Patent Publication Number: US-9411430-B2

Title: Optical touch screen using total internal reflection

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/764,812, entitled OPTICAL TOUCH SCREEN SYSTEMS USING TOTAL INTERNAL REFLECTION, filed on Feb. 12, 2013 by inventors Stefan Holmgren, Lars Sparf, Remo Behdasht, Thomas Eriksson, Michael Lawrence Elyan, Joseph Shain, Anders Jansson, Robert Pettersson and John Karlsson, the contents of which are hereby incorporated herein in their entirety. U.S. application Ser. No. 13/764,812 claims priority benefit of U.S. Provisional Patent Application Ser. No. 61/609,325, entitled RESILIENT LIGHT-BASED TOUCH SURFACE, filed on Mar. 11, 2012 by inventors Thomas Eriksson and Michael Elyan, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/764,812 is a continuation-in-part of U.S. application Ser. No. 13/424,592, entitled LIGHT-BASED FINGER GESTURE USER INTERFACE, filed on Mar. 20, 2012 by inventors Thomas Eriksson, Per Leine, Jochen Laveno Mangelsdorff, Robert Pettersson and Anders Jansson, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/764,812 is a continuation-in-part of U.S. application Ser. No. 13/424,543, entitled OPTICAL ELEMENTS WITH ALTERNATING REFLECTIVE LENS FACETS, filed on Mar. 20, 2012 by inventors Stefan Holmgren, Lars Sparf, Magnus Goertz, Thomas Eriksson, Joseph Shain, Anders Jansson, Niklas Kvist, Robert Pettersson and John Karlsson, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/424,592 claims priority benefit of U.S. Provisional Application Ser. No. 61/564,868, entitled LIGHT-BASED FINGER GESTURE USER INTERFACE, filed on Nov. 30, 2011 by inventors Thomas Eriksson, Per Leine, Jochen Laveno Mangelsdorff, Robert Pettersson and Anders Jansson, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/424,543 claims priority benefit of U.S. Provisional Application Ser. No. 61/564,164, entitled OPTICAL ELEMENTS WITH ALTERNATIVE REFLECTIVE LENS FACETS, filed on Nov. 28, 2011 by inventors Stefan Holmgren, Lars Sparf, Thomas Eriksson, Joseph Shain, Anders Jansson, Niklas Kvist, Robert Pettersson and John Karlsson, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/424,543 claims priority benefit of PCT Application No. PCT/US11/29191, entitled LENS ARRANGEMENT FOR LIGHT-BASED TOUCH SCREEN, filed on Mar. 21, 2011 by inventors Magnus Goertz, Thomas Eriksson, Joseph Shain, Anders Jansson, Niklas Kvist, Robert Pettersson, Lars Sparf and John Karlsson, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/424,543 is a continuation-in-part of U.S. application Ser. No. 12/371,609, now U.S. Pat. No. 8,339,379, entitled LIGHT-BASED TOUCH SCREEN, filed on Feb. 15, 2009 by inventors Magnus Goertz, Thomas Eriksson and Joseph Shain, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/424,543 is a continuation-in-part of U.S. application Ser. No. 12/486,033, entitled USER INTERFACE FOR MOBILE COMPUTER UNIT, filed on Jun. 17, 2009 by inventors Magnus Goertz and Joseph Shain, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/424,543 is a continuation-in-part of U.S. application Ser. No. 12/760,567, entitled OPTICAL TOUCH SCREEN SYSTEMS USING REFLECTED LIGHT, filed on Apr. 15, 2010 by inventors Magnus Goertz, Thomas Eriksson and Joseph Shain, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 13/424,543 is a continuation-in-part of U.S. application Ser. No. 12/760,568, entitled OPTICAL TOUCH SCREEN SYSTEMS USING WIDE LIGHT BEAMS, filed on Apr. 15, 2010 by inventors Magnus Goertz, Thomas Eriksson and Joseph Shain, the contents of which are hereby incorporated herein in their entirety. 
     PCT Application No. PCT/US11/29191 claims priority benefit of U.S. Provisional Application Ser. No. 61/379,012, entitled OPTICAL TOUCH SCREEN SYSTEMS USING REFLECTED LIGHT, filed on Sep. 1, 2010 by inventors Magnus Goertz, Thomas Eriksson, Joseph Shain, Anders Jansson, Niklas Kvist and Robert Pettersson, the contents of which are hereby incorporated herein in their entirety. 
     PCT Application No. PCT/US11/29191 claims priority benefit of U.S. Provisional Application Ser. No. 61/380,600, entitled OPTICAL TOUCH SCREEN SYSTEMS USING REFLECTED LIGHT, filed on Sep. 7, 2010 by inventors Magnus Goertz, Thomas Eriksson, Joseph Shain, Anders Jansson, Niklas Kvist and Robert Pettersson, the contents of which are hereby incorporated herein in their entirety. 
     PCT Application No. PCT/US11/29191 claims priority benefit of U.S. Provisional Application Ser. No. 61/410,930, entitled OPTICAL TOUCH SCREEN SYSTEMS USING REFLECTED LIGHT, filed on Nov. 7, 2010 by inventors Magnus Goertz, Thomas Eriksson, Joseph Shain, Anders Jansson, Niklas Kvist, Robert Pettersson and Lars Sparf, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 12/486,033 claims priority benefit of U.S. Provisional Application Ser. No. 61/132,469, entitled IMPROVED KEYPAD FOR CHINESE CHARACTERS, filed on Jun. 19, 2008 by inventors Magnus Goertz, Robert Pettersson, Staffan Gustafsson and Johann Gerell, the contents of which are hereby incorporated herein in their entirety. 
     U.S. application Ser. No. 12/760,567 claims priority benefit of U.S. Provisional Application Ser. No. 61/169,779, entitled OPTICAL TOUCH SCREEN, filed on Apr. 16, 2009 by inventors Magnus Goertz, Thomas Eriksson and Joseph Shain, the contents of which are hereby incorporated herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The field of the present invention is light-based touch screens. 
     BACKGROUND OF THE INVENTION 
     Many consumer electronic devices are now being built with touch sensitive screens, for use with finger or stylus touch user inputs. These devices range from small screen devices such as mobile phones and car entertainment systems, to mid-size screen devices such as notebook computers, to large screen devices such as check-in stations at airports. 
     Most conventional touch screen systems are based on resistive or capacitive layers. Such systems are not versatile enough to offer an all-encompassing solution, as they are not easily scalable. 
     Reference is made to  FIG. 1 , which is a prior art illustration of a conventional touch screen system. Such systems include an LCD display surface  606 , a resistive or capacitive overlay  801  that is placed over the LCD surface, and a controller integrated circuit (IC)  701  that connects to the overlay and converts inputs from the overlay to meaningful signals. A host device (not shown), such as a computer, receives the signals from controller IC  701 , and a device driver or such other program interprets the signals to detect a touch-based input such as a key press or scroll movement. 
     Reference is made to  FIG. 2 , which is a prior art illustration of a conventional resistive touch screen. Shown in  FIG. 2  are conductive and resistive layers  802  separated by thin spaces. A PET film  803  overlays a top circuit layer  804 , which overlays a conductive coating  806 . Similarly, a conductive coating  807  with spacer dots  808  overlays a bottom circuit layer  805 , which overlays a glass layer  607 . When a pointer  900 , such as a finger or a stylus, touches the screen, a contact is created between resistive layers, closing a switch. A controller  701  determines the current between layers to derive the position of the touch point. 
     Advantages of resistive touch screens are their low cost, low power consumption and stylus support. 
     A disadvantage of resistive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is that pressure is required for touch detection; i.e., a pointer that touches the screen without sufficient pressure goes undetected. As a consequence, resistive touch screens do not detect finger touches well. Another disadvantage is that resistive touch screens are generally unreadable in direct sunlight. Another disadvantage is that resistive touch screens are sensitive to scratches. Yet another disadvantage is that resistive touch screens are unable to discern that two or more pointers are touching the screen simultaneously, referred to as “multi-touch”. 
     Reference is made to  FIG. 3 , which is a prior art illustration of a conventional surface capacitive touch screen. Shown in  FIG. 3  is a touch surface  809  overlaying a coated glass substrate  810 . Two sides of a glass  811  are coated with a uniform conductive indium tin oxide (ITO) coating  812 . In addition, a silicon dioxide hard coating  813  is coated on the front side of one of the ITO coating layers  812 . Electrodes  814  are attached at the four corners of the glass, for generating an electric current. A pointer  900 , such as a finger or a stylus, touches the screen, and draws a small amount of current to the point of contact. A controller  701  then determines the location of the touch point based on the proportions of current passing through the four electrodes. 
     Advantages of surface capacitive touch screens are finger touch support and a durable surface. 
     A disadvantage of surface capacitive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is a limited temperature range for operation. Another disadvantage is a limited capture speed of pointer movements, due to the capacitive nature of the touch screens. Another disadvantage is that surface capacitive touch screens are susceptible to radio frequency (RF) interference and electromagnetic (EM) interference. Another disadvantage is that the accuracy of touch location determination depends on the capacitance. Another disadvantage is that surface capacitive touch screens cannot be used with gloves. Another disadvantage is that surface capacitive touch screens require a large screen border. As a consequence, surface capacitive touch screens cannot be used with small screen devices. Yet another disadvantage is that surface capacitive touch screens are unable to discern a multi-touch. 
     Reference is made to  FIG. 4 , which is a prior art illustration of a conventional projected capacitive touch screen. Shown in  FIG. 4  are etched ITO layers  815  that form multiple horizontal (x-axis) and vertical (y-axis) electrodes. Etched layers  815  include outer hard coat layers  816  and  817 , an x-axis electrode pattern  818 , a y-axis electrode pattern  819 , and an ITO glass  820  in the middle. AC signals  702  drive the electrodes on one axis, and the response through the screen loops back via the electrodes on the other axis. Location of a pointer  900  touching the screen is determined based on the signal level changes  703  between the horizontal and vertical electrodes. 
     Advantages of projective capacitive touch screens are finger multi-touch detection and a durable surface. 
     A disadvantage of projected capacitive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is their high cost. Another disadvantage is a limited temperature range for operation. Another disadvantage is a limited capture speed, due to the capacitive nature of the touch screens. Another disadvantage is a limited screen size, typically less than 5″. Another disadvantage is that surface capacitive touch screens are susceptible to RF interference and EM interference. Yet another disadvantage is that the accuracy of touch location determination depends on the capacitance. 
     Conventional optical touch screens project light beams from one edge of the screen, over and across the screen surface to where photo detectors detect the uninterrupted beams. Touches are detected when an object placed on the screen blocks one or more of the projected light beams, and some of the photo detectors do not detect the expected light. 
     A disadvantage of conventional optical touch screens is that they require a raised bezel around the screen in order to project the light beams across the screen. This requirement is incompatible with some product designs that require a completely flat upper surface with the edges of the device being flush with the screen surface. 
     Another disadvantage of conventional optical touch screens is an artifact known as “ghosting”. Ghosting is manifested when a pointer such as a finger completely blocks a light beam, and a second pointer situated inside the shadow of the blocked beam goes undetected, since the second pointer does not affect the amount of light that reaches the detector. 
     It would thus be beneficial to provide touch screens that overcome the disadvantages of conventional resistive and capacitive touch screens described above, while enabling flush device designs and detecting multiple objects in a single beam&#39;s path. 
     SUMMARY OF THE DESCRIPTION 
     Aspects of the present invention provide light-based touch screens with light beams directed over and across a display though a solid or liquid layer covering the display, for which locations of two or more pointers touching the screen simultaneously may be unambiguously inferred. 
     There is thus provided in accordance with an embodiment of the present invention a touch screen for a computing device, including a housing, a layer of light-transmissive material mounted in the housing, including an upper surface that is exposed to be touched by one or more objects from outside of the housing, a plurality of light emitters mounted in the housing underneath the upper surface, for emitting light beams, a first lens assembly for directing the light beams emitted by the light emitters into the layer at an angle such that the light beams, when entering the layer, remain confined to the layer by total internal reflection at the upper and lower surfaces of the layer when the light beams are not absorbed by any of the objects touching the upper surface, a plurality of light detectors mounted in the housing underneath the upper surface, for detecting light beams and for generating outputs indicating the amounts of light detected, a second lens assembly for directing light beams at a surface of the layer towards one or more of the light detectors, and a calculating unit, mounted in the housing and connected to the light receivers, for determining respective one or more locations of the one or more objects touching the upper surface, based on outputs of the light detectors, wherein light beams in the layer are partially absorbed at the upper surface when they come into contact with any of the objects. 
     There is additionally provided in accordance with an embodiment of the present invention a touch screen for a computing device, including a housing, a layer of light-transmissive material mounted in the housing, including an upper surface that is exposed for touch by one or more objects from outside of the housing, a plurality of light emitters mounted in the housing underneath the upper surface, for emitting light beams, a first lens assembly mounted in the housing for directing the light beams emitted by the light emitters into the layer at an angle such that the light beams, when entering the layer, remain confined to the layer by total internal reflection at the upper and lower surfaces of said layer, a plurality of light detectors mounted in the housing underneath the upper surface, for detecting light beams and for generating outputs indicating the amounts of light detected, a second lens assembly mounted in the housing for directing light beams at a surface of the layer towards one or more of the light detectors, and a calculating unit, mounted in the housing and connected to said light receivers, for determining respective one or more locations of the one or more objects touching the upper surface, based on outputs of the light detectors, wherein light beams in the layer are scattered back into the layer at the upper surface when they come into contact with any of the objects, so that the second lens assembly directs them to more of the light detectors than had they not been scattered. 
    
    
     
