Display arrangement and method for operating a display arrangement

Abstract

The invention concerns a display arrangement (1) comprising a pixel array with a plurality of pixels (10), at least some of the plurality of pixels each comprising first and a second μLED for light in the red spectrum, a third μLED for light in the blue spectrum and a fourth μLED for light in the green spectrum. The first and the second μLED are separated by one of the third and fourth LED. A control unit (13) is configured to operate the at least some of the plurality of pixels in a display mode, in which at least one of the first and second µLED, the third μLED and fourth μLED are biased in forward direction; and further configured to operate a first subset of at least some of the plurality of pixels in a sensing mode, in which the fourth μLED of the first subset are biased in forward direction; and pixels of a second subset partially adjacent to the pixels of the first subset in an adjusted sensing mode, in which at least one of the first and second μLEDs of the second subset are in reverse direction.

Description

[0001] DISPLAY ARRANGEMENT AND METHOD FOR OPERATING A DISPLAY ARRANGEMENT

[0002] The present application claims priority of DE patent application No DE 10 2023 124 916 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 pLEDs and a method for operating a display arrangement . The present invention also concerns a fingerprint application for such display .

[0003] BACKGROUND

[0004] Modern mobiles provide a high-resolution, dynamic , and intuitive display technology, which not only presents information with exceptional clarity, but also facilitates user interaction .

[0005] 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 pLEDs , in which each pixel comprises at least three subpixels . Each subpixel comprises at least one pLED responsible for emitting light in one of the green, blue and red portion of the visible spectrum, also referred to as green pLED, blue pLED and red pLED . Those pLEDs are often configured together with a second, redundant , pLED in order to avoid the need of repairing or replace damaged or non-f unctional pLEDs .

[0006] The expression "pLEDs" refers to an optoelectronic device that comprises a size with less than 50 pm in diameter, and particularly between 2 pm and 25 pm, although these specifications may vary . pLEDs 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 pLEDs 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, like PPG measurements as well as scanner of pictures or other obj ects being placed in front of the displays . 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 .

[0007] Current technologies , particularly in the area of fingerprint and PPG measurements have some drawbacks , when such sensors utilize optical information . Particularly, many conventional technologies suffer from a relatively small resolution, which can be cumbersome to correctly identify the fingerprint or more generally fulfil the desired functionality . Likewise for PPG measurements , the conventional implementations are rather inflexible and not necessarily suited in varying environments , e . g . when a user' s finger is placed somewhere on a larger display with multipurpose functionality or moving throughout the measurement .

[0008] It is therefore an obj ect 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 .

[0009] SUMMARY OF THE INVENTION

[0010] This and other obj ects are addressed by the subj ect matter of the independent claims . Features and further aspects of the proposed principles are outlined in the dependent claims .

[0011] The inventors propose a new bi-directions display arrangement , in which optoelectronic devices forming subpixels of a plurality of pixels , said subpixels implemented as pLEDs 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 forward direction, such that they emit light . However, it has been found that pLEDs and optoelectronic devices can also be operated as photodetectors , when being operated with a zero bias or a reverse bias . Consequently, when biased in reverse direction, these pLEDs , 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 j ust reading out any photo current . Hence pixels , subpixels or pLEDs switched into zero bias or reverse bias are used as detectors .

[0012] The inventors now propose to implement the pixels with the individual subpixels and its pLEDs 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 .

[0013] For example , one may use the pLEDs for green light emission of some pixels in emission mode , while operating pLEDs for red light in other pixels in sensing mode , thereby implementing a fingerprint or vital signs monitoring detector . Utilizing a RGBR scheme for the respective pixels , one can not only effectively double the current display resolution for such vital signs monitoring applications , but also utilize it in a more flexible way, improving on the signal-to-noise ratio by evaluating the captured light from pLEDs arranged at various distances .

[0014] 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 pattern can provide a compensation in case of dirt or dust on the display or the obj ect to be scanned, i . e . during a PPG measurement . Furthermore , this PPG measurement principle can be combined with a fingerprint sensing technique to evaluate possible fraud signals . 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 adj usted RGBR configuration . Such a configuration is characterized by two pLEDs configured to emit light in the red spectrum; a third pLED configured to emit light in the blue spectrum; and a fourth pLED configured to emit light in the green spectrum. The first and the second pLED are separated by one of the third and fourth pLED .

[0015] In accordance with the proposed principle , the display arrangement comprises a control unit . The control unit is configured to operate 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 a forward direction . The control unit of 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 pLEDs of the first subset are biased in a forward direction . The control unit is configured to operate pixels of a second subset , said pixels being at least partially adj acent to the pixels of the first subset in an adj usted sensing mode , in which at least one of the first and second pLEDs of the second subset are biased in reverse direction, and the third and fourth pLEDs are either biased below the threshold voltage , unbiased or in biased in reverse direction .

[0016] The expression "unbiased" means that those pLEDs 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 pLEDs are effectively switched off .

[0017] The control unit is further configured to record signals from the at least one of the first and second pLEDs of the second subsets during the adj usted sensing mode , said signals indicative of a photo current ; and to provide a Fourier transformation, e . g . a Fast Fourier Transformation ( FFT ) , from at least some of the recorded signals to obtain a PPG signal therefrom . The FFT obtains the frequency spectrum from the time series of the recorded signal . The display according to the proposed principle implements a PPG sensor or more generally vital sign monitoring by using reverse biased pLEDs . The in-display sensor requires only one red pLED per pixel to reach a sufficient resolution for the PPG measurement . However, with an additional red pLED, the display arrangement can reach a resolution of more than 400ppi , effectively doubling the display resolution . This allows for minimal additional costs in a standard display manufacturing process , a substantial increase in flexibility and offers a wide range of dual use applications and their combinations .

