Patent Publication Number: US-10319287-B2

Title: Method for operating bi-directional display

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 nationalization of PCT/EP2015/071116 having an international filing date of Sep. 15, 2015, the entire contents of which are hereby incorporated by reference, which in turn claims priority under 35 USC § 119 to German patent application 10 2014 113 577.6 filed on Sep. 19, 2014, the contents of both are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The invention relates to a method for operating a bidirectional display, on which both an array of light-generating image elements and an array of light-detecting elements are arranged. The light-detecting elements of a bidirectional component are also referred to as active pixel sensors (abbreviated to APS) or as a pixel cell. In particular, the invention relates to a method for driving APSs. 
     BACKGROUND 
     Besides CCDs (charge-coupled devices), image sensing devices based on CMOS technology constitute a widespread variant of image sensors. Sensors based on CMOS technology have the advantage over CCDs that electronic circuits can be co-integrated very simply on a chip, which makes complex system-on-chip solutions possible. 
     DE 10 2006 030 541 A1 describes an arrangement in which electromagnetic radiation-emitting elements and electromagnetic radiation-detecting elements are located together on a chip. In this case, the two element types may be arranged in a matrix on the chip. A disadvantagous effect of this is that the immediately adjacent arrangement of electromagnetic radiation-emitting elements and electromagnetic radiation-detecting elements leads to cross-coupling. 
     WO 2012/163312 A1 discloses bidirectional displays on which a plurality of light-generating image elements and a plurality of light-detecting elements are arranged in the form of an array. In this case, the light-generating image elements as a whole may, for example, function as a display surface of a display and the light-detecting elements as a whole may, for example, function as a sensor of a camera. WO 2012/163312 A1 furthermore describes different driving variants for the elements, which are intended to solve the problem of direct crosstalk from light-generating image elements to adjacent light-detecting elements by driving the light-generating image elements and adjacent light-detecting elements successively. Light-generating image elements and adjacent light-detecting elements are therefore actively effective only alternately in succession. 
     When a light-detecting element is activated, a single full exposure phase is always followed by a readout phase of this light-detecting element. All the light-detecting elements are driven in the same way. In relation to the light-detecting elements as a whole, and therefore the function as a camera, only the functionality of the shutter of the camera is varied. Driving for both a global shutter and for a rolling shutter is described. 
     When the light-generating image elements of a bidirectional device function as a display, the driving variants disclosed in WO 2012/163312 A1 reach their limitations. In particular, the inactivity of the light-generating image elements during an exposure time of light-detecting elements, which requires a particular length for a good signal quality, may lead to perceptible image perturbations, or at least to a visible brightness loss. The sensitivity of the detector elements in turn cannot be increased arbitrarily since ever-higher resolutions are required in display applications, while the chip area used should remain as small as possible for cost reasons, so that the space for detector elements or circuit-technology measures to increase the sensitivity thereof are thus greatly restricted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of a structure of a light-emitting element; 
         FIG. 2  shows an equivalent circuit diagram of a light-emitting element; 
         FIG. 3  shows a schematic representation of a phase sequence for driving a light-emitting element according to the prior art; 
         FIG. 4  shows a schematic representation of a phase sequence for the inventive operation of a light-detecting element and associated light-generating image elements in two variants; and 
         FIG. 5  shows a schematic representation of a phase sequence of a multiplicity of light-detecting elements arranged in rows, in variants. 
     
    
    
     DETAILED DESCRIPTION 
     The technical object of the invention is therefore to provide a method for driving a bidirectional display, by means of which the disadvantages from the prior art can be overcome. In particular, light-generating image elements and light-detecting elements are intended to be driven with the method according to the invention in such a way that a mutual influence is at least reduced, and the performance perceptible to the human eye should as far as possible not be reduced. 
     In the method according to the invention, a bidirectional display comprising a substrate, on which a two-dimensional display array consisting of a multiplicity of light-generating image elements and a two-dimensional sensor array consisting of a multiplicity of light-detecting elements are formed, each light-detecting element being assigned at least one light-generating image element, is operated in such a way that the exposure phase of a light-detecting element between two successive readout phases of the light-detecting element is subdivided into at least two exposure subphases chronologically separated from one another, and the at least one light-generating image element assigned to the light-detecting element is activated at least temporarily between the two exposure subphases of the light-detecting element. Preferably, the exposure phase of the light-detecting element is subdivided into more than just two exposure subphases, and the light-generating image element is respectively activated at least temporarily between two successive exposure subphases. The method according to the invention therefore offers the advantage that the inactive time of a light-generating image element extends no longer continuously over the entire duration of a full exposure phase of a light-detecting element, but now only over fractions of an exposure phase. The activation of a light-generating image element at shorter time intervals compared with the prior art leads to an improved image quality. Furthermore, the exposure subphases of a light-detecting element and emit phases of a light-generating image element are also carried out in succession in the method according to the invention, so that cross-coupling of light-generating and light-detecting elements is prevented. The term emit phases is in this case intended to mean the active phases of a light-generating image element, i.e. those phases during which the light-generating element emits light. 