       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 prior art illustration of a conventional touch screen system; 
         FIG. 2  is a prior art illustration of a conventional resistive touch screen; 
         FIG. 3  is a prior art illustration of a conventional surface capacitive touch screen; 
         FIG. 4  is a prior art illustration of a conventional projected capacitive touch screen; 
         FIG. 5  is a simplified illustration of a touch screen detection channel, in accordance with an embodiment of the present invention; 
         FIG. 6  a simplified illustration of light beams orientated along a screen axis in a touch screen system, in accordance with an embodiment of the present invention; 
         FIG. 7  is a simplified illustration of light beams spread across a screen like a fan in a touch screen system, in accordance with an embodiment of the present invention; 
         FIG. 8  is a simplified illustration of combined transmitter-receiver elements distributed along a screen edge that create circular, or arc-shaped detection zones, in accordance with an embodiment of the present invention; 
         FIG. 9  is an illustration of a portion of a touch screen including a plurality of emitters that are positioned close together, wherein light is guided by fiber optic light guides to locations along a first screen edge, in accordance with an embodiment of the present invention; 
         FIG. 10  is a diagram of a touch screen having 16 emitters and 16 receivers, in accordance with an embodiment of the present invention; 
         FIGS. 11-13  are diagrams of the touch screen of  FIG. 10 , showing detection of two pointers that touch the screen simultaneously, in accordance with an embodiment of the present invention; 
         FIGS. 14 and 15  are diagrams of a touch screen that detects a two finger glide movement, in accordance with an embodiment of the present invention; 
         FIG. 16  is a circuit diagram of the touch screen from  FIG. 10 , in accordance with an embodiment of the present invention; 
         FIG. 17  is a simplified diagram of a light-based touch screen system, in accordance with an embodiment of the present invention; 
         FIG. 18  is a simplified cross-sectional diagram of the touch screen system of  FIG. 17 , in accordance with an embodiment of the present invention; 
         FIG. 19  is a simplified illustration of an arrangement of emitters, receivers and optical elements that enable a touch screen system to determine a precise location of a fingertip touching the screen, in accordance with an embodiment of the present invention; 
         FIG. 20  is a simplified illustration of an arrangement of emitters, receivers and optical elements that enable a touch screen system to detect a pointer that is smaller than the sensor elements, including inter alia a stylus, in accordance with an embodiment of the present invention; 
         FIG. 21  is a simplified diagram of a touch screen with wide light beams covering the screen, in accordance with an embodiment of the present invention; 
         FIG. 22  is a simplified illustration of a collimating lens, in accordance with an embodiment of the present invention; 
         FIG. 23  is a simplified illustration of a collimating lens in cooperation with a light receiver, in accordance with an embodiment of the present invention; 
         FIG. 24  is a simplified illustration of a collimating lens having a surface of micro-lenses facing an emitter, in accordance with an embodiment of the present invention; 
         FIG. 25  is a simplified illustration of a collimating lens having a surface of micro-lenses facing a receiver, in accordance with an embodiment of the present invention; 
         FIG. 26  is a simplified diagram of an electronic device with a wide-beam touch screen, in accordance with an embodiment of the present invention; 
         FIG. 27  is a diagram of the electronic device of  FIG. 26 , depicting overlapping light beams from one emitter detected by two receivers, in accordance with an embodiment of the present invention; 
         FIG. 28  is a diagram of the electronic device of  FIG. 26 , depicting overlapping light beams from two emitters detected by one receiver, in accordance with an embodiment of the present invention; 
         FIG. 29  is a diagram of the electronic device of  FIG. 26 , showing that points on the screen are detected by at least two emitter-receiver pairs, in accordance with an embodiment of the present invention; 
         FIG. 30  is a simplified diagram of a wide-beam touch screen, showing an intensity distribution of a light signal, in accordance with an embodiment of the present invention; 
         FIG. 31  is a simplified diagram of a wide-beam touch screen, showing intensity distributions of overlapping light signals from two emitters, in accordance with an embodiment of the present invention; 
         FIG. 32  is a simplified diagram of a wide-beam touch screen, showing intensity distributions of two sets of overlapping light signals from one emitter, in accordance with an embodiment of the present invention; 
         FIG. 33  is a simplified diagram of a wide beam touch screen with emitter and receiver lenses that do not have micro-lens patterns, in accordance with an embodiment of the present invention; 
         FIGS. 34 and 35  are simplified diagrams of a wide-beam touch screen with emitter and receiver lenses that have micro-lens patterns, in accordance with an embodiment of the present invention; 
         FIG. 36  is a simplified diagram of a wide-beam touch screen with emitter and receiver lenses that do not have micro-lens patterns, in accordance with an embodiment of the present invention; 
         FIG. 37  is a simplified diagram of a wide beam touch screen, with emitter and receiver lenses that have micro-lens patterns, in accordance with an embodiment of the present invention; 
         FIG. 38  is a simplified diagram of two emitters with lenses that have micro-lens patterns integrated therein, in accordance with an embodiment of the present invention; 
         FIG. 39  is a simplified diagram of two receivers with lenses that have micro-lens patterns integrated therein, in accordance with an embodiment of the present invention; 
         FIG. 40  is a simplified diagram of a side view of a single-unit light guide, in the context of an electronic device with a display and an outer casing, in accordance with an embodiment of the present invention; 
         FIG. 41  is a simplified diagram of side views, from two different angles, of a lens with applied feather patterns on a surface, in accordance with an embodiment of the present invention; 
         FIG. 42  is a simplified diagram of a portion of a wide-beam touch screen, in accordance with an embodiment of the present invention; 
         FIG. 43  is a top view of a simplified diagram of light beams entering and exiting micro-lenses etched on a lens, in accordance with an embodiment of the present invention; 
         FIG. 44  is a simplified diagram of a side view of a dual-unit light guide, in the context of a device having a display and an outer casing, in accordance with an embodiment of the present invention; 
         FIG. 45  is a picture of light guide units, within the context of a device having a PCB and an outer casing, in accordance with an embodiment of the present invention; 
         FIG. 46  is a top view of the light guide units of  FIG. 45 , in accordance with an embodiment of the present invention; 
         FIG. 47  is a simplified diagram of shift-aligned emitters and detectors for a light-based touch screen, for detecting finger touches, in accordance with an embodiment of the present invention; 
         FIG. 48  is a simplified illustration of finger touch detection on the screen of  FIG. 47 , in accordance with an embodiment of the present invention; 
         FIG. 49  is a simplified diagram of a side view cutaway of a light guide within an electronic device, in accordance with an embodiment of the present invention; 
         FIG. 50  is a simplified diagram of a side view cutaway of a portion of an electronic device and an upper portion of a light guide with at least two active surfaces for folding light beams, in accordance with an embodiment of the present invention; 
         FIG. 51  is a simplified drawing of a section of a transparent optical touch light guide, formed as an integral part of a protective glass covering a display, in accordance with an embodiment of the present invention; 
         FIG. 52  is a simplified illustration of the electronic device and light guide of  FIG. 50 , adapted to conceal the edge of the screen, in accordance with an embodiment of the present invention; 
         FIG. 53  is a simplified diagram of a light guide that is a single unit extending from opposite an emitter to above a display, in accordance with an embodiment of the present invention; 
         FIG. 54  is a simplified diagram of a dual-unit light guide, in accordance with an embodiment of the present invention; 
         FIG. 55  is a simplified diagram of a touch screen device held by a user, in accordance with an embodiment of the present invention; 
         FIG. 56  is a simplified diagram of a touch screen with wide light beams covering the screen, in accordance with an embodiment of the present invention; 
         FIGS. 57-59  are respective simplified side, top and bottom views of a light guide in the context of a device, in accordance with an embodiment of the present invention; 
         FIG. 60  is a simplified illustration of a touch screen surrounded by emitters and receivers, in accordance with an embodiment of the present invention; 
         FIG. 61  is a simplified illustration of an optical element with an undulating angular pattern of reflective facets, shown from three angles, in accordance with an embodiment of the present invention; 
         FIG. 62  is a simplified illustration of an optical element reflecting, collimating and interleaving light from two neighboring emitters, in accordance with an embodiment of the present invention; 
         FIG. 63  is a simplified diagram of a multi-faceted optical element, in accordance with an embodiment of the present invention; 
         FIG. 64  is a simplified graph showing the effect of various reflective facet parameters on light distribution for nine facets, in accordance with an embodiment of the present invention; 
         FIG. 65  is a simplified illustration of a touch screen with a wide light beam crossing the screen, in accordance with an embodiment of the present invention; 
         FIG. 66  is a simplified illustration of a touch screen with two wide light beams crossing the screen, in accordance with an embodiment of the present invention; 
         FIG. 67  is a simplified illustration of a touch screen with three wide light beams crossing the screen, in accordance with an embodiment of the present invention; 
         FIG. 68  is a simplified graph of light distribution of a wide beam in a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 69  is a simplified illustration of detection signals from three wide beams as a fingertip moves across a screen, in accordance with an embodiment of the present invention; 
         FIGS. 70-72  are simplified graphs of light distribution in overlapping wide beams in a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 73  is a simplified graph of detection signals from a wide beam as a fingertip moves across a screen at three different locations, in accordance with an embodiment of the present invention; 
         FIG. 74  is a simplified diagram of four optical elements and four neighboring emitters, in accordance with an embodiment of the present invention; 
         FIG. 75  is a simplified diagram of a diffractive surface that directs beams from two emitters along a common path, in accordance with an embodiment of the present invention; 
         FIG. 76  is a simplified diagram of a touch screen surrounded with alternating emitters and receivers, in accordance with an embodiment of the present invention; 
         FIG. 77  is a simplified illustration of a touch screen surrounded with alternating emitters and receivers, and a wide beam crossing the screen, in accordance with an embodiment of the present invention; 
         FIG. 78  is a simplified illustration of a touch screen surrounded with alternating emitters and receivers and two wide beams crossing the screen, in accordance with an embodiment of the present invention; 
         FIG. 79  is a simplified illustration of a touch screen surrounded with alternating emitters and receivers and three wide beams crossing the screen, in accordance with an embodiment of the present invention; 
         FIG. 80  is a simplified illustration of a collimating optical element reflecting and interleaving light for an emitter and a neighboring receiver, in accordance with an embodiment of the present invention; 
         FIGS. 81-84  are illustrations of multi-touch locations that are ambiguous vis-à-vis a first orientation of light emitters, in accordance with an embodiment of the present invention; 
         FIGS. 85-87  are illustrations of the multi-touch locations of  FIGS. 81-83  that are unambiguous vis-à-vis a second orientation of light emitters, in accordance with an embodiment of the present invention; 
         FIG. 88  is a simplified illustration of a touch screen with light beams directed along four axes, in accordance with an embodiment of the present invention; 
         FIG. 89  is a simplified illustration of an alternate configuration of light emitters and light receivers with two grid orientations, in accordance with an embodiment of the present invention; 
         FIG. 90  is a simplified illustration of a configuration of alternating light emitters and light receivers, in accordance with an embodiment of the present invention; 
         FIG. 91  is a simplified illustration of two wide light beams from an emitter being detected by two receivers, in accordance with an embodiment of the present invention; 
         FIG. 92  is a simplified illustration of two wide beams and an area of overlap between them, in accordance with an embodiment of the present invention; 
         FIG. 93  is a simplified illustration of a touch point situated at the edges of detecting light beams, in accordance with an embodiment of the present invention; 
         FIG. 94  is a simplified illustration of a finger-sized touch point in a screen designed for finger touch detection, in accordance with an embodiment of the present invention; 
         FIG. 95  is a simplified illustration of an emitter along one edge of a display screen that directs light to receivers along two edges of the display screen, in accordance with an embodiment of the present invention; 
         FIGS. 96 and 97  are simplified illustrations of a lens for refracting light in three directions, having a lens surface with a repetitive pattern of substantially planar two-sided and three-sided recessed cavities, respectively, in accordance with embodiments of the present invention; 
         FIGS. 98-100  are simplified illustrations of a touch screen surrounded with alternating emitters and receivers and diagonal wide beams crossing the screen, in accordance with an embodiment of the present invention; 
         FIG. 101  is a simplified graph of light distribution across a diagonal wide beam in a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 102  is a simplified graph of light distribution across three overlapping diagonal wide beams in a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 103  is a simplified graph of touch detection as a finger glides across three overlapping diagonal wide beams in a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 104  is a simplified graph of detection signals from a diagonal wide beam as a fingertip moves across the screen at three different locations, in accordance with an embodiment of the present invention; 
         FIG. 105  is a simplified illustration of a first embodiment for a touch screen surrounded with alternating emitters and receivers, whereby diagonal and orthogonal wide beams crossing the screen are detected by one receiver, in accordance with an embodiment of the present invention; 
         FIG. 106  is a simplified illustration of a second embodiment for a touch screen surrounded with alternating emitters and reciters, whereby diagonal and orthogonal wide beams crossing the screen are detected by one receiver, in accordance with an embodiment of the present invention; 
         FIG. 107  is a simplified illustration of a user writing on a prior art touch screen with a stylus; 
         FIG. 108  is a simplified illustration of light beams detecting location of a stylus when a user&#39;s palm rests on a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 109  is a simplified illustration of a frame surrounding a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 110  is a simplified illustration of a first embodiment of emitters, receivers and optical elements for a corner of a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 111  is a simplified illustration of a second embodiment of emitters, receivers and optical elements for a corner of a touch screen, in accordance with an embodiment of the present invention; 
         FIG. 112  is an illustration of optical components made of plastic material that is transparent to infrared light, in accordance with an embodiment of the present invention; 
         FIG. 113  is a simplified diagram of a side view of a touch screen with light guides, in accordance with an embodiment of the present invention; 
         FIG. 114  is an illustration of a touch screen with a block of three optical components on each side, in accordance with an embodiment of the present invention; 
         FIG. 115  is a magnified illustration of one of the emitter blocks of  FIG. 114 , in accordance with an embodiment of the present invention; 
         FIG. 116  is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention; 
         FIG. 117  is a simplified illustration of a touch scattering internally reflected light in a screen assembly having a cover glass, in accordance with an embodiment of the present invention; 
         FIG. 118  is a simplified illustration of a touch object absorbing internally reflected light in a screen assembly having a cover glass, in accordance with an embodiment of the present invention; 
         FIG. 119  is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention; 
         FIG. 120  is a simplified illustration of a light beam path in the touch screen assembly of  FIG. 119 , in accordance with an embodiment of the present invention; 
         FIG. 121  is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention; 
         FIG. 122  is a simplified illustration of emitters and receivers detecting two diagonal touch points, in accordance with an embodiment of the present invention; 
         FIG. 123  is a simplified illustration of emitters and receivers detecting three touch points, in accordance with an embodiment of the present invention; 
         FIG. 124  is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention; 
         FIG. 125  is a flowchart of a method for disambiguating multiple touch detection signals in accordance with an embodiment of the present invention; 
         FIG. 126  is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention; 
         FIG. 127  is an illustration of a touch screen having a long thin light guide along a first edge of the screen, for directing light over the screen, and having an array of light receivers arranged along an opposite edge of the screen for detecting the directed light, and for communicating detected light values to a calculating unit, in accordance with an embodiment of the present invention; 
         FIG. 128  is an illustration of a touch screen having an array of light emitters along a first edge of the screen for directing light beams over the screen, and having a long thin light guide for receiving the directed light beams and for further directing them to light receivers situated at both ends of the light guide, in accordance with an embodiment of the present invention; 
         FIG. 129  is an illustration of two light emitters, each emitter coupled to each end of a long thin light guide, in accordance with an embodiment of the present invention; 
         FIGS. 130-133  are illustrations of a touch screen that detects occurrence of a hard press, in accordance with an embodiment of the present invention; 
         FIGS. 134 and 135  are bar charts showing increase in light detected, when pressure is applied to a rigidly mounted 7-inch LCD screen, in accordance with an embodiment of the present invention; 
         FIGS. 136 and 137  are illustrations of opposing rows of emitter and receiver lenses in a touch screen system, in accordance with an embodiment of the present invention; 
         FIG. 138  is a simplified illustration of a technique for determining a touch location, by a plurality of emitter-receiver pairs in a touch screen system, in accordance with an embodiment of the present invention; 
         FIG. 139  is an illustration of a light guide frame for the configuration of  FIGS. 136 and 137 , in accordance with an embodiment of the present invention; 
         FIG. 140  is a simplified flowchart of a method for touch detection for a light-based touch screen, in accordance with an embodiment of the present invention; 
         FIGS. 141-143  are illustrations of a rotation gesture, whereby a user places two fingers on the screen and rotates them around an axis; 
         FIGS. 144-147  are illustrations of touch events at various locations on a touch screen, in accordance with an embodiment of the present invention; 
         FIGS. 148-151  are respective bar charts of light saturation during the touch events illustrated in  FIGS. 144-147 , in accordance with an embodiment of the present invention; 
         FIG. 152  is a simplified flowchart of a method for determining the locations of simultaneous, diagonally opposed touches, in accordance with an embodiment of the present invention; 
         FIG. 153  is a simplified flowchart of a method for discriminating between clockwise and counter-clockwise gestures, in accordance with an embodiment of the present invention; 
         FIG. 154  is a simplified flowchart of a method of calibration and touch detection for a light-based touch screen, in accordance with an embodiment of the present invention; 
         FIG. 155  is a picture showing the difference between signals generated by a touch, and signals generated by a mechanical effect, in accordance with an embodiment of the present invention; 
         FIG. 156  is a simplified diagram of a control circuit for setting pulse strength when calibrating a light-based touch screen, in accordance with an embodiment of the present invention; 
         FIG. 157  is a plot of calibration pulses for pulse strengths ranging from a minimum current to a maximum current, for calibrating a light-based touch screen in accordance with an embodiment of the present invention; 
         FIG. 158  is a simplified pulse diagram and a corresponding output signal graph, for calibrating a light-based touch screen, in accordance with an embodiment of the present invention; 
         FIG. 159  is an illustration showing how a capillary effect is used to increase accuracy of positioning a component, such as an emitter or a receiver, on a printed circuit board, in accordance with an embodiment of the present invention; 
         FIG. 160  is an illustration showing the printed circuit board of  FIG. 159 , after having passed through a heat oven, in accordance with an embodiment of the present invention; 
         FIG. 161  is a simplified illustration of a light-based touch screen and an ASIC controller therefor, in accordance with an embodiment of the present invention; 
         FIG. 162  is a circuit diagram of a chip package for a controller of a light-based touch screen, in accordance with an embodiment of the present invention; 
         FIG. 163  is a circuit diagram for six rows of photo emitters with 4 or 5 photo emitters in each row, for connection to the chip package of  FIG. 162 , in accordance with an embodiment of the present invention; 
         FIG. 164  is a simplified illustration of a touch screen surrounded by emitters and receivers, in accordance with an embodiment of the present invention; 
         FIG. 165  is a simplified application diagram illustrating a touch screen configured with two controllers, in accordance with an embodiment of the present invention; 
         FIG. 166  is a graph comparing scan sequence performance using a conventional chip vs. a dedicated controller of the present invention; 
         FIG. 167  is a simplified illustration of a touch screen having a shift-aligned arrangement of emitters and receivers, in accordance with an embodiment of the present invention; 
         FIG. 168  is a simplified diagram of a touch screen having alternating emitters and receivers along each screen edge, in accordance with an embodiment of the present invention; 
         FIG. 169  is a simplified illustration of a touch surface with a flexible compressible layer on top of the surface, in accordance with an embodiment of the present invention; 
         FIG. 170  is a magnified view of the touch surface of  FIG. 169 , in accordance with an embodiment of the present invention; 
         FIG. 171  is a simplified illustration of an object pressing down on the flexible compressible layer of the touch surface of  FIG. 169 , and creating an impression thereon, in accordance with an embodiment of the present invention; 
         FIG. 172  is a simplified illustration of an alternative touch surface with a flexible compressible layer on top of the surface, in accordance with an embodiment of the present invention; 
         FIG. 173  is a simplified illustration of an object pressing down on the flexible compressible layer of the touch surface of  FIG. 172 , and creating an impression thereon, in accordance with an embodiment of the present invention; and 
         FIG. 174  is a simplified illustration of another alternative touch surface with a flexible compressible layer on top of the surface, in accordance with an embodiment of the present invention. 
     
    
    
     For reference to the figures, the following index of elements and their numerals is provided. Elements numbered in the 100&#39;s generally relate to light beams, elements numbered in the 200&#39;s generally relate to light sources, elements numbered in the 300&#39;s generally relate to light receivers, elements numbered in the 400&#39;s and 500&#39;s generally relate to light guides, elements numbered in the 600&#39;s generally relate to displays, elements numbered in the 700&#39;s generally relate to circuit elements, elements numbered in the 800&#39;s generally relate to electronic devices, and elements numbered in the 900&#39;s generally relate to user interfaces. Elements numbered in the 1000&#39;s are operations of flow charts. 
     Similarly numbered elements represent elements of the same type, but they need not be identical elements. 
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to light beams 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 100-102 
                 Light beams 
               
               
                   
                 105, 106 
                 Reflected light beam 
               
               
                   
                 107-109 
                 Arc of light output from light source 
               
               
                   
                 110 
                 Dist between centers of two beams 
               
               
                   
                 111 
                 Dist from emitter/rcvr to opt element 
               
               
                   
                 112 
                 Refracted beam 
               
               
                   
                 113-117 
                 Blocked light beams 
               
               
                   
                 120 
                 Light beams (full intensity) 
               
               
                   
                 121 
                 Light beams (partial intensity) 
               
               
                   
                 122 
                 Scattered light beams 
               
               
                   
                 123 
                 Absorbed light beams 
               
               
                   
                 142 
                 Arc of light output from light source 
               
               
                   
                 143 
                 Arc of light input to light receiver 
               
               
                   
                 144 
                 Wide light beams 
               
               
                   
                 145-148 
                 Edge of wide light beam 
               
               
                   
                 151-154 
                 Light beams 
               
               
                   
                 158 
                 Wide light beam 
               
               
                   
                 167-169 
                 Wide light beam 
               
               
                   
                 170-172 
                 Signals received by light receivers 
               
               
                   
                 173 
                 Beam from 1 emitter to 2 receivers 
               
               
                   
                 174 
                 Beam from 1 emitter to 1 st  receiver 
               
               
                   
                 175 
                 Beam from 1 emitter to 2 nd  receiver 
               
               
                   
                 176 
                 Beam from emitter to 1 st  receiver 
               
               
                   
                 177 
                 Beam from emitter to 2 nd  receiver 
               
               
                   
                 178 
                 Beam from 1 emitter to 1 st  receiver 
               
               
                   
                 179 
                 Beam from 1 emitter to 2 nd  receiver 
               
               
                   
                 182 
                 Beam from 1 emitter to 2 receivers 
               
               
                   
                 183-187 
                 Middle of arc of light 
               
               
                   
                 190 
                 Light beams output from light source 
               
               
                   
                 191 
                 Light beams input to light receiver 
               
               
                   
                 192 
                 Arcs of light 
               
               
                   
                 193 
                 Wide light beam from two sources 
               
               
                   
                 194-196 
                 Arcs of light 
               
               
                   
                 197 
                 Reflected light beam 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to light sources 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 200-213 
                 Light emitters 
               
               
                   
                 220 
                 LED cavity 
               
               
                   
                 230 
                 Combined emitter-receiver elements 
               
               
                   
                 231-233 
                 Combined emitter-receiver elements 
               
               
                   
                 235-241 
                 Light emitters 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to light receivers 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 300-319 
                 Light receivers 
               
               
                   
                 394 
                 Light receiver 
               
               
                   
                 398 
                 Light receiver/light emitter 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to light guides 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 400 
                 Lens 
               
               
                   
                 401, 402 
                 Fiber optic light guides 
               
               
                   
                 407 
                 Raised reflector bezel 
               
               
                   
                 408 
                 Cutout 
               
               
                   
                 437, 438 
                 Reflector &amp; lens 
               
               
                   
                 439-443 
                 Lens 
               
               
                   
                 444 
                 Micro-lenses 
               
               
                   
                 445 
                 Surface with fan of micro-lenses 
               
               
                   
                 450 
                 Light guide 
               
               
                   
                 451, 452 
                 Internally reflective surface 
               
               
                   
                 453, 454 
                 Light guide surface 
               
               
                   
                 455 
                 Light guide 
               
               
                   
                 456 
                 Internally reflective surface 
               
               
                   
                 457 
                 Collimating lens &amp; reflective surface 
               
               
                   
                 458 
                 Micro-lenses 
               
               
                   
                 459 
                 Light guide surface 
               
               
                   
                 460 
                 Surface with fan of micro-lenses 
               
               
                   
                 461 
                 Lens 
               
               
                   
                 462 
                 Micro-lenses 
               
               
                   
                 463 
                 Upper portion of light guide 
               
               
                   
                 464 
                 Lower portion of light guide 
               
               
                   
                 465 
                 Light guide surface 
               
               
                   
                 466 
                 Surface with parallel row micro-lenses 
               
               
                   
                 467 
                 Parallel row pattern of micro-lenses 
               
               
                   
                 468 
                 Light guide 
               
               
                   
                 469, 470 
                 Internally reflective surface 
               
               
                   
                 471 
                 Light guide surface 
               
               
                   
                 472 
                 Light guide 
               
               
                   
                 473 
                 Internally reflective surface 
               
               
                   
                 474 
                 Light guide surface 
               
               
                   
                 475 
                 Focal line of a lens 
               
               
                   
                 476 
                 Light guide 
               
               
                   
                 477 
                 Internally reflective surface 
               
               
                   
                 478 
                 Light guide surface 
               
               
                   
                 479 
                 Light guide 
               
               
                   
                 480 
                 Internally reflective surface 
               
               
                   
                 481 
                 Light guide surface 
               
               
                   
                 482 
                 Black plastic transmissive element 
               
               
                   
                 483 
                 Light guide 
               
               
                   
                 484 
                 Surface with fan of micro-lenses 
               
               
                   
                 485 
                 Upper portion of light guide 
               
               
                   
                 486 
                 Lower portion of light guide 
               
               
                   
                 487 
                 Surface with parallel row micro-lenses 
               
               
                   
                 488, 489 
                 Optical component 
               
               
                   
                 490-492 
                 Surface of optical component 
               
               
                   
                 493 
                 Multi-faceted reflective surface 
               
               
                   
                 494-497 
                 Optical component 
               
               
                   
                 498, 499 
                 Light guide 
               
               
                   
                 500-501 
                 Emitter optical component block 
               
               
                   
                 502-503 
                 Receiver optical component block 
               
               
                   
                 504 
                 Emitter lenses 
               
               
                   
                 505 
                 Receiver lenses 
               
               
                   
                 506, 507 
                 Emitter optical component 
               
               
                   
                 508-510 
                 Receiver optical component 
               
               
                   
                 511 
                 Emitter optical components 
               
               
                   
                 512 
                 Receiver optical components 
               
               
                   
                 513 
                 Optical component/temporary guide 
               
               
                   
                 514 
                 Long thin light guide 
               
               
                   
                 515 
                 Light guide reflector 
               
               
                   
                 516 
                 Micro-lenses 
               
               
                   
                 517 
                 Light scatterer strip 
               
               
                   
                 518, 519 
                 Light guides 
               
               
                   
                 520, 521 
                 Protruding lips on light guides 
               
               
                   
                 522, 523 
                 Relative position of light guide element 
               
               
                   
                 524 
                 Clear, flat glass 
               
               
                   
                 525 
                 Collimating lens 
               
               
                   
                 526 
                 Clear flat glass with micro-lens surface 
               
               
                   
                 527 
                 Lens with pattern of refracting surfaces 
               
               
                   
                 528 
                 Micro-lens pattern 
               
               
                   
                 530-534 
                 Opt element with multi-faceted surface 
               
               
                   
                 541 
                 Optical element surface 
               
               
                   
                 542 
                 Multi-faceted reflective surface 
               
               
                   
                 545-549 
                 Reflective facets 
               
               
                   
                 550-552 
                 Lens section in multi-lens assembly 
               
               
                   
                 555, 556 
                 Air gap 
               
               
                   
                 559 
                 Connector joining lens section 
               
               
                   
                 560 
                 Diffractive surface 
               
               
                   
                 561 
                 Optically clear transfer tape 
               
               
                   
                 562 
                 Reflective facet 
               
               
                   
                 563 
                 Air gap 
               
               
                   
                 564, 565 
                 Light guide 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to displays 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 600 
                 Screen glass 
               
               
                   
                 606 
                 LCD display (prior art) 
               
               
                   
                 607 
                 Screen glass (prior art) 
               
               
                   
                 635-637 
                 Display 
               
               
                   
                 638 
                 Protective glass 
               
               
                   
                 639 
                 Daylight filter sheet 
               
               
                   
                 640 
                 Protective glass 
               
               
                   
                 641 
                 Daylight filter sheet 
               
               
                   
                 642, 643 
                 Display 
               
               
                   
                 646 
                 Cover glass 
               
               
                   
                 650 
                 Resilient flexible layer 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to circuit elements 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 700 
                 Printed circuit board 
               
               
                   
                 701 
                 Controller integrated circuit (pr. art) 
               
               
                   
                 702 
                 AC input signal (prior art) 
               
               
                   
                 703 
                 Output signal (prior art) 
               
               
                   
                 720 
                 Shift register for column activation 
               
               
                   
                 730 
                 Shift register for column activation 
               
               
                   
                 731 
                 Chip package 
               
               
                   
                 732, 733 
                 Signal conducting pins 
               
               
                   
                 736 
                 Input/output pins 
               
               
                   
                 737 
                 Chip select pin 
               
               
                   
                 740 
                 Emitter driver circuitry 
               
               
                   
                 742 
                 Emitter pulse control circuitry 
               
               
                   
                 750 
                 Detector driver circuitry 
               
               
                   
                 753 
                 Detector signal processing circuitry 
               
               
                   
                 755 
                 Detector current filter 
               
               
                   
                 756 
                 Analog-to-digital convertor 
               
               
                   
                 759 
                 Controller circuitry 
               
               
                   
                 760, 761 
                 Electrical pad 
               
               
                   
                 762, 763 
                 Printed circuit board 
               
               
                   
                 764 
                 Guide pin 
               
               
                   
                 765 
                 Solder pad 
               
               
                   
                 766 
                 Component solder pad 
               
               
                   
                 767 
                 Solder pads after heat oven 
               
               
                   
                 768, 769 
                 Notch in optical component/guide 
               
               
                   
                 770 
                 Calculating unit 
               
               
                   
                 772 
                 Host processor 
               
               
                   
                 774 
                 Touch screen controller 
               
               
                   
                 775 
                 Serial Peripheral Interface (SPI) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to touch-based electronic devices 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 800 
                 Touch screen 
               
               
                   
                 801 
                 Touch overlay (prior art) 
               
               
                   
                 802 
                 Conductive &amp; resistive layers (pr. art) 
               
               
                   
                 803 
                 PET film (prior art) 
               
               
                   
                 804 
                 Top circuit layer (prior art) 
               
               
                   
                 805 
                 Bottom circuit layer (prior art) 
               
               
                   
                 806, 807 
                 Conductive coating (prior art) 
               
               
                   
                 808 
                 Spacer dot (prior art) 
               
               
                   
                 809 
                 Touch surface (prior art) 
               
               
                   
                 810 
                 Coated glass substrate (prior art) 
               
               
                   
                 811 
                 Glass substrate (prior art) 
               
               
                   
                 812 
                 Conductive ITO coating (prior art) 
               
               
                   
                 813 
                 Silicon dioxide hard coating (prior art) 
               
               
                   
                 814 
                 Electrode (prior art) 
               
               
                   
                 815 
                 Etched ITO layers (prior art) 
               
               
                   
                 816, 817 
                 Hard coat layer (prior art) 
               
               
                   
                 818 
                 x-axis electrode pattern (prior art) 
               
               
                   
                 819 
                 y-axis electrode pattern (prior art) 
               
               
                   
                 820 
                 ITO glass (prior art) 
               
               
                   
                 826 
                 Electronic device 
               
               
                   
                 827-832 
                 Device casing 
               
               
                   
                 841, 842 
                 Resilient members 
               
               
                   
                 843 
                 Flex air gap 
               
               
                   
                 844-847 
                 Image sensors 
               
               
                   
                 849 
                 Screen frame 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Elements generally related to user interfaces 
               
            
           
           
               
               
               
            
               
                   
                 Element 
                 Description 
               
               
                   
               
               
                   
                 900-902 
                 Pointer/finger/thumb/stylus 
               
               
                   
                 905 
                 Detected touch area 
               
               
                   
                 910-912 
                 Light signal attenuation area 
               
               
                   
                 920, 921 
                 Light signal attenuation gradient 
               
               
                   
                 925-927 
                 Path across a wide beam 
               
               
                   
                 930 
                 Hand 
               
               
                   
                 931 
                 Stylus 
               
               
                   
                 932 
                 Drawn line 
               
               
                   
                 971, 972 
                 Touch points 
               
               
                   
                 973-976 
                 Light signal attenuation area 
               
               
                   
                 977 
                 Point on lens 
               
               
                   
                 980 
                 Touch point 
               
               
                   
                 981, 982 
                 Point on lens 
               
               
                   
                 989, 990 
                 Pin 
               
               
                   
                 991-993 
                 Active touch area 
               
               
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION 
     Aspects of the present invention relate to light-based touch screens. 
     For clarity of exposition, throughout the present specification the term “touch screen” is used as a generic term to refer to touch sensitive surfaces that may or may not include an electronic display. As such, the term “touch screen” as used herein includes inter alia a mouse touchpad as included in many laptop computers, and the cover of a handheld electronic device. The term “optical touch screen” is used as a generic term to refer to light-based touch screens, including inter alia screens that detect a touch based on the difference between an expected light intensity and a detected light intensity, where the detected light intensity may be greater than or less than the expected light intensity. 
     For clarity of exposition, throughout the present specification, the term “emitter” is used as a generic term to refer to a light emitting element, including inter alia a light-emitting diode (LED), and the output end of a fiber optic or tubular light guide that outputs light into a lens or reflector that directs the light over a display surface. The term “receiver” is used as a generic term to refer to a light detecting element, including inter alia a photo diode (PD), and the input end of a fiber optic or tubular light guide that receives light beams that traversed a display surface and directs them to a light detecting element or to an image sensor, the image sensor being inter alia a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor. 
     A general principle underlying touch detection is that an object such as a finger, when placed on a screen, changes the coupling of light between a transmitter and a receiver. The position of the finger is calculated by determining how a signal changed and which transmitters and receivers are affected. By pulsing transmitters one at a time, it is determined which transmitter sent light to a given receiver. The information necessary for touch detection is a signal indicating whether a finger is touching the screen, and a signal indicating where the touch is located. 
     Reference is made to  FIG. 5 , which is a simplified illustration of a touch screen detection channel, in accordance with an embodiment of the present invention. As shown in  FIG. 5 , between each transmitter and each receiver there is a channel for conducting signals. A channel signal indicates if there is a touch or not going through the channel. As explained below, when multiple touches occur at the same time, the signal also encodes the number of touches in the channel. This case is commonly referred to as “multi-touch”. There are two types of channels; namely,
         A. channels for which a finger activates a signal between the transmitter and the receiver; and   B. channels for which a finger blocks a signal between the transmitter and the receiver.       

