Abstract:
A touch screen assembly including a glass screen, LEDs, photo diodes, a transparent plastic frame surrounding the screen, and a light guide that guides light emitted by the LEDs to the photo diodes along light paths that go under the frame on one side, over the screen, and under the frame on the opposite side, and a processor operative to selectively activate LEDs and photo diodes, to identify location of an object touching the screen, based on amounts of light detected by activated photo diodes when light emitted by activated LEDs is blocked along its light path by the object, and to recognize the object touching the frame, based on amounts of light detected by activated photo diodes when light emitted by activated LEDs is absorbed along its light path by the object, thereby providing light-based touch sensitivity to the screen and to the frame.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority benefit of U.S. Provisional Patent Application No. 62/054,353, entitled INTEGRATED LIGHT GUIDE AND TOUCH SCREEN FRAME AND MULTI-TOUCH DETERMINATION METHOD, filed on Sep. 23, 2014 by inventors Saska Lindfors, Robert Pettersson, John Karlsson and Thomas Eriksson. 
    
    
     FIELD OF THE INVENTION 
     The field of the present invention is light-based touch screens. 
     BACKGROUND OF THE INVENTION 
     In computing, multi-touch refers to a touch sensing surface&#39;s ability to recognize the presence of two or more points of contact with the surface. This plural-point awareness is often used to implement advanced functionality such as pinch to zoom or activating predefined programs (Wikipedia, “multi-touch”). The Windows 8 operating system from Microsoft Corporation requires a touch screen supporting a minimum of 5-point digitizers. WINDOWS is a registered trademark of Microsoft Corporation. 
     The present invention relates to light-based touch sensitive surfaces. Light-based touch sensitive surfaces surround the surface borders with light emitters and light detectors to create a light beam grid above the surface. An object touching the surface blocks a corresponding portion of the beams. 
     Reference is made to  FIG. 1 , which is a diagram of a prior art, light-based touch screen having 16 LEDs and 16 PDs. Screen  801  in  FIG. 1  is surrounded by emitters  101  along two edges and photodiode (PD) receivers  201  along the remaining two edges, which together emit and receive a lattice of light beams  300  covering the screen. 
     Light-based touch detection systems are unable to accurately recognize many instances of two or more points of contact with the surface. Reference is made to  FIGS. 2 and 3 , which are illustrations of instances of ambiguous multi-touch detections in prior art touch screens.  FIGS. 2 and 3  show different instances of two, diagonally opposed touches  901  and  902  that are ambiguous vis-à-vis the light grid of  FIG. 1 . As shown in  FIGS. 2 and 3 , the same light beams are blocked in both instances. 
     There is further ambiguity when more than two objects touch the screen simultaneously. Reference is made to  FIGS. 4 and 5 , which are illustrations of instances of ghosted touches in prior art touch screens. The two-touch cases shown in  FIGS. 2 and 3  are also ambiguous vis-à-vis the three-touch case,  901 - 903 , shown in  FIG. 4 , and vis-à-vis the four-touch case,  901 - 904 , shown in  FIG. 5 . In each of the cases illustrated in  FIGS. 2-5 , row and column PDs a-h show an absence of light in the same locations. The ambiguity illustrated in  FIGS. 4 and 5  is caused by “ghosting”, which refers to an effect where the shadow of a first object obscures a second object and prevents the second object from being detected. 
     Reference is made to  FIG. 6 , which is an illustration of a prior art method of disambiguating multi-touch detections in prior art touch screens. The real touches shown in  FIG. 6  are at locations A and B. They both block light in the X and Y directions, and this is detected on the right and bottom edges. However, it is possible to add touches at locations C and D without any effect to the light blocking pattern. With the available data it is not possible to determine what the real touch points are, and whether there are in fact 2, 3 or 4 of them. 
     As shown in  FIG. 6 , prior art methods of disambiguating multi-touches rely on additional light beam directions, to create a less ambiguous blocking pattern. For example, adding diagonally-oriented beams (arrow  1 ) to the scenario in  FIG. 6  reveals that C and D are not touch locations because diagonal beams would pass unblocked. On the other hand, if there were really three touches, say, A, B and C, it would not be possible to distinguish whether the third touch is at C or D or both. In order to do so, a second diagonal direction is added (arrow  2 ). 
       FIGS. 1-6  are simplified in that they show touch sizes that are uniform and locations that are neatly aligned with the light beams. In reality, though, some touches may partially block more than one beam while still shadowing each other. Even with a large number of beam directions some touch configurations generate a blocking pattern with ambiguous or surplus touch location candidates. Furthermore, the detection data is not perfect which makes it difficult to perform direct geometric calculations on the touch locations. 
     Reference is made to  FIG. 7 , which is a prior art blocking pattern of light intensity from measured five-touch data. Each blocked light beam has been shadowed according to its blocking level. It is easy to see, based on the blocking pattern, that there could be more than ten touches. The real touches are not even placed on the most blocked locations. Had they not been highlighted in  FIG. 7 , it would have been impossible to pinpoint them from the blocking pattern. 
     The present invention also relates to integrated casings for optical touch screen displays in which the outer casing, or housing, for the display is a light guide for guiding light from emitters mounted within the housing over and across the screen surface, and for guiding beams that have crossed the screen surface to light receivers mounted within the housing. 
     SUMMARY 
     Embodiments of the present invention provide improved multi-touch detection methods that eliminate ghosting and unambiguously identify touch locations, based on blocked light beams. The methods of the present invention also detect moving multi-touch locations. 
     Embodiments of the present invention also provide an integrated display frame and light guide which wraps around the outside to the front of the monitor or All-in-One (AiO) device. In some embodiments the light guide is made of clear plastic making the frame almost invisible at first glance. 
