and generating electron-hole pairs. These µLEDs are usually used for red light emission, but they can be adapted as photodetectors in the green portion of the spectrum. As illustrated, the quantum efficiency for µLEDs in reverse bias mode within that particular range of the spectrum is about 0.2, which is sufficient to act as photodetectors. Consequently, µLEDs configured to emit light in the red spectrum can be operated in reverse bias mode and used to detect photons having a wavelength between 500 nm and 550 nm. Figure 5 illustrates a similar example for µLEDs configured to emit light in the blue range of the visible spectrum around 450 nm. Straight curve K1 illustrates their emission spectrum. However, µLEDs based on a gallium nitride but used to emit green light comprise a quantum efficiency for detecting such emitted photons at around 0.15, see dotted curve K2 in Figure 2, which is lower than for µLEDs configured to emit blue light. Still, this allows to use µLED based on nitrides to be operated in reverse bias mode to act as a photodetector of blue light. The dotted curve K2 illustrates the respective quantum efficiency over the presented wavelength. Figure 6 finally illustrates the initial spectrum of a red, green and blue light µLEDs, see straight curves K1 to K3 as well as the corresponding quantum efficiency when switched into reverse bias mode. The dotted lines K4 to K6 correspond to quantum efficiency in the reverse bias mode, when the µLEDs work as photodetectors. As shown, the µLEDs based on phosphide for red light emission can be used to detect photons within the red spectrum, the green spectrum as well as the blue spectrum, although the latter is usually not done by such µLEDs. The µLEDs configured to emit green light could be used in reverse bias mode to detect light within the blue spectrum, although the respective µLEDs for blue light are far more suited for such purpose due to its higher external quantum efficiency of approximately 0.25 compared to 0.15 for green light. µLEDs configured to emit red light are used to detect green light within the spectrum of 500 nm to 550 nm, as illustrated by the dotted curve K5. A second µLED based on phosphide can also be used to detect green. Hence, in a pixel one may provide two µLEDs configured to emit red light, wherein one of those µLEDs is now covered with a color filter being transparent within the desired spectrum between 500 nm and 550 nm to act as a very specific photodetector for green light. The second µLED configured to emit red light can be used as an emitter as well as a detector of green light. This practically doubles the overall resolution when detecting green light reflected from a user’s finger. The second red color filter may not be necessary, as one can correct a detected signal in those µLEDs with the photo signal obtained by the µLEDs covered with a green filter. In this regard, Figure 11 illustrates the wavelength-quantum efficiency diagram for µLEDs used as a photodetector for blue light, as well as two a µLEDs used as detectors for green and red light, respectively, the latter with a respective color filter on top. The quantum efficiency in each case is between 0.15 and 0.2, which is sufficient for scanning applications. Consequently, a pixel with four µLEDs as subpixels can be used either as emitters for generating light of the desired color as well as a photodetector to detect photons within the visible spectrum. Consequently, no additional photodetectors are required. Moreover, this arrangement allows a variety of different applications by operating the respective µLEDs in each pixel separately, as indicated already in the first three embodiments. The arrangement and structure of the respective pixels, including the µLEDs associated with the pixel, are illustrated in Figure 7 to Figure 10. Figure 7 illustrates an arrangement of µLEDs, representing subpixels in a pixel 10 in a diagonal arrangement. In total, each pixel 10 consists of five µLEDs, wherein two of the µLEDs 103 used as respective detectors. More particularly, pixel 10 comprises a first µLED 100r configured to emit red light, a second µLED 100g configured to emit light within the green spectrum, as well as µLED 100b configured to emit light within the blue spectrum. The three µLEDs 100r, 100g and 100b are arranged along a virtual line, being separated by a certain distance. Between the red µLED and the green µLED as well as between the green µLED and the blue µLED, another optoelectronic device 103 is arranged. The devices 103 are configured to act as respective detectors and are biased in reverse mode during operation of the device. Depending on the application, the three µLEDs can be used to emit light for display. This is illustrated in Figure 7A. The detectors 103 are usually switched off during such operation. Depending on the application, optoelectronic devices 103 of adjacent or neighboring pixels can be operated in reverse bias mode, in which the µLEDs 100r, 100g and 100be are not operated. Thus, the direct incident light is limited, and the photodetectors will detect light being reflected from an object. Figure 7B illustrates a further embodiment, in which an infrared light emitter 102 is arranged in each pixel. This will allow illuminating an object using infrared light, which can subsequently be detected by photodetectors 103. For this purpose, the photodetectors 103 can be configured with respective color filters covering the main detection surface to only allow the reflected light from the infrared emitter 102 to reach the detectors. Figure 8 illustrates two pixels, each of those having three µLEDs 100r, 100g, and 100b configured to emit respective colored light as well as three µ-photodetectors 103. The photodetector can be selectively activated or deactivated during emission of the µLEDs allowing to detect portions of the emitted light reflected from an object. Similar to the previous embodiment, the photodetectors as well as the µLEDs can be arranged in a virtual diagonal line through the respective pixels 10. Figure 9A illustrates a slightly different arrangement, in which dedicated µ-photodetectors 104 are located in each pixel. These discrete µ-photodetectors can either be placed separately or be embedded into the back plane of the main display during the manufacturing of the display arrangement. In some aspects, the photodetectors are mass transferred onto the display back plane, similar to the transfer process of the µLEDs. The µ-photodetectors 104 are suited to detect mainly light within a certain range of the visible spectrum, for example, the green range. This will allow to further implement certain applications like fingerprints sensing with a high degree of resolution. Consequently, the µ-photodetectors can be based on silicon, Si, arsenide, phosphide or other materials different from the materials used for the µLEDs. Figure 9B and 9C illustrate yet another example for fingerprints or a vital parameter sensing like PPG measurements. The respective pixels and µLEDs are operated in an emission mode in Figure 9B. µLEDs 100r, 100g as well as 100b are supplied in forward direction, while the second µLEDs configured to emit red light 101r is switched off. Due to the fact that usually such pixels require redundant µLEDs anyway, no significant complexity is added. In sensing mode, only the µLED 100g is emitting light while µLEDs 100r and 101r are switched into reverse bias direction. Figure 9C illustrates this so-called sensing mode, suitable for fingerprint sensing as well as PPG measurements or other applications. In such a mode, the blue µLED 100b is switched off and the green LED 100g is operated in a forward mode to emit light in the green spectrum. The light is reflected from an object, for example, a finger or any skin portion of the user, said portion being placed on the display. A portion of the reflected light reaches theμLEDs lOOr and lOlr. Those μLEDs are switched in reverse bias direction to act as photodetectors for the reflected green light portion. Due to the fact that thoseμLEDs lOOr and lOlr are operated in the same manner in the respective pixels, the overall resolution of detectors is effectively doubled and, more particular, the signal-to-noise ratio improved.