     For channel A, a low signal, near 0, indicates no touch, and a high signal indicates a touch. Channel A extends to a channel A′, which detects more than one touch per channel. For channel A′, high signal values occur at different levels corresponding to the number of touches, where each touch added to the channel increases the signal by one step. 
     For channel B, a high signal indicates no touch, and a low signal, near 0, indicates a touch. Channel B extends to a channel B′, which divides the signal value into multiple ranges or steps. Each additional touch in the channel decreases the signal by one step. 
       FIGS. 6-8  illustrate different orientations of detection channels covering a screen. Reference is made to  FIG. 6 , which is a simplified illustration of light beams orientated along a screen axis in a touch screen system, in accordance with an embodiment of the present invention.  FIG. 6  shows detection channels  100  along the width of screen  635 . Each channel begins at an emitter  200  at one edge of screen  635 , and ends at a respective receiver  300  at the opposite edge of screen  635 . 
     Reference is made to  FIG. 7 , which is a simplified illustration of light beams spread across a screen like a fan in a touch screen system, in accordance with an embodiment of the present invention.  FIG. 7  shows detection channels  100  spread out like a fan across a screen. All of the channels begin at a single emitter  200  in one corner of the screen, and each channel ends at a respective receiver  300  at an opposite edge of the screen. 
     Reference is made to  FIG. 8 , which is a simplified illustration of combined transmitter-receiver elements distributed along a screen edge that create circular, or arc-shaped detection zones, in accordance with an embodiment of the present invention.  FIG. 8( a )  shows detection channels as circular zones  194 - 196  on screen  635 . Each detection channel is created by an emitter-receiver element  231 - 233  that emits an arc of light  194 - 196  and detects the reflection of an object inserted into the arc of light. The detection is shown in  FIG. 8( b ) , where an object  900  inserted into arc  194  reflects light beam  197  back onto emitter-receiver element  231 . 
     Reference is made to  FIG. 9 , which is an illustration of a portion of a touch screen including a plurality of emitters  201 - 203  that are positioned close together, wherein light is guided by fiber optic light guides  401  to locations along a first screen edge, in accordance with an embodiment of the present invention. The portion of the touch screen also includes a plurality of receivers  301 - 305  that are positioned close together, wherein light is guided thereto by fiber optic light guides  402  from locations along a second screen edge. 
     According to embodiments of the present invention, a light-based touch screen includes one or more emitters, including inter alia infra-red or near infra-red light-emitting diodes (LEDs), and a plurality of receivers, including inter alia photo diodes (PDs), arranged along the perimeter surrounding the touch screen or touch surface. The emitters project light substantially parallel to the screen surface, and this light is detected by the receivers. 
     In some embodiments, the projected light is transmitted through air above the screen surface. A pointer, such as a finger or a stylus, placed over a portion of the screen blocks some of the light beams, and correspondingly some of the receivers detect less light intensity. In other embodiments, the projected light is transmitted through an optically transmissive layer above the screen surface. The projected light traverses the screen without exiting this layer due to total internal reflection. A pointer, such as a finger or a stylus, that touches this layer, absorbs and/or scatters some of the light beams and, correspondingly, some of the receivers detect less light intensity. 
     In each of these embodiments, the geometry of the locations of the emitters and receivers, and the detected light intensities, determine the screen coordinates of the pointer. The emitters and receivers are controlled for selective activation and de-activation by a controller. Generally, each emitter and receiver has I/O connectors, and signals are transmitted to specify which emitters and which receivers are activated. 
     In an embodiment of the present invention, plural emitters are arranged along two adjacent sides of a rectangular screen, and plural receivers are arranged along the other two adjacent sides. In this regard, reference is now made to  FIG. 10 , which is a diagram of a touch screen  800  having 16 emitters  200  and 16 receivers  300 , in accordance with an embodiment of the present invention. Emitters  200  emit infrared or near infra-red light beams across the top of the touch screen, which are detected by corresponding receivers  300  that are directly opposite respective emitters  200 . When a pointer touches touch screen  800 , it diminishes the amount of light that reaches some of receivers  300 , either by obstructing a portion of the beam, or by absorbing and/or scattering a portion of the beam as described above. By identifying, from the receiver outputs, which light beams have been blocked or reduced by the pointer, the pointer&#39;s location can be determined. 
     Reference is now made to  FIGS. 11-13 , which are diagrams of touch screen  800  of  FIG. 10 , showing detection of two pointers,  901  and  902 , that touch the screen simultaneously, in accordance with an embodiment of the present invention. When two or more pointers touch the screen simultaneously, this is referred to as a “multi-touch.” Pointers  901  and  902 , which are touching the screen, block light from reaching some of receivers  300 . In accordance with an embodiment of the present invention, the locations of pointers  901  and  902  are determined from the crossed lines of the infra-red beams that the pointers block. In distinction, prior art resistance-based and capacitance-based touch screens are generally unable to detect a multi-touch. 
     When two or more pointers touch screen  800  simultaneously along a common horizontal or vertical axis, the positions of the pointers are determined by the receivers  300  that are blocked. Pointers  901  and  902  in  FIG. 11  are aligned along a common vertical axis and block substantially the same receivers  300  along the bottom edge of touch screen  800 ; namely the receivers marked a, b, c and d. Along the left edge of touch screen  800 , two different sets of receivers  300  are blocked. Pointer  901  blocks the receivers marked e and f, and pointer  902  blocks the receivers marked g and h. The two pointers are thus determined to be situated at two locations. Pointer  901  has screen coordinates located at the intersection of the light beams blocked from receivers a-d and receivers e and f; and pointer  902  has screen coordinates located at the intersection of the light beams blocked from receivers a-d and receivers g and h. 
     Pointers  901  and  902  shown in  FIGS. 12 and 13  are not aligned along a common horizontal or vertical axis, and they have different horizontal locations and different vertical locations. From the blocked receivers a-h, it is determined that pointers  901  and  902  are diagonally opposite one another. They are either respectively touching the top right and bottom left of touch screen  800 , as illustrated in  FIG. 12 ; or else respectively touching the bottom right and top left of touch screen  800 , as illustrated in  FIG. 13 . 
     For light-based touch screens that use total internal reflection, discriminating between  FIG. 12  and  FIG. 13  is resolved by analyzing increases in light detection due to scattered light. This analysis is described in detail below with reference to  FIG. 122 . 
     Determining locations of a diagonally oriented multi-touch is further discussed below with reference to shift-aligned arrangements of emitters and receivers, and with reference to light beams directed along four axes. An additional method of resolving ambiguous multi-touches is described with reference to fast scan frequencies enabled by the ASIC controller discussed hereinbelow. 
     Reference is now made to  FIGS. 14 and 15 , which are diagrams of a touch screen  800  that detects a two-finger glide movement, in accordance with an embodiment of the present invention. The two-finger glide movement illustrated in  FIGS. 14 and 15  is a diagonal pinch gesture that brings pointers  901  and  902  closer together. The direction of the glide is determined from changes in which receivers  300  are blocked. As shown in  FIGS. 14 and 18 , blocked receivers are changing from a and b to receivers  300  more to the right, and from c and d to receivers  300  more to the left. Similarly, blocked receivers are changing from e and f to receivers  300  more to the bottom, and from g and h to receivers  300  more to the top. For a two-finger glide in the opposite direction, i.e., a spread, or reverse-pinch, gesture, that moves pointers  901  and  902  farther apart, the blocked receivers change in the opposite directions. 
     When pointers  901  and  902  are aligned along a common vertical or horizontal axis, there is no ambiguity in identifying two-finger glide patterns. When pointers  901  and  902  are not aligned in a common vertical or horizontal axis, there may be ambiguity in identifying glide patterns, as illustrated in  FIGS. 14 and 15 . In case of such ambiguity, and as described hereinabove with reference to  FIGS. 12 and 13 , discriminating between  FIG. 14  and  FIG. 15  is resolved by analyzing increases in light detection due to scattered light, as described in detail below with reference to  FIG. 122 . 
     Reference is made to  FIG. 16 , which is a circuit diagram of touch screen  800  from  FIG. 10 , in accordance with an embodiment of the present invention. The emitters and receivers are controlled by a controller (not shown). The emitters receive respective signals LED00-LED15 from switches A, and receive current from VROW and VCOL through current limiters B. The receivers receive respective signals PD00-PD15 from shift register  730 . Receiver output is sent to the controller via signals PDROW and PDCOL. Operation of the controller, of switches A and of current limiters B is described in applicant&#39;s co-pending application, U.S. application Ser. No. 12/371,609 filed on Feb. 15, 2009, now U.S. Pat. No. 8,339,379, and entitled LIGHT-BASED TOUCH SCREEN, the contents of which are hereby incorporated by reference. 
     According to an embodiment of the present invention, the emitters are controlled via a first serial interface, which transmits a binary string to a shift register  720 . Each bit of the binary string corresponds to one of the emitters, and indicates whether to activate or deactivate the corresponding emitter, where a bit value “1” indicates activation and a bit value “0” indicates deactivation. Successive emitters are activated and deactivated by shifting the bit string within shift register  720 . 
     Similarly, the receivers are controlled by a second serial interface, which transmits a binary string to a shift register  730 . Successive receivers are activated and deactivated by shifting the bit string in shift register  730 . Operation of shift registers  720  and  730  is described in U.S. application Ser. No. 12/371,609 referenced above. 
     Reference is made to  FIG. 17 , which is a simplified diagram of a light-based touch screen system, in accordance with an embodiment of the present invention. A first portion of the light emitted by emitter  200  is directed through air above a cover glass that covers the display. A second portion of the light emitted by emitter  200  is directed into the cover glass. The second portion of the light is guided by total internal reflection. A small infrared transparent frame  407  surrounds the display to reflect the first portion of light beams between emitters  200  and receivers positioned on opposite sides of the screen. When a pointer, such as a finger or a stylus, touches the cover glass at a specific area  905 , one or more light beams generated by emitters  200  are obstructed; specifically, the first portion of beams are blocked by the finger, and the second portion of beams are at least partially absorbed by the finger. The obstructed light beams are detected by corresponding decreases in light received by one or more of the receivers, which is used to determine the location of the pointer. 
     Reference is made to  FIG. 18 , which is a simplified cross-sectional diagram of the touch screen system of  FIG. 17 , in accordance with an embodiment of the present invention. Shown in  FIG. 18  is a cross-sectional view of a section A-A of an LCD display  635 , a cover glass  646 , and its surrounding infrared transparent frame  407 . The cross-sectional view shows an emitter  200  emitting light  100  that is reflected by a cut-out  408  in frame  407 , and directed substantially parallel over the display surface. The cross-sectional view also shows emitted light  103  that is internally reflected in cover glass  646 , across the display surface. As a finger  900  approaches cover glass  646 , some of the light,  101 , emitted by the emitters and directed over the location of the near touch is blocked by the finger, and some of the light,  102 , passes between the fingertip and the cover glass. The reduction in detected light is substantially linear as the finger draws closer to the cover glass. The internally reflected portion of the light  103  is unaffected by the approaching finger. When finger  900  touches the display surface, all of the light emitted by the emitters and directed through air above the touch location, e.g., beams  101  and  102 , is blocked by finger  900 . In addition, a significant portion of the internally reflected light  103  is absorbed and/or scattered by the finger, causing a sudden drop in the amount of detected light when the finger touches the cover glass. This provides an indication as to when contact was first made. 
     Touch Screen System Configuration No. 1 
     Reference is made to  FIG. 19 , which is a simplified illustration of an arrangement of emitters, receivers and optical elements that enable a touch screen system to determine a precise location of a fingertip touching the screen, in accordance with an embodiment of the present invention. Shown in  FIG. 19  are a mirror or optical lens  400 , an emitter  200 , a wide reflected light beam  105 , a pointer  900  and a receiver  300 . Mirror or optical lens  400  generates a wide light beam that is focused onto receiver  300  by a second mirror or optical lens. The wide beam makes it possible to sense an analog change in the amount of light detected at receiver  300  when a pointer blocks a portion of the wide beam. In some embodiments the mirror or optical lens  400  distributes light at approximately uniform intensity along the width of beam  105 . Thus, as a fingertip passes across wide beam  105 , it blocks increasing amounts of the beam, and the amount of light blocked is linearly proportional to the width of the blocked portion of the beam. The fingertip is slightly wider than each wide beam, such that the fingertip is detected by at least two adjacent wide beams. The precise location of the finger is determined by interpolating the detection signals in adjacent beams. In systems where wide beam  105  is directed through air over screen  800 , pointer  900  in  FIG. 19  blocks only a portion of wide beam  105 . In systems where beam  105  is directed by total internal reflection through a cover glass placed above screen  800 , when pointer  900  touches the cover glass it absorbs and/or scatters a portion of wide beam  105 . In addition to enabling precise detection of a fingertip, the wide beam also enables mounting the emitters far apart from one another, and mounting the receivers far apart from one another. Consequently, this reduces the bill of materials by requiring fewer emitters and fewer receivers. 
     Reference is made to  FIG. 20 , which is a simplified illustration of an arrangement of emitters, receivers and optical elements that enable a touch screen system to detect a pointer that is smaller than the sensor elements, including inter alia a stylus, in accordance with an embodiment of the present invention. Shown in  FIG. 20  are a mirror or optical lens  400 , an emitter  200 , a wide reflected light beam,  105 , a pointer  900  a receiver  300 . Mirror or optical lens  400  generates a wide light beam that is focused onto receiver  300  by a second mirror or optical lens. The wide beam enables sensing of an analog change in the amount of light detected at receiver  300  when a pointer  900  blocks a portion of the wide beam, either by directly blocking the beam path when the beam travels through air above the display, or by absorbing a portion of the beam when the beam is guided, by total internal reflection, through a cover glass. A detailed discussion of the absorption using a cover glass is provided below with reference to configuration no. 6. Pointer  900 , as shown in  FIG. 20 , blocks only a portion of wide beam  105 , indicated by beam  106  being blocked by the tip of pointer  900 . In some embodiments, the mirror or optical lens  400  distributes light at graduating intensities along the width of beam  105 , with a weak signal at the edges linearly increasing to a maximum intensity at the center. Thus, as a stylus passes across the wide beam it blocks different amounts of the beam, and the amount of blocked light depends on the location of the stylus within the width of the beam. Such embodiments are described below, with reference to  FIGS. 30 and 31 , in which beams from two emitter-receiver pairs along one axis overlap and provide two detection signals for the stylus. This enables determination of whether the stylus is in the right half or in the left half of the beam. The wide beam also enables mounting emitters far apart from one another, and mounting receivers far apart from one another. In turn, this reduces the bill of materials by requiring fewer emitters and fewer receivers. 
     Without the wide beam, there are generally spaces between beams that go undetected, making it impossible to distinguish between a user dragging a fine-point stylus across the beams, and the user tapping on different beams with a fine-point stylus. Moreover, with widely spaced narrow beams the pointer touch must be very precise in order to cross a narrow beam. 
     Reference is made to  FIG. 21 , which is a simplified diagram of a touch screen with wide light beams covering the screen, in accordance with an embodiment of the present invention. Touch screen systems using wide beams are described in applicant&#39;s provisional patent application, U.S. Application Ser. No. 61/317,255 filed on Mar. 24, 2010 and entitled OPTICAL TOUCH SCREEN WITH WIDE BEAM TRANSMITTERS AND RECEIVERS, the contents of which are hereby incorporated by reference. 
     The emitters and receivers shown in  FIG. 21  are spaced relatively widely apart. Generally, the emitters are not activated simultaneously. Instead, they are activated one after another, and the coverage areas of their light beams are substantially connected. 
       FIG. 21  shows a top view and a side view of a touch system having a touch screen or touch surface  800 . The touch system provides touch-sensitive functionality to a surface irrespective of whether or not the surface includes a display screen. Moreover, a physical surface is not required; the light beams may be projected though the air, and the location of a pointer in mid-air that breaks the light beams may be detected. In an alternative embodiment, a cover glass is used to guide the light by total internal reflection, and a touch absorbs a portion of the internally reflected light. Absorption using a cover glass is described in detail below with reference to configuration no. 6. 
     Also shown in  FIG. 21  are emitters  200 , reflectors  437  and  438 , and receivers  300  coupled with a calculating unit  770 . Emitters  200  and receivers  300  are positioned beneath screen  800 . Emitters  200  project arcs  142  of light under screen  800  onto reflectors  437 . The distance between emitters  200  and reflectors  437  is sufficient for an arc to spread into a wide beam at a reflector  437 . In various embodiments of the present invention, the distance between emitters  200  and reflectors  437  may be approximately 4 mm, 10 mm, 20 mm or greater, depending on factors including inter alia the widths of the wide beams, the required touch resolution, the emitter characteristics and the optical reflector characteristics. 
     Reflectors  437  collimate the light as wide beams  144  across a swath of screen surface. As explained above, in systems intended for finger touch, it is of advantage to distribute light uniformly across the width of the beam, whereas in systems intended for stylus touch it is of advantage to distribute light at different intensities across the width of the beam. Nevertheless, systems that distribute light at different intensities along the width of the beam may precisely determine the location of a finger touch based on the portion of the beam that is blocked, if the intensity distribution across the beam is known. A wide beam  144  reaches a reflector  438 , which (i) redirects the light beam below screen  800 , and (ii) narrows the wide beam  144  into an arc  143 . As such, wide beam  144  converges onto the surface of one of receivers  300  below the surface of screen  800 . The light intensity detected by each of receivers  300  is communicated to calculating unit  770 . 
     The configuration of  FIG. 21  is of advantage in that the wide light beams cover the entire screen surface, thereby enabling touch sensitive functionality anywhere on the screen. Additionally, the cost of materials for the touch screen is reduced, since relatively few emitter and receiver components are required. 
     Touch Screen System Configuration No. 2 
     Configurations 2-6 use multiple emitter-receiver pairs to precisely identify a touch position. In some of the configurations described hereinabove there are opposing rows of emitters and receivers, each emitter being opposite a respective receiver. In configurations 2 and 3 the emitters are shift-aligned with the receivers. For example, each emitter may be positioned opposite a midpoint between two opposing receivers. Alternatively, each emitter may be off-axis aligned with an opposite receiver, but not opposite the midpoint between two receivers. 
     Embodiments of the present invention employ two types of collimating lenses; namely, (i) conventional collimating lenses, and (ii) collimating lenses coupled with a surface of micro-lenses that refract light to form multiple wide divergent beams. When a light source is positioned at the focus of a conventional collimating lens, the lens outputs light in substantially parallel beams, as illustrated inter alia in  FIGS. 19-21 . When a light source is positioned between a conventional collimating lens and its focus, the lens outputs a wide beam, the outer edges of which are not parallel to each other, as illustrated inter alia in  FIGS. 27-30 . 
     Reference is made to  FIG. 22 , which is a simplified illustration of a collimating lens in cooperation with a light emitter, in accordance with an embodiment of the present invention. Shown in  FIG. 22  is (A) a light emitter  200  transmitting light beams  190  through a flat clear glass  524 . Beams  190  are unaltered by the glass. 
     Also shown in  FIG. 22  is (B) an emitter positioned at the focus of a collimating lens  525 . Beams  190  are collimated by lens  525 . 
     Also shown in  FIG. 22  is (C) an emitter  200  positioned between collimating lens  525  and the lens&#39; focus. Beams  190  are partially collimated by lens  525 ; i.e., the output wide beams are not completely parallel. 
     Reference is made to  FIG. 23 , which is a simplified illustration of a collimating lens in cooperation with a light receiver, in accordance with an embodiment of the present invention. Shown in  FIG. 23  is (A) substantially parallel light beams  191  transmitted through a flat clear glass  524 . Beams  191  are unaltered by the glass. 
     Also shown in  FIG. 23  is (B) a receiver  300  positioned at the focus of collimating lens  525 . Beams  191  are refracted onto receiver  300  by collimating lens  525 . 
     Also shown in  FIG. 23  is (C) a receiver  300  positioned between collimating lens  525  and the lens&#39; focus. Beams  191  are collimated by lens  525 , but because receiver  300  is not at the lens focus, the beams do not converge thereon. 
     Collimating lenses coupled with an outer surface of micro-lenses, which face away from emitters or receivers, transmit light in two stages. As light passes through the bodies of the lenses, light beams are collimated as with conventional collimating lenses. However, as the light passes through the surface of micro-lenses, the light is refracted into multiple wide divergent beams, as illustrated inter alia in  FIGS. 34, 35 and 37-39 . In  FIGS. 38 and 39 , collimating lenses  439  and  440  are shown having micro-lens surfaces  444 . In  FIG. 38 , light emitters  201  and  202  are positioned within the focal distance of collimating lenses  439  and  440 , and wide light beams from the emitters are shown entering lenses  439  and  440 . Light is collimated as it passes through the lens, as with conventional collimating lenses. When the collimated light passes through micro-lens surface  444 , it is refracted into multiple wide divergent beams, three of which are illustrated in  FIG. 38 . In  FIG. 39 , light receivers  301  and  302  are positioned within the focal distance of the collimating lenses, and light beams are shown entering lenses  439  and  440  through micro-lens surface  444 . The incoming beams are refracted into wide divergent beams inside the lens bodies. The refracted beams are directed by the collimating portions of lenses  439  and  440 , which concentrate the beams onto light receivers  301  and  302 . 
     Reference is made to  FIG. 24 , which is a simplified illustration of a collimating lens having a surface of micro-lenses facing an emitter, in accordance with an embodiment of the present invention.  FIG. 24  shows (A) a flat glass  526  having micro-lenses etched on a surface facing an emitter  200 . Light beams  190  enter glass  526  at various angles. At each entry point, a micro-lens refracts an incoming beam into a wide arc  192 . Lines  183  show how the middle of each arc is oriented in a different direction, depending on the angle of approach of the beam into glass  526 . 
       FIG. 24  also shows (B) a collimating lens  527  having micro-lenses etched on a surface facing an emitter  200 . A focus point of the lens, without the micro-lenses, is determined, and emitter  200  is positioned at that point. Light beams  190  enter collimating lens  527  at various angles. At each entry point, a micro-lens refracts the incoming beams into a wide arc  192 . Lines  184  show how the middle of each arc is oriented in the same direction, irrespective of the angle of approach of the beams into collimating lens  527 . This type of lens is referred to as a “multi-directional collimating lens”, because it outputs arcs of light, not parallel beams, but all of the arcs are substantially uniformly directed. 
       FIG. 24  also shows (C) the same collimating lens  527 , but with emitter  200  positioned between the lens and the focus point. The output arcs  192  are oriented in directions between those of the arcs of (A) and the arcs of (B), indicated by lines  185 . 
     Reference is made to  FIG. 25 , which is a simplified illustration of a collimating lens having a surface of micro-lenses facing a receiver, in accordance with an embodiment of the present invention.  FIG. 25  shows (A) a flat glass  526  having micro-lenses etched on a surface facing a receiver  300 . Light beams  191  are shown entering glass  526  as parallel beams. At each exit point, a micro-lens refracts a beam into a wide arc  192 . Lines  186  show how the middle of each arc is oriented in the same direction. The arcs do not converge on receiver  300 . 
       FIG. 25  also shows (B) a multi-directional collimating lens  527  having micro-lenses etched on a surface facing receiver  300 . A focus point of the lens, without the micro-lenses, is determined, and receiver  300  is positioned at that point. Light beams  191  enter lens  527  as substantially parallel beams. At each exit point, a micro-lens refracts an incoming beam into a wide arc  192 . Lines  187  show how the middle of each arc is oriented towards receiver  300 . 
       FIG. 25  also shows (C) the same lens  527 , but with receiver  300  positioned between the lens and the focus point. 
     As used through the present specification, the term “collimating lens” includes a multi-directional collimating lens. 
     Reference is made to  FIG. 26 , which is a simplified diagram of an electronic device with a wide-beam touch screen, in accordance with an embodiment of the present invention. Shown in  FIG. 26  is an electronic device  826  with two emitters,  201  and  202 , and three receivers,  301 ,  302  and  303 , the emitters and receivers being placed along opposite edges of a display  636 . Light intensities detected at each of receivers  301 ,  302  and  303 , are communicated to a calculating unit  770 . Each emitter and receiver uses a respective primary lens, labeled respectively  441 ,  442 ,  443 ,  439  and  440 . Emitters and receivers use the same lens arrangement, to ensure that light emitted by an emitter and re-directed by an emitter lens, is reverse-directed by an opposing lens onto a receiver. 
     It is desirable that the light beam from each emitter covers its two opposite receiver lenses. Such a condition is achieved by positioning each emitter between its lens and its lens&#39; focal point. As such, the emitter is not in focus and, as a result, its light is spread, instead of being collimated, by its lens. Each receiver is similarly positioned between its lens and its lens&#39; focal point. 
     Reference is made to  FIG. 27 , which is a diagram of electronic device  826  of  FIG. 26 , depicting overlapping light beams from one emitter detected by two receivers, in accordance with an embodiment of the present invention. Shown in  FIG. 27  are two wide light beams from emitter  201 , one of which is detected at receiver  301  and another of which is detected at receiver  302 , respectively. The left and right sides of the one beam are marked  145  and  146 , respectively, and the left and right sides of the other beam are marked  147  and  148 , respectively. The shaded area in  FIG. 27  indicates the area on display  636  at which a touch blocks a portion of both wide beams. As such, a touch in this area is detected by two emitter-receiver pairs; namely,  201 - 301  and  201 - 302 . The touch blocks a portion of both wide beams, either by directly blocking the beam path when the beam travels through air above the display, or by absorbing a portion of the beam if guided by total internal reflection through a cover glass. Absorption using a cover glass is described in detail below with reference to configuration no. 7. 
     Reference is made to  FIG. 28 , which is a diagram of electronic device  826  of  FIG. 26 , depicting overlapping light beams from two emitters detected by one receiver, in accordance with an embodiment of the present invention. Shown in  FIG. 28  are wide beams, one from emitter  201  and another from emitter  202 , that are both detected at receiver  302 . The left and right sides of the one beam are marked  145  and  146 , respectively, and the left and right sides of the other beam are marked  147  and  148 , respectively. The shaded area in  FIG. 28  indicates the area on display  636  at which a touch blocks a portion of both wide beams. As such, a touch in this area is detected by two emitter-receiver pairs; namely,  201 - 302  and  202 - 302 . 
     Reference is now made to  FIG. 29 , which is a diagram of the electronic device  826  of  FIG. 26 , showing that points on the screen are detected by at least two emitter-receiver pairs, in accordance with an embodiment of the present invention.  FIG. 29  shows the wide beams of  FIGS. 27 and 28 , and illustrates that touches in the shaded wedges on display  636  are detected by at least two emitter-receiver pairs. The two emitter-receiver pairs are either one emitter with two receivers, as in  FIG. 27 , or two emitters with one receiver, as in  FIG. 28 . More specifically, touches that occur near the row of emitters are generally detected by the former, and touches that occur near the row of detectors are generally detected by the latter. By surrounding the screen with similarly arranged emitters, lenses and receivers, any point may be similarly detected by two emitter-receiver pairs. 
     Reference is made to  FIG. 30 , which is a simplified diagram of a wide-beam touch screen, showing an intensity distribution of a light signal, in accordance with an embodiment of the present invention. Shown in  FIG. 30  is a wide angle light beam emitted by emitter  201  into lens  439 . The light beam crosses over display  636  and substantially spans lenses  441  and  442 . The light is detected at receivers  301  and  302 . 
     Shown in  FIG. 30  is a graph of detected light intensity. Total detected light corresponds to a shaded area under the graph. An object touching the screen blocks a portion of this light. If the object touching the screen moves across the wide beam, from left to right, the amount of blocked light increases, and correspondingly the total detected light decreases, as the object progresses from the left edge of the beam to the center of the beam. Similarly, the amount of blocked light decreases, correspondingly the total detected light increases, as the object progresses from the center of the beam to the right edge of the beam. 
     It is noted that the detected light intensities at the edges of the light beam are strictly positive, thus ensuring that a touch at these edges is detected. 
     Reference is made to  FIG. 31 , which is a simplified diagram of a wide-beam touch screen, showing intensity distributions of overlapping light signals from two emitters, in accordance with an embodiment of the present invention.  FIG. 31  shows light detected from emitters  201  and  202 . A touch point  980  on display  636  blocks light from these emitters differently. Area  973  indicates attenuation of light from emitter  201  by touch point  980 , and the union of areas  973  and  974  corresponds to the attenuation of light from emitter  202  by point  980 . By comparing the light attenuation the two emitter-receiver pairs,  201 - 302  and  202 - 302 , a precise touch coordinate is determined. 
     Reference is made to  FIG. 32 , which is a simplified diagram of a wide-beam touch screen, showing intensity distributions of two sets of overlapping light signals from one emitter, in accordance with an embodiment of the present invention. As shown in  FIG. 32 , touch point  980  is inside the area detected by emitter-receiver pair  201 - 301  and emitter-receiver pair  201 - 302 . The attenuation of the light signal at receiver  302 , depicted as area  976 , is greater than the attenuation at receiver  301 , depicted as area  975 . By comparing the light attenuation in the two emitter-receiver pairs,  201 - 301  and  201 - 302 , a precise touch coordinate is determined. 
     Determining the position of touch point  980  requires determining a position along an axis parallel to the edge along which the emitters are positioned, say, the x-axis, and along an axis perpendicular to the edge, say, the y-axis. In accordance with an embodiment of the present invention, an approximate y-coordinate is first determined and then, based on the expected attenuation values for a point having the thus determined y-coordinate and based on the actual attenuation values, a precise x-coordinate is determined. In turn, the x-coordinate thus determined is used to determine a precise y-coordinate. In cases where the touch point  980  is already touching the screen, either stationary or in motion, previous x and y coordinates of the touch point are used as approximations to subsequent x and y coordinates. Alternatively, only one previous coordinate is used to calculate a first subsequent coordinate, with the second subsequent coordinate being calculated based on the first subsequent coordinate. Alternatively, previous coordinates are not used. 
     Reference is made to  FIG. 33 , which is a simplified diagram of a wide-beam touch screen with emitter and receiver lenses that do not have micro-lens patterns, in accordance with an embodiment of the present invention. Shown in  FIG. 33  is an electronic device  826  with a display  636 , emitters  201  and  202 , corresponding emitter lenses  439  and  440 , receivers  301 ,  302  and  303 , and corresponding receiver lenses  441 ,  442  and  443 . Two light beams,  151  and  152 , from respective emitters  201  and  202 , arrive at a point  977  that is located at an outer edge of lens  442 . Since beams  151  and  152  approach point  977  at different angles of incidence, they do not converge on receiver  302 . Specifically, light beam  152  arrives at receiver  302 , and light beam  151  does not arrive at receiver  302 . 
     In order to remedy the non-convergence, a fine pattern of micro-lenses is integrated with the receiver lenses, at many points long the surfaces of the lenses. The micro-lenses distribute incoming light so that a portion of the light arriving at each micro-lens reaches the receivers. In this regard, reference is made to  FIGS. 34 and 35 , which are simplified diagrams of a wide-beam touch screen with emitter and detector lenses that have micro-lens patterns, in accordance with an embodiment of the present invention.  FIG. 34  shows incoming beam  151  being spread across an angle θ by a micro-lens at location  977 , thus ensuring that a portion of the beam reaches receiver  302 .  FIG. 35  shows incoming beam  152  being spread across an angle ψ by the same micro-lens at location  977 , thus ensuring that a portion of this beam, too, reaches receiver  302 . By arranging the micro-lenses at many locations along each receiver lens, light beams that enter the locations from different angles are all detected by the receiver. The detected light intensities are communicated to a calculating unit  770  coupled with the receivers. 
     Reference is made to  FIG. 36 , which is a simplified diagram of a wide-beam touch screen with emitter and receiver lenses that do not have micro-lens patterns, in accordance with an embodiment of the present invention. Shown in  FIG. 36  is an electronic device  826  with a display  636 , emitters  201  and  202 , corresponding emitter lenses  439  and  440 , receivers  301 ,  302  and  303 , and corresponding receiver lenses  441 ,  442  and  443 . Two light beams emitted by emitter  201  and detected by respective receivers  301  and  302 , are desired in order to determine a precise location of touch point  980 . However, lens  439 , without micro-lens patterns, cannot refract a beam crossing point  980  to receiver  301 . I.e., referring to  FIG. 36 , lens  439  cannot refract beam  153  as shown. Only the beam shown as  154 , crossing point  980 , is detected. 