     There is thus provided in accordance with an embodiment of the present invention a touch screen assembly including a display screen, a plurality of infra-red LEDs operative to emit light when activated, a plurality of photo diodes operative to detect amounts of light received when activated, a transparent plastic frame including an exposed upper edge along the entire frame perimeter, vertically straight inner walls, extending from below the display screen to the exposed upper edge of the frame, along the entire frame perimeter, and internally reflective facets for directing light, emitted by the infra-red LEDs, along light paths that travel upward through one side of the frame along the height of the inner walls, over the display screen, downward through the opposite side of the frame along the height of the inner walls, and onto the photo diodes, and a processor coupled with the infra-red LEDs and the photo diodes, operative to selectively activate the infra-red LEDs and the photo diodes, to identify a location of an object touching the display screen from above, based on amounts of light detected by activated photo diodes when light emitted by activated infra-red LEDs is blocked along its light path by the object. 
     There is additionally provided in accordance with an embodiment of the present invention a touch screen assembly, including a curved display screen, a plurality of LEDs, mounted underneath the display screen, operative to emit light when activated, a plurality of photo diodes, mounted underneath the display screen, operative to detect amounts of light received when activated, a frame including internally reflective surfaces that guide light emitted by the LEDs along light paths that travel upwards, across the display screen in segments that follow the contour of the display screen, and downwards to the photo diodes, wherein the frame is oriented such that some of the light paths are incident upon and reflect off of the display screen while crossing said display screen, and a processor coupled with the LEDs and the photo diodes, operative to selectively activate the LEDs and the photo diodes, and to identify a location of an object touching the display screen, based on amounts of light detected by activated photo diodes when light emitted by activated LEDs is blocked by the object along its light path. 
     There is further provided in accordance with an embodiment of the present invention a touch screen assembly including a display screen, a plurality of infra-red LEDs operative to emit light when activated, a plurality of photo diodes operative to detect amounts of light received when activated, a transparent plastic frame surrounding the display screen on four sides that guides light emitted by the infra-red LEDs to the photo diodes along light paths that travel into the frame on one side, over the display screen, and into the frame on the opposite side, and a processor coupled with the infra-red LEDs and the photo diodes, operative to selectively activate the infra-red LEDs and the photo diodes, to identify a location of an object touching the display screen from above, based on amounts of light detected by activated photo diodes when light emitted by activated infra-red LEDs is blocked along its light path by the object, and to recognize the object touching the frame, based on amounts of light detected by activated photo diodes when light emitted by activated infra-red LEDs is absorbed along its light path by the object, thereby providing light-based touch sensitivity to the display screen and to the frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings in which: 
         FIG. 1  is a diagram of a prior art, light-based touch screen having 16 LEDs and 16 PDs; 
         FIGS. 2 and 3  are illustrations of instances of ambiguous multi-touch detections in prior art touch screens; 
         FIGS. 4 and 5  are illustrations of instances of ghosted touches in prior art touch screens; 
         FIG. 6  is an illustration of a prior art method of disambiguating multi-touch detections in prior art touch screens; 
         FIG. 7  is a prior art pattern of light intensity from measured five-touch data; 
         FIG. 8  is an illustration of a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 9  is an illustration of a light beam from one emitter projected across a touch screen, in accordance with an embodiment of the present invention; 
         FIGS. 10 and 11  are illustrations of portions of the light beam from one emitter detected by two different receivers, in accordance with an embodiment of the present invention; 
         FIGS. 12-14  are illustrations of sets of detected light beams, in accordance with an embodiment of the present invention; 
         FIGS. 15-17  are illustrations of touch detections in different sets of light beams, in accordance with an embodiment of the present invention; 
         FIG. 18  is an illustration of touch detections in overlapping light beams being analyzed together, in accordance with an embodiment of the present invention; 
         FIG. 19  is an illustration of a set of five touch coordinates being tracked, in accordance with an embodiment of the present invention; 
         FIG. 20  is a simplified illustration of a touch screen display surrounded by an exposed light guide frame, in accordance with an embodiment of the present invention; 
         FIGS. 21 and 22  are magnified views of a corner of the touch screen of  FIG. 20 , in accordance with an embodiment of the present invention; 
         FIGS. 23 and 24  are exploded views of the touch screen of  FIG. 20 , in accordance with an embodiment of the present invention; 
         FIGS. 25-27  are cutaway views of the touch screen of  FIG. 20 , illustrating a light beam path from an emitter to crossing above the display to a receiver, in accordance with an embodiment of the present invention; 
         FIG. 28  is a magnified view of a corner of the touch screen of  FIG. 20  viewed from below, with the screen&#39;s rear support removed, in accordance with an embodiment of the present invention; 
         FIG. 29  is a front view of a touch screen display for an all-in-one computer, showing light paths from top to bottom in accordance with an embodiment of the present invention; 
         FIG. 30  is a front view of the touch screen display of  FIG. 29 , showing light paths from left to right in accordance with an embodiment of the present invention; 
         FIG. 31  is a side view of the touch screen display of  FIG. 29 , in accordance with an embodiment of the present invention; 
         FIG. 32  is a back view of the touch screen display of  FIG. 29 , in accordance with an embodiment of the present invention 
         FIG. 33  is an exploded cutaway back view of the touch screen display of  FIG. 29  and a PCB strip for LEDs underneath the display, in accordance with an embodiment of the present invention; and 