[0018] Additionally, structured illumination is used to increase the signal- to-noise ratio or SNR in vital sign monitoring . Both red pLEDs in a pixel at various distances from the light source are used for sensing . A structured illumination is used to reach deeper layers of the s kin and therefor increase the signal-to-noise ratio . In the fingerprint , the patterned illumination allows preventing crosstalk . Also , in fingerprint , the pattern / anti-pattern illumination is needed to get the full image . A structured illumination scheme can be used to account and compensate in-plane cross talk from the illuminating green pLED to the sensing red pLEDs and to increase the signal-to-noise ratio . Likewise , PPG measurement can be utilized to detect fraud during measurements of other information, i . e . fingerprint sensing . Different measurements patterns for such applications are possible .

[0019] The display arrangement comprises a control unit coupled to the pixel array for operating the at least some of the plurality of pixels . The pLEDs can be attached to a backplane , said backplane including the control unit as well as current and voltage sources to supply each pixel and subpixel individually .

[0020] The display arrangement can be configured to execute certain illumination patterns , that is operating certain pixels in sensing mode while also executing corresponding adj usted 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 pLED configured to emit green light , while utilizing the first and second pLEDs in said pixel in reverse bias direction as well as adj acent or neighboring in an adj usted sensing mode , in which those pLEDs are operated in reverse bias direction acting as detectors for green light reflected at a user' s finger .

[0021] 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 adj usted sensing mode , in which at least one of the first and second pLEDs of the first subset are biased in reverse direction . For the pixels operated in adj usted sensing mode , the third and fourth pLEDs of the second subset are substantially not biased in forward direction or biased less than the threshold voltage .

[0022] In some aspects , the display arrangement is configured to operate the second subset of at least some of the plurality of pixels in the sensing mode , in which one of the third and fourth pLEDs of the second subset are biased in forward direction; and at least one of the first and second pLEDs of the second subset are biased in reverse direction or not biased at all ; 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 adj usted sensing mode , in which at least one of the first and second pLEDs of the first subset are biased in reverse direction and the third and fourth pLEDs of the first subset are substantially not biased in forward direction or biased less than the threshold voltage .

[0023] 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 emission mode is significantly smaller than pixels operating in 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 for red light of said pixels are either switched off or in reverse biased direction and other pixels in said area are controlled to operate in adj usted sensing mode .

[0024] 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 adj usted sensing mode .

[0025] Some aspects concern the arrangement of the various pixels , and more particularly the operation of those pixels . As stated, pixels are operated in sensing and adj usted sensing mode , thus implementing some form of detector arrays with photodetectors arranged at various distances from the source . In some aspects , pixels of the second subset ( those with are operated in adj usted sensing mode ) are arranged around a centrally located pixel of the first subset . The pixels of the second subset can be arranged in the form of a square or rectangle . In some aspects , the pixels of the second subset from a plurality of squares centrally arranged around one or more pixels of the first subset . Each pixel of a certain square comprises the same distance from the center . Consequently, this approach enables a substantial symmetrical detection of light reflected from a user at various distances .

[0026] Such an approach is useful , as it has been found that a PPG signal has a strong dependence on the distance between the detector and the light source . By evaluating the signal from various pixels at different distances from the light source , one can find a suitable signal , e . g . with a large signal-to-noise ratio .

[0027] In some aspects , two pixels of the first subset are separated by at least two pixels of the second subset . If the pixels are arranged in rows and columns , the number of pixels separating two pixels of the first subset can vary in the two different directions . More generally speaking , the number of pixels of the second subset separating two pixels of the first subset per row can be different from the number of pixels of the second subset separating two pixels of the first subset per column . A pattern of pixels of the first subset and the second subset , respectively, may comprise in some instances a periodicity in a first direction and a second periodicity in a second direction .

[0028] Different periodicity is suitable as they allow optimization of illumination and detection patterns depending on the actual application . For example , one may use the first periodicity for a PPG signal and the second periodicity for a different vital sign monitoring signal or another application . In some aspects , a pixel of the first subset is surrounded by one of eight , 16 or 24 and 32 pixels of the second subset . As a result , and in some instances , the number of pixels in the second subset exceeds the number of pixels of the first subset by at least a factor of four .

[0029] Not all pixels of the first or second subset are operated at once in some instances . For example , the display arrangement and the control unit can be configured to operate pixels of the first subset can be intermittently . This reduces stray light from those pixels . In addition, pixels of the first subset can be operated to form a timely varying pattern . The timely varying pattern can be used to further improve the signal-to-noise ratio by evaluating stray light from pixels very close or very far away from the actual light source .

[0030] Some aspects concern the control unit . In some instances , the control unit is configured to obtain at least one parameter indicative of a signal strength, in particular a signal / noise ratio in dependence of a distance between pixels of a first subset and pixels of the second subset recording the signal . It has been found that the PPG signal strongly correlates with the distance between the photodetector and the light source . It is therefore suitable , if the control unit is configured, to associate recorded signals from pixels of the second subset with a distance between the first subset and the pixel of the second subset recording the signal .

[0031] As already stated, various pixels of the first subset can be operated intermittently . The control unit is therefore configured in some instances to operate a plurality of first subsets of at least some of the plurality of pixels at different times in sensing mode . Likewise , the control unit can be configured to operate pixels of the first subset at different times in sensing mode . In this regard, the control unit operates the first subset in the adj usted sensing mode after operating the first subset in the sensing mode and operates some pixels of the second subset in the sensing mode after operating those pixels in an adj usted sensing mode .

[0032] In some aspects , the control unit is configured to obtain information about the location of the user' s finger, or any other obj ect being placed on the display arrangement and select pixels of the first and second subsets accordingly .