[0034] In some instances, the detectorμLEDs lOlr can comprise a green color filter transparent for green color only. This would allow compensating for certain noise or other undesired signal portions, by comparing the photo current provided byμLEDs lOOr and lOlr in each pixel and / or adjacent pixels. Using a pixel arrangement with two respectiveμLEDs configured to emit red light and operating them in reverse bias direction results in resolution for fingerprints sensing of higher than 600 dots per inch. At the same time, each pixel lOOr (and even lOlr) can be used for normal display of light.

[0035] Figure 10 illustrates a further example of such an arrangement, which is suitable for directing light when the respectiveμLEDs are operated in reverse bias direction, acting as photodetectors. In this configurationμ,LEDs 100r' is configured to emit red light and further comprises a red color filter covering the main emission surface. The red color filter is transparent to red light, but absorbs all reflected lights in other portions of the visible spectrum.

[0036] A further μLED100g is configured to emit green light and another μLED 100b is configured to emit blue light. Furthermore, a fourth μLED101r' is configured to emit red light. Similar to μLED100r', this μLED101r' comprises a color filter transparent for photons in the green range of the visible spectrum, covering the main emission surface of the respective μLED. During emission mode depicted in the right picture of Figure 10, theμLEDs 100r', 100g and 100b are operated in a normal forward bias direction that is acting as emitter for the respective pixel. The second μLED101r' is usually switched off in such operation. In sensing mode, in which the different colors of an object are to be scanned and detected, the µLED 100g for the green color mode is switched off, while the remaining µLEDs 100r’ and 101r’ as well as 100b are operated in reverse bias direction. In such operation, the respective µLEDs detect light in their respective portion of the visible spectrum. In this regard, it should be noted that it is not necessary to illuminate an object being placed on the display arrangement to be scanned. Rather, natural light or if the object emits light on its own can be detected by µLEDs 100b, 100r’ and 101r’. Depending on the various applications, the arrangement of µLEDs and subpixel in each pixel 10 can be optimized and suited for the respective display and directional operation mode. Figure 12 to 15 illustrate various configurations and arrangements of subpixels in such a display. Each pixel contains four µLEDs in Figure 12, to configured to emit light in the red spectrum, one µLED in each pixel is configured to emit light in the blue spectrum and one µLEDs configured to emit light in the green spectrum. These µLEDs are arranged along a virtual line, wherein the µLEDs configured to emit light in the red spectrum are separated by at least one of the green and blue LEDs. This configuration is also referred to as RGBR pixel. As shown herein, the pixels have the virtual shape of a parallelogram arranged on the display structure. In a slightly different arrangement depicted in Figure 13, each pixel comprises the shape of a rectangle of equal size. The respective subpixels and µLEDs in each pixel are arranged on the virtual diagonal line in each of those pixels. Similar to the previous embodiment, the red µLEDs are separated by a green µLEDs or blue µLEDs vis-à-vis the pixel borders. A different configuration is presented in Figure 14, in which the subpixels and µLEDs are arranged in a rectangular form, with the two µLEDs 100r and 101r being separated by a large distance. Similar to the previous embodiment, the red µLED 100r and 101r are both separated by respective green or blue µLED. Figure 15 finally illustrates a diamond shape configuration, in which the respective µLEDs and subpixels in each pixel 10 form a diamond like structure with the µLEDs 100r and 101r being arranged on a virtual horizontal line and the pixels 100g and 100b being arranged alternating on a respective virtual vertical line. Figure 16 illustrates a wavelength-transmittance diagram of several color filters F1, F2 and F3 usable and suited for the display arrangement in accordance with the proposed principle. The color filters F1, F2 and F3 comprise different