         FIG. 34  is an assembled view corresponding to the exploded view of  FIG. 33 , in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present invention relate to light-based touch screens and light-based touch surfaces. Throughout this specification, the terms “touch screen” and “touch sensitive surface” include touch sensitive electronic displays and touch surfaces that do not include an electronic display, inter alia, a mouse touchpad as included in many laptop computers and the back cover of a handheld device. They also include airspace enclosed by the rectangular emitter-detector sensor frame provided by the present invention. They also include airspace bordered on only one edge by a linear emitter-detector array whereby light projected into the airspace by the emitters is reflected by the touch object onto the reflectors. 
     According to embodiments of the present invention, a light-based touch sensor includes a plurality of infra-red or near infra-red light-emitting diodes (LEDs) arranged along two adjacent edges of a rectangular touch sensitive surface, as defined above, and a plurality of photodiodes (PDs) arranged along the two remaining adjacent edges. When light projected by the LEDs is blocked by an inserted object, such as a finger or a stylus, the absence of expected light is detected by the PDs. The LEDs and PDs are controlled for selective activation and de-activation by a controller. Generally, each LED and PD has I/O connectors, and signals are transmitted to specify which LEDs and which PDs are activated. 
     Reference is made to  FIG. 8 , which is an illustration of a touch screen, in accordance with an embodiment of the present invention.  FIG. 8  shows an arrangement of emitters  100  and receivers  200  along opposite edges of touch screen  500 . Emitter lenses  401  and receiver lenses  402  are also shown. Vertical line  907  shows that the emitters  100  are shift-aligned with the receivers  200  and that emitter lenses  401  are shift-aligned with receiver lenses  402 . Although  FIG. 8  shows emitters and receivers along only two edges of screen  500 , a similar arrangement of emitters and receivers is present along the remaining two edges as well. 
     Reference is made to  FIG. 9 , which is an illustration of a light beam from one emitter projected across a touch screen, in accordance with an embodiment of the present invention.  FIG. 9  shows how emitter lenses  401  spread the light from each emitter  100  into a wide beam. Wide beam  301  is projected by emitter E 5 . Receiver lenses  402  provide a wide viewing angle for each receiver  200 , similar in scope to that illustrated by wide beam  301 . The emitters  100  are listed as E 0 -E 11  and the receivers  200  are listed as R 0 -R 12 . 
     Reference is made to  FIGS. 10 and 11 , which are illustrations of portions of the light beam from one emitter detected by two different receivers, in accordance with an embodiment of the present invention.  FIG. 10  shows that a portion  302  of a wide beam  301  from each emitter is detected by a respective receiver. Thus, wide beam  302  is the portion of wide beam  301  detected by receiver R 5 . Wide beam  302  is shaped as a parallelogram that spans the width of the emitter lens associated with emitter E 5  and the width of the receiver lens associated with receiver R 5 . 
       FIG. 11  shows that another portion  303  of wide beam  301  from each emitter is detected by a different respective receiver. Thus, wide beam  303  is the portion of wide beam  301  detected by receiver R 6 . Wide beam  303  is shaped as a parallelogram that spans the width of the emitter lens associated with emitter E 5  and the width of the receiver lens associated with receiver R 6 . 
     Reference is made to  FIGS. 12-14 , which are illustrations of sets of detected light beams, in accordance with an embodiment of the present invention.  FIG. 12  shows that a set of beams is formed by portion  303  of each emitter beam detected by a respective receiver. The set of beams  303  provides continuous detection of an inserted pointer anywhere between the first and last beam  303 , as neighboring beams  303  are adjacent one another. 
       FIG. 13  shows that a second set of beams is formed by portion  302  of each emitter beam detected by a different respective receiver than that of beam  303 . This set of beams  302  also provides continuous detection of an inserted pointer anywhere between the first and last beam  302 , as neighboring beams  302  are adjacent one another. 
       FIG. 14  shows a third set of beams formed by portion  304  of each emitter beam detected by a different respective receiver than those of beams  302  and  303 . Thus, whereas light from each emitter is detected by multiple receivers, in some embodiments of the present invention the beams are grouped into sets of similar beams. Namely, each set is formed by pairing each emitter with a respective one of the receivers, where the offset between each emitter and its respective receiver is the same. 
     One advantage of grouping the beams this way is that beams within each group do not intersect. Therefore, when multiple objects are detected within a set of beams the system can identify their relative placement to one another along the axis that the emitters are situated along. In contrast, when analyzing a many-to-many configuration of detection beams in which intersecting beams are analyzed together it is often not possible to determine the relative positions of the detected objects. For example, when two objects a and b are detected by two intersecting beams, it is not known whether a is to the left of b or to the right, because it is unknown if a and b are situated above or below the point of intersection between the two detecting beams. This advantage is elaborated below. 
     According to embodiments of the present invention a plurality of light pulse emitters, E 1 , . . . , E n , is arranged such that each emitter, E, transmits light pulses, denoted E(θ 1 ), . . . , E(θ n ), that are directed in directions, θ 1  . . . θ n , outward from an edge of the display and over the display, for detection by n light pulse detectors situated along the perimeter of the display. Each light pulse E(θ) is directed through a lens to create a wide light pulse such that light pulses from neighboring emitters E m  and E m+1 , directed at a same angle θ b , are substantially contiguous. Each such set of parallel beams is thus denoted E 1 (θ a ), . . . , E n (θ a ). 
     In response to a single touch on the display each set of parallel beams identifies a respective touch location, corresponding to one or more light pulses E i (θ j ) that is at least partially blocked by the touch, and having (a) a normalized touch value between 0 and 1, denoted W(E i (θ j )), according to the percentage of blockage of light pulse E i (θ j ) by the touch, and (b) a respective screen coordinate, denoted X(E i (θ j ). A touch coordinate, denoted XT, is calculated by interpolating the screen coordinates of the identified touch locations according to the identified touch locations&#39; normalized touch values,
 