[0033] In some aspects , the respective pLEDs are evenly distributed across the display arrangement area or a portion thereof . In some other aspects , the pLEDs 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 , pLEDs of each pixel are located on the edges of a virtual rectangle . Likewise , in some aspects , the pLEDs are arranged in a diamond like shape . As some application can make use of and benefit from the pLEDs configured to emit light in the red portion of the visible spectrum, these pLEDs should be placed spaced apart . Consequently, those pLEDs are separated by at least one of the green and the blue pLED . This distance may effectively increase the resolution for such pLEDs when being operated in sensing mode , suitable for example in fingerprint or vital signs monitoring applications .

[0034] Some aspects concern the utilization of pLEDs as light emitters and photodetectors . In some aspects , the first and second pLED 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 pLEDs 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 pLEDs are used as photodetectors .

[0035] In some aspects , it is suitable to ensure , that at least one of the first and second pLED (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 pLED, said one of the first and second pLED biased in reverse direction in the sensing mode or the adj usted sensing mode .

[0036] Some aspects concern the color filter covering the surface of one of the first and second pLEDs . 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 pLED configured to emit light in the red portion of the visible spectrum, the pLED can be used as a detector for green light when operated reversely biased .

[0037] 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 .

[0038] Different embodiments for arranging the color filter above the respective pLEDs are possible . In some aspects , the color filters are arranged on a light emission surface of the respective first or second pLEDs . 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 pLEDs . In some aspects , an area of at the color filters is larger than an emission area of the pLED associated with the color filter . This will prevent stray light from reaching the pLED' s surface when it is operated in sensing mode . In this regard, the pLED' s can be embedded in recesses of a backplane surface . A transparent cover can be arranged on top of the recesses , thereby forming an even surface . The backplane may comprise respective contact areas in the recesses to contact the pLEDs arranged therein .

[0039] In some aspects , the display arrangement further comprises a glass cover arranged above light emission surfaces of the respective pLEDs . 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 .

[0040] Some other aspects concern a method for operating a display arrangement comprising a plurality of pixels . Each pixel comprises two pLEDs configured to emit light in the red spectrum, Furthermore , the pixels also comprise a third pLED configured to emit light in the blue portion of the spectrum and a fourth pLED configured to emit light in the green portion of the visible spectrum.

[0041] 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 pLED, the third pLED and fourth pLED 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 the fourth pLEDs of the first subset is biased in a forward direction . Pixels of a second subset however , said pixels arranged at various distances from pixels of the first subset are biased in reverse direction and in an adj usted sensing mode . Said pixels of the second subset comprise at least one of the first and second pLEDs being biased in the reverse direction . In accordance with the proposed principle , signals from the at least one of the first and second pLEDs of the second subsets during adj usted sensing mode are recorded, said signals indicative of a photo current ; and subsequently processed to obtain a PPG signal therefrom. The proposed method therefore facilitates to record photo current signals from pLEDs arranged at various and different distances from the pixels of the first subset , that is from the light source . As a result , the signal-to-noise ratios at various distances can be obtained to optimize the measurement .

[0042] In some instances and during a third time period, the second subset of at least some of the plurality of pixels is operated in the sensing mode , in which one of the third and fourth pLEDs of the second subset are biased in forward direction; and optionally at least one of the first and second pLEDs of the second subset are biased in reverse direction; during the third time period, the first subset of at least some of the plurality of pixels is operated in an adj usted sensing mode , in which at least one of the first and second pLEDs of the first subset are biased in reverse direction and the third and fourth pLEDs of the first subset are substantially not biased in forward direction or biased less than the threshold voltage .

[0043] The distance between the pLEDs acting as photodetectors may vary and can range from the pLEDs in the same pixels to pLEDs in adj acent pixels and even further away . For example , in some instances pixels ( and their respective pLEDs ) operated in adj usted sensing mode as photodetectors are at least two pixels away from a pixel , whose pLED 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 .

[0044] 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 adj usted sensing mode . It is also possible to operate some pixels of the second subset in the sensing mode after operating those pixels in an adj usted sensing mode . In such case , pixels of the first subset may then be operated in adj usted 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 adj usted sensing mode .

[0045] 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 adj usted sensing mode .

[0046] In some aspects , the first subset and / or the second subset are operated such that they respectively form a regular pattern, in particular a chess pattern . In some aspects , the pixels of the first subset operated in sensing mode are surrounded by eight , 16 , 24 or 32 pixels of the second subset operated in adj usted sensing mode . In some aspects , pixels of the first subset are operated such that each pixel of the first subset is surrounded by at least two pixels of another subset .

[0047] In some aspects , pixels of the first subset are operated with a first periodicity in a first direction and a second periodicity in a second direction .

[0048] The proposed displays as well as the proposed method require an RGBR configuration with two pLEDs configured to emit light in the red portion of the visible spectrum. Depending on the implementation, pixels may contain a redundant pLED for the red, green or blue . Those can be reutilized 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 pLEDs have a sufficiently high quantum efficiency for capturing photons in the green spectrum. However , the pLEDs 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 second pLEDs . Said color filter comprises a large transmittance in the green spectrum arranged over one of the first and second pLED, said one of the first and second pLED biased in reverse direction in the sensing mode or the adj usted sensing mode .

[0049] Some further aspects concern the subsequent process steps to obtain a PPG signal . In some aspects , the method further comprises the steps of detecting a photo current of the first and second pLEDs of the first and / or second subsets and deriving PPG information therefrom. Likewise , in some instances , a first photo current is recorded from at least one pLED of the second subset at a first distance from pixels of the first subset and a second photo current is recorded from at least one pLED of the second subset at a second distance from pixels of the first subset . Both, first and second photo currents can be compared and a PPG signal derived therefrom . This aspect deals with the observation that the portion of reflected light at various distances originates from various depth of a user' s skin . Hence , the recorded signals can be processed associated with different distances from pixels operated in sensing mode .