transmittance in their respective ranges, whereas the transmittance outside the desired range significantly drops below 20%. This allows to effectively block and absorb light outside the respective large transmittance area to prevent a corresponding reaction of the photodetectors. Figure 17 illustrates a view of a portion of the display arrangement in accordance with the proposed principle to show some aspects of the display and sensing mode side-by-side. The display arrangement comprises a plurality of pixels 10, which comprise a total of four separate subpixels implemented is µLEDs. These separate subpixels are implemented in the RGBR configuration with two µLEDs 100r and 101r configured to emit red light, a µLED 100g configured to emit green light as well as a µLED 100b configured to emit blue light. The pixels are covered by a cover structure 300 comprising an anti-reflective coating 303 attached to a glass 302. A cover glass comprises a contrast enhancement layer 301 on top of the cover structure 300 forming the outer layer. In a display mode depicted on the left part of Figure 17, the three µLEDs 100r, 100g and 100b will emit light in accordance with the respective control and supply signals to provide a colored start. The second µLED 101r configured to emit red light is switched off for the display and emission mode. During sensing amount depicted in the right drawing of Figure 17, the µLEDs 100r and 100b are switched off and the µLED 100g for emitting green light is activated. As depicted, the µLED emits green light towards the glass cover 300. A portion of that light is reflected back as reflected light RL due to the mismatch of the reflective index between air and layer 301 and may reach the top surface of µLEDs 100r and 101r. The µLEDs are switched into the reverse bias direction, thereby acting as photodetectors. As illustrated previously, the quantum efficiency of both µLEDs is sufficient to capture any green photons coming from the reflected portion, thereby causing photo current signal in both µLEDs. Depending on the reflection direction, a plurality of red µLEDs is now able to capture the reflected light portion and create a corresponding signal. Figure 18 illustrates an embodiment, in which a user places his or her finger on to the top surface and more particularly onto the contrast enhancement layer 301. The finger’s skin comprises a roughened, an uneven surface with valleys and ridges corresponding to a fingerprint to be sensed. As illustrated, the plurality of pixels 10 are activated in sensing mode such that the green µLED emits light towards the glass cover. The light is partially absorbed or scattered away as scattered light SL and partially reflected as indicated by the dotted arrows reaching the top surface of the red µLEDs switched into reverse by direction. As a result, some green light is reflected and subsequently detected by the red µLEDs in reverse bias direction, indicated by the dotted arrows FL and straight arrows RL. A first portion given by the straight arrows is reflected directly from the glass cover, as there is still an air interface between the glass cover and air. This is due to the fingerprint valleys, as indicated. A second portion FL of reflected light comes from the finger ridges in the fingerprint profile. Hence, the amount of reflected light differs depending on whether the fingerprint profile is touching the surface with the ridges or not due to the valleys. Consequently, the refractive index of the human tissue and the glass is different, resulting in a corresponding fingerprint light FL being captured by the red µLEDs. More particularly, touching points corresponding to the ridges of different profile comprises a matching refractive index of the human tissue and the cover glass. This index matching leads to a reduced back reflection of the cover glass top surface, as indicated. Consequently, the reflected light portion is larger in areas of valleys than the corresponding areas of the fingerprint light FL. There is a high contrast due to the losses of the back reflection in the contact points between the fingerprint profile ridges and the corresponding valleys indicated in Figure 18. By changing the illumination pattern of the greenμLEDs during the sensing process, possible irregularities and noise effects can be compensated.

