Abstract:
A touch screen display device includes a plurality of display pixels for generating an image, emitters interspersed among the display pixels and emitting light, and detectors interspersed among the display pixels for detecting light. The light from the emitters is coupled into a transparent substrate to reach the front surface which transmits light incident at an angle smaller than a critical angle and which totally internally reflects light which is incident at an angle greater than the critical angle. The display device further includes processing means coupled to the emitters and detectors for detecting the light reaching each detector from different specific emitters and determining the light that is received by each detector that may be due to direct reflection from a near-field object, and the light that is totally internally reflected and which may be frustrated by a touching object.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a touch screen display device, particularly, though not exclusively, to optical (including infra-red) touch screen display screens, which may be integrated as part of an Organic Light Emitting Diode (OLED) display device. 
     2. Related Technology 
     It is known to provide infra-red touch screen technology integrated into Liquid Crystal Diode (LCD) displays. However, such technology can only detect the presence of an object touching the screen; it does not actually provide images of the objects, hence does not provide information regarding their size and shape and cannot, for example, recognize bar codes. Furthermore, it has proved difficult to integrate infra-red emitters and detectors onto a single back plane together with the LCDs. In general, the infra-red emitters and detectors are positioned behind the LCD plane, which is more complicated, and therefore costly, to manufacture. 
     Near field optical touch screens are known, in which the presence of a near field object, i.e. an object that is close to, but not actually touching, the display screen is detected using proximity detection. Optical touch screens which detect the presence of an object that actually touches the display screen are also known, where the touching object is detected using Frustrated Total Internal Reflection (FTIR). In this case, the light from the emitter is totally internally reflected from the surface of the display and is detected by detectors positioned in a line from that emitter. If an object touches the surface of the display, the total internal reflection is frustrated, since the light is absorbed by the object touching the surface of the screen. In this way, by having a plurality of rows and columns of detectors, the position of the object can be determined by detecting in which row and which column the total internal reflection is frustrated. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved touch screen display device. 
     According to a first aspect of the present invention, there is provided a touch screen display device comprising a plurality of display pixels and a transparent body having a front surface, a plurality of emitters interspersed among the display pixels and emitting electromagnetic radiation so as to reach the front surface through the transparent body, and a plurality of detectors interspersed among the display pixels for detecting electromagnetic radiation, wherein the electromagnetic radiation which is emitted by an emitter at an angle smaller than the critical angle to the normal to said front surface off the transparent body is transmitted through said front surface and the electromagnetic radiation which is emitted by the emitter at an angle greater than the critical angle to the normal to said front surface is totally internally reflected, the touch screen display device further comprising processing apparatus coupled to the emitters and detectors operable to identify the electromagnetic radiation reaching each detector from different specific emitters and to discriminate between a. electromagnetic radiation that may be received by each detector by reflection from an object adjacent but not touching said front surface, and b. electromagnetic radiation the total internal reflection of which may be frustrated by an object touching said front surface. 
     According to a second aspect of the present invention, there is provided a method of discriminating between an object touching the surface of a display and an object adjacent, but not touching the display, the method comprising:
         illuminating a transparent body defining the said surface of the display with electromagnetic radiation through the transparent body, the electromagnetic radiation having a range of incident angles to the surface, the range of incident angles including angles greater and smaller than the critical angle for total internal reflection;   measuring the intensity of electromagnetic radiation coming back from the surface of the transparent body; and   identifying a reduction in the measured intensity as the presence of an object in contact with the surface, and identifying an increase in the measured intensity as the presence of an object adjacent but not touching said surface.       

     The present invention is particularly suitable for integration as part of an Organic Light Emitting Diode (OLED) display device. 
     Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colourful, fast switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. 
     Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colours, depending upon the materials used. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of small molecule based devices are described in U.S. Pat. No. 4,539,507 and examples of dendrimer-based materials are described in WO99/21935 and WO02/067343. 
     A basic structure of a typical organic LED involves a glass or plastic substrate supporting a transparent anode layer comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer, an electroluminescent layer and a cathode. The electroluminescent layer, may comprise, for example, PEDOT: PSS (polystyrene-sulphorate—doped polyethylene—dioxythiophene). The cathode layer typically comprises a low work function metal such as calcium and may include an additional layer immediately adjacent electroluminescent layer, such as a layer of aluminium, for improved electron energy level matching. Contact wires to the anode and the cathode respectively provide a connection to a power source. The same basic structure may also be employed for small molecule devices. In this structure, light can be emitted through the transparent anode and substrate and devices with this structure are referred to as “bottom emitters”. Devices which emit through the cathode may also be constructed, for example, by keeping the thickness of the cathode layer to less than around 50-100 mm so that the cathode is substantially transparent. 
     Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multi-coloured display may be constructed using groups of red, green and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a CRT picture, to give the impression of a steady image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particular embodiments of the invention will now be more fully described, by way of example, with reference to the drawings, of which: 
         FIG. 1  shows a schematic diagram of a touch screen forming part of a display device according to one embodiment of the present invention; 
         FIG. 2  shows the touch screen of  FIG. 1  with a near field object; 
         FIG. 3  shows the touch screen of  FIG. 1  with a touching object; 
         FIG. 4  shows a schematic diagram of a touch screen display device according to a second embodiment of the present invention; and 
         FIG. 5  shows a schematic plan view of two rows of a array forming part of the touch screen display device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Thus, as shown in  FIG. 1 , a touch screen  1  forming part of an OLED touch screen display device according to a first embodiment of the present invention is formed by a transparent substrate  2  having a front surface  3  and a back plane  4  on which are formed a first array of OLED emitters and a second array of detectors. In  FIG. 1 , for simplicity, only one emitter is shown, together with a plurality of detectors, to enable the operation of the device to be more easily explained. As shown, the back plane  4  of the transparent substrate  2  is provided with an OLED emitter  5  and a number of detectors  6 - 11  arranged either side of the emitter  5 . Light from the OLED emitter  5  is emitted over a wide range of angles up to 180°, although a coverage of approximately 120°, is shown for clarity. Light that is emitted at angles greater than the critical angle for the transparent substrate is totally internally reflected from the front surface  3  of the transparent substrate  2  and reaches detectors  6 ,  7 ,  10 ,  11 , as shown, whereas light that is emitted by the emitter  5  at an angle smaller than the critical angle is transmitted through the substrate  2  and then diverges after exiting the front surface  3  of the substrate  2 . 
     When an object  12 , such as a user&#39;s finger, approaches the touch screen, but without touching it, it becomes a near field object, as shown in  FIG. 2 . The near field object  12 , which is located within the light that passes through the substrate, reflects back some of that light. As shown, the near field object  12  is positioned off-center with respect to the emitter  5 , so more light is reflected to detector  8  (as shown by large arrow  13 ) and less light reaches detector  9  (as shown by small arrow  14 ). Thus, the presence and location of near field object can be determined by detecting increases in light received at a number of detectors from different emitters. 
     Turning now to  FIG. 3 , there is shown an object  12 , such as a user&#39;s finger, which is touching the front surface  3  of the transparent substrate  2  at a point where light from the emitter  5  is totally internally reflected from the front surface  3 . In this case, the light which would otherwise reach detector  6  is prevented from doing so, because the total internal reflection is frustrated by the touching object  12 . It will be apparent, therefore, that measuring the reduction in light received at a detector will enable the presence of a touching object to be determined. Furthermore, by detecting such reductions at a number of detectors from different emitters, the location of the touching object can be determined. 
     Although the above explanation has been made showing the light from one emitter to several detectors, it will be apparent that the situation is analogous when considering the light received at one detector from a plurality of emitter, with the light received from neighbouring emitters being due to reflection from near-field objects and light from further objects being from TIR, with frustration of that TIR light being due to touching objects. Thus, by constructing arrays of emitters and detectors, two digital representations can be produced through signal analysis—one for near-field objects and one for touching objects. Of course, a touching object can be considered as a (very) near-field object and will produce similar reflected light. However, a near-field object will not frustrate totally internally reflected light (unless it is extremely close to being in contact with the front surface), so the two types of objects can be discriminated by having both types of information. 
     As shown in  FIG. 4 , a touch screen display device according to a second embodiment of the present invention, has a back plane  21  on which are provided display pixels  22 , emitters  23  and  24  and detectors  25 ,  26  and  27 . In this embodiment, the back plane is a separate structural member from a transparent substrate  28  and is connected thereto by structural side elements  29  and  30 . The display pixels  22  are preferably OLED display pixels and the emitters  23  and  24  are also preferably OLED emitters. The detectors may be organic photodetectors, such as phototransistors or photodiodes. 
     The light emitted by the display pixels  22 , as shown by dotted lines  31 , is coupled by any suitable coupling means  32 , such as a conformal optically transparent material, to the transparent substrate  28 . This light will, in general, be transmitted through the transparent substrate  28  and pass through a front surface  33  thereof, to produce a display image for viewing from the front of the touch screen display device. Similarly, light from emitters  23  and  24 , as shown by long dashed lines  34 , is coupled by a suitable coupling means (not shown for clarity), such as a conformal optically transparent material, to the transparent substrate  28 . This light will, partly, be transmitted through the transparent substrate  28  and pass through the front surface  33  thereof, as described above with reference to  FIGS. 1 to 3 , and will partly be totally internally reflected at the front surface  33 . The light that is totally internally reflected will be received by the detectors  25 ,  26  and  27 . For ease of understanding, the light received by detector  25  is shown as dash and single dotted lines  35 , irrespective of whether it is emitted by emitter  23  or emitter  24 . Similarly, light received by detector  26  is shown as short dashed lines  36  and light received by detector  27  is shown as dash and double dotted lines  36 . Again, suitable coupling means (not shown for clarity), such as a conformal optically transparent material, are provided to couple light between the transparent substrate  28  and the detectors  25 ,  26  and  27 . 