[0050] In some aspects , the PPG signal is sued to trigger and monitor other applications . For example , in some aspects , the PPG signal is compared with a threshold and in response to the comparison, fingerprint information is also derived from the detected photocurrent . In other words , the PPG signal is used to check the validity of the fingerprint information, e . g . if the information is obtained from living tissue or not . Some aspects relate to rdentrf ication of the user' s frnger prior to performing such measurement . In some aspects , a coverage of a user' s finger on an area of a display is detected . This can be done by rapidly scanning through the display and looking for spatially dependent a strong decrease in the photo current . Such a strong decrease indicates coverage , as the user' s finger will block the light in such an area . In response to detecting a coverage , pixels in said area can be selected pixels of the first subset . Other pixels in said area are selected as pixels of the second subset .

[0051] SHORT DESCRIPTION OF THE DRAWINGS 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

[0052] Figure 1 shows a first embodiment of a display arrangement in accordance with some aspects of the proposed principle ;

[0053] Figure 2 illustrates a second embodiment of a display arrangement in accordance with some aspects of the proposed principle ;

[0054] Figure 3 shows a third embodiment of a display arrangement in accordance with some aspects of the proposed principle ;

[0055] Figure 4 illustrates a wavelength efficiency diagram for light in the green spectrum to outline some aspects of the proposed principle ;

[0056] Figure 5 shows a wavelength-efficiency diagram for light in the blue spectrum to outline some aspects of the proposed principle ;

[0057] Figure 6 illustrates a wavelength-efficiency diagram to outline some aspects of the proposed principle ;

[0058] Figure 7 illustrates a first embodiment of a pixel in accordance with some aspects ; Figure 8 shows a second embodiment of a pixel in accordance with some aspects ;

[0059] Figures 9A to 9C show embodiments of a pixel in display or sensing mode in accordance with some aspects ;

[0060] Figure 10 illustrates another embodiment of a pixel in a display and sensing mode in accordance with some aspects ;

[0061] Figure 11 shows a wavelength quantum efficiency diagram for pLEDs covered by a color filter in accordance with some aspects ;

[0062] Figure 12 illustrates a first embodiment of a plurality of pixels including an arrangement of their respective pLEDs in accordance with some aspects ;

[0063] Figures 13 to 15 illustrate further embodiments of a plurality of pixels including an arrangement of their respective pLEDs in accordance with some aspects ;

[0064] Figure 16 shows a wavelength-transmittance diagram for different color filters in accordance with some aspects of the proposed principle ;

[0065] Figure 17 illustrates some aspects of measuring a PPG signal in a display arrangement indicating some aspects of the proposed principle ;

[0066] Figures 18A and B show some aspects of measuring a PPG signal in a display arrangement indicating some aspects of the proposed principle ;

[0067] Figure 18C shows a power spectrum for a PPG measurement obtained with the proposed method .

[0068] DETAILED DESCRIPTION

[0069] 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 .

[0070] 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 .

[0071] Figure 1 illustrates a display arrangement in accordance with some aspects of the proposed principle .

[0072] The display arrangement 1 comprises an array of pixels 10 , which are arranged in rows and columns on a respective substrate or backplane 12 . The pixels 10 each comprise a plurality of pLEDs capable of generating light in various portions of the visible spectrum . For the purpose of simplicity, a pLED or optoelectronic device configured to emit light in the red portion of the visible spectrum is referred to as a red pLED . Likewise , a green pLED is an optoelectronic device configured to emit light in the green portion of the visible spectrum and a blue pLED is configured to emit blue light in operation thereof .

[0073] In this particular case , each pixel 10 comprises pLEDs in an RGBR configuration in accordance with the proposed principle . That is , two pLEDs are configured to emit red light , and one pLED each is configured to emit blue light as well as green light , respectively . The individual pLEDs can either be integrated and embedded into backplane 12 using monolithic integration, or be attached to backplane 12 using certain bonding techniques . Backplane 12 comprises a plurality of integrated circuits , which are configured to control the respective pLEDs in each of the pixels 10 individually to act as emitter and partially as detectors in accordance with the proposed principle .

[0074] The individual pixels and pLEDs are connected to a control unit 13 via the backplane 12 . Control unit 13 can also be integrated into backplane 12 . Control unit 13 is configured to supply the necessary current signals to individually address the pLEDs 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 .

[0075] At least some of the pixels 10 , their subpixels and respective pLEDs 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 obj ect arranged directly in front of the display arrangement and derive information from those captured signals using various processing techniques .

[0076] In such application, various pixels are used to illuminate an obj ect , while other pixels in the vicinity are utilized as photodetectors to detect and capture the light portions of the emitted light reflected at an obj ect opposite the various pixels . Apart from scanning the obj ect , 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 . The latter 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 .

[0077] 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 the pLEDs configured to emit light in the red spectrum is covered by a red color filter . Likewise , the second pLED 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 second pLEDs is transparent for light within the green spectrum . The respective transmission curves for the color filters are explained in detail a further below .

[0078] The functionality of such pixel 10 is now as follows . In the emission mode of the display arrangement , the control unit controls the pLEDs 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 pLED for red light is switched off in this mode . The control unit 13 controls the mixing of light with the respective portions and controls the on-off ratio of the pLEDs . 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 .