     In order to remedy this detection problem, micro-lenses are integrated with the emitter lenses at many points along the surface of the lenses. The micro-lenses distribute outgoing light so that a portion of the light reaches the desired receivers. In this regard, reference is made to  FIG. 37 , which is a simplified diagram of a wide beam touch screen, with emitter and receiver lenses that have micro-lens patterns, in accordance with an embodiment of the present invention.  FIG. 37  shows that a portion of light exiting from micro-lens location  982  reaches multiple receivers. As such, a touch at point  980  is detected by receivers  301  and  302 . It will be noted from  FIGS. 36 and 37  that the beams passing through point  980  are generated by micro-lenses at different locations  981  and  982 . Light intensity values detected by the receivers of  FIGS. 36 and 37  are communicated to a calculating unit  770 . 
     Micro-lens patterns integrated with emitter and receiver lenses thus generate numerous overlapping light beams that are detected. Each point on the touch screen is traversed by multiple light beams from multiple micro-lenses, which may be on the same emitter lens. The micro-lenses ensure that the multiple light beams reach the desired receivers. Reference is made to  FIG. 38 , which is a simplified diagram of two emitters,  201  and  202 , with respective lenses,  439  and  440 , that have micro-lens patterns  444  integrated therein, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 39 , which is a simplified diagram of two receivers,  301  and  302 , with respective lenses,  439  and  440 , that have micro-lens patterns  444  integrated therein, in accordance with an embodiment of the present invention. 
     When the light beams are guided across the screen through a cover glass by total internal reflection, as described below with reference to configuration no. 7, the lenses are not exposed to the user. However, when the light beams are directed above the screen through air, the outermost surfaces of the lenses are visible to the user, and it may be less aesthetic to have the micro-lenses on these exposed surfaces, in order that the visible surfaces appear smooth. Moreover, outermost surfaces are susceptible to scratching and to accumulation of dust and dirt, which can degrade performance of the micro-lenses. As such, in embodiments of the present invention, the micro-lenses are integrated on surfaces that are not exposed to the user, as shown below in  FIGS. 40, 41 and 44 . Although these lenses have particular advantages for through-air beams, the lenses described in  FIGS. 40, 41 and 44  may also be connected to a cover glass and used when light beams are guided across the screen through the cover glass by total internal reflection. 
     Reference is made to  FIG. 40 , which is a simplified diagram of a side view of a single-unit light guide, in the context of an electronic device having a display and an outer casing, in accordance with an embodiment of the present invention. Shown in  FIG. 40  is a cut-away of a portion of an electronic device with a display screen  637 , an outer casing  827  above screen  637 , and an emitter  200  below screen  637 . A light guide  450  receives light beams  100  and reflects them above screen  637  so that they travel across the surface of screen  637  for detection. Light guide  450  includes internal reflective surfaces  451  and  452  for projecting light beams  100  above the surface of screen  637 . A section  445  of light guide  450  serves as a primary lens to collimate light beams  100  when they are received. The surface of section  445  that faces emitter  200 , indicated in bold, has patterns of micro-lenses etched thereon. As such, the micro-lenses are not visible to a user, and are protected from damage and dirt. 
     The surface of section  445  has a feather pattern for scattering incoming light beams  100  from an emitter  200 . Reflective surfaces  451  and  452  reflect light beams  100 . Reflective surface  451  is concave, and reflective surface  452  is a flat reflector oriented at a 45° angle with respect to incoming light beams  100 . 
     Light beams  100  exit light guide  450  through flat surface  453 . Surface  454  serves to connect light guide  450  to outer casing  827 . Surface  454  is located above the plane of active light beams used by the touch system, and is angled for aesthetic purposes. 
     The reflective characteristics of surface  452  require that dust and dirt not accumulate on surface  452 , and require that outer casing  827 , which may be made inter glia of metal or plastic, not make contact with surface  452 ; otherwise, reflectivity of surface  452  may be impaired. As such, outer casing  827  is placed above surface  452 , thereby protecting surface  452  from dust and dirt, and outer casing  827  is not flush with surface  452 , so that casing material does not touch surface  452 . Being a flat reflector at a 45° angle relative to incoming light beams, surface  452  is positioned above the upper surface of display  637 . As such, the device height, H 3 , above display  637  due to light guide  450 , comprises the height, H 1 , of surface  452  plus the thickness, H 2 , of outer casing  827 . 
     At the receiving side, a light guide similar to  450  is used to receive light beams  100  that are transmitted over screen  637 , and to direct them onto corresponding one or more receivers. Thus, light beams enter light guide  450  at surface  453 , are re-directed by surface  452  and then by surface  451 , and exit through the micro-lens patterned surface of section  445  to one or more receivers. At the receiving side, the surface of section  445  has a pattern that scatters the light beams as described hereinabove. 
     Reference is made to  FIG. 41 , which is a simplified diagram of side views, from two different angles, of a lens with applied feather patterns on a surface, in accordance with an embodiment of the present invention. Shown in  FIG. 41  is a light guide  455  having an internal reflective section  456 , an internal collimating lens  457 , and etched micro-lenses  458 . Light beams  101  entering light guide  455  at lens  457  exit the light guide through a surface  459  as light beams  105 . 
     Similar light guides are used for receiving beams that have traversed the screen, to focus them onto receivers. In this case, light beams enter at surface  459 , are reflected below the screen surface by internal reflective section  456 , are re-focused onto a receiver by collimating lens  457 , and re-distributed by micro-lenses  458 . In general, the same lens and micro-lenses are used with an emitter and a detector, in order that the light beam be directed at the receiving side in reverse to the way it is directed at the emitting side. 
     Collimating lens  457  has a rounded bottom edge, as shown at the bottom of  FIG. 41 . In order to properly refract incoming light on the emitter side, the micro-lenses  458  are formed in a feather pattern, spreading as a fan, as shown at the bottom of  FIG. 41  and in  FIG. 42 . 
     Reference is made to  FIG. 42 , which is a simplified diagram of a portion of a wide-beam touch screen, in accordance with an embodiment of the present invention. A feather pattern  460  is shown applied to the surface of a lens  461 . A similar neighboring lens is associated with an emitter  200  emitting a wide beam  158 . 
     Reference is made to  FIG. 43 , which is a top view of light beams entering and exiting micro-lenses etched on a lens, in accordance with an embodiment of the present invention. Substantially collimated light beams  101  are shown in  FIG. 43  entering micro-lenses  462  and being refracted to light beams  102 , such that each micro-lens acts as a light source spreading a wide beam across a wide angle. 
     Touch Screen System Configuration No. 3 
     Several challenges arise in the manufacture of the micro-lenses in configuration no. 2. One challenge is the difficulty of accurately forming the fan-shaped feather pattern of micro-lenses. It is desirable instead to use micro-lenses arranged parallel to one another, instead of the fan/feather pattern. 
     A second challenge relates to the mold used to manufacture the light guide in configuration no. 2. Referring to  FIG. 40 , it is desirable that the outer surface of section  445 , facing emitter  200 , be vertical, so that the front surface of section  445  is parallel with the straight back surface portion of light guide  450 . However, it is difficult to manufacture exactly parallel surfaces. Moreover, if the light guide  450  were to be wider at its bottom, then it would not be easily removable from its mold. As such, the two surfaces generally form a wedge, and the surface of section  445  facing emitter  200  is not perfectly vertical. To compensate for this, the micro-lenses are arranged so as to be perpendicular to a plane of incoming light beams. 
     A third challenge is the constraint that, for optimal performance, the micro-lenses be positioned accurately relative to their corresponding emitter or receiver. The tolerance for such positioning is low. As such, it is desirable to separate section  445  of the light guide so that it may be positioned accurately, and to allow more tolerance for the remaining portions of the light guide as may be required during assembly or required for robustness to movement due to trauma of the electronic device. 
     Configuration no. 3, as illustrated in  FIGS. 44-46 and 54 , serves to overcome these, and other, challenges. 
     Reference is made to  FIG. 44 , which is a simplified diagram of a side view of a dual-unit guide, in the context of an electronic device having a display  637  and an outer casing  827 , in accordance with an embodiment of the present invention. Shown in  FIG. 44  is an arrangement similar to that of  FIG. 40 , but with light guide  450  split into an upper portion  463  and a lower portion  464 . The micro-lenses are located at an upper surface  466  of lower portion  464 . As such, the micro-lenses are not embedded in the collimating lens portion of light guide  464 . 
     In configuration no. 2, the curved shape of the collimating lens necessitated a fan/feather pattern for the micro-lenses etched thereon. In distinction, in configuration no. 3 the micro-lenses are etched on rectangular surface  466 , and are arranged as parallel rows. Such a parallel arrangement, referred to herein as a “tubular arrangement”, is shown in  FIG. 46 . Specifically, a parallel series of micro-lenses  467  are shown along an upper surface of light guide  464  in  FIG. 46 . 
     An advantage of configuration no. 3 is that the flat upper surface of the light guide may be molded as nearly parallel with the screen surface as possible, since the mold is one flat surface that lifts off the top of light guide  464 . Furthermore, in configuration no. 3, only portion  464  of the light guide has a low tolerance requirement for positioning. Portion  463  has a higher tolerance, since its surfaces are not placed at a focal point of an element. 
     As shown in  FIG. 44 , light beams  100  emitted by emitter  200  enter light guide unit  464  at surface  465 , are reflected by reflective surface  451  through surface  466 , and into light guide unit  463 . Inside light guide unit  463 , light beams  100  are reflected by surface  452 , and exit through surface  453  over display  637 . 
       FIG. 44  indicates that the height, H 3 , added by the light guide over display  637  comprises the sum of the height, H 1 , of internal reflective surface  452 , and the height, H 2 , of the thickness of outer casing  827 . 
     Reference is made to  FIG. 45 , which is a picture of light guide units  463  and  464 , within the context of a device having a PCB  700  and an outer casing  827 , in accordance with an embodiment of the present invention. The tubular pattern on the upper surface of light guide unit  464  is a fine pattern. In order for this pattern to distribute the light beams correctly, light guide  464  is placed precisely relative to its respective LED or PD. By contrast, light guide unit  463  has a flat reflective surface and, as such, does not require such precision placement.  FIG. 45  indicates the relative positioning of light guide units  463  and  464 . Their alignment is represented by a distance  523 , and has a tolerance of up to 1 mm. A distance  522  represents the height between the light guide units. 
     Reference is made to  FIG. 46 , which is a top view of light guide units  463  and  464  of  FIG. 45 , in accordance with an embodiment of the present invention. Tubular pattern  467  appears on the upper surface of light guide unit  464 . 
     Touch Screen System Configuration No. 4 
     Configuration nos. 2 and 3 relate to detection of a small touch area  980  in  FIGS. 31, 32, 36 and 37 . A typical use case for such touch area size is stylus input. However when the expected use generates a relatively large touch area, such as the area of a fingertip, a precise touch location may be determined without the micro-lenses used in configurations 2 and 3. 
     Reference is made to  FIG. 47 , which is a simplified diagram of shift-aligned emitters and detectors for a light-based touch screen, for detecting finger touches, in accordance with an embodiment of the present invention.  FIG. 47  shows a system intended for finger touch. Light from an emitter  210  is distributed uniformly across the wide beam that spans lenses  441  and  442 . Aside from the uniform distribution of light, the system of  FIG. 47  is the same as the system of  FIG. 30 . 
     Reference is made to  FIG. 48 , which is a simplified illustration of finger touch detection on the screen of  FIG. 47 , in accordance with an embodiment of the present invention.  FIG. 48  shows a large touch object  980  detected by two detection channels,  201 - 301  and  201 - 302 . Light from emitter  201  is substantially collimated, such that the left half of the beam from emitter  201  reaches detector  301 , and the right half of the beam from emitter  201  reaches detector  302 . A significant portion of each channel is blocked by pointer  980 . The amount of light blocked from each detector is shown as portions  975  and  976 . The center of the blocking pointer  980  is determined by interpolating these two amounts. In addition, because neighboring light beams  201 - 301  and  201 - 302  detect the touch, pointer  980  lies on the border between these two beams. Thus, the leftmost edge of pointer  980  is determined based on the portion of beam  201 - 301  that is blocked. Similarly, the rightmost edge of pointer  980  is determined based on the portion of beam  201 - 302  that is blocked. As such, the segment, or in two dimensions—the area, covered by the pointer is determined. 
     Touch Screen System Configuration No. 5 
     Configuration no. 5 uses a reflective light guide and lens that reduce the height of a light guide above a display. The reflective light guide and lens of configuration no. 5 are suitable for use with the feather pattern lenses of configuration no. 2, with the tubular pattern lenses of configuration no. 3, with the collimating lenses of configuration no. 4, and also with the alternating reflective facets of configuration no. 6. Many electronic devices are designed with a display surface that is flush with the edges of the devices. This is often an aesthetic feature and, as such, when integrating light-based touch screens with electronic devices, it is desirable to minimize or eliminate the raised rims. Less visibly prominent rims result in sleeker, more flush outer surfaces of the devices. 
     Moreover, in light-based touch screens, the raised rim occupies a width around the display, beyond the edges of the display. Many electronic devices are designed with display surfaces that seamlessly extend to the edges of the devices. This is often an aesthetic feature and, as such, when integrating light-based touch screens with electronic devices, it is desirable to design the reflective raised rims in such a way that they appear as seamless extensions of the display. 
     Configuration no. 5 achieves these objectives when light beams are projected over air above the touch surface, by reducing bezel height and providing a seamless transition between a display edge and an outer border of a device, resulting in a more appealing aesthetic design. The light guide of configuration no. 5 integrates with an outer casing having an elongated rounded edge, thereby softening sharp angles and straight surfaces. 
     Configuration no. 5 employs two active mirror surfaces; namely, a parabolic reflective surface that folds and focuses incoming light to a focal location, and an elliptical refractive surface that collects light from the focal location and collimates the light into beams across the screen. 
     Reference is made to  FIG. 49 , which is a simplified diagram of a side view of a light guide within an electronic device, in accordance with an embodiment of the present invention. Shown in  FIG. 49  is a light guide  468  between an outer casing  828  and a display  637 . Light beams from an emitter  200  enter light guide  468  through a surface  445 . A feather pattern of micro-lenses is present on a lower portion of surface  445 , in order to scatter the light beams  100 . Light beams  100  are reflected by an internal concave reflective surface  469  and by a parabolic reflective surface  470 , and exit light guide  468  through an elliptical refractive surface  471 . Elliptical refractive surface  471  redirects at least a portion of light beams  100  in a plane parallel with the surface of display  637 . Light beams  100  are received at the other end of display  637 , by a similar light guide that directs the beams onto a light receiver  300 . The light intensity detected by light receiver  300  is communicated to a calculating unit  770 . 
     Reference is made to  FIG. 50 , which is a simplified diagram of a side view cutaway of a portion of an electronic device and an upper portion of a light guide with at least two active surfaces for folding light beams, in accordance with an embodiment of the present invention. Shown in  FIG. 50  is an upper portion of a light guide  472 . Surface  473  is part of a parabola, or quasi-parabola, or alternatively is a free form, having a focal line  475 . Focal line  475 , and surfaces  473  and  474  extend along the rim of display  637 . Surface  474  is part of an ellipse, or quasi-ellipse, or alternatively a free form, having focal line  475 . 
     On the emitter side, light beams enter the light guide, and parabolic mirror  473  reflects the beams to a focal point inside the light guide. Refracting elliptical lens  474  has the same focal point as parabolic mirror  473 . Elliptical lens  474  refracts the light from the focal point into collimated light beams over display  637 . On the receiver side, collimated light beams enter the light guide, and are refracted by elliptical lens  474  into a focal point. Parabolic mirror  473  reflects the beams from the focal point inside the light guide, to collimated output beams. 
     Surface  469  in  FIG. 49  folds light beams  100  upwards by 90°. Surface  469  is formed as part of a parabola. In one embodiment of the present invention, surface  469  is corrected for aberrations due to input surface  445  being slightly inclined rather than perfectly vertical, and also due to the light source being wider than a single point. 
     Surfaces  469  and  470  use internal reflections to fold light beams. Thus these surfaces need to be protected from dirt and scratches. In  FIG. 50 , surface  473  is protected by outer casing  829 . The lower portion (now shown) of light guide  472  is deep within the electronic device, and is thus protected. 
     Using configuration no. 5, substantially all of reflective surface  473  is located below the upper surface of display  637 . Thus, this configuration adds less height to an electronic device than does configuration no. 2, when projecting light beams through the air above the touch surface. Referring back to  FIG. 49 , the height, H 3 ′, added by the light guide in the present configuration is approximately the thickness, H 2 , of the outer casing, which is less than the corresponding height, H 3 , in configuration no. 2. Moreover, the convex shape of surface  471  of  FIG. 40  and surface  474  of  FIG. 50  is easier for a user to clean than is the perpendicular surface  453  of  FIG. 40 . Thus a user can easily wipe away dust and dirt that may accumulate on display  637  and on surface  471 . It is noted that configuration no. 5 eliminates the need for surface  454  of  FIG. 40 , since outer casing  828  is flush with the height of surface  471 , instead of being above it. 
     The convex shape of surface  471  of  FIG. 49  makes the bezel less visibly prominent than does the perpendicular surface  453  of  FIG. 40 . 
     Some electronic devices are covered with a flat sheet of glass that extends to the four edges of the device. The underside of the glass is painted black near the devices edges, and the display is viewed through a clear rectangular window in the middle of the glass. Examples of such devices include the IPHONE®, IPOD TOUCH® and IPAD®, manufactured by Apple Inc. of Cupertino, Calif., and also various models of flat-panel computer monitors and televisions. In some cases, the light guides surrounding the various touch screens described herein may appear non-aesthetic, due to (a) the light guide being a separate unit from the screen glass and thus the border between them is noticeable, and (b) the light guide extending below the screen and thus, even if the underside of the light guide is also painted black, the difference in heights between the bottom of the light guide and the screen glass is noticeable. Embodiments of the present invention employ a two-unit light guide to overcome this problem. 
     In one such embodiment, the upper unit of the light guide is merged with the screen glass. In this regard, reference is made to  FIG. 51 , which is a simplified drawing of a section of a transparent optical touch light guide  476 , formed as an integral part of a protective glass  638  covering a display  637 , in accordance with an embodiment of the present invention. A daylight filter sheet  639  on the underside of protective glass  638  serves, instead of black paint, to hide the edge of display  637 , without blocking light beams  100 . Light guide  476  has an outer elliptical surface  478  and an inner parabolic surface  477 , and merges smoothly with an outer casing  830 . Light beams  100  pass through light guide  476  as in  FIG. 50 . 
     In some cases, the cost of manufacturing a protective glass cover with an integrated reflective lens may be expensive. As such, in an alternative embodiment of the present invention, a black object is placed between the upper and lower units of the light guide. The height of the black object is aligned, within the electronic device, with the height of the black paint on the underside of the protective glass. In this regard, reference is made to  FIG. 52 , which is a simplified illustration of the electronic device and light guide of  FIG. 50 , adapted to conceal the edge of the screen, in accordance with an embodiment of the present invention. Shown in  FIG. 52  is black paint, or alternatively a daylight filter sheet  641 , on the underside of protective glass  640 , covering display  637 . A black plastic element  482  is aligned with black paint/daylight filter sheet  641 , so that the edge of protective glass  640  is not discernable by a user. Black plastic element  482  transmits infra-red light to allow light beams  100  to pass through. 
     Reference is made to  FIG. 53 , which is a simplified diagram of a light guide  483  that is a single unit extending from opposite an emitter  200  to above a display  637 , in accordance with an embodiment of the present invention. A portion of an outer casing  832  is shown flush with the top of light guide  483 . The lower portion of light guide  483  has a feather pattern of micro-lenses  484  to scatter the light beams arriving from emitter  200 . At the receiving side, the light beams exit through the bottom of a light guide similar to light guide  483 , towards a receiver. The same feather pattern  484  breaks up the light beams en route to the receiver. 
     Reference is made to  FIG. 54 , which is a simplified diagram of a dual-unit light guide, in accordance with an embodiment of the present invention. Shown in  FIG. 54  is a light guide with an upper unit  485  and a lower unit  486 . A portion of an outer casing  832  is flush with the top of light guide unit  485 . A display  637  is shown to the right of light guide unit  485 . The top surface of light guide unit  486  has a tubular pattern of micro-lenses  487  to break up light beams arriving from an emitter  200 . At the receiving side, the light beams exit through the bottom of a light guide similar to the light guide shown in  FIG. 54 , towards a receiver. The same tubular pattern  487  breaks up the light beams en route to the receiver. 
     As explained hereinabove with reference to  FIGS. 40 and 49 , the positioning of light guide unit  486  with tubular pattern  487  requires high precision, whereas the positioning of light guide unit  485  does not require such precision. The effect of tubular pattern  487  on the light beams depends on its precise placement relative to its respective emitter or receiver. The active surfaces in light guide unit  485  are more tolerant, since they are largely self-contained; namely, they are both focused on an internal focal line, such as focal line  475  of  FIG. 50 . 
     It is noted that placement of emitters and receivers underneath a device screen, and placement of a collimating reflective element opposite each emitter or receiver, imposes restrictions on the thickness of the device. A first restriction is that the thickness of the device be at least the sum of the screen thickness and the emitter or receiver thickness. A second restriction is that in order to properly collimate light that is reflected upward above the screen, the reflective element opposite the emitter or receiver be curved into a convex “smile” shape, as shown inter alia in  FIGS. 41 and 42 . The convex shape adds to the total thickness of the device. 
     Designers of tablets and e-book readers strive to achieve as slim a form factor as possible. As such, according to an embodiment of the present invention, the receivers and collimating lenses are placed inside a border surrounding the screen, instead of being placed underneath the screen. This is particularly feasible for tablets and e-book readers that provide a non-screen border area for holding the device. 
     Reference is made to  FIG. 55 , which is a simplified diagram of a touch screen device held by a user, in accordance with an embodiment of the present invention. Shown in  FIG. 55  is a device  826  with a touch screen  800  surrounded by a frame  840  held by hands  930 . 
     Reference is made to  FIG. 56 , which is a simplified diagram of a touch screen with wide light beams covering the screen, in accordance with an embodiment of the present invention.  FIG. 56  shows a top view and a side view of a touch system with a touch screen  800 , in the context of an electronic device such as a tablet or an e-book reader.  FIG. 56  also shows emitters  200  and receivers  300 , each coupled with a pair of lenses  550  and  551 , separated by an air gap  555 , for collimating light. The side view shows a device casing  827  and a frame  849  surrounding touch screen  800 . Frame  849  provides a grip for a user to hold the device, and is wide enough to encase elements  200 ,  300 ,  550  and  551 . 
     Light is more efficiently collimated over a short distance using multiple air-to-plastic interfaces than with a solid lens. The emitter, receiver and lenses are substantially coplanar with the surface of touch screen  800 . The flat non-curved profile of lenses  500  and  551  along the height of the device is lower than the profile of the lenses of  FIGS. 41 and 38 , due to the fact that in the case of lenses  500  and  551  light is projected only along the plane of the screen surface. The only height added to the device form factor is the height of the bezel, or lens  551 , above touch screen  800  for directing light across the screen. If micro-lens patterns are used, e.g., to create overlapping beams, then a third lens is added that includes the micro-lens patterns. Alternatively, the micro-lens patterns may be formed on one of the two lenses  500  and  551 . 
     Reference is made to  FIGS. 57-59 , which are respective simplified side, top and bottom views of a light guide in the context of a device, in accordance with an embodiment of the present invention.  FIG. 57  is a side view showing a display  635  and a side-facing emitter  200  that is substantially coplanar with display  635 . A multi-lens assembly reflects light above display  635  and outputs a wide beam.  FIG. 57  shows the multi-lens assembly with three sections  550 - 552  separated by air gaps  555  and  556 . Sections  550  and  551  are connected beneath air gap  555  and form part of a rigid frame that surrounds display  635 . The frame includes a cavity  220  for accommodating side-facing emitter  200  or a similar shaped receiver. Lens sections  550  and  551  together produce a wide collimated beam as described hereinabove. Lens section  552  includes a tubular pattern of micro-lenses as described hereinabove with reference to  FIGS. 45 and 46 .  FIG. 57  shows rays of a beam  105  crossing above display  635 . A PCB  700  forms a substrate for supporting emitters  200 , display  635 , and the light guide frame. 
       FIG. 58  is a top view showing lens sections  550 - 552  separated by air gaps  555  and  556 .  FIG. 58  shows three collimated beams  105 , to illustrate how lens sections  550  and  551  collimate a wide light beam.  FIG. 58  also shows small connectors  559  that connect lens section  552  to the rigid frame formed by lens sections  550  and  551 . As such, all three sections  550 - 552  may be formed from a single piece of plastic. 
       FIG. 59  is a bottom view showing lens section  500  with emitter/receiver cavities  220  containing three emitters  200 . 
     Touch Screen System Configuration No. 6 
     In accordance with an embodiment of the present invention, high resolution touch sensitivity is achieved by combining two or more emitter-receiver pair signals that span a common area, as described hereinabove with reference to configurations nos. 2 and 3. Configuration no. 6 provides alternative optical elements and alternative arrangements of emitters and receivers for providing overlapping detection. 
     Various approaches may be used to provide overlapping detection beams. One approach is to provide two separate wide beams that are projected at slightly different heights across the screen. Both beams cover a common screen area, and thus provide multiple detection signals for touches in that area. Another approach is to provide optical elements that interleave rays of two wide beams when both beams are activated at once, which can be achieved using diffractive structures to interleave minute rays from two beams, or using slightly larger alternating facets to interleave beams on the order of 0.1-0.6 mm from two sources. Generally, the two beams are activated separately. As such, they cover a common screen area but are not actually interleaved. This latter alternative is described in what follows. 
     Reference is made to  FIG. 60 , which is a simplified illustration of a touch screen  800  surrounded by emitters and receivers, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 61 , which is a simplified illustration of an optical element  530  with an undulating angular pattern of reflective facets, shown from three angles, in accordance with an embodiment of the present invention. Shown in  FIG. 61  are three views, (a), (b) and (c), of optical element  530 . Light from the emitters enters optical element  530  as wide angled overlapping beams.  FIG. 61  shows emitters  200 - 202  facing a surface  541  of element  530 . Wide beams  107 - 109  from respective emitters  200 - 202  enter element  530  through surface  541 .  FIG. 61  also shows the distance, or pitch, between neighboring emitter elements. 
     Each of wide beams  107 - 109  spans two pitches and, as such, the wide beams overlap in the area between neighboring emitters. A surface  542  of element  530  is formed as a wave-like pattern of facets, alternatingly directed at neighboring emitters.  FIG. 61( c )  shows alternating shaded and non-shaded facets on surface  542 . In element  530  between emitters  200  and  201 , shaded facets aimed at emitter  200  are interleaved with non-shaded facets aimed at emitter  201 . In element  530  between emitters  201  and  202 , shaded facets aimed at emitter  202  are interleaved with non-shaded facets aimed at emitter  201 . 
     Reference is made to  FIG. 62 , which is a simplified illustration of an optical element reflecting, collimating and interleaving light from two neighboring emitters, in accordance with an embodiment of the present invention. As shown in  FIG. 62 , each reflective facet of element  530  collimates rays from its corresponding emitter, thereby interleaving collimated rays from two emitters.  FIG. 62  shows optical element  530  reflecting and collimating light from two neighboring emitters  200  and  201 . Alternating facets of element  530  focus on these two elements. By interleaving collimated rays, element  530  collimates light from two emitters across the screen in overlapping wide beams. Elements  530  at an opposite screen edge direct the wide beams onto respective receivers. 
     Each facet on surface  542  is precisely angled to focus on its element. The surface areas of each facet are also configured so that sufficient amounts of light are provided for detection. 
     Alternative embodiments of optical element  530  collimate and interleave incoming wide beams through refraction instead of reflection. In such case, the wave-like multi-faceted surface is situated at an input or output surface of optical element  530 . In the case of reflecting facets, the facets re-direct light inside the optical element. 
     At times, it is desirable to run a touch screen in a low frequency mode, e.g., in order to save power. Configuration no. 6 enables an accurate low-frequency scan mode. In accordance with an embodiment of the present invention, two detection signals along a screen axis are provided for each touch location. In low frequency mode, during a first scan every other emitter-receiver pair is activated, thus activating only half of the pairs along only one screen axis, but nevertheless covering the entire screen. During a second scan, the remaining emitter-receiver pairs along this axis are activated. As such, odd emitter-receive pairs are first activated, then even emitter-receiver pairs, thus providing two full screen scans and spreading usage evenly across all emitter and receiver elements. In order to keep power consumption at a minimum, only emitter-receiver pairs along the shorter edge of a rectangular screen are activated. 
     In an alternative embodiment of the present invention both axes of a screen are scanned, and each scanned axis provides initial touch information about the screen. As such, instead of sequentially activating multiple scans of a single axis, in the alternative embodiment sequential activation of scans of separate axes are activated. A sequence of four scans are activated at four sampling intervals; namely, (i) a first half of the emitter-receiver pairs along a first screen axis are scanned; (ii) a first half of the emitter-receiver pairs along a second screen axis are activated, (iii) the second half of the emitter-receiver pairs along the first screen axis are activated, and (iv) the second half of the emitter-receiver pairs along the second screen axis are activated. 
     Design of Reflective Elements 
     A goal in designing alternating reflective or refractive facets of an optical element, is to generate a light distribution that provides good gradients as a basis for interpolation, by way of a linear signal gradient, S(x), from an emitter to a receiver. A number of parameters affect the light distribution. 
     Reference is made to  FIG. 63 , which is a simplified diagram of a multi-faceted optical element  530 , in accordance with an embodiment of the present invention. Shown in  FIG. 63  are parameters that control light from each facet of the optical element, as described in what follows. 
     The light intensity distribution depends on a polar angle, θ, in accordance with the third power, cos 3  θ. The angle θ is a function of distance  110  between beams of a single emitter or receiver element that go to different facets, and of distance  111  between the emitter or receiver element and element  530 . 
     The facet width, B, is a readily adjustable parameter. 
     The Fresnel loss, F, is the amount of light lost due to reflection caused by the refractive index of element  530 , when a beam enters optical element  530 . Variation of Fresnel loss F between different angles θ under Brewster&#39;s angle is less than 1%, and is therefore negligible. 
     Facet beam width, Y, is the total width covered by a single facet beam. The alternating facets generate gaps in the light from emitter  201 , as neighboring facets are focused on neighboring emitter  202 . Light from each facet covers the gaps. Facet beam width, Y, depends on facet width B and on the widths of neighboring facets.  FIG. 63  shows facets  545 ,  547  and  549  aimed at emitter  201  and respective facet-beam widths Y 545 , Y 547  and Y 549  that together cover the neighboring facets  548  and  546  aimed at emitter  202 . 
     Reference is made to  FIG. 64 , which is a simplified graph showing the effect of reflective facet parameters θ, Y and B on light distribution for nine facets, in accordance with an embodiment of the present invention. The graph of  FIG. 64  also shows actual light distribution, and a reference linear function. As seen in  FIG. 64 , the actual light distribution signal is approximately linear. The data in the graph is normalized based on the central facet, located at location 0 on the x-axis, being assigned a value of 1 in all aspects. As such, the facet width B is labeled Bnorm in the graph, and facet widths are normalized relative to the width of the central facet. Generally, the angular parameter θ provides a sloped curve, which is flat for small values of θ, as seen in  FIG. 64  in the flat portion of the e curve, labeled cos 3, between positions 0 and 2 along the x-axis. The gradient for small θ is increased by adjusting parameter B, which in turn affects parameter Y, labeled Yfactor. The complete signal is labeled signal in the graph, and it is approximately linear. 
     Light intensity for facet k, as a function of parameters θ, B, F and Y, is described in accordance with 
                         S   k       S   1       =           cos   3     ⁡     (     θ   k     )           cos   3     ⁡     (     θ   1     )         ·       B   k       B   1       ·       F   k       F   1       ·       Y   k       Y   1           ,           (   1   )               
where the lighting of facet k is normalized based on θ=0 for the central facet.
 