 XT=ΣW ( E   i (θ j ))* X ( E   i (θ j )  (1)
 
When an unambiguous pattern of one or more touches is detected, an interpolation of touch locations from even only a few sets of parallel beams, e.g., 2-6 sets, provides a highly accurate touch coordinate. The fewer touch locations required for interpolation the faster the scanning speed and/or the lower the power required per screen scan. When the possibility of ghosting is determined present in one or more locations on the screen, the system scans additional sets of parallel beams E 1 (θ a ), E n (θ a ), E 1 (θ b ), . . . E n (θ b ), . . . E 1 (θ m ), . . . E n (θ m ). In some embodiments, the area containing the potential ghosting is identified, and only a subset of each additional set of beams that traverses the potential ghosting area is activated. The additional sets of beams resolve the ghosting ambiguity in two stages. The first is explained presently, and the second is described beginning at the section entitled Ghost Point Elimination.
 
     Using few sets of parallel beams, e.g., two sets, for each of the x and y axes, touch locations are detected. Discussion now turns to resolving whether the detected touch is caused by one touch object or by a multi-touch, namely, a plurality of separate simultaneous touches. One axis—the x-axis, is discussed, but a similar process is performed for the y-axis. When a possible multi-touch is detected, the screen is logically divided into strips parallel to the axis whose coordinates the system is analyzing. Thus, when determining x-axis coordinates, the screen is logically divided into strips each strip crossing the width of the screen at a different y coordinate. A segment within each strip, containing the possible multi-touch, is identified based on the initial few sets of parallel beams. Next, the system determines which beams, in each additional set of parallel beams, cross the thus identified segment. The result is a table looking like this: 
     
       
         
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Start and end emitters whose 
               
               
                   
                 beams cross the identified segment 
               
             
          
           
               
                 Strip y-coordinate 
                 Θ 1  Set 
                 Θ 2  Set 
                 Θ 3  Set 
                 . . .  
                 Θ n  Set 
               
               
                   
               
               
                 0 
                 {E 3 -E 7 } 
                 {E 3 -E 7 } 
                 {E 5 -E 10 } 
                 . . .  
                 . . .  
               