[0037] Particular in the RGBR configuration as illustrated earlier, the overall amount of redμLEDs capable of measuring and detecting the reflected light portion RL and FL from the fingerprint effectively increases the overall resolution.

[0038] However, as a human tissue is highly scattering green light, scattering leads also to a halo around the back reflected light from a single pixel illumination. During illumination of a larger area, halos of all pixels with overlap and the cross scattering will reduce the overall contrast. Hence, an illumination pattern is proposed to reduce the parasitic effect of scattering. Such pattern illumination is for example presented already in regard to Figures 1, 2 and 3, wherein the first number of subpixels within certain pixels are activated in forward direction while other subpixels, namely the red subpixels in the adjacent pixels are switched into reverse bias direction. Such a pattern can change, and for example, move through the area to be scanned. Similarly, different patterns can be activated and deactivated to overall measure and calculate every diffuse back scattering and subsequently compensate the halos of respective pixels.

[0039] Figures 19A and 19B illustrate a respective approach, in which the illumination pattern is inverse in both drawings. As illustrated in the middle pattern inversion view, the scattering effect results in an increased contrast of the overall image when both images are simply added. Consequently, the ridges and valleys of the fingerprint can be identified more clearly. As shown in Figures 19A and 19B, this pattern inversion works on a number of patterns with specific sizes. The pattern periodicity as well as the sizes of the pattern can be adjusted to fit the desired needs, and particularly to fit the sizes of the respective fingerprint. Figure 20 illustrates a pattern periodicity for optimal contrast found by this method. The pattern periodicity in this regard is dependent on the size of the ridges of a fingerprint and lies between 120 pm and approximately 200 pm. In such an area, the contrast would be approximately 0.038 on an artificial scale. The overall contrast is reduced below said pattern periodicity as well as above said pattern periodicity, whereas a strong increase happens with pattern periodicity smaller than 120 pm, while the decline of contrast is smaller for patterns with a periodicity larger than 200 pm. Nevertheless, in usually display applications with 300 dpi, the use of bothμLEDs configured to emit red light in reverse bias direction effectively increases the resolution and helps to capture and collect the respective back reflected light.

[0040] Apart from the present pattern illustrated herein in the form of chess board structure, other patterns can be used.

[0041] For example, the pattern can move through the area of interest similar to conventional scanners, in which a light strip, in this case a green light strip, moves through the area of interest from left to right, right to left, or up to bottom and vice. This may reduce the overall halo effect and provide better signal-to-noise ratio. Likewise, it is possible to detect the location of the finger being put on the glass display by evaluating the overall amount of light reaching theμLEDs being switched in reverse direction.

[0042] In a subsequent step, after the location of the finger has been determined, the center of that covered area can be determined and from thereon, various patterns are generated. These include, but are not limited to a star, a circle, a rectangle, triangle and the like. These patterns are inverted to obtain a better contrast. Likewise, certain patterns can "grow" or "shrink", that is increase or decrease its size over time. The overall contrast during the scanning is significantly improved.