     The components (display pixels  22 , emitters  23  and  24  and detectors  25 ,  26  and  27 ) on the back plane  21  are connected to on-panel control circuitry  37  for controlling their operation. The on-panel control circuitry  37  is coupled to off-panel operational circuitry, including off-panel drivers  38 , and processing circuitry  39 . The processing circuitry  39  is used to analyse the light detected from the detectors  25 ,  26  and  27  and to produce a digital representation  41  (map) of the touch screen and any near field objects adjacent thereto and a digital representation  42  (map) of the touch screen and any touching objects adjacent thereto. An image generator  40  is coupled to the off-panel drivers  38  to control the off-panel drivers  38  to control the OLED display pixels  22  to produce the image(s) for display. 
     Turning now to  FIG. 5 , there is shown part of an array of display pixels  22 , emitters  23  and  24  and detectors  25 ,  26  and  27  on the back plane  21 . In this case, two rows are shown connected to separate on-off driver inputs  43  and  44 . Both rows of components are also connected to the on-panel control circuitry  37 . As shown, each of the components in a row are also connected to column driver inputs  45 . By choosing to enable one particular row driver input  43  or  44  and using appropriate column driver inputs  45 , each of the components of that row can be controlled, even when, as shown, all components of a particular column are coupled together to the same column driver input  45 . 
     One example of how the device may be used to determine touching objects is shown in the top row of  FIG. 5  (connected to row driver input  43 ). In this case, light ( 34 ,  35 ) from emitters  23  and  24  that is totally internally reflected at the front surface  33  of the transparent substrate  28  reaches detectors  25  and  26  in the usual manner. However, a touching object  46  that touches the front surface  33  of the transparent substrate  28  between detectors  26  and  27  frustrates the totally internally reflected light  36  so that it does not reach detector  27 . Thus, the fact that light from emitters  23  and  24  reaches detector  26 , but does not reach detector  27  means that it is frustrated by a touching object  46  located between detectors  26  and  27 . It will be apparent that although the above example has been described with respect to only one row, by making similar measurements in a two dimensional array, for example, by switching emitters in only one row on, but having detectors in several adjacent rows on, can allow more accurate two-dimensional analysis to be made. 
     Of course, as described above, by determining how much light is received, and whether it is increased or decreased, the difference between frustrated totally internally reflected light and reflected light can be determined, thus providing information as to which emitters are “blocked” by the touching object, and which emitters have had light reflected by a near-field object. 
     A second example of how the device may be used in a more complicated manner to determine touching and near-field objects by determining at each detector, which emitter the light received at that detector was emitted from. This can be achieved by modulating the light emitted from each of the detectors at a different frequency. In this example, since there are two emitters  23  and  24 , the light from emitter  23  can be modulated as a first square wave  47  and the light from emitter  24  can be modulated as a second square wave  48 , having half the frequency of the first square wave  47 . The combination of the first and second square waves  47  and  48  produces the combined waveform  49 . It will thus be apparent that, depending on which modulated waveform is received at a particular detector, the position of the touching object can be determined. Furthermore, if a combined waveform is received, especially if there are more than two emitters and the combined waveform has light from a number of different emitters, it can be filtered using appropriate high and low pass filters to determine the amount of light received at each frequency, and therefore from each different emitter. 
     Once it has been determined, for each detector, whether light from a particular emitter has been received or not, a matrix providing the results for each emitter and detector in a particular row (or other set) can be generated. Thus, by comparing one matrix with another matrix generated later in time, changes due to reflection of light by near-field objects and frustration due to touching objects can be determined. It will be apparent, of course, that depending on how far the totally internally reflected light can travel, some results of emitter/detector pairs can be disregarded since they will never be positive, if the pair are too far apart. This would reduce the amount of data within a matrix that needed to be analysed by the processing circuitry to produce the digital representations (maps) of the touch screen showing any near-field objects and touching objects respectively. 
     It will be appreciated that further processing circuitry (not shown) can be used to analyse the digital representations  41  and  42  to provide a temporal analysis of the size, shape and location of the near-field and touching objects to be able to determine their lateral movement. By correlating the digital representations of the near field and touching objects and their movements, the processing circuitry can determine which of the near field objects have moved towards the touch screen to become touching objects and which touching objects have moved away from the touch screen to become near field objects. This allows the processing apparatus to utilize the lateral movement of a near field object to control movement of a virtual object, such as a computer cursor, and to utilize the movement of the near field object towards the touch screen to become a touching object as actuation of the cursor (i.e. a “click”). In this way, the system can respond to either movement of a cursor due to movement of a near field object or to actuation of the cursor due to the object touching the touch screen. 
     It will be appreciated that although only two particular embodiments of the invention have been described in detail, various equivalent means, modifications and improvements will be immediately apparent to a person skilled in the art without departing from the scope of the present invention.