               
                 1 
                 . . .  
                 . . .  
                 . . .  
                 . . .  
                 . . .  
               
               
                 . . . 
                 . . .  
                 . . .  
                 . . .  
                 . . .  
                 . . .  
               
               
                 Max y 
                 . . .  
                 . . .  
                 . . .  
                 . . .  
                 . . . 
               
               
                   
               
             
          
         
       
     
     Next, the system activates the sets of additional beams in the table. As mentioned above, neighboring parallel beams in each set are substantially contiguous. Thus, a case of two touches is distinguished from a case of one touch if the pattern of touch detections within a series of neighboring parallel beams has more than one peak. This is explained with reference to  FIGS. 15-17 . 
     Reference is made to  FIGS. 15-17 , which are illustrations of touch detections in different sets of light beams, in accordance with an embodiment of the present invention.  FIG. 15  illustrates a case in which multiple blocked beams indicate a possible situation of multiple touches. As described above, the screen is divided into strips having different y-axis coordinates, and for each strip, a segment containing the touch detections is identified. In  FIG. 15  the borders of the Y 0  segment are indicated by arrows  905  and  906 . Within this segment lie the detections of beams from emitters E 3 -E 7  in beam set ⊖ 1 . In  FIGS. 15-17  the detection signal, i.e., the normalized touch value between 0 and 1 for each beam within the segment, is indicated above its respective receiver. Thus, the detections in  FIG. 15  are 0.9, 1.0, 1.0, 1.0, 0.1, for beams from emitters E 3 -E 7 , respectively. A graph mapping these detections is rendered in the upper portion of  FIGS. 15-17 . The graph in  FIG. 15  shows that the resulting detection signal pattern has only one peak. 
       FIG. 16  shows the detections of beams from emitters E 3 -E 7  in beam set ⊖ 2 . The resulting detection signal pattern has only one peak. 
       FIG. 17  shows the detections of beams from emitters E 5 -E 10  in beam set ⊖ 3 . For this set of beams the normalized touch values are 0.6, 1.0, 0.65, 1.0, 1.0, 0.05, respectively, and the resulting detection signal pattern has two peaks, indicating that there is a gap between touches somewhere within the detected wide beam generated by emitter E 7 . The interruption between the two peaks is caused by a normalized touch value of 0.65. A beam having a normalized touch value of 0.65, when considered alone, indicates that a touch object is present. However, when considered in the context of its neighboring beams, the fact that this touch detection caused a trough in the graph between its two neighboring detections of 1.0, indicates that there are two separate touches in this case. 
     In prior art touch detection systems that activate many-to-many emitter-receiver pairs, certain ambiguities remain. For example, two intersecting beams are blocked by two touches  910  and  911 , respectively. It is unclear whether the touches are located above the intersection point—and thus touch point  910  is to the left of touch point  911 , or below the intersection point in which case touch point  910  is to the right of touch point  911 . 
     Reference is made to  FIG. 18 , which is an illustration of touch detections in overlapping light beams being analyzed together, in accordance with an embodiment of the present invention.  FIG. 18  shows an instance of ambiguity. In  FIG. 18  all detections are combined into the graph at the top of the figure. The graph shows multiple peaks, making it difficult to determine how many touches exist. One of the factors contributing to the construction of this graph is that it is unclear how to order the data when combining signals for different intersecting beams, since the touch (or touches) contributing to the blockage can lie anywhere along the beam—namely, above or below any intersection between two beams. 
     Embodiments of the present invention resolve these ambiguities by analyzing each set of parallel beams separately. Within each set of beams, the beams do not intersect. Therefore, the ambiguity resulting from intersecting blocked beams does not exist. 
     Ghost Point Elimination 
     The blocking pattern, designated  m , shown in  FIG. 7 , is essentially an array of light intensities, m k , at discrete locations, k=1, . . . , N. The basic idea behind methods embodying the subject invention is that each touch must affect the detected blocking pattern. If a touch does not contribute to the light blocking in any way, it must be completely hidden by other touches and cannot be detected. Adding or removing a visible touch always alters the blocking pattern, which is a complex nonlinear function of all present touches. 
     In order to remove ghost touches a blocking pattern is calculated based on a candidate touch combination. The geometry of the light transmitters and receivers together with the candidate touch combination is used to model a blocking pattern, m,
 
   m =f (   x ,  y   ),  (2)
 
where  x =(x 1 , . . . , x n ) and  y =(y 1 , . . . , y n ) are coordinate vectors for n candidate touches, n being a number greater than or equal to one. The problem of multi-touch is thus the “inverse problem”, of finding unknown touch coordinate vectors,  x  and  y , corresponding to a detected blocking pattern,  d . EQ. (2) enables comparing a predicted blocking pattern,  m , based on current candidate touches,  x  and  y , to an actual detected blocking pattern,  d , and measuring an error between the predicted blocking pattern and the actual detected blocking pattern.
 