[0079] In a second mode of operation, various pLEDs are reversely biased, thereby acting as photodetectors . More particularly, at least some of the first pLEDs configured to emit red light with the red color filter placed upon the emission surface are switched in a reverse bias mode to act as photodetectors . Due to the red color filter , these pLEDs only detect light within the red spectrum . Finally, some of the second pLEDs covered by the green color filter are switched into reverse bias mode to act as a photodetector of green light , as those pLEDs 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 third pLEDs configured to emit green light are partially switched on in the sensing mode to emit green light . The light illuminates an obj ect placed on the cover of the display arrangement , like a user' s finger . A portion of the light is reflected towards the other pLEDs and captured by the pLEDs . Due to the red color filter and the pLED for blue light being relatively insensitive to such portion of the spectrum, the signals of those pLEDs can be used to compensate for possible noise . The second pLED covered with the green color filter captures the reflected light . Since the reflected light varies with the heartbeat , the signal change over time can be processed to obtain heartbeat and other information . Moreover , it has been found that reflection of green light is dependent on the distance between the location of the light source and the location of the measurement . Hence , at certain distances the signal- to-noise ratio is large while at other distances the ratio deteriorates . However, the optimal distance is often individual and may depend on the s kin type , s kin irregularities , pressure of the finger onto the display glass and other aspects .

[0080] The control unit may now obtain various measurement with pLEDs placed at different distances from the first pLEDs as well as different measurement pattern to obtain an optimal signal-to— noise ratio . Those measurements over a certain period of time , e . g . several seconds , are then transformed by a Fast Fourier Transformation to obtain the signal spectrum . The location of the various maxima, its distance towards each other and their respective phases can be used to obtain a PPG signal and provide other vital sign monitoring .

[0081] 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 pLED now 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 pLED covered by a green color filter is dual used, as a display and as a fingerprint . Moreover, in some instances , the first pLED 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 pLED .

[0082] 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 and a PPG measurement is performed .

[0083] 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 respective pLEDs . 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 green pLEDs are used for emission to illuminate the finger to be scanned, while other portions of pixels are operated in an adj usted sensing mode .

[0084] 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 adj usted 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 pLED emits green light , while the blue pLED is switched off and the pLED suitable to emit red light are switched into reverse bias direction . The emitted green light is reflected from the obj ect being located directly above the display arrangement and is detected by the red subpixels of first portion Pl of pixels at various distances from the emitting pixel P2 . These subpixels of portion Pl are operated in reverse bias direction, acting as respective photodetectors .

[0085] 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 obj ect 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 adj usted sensing mode ( that is the red pLEDs are biased in a reversed bias operation while the green and blue pLED 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 green light .

[0086] Moreover , in Figure 2 , the pixels P2 are emitting green light not all at once but sequentially, i . e . each pixel for a coupled of a few seconds . Alternatively, only every second or third row of pixels P2 emit light during a first time period . During a subsequent second time period, the row changes , thus that rows of pixel P2 emit green light in an alternating manner . Likewise , pixels P2 not emitting during a certain time period can be switched by the control unit into adj usted sensing mode .

[0087] Such switch between sensing and adj usted 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 , s kin irregularities and the like can be recognized and subsequently corrected . Capturing the reflected light at various distances enables finding the maximal reflected signal and improves a signal-to-noise ratio .

[0088] 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 adj usted 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 captured photocurrent to obtain a good scan of the obj ect arranged above the display . The pattern can change and "move across the display" similar to conventional scanners

[0089] Figure 3 illustrates another possibility of such a configuration . In this embodiment , a first portion Pl of pixels is set into adj usted sensing mode . Each direct adj acent pixel of such pixel portion Pl is either switched off or set into adj usted 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 adj usted sensing mode can alternate as well , whereas pixel portions Pl are subsequently set into sensing mode , while pixel portions P2 are switched into the adj usted 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 adj usted sensing patterns are possible for the display arrangement in accordance with the proposed principle to provide the necessary application .

[0090] In both embodiments , the control unit 13 captures the signal and processes the detected photo current in accordance with the above- mentioned principle .

[0091] 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 adj usted 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 .

[0092] As stated before , the proposed principle lies on the dual use of the respective optoelectronic devices and pLEDs , both for light emission and light detection . Consequently, the pLEDs 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 pLEDs based on gallium nitride used for blue and green emission, as well as the phosphide or arsenide material system used for red emission .

[0093] Figure 4 illustrates a wavelength-quantum efficiency diagram for a pLED configured to emit green light with the wavelengths between 500 nm and 550 nm . This is given by straight curve KI . The base material system for such a pLEDs is usually based on gallium nitride .

[0094] However, as illustrated, for pLEDs 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 pLEDs 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 pLEDs in reverse bias mode within that particular range of the spectrum is about 0 . 2 , which is sufficient to act as photodetectors . Consequently, pLEDs 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.

[0095] Figure 5 illustrates a similar example for pLEDs configured to emit light in the blue range of the visible spectrum around 450 nm. Straight curve KI illustrates their emission spectrum . However, pLEDs 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 pLEDs configured to emit blue light . Still , this allows to use pLED 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 .

[0096] Figure 6 finally illustrates the initial spectrum of a red, green and blue light pLEDs , see straight curves KI 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 pLEDs work as photodetectors .

[0097] As shown, the pLEDs 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 pLEDs . The pLEDs configured to emit green light could be used in reverse bias mode to detect light within the blue spectrum, although the respective pLEDs 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 . pLEDs 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 pLED based on phosphide can also be used to detect green . Hence , in a pixel one may provide two pLEDs configured to emit red light , wherein one of those pLEDs 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 pLED 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 for PPG measurements , but also allows for a combination of such measurement with other application without losing spatial resolution . The second red color filter may not be necessary, as one can correct a detected signal in those pLEDs with the photo signal obtained by the pLEDs covered with a green filter .

[0098] In this regard, Figure 11 illustrates the wavelength-quantum efficiency diagram for pLEDs used as a photodetector for blue light , as well as two a pLEDs 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 pLEDs 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 pLEDs in each pixel separately, as indicated already in the first three embodiments . The arrangement and structure of the respective pixels , including the pLEDs associated with the pixel , are illustrated in Figure 7 to Figure 10 .