[0043] Furthermore, depending on the scanning directions, additional information can be obtained like a PPG signal from the reflected and captured signals in the photodetectors. The PPG signal is to evaluate whether the object placed on the glass cover is a real finger from a living user or a fraudulent signal. Consequently, different patterns suitable for fingerprint sensing as well as PPG sensing can be used in quick succession to obtain the desired results.

[0044] LIST OF REFERENCES

[0045] 1 display arrangement 10 pixel

[0046] 10', 10' adjacent pixel 10''' adjacent pixel 12 back plane 13 control unit lOOr subpixel, red μLED 100r' subpixel, red μLEDwith color filter 100g subpixel, green μLED 100b subpixel, blue μLED lOlr subpixel red μLED 101r' subpixel red μLEDwith color filter 102 subpixel infrared μLED 103 photodetector 104 p-photodetector 300 cover structure, glass cover 301 contrast enhancement layer 302 glass 303 anti-reflective coating Pl, Pl' pixel P2, P2' pixel KI, K2 curves K3, K4 curves K5, K6 curves RL, FL reflected light portion SL scattered light

Claims

CLAIMS1. Display arrangement comprising a pixel array having a plurality of pixels arranged in rows and columns, at least some of the plurality of pixels each comprising: o a first and a second μLED, each configured to emit light in the red spectrum; o a third μLEDconfigured to emit light in the blue spectrum; o a fourth μLED configured to emit light in the green spectrum; wherein the first and the second μLEDare separated by one of the third and fourth μLED; wherein the display arrangement is configured to operate the at least some of the plurality of pixels in an emission mode, in which at least one of the first and second μLED, the third μLED and fourth μLEDare biased in forward direction; and wherein the display arrangement is configured to operate a first subset of at least some of the plurality of pixels in a sensing mode, in which one of the third and fourthμLEDs of the first subset are biased in forward direction; and at least one of the first and second μLEDs of at least some of the first subset are biased in reverse direction or zero biased.

2. Display arrangement according to claim 1, wherein the display arrangement is configured to operate, -while operating the first subset in the sensing mode- a second subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and second μLEDs of the second subset are biased in reverse direction or zero biased and the third and fourthμLEDs of the second subset are substantially not biased in forward direction or biased less than the threshold voltage; wherein optionally pixels of the second subset of at least some of the plurality of pixels are located adjacent to the first subset.

3. Display arrangement according to any of the preceding claims, wherein the display arrangement is configured to operate a second subset of at least some of the plurality of pixels in the sensing mode, in which one of the third and fourthμLEDs of the second subset are biased in forward direction; and at least one of the first and second μLEDs of the second subset are biased in reverse direction or zero biased; and wherein -when operating the second subset of at least some of the plurality of pixels in the sensing mode-, operating first subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and secondμLEDs of the first subset are biased in reverse direction or zero biased and the third and fourthμLEDs of the first subset are substantially not biased in forward direction or biased less than the threshold voltage.

4. Display arrangement according to any of the preceding claims, wherein the display arrangement is configured to operate pixels of the first subset to form a regular pattern, particularly a chess pattern; and / or the display arrangement is configured to operate pixels of the second subset to form a regular pattern, particularly a chess pattern.

5. Display arrangement according to any of the preceding claims, wherein the display arrangement is configured to operate the first and second subsets alternatively in the sensing mode and the adjusted sensing mode.

6. Display arrangement according to any of the preceding claims, further comprising a color filter having a large transmittance in the green spectrum arranged over one of the first and second μLED, said one of the first and second μLEDbiased in reverse direction or zero biased in the sensing mode or the adjusted sensing mode.

7. Display arrangement according to claim 6, wherein a transmittance of the color filter is larger than 60% for light of a wavelength between 512 nm and 565 nm, and particularlylarger than 70 % for light of a wavelength between 520 nm and 560 nm and particularly larger than 75 % for light of a wavelength of 540 nm; and / or a transmittance of the color filter is less than 30% for light of a wavelength above 600 nm and below 475 nm, and particularly less than 20 % for light of a wavelength less than 450 nm.

8. Display arrangement according to claim 6 or 7, wherein the color filter is arranged on a light emission surface of the respective first and second μLEDs; and / or the color filter is arranged on a glass cover arranged above light emission surfaces of the respectiveμLEDs ; and / or an area of the color filter is larger than an emission area of the μLEDassociated with the color filter.