     There are different possibilities for an error metric, but calculating the norm of the difference in blocking patterns was found empirically to work well:
 
 e=Σ   k=1   N   |d   k   −m   k |,  (3)
 
where  d  and  m  are the detected and modelled blocking vectors, respectively. In an ideal case the matching error, e, according to EQ. (3), would become zero for correct candidate touch combination,  x  and  y . In practice, though, the modeling and detection inaccuracies prevent this, and the calculated matching error, e, serves as a metric indicating whether one candidate touch combination is better than another. In the simplest form this can be used to test for potential ghost touch by calculating the matching error with and without that potential ghost touch location included in x and y. The most straight forward and robust approach is to calculate the matching error for all combinations of touch candidates and then pick the combination with lowest error as the right one.
 
Tracking
 
     The matching error can be used as a metric to perform touch tracking, for moving touch locations. The method consists of three steps:
         1. Update touch coordinates,  x =(x 1 , . . . , x n ) and     y=(y 1 , . . . , y n ), based on previous values and based on estimated speeds of motion,  v =(v 1 , . . . , v n ) of the touch coordinates, to form a new set of candidate coordinates.   2. Perform an optimization on the touch coordinates,  x  and  y , to find the set which minimizes the matching error.   3. Update the touch coordinates,  x  and  y , and the speed estimates.       

     Reference is made to  FIG. 19 , which is an illustration of a set of five moving touch coordinates being tracked with the above method, in accordance with an embodiment of the present invention. A circle in  FIG. 19  denotes an actual coordinate, and a cross in  FIG. 19  denotes the result of the above method. The number of light beams is pruned down to 100 from a total of about 300, by estimating from the model which beams are most likely to contribute to the matching error. 
     The optimization used in  FIG. 19  is based on steepest descent. Other optimization algorithms, or a particular filter, may be used to perform the optimization step, and to find the best matching touch coordinates, Y and y. 
     Improving Touch Accuracy 
     Optimization of the matching error can alternatively be used to improve touch accuracy in situations where the detection algorithm does not perform well. This may happen close to edges or corners where there are fewer light beams available for coordinate calculation. 
     The optimization approach can be extended to cover inaccuracies due to finite beam acquisition time. In a multi-touch system based on tens of beams per touch, the acquisition may take long enough for the touches to move a significant distance. This can be taken into account by including estimated touch speeds, as additional unknowns, into the blocking pattern model
 
   m =f (   x ,  y ,  v   ).  (4)
 
Each touch is assumed to be moving, and the sequence of measuring the beam data is included in the model for m.
 