[0099] Figure 7 illustrates an arrangement of pLEDs , representing subpixels in a pixel 10 in a diagonal arrangement . In total , each pixel 10 consists of five pLEDs , wherein two of the pLEDs 103 used as respective detectors . More particularly, pixel 10 comprises a first pLED l OOr configured to emit red light , a second pLED 100g configured to emit light within the green spectrum, as well as pLED 100b configured to emit light within the blue spectrum.

[0100] The three pLEDs lO Or , 100g and 100b are arranged along a virtual line , being separated by a certain distance . Between the red pLED and the green pLED as well as between the green pLED and the blue pLED, 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 pLEDs 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 adj acent or neighboring pixels can be operated in reverse bias mode , in which the pLEDs l OOr, 100g and l OObe are not operated . Thus , the direct incident light is limited, and the photodetectors will detect light being reflected from an obj ect .

[0101] Figure 7B illustrates a further embodiment , in which an infrared light emitter 102 is arranged in each pixel . This will allow illuminating an obj ect 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 .

[0102] Figure 8 illustrates two pixels , each of those having three pLEDs lO Or , 100g , and 100b configured to emit respective colored light as well as three p-photodetectors 103 . The photodetector can be selectively activated or deactivated during emission of the pLEDs allowing to detect portions of the emitted light reflected from an obj ect . Similar to the previous embodiment , the photodetectors as well as the pLEDs can be arranged in a virtual diagonal line through the respective pixels 10 .

[0103] Figure 9A illustrates a slightly different arrangement , in which dedicated p-photodetectors 104 are located in each pixel . These discrete p-photodetectors can either be placed separately or be embedded into the backplane of the main display during the manufacturing of the display arrangement . In some aspects , the photodetectors are mass transferred onto the display backplane , similar to the transfer process of the pLEDs . The p-photodetectors 104 are suited to detect mainly light within a certain range of the visible spectrum, for example , the green range .

[0104] This will allow to further implement certain applications like PPG measurement in combination with fingerprint sensing with a high degree of resolution . Consequently, the p-photodetectors can be based on silicon, Si , arsenide , phosphide or other materials different from the materials used for the pLEDs . Figure 9B and 9C illustrate yet another example for fingerprints or a vital parameter sensing like PPG measurements . The respective pixels and pLEDs are operated in an emission mode in Figure 9B . pLEDs lO Or , 100g as well as 100b are supplied in forward direction, while the second pLEDs configured to emit red light lO lr is switched off . Due to the fact that usually such pixels require redundant pLEDs anyway, no significant complexity is added . In sensing mode , only the pLED 100g is emitting light while pLEDs lO Or and lO lr are switched into reverse bias direction .

[0105] 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 pLED 100b is switched off and the green LED 100g is operated in a forward mode to emit light in the green spectrum .

[0106] The light is reflected from an obj ect , 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 the pLEDs lO Or and lO lr . Those pLEDs are switched in reverse bias direction to act as photodetectors for the reflected green light portion . Due to the fact that those pLEDs lO Or and l Olr 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 . The control unit utilizes the generated photo current and performs a fast Fourier transformation to obtain the frequency spectrum.

[0107] In some instances , the detector pLEDs l Olr 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 by pLEDs l OOr and l Olr in each pixel and / or adj acent pixels . Using a pixel arrangement with two respective pLEDs 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 lO Or ( and even lO lr ) can be used for normal display of light . Figure 10 illustrates a further example of such an arrangement , which is suitable for directing light when the respective pLEDs are operated in reverse bias direction, acting as photodetectors . In this configuration, pLEDs 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 .

[0108] A further pLED 100g is configured to emit green light and another pLED 100b is configured to emit blue light . Furthermore , a fourth pLED 101r' is configured to emit red light . Similar to pLED 100r' , this pLED 101r' comprises a color filter transparent for photons in the green range of the visible spectrum, covering the main emission surface of the respective pLED . During emission mode depicted in the right picture of Figure 10 , the pLEDs 100r' , 100g and 100b are operated in a normal forward bias direction that is acting as emitter for the respective pixel . The second pLED 101r' is usually switched off in such operation . In sensing mode , in which the different colors of an obj ect are to be scanned and detected, the pLED 100g for the green color mode is switched off , while the remaining pLEDs 100r' and 101r' as well as 100b are operated in reverse bias direction . In such operation, the respective pLEDs 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 obj ect being placed on the display arrangement to be scanned . Rather , natural light or if the obj ect emits light on its own can be detected by pLEDs 100b, 100r' and 101r' .

[0109] Depending on the various applications , the arrangement of pLEDs and subpixel in each pixel 10 can be optimized and suited for the respective display and directional operation mode .

[0110] Figure 12 to 15 illustrate various configurations and arrangements of subpixels in such a display . Each pixel contains four pLEDs in Figure 12 , to configured to emit light in the red spectrum, one pLED in each pixel is configured to emit light in the blue spectrum and one pLEDs configured to emit light in the green spectrum . These pLEDs are arranged along a virtual line , wherein the pLEDs 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 .

[0111] In a slightly different arrangement depicted in Figure 13 , each pixel comprises the shape of a rectangle of equal size . The respective subpixels and pLEDs in each pixel are arranged on the virtual diagonal line in each of those pixels . Similar to the previous embodiment , the red pLEDs are separated by a green pLEDs or blue pLEDs vis-a-vis the pixel borders . A different configuration is presented in Figure 14 , in which the subpixels and pLEDs are arranged in a rectangular form, with the two pLEDs l OOr and l Olr being separated by a large distance . Similar to the previous embodiment , the red pLED lO Or and lO lr are both separated by respective green or blue pLED .