     In the subdivision of the exposure phase into exposure subphases according to the invention, the exposure subphases are preferably selected to be of equal length, although as an alternative they may also be set with a different length. Likewise, the time intervals between successive exposure subphases may be set to have the same length or, alternatively, different lengths. 
     The present invention will be explained in more detail below with the aid of exemplary embodiments 
     Bidirectional displays for which the method may be used are, for example, described in WO 2012/163312 A1. 
     Such a bidirectional display comprises both a multiplicity of light-generating image elements and a multiplicity of light-detecting elements, which are conventionally interleaved with one another in the manner of an array having a number of rows and columns. 
       FIG. 1  schematically represents the structure of a light-detecting element, and  FIG. 2  represents it as an equivalent circuit diagram. Such a light-detecting element comprises at least the following components: a photodetector PD, a reset switch T 1 , a transfer switch T 2 , a memory T 3  and a select switch T 4 , which are electrically interconnected with one another via the nodes n 0 , n 1 , n 2  and n 3 . Optionally, a capacitor element C 1  may also be interconnected with node n 1 . 
       FIG. 3  schematically shows a phase sequence with which a light-detecting element of a bidirectional display according to the prior art is driven. In this case, the reset switch T 1  is driven with the signal “res”, the transfer switch T 2  with the signal “tr” and the select switch T 4  with the signal “sel”. For all subsequently described control signals of the switches, for reasons of clarity, highly active signals are assumed, i.e. the switches are closed—i.e. connected—when a signal reaches the state high or “1”, and opened—i.e. disconnected—when the control signal reaches the level low or “0”. As is known from circuit technology, a switch may however alternatively also be closed with a low signal and opened with a high signal. 
     At an initial instant, according to the prior art as per  FIG. 3 , a reset phase is started by closing the reset switch T 1  and the transfer switch T 2  by a respective high signal at the signal inputs “res” and “tr”, while the select switch T 4  is open. As a result of this, the reset reference voltage “V ref, res ” is connected through to the nodes n 0  and n 1 . With the opening of the reset switch T 1  because of a low signal at the signal input “res”, the reset phase ends and at the same time an exposure phase starts. Owing to the electrical current flowing through the photodetector PD, the electrical voltage at the nodes n 0  and n 1  is reduced until the transfer switch T 2  is opened by means of a low signal at the signal input “tr” and a full exposure phase is thereby ended. A corresponding voltage value is now stored in the memory T 3  and is read out via a data line “data” during a readout phase by the select switch T 4  being closed by means of a high signal at the signal input “sel”. After opening of the select switch by means of a low signal at the signal input “sel”, the entire cycle begins again with a further reset phase at a new initial instant. According to the prior art, light-generating image elements assigned to the light-detecting element remain inactive throughout an entire exposure phase of the light-detecting element. 
     According to the invention, a full exposure phase of a light-detecting element is subdivided into a plurality of chronologically separated exposure subphases, and furthermore light-generating image elements assigned to the light-detecting element are at least temporarily activated between the exposure subphases. The image elements assigned to a light-detecting element are necessarily light-generating image elements that are adjacent to the light-detecting element, although it is necessary to prevent cross-coupling especially of neighbouring elements of the two element types. As a minimum, a light-detecting element is also assigned only one light-generating image element. In another embodiment, there may also be a plurality of light-detecting elements to which one and the same light-generating image element is assigned. 
       FIG. 4  shows a schematic representation of a phase sequence for the inventive operation of a light-detecting element as represented by way of example in  FIGS. 1 and 2 , and light-generating image elements assigned to the light-detecting element, on a bidirectional display. For such a phase sequence,  FIG. 4  represents two variants, a variant V 1   a  and a variant V 1   b . In this case as well, the reset switch T 1  is driven with the signal “res”, the transfer switch T 2  with the signal “tr” and the select switch T 4  with the signal “sel”. 