     This technique reduces the need for parallel beam acquisition, and consequently reduces total hardware cost of the multi-touch solution. 
     Touch Screen Assembly 
     According to embodiments of the present invention, a light guide for directing light beams for touch detection over and across the touch screen surface forms an exposed, outer frame of the screen. Embodiments of the invention are designed for desktop displays and All-in-One devices where there is little risk of trauma to the screen frame in the course of normal use, as compared to mobile phones, laptops and tablets. Nonetheless, the present invention is applicable to mobile phones, laptops and tablets as well. 
     Reference is made to  FIG. 20 , which is a simplified illustration of a touch screen display surrounded by an exposed light guide frame, in accordance with an embodiment of the present invention.  FIG. 20  shows a touch screen display monitor according to the teachings of the present invention, featuring a display  501  and an exposed surrounding frame  403  that serves as a light guide for directing near-infrared light beams from emitters located underneath the display out over the exposed surface of the display and back under the display to receivers. Light guide frame  403  has a very thin profile that extends upward along the outer edges of display  501 . Preferably, display  501  has no stiffeners or structural support elements above its outer edges. This absence of metal overlays enables using the full upper surface of the display for rendering pixels, allowing the rendered image to span the full width and height of the display. The thin profile of light guide frame  403  makes this device very attractive. Furthermore, light guide frame  403  can be manufactured of transparent plastic that makes it seem as if the rendered image extends to the outer edges of the desktop or All-in-One display. 
     In other embodiments, visible light LEDs are provided to illuminate light guide frame  403 . In one embodiment, the system identifies the color of the outermost pixels on the display and illuminates light guide frame  403  with a similar color so that the frame is adapted to visually blend into the rendered image. 
     Reference is made to  FIGS. 21 and 22 , which are magnified views of a corner of the touch screen of  FIG. 20 , in accordance with an embodiment of the present invention. Frame  403  extends above display  501 , but is very thin, so it adds very little to the width and length of the device. Frame  403  extends above display  501  in order to project and receive the near-infrared light beams above and across the screen surface. 
       FIG. 22  shows an enlarged view of one corner of the touch screen of  FIG. 20 , viewed from outside frame  403 . Two internally reflective surfaces,  404  and  405 , are shown for directing the near-infrared light beams from underneath display  501  across the screen surface. 
     Reference is made  FIGS. 23 and 24 , which are exploded views of the touch screen of  FIG. 20 , in accordance with an embodiment of the present invention. As can be seen in  FIG. 23 , display  501  is inserted into frame  403  from above; from below, PCB  502  on which the near-infrared emitters and receivers are mounted is inserted into frame  403  as are lenses  407 . Frame  403  is divided into an upper cavity and a lower cavity by floor  803 . Display  501  is housed in the upper cavity, and PCB  502  and lenses  407  are housed in the lower cavity. In some embodiments, display  501  has some exposed electronics or mechanical stiffeners along one edge. The exposed electronics or mechanical stiffeners are covered by light guide  406  that transmits light exiting the upper edge of light guide  403  over the exposed electronics or mechanical stiffeners. In other embodiments, the display pixels extend to all four edges of display  501 . In these other embodiments, light guide  406  is not used. The touch screen is supported and kept rigid by rear support  802 . 
       FIG. 24  shows light guide frame  403 , PCB  502 , emitter and receiver lenses  407  and rear support  802 . PCB  502  is formed by four PCB strips situated along the four edges of frame  403 . Each PCB strip has a row of either emitters or receivers mounted on its underside, not shown in  FIG. 24 . At the corner, where two PCB strips meet, there is not sufficient room to mount both an emitter and a receiver, so one of these two elements, e.g., the emitter, is mounted on the topside of PCB  502  and coupled to lens  408  that redirects this element&#39;s light beam underneath PCB  502  so that this beam enters its respective lens  407 . 
     Reference is made to  FIGS. 25-27 , which are cutaway views of the touch screen of  FIG. 20 , illustrating a light beam path from an emitter crossing above the display to a receiver, in accordance with an embodiment of the present invention.  FIG. 25  shows light guide frame  403 , its upper and lower internally reflective surfaces  404  and  405  and floor  803 . Display  501  is above floor  803  and PCB  502  is mounted on the underside of floor  803 . A light emitter  102  is shown mounted on PCB  502 . The path of light beam  305  is illustrated entering lens  407 , being reflected by surfaces  405  and  404 , through light guide  406 , and out across the upper surface of screen  501 . 
       FIG. 26  shows the path of light beam  305  toward receiver  202 . As mentioned above, in the illustrated example light guide  406  is situated along only one edge of display  501 . Thus, light guide  406  is present in  FIG. 25  and absent in  FIG. 26 . 
       FIG. 27  shows the path of light beam  305  across the display, from an emitter to a receiver. As mentioned hereinabove, in the illustrated example light guide  406  is situated along only one edge of display  501  the light beam is wide and reaches multiple receivers. Thus,  FIG. 27  is a simplified illustration of only the functionality of light guide frame  403 . 
     Reference is made to  FIG. 28 , which is a magnified view of a corner of the touch screen of  FIG. 20  viewed from below, with the screen&#39;s rear support removed, in accordance with an embodiment of the present invention.  FIG. 28  shows the underside of light frame  403  in the touch screen of  FIG. 20 , and two PCBs  502 , one on which emitters  103  are mounted and the other on which receivers  203  are mounted. Each emitter and receiver has a respective lens  407 . Processor  503 , shown mounted on PCB  502 , is coupled with said LEDs  103  and PDs  203 . 
     In some embodiments, light guide frame  403  is transparent, and furthermore, visible-light LEDs are mounted within frame  403 , e.g., on PCB  502 , to illuminate the frame, under certain conditions. In some embodiments, the frame is illuminated to notify the user, e.g., upon receiving an email or a video call the frame is illuminated a specific color. 