[0112] Figure 15 finally illustrates a diamond shape configuration, in which the respective pLEDs and subpixels in each pixel 10 form a diamond like structure with the pLEDs l OOr and l Olr being arranged on a virtual horizontal line and the pixels 100g and 100b being arranged alternating on a respective virtual vertical line .

[0113] Figure 16 illustrates a wavelength-transmittance diagram of several color filters Fl , F2 and F3 usable and suited for the display arrangement in accordance with the proposed principle . The color filters Fl , 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 .

[0114] Figure 17 illustrates a PPT signal sensor diagram indicating that the sensor signal , that is the photo current , is increasing, and therefore a PPT signal strength is increasing with an increasing distance between the emitter and the respective sensor . That means that an increased distance of the pixels of the second subset being operated in adj usted sensing mode from pixels of the first subset being operated in sensing mode results in a stronger overall PPG signal . This can be explained by the right drawing of Figure 17 , indicating the penetration depth of the emitted light by pixel 10 into the skin layers . Light portions reflected directly at the top s kin layer usually reaches the detector 10 ' closest to the respective emitter 10 .

[0115] With increasing penetration depth into the various skin layers , the reflected light reaches detectors 10 ' ' and 10 ' ' ' , which are also distanced further away from the respective emitter . As blood plasma and blood cells are usually deeper below the skin surface , sensor 10 ' ' ' will most likely record a signal , which provides a good signal-to- noise ratio in regard to the PPG information . Depending on the overall thickness of the s kin layers , irregularities of the skin surface as well as the skin type and other parameters , it is useful to provide a plurality of photodetectors at different distances from the respective emitter . Consequently, the method proposes obtaining respective photo current from a plurality of pixels of the second subset , with pixels of said subset arranged at various distances from the respective emitter .

[0116] Figure 18A illustrates various arrangements in a top view similar to the previous embodiments . In some aspects , the two emitters can be arranged and separated by a single detector . By providing light to the skin surface , the central detector in a single unit can obtain the respective reflected portions from both emitters . This structure can be expanded into an array having a plurality of emitters 10 and detectors 10 ' implemented as various pixels oriented in a chess like pattern . In an alternative embodiment , various pixels in the sensing mode can also be arranged together as a full density emitter, which is subsequently surrounded by a plurality of pixels of the second subset shown in here . Alternatively, in the right drawing of Figure 18 , the emission areas arranged in a chess pattern can be surrounded by respective detectors . Hence , each emitter is surrounded by pixels of second subsets acting as photodetectors . Figure 18B illustrate in the side view the potential light portion and being reflected within the skin certain depth reaching the detector 10 ' . The embodiment illustrates that the detector is symmetrically arranged between the two photo emitters 10 , thereby providing a substantially asymmetrical signal

[0117] After the signals have been recorded, and pre-processed, a Fast Fourier Transformation or FFT is performed to provide the power spectrum illustrated in Figure 18C . The first maximum, approximately at 2Hz corresponds to the heartbeat ratio of the signal . By evaluating the phase correlation between the first maximum and the respective harmonics illustrated herein at approximately 4 Hz and 6 Hz , respectively, one can obtain additional information related to certain parameters and characteristics of the blood, from which the PPG information is obtained .

[0118] By changing the respective patterns of illumination and detection, that is the patterns of the first subset and the second subset , one can not only derive a good signal-to-noise ratio but also obtain additional information useful in other applications .