     TABLE I lists parameters for each facet in a series of nine facets that are focused on one emitter or receiver element. In TABLE I, x-pos denotes the distance in millimeters from the central facet, B denotes the facet width in millimeters, B-norm denotes the normalized facet width, based on the central facet having a width of 1, Yfactor denotes the facet beam width, normalized to the width of the central facet beam, Signal denotes the normalized signal value for each facet, and Line denotes signal values for a reference straight line. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Facet parameters for nine facets 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Facet no. 
                 x-pos 
                 B 
                 B-norm 
                 Yfactor 
                 cos 3 θ 
                 Signal 
                 Line 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 0 
                 0.66 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 2 
                 1.265 
                 0.59 
                 0.893939 
                 1.065574 
                 0.973981 
                 0.927774 
                 0.913516 
               
               
                 3 
                 2.46 
                 0.56 
                 0.848485 
                 1.11588 
                 0.907237 
                 0.858978 
                 0.831817 
               
               
                 4 
                 3.605 
                 0.55 
                 0.833333 
                 1.150442 
                 0.817261 
                 0.78351 
                 0.753537 
               
               
                 5 
                 4.725 
                 0.55 
                 0.833333 
                 1.171171 
                 0.717801 
                 0.700557 
                 0.676966 
               
               
                 6 
                 5.835 
                 0.57 
                 0.863636 
                 1.160714 
                 0.618698 
                 0.620205 
                 0.601079 
               
               
                 7 
                 6.965 
                 0.59 
                 0.893939 
                 1.135371 
                 0.524528 
                 0.532371 
                 0.523824 
               
               
                 8 
                 8.13 
                 0.62 
                 0.939394 
                 1.087866 
                 0.438568 
                 0.448188 
                 0.444177 
               
               
                 9 
                 9.35 
                 0.64 
                 0.969697 
                 1.027668 
                 0.362027 
                 0.360769 
                 0.360769 
               
               
                   
                 10 
               
               
                   
               
            
           
         
       
     