     Furthermore, the infrared light used for touch detection is transmitted through frame  403  by total internal reflection (TIR). As such, when an object having a higher index of refraction than the frame touches the frame, the object absorbs a portion of the transmitted infrared light. This absorption is detected as a reduction in expected light at the receivers. Significantly, this reduction occurs only to light beams along one axis, not both axes, when a user touches one edge of the frame. Thus, a touch on the frame is distinguished from a touch on the screen which causes detections on two axes. This touch gesture, namely, a touch or tap on the outside of frame  403 , is used in some embodiments to activate a function, e.g., to open the Charms bar in certain Windows operating systems. WINDOWS® is a registered trademark of Microsoft Corporation. In other embodiments, when a notification of an incoming email or video call is received, this gesture opens the email or the video call. In another embodiment, a tap on the frame wakes up the computer from sleep mode. This can be communicated to the user by illuminating the frame in response to the tap. 
     Still further, a swipe gesture along an outer edge of frame  403  is also detectable because as the user glides his finger along the edge his finger absorbs infrared light beams from different emitters or receivers. Therefore, swipe gestures are also enabled on the outer perimeter of the frame. 
     An advantage of the touch screen assembly described above is the ability to accommodate light-based touch sensitivity for curved display screens. A technical challenge arises when trying to emit light from one edge of a curved screen to an opposite edge. In general, it is not possible for light to travel from one edge of the screen to the opposite edge along a single plane, since the edges are not co-planar. Furthermore, even if two opposite edges are coplanar, use of a single plane to direct light over a curved screen leads to touch detection errors due to the screen dipping underneath the plane, as shown in  FIG. 31  below. Moreover, the frame that guides the emitted light is generally only able to guide the light at a fixed orientation relative to the screen surface. In order to overcome this technical challenge, special light paths that follow the contour of the curved screen and bounce light reflectively off of the screen surface are used. Specifically, the frame is oriented so as to guide the light emitted by the LEDs along light paths that travel across the screen in segments that follow the contour of the display screen, which are incident upon and reflect off of the display screen one or more times, while crossing the screen. 
     Reference is made to  FIG. 29 , which is a front view of a touch screen display  804  for an all-in-one computer, showing light paths  310 - 312  from top to bottom in accordance with an embodiment of the present invention. The screen surface of  FIG. 29  has a curved cross-section in the left-to-right direction and a straight cross-section in the top-to-bottom direction. In one embodiment of the present invention, the screen has a 34″ diagonal. 
     As shown in  FIG. 29 , emitted light travels from the top edge of the screen along light paths that may have one or more reflections off of the screen surface until they reach the bottom edge. Specifically, the light emitted from the top edge is directed parallel to the screen surface. The light oriented parallel to the top-to-bottom dimension reaches the opposite edge in a straight line. However, the light that is skewed towards the left or right edges encounters one or more reflections off of the screen surface, depending on the amount of the skew, due to the curvature of the surface in the left-to-right direction.  FIG. 29  shows three top-to-bottom light paths; namely, a center light path  311  with no reflections, a light path  312  skewed leftward with one reflection, and a light path  310  skewed rightward with two reflections.  FIG. 29  shows emitter location  105  and receiver locations  208 - 210 . 
     Reference is made to  FIG. 30 , which is a front view of touch screen display  804  of  FIG. 29 , showing light paths  306 - 309  from left to right in accordance with an embodiment of the present invention. The light paths shown in  FIG. 30  reflect multiple times off of the screen surface, following the contour of the surface, and eventually make their way to the right edge.  FIG. 30  shows emitter location  104  and receiver locations  204 - 207 . 
     Although the left-to-right light paths could have been generated as straight lines, since the left and right edges are co-planar in the screen of  FIG. 30 , use of such light paths would, however, lead to touch detection errors, as explained in what follows. 
     Reference is made to  FIG. 31 , which is a side view of the touch screen display of  FIG. 29 , in accordance with an embodiment of the present invention.  FIG. 31  shows imaginary straight line paths  908 - 911  and an imaginary finger  912  blocking the paths.  FIG. 31  also shows emitter location  104  and receiver location  205 .  FIG. 31  also shows light path  313  which follows the contour of the curved screen surface, and emitter locations  106 - 108  at the left edge of imaginary straight line paths  909 - 911 , respectively. Were these straight line light paths to be used instead of the light paths of  FIG. 30 , which follow the contour of the curved screen surface, then two types of touch detection errors would arise. A first error, designated “false positive” in  FIG. 31 , arises when a finger blocks the straight line path while not touching the screen. Were straight line light paths to be used, a touch would be detected when in fact the finger does not touch the screen. A second error, referred to as “parallax effect”, arises when the finger is slanted non-orthogonal to the screen surface, skewed towards the top or towards the bottom of the surface. Were straight line light paths to be used, a touch would be detected at a location on the screen below the middle of the finger instead of at the bottom of the finger. As such, it may be appreciated that use of the light paths of  FIG. 30 , which follow the contour of the curved screen, reflecting off of the screen, avoids both of these types of errors. 
     Reference is made to  FIG. 32 , which is a back view of the touch screen display of  FIG. 29 , in accordance with an embodiment of the present invention. 
     Reference is made to  FIG. 33 , which is an exploded cutaway back view of the touch screen display of  FIG. 29  and a PCB strip  504  for LEDs underneath the display, in accordance with an embodiment of the present invention. Also shown in  FIG. 33  are lenses  409  for the LEDs that guide light from the LEDs into light guide frame  403 , floor  803  and LED lenses  410  along the left edge of the screen. 
     Reference is made to  FIG. 34 , which is an assembled view corresponding to the exploded view of  FIG. 33 , in accordance with an embodiment of the present invention. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific exemplary embodiments without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.