[0119] LIST OF REFERENCES

[0120] 1 display arrangement

[0121] 10 pixel

[0122] 10' , 10' ' adjacent pixel

[0123] 10' ' ' adjacent pixel

[0124] 12 backplane

[0125] 13 control unit lOOr subpixel, red pLED

[0126] 100r' subpixel, red pLED with color filter

[0127] 100g subpixel, green pLED

[0128] 100b subpixel, blue pLED lOlr subpixel red pLED

[0129] 101r' subpixel red pLED with color filter

[0130] 102 subpixel infrared pLED

[0131] 103 photodetector

[0132] 104 p-photodetector

[0133] 300 cover structure, glass cover

[0134] 301 contrast enhancement layer

[0135] 302 glass

[0136] 303 anti-reflective coating

[0137] Pl, Pl' pixel

[0138] P2 , P2' pixel

[0139] KI, K2 curves

[0140] K3, K4 curves

[0141] K5 , K6 curves

[0142] RL, FL reflected light portion

[0143] 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 pLED, each configured to emit light in the red spectrum; o a third pLED configured to emit light in the blue spectrum; o a fourth pLED configured to emit light in the green spectrum; wherein the first and the second pLED are separated by one of the third and fourth LED; a control unit electrically coupled to the at least some of the plurality of pixels ; wherein the control unit is configured to operate the at least some of the plurality of pixels in a display mode , in which at least one of the first and second pLED, the third pLED and fourth pLED are biased in forward direction; and wherein the control unit is configured to : o operate a first subset of at least some of the plurality of pixels in a sensing mode , in which the fourth pLED of the first subset are biased in forward direction; and o operate pixels of a second subset , said pixels being at least partially adj acent to the pixels of the first subset , in an adj usted sensing mode , in which at least one of the first and second pLEDs of the second subset are biased in reverse direction or zero biased; wherein the control unit is configured to record signals from the at least one of the first and second pLEDs of the second subsets during adj usted sensing mode , said signals indicative of a photo current ; and wherein the control unit is configured to provide a Fourier transformation from at least some of the recorded signals to obtain a PPG signal therefrom .2 . Display arrangement according to claim 1 , wherein the display arrangement is configured to operate during the sensing mode at least one of the first and second pLEDs of the first subset biased in reverse direction or zero biased .3 . Display arrangement according to any of the preceding claims , wherein pixels of the second subset are : arranged around a centrally located pixel of the first subset ; arranged in rows and columns , wherein two pixels of the first subset are separated by at least two pixels of the second subset ; arranged in a pattern that comprises a first periodicity in a first direction and a second periodicity in a second direction; arranged such that a pixel of the first subset is surrounded by one of four and eight pixels of the second subset .4 . Display arrangement according to any of the preceding claims , wherein the number of pixels in the second subset exceed the number of pixels of the first subset by at least a factor of four .5 . Display arrangement according to any of the preceding claims , wherein the control unit is configured to obtain at least one parameter indicative of a signal strength, in particular a signal / noise ratio in dependence of a distance between pixels of a first subset and pixels of the second subset recoding the signal .6 . Display arrangement according to any of the preceding claims , wherein control unit is configured to obtain recorded signals from pixels of the second subset , said recorded signal associated with a distance between the first subset and the pixel of the second subset recording the signal .7 . Display arrangement according to any of the preceding claims , wherein the control unit is configured to : operate a plurality of first subsets of at least some of the plurality of pixels at different times in sensing mode ; and / or operate the first subset in the adj usted sensing mode after operating the first subset in the sensing mode and operate somepixels of the second subset in the sensing mode after operating said pixels in the adj usted sensing mode .8 . Display arrangement according to any of the preceding claims , wherein the first and second pLED in each of the at least some of the plurality of pixels are separated by one of the third and fourth pLED; and / or three pLEDs in each of the at least some of the plurality of pixels are substantially arranged along a virtual line .9 . Display arrangement according to any of the preceding claims , wherein the first and second pLED comprise 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 .10 . Display arrangement according to any of the preceding claims , wherein a color filter comprising a large transmittance in the green spectrum is arranged over one of the first and second of the two pLEDs ; and wherein said color filter comprises : a transmittance 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; and / or a transmittance 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.11 . Display arrangement according to claim 10 , wherein the color filters are arranged on a light emission surface of the respective first and second pLEDs ; and / or wherein an area of the color filter is larger than an emission area of the pLED associated with the color filter .12 . Display arrangement according to any of the preceding claims , further comprising : a transparent cover that is arranged above the main emission surfaces of the plurality of pixels arranged in rows and columns , wherein the transparent cover comprises a plurality of color filters located opposite emission areas of one of the first and second pLEDs .13 . Method of operating a display arrangement comprising a plurality of pixels , each having : two pLEDs configured to emit light in the red spectrum; a third pLED configured to emit light in the blue spectrum; a fourth pLED configured to emit light in the green spectrum; wherein the first and the second pLED are separated by one of the third and fourth pLED; 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 pLED, the third pLED and fourth pLED are 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 the fourth pLED of the first subset are biased in forward direction; and o pixels of a second subset , said pixels being at least partially adj acent to the pixels of the first subset , in an adj usted sensing mode , in which at least one of the first and second pLEDs of the second subset are biased in reverse direction or zero biased; recording signals from the at least one of the first and second pLEDs of the second subsets during adj usted sensing mode , said signals indicative of a photo current ; and processing the recorded signals to obtain a PPG signal therefrom .14 . Method according to claim 13 , further comprising the steps of operating during a third time period :the second subset of at least some of the plurality of pixels in the sensing mode , in which one of the third and fourth pLEDs of the second subset are biased in forward direction; and optionally at least one of the first and second pLEDs of the second subset are biased in reverse direction or zero biased; and the first subset of at least some of the plurality of pixels in an adj usted sensing mode , in which at least one of the first and second pLEDs of the first subset are biased in reverse direction or zero biased and the third and fourth pLEDs of the first subset are substantially not biased in forward direction or biased less than the threshold voltage .15 . Method according to any of claims 13 to 14 , 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 ; and / or operating the first subset with a first periodicity in a first direction and a second periodicity in a second direction; operating pixel of the first subset in sensing mode such that pixels of the first subset is surrounded by eight , 16 , 24 or 32 pixels of the second subset .16 . Method according to any of claims 13 to 15 , wherein the step of operating comprises operating the first and second subsets alternatively in the sensing mode and the adj usted sensing mode .17 . Method according to any of claims 13 to 16 , wherein wherein the step of operating comprises : operating a plurality of first subsets of at least some of the plurality of pixels at different times in sensing mode ; and / oroperating the first subset in the adj usted sensing mode after operating the first subset in the sensing mode and operate some pixels of the second subset in the sensing mode after operating pixels in the adj usted sensing mode .18 . Method according to any of claims 13 to 16 , wherein one of the first and second pLEDs of the first and / or second subset biased in reverse direction or zero biased in sensing or adj usted 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 pLED, said one of the first and second pLED biased in reverse direction or zero biased in the sensing mode or the adj usted sensing mode .19 . Method according to any of claims 13 to 17 , wherein recording signals comprising the steps of : detecting a photo current of the first and second pLEDs of the first and / or second subsets and deriving a PPG signal therefrom .20 . Method according to any of claims 13 to 18 , wherein recording signals comprising the steps of : detecting a first photo current from at least one pLED of the second subset at a first distance from pixels of the first subset ; detecting a second photo current from at least one pLED of the second subset at a second distance from pixels of the first subset ; comparing the first and second photo current .21 . Method according to claims 18 or 19 , wherein processing the recorded signals comprises : processing recorded signals associated with different distances from pixels operated in sensing mode .22 . Method according to claims 18 or 19 , comprising the steps of : comparing the PPG signal with a threshold and in response to the comparisonderiving fingerprint information from the detected photocurrent .23 . Method according to any of claims 13 to 21 , further comprising : detecting a coverage of a user' s finger on an area of a display and in response to detecting a coverage selecting pixels of said area as pixels of the first subset and selecting other pixels as pixels of the second subset .24 . Use of a display arrangement according to any of claims 1 to 12 as : - an in-display fingerprint sensor and an in-display PPG sensor .

Images