     TABLE II lists parameters for a series of alternating facets focused on two neighboring elements, such as an emitter and a neighboring receiver. In TABLE II, facets nos. 1-5 are focused on an emitter, and facets nos. 6-9 are focused on a neighboring receiver. Three values are listed for each facet; namely, its width, B, its location, x-pos, along the x-axis relative to the center of the central facet for the emitter, and the location, border_pos, of the facet&#39;s outer edge. All facet values are specified in millimeters. 
                     TABLE II                  Nine alternating facets                                     Facet no.   B   x-pos   border pos                                                 1   0.66   0   0.33           9   0.64   0.65   0.97           2   0.59   1.265   1.56           8   0.62   1.87   2.18           3   0.56   2.46   2.74           7   0.59   3.035   3.33           4   0.55   3.605   3.88           6   0.57   4.165   4.45           5   0.55   4.725   5                   5                        
Signals Generated by Element  530 
 
     Reference is made to  FIG. 65 , which is a simplified illustration of a touch screen with a wide light beam crossing the screen, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 66 , which is a simplified illustration of a touch screen with two wide light beams crossing the screen, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 67 , which is a simplified illustration of a touch screen with three wide light beams crossing the screen, in accordance with an embodiment of the present invention. As shown in  FIG. 65 , a screen  800  is surrounded with emitters and receivers. A wide beam  167  is shown representing a wide detection area on screen  800 , that is detected by an emitter-receiver pair  200 - 300 . Wide beam  167  is generated by optical elements, such as element  530  described hereinabove but not shown in  FIGS. 65-67 . A first element  530  collimates light from emitter  200 , and a second element  530  focuses wide beam  167  onto receiver  300 . A graph  910  shows the gradient of signal intensities detected across the width of wide beam  167 . 
       FIG. 66  shows neighboring wide beams  168  and  169 , representing wide detection areas on screen  800  detected by respective emitter-receiver pairs  201 - 301  and  202 - 302 . Respective graphs  911  and  912  illustrate the gradient of signal intensities detected across the widths of wide beams  168  and  169 . 
       FIG. 67  shows the three wide beams of  FIGS. 5 and 66 . As seen in  FIG. 67 , the left half of beam  167  is overlapped by half of beam  168 , and the right half of beam  167  is overlapped by half of beam  169 . The intensity gradients in graphs  910 - 912  indicate that a touch at any location along the width of beam  167  is detected along two gradients of two overlapping wide beams. Similarly, a touch at any location on the screen is detected in both the vertical and the horizontal axis along two gradients of two overlapping wide beams on each axis. A precise touch coordinate is calculated by interpolating touch locations of the two signals based on the detection signal gradients.  FIG. 62  shows the light signal attenuation gradients  920  and  921  across the widths of the two overlapping beams. Light signal attenuation gradient  920  corresponds to the beam emitted from emitter element  200 , and light signal attenuation gradient  921  corresponds to the beam emitted from emitter element  201 . As such, the beam has maximum intensity directly above the element, and tapers off at either side. Having two different sloping gradients for the overlapping beams is of advantage for calculating a precise touch location, as described hereinbelow. 
     Reference is made to  FIG. 68 , which is a simplified graph of light distribution of a wide beam in a touch screen, in accordance with an embodiment of the present invention. The lower portion of  FIG. 68  shows a path across wide beam  167 , and the upper portion of  FIG. 68  is a graph depicting signal intensity distribution along this path. The graph&#39;s x-axis represents the horizontal screen dimension in units of millimeters. The graph&#39;s y-axis represents the baseline signal intensity detected by emitter-receiver pair  200 - 300  situated at 10 mm along the screen axis. The signal corresponds to a screen with emitter and receiver elements arranged at a pitch of 10 mm. As such, the detected wide beam spans 20 mm. The spikes in the graph are caused by the alternating facets of optical element  530  describe above, which alternately focus rays at neighboring elements. As such, spikes correspond to facets belonging to the measured emitter-receiver pair, and the neighboring troughs correspond to facets belonging to a neighboring emitter-receiver pair. Despite these spikes, detection signals of a finger or another object along the measured screen axis have a relatively smooth gradient along the entire 20 mm span of the beam since the finger is wider than the narrow spike and trough channels. As such, a finger blocks a series of spikes which remain substantially uniform as the finger slides long the screen axis. E.g., a fingertip is approximately 6 mm wide, whereas there are 8-9 spikes in 10 mm in the graph of  FIG. 68 . 
     Reference is made to  FIG. 69 , which is a simplified illustration of detection signals from three wide beams as a fingertip moves across a screen, in accordance with an embodiment of the present invention. Shown in  FIG. 69  are three detection signals of a fingertip as it moves across three neighboring wide beams along a screen axis. From each of the signals it is apparent that as the finger enters a wide beam, the finger blocks a small portion of the beam. As the finger moves along the axis toward the center of the beam, it blocks progressively more of the beam until it blocks roughly 40% of the beam intensity, indicated in the graph by a minimum detection of 60% of the expected baseline signal. As the finger moves further along, it blocks progressively less of the beam. The shape of the detection curve is relatively smooth, despite the peaks and troughs in the light beam shown in  FIG. 68 . There are slight fluctuations along the detection curves of  FIG. 69  that are at least partially due to the peaks, but these fluctuations are minimal and do not significantly distort the trend of the signal. 
     Reference is made to  FIGS. 70-72 , which are simplified graphs of light distribution in overlapping wide beams in a touch screen, in accordance with an embodiment of the present invention. Taken together,  FIGS. 68 and 70-72  show a light distribution across three neighboring wide light beams on a screen with emitter-receiver pairs spaced 10 mm apart. As seen in these figures, the facets of optical element  530  provide overlapping touch detection by two emitter-receiver pairs.  FIG. 70  shows the light signal from an emitter-receiver pair situated at location 0 along the measured screen axis.  FIG. 71  shows the light signal from an emitter-receiver pair situated at a location 20 mm along the measured screen axis.  FIG. 72  shows the light signals from the three emitter-receiver pairs of  FIGS. 68, 70 and 71 , and shows how these light beams cover overlapping areas of the screen surface.  FIG. 69  shows three detection signals for the three emitter-receiver pairs of  FIG. 72 , as a fingertip moves along the screen axis. 
     Touch detection signals are less smooth when using a fine-point stylus than when using a finger. E.g., a 2 mm stylus tip moving across a screen generates more fluctuations in a detection signal than does a 6 mm finger, since the stylus tip covers fewer peaks in the light signal and, therefore, moving in and out of a signal peak changes a larger part of the blocked signal. Nevertheless, embodiments of the present invention overcome this drawback and determine stylus touch locations with a high level of accuracy, by interpolating multiple detection signals. 
     Reference is made to  FIG. 73 , which is a simplified graph of detection signals from a wide beam as a fingertip moves across a screen at three different locations, in accordance with an embodiment of the present invention. Shown at the bottom of  FIG. 73  are three paths  925 - 927  traced by a finger across a wide beam  167 . Path  925  is near LED  200 , path  926  is mid-screen, and path  927  is near a PD  300 . The graph in the upper portion of  FIG. 73  shows three detection signals of a fingertip as it traverses the three paths  925 - 927 , labeled in the graph legend as LED edge, Midscreen and PD edge, respectively. The three detection signals in the graph are substantially overlapping. As such, the signal is uniformly detected along its depth, and the signal varies as a function of the touch along only one axis of the screen. Thus determining a touch location along a first axis is independent of the detection signal along a second axis. Moreover, the intensity of the signal is uniform along the second axis, making the signal robust. 
     Supporting Various Screen Sizes 
     Some embodiments of configuration no. 6 include optical elements with alternating facets that are focused on two neighboring light emitting or receiving elements. When such an optical element is separate from the light emitters or receivers, the emitters or receivers are generally spaced at a particular pitch. When such an optical element is formed as a rigid module together with an emitter or a receiver, the embedded emitter or receiver is precisely positioned with respect to the reflective facets. The facets aimed at a neighboring module, are aimed in accordance with the embedded emitter or receiver in the neighboring module that is similarly situated in its module. Such positioning potentially restricts the size of a screen to integral multiples of the pitch. E.g., with a pitch of 10 mm between emitters, the screen dimensions must be integral multiples of 10 mm. Embodiments of the present invention are able to overcome this restriction, as described in what follows. 
     Reference is made to  FIG. 74 , which is a simplified diagram of four optical elements and four neighboring emitters, in accordance with an embodiment of the present invention. Shown in  FIG. 74  are four optical elements  531 - 534  arranged in a row. Each element is positioned opposite a respective one of emitters  200 - 203 . The same configuration is assembled for receivers, or for alternating emitters and receivers. In the case of receivers, emitters  200 - 203  are replaced by receivers; and in the case of alternating emitters and receivers, emitters  200  and  202  are replaced by receivers. 
     Optical elements  531 ,  532  and  534  are all of the same width, e.g., 10 mm; i.e., w 1 =w 2 =w 4 . The pitch, P 1 , between emitters  200  and  201  is a standard distance, e.g., 10 mm. The facets of optical element  531  are constructed for emitters that are at a standard pitch of 10 mm. Pitches P 2  and P 3  may be nonstandard. By enabling a device manufacturer to insert a single emitter at a non-standard pitch, the manufacturer can accommodate any screen size. The width, w 3 , of optical element  533  is customized for a non-standard screen size; e.g., for a screen length of 96 mm, w 3  is 6 mm instead of 10 mm, and pitches P 2  and P 3  are each 8 mm. Optical element  532  is a hybrid element—the left half of element  532  has facets aimed at emitters  200  and  201 , which are positioned according to a standard 10 mm pitch, and the right half of element  532  is special having facets aimed at emitters  201  and  202 , where emitter  202  has a non-standard placement. Optical element  534  is also a hybrid element, as its left half has facets aimed at emitters  202  and  203 , whereas its right half is aimed at two standard pitch emitters. Optical element  533  is non-standard throughout—it is not as wide as the standard elements and has every other of its facets aimed at emitter  202 . In this example, the width of the beam from emitter  202  is roughly 16 mm, as compared to the standard 20 mm width. As such, emitter  202  is placed slightly closer to optical element  533 . 
     Diffractive Surfaces 
     As described hereinabove, diffractive surfaces are used in embodiments of the present invention to direct beams from two emitters along a common path. Reference is made to  FIG. 75 , which is a simplified diagram of a diffractive surface that directs beams from two emitters along a common path, in accordance with an embodiment of the present invention. Shown in  FIG. 75  are emitters  200  and  201  emitting arcs of light  107  and  108  into two collimating lenses  525 . Wide beams  167  and  168  exit lenses  525  and enter refractive surface  560 , which directs both beams  167  and  168  into a wide beam  193  that crosses the screen. A similar optical arrangement splits wide beam  193  onto two receivers at the opposite screen edge. Each emitter is activated separately with a respective opposite receiver. Beams from the two emitters have different signal gradients along the width of beam  193 , as explained hereinabove. The two detection signals are used to calculate a touch location from EQS. (2) and (3) provided hereinbelow. 
     Parallel Overlapping Beams 
     As described hereinabove, parallel wide beams projected at slightly different heights over a screen are used in alternative embodiments of the present invention, to provide multiple detection signals for a touch event on the screen. 
     Alternating Emitters and Receivers 
     In an alternative embodiment of the present invention, emitters and receivers are positioned alternately along each screen edge. Reference is made to  FIG. 76 , which is a simplified diagram of a touch screen surrounded with alternating emitters and receivers, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 77 , which is a simplified illustration of a touch screen surrounded with alternating emitters and receivers, and a wide beam crossing the screen, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 78 , which is a simplified illustration of a touch screen surrounded with alternating emitters and receivers and two wide beams crossing the screen, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 79 , which is a simplified illustration of a touch screen surrounded with alternating emitters and receivers and three wide beams crossing the screen, in accordance with an embodiment of the present invention.  FIGS. 77-79  show overlapping wide beams, similar to those of  FIGS. 65-67  described hereinabove. 
     Reference is made to  FIG. 80 , which is a simplified illustration of a collimating optical element reflecting and interleaving light for an emitter and a neighboring receiver, in accordance with an embodiment of the present invention.  FIG. 80  shows optical element  530  interleaving neighboring light beams, wherein a first beam is outgoing from emitter  200  and a second beam is incoming to neighboring receiver  301 .  FIG. 80  also shows signal gradient  920  for the first beam and signal gradient  921  for the second beam. When a touch is detected on both beams, the sloping gradients enable determination of a precise touch location by interpolation, as described hereinbelow. 
     As indicated hereinabove with reference to  FIG. 73 , the detection signal does not vary with depth of touch location within a wide beam. Therefore, the opposing directions of the adjacent overlapping wide beams do not affect the touch detection signal. In turn, this enables interpolating signals from overlapping beams without regard for direction of each beam. 
     As explained above with reference to configuration no. 4, when a touch pointer is expected to be wide, such as a finger, the wide beams need not overlap, since it is expected that the finger span at least a portion of two neighboring beams, assuming the beams are slightly less wide than the finger. Moreover, the finger covers each beam beginning at one of the beam&#39;s edges. Therefore, even when the beam distributes light evenly across the beam&#39;s width, the system is able to determine the blocked portion of each beam. E.g., if 50% of the beam is blocked, the finger is blocking half of the beam; and if 25% of the beam is blocked, the finger is blocking one quarter of the beam. This is different than the attenuated signal gradients  920  and  921  of  FIG. 80 . The location of the finger is then determined by interpolating signals from two or more neighboring beams. 
     Multi-Touch Detection 
     Multi-touch locations are often difficult to identify unambiguously via light emitters that emit light in directions parallel to two axes. Reference is made to  FIGS. 81-84 , which are illustrations of multi-touch locations that are ambiguous vis-à-vis a first orientation of light emitters, in accordance with an embodiment of the present invention. As shown in  FIGS. 81 and 82 , there is ambiguity in determining the locations of a diagonally oriented multi-touch. There is further ambiguity if a multi-touch includes more than two pointers. For example, the two-touch cases shown in  FIGS. 81 and 82  are also ambiguous vis-à-vis the three-touch case shown in  FIG. 83  and vis-à-vis the four-touch case shown in  FIG. 84 . In each of these cases, row and column indicators a-h show an absence of light in the same locations. Such ambiguity is caused by “ghosting”, which refers to an effect where the shadow of one pointer obscures a portion of another pointer. 
     In accordance with an embodiment of the present invention, ghosting is resolved by use of two sets of grid orientations for touch detection. 
     Reference is made to  FIGS. 85-87 , which are illustrations of the multi-touch locations of  FIGS. 81-83  that are unambiguous vis-à-vis a second orientation of light emitters, in accordance with an embodiment of the present invention. Use of an arrangement of alternating emitters and receivers, as described hereinabove with reference to  FIGS. 76 and 77 , and use of additional optical elements to generate two sets of detection axes, provide important advantages. One advantage is generating a robust set of overlapping wide beams, whereby multiple detection signals may be interpolated in order to determine touch coordinates with high precision. Another advantage is generating overlapping wide beams on the second axis set, such that touch detection on the second axis set is also precise. 
     A dual-unit light guide is described hereinabove with reference to  FIGS. 45 and 46 . As described there, the lower portion  464  of the light guide contains reflective facets or lenses that are focused on the emitters and receivers, and the upper portion  463  includes reflective surface and lenses that do not require precision placement vis-à-vis the emitters and receivers. In configuration no. 6, the alternating reflective or refractive facets form part of the lower portion. A three-sided refractive cavity for distributing light beams in three directions is formed as part of the upper portion. In configuration no. 6, use of micro-lenses  467  is not required. Alternatively, the alternating facets are formed in transparent plastic modules that include an emitter or receiver, as described hereinbelow with reference to  FIG. 112 . An arrangement of these modules replaces lower portion  464 , and upper portion  463  remains. 
     Reference is made to  FIG. 88 , which is a simplified illustration of a touch screen with light beams directed along four axes, in accordance with an embodiment of the present invention. Shown in  FIG. 88  is a row of light emitters  200  along the top edge of a screen  800 , and a row of light receivers  300  along the bottom edge of screen  800 . The left and right edges of screen  800  include opposing rows of combined emitter-receiver elements  230 . Elements  230  act as emitters and as receivers. In an embodiment of the present invention, an emitter and a receiver are combined in a single unit, such as the reflective and transmissive sensor manufactured by Vishay Corporation of Malvern, Pa. In another embodiment of the present invention, an LED is used for both light emission and detection. An integrated circuit that both emits and detects light using an LED and a current limiting resistor, is described in Dietz, P. H., Yerazunis, W. S. and Leigh, D. L., “Very low cost sensing and communication using bidirectional LEDs”, International conference on Ubiquitous Computing (UbiComp), October, 2003. 
     Reference is made to  FIG. 89 , which is a simplified illustration of an alternate configuration of light emitters and light receivers with two grid orientations, in accordance with an embodiment of the present invention. Shown in  FIG. 89  are light emitters  200  in an alternating pattern with light receivers  300  around a screen perimeter. Light emitted by each emitter is detected by two receivers at an opposite screen edge, the two receivers being separate by an emitter therebetween. 
     In order that the light from an emitter arrive at the outer edges of two opposite receivers, the wide beams emitted from each emitter must span a distance of three optical lenses. This is in contrast to the configuration described above with shift-aligned emitters and receivers, where the two receivers that detect light from a common emitter are positioned adjacent one another, and thus the wide beams emitted from each emitter need only span a distance of two optical lenses. 
     Reference is made to  FIG. 90 , which is a simplified illustration of a configuration of alternating light emitters and light receivers, in accordance with an embodiment of the present invention. As shown in  FIG. 90 , emitter  201  is situated between receivers  303  and  304  along the bottom screen edge, and emitter  202  is situated between receivers  301  and  302  along the top screen edge. Light from emitter  201  is detected by receivers  301  and  302 , and light from emitter  202  is detected by receivers  303  and  304 . 
     Reference is made to  FIG. 91 , which is a simplified illustration of two wide light beams from an emitter being detected by two receivers, in accordance with an embodiment of the present invention. Shown in  FIG. 91  are two wide beams from emitter  201  that exit lens  440  and arrive at lenses  441  and  443  for detection by receivers  301  and  302 , respectively. One wide beam is bordered by edges  145  and  146 , and the other wide beam is bordered by edges  147  and  148 . A cross-hatched triangular area indicates an overlap where a touch is detected at receivers  301  and  302 . 
     Reference is made to  FIG. 92 , which is a simplified illustration of two wide beams and an area of overlap between them, in accordance with an embodiment of the present invention. One wide beam, from emitter  201 , exits lens  440  and arrives at lens  441  for detection by receiver  301 . The wide beam is bordered by edges  145  and  146 . Another wide beam, from emitter  202  to receiver  303 , is bordered by edges  147  and  148 . A cross-hatched diamond-shaped area indicates an overlap where a touch is detected at receivers  301  and  303 . 
     It will thus be appreciated by those skilled in the art that any location on the screen is detected by two emitter-detector pairs, when the emitter-detector pairs are situated at opposite screen edges and, as such, an accurate touch location may be calculated as described hereinabove. 
     Reference is made to  FIG. 93 , which is a simplified illustration of a touch point  980  situated at the edges of detecting light beams, in accordance with an embodiment of the present invention.  FIG. 93  shows that it is desirable that the light beams extend to the edges of the emitter and receiver lenses, in order to accurately determine the location of touch point  980 . 
     Reference is made to  FIG. 94 , which is a simplified illustration of a finger-sized touch point in a screen designed for finger touch detection, in accordance with an embodiment of the present invention.  FIG. 94  shows a large touch point  980 , such as a finger touch, and alternating beams  201 - 301 ,  202 - 302  and  203 - 303 . A detection signal is shown next to each detector in the form of a rectangle, indicating a uniform light distribution along the width of the beam. Beams  201 - 301  and  202 - 302  have portions blocked by pointer  980 . The location of pointer  980  is determined based on the blocked portions of beams  201 - 301  and  202 - 302 . In this case, the beams are neatly collimated and span only one lens, not three. 
     Reference is made to  FIG. 95 , which is a simplified illustration of an emitter along one edge of a display screen that directs light to receivers along two edges of the display screen, in accordance with an embodiment of the present invention. Shown in  FIG. 95  are a first pair of light beams emitted from an emitter  200  at one edge of a display screen to receivers  300  and  301  along the opposite edge of the display screen, and a second pair of light beams emitted from emitter  200  to receivers  302  and  303  along the adjacent left edge of the display screen. A third pair of light beams (not shown) is emitted from emitter  200  to receivers at the adjacent right edge of the display screen. The second and third pairs of light beams are each oriented at an angle of approximately 45° relative to the first pair of light beams. 
     Also shown in  FIG. 95  is a lens  439 , used to refract light from emitter  200  to lenses  442  and  443 , which are oriented at approximately 45° to the left of lens  439 . In an embodiment of the present invention, lens  439  is made of a plastic material, which has an index of refraction on the order of 1.4-1.6. As such, an angle of incidence of approximately 84° is required in order for the light to be refracted at an angle of 45°. However, for such a large angle of incidence, the amount of light lost due to internal reflection is large. In order to improve throughput, two air/plastic interfaces are used to achieve an angle of refraction of approximately 45°, as described hereinabove. 
     Tri-Directional Micro-Lenses 
     Reference is made to  FIGS. 96 and 97 , which are simplified illustrations of a lens for refracting light in three directions, having a lens surface with a repetitive pattern of substantially planar two-sided and three-sided recessed cavities, respectively, in accordance with embodiments of the present invention. The flat surface opposite the emitter or receiver is distal to the emitter or receiver in  FIG. 96  forming a three-sided cavity, and is proximal thereto in  FIG. 97  separating two two-sided cavities. 
     Such three-sided lenses are used in several embodiments. In a first embodiment, the lens is used without an additional optical component with alternating facets for interleaving neighboring beams. In this embodiment, wide beams cover the screen but do not necessarily overlap to provide two or more detection signals for interpolation. A typical use case for this embodiment is finger input, but not stylus input. The tri-directional lens enables detection on four different axes, to eliminate ambiguity and ghosting in multi-touch cases. The tri-directional lens also provides additional touch location information; namely, four axes instead of two, and the additional information increases the precision of the touch location, even for a single touch. 
     In a second embodiment, the lens is used with an additional optical component with alternating facets for interleaving neighboring beams, or with an alternative arrangement providing overlapping detection signals. In this embodiment, overlapping wide beams provide two or more detection signals for interpolation. Typical use cases for this embodiment are finger and stylus input. The tri-directional lenses and the interleaving facets may be formed in two distinct components. The interleaving facets component is positioned closer to its emitter or receiver than the tri-directional component, since the tolerance for imprecise placement of the interleaving facets component is low, whereas the tolerance for imprecise placement of the tri-directional lens component is high. Alternatively, the tri-directional lenses and the interweaving facets may be formed in a single rigid component. For example, a diffractive grating interleaves signals from two sources and also splits the beams in three directions. 
     Shown in  FIG. 96  is a lens  527  with a pattern of micro-lenses  528  on its bottom surface. The micro-lens pattern shown in  FIG. 96  has three substantially planar sides, each side refracting light in a different direction. The pattern of micro-lenses  528  form a saw-tooth repetitive pattern along the bottom edge of the upper section of the lens. The three walls of each micro-lens  528  are slightly curved, in order to spread the light in a wider arc as it exits the lens toward an intended receiver. 
     A collimating lens section (not shown) is situated beneath lens  527 , to direct the light in parallel beams into micro-lenses  528 . 