9. Display arrangement according to any of the preceding claims, further comprising: a control device coupled to the pixel array for operating the at least some of the plurality of pixels.10.Display arrangement according to any of the preceding claims, wherein the first and second μLED in each of the at least some of the plurality of pixels are separated by one of the third and fourth μLED.11.Display arrangement according to any of the preceding claims, wherein threeμLEDs in each of the at least some of the plurality of pixels are substantially arranged along a virtual line.12.Display arrangement according to any of the preceding claims, wherein the first and second μLEDcomprise an external quantum efficiency in reverse bias of larger than 0.15 for light of a wavelength between 500 nm and 535 nm and between 575 nm and 620 nm; and particularly larger than 0.2 for light of a wavelength between 500 nm and 535 nm and larger than 0.14 for light of a wavelength between 480 nm and 620 nm.13.Display arrangement according to any of the preceding claims, further comprising: a glass cover arranged above light emission surfaces of the respectiveμLEDs ; and optionally a contrast enhancement layer arranged on the glass cover facing away from the light emission surfaces.14.Method of operating a display arrangement comprising a plurality of pixels, each having: twoμLEDs configured to emit light in the red spectrum; a third μLEDconfigured to emit light in the blue spectrum; a fourth μLED configured to emit light in the green spectrum; wherein the first and the second μLED are separated by one of the third and fourth μLED; the method comprising the steps of: operating during a first time period the at least some of the plurality of pixels in an emission mode, in which at least one of the first and second μLED, the third μLEDand fourth μLEDare biased in forward direction; and operating during a second time period a first subset of at least some of the plurality of pixels in a sensing mode, in which one of the third and fourthμLEDs of the first subset are one of unbiased or biased in forward direction; and at least one of the first and second μLEDs of the first subset are biased in reverse direction or zero biased.15.Method according to claim 14, wherein, - while operating the first subset in the sensing mode-, operating a second subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and secondμLEDs of the second subset are biased in reverse direction or zero biased and the third and fourthμLEDs of the second subset are substantially not biased in forward direction or biased less than the threshold voltage.16.Method according to claim 15, wherein pixels of the second subset of at least some of the plurality of pixels are located adjacent to pixels of the first subset.17.Method according to claim 14, further comprising the steps of: operating a second subset of at least some of the plurality of pixels in the sensing mode, in which one of the third and fourthμLEDs of the second subset are biased in forward direction; and at least one of the first and second μLEDs of the second subset are biased in reverse direction or zero biased; and wherein -when operating the second subset of at least some of the plurality of pixels in the sensing mode-, operating the first subset of at least some of the plurality of pixels in an adjusted sensing mode, in which at least one of the first and second μLEDs of the first subset are biased in reverse direction or zero biased and the third and fourthμLEDs of the first subset are substantially not biased in forward direction or biased less than the threshold voltage.18.Method according to any of claims 14 to 17, wherein the step of operating the first subset in sensing mode and / or the second subset in sensing mode comprises:- operating the first subset such to form a regular pattern, in particular a chess pattern; and / or operating pixels of the second subset to form a regular pattern, particularly a chess pattern; and / or operating the first subset such that each pixel of the first subset is surrounded by at least two pixels of another subset.19.Method according to any of claims 14 to 18, operating the first and second subsets alternatively in the sensing mode and the adjusted sensing mode.20.Method according to any of claims 14 to 19, wherein one of the first and second μLEDsof the first and / or second subset biased in reverse direction or zero biased in sensing or adjusted sensing mode comprises a color filter covering its emission surface, said color filter having a large transmittance in the green spectrum arranged over one of the first and second μLED, said one of the first and second μLED biased in reverse direction or zero biased in the sensing mode or the adjusted sensing mode.21.Method according to any of claims 14 to 20, comprising the steps of: detecting a photo current of the first and second μLEDs of the first and / or second subsets and - deriving fingerprint information therefrom;22.Method according to claims 21, comprising the steps of: deriving a PPG signal from the detected photocurrent; comparing the PPG signal with a threshold and in response to the comparison deriving fingerprint information therefrom.23.Use of a display arrangement according to any of claims 1 to 13 as: an in-display fingerprint sensor and an in-display PPG sensor.

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