     In variant V 1   a , at an initial instant, an initial reset phase beginning with a repeating cycle is started by closing the reset switch T 1  and the transfer switch T 2  by a respective high signal at the signal inputs “res” and “tr”, while the select switch T 4  is open. As a result of this, the reset reference voltage “V ref, res ” is connected through to the nodes n 0  and n 1 . With the opening of the reset switch T 1 , this reset phase ends and at the same time an exposure subphase  1 , which lasts only a fraction of a full exposure phase, starts. The exposure subphase  1  is ended by opening the transfer switch T 2 . This is immediately followed by an emit phase, in which the image elements assigned to the light-detecting element emit light. With closure of the reset switch T 1 , a further reset phase starts. Since the transfer switch T 2  in this case remains open, only the node n 0  is reset to the reset reference voltage “V ref, res ”. The reset phase is ended with opening of the reset switch T 1 , and at the same time an exposure subphase  2  is started with closure of the transfer switch T 2 . Because the reset T 1  is now open and the transfer switch T 2  is closed, charge equilibration takes place between the node n 0  (the node of the photodetector PD) and the node n 1  (memory node of the last exposure subphase). At the same time, the photodetector PD causes a charge modification at the now short-circuited nodes n 0  and n 1 . The exposure subphase  2  ends with opening of the transfer switch, and is again followed by an emit phase during which the light-generating image elements assigned to the light-detecting element emit light. The sequence of reset phase, exposure subphase and emit phase is subsequently continued until an exposure subphase N, with which a desired signal level for the exposure is finally reached and a full exposure phase is therefore ended. The exposure phase N may optionally also be followed by another emit phase, or alternatively a readout phase starts immediately after the exposure subphase N with closure of the select switch T 4 . The readout phase, in which the value stored in the memory T 3  is read out via the data line “data”, ends with opening of the select switch T 4 . Following this, a new exposure cycle starts with an initial reset phase. 
     Variant V 1   b  differs from variant V 1   a  only in that a pause is inserted between an exposure subphase and a proceeding reset phase in variant V 1   b , starting with the exposure subphase  2 . In this way, it is possible to prevent cross-coupling of the reset phase to the exposure phase, which could lead to the stored value being influenced by preceding exposure sections. In the schematically represented phase sequences according to  FIG. 4 , there is an emit phase, in which light-generating image elements are activated, primarily between an exposure subphase and a reset phase. It should be explicitly mentioned here that the protective scope of the invention is not limited to the light-generating image elements having to be activated for the emission of light immediately with the end of an exposure subphase and therefore following the start of an emit phase. The activation of the light-generating image elements may also take place during an emit phase with a pause from the preceding exposure subphase. Likewise, the protective scope of the invention includes embodiments in which the activation of light-generating image elements extends beyond a schematically represented emit phase into a subsequent reset phase. What is essential for avoiding cross-coupling is merely that the activation of light-generating image elements does not coincide with an exposure subphase of an associated light-detecting element. 
     The inventive operation of a bidirectional display has been described above merely with reference to one light-detecting element and associated light-generating image elements. However, a bidirectional display often consists of a multiplicity of light-detecting elements and a multiplicity of light-generating image elements, which are arranged on the bidirectional display preferably while being interleaved with one another in a number of rows and columns of an array. As an alternative, the display array and the sensor array may also be arranged next to one another. According to the invention, each of the light-detecting elements and the image elements respectively assigned to these elements are driven according to the phase sequence described above. The readout of the individual pixel cells is in this case carried out by addressing them, for example, according to known method steps via a row line and forwarding the value of the desired pixel cell to external signal processing via a column line. 
       FIG. 6  shows a schematic representation of three variants of phase sequences for the inventive driving of a bidirectional display, in which a multiplicity of light-detecting elements and light-generating image elements are arranged in the form of a pixel matrix. In variant V 2   a , the individual phases of light-detecting elements of all rows are carried out simultaneously. Only the readout phases of the rows are carried out successively with a time offset. If all the light-detecting elements of a bidirectional display as a whole are considered as a sensor of a camera, the phase sequence of the pixel rows as represented in variant V 2   a  corresponds to a so-called global shutter. The phase sequences represented in variants V 2   b  and V 2   c , with a time offset from row to row, on the other hand correspond to the principle of a so-called rolling shutter. Variants V 2   b  and V 2   c  differ only in that an emit phase is again carried out after a final exposure subphase before the readout phase in a row begins in variant V 2   b . In variant V 2   c , conversely, the readout takes place immediately after the final exposure subphase.