     In some embodiments of the present invention, lens  527  is part of a two-lens arrangement, with lens  527  forming the upper of the two lenses, farther from the emitter or receiver, and nearer to the screen surface. In distinction, the two-section lens shown in  FIG. 45  has a micro-lens pattern on the top of the lower section. 
     In order to properly interleave collimated beams from the alternating facets component, the pitch of the three-sided cavities needs to be much smaller than the pitch of the alternating facets. Ideally, the pitch of the cavities should be made as small as possible. With alternating facets of about 0.6 mm, the cavities should be 0.2 mm or smaller. The dihedral angle between each pair of adjacent planes is approximately 122°, to achieve a 45° refraction using plastic having a refractive index of 1.6. However, different angles may be desired for a different set of diagonal axes, or plastic having a different refractive index may be desired, in which case the dihedral angle will be different. 
     As shown in  FIG. 96 , incoming collimated light is refracted through two air/plastic interfaces, to emerge at an angle of refraction that is approximately 45°. The first interface, along an inner plane of the micro-lens, refracts the incoming light to an angle of refraction that is approximately 58°, and the second interface refracts the light to emerge at an angle of refraction that is approximately 45°. 
     Reference is made to  FIGS. 98-100 , which are simplified illustrations of a touch screen surrounded with alternating emitters and receivers and diagonal wide beams crossing the screen, in accordance with an embodiment of the present invention.  FIGS. 98 and 99  show diagonal wide beams from emitter  200  and  201  to receiver  300 , and a corresponding signal gradient  910 .  FIG. 100  shows diagonal wide beams from emitters  202  and  204  to receivers  302  and  304 , and corresponding signal gradients  911  and  912 . These wide beams overlap wide beam  167  of  FIG. 95 , thereby providing multiple touch detections for interpolation. 
     Reference is made to  FIG. 101 , which is a simplified graph of light distribution across a diagonal wide beam in a touch screen, in accordance with an embodiment of the present invention. The lower portion of  FIG. 101  shows a wide beam  167  and a path  925  crossing this beam according to a second axis system. If the pitch between elements is 1 unit, then the width of this beam is 1/√2 units. Thus if the pitch between elements is 10 mm, then the beams along the diagonal axes are approximately 7 mm across. The upper portion of  FIG. 101  shows the distribution of light across wide beam  167 . The signal spans across approximately 14 mm of the diagonal beam, as compared with 20 mm of the vertical beam in  FIG. 69 . As described above with reference to  FIG. 68 , the signal gradient across the width of the beam enables interpolating multiple detection signals to determine a precise touch position. 
     Reference is made to  FIG. 102 , which is a simplified graph of light distribution across three overlapping diagonal wide beams in a touch screen, in accordance with an embodiment of the present invention.  FIG. 102  shows a signal distribution across three overlapping beams in a second axis system, similar to  FIG. 72 . Different widths are covered by these two sets of beams. 
     Reference is made to  FIG. 103 , which is a simplified graph of touch detection as a finger glides across three overlapping diagonal wide beams in a touch screen, in accordance with an embodiment of the present invention.  FIG. 103  shows how reception of a finger passing across three adjacent overlapping beams is detected by each beam. The maximum detection signal is approximately 40% of the baseline signal intensity, and this occurs when the finger is in the middle of the beam. In this case, the finger blocks approximately 60% of the total light of the beam. This is greater than the amount of light blocked by the same finger in  FIG. 69 ; namely, 40%. The difference is due to the diagonal beam being narrower than the vertical beam. Therefore a 6 mm fingertip blocks a greater portion of light in the beam. The detection signals are substantially smooth and robust for determining touch locations. 
     Reference is made to  FIG. 104 , which is a simplified graph of detection signals from a diagonal wide beam as a fingertip moves across the screen at three different locations, in accordance with an embodiment of the present invention.  FIG. 104  shows that touch detection remains stable along depth of a wide beam, and varies only according to its location across the width of the beam, as described hereinabove with reference to  FIG. 73 . 
     Reference is made to  FIG. 105 , which is a simplified illustration of a first embodiment for a touch screen surrounded with alternating emitters and receivers, whereby diagonal and orthogonal wide beams crossing the screen are detected by one receiver, in accordance with an embodiment of the present invention.  FIG. 105  shows an embodiment with an equal number of elements positioned along each screen edge. Three beams  167 - 169  are shown for one receiver  300 ; namely, one directed to an opposite emitter  200  and the other two directed to emitters  201  and  202  on adjacent screen edges. The diagonal beams generate two axes that are not perpendicular to one another. 
     Reference is made to  FIG. 106 , which is a simplified illustration of a second embodiment for a touch screen surrounded with alternating emitters and reciters, whereby diagonal and orthogonal wide beams crossing the screen are detected by one receiver, in accordance with an embodiment of the present invention.  FIG. 106  shows an embodiment with different numbers of elements positioned along adjacent screen edges. Three beams  167 - 169  are shown for one receiver  300 ; namely, one directed to an opposite emitter  200 , and the other two directed at substantially 45° angles to emitters  201  and  202 , one of which is on an opposite edge and another of which is positioned on an adjacent edge. These diagonal beams generate two axes that are perpendicular to one another. 
     Palm Rejection 
     When a user rests his hypothenar muscles, located on the side of his palm beneath his little finger, on a touch screen when writing with a stylus, ghosting generally occurs. This part of the palm blocks a large area of the touch screen, and often blocks a series of light beams along the screen&#39;s vertical axis, thereby hiding the stylus&#39; touch position along the vertical axis. 
     Reference is made to  FIG. 107 , which is a simplified illustration of a user writing on a prior art touch screen with a stylus. Shown in  FIG. 107  is a hand  930  holding a stylus  931 , and drawing a line  932  on a touch screen  800 . The user&#39;s palm is resting on screen  800 , blocking two series of light beams depicted as dotted lines; namely, a series  113  along the screen&#39;s horizontal axis, and a series  114  along the screen&#39;s vertical axis. The location of the stylus tip on the vertical axis is within series  114 . Beam  115  does detect the tip of the stylus, but it only provides a horizontal axis location. 
     Embodiments of the present invention overcome the drawback illustrated in  FIG. 107 . Reference is made to  FIG. 108 , which is a simplified illustration of light beams detecting location of a stylus when a user&#39;s palm rests on a touch screen, in accordance with an embodiment of the present invention. By providing two sets of detection axes; namely, an orthogonal set and a diagonal set, a two-dimensional location of a stylus is determined.  FIG. 108  shows that beams  115  and  116  uniquely detect a stylus. Since each detection comprises overlapping wide beams whose signals are interpolated, as described hereinabove, the stylus position is determined with high precision, despite beams  115  and  116  not being perpendicular to one another. When the bottom of the user&#39;s palm does not block diagonal beam  117 , then beam  117  also detects the stylus location separately from the palm. In such case, beams  116  and  117  are used to detect the stylus location. Alternatively, all three detecting beams  115 - 117  may be used. 
     Another challenge that arises with touch screens that support both stylus and finger input arises when a user places his palm on the screen in order to write with a stylus, is misinterpretation of the initial contact between palm and screen as being a tap on an icon, in response to which the device launches an unintended application whose icon was tapped. Once the palm is resting on the screen, an area of contact is used to reject the palm touch as a screen tap. Nevertheless, the initial contact may cover a small surface area of the screen and thus be misinterpreted as a screen tap. 
     According to embodiments of the present invention, light beams above the screen are used to detect a palm as it approaches the screen. In one embodiment this is accomplished by projecting light from each emitter at several heights above the screen, as illustrated in  FIG. 18  showing an approaching finger  900  blocking beam  101  but not beam  102 . In another embodiment, multiple layers of emitters and receivers are arranged around the screen, and used to detect objects at different heights above the screen, as described hereinabove with reference to a user input gesture cavity and, in particular, with the cavity frame folded on top of the screen. 
     Reference is made to  FIG. 109 , which is a simplified illustration of a frame surrounding a touch screen, in accordance with an embodiment of the present invention.  FIG. 109  shows a frame  849  surrounding a touch screen, similar to frame  849  of  FIG. 55 . Two stacked rows of emitters  200  and receivers  300  are provided in the frame. When assembled together with a display in an electronic device, the stacked rows of emitters and receivers are raised above the display surface and provide object detection at two heights, namely, on the screen by the lower row of emitters and receivers, and above the screen by the upper row of emitters and receivers. When a user&#39;s palm begins to touch the screen, a large palm area is detected hovering above the screen. This enables the device to determine that a palm is approaching the screen, and that any screen tap is inadvertent. 
     In another embodiment of the present invention, only one row of emitters and receivers is provided for detecting a palm hovering above the screen, and touches on the screen are detected by conventional detection systems imposed on the display including inter alia capacitive or resistive touch sensors. 
     According to an embodiment of the present invention, a user interface disables screen taps for activating functions when a palm is detected. When the palm is detected, the user interface is configured to launch applications in response to a user touching an icon and gliding his finger away from the touched location along the touch screen. I.e., two sets of user interface gestures are provided. When no palm is detected, the first set of gestures is used. With the first set of gestures, a tap on an icon activates an application or function associated with the icon. When a palm is detected hovering above the screen, the second set of gestures is used. With the second set of gestures, the user is required to touch an icon and then glide his finger away from the touch location along the touch screen in order to activate the application or function associated with the icon. In this way, the device does not launch an unintended application when a user places his palm on the screen. The second set of gestures does not disable activation of icons; it enables the user to activate the application or function associated with the icon, if he desires to do so, by a touch and glide gesture. 
     Situating Elements around Corners 
     Screen corners present several challenges for arranging emitters and receivers. One challenge is that two emitters need to be placed in the same location—one for each screen edge. The challenge is complicated by the layout illustrated in  FIG. 44 , whereby the emitter and receiver elements are positioned under the screen surface, and therefore the rectangle formed by these elements is smaller than the frame of lenses surrounding the screen. One approach to overcoming this challenge is placement of two emitters at approximately the same location on the PCB, with one of the emitters placed on the top surface of the PCB and the other emitter placed on the bottom surface of the PCB. However, this approach introduces complications with connectors and positioning of optical elements. 
     Another challenge is extending overlapping beams to the edges of the screen. Although the emitters and receivers are underneath the screen, touch detection covers the entire area bordered by the inner edges of the optical elements that surround the screen. 
     Embodiments of the present invention provide arrangements that are suitable for use with orthogonal and diagonal detection axes, as described hereinabove. Reference is made to  FIG. 110 , which is a simplified illustration of a first embodiment of emitters, receivers and optical elements for a corner of a touch screen, in accordance with an embodiment of the present invention.  FIG. 110  shows a first corner arrangement of emitter or receiver elements and their respective optical elements. Receivers  300 - 303  and emitters  200 - 202  are arranged alternatingly along two adjacent screen edges. Solid lines indicate light beams from the emitters, and dashed lines indicate light beams arriving at the receivers. Emitters and receivers  300 ,  200 ,  302 ,  202  and  303  are positioned according to a standard pitch, and optical elements  530  are configured accordingly. Receiver  301  and emitter  201  are oriented at an angle, and their wide beams are divided such that half of a beam traverses the screen in a first direction, e.g., along the screen&#39;s vertical axis, and the other half of the beam traverse the screen in a second direction, e.g., along the screen&#39;s horizontal axis. Moreover, in embodiments that include a second lens having three-sided cavities for splitting beams, as described hereinabove, half of the wide beam is split into a first pair of diagonal beams that originate along one screen edge, and the other half of the beam is split into a second pair of diagonal beams that originate along an adjacent screen edge. A hybrid optical element  531  is provided in order to overlap beams for emitter  201  and receiver  302 . Optical element  531  is referred to as a “hybrid optical element” because the right half of the element is the same as the right half of element  530 , but a portion of the reflective or refractive facets on the left half are directed at the non-standard location and orientation of emitter  201 . Similarly, a hybrid optical element  532  is provided in order to overlap beams for emitter  200  and receiver  301 . The lower half of hybrid optical element  532  is similar to the left half of element  530 . Both halves of corner element  533  are uniquely configured; namely, the left half overlaps beams for emitter  201  and receiver  301 , and the right half overlaps beams for emitter  201  and receiver  302 . Both halves of corner optical element  534  are also uniquely configured for emitters  200  and  201  and for receiver  301 . 
     Reference is made to  FIG. 111 , which is a simplified illustration of a second embodiment of emitters, receivers and optical elements for a corner of a touch screen, in accordance with an embodiment of the present invention.  FIG. 111  shows an alternative corner arrangement of emitter or receiver elements and their respective optical elements. In the arrangement shown in  FIG. 111 , only one emitter  201  is placed at a non-standard pitch and orientation. Standard optical elements  530  are used together with hybrid optical elements  531  and  532  and unique corner optical elements  533 . Optical elements  531 - 533  are configured for the emitter-receiver arrangement shown, and are therefore different than elements  531 - 533  of  FIG. 110 . 
     Integrated Modules 
     In general, there is low tolerance for assembly errors for touch systems using alternating reflective or refractive facets aimed at two foci. An offset in placement of an emitter or a receiver causes it to be out of the reflective facet&#39;s focus, which can degrade accuracy and performance of such systems. In accordance with an embodiment of the present invention, rigid modular blocks containing reflective or refractive facets and an emitter or a receiver are prepared, in order to ensure the required assembly precision. Such modular blocks are useful for simplifying the process of integrating touch screen components, and for minimizing the tolerance chain for a manufacturer. These modular blocks are formed so as to be easily positioned together in a row along an edge of a display, for fast assembly of a touch screen. The high tolerance requirements of placing an emitter or receiver in exactly the correct position vis-à-vis the reflective or refractive facets, are handled during manufacture of the modular blocks, thus removing the burden of high precision assembly from a device manufacturer. 
     Simplified manufacturing is achieved by integrating optical elements and electronic components into a single unit. As such, complex surfaces may be gathered into one component, thereby reducing the need for high assembly tolerances. 
     Reference is made to  FIG. 112 , which is an illustration of optical components made of plastic material that is transparent to infrared light, in accordance with an embodiment of the present invention. Shown in  FIG. 112  is an optical component  488  that includes a forward-facing LED  236 , and electronics to handle the LED signal. Optical component  488  is connected to electrical pads  760  and  761 . Optical component  488  is used to transmit collimated light beams from two emitters; namely, emitter  235  and emitter  236 . Emitter  235  is included in a neighboring optical component  489 . In the alternating emitter-receiver embodiment described hereinabove, optical component  488  is used to transmit collimate light beams for one emitter and one receiver. E.g., neighboring module  489  includes a receiver instead of emitter  235 . 
     Light beams from emitter  235  exit optical component  489  through a tight-fitting surface  491 , and enter optical component  488  through a tight-fitting surface  490 .  FIG. 105  shows non-parallel light beams from emitters  235  and  236  hitting alternating facets on a wave-like multi-faceted reflective surface  493 . Components  488  and  489  are substantially identical, and fit together. A device manufacturer can thus use these components as building blocks to create a touch screen, by arranging a series of these building blocks in a row along each edge of the display. Typical arrangements are (a) two adjacent display edges are lined with emitter components, and the other two edges are lined with receiver components, and (b) all four display edges are lined with alternating emitter/receiver components, i.e., each emitter has a neighboring receiver. Indeed, the emitter and receiver components, being of substantially identical shape, can be positioned together in the same row. 
     An optical component  494  is similar to optical component  488 , except that an LED  237  is side-facing instead of forward-facing.  FIG. 112  shows collimated light beams  100  exiting optical component  494 . Pins  989  and  990  guide optical component  494  on a printed circuit board. 
     Optical component  495  is optical component  488  as viewed from the front.  FIG. 112  shows collimated light beams  100  exiting optical component  495 . 
     Similar optical components (not shown) are also provided for receiving light beams that traverse the screen surface. For these components, the emitters are replaced by receivers, and the electrical components handle the receiver signals. Such optical components receive collimated light beams, and direct the beams onto two different receivers. 
     Reference is made to  FIG. 113 , which is a simplified diagram of a side view of a touch screen with light guides, in accordance with an embodiment of the present invention. Shown in  FIG. 113  are a display  642 , an optical element  496 , a photo diode  394  within optical element  496 , an optical element  497 , and an emitter  238  within optical element  497 . Optical elements  496  and  497  are connected to a printed circuit board  762 . Emitter  238  emits non-parallel light beams and, as described hereinabove with reference to  FIG. 112 , the non-parallel beams are converted into collimated beams, or substantially collimated beams, before exiting optical element  497 . Another portion of the non-parallel beams are collimated by a neighboring module, not shown in  FIG. 112 . The beams  100  that exit optical element  497  are directed upwards and are reflected over display  642  by a light guide  498 . In an embodiment of the present invention, three-way refracting cavities are etched, or otherwise formed, on the lower surface of optical element  498  to refract the light beams in three directions in order to provide two coordinate systems for determining a touch location. The light beams  100  enter a light guide  499  on the opposite side of screen  642 , and are reflected below display  642  into optical element  496 . In embodiments supporting the two coordinate systems, the three-way refracting cavities are present on the underside of optical element  499  as well. As described hereinabove, optical element  496  and its neighboring optical element, not shown, focus the incoming light beams on photo diode  394 . In one embodiment of the present invention, the light guides  498  and  499  are constructed as a frame that surrounds display  642 . 
     In the touch screen of  FIG. 113 , two types of light beam redirection occur. A first redirection requires multiple facets directed at a single focus point. A second redirection uniformly redirects incoming beams at a 90° angle, or folds incoming light beams into a narrow waist or focus, as described hereinabove with reference to configuration no. 5. In some embodiments, the collimated beams are refracted in three directions, in between the first and second redirections, by refracting cavities. 
     The first type of redirection requires that the emitter or receiver be positioned at a specific location relative to the focal point of many facets. As such, the positioning of the emitter or receiver and its reflective surfaces, is sensitive to variations in placement. Thus the assembly of the emitter or receiver, together with its corresponding surface of reflective facets, has a low tolerance of error. The second type of redirection, involving reflection and, in some cases, uniform refraction in three directions, is robust to variations in position of the reflector and to the pattern of refracting cavities located in the light guide. Thus assembly of this portion of the light guide has a high tolerance for error. 
     The light guides that reflect light above the screen surface may be manufactured separately and assembled with other touch screen components. Thus in  FIG. 113  light guides  498  and  499  are shown separate from optical elements  496  and  497 . 
     Reference is made to  FIG. 114 , which is an illustration of a touch screen with a block of three optical components on each side, in accordance with an embodiment of the present invention. Blocks  500  and  501  are emitters, and blocks  502  and  503  are receivers. The blocks create an active area  991 , where an x-y touch position of a stylus or finger may be calculated based on detected blocked light. Adding more optical components of the same type to each block serves to enlarge the active area that is created. 
     Reference is made to  FIG. 115 , which is a magnified illustration of one of the emitter blocks of  FIG. 114 , in accordance with an embodiment of the present invention. Shown in  FIG. 115  are three emitters  239 ,  240  and  241 , that emit respective wide beams  167 ,  168  and  169  from one edge of a screen, which are read as respective signals  170 ,  171  and  172 . The signal gradients are indicated by their diagonal orientations. At the opposite edge of the screen, signals  170 ,  171  and  172  are each redirected onto respective receivers by respective optical components. An accurate position of an object, such as a finger or stylus, touching the screen, is then determined based on values of blocked light at the receivers, as described below. 
     Touch Screen System Configuration No. 7 
     Configuration no. 7 uses total internal reflection in a touch screen. Whereas in configurations 1-6 the light beams travel in air above the screen surface, in configuration no. 7 the light beams travel through a sheet of glass or plastic that is transmissive to the wavelengths used in the touch detection system. In other embodiments, the light travels through a liquid or gel layer that is transmissive to the wavelengths used in the touch detection system. 
     Total internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle, with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, no light passes through the boundary and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs. 
     When a light beam crosses a boundary between materials with different refractive indices, the light beam is partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e., the ray is closer to being parallel to the boundary) than the critical angle—the angle of incidence at which light is refracted such that it travels along the boundary—then the light stops crossing the boundary altogether and instead is totally reflected back internally. This only occurs where light travels from a medium with a higher refractive index to one with a lower refractive index. For example, it occurs when passing from glass to air. 
     A touch screen according to configuration no. 7 has a glass or plastic sheet or pane above the display screen referred to as a cover glass. Alternatively, a gel layer or a liquid filled sac is placed over the display. The cover glass, or gel or liquid filled sac material is transparent to light at the wavelength used. Typically optical touch systems use wavelengths in the near infrared range, i.e., wavelengths below 1100 nm, e.g., 940 nm. A narrow air gap is provided between the cover glass and the display such that both the upper and lower surfaces of the cover glass, exposed to air, internally reflect light inside the cover glass. Using any of the collimating lenses described above with reference to configurations 1-6, light enters the cover glass from below at an angle larger than the critical angle with respect to the normal to the cover glass surface, and is directed through the cover glass by total internal reflection. A finger touching the cover glass from above absorbs a portion of the light inside the cover glass at the touched location. In addition, the finger also scatters a portion of the light inside the cover glass at the touched location. Both of these actions diminish the amount of light that arrives at a respective detector, and the detector measurement is used as described herein to calculate the location of the touch. 
     Reference is made to  FIG. 116 , which is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention.  FIG. 116  shows a screen  635 , a cover glass  646 , emitters  200 , emitter lenses  564 , receivers  300  and receiver lenses  565 .  FIG. 116  shows a view from above, and a cross section along the line A-A. The emitters and receivers in  FIG. 116  are shift-aligned, and the lenses ensure that light from each emitter reaches two opposite receivers. 
     Reference is made to  FIG. 117 , which is a simplified illustration of a touch object scattering internally reflected light in a screen assembly having a cover glass, in accordance with an embodiment of the present invention.  FIG. 117  shows scattering of light by an object touching the screen. Light beams  120  from emitter  201  to receivers  301  and  302  are internally reflected inside cover glass  646  and scattered by a touch object  900 .  FIG. 117  shows scattered beams  122 . However, a portion of the detection channel beams  121  beneath touch object  900  arrive at receivers  301  and  302 . 
     Reference is made to  FIG. 118 , which is a simplified illustration of a touch object absorbing internally reflected light in a screen assembly having a cover glass, in accordance with an embodiment of the present invention.  FIG. 118  shows absorption of light by a finger  900  touching the screen. The light beams from emitter  200  to receiver  300  are internally reflected inside cover glass  646  and are partially absorbed into finger  900 . 
     Reference is made to  FIG. 119 , which is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention.  FIG. 119  shows the lens arrangement of  FIGS. 60 and 61  modified for a system according to configuration no. 7. As in  FIG. 60 , an LED  200  is coupled with a pair of lenses  550  and  551 , separated by an air gap  555 , for collimating light. In addition,  FIG. 119  includes cover glass  646  that receives light beams emitted by emitter  200  and transmits the beams through cover glass  646  using total internal reflection. This lens differs from that of  FIG. 60  in that it is entirely below the height of the device. Also, lens  551  includes an additional reflective facet  562  for guiding light into the underside of cover glass  646  at a suitable angle α, i.e., at an angle less than the critical angle with respect to the cover glass surface, as illustrated in  FIG. 120 . For example, the critical angle for a glass-to-air boundary is roughly 46° so a suitable angle α in this case is 40°. The actual refractive indices of the cover glass material, air, and a low-index adhesive, if such is used to laminate the cover glass to the display, determine the critical angle for each embodiment. Lens  551  is connected to cover glass  646  using an optically clear transfer tape  561 , e.g., TESA 69304 optically clear pure acrylic adhesive manufactured by TESA Corp. of Charlotte, N.C., so that this reflected light enters cover glass  646 . A similar arrangement at an opposite edge of the screen guides the light out of cover glass  646  and onto one or more respective photo detectors. In other embodiments light guide  551  is formed as a unitary molded plastic unit together with cover glass  646  and adhesive transfer tape  561  is not used. In an alternative embodiment light enters cover glass  646  from the side instead of from underneath, as illustrated in  FIG. 121 . 
     Reference is made to  FIG. 120 , which is a simplified illustration of a light beam path in the touch screen assembly of  FIG. 119 , in accordance with an embodiment of the present invention. Reference is also made to  FIG. 121 , which is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention.  FIG. 120  shows the path of light beam  151  from emitter  200  through lenses  550  and  551  and into cover glass  646  at a suitable angle α where it propagates by virtue of total internal reflection.  FIG. 121  shows air gap  563  between cover glass  646  and display  635 . This air gap creates the necessary medium boundary on the underside of the cover glass for total internal reflection inside cover glass  646 . Alternatively, the necessary medium boundary for total internal reflection is provided by laminating the cover glass to the screen using a suitable low-index adhesive. Cover glass  646  is preferably 1-2 mm thick. 
     Configuration 7 differs from configurations 1-6 in the amount of light blocked during a touch. In general, a touch in configuration nos. 1-6 blocks more of the beam than a comparable touch in configuration no. 7. However principles such as gradations of intensity along the width of the beams described hereinabove, inter alia with reference to  FIGS. 47 ,  48  and  68 - 76 , are the same in configuration no. 7 as they are in configuration nos. 1-6. Therefore the methods for interpolating signals described herein are applicable to configuration no. 7, once the lower amount of light beam blockage in configuration no. 7 is accounted for. The arrangement of divergent beams where three divergent wide beams are directed out of each emitter, as described in detail with reference to  FIGS. 79-83, 91-92 and 99-109 , is also applicable to configuration no. 7 with the divergent beams all being directed into the cover glass at an appropriate angle for total internal reflection. 
     Configuration no. 7 enables designing a device without a protruding bezel around the screen. This is an advantage over configurations 1-6 in terms of design. 
     Another advantage relates to multi-touch detection. In configurations 1-6, when two or more objects are inserted into a light beam path simultaneously, the light beam shadow patterns no longer correspond to unique finger positions, and therefore the signal pattern is ambiguous. Examples of different touch patterns that produce the same shadow signal are shown in  FIGS. 85-87 . In systems using total internal reflection, each touch results in a further partial reduction in the signal so that the number of touches between transmitter and receiver may be calculated. 
     This calculation is simplest if the system assumes that only one type of item may be placed on the screen. Otherwise, a thick finger could be mistaken for two thin fingers, for example. However, in many cases there is a delay between each of the touches in a multi-touch gesture. When the system detects incremental steps in the magnitude of the touch detection signal, it indicates that the signal is generated by multiple touches as opposed to a large touch object. Thus when the system samples the screen at a high frequency, such that new samples are generated as each additional touch is added, the system determines that an additional touch occurred due to the further partial reduction in the signal. In particular, when using the controller described below, the system can sample the screen at rates of up to 1000 Hz, enabling discriminating between touches that occur at almost the same time. 
     Another advantage provided by configuration no. 7 relates to scattering of light by the touch object. A finger touching the screen scatters a portion of the light inside the cover glass at the touched location. The lenses  550  and  551  collimate the light from the emitter  200  and direct it at one or more respective detectors. The scattering of light by a touch object results in the light reaching other detectors. The collimating lenses associated with the detectors direct light scattered from a point along the collimated path onto the detectors. Therefore, in cases that may indicate multi-touch, the system polls additional detectors and resolves the multi-touch locations based on the detection of scattered light. 
     An example illustrates this advantage, with reference to  FIG. 122 , which is a simplified illustration of emitters and receivers detecting two diagonal touch points, in accordance with an embodiment of the present invention.  FIG. 122  shows an arrangement of emitters  200  and PD receivers  300  surrounding a touch screen according to configuration no. 7. Two touch points  971  and  972  are drawn. Emitters  200  and PD receivers  300  are both numbered  1 - 16 . A reduction in expected light occurs at PD numbers  2 ,  7 ,  11  and  15  and, as already explained, this signal pattern is not unique to these two touch points. In this case the system activates emitter number  2  and samples two PDs: PD number  11  and PD number  15 . Each detector has an associated collimating lens not shown in the figure. In the touch pattern drawn, PD number  15  will detect a greater amount of scattered light (by touch point  971 ) than PD number  11  because touch point  971  is situated along the path of the collimating lens associated with PD number  15  but not along the path of the collimating lens associated with PD number  11 . Similarly, the system activates emitter number  15  and samples two PDs: PD number  2  and PD number  7 . In the touch pattern drawn, PD number  2  will detect a greater amount of scattered light (by touch point  971 ) than PD number  7 . Based on this, the system determines that the touches are at the locations shown and not at the opposite corners of the screen. 
     Reference is made to  FIG. 123 , which is a simplified illustration of emitters and receivers detecting three touch points, in accordance with an embodiment of the present invention. Continuing the example of  FIG. 122 ,  FIG. 123  adds a third touch point  980 . In this case, PD number  7  detects a significantly greater reduction in expected light from its respective emitter (number  7 ) than PD number  2  detects from its respective emitter (number  2 ). This is because two touches ( 972 ,  980 ) absorb light from emitter  7 , but only one touch ( 971 ) absorbs light from emitter  2 . Based on this, the system determines that the touches are at the locations shown. 
     Reference is made to  FIG. 124 , which is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention. With respect to scattered light, in the touch screen of  FIG. 123 , during activation of emitter  15 , detectors  7  and  2  detect scattered light from touch points  971  and  980  respectively; whereas during activation of emitter  2 , detector  15  detects scattered light from touch point  971 , and detector  11  detects much less scattered light. This indicates that touch point  980  is at the bottom right of the screen and not at the upper left corner. 
     In general, an activation sequence of emitters and detectors aimed at detecting scattered light may be employed selectively. For example, it may be performed only when a potential ghosted gesture is possible. Also, the sequence aimed at detecting scattered light may be limited to activating only those emitter-detector pairs that are likely to resolve the ghosting; e.g., when the detection pattern of  FIGS. 122 and 123  occurs, only activating emitter-detector pairs  15 - 2 ,  15 - 7 ,  2 - 11  and  2 - 15 . 
     Reference is made to  FIG. 125 , which is a flowchart of a method for disambiguating multiple touch detection signals in accordance with an embodiment of the present invention. At step  1060  emitter-receiver pairs around a touch screen are scanned. A reduction in expected light indicates a touch along the emitter-receiver signal paths. If at least two x-coordinate channels and at least two y-coordinate channels detect a touch, the system proceeds through steps  1063 - 1065  to correctly pair the x, y coordinates. At step  1063  the system pairs each x coordinate emitter with all of the detected y-coordinate receivers to create a second set of touch detection channels. In these channels, a touch is indicated by an increase in detected light that results from light being scattered by the touch object. At step  1064  the system pairs each y-coordinate emitter with all of the detected x-coordinate receivers to create a similar set of touch detection channels. At step  1065  the system determines the most likely touch, based on steps  1063  and  1064 , and outputs the touch coordinates at step  1062 . 
     If only one x-coordinate or only one y-coordinate is returned at step  1061  the system outputs touch coordinates at step  1062  as follows: if one x-coordinate and one y-coordinate are detected, then output one x,y touch coordinate. If multiple coordinates are detected along one axis, pair each of these one axis coordinates with the single x or y-coordinate on the other axis. 
     Touch Screen System Configuration No. 8 
     Configuration no. 8 combines over-air light beams, as in configurations 1-6, with total internal reflection light beams as described in configuration 7. In certain embodiments, each emitter-detector pair activation includes a first portion of light traveling in air above the screen, and a second portion of light traveling through a cover glass. Both portions of light originate at the emitter and arrive at the detector. 
     Reference is made to  FIG. 126 , which is a simplified illustration of a touch screen assembly having a cover glass, in accordance with an embodiment of the present invention.  FIG. 126  shows two light beams  151  and  152  in a touch system according to configuration no. 8. Both beams originate at emitter  200  and both beams converge onto an opposite detector not shown in the figure. Beam  151  is directed into cover glass  646  from beneath, and beam  152  is directed over air, above cover glass  646 . Light guide  498  guides both beams from emitter  200 . 
     Configuration no. 8 has several advantages. This configuration detects a hovering object that blocks a portion of the over-air beam. However, a hovering object does not affect the total internal reflection; actual contact with the cover glass is required to frustrate the total internal reflection. As such, there is a significant drop in the signal when contact occurs. This enables the system to clearly distinguish a hover gesture from a touch gesture. 
     Another advantage is that configuration no. 8 has two detection systems: the over-air beams and the total internal reflection beams. When one of these systems is impaired, the other system provides touch detection. For example, a narrow stylus point is accurately traced by the over-air beams as described hereinabove, but the narrow stylus point does not absorb or frustrate much of the total internal reflection beams. 
     Yet another advantage is that the total internal reflection system is available to resolve ghosted gestures, as explained above with reference to configuration no. 7. 
     Touch Screen System Configuration No. 9 
     Configuration no. 9 uses a reduced number of components by coupling an emitter or a receiver to one end of a long thin light guide situated along an edge of the screen. Such a light guide is described in U.S. Pat. No. 7,333,095 entitled ILLUMINATION FOR OPTICAL TOUCH PANEL. 
     Reference is made to  FIG. 127 , which is an illustration of a touch screen having a long thin light guide  514  along a first edge of the screen, for directing light over the screen, and having an array of light receivers  300  arranged along an opposite edge of the screen for detecting the directed light, and for communicating detected light values to a calculating unit  770 , in accordance with an embodiment of the present invention. Light emitters  200  are coupled to both ends of light guide  514 . Light guide  514  is positioned along one edge of a touch screen  800 . Light is emitted into light guide  514  along a screen edge, and is re-directed across the screen surface by a reflector  515 . A plurality of receivers  300  is situated along the opposite edge of touch screen  800 , to enable multiple receivers to detect a touch, as described hereinabove with reference to configuration nos. 2 and 3. 
     Reference is made to  FIG. 128 , which is an illustration of a touch screen having an array of light emitters  200  along a first edge of the screen for directing light beams over the screen, and having a long thin light guide  514  for receiving the directed light beams and for further directing them to light receivers  300  situated at both ends of light guide  514 , in accordance with an embodiment of the present invention. Detected light values at receiver  300  are communicated to a calculating unit (not shown). According to another embodiment of the present invention, only one light receiver  300  is coupled to one end of light guide  514 . Light guide  514  is positioned along one edge of a touch screen  800 . A plurality of emitters is situated along the opposite edge of the touch screen, to enable receiver(s)  300  to detect a touch based on serial activation of multiple emitters, as described hereinabove with reference to configuration nos. 2 and 3. Light emitted across the screen surface is re-directed by a reflector  515 . Light is received into light guide  514  along the screen edge and is directed through the length of light guide  514  onto a receiver  300 . 
     Reference is made to  FIG. 129 , which is an illustration of two light emitters,  201  and  202 , each emitter coupled to an end of a long thin light guide  514 , in accordance with an embodiment of the present invention. Light guide  514  is positioned along one edge of a touch screen. Light  100  is emitted into light guide  514  along a screen edge, and is re-directed across the screen surface by a reflector  515 . A plurality of receivers is situated along the opposite edge of the touch screen, to enable multiple receivers to detect a touch, as described hereinabove with reference to configuration nos. 2 and 3. Each emitter  201  and  202  is activated separately, and the receivers thus detect a touch based on blocked light from each of the two emitters. The amount of light  100  emitted at any given location along the length of the light guide decreases as a function of the distance between the location and the emitter. As such, different amounts of detected light from each emitter  201  and  202  are used to calculate the precise location of a touch, as described hereinabove with reference to configuration nos. 2 and 3. 
     Embodiments of the present invention improve upon the light guide of U.S. Pat. No. 7,333,095, by etching or otherwise forming micro patterns  516  on the outer surface of the light guide, in order to widely refract outgoing light beams  101  of  FIG. 127 , or incoming light beams  102  of  FIG. 106 , as described hereinabove with reference to configuration nos. 2 and 3. Micro patterns  516  are a uniform substantially parallel pattern of grooves along light guide  514 , and are simpler to form than the fan pattern described hereinabove with reference to configuration no. 2. Light guide  514  also includes a light scatterer strip  517  inside of light guide  514 . Micro patterns  516  and light scatterer strip  517  appear in  FIGS. 127 and 128 . 
     Touch Screen System Configuration No. 10 
     Configuration no. 10 enables detecting pressure on a touch screen, as applied during a touch operation. Detecting pressure enables discrimination between a light touch and a hard press, and is useful for user interfaces that associate separate actions to a touch and a press. E.g., a user may select a button or icon by touching it, and activate the function associated with the button or icon by pressing on it. Such a user interface is described in applicants&#39; co-pending U.S. application Ser. No. 12/486,033, entitled USER INTERFACE FOR MOBILE COMPUTER UNIT. 
     In some embodiments of the present invention, a touch enabled device includes a base plane, such as a PCB, a light guide frame rigidly mounted on the base plane, and a resilient member attached to the base plane to suspend or “float” a non-rigidly mounted touch screen inside the light guide frame. A press on the touch screen deflects the floating touch screen along a z-axis, exposing more of the light guide frame. A light guide frame reflector, which directs light over the screen as described hereinabove, is formed so that the exposure allows more light to traverse the screen. In this way, when a hard press on the screen occurs, many of the receivers detect a sudden increase in detected light. Moreover, detection of a hard press may be conditioned upon a touch being detected at the same time, thus preventing false detection of a hard press due to a sudden increase in ambient light. When the downward pressure is released, the resilient member returns the screen to its original position within the light guide frame. 
     Reference is made to  FIGS. 130-133 , which are illustrations of a touch screen  800  that detects occurrence of a hard press, in accordance with an embodiment of the present invention.  FIG. 130  shows touch screen  800  in rest position, screen  800  being supported by resilient supporting members  841  and  842  that create a flex air gap  843 , which are mounted on a printed circuit board  700 .  FIG. 130  shows two light guides,  518  and  519 , one on either side of screen  800 , for directing light  100  from an emitter  200  over screen  800  to a receiver  300 . Only a small upper portion of each light guide  518  and  519  extends above screen  800 . Receiver  300  communicates detected light intensities to a calculating unit (not shown). 
       FIG. 133  shows a finger  900  pressing down on the screen, causing members  841  and  842  to compress and to narrow flex air gap  843 . As a result, a larger portion of light guides  518  and  519  are exposed above screen  800 , thus allowing (a) more light  100  from emitter  200  to traverse screen  800  and be detected by receiver  300 , and (b) more ambient light  101  to reach receiver  300 . In various embodiments, either or both of these increases in detected light are used to indicate a hard press. In other embodiments, the amount of downward pressure applied is determined based on the amount of additional detected light, thus enabling discrimination between more hard and less hard touches. 
     In some embodiments, the light guide frame includes protruding lips  520  and  521 , shown in  FIG. 132 , that extend over the edges of screen  800 , to counter balance the upward force of resilient members  841  and  842  when no downward pressure is applied to screen  800 . Resilient members  841  and  842  may comprise inter alia a flexible mounting material, a torsion spring, an elastic polymer body, or a hydraulic suspension system.  FIG. 133  shows emitters  200 , receivers  300  coupled with calculating unit  770 , and resilient members  841  and  842  arranged on a single PCB  700 . 
     In other embodiments, the touch screen is not displaceable relative to the frame. However, the screen flexes or bends somewhat in response to a hard press. The bending of the screen causes a sudden increase in detected light in many of the receivers, indicating a hard press on the screen. As indicated hereinabove, detection of a hard press may be conditioned upon a touch also being detected at the same time, thus preventing false detection of a hard press in response to trauma to the device. 
     Reference is made to  FIGS. 134 and 135 , which are bar charts showing increase in light detected, when pressure is applied to a rigidly mounted 7-inch LCD screen, in accordance with an embodiment of the present invention. The bar charts show the amount of light detected from each emitter along one edge of the screen when a soft touch occurs ( FIG. 134 ), and when a hard touch occurs ( FIG. 135 ). The light emitters and light receivers are shift-aligned, so that light from each emitter is detected by two receivers. As such, two bars are shown for each emitter, indicating the light detected by each of the two receivers. Both bars indicate that a touch is detected at receivers opposite LED  4 , where no light is detected. The bar charts show that more light is detected from neighboring emitters in the case of a hard touch, than in the case of a soft touch. 
     Operation of Configurations Nos. 2 and 3 
     The following discussion relates to methods of operation for arrangements of the optical elements shown in configurations nos. 2 and 3, around a touch screen, used in conjunction with the cover glass described above with reference to configurations nos. 6 and 7, to achieve accurate touch detection based on total internal reflection. These methods are of advantage for pen and stylus support, which have fine touch points, and provide highly accurate touch location determination for single-finger and multi-finger touches as well. 
     Reference is made to  FIGS. 136 and 137 , which are illustrations of opposing rows of emitter lenses and receiver lenses in a touch screen system, in accordance with an embodiment of the present invention. Positioned behind each emitter and receiver lens is a corresponding respective light emitter  200  or light receiver  300 . As shown in  FIG. 138 , each emitter  200  is positioned opposite two receivers  300  that detect light beams emitted by the emitter. Similarly, each receiver  300  is positioned opposite two emitters  200 , and receives light beams emitted from both emitters. 
       FIG. 136  shows (A) a single, full beam  173  from an emitter  200  that spans two receivers  300 ; (B) the portion of the full beam, designated  174 , detected by the left one of the two receivers  300 ; (C) the portion of the full beam, designated  175 , detected by the right one of the two receivers  300 ; (D) multiple beams  176 , for multiple emitters  200 , covering the touch screen, and (E) multiple beams  177 , for multiple emitters  200 , covering the touch screen. Generally, each emitter  200  is activated alone. Precision touch detection is described hereinbelow, wherein a touch point is detected by multiple beams. It will be appreciated from (D) and (E) that points on the screen are detected by at least one beam  176  and one beam  177 . 
     To conserve power, when the touch screen is idle only one set of beams, namely, beams  176  or beams  177 , are scanned in a scanning sweep, and only for the axis with the smallest number of emitters  200 . The scanning toggles between beams  176  and beams  177 , and thus two scanning sweeps along the axis activate every emitter-receiver pair along the axis. The other axis, with the larger number of emitters, is only scanned when either a touch is present, or when a signal differs from its reference value by more than an expected noise level, or when an update of reference values for either axis is being performed. Reference values are described in detail hereinbelow. 
       FIG. 137  shows (A) an emitter  201  sending light to a receiver  301  at an angle of 15° to the left; (B) emitter  201  sending light to a receiver  302  at an angle of 15° to the right; (C) emitter  202  sending light to receiver  302  at an angle of 15° to the left; and (D) a microstructure refracting incoming light. The emitter lenses and receiver lenses shown in  FIG. 137  are equipped with the microstructure shown in (D), in order (i) to emit light in both left and right directions from multiple locations along the emitter lens surface, and (ii) to ensure that light received at any angle of incidence at any location along the receiver lens surface is detected by the receiver. 
     Reference is made to  FIG. 138 , which is a simplified illustration of a technique for detecting a touch location, by a plurality of emitter-receiver pairs in a touch screen system, in accordance with an embodiment of the present invention. Shown in  FIG. 138  is an optical emitter lens  506  of width k, positioned opposite two optical receiver lenses  508  and  509 , each of width k, on a touch screen. A pointer,  900 , touching the screen blocks a portion of the light beam emitted from optical emitter lens  506 . Optical emitter lens  506  emits overlapping beams that cover both optical receiver lenses  508  and  509 . The spread angle of the wide beam depends on the screen dimensions, and on the lens width, k, along the x-axis. Another optical emitter lens  507  is also shown, shifted by half an element width, m, below an optical receiver lens  510 . 
     In accordance with an embodiment of the present invention, at least one surface of optical emitter lens  506  is textured with a plurality of ridges. Each ridge spreads a beam of light that spans the two opposing receiver lenses  508  and  509 . As such, light from each of many points along the surface of optical emitter lens  506  reaches both opposing receiver lenses  508  and  509 , and the light beams detected by adjacent receivers overlap. In configuration no. 2 these ridges form a feather pattern, and in configuration no. 3 these ridges form a tubular pattern. 
     In accordance with an embodiment of the present invention, the ridges form micro-lenses, each having a pitch of roughly 0.2-0.5 mm, depending on the touch screen configuration. In the case of a feather pattern, the ridges form a fan, and their pitch narrows as the ridges progress inward and become closer together. In the case of a tubular pattern, the pitch of each micro-lens remains constant along the length of the micro-lens. 
     At least one surface of each receiver lens  508  and  509  is similarly textured, in order that at least a portion of light arriving at each of many points along the receiver lens surface, arrive at the receiver photo diode. 
     In accordance with an embodiment of the present invention, the output x and y coordinates are filtered temporally and spatially. The following discussion relates to determination of the x-coordinate, and it will be appreciated by those skilled in the art that the same method applies to determination of the y-coordinate. 
     Configurations nos. 2 and 3 show that a touch location is detected by at least two emitter-receiver pairs.  FIG. 138  shows two such emitter-receiver pairs,  506 - 508  and  506 - 509 , detecting a touch location of object  900  along the x-axis. In  FIG. 138 , beams  506 - 508  are denoted by beam  178 , and beams  506 - 509  are denoted by beam  179 .  FIG. 138  shows three detection areas; namely, (i) the screen area detected by emitter-receiver pair  506 - 508 , drawn as a wedge filled with right-sloping lines, (ii) the screen area detected by emitter-receiver  506 - 509 , drawn as a wedge with left-sloping lines, and (iii) the screen area detected by both emitter-receiver pairs  506 - 508  and  506 - 509 , drawn as a wedge with a crosshatch pattern. The left and right borders of this third screen area are shown as lines X 1  and X 2 , respectively. 
     In order to determine the x-coordinate X p  of object  900 &#39;s touch location (X p , Y p ), an initial y-coordinate, Y initial , is determined corresponding to the location along the y-axis of the emitter-receiver pair having the maximum touch detection signal among all emitter-receiver pairs along the y-axis. In  FIG. 138 , this emitter-receive pair is  507 - 510 . The lines designated X 1  and X 2  in  FIG. 138  are then traversed until they intersect the line y=Y initial  at locations (X a , Y initial ) and (X b , Y initial ). Coordinates X a  and X b  are shown in  FIG. 138 . The x-coordinate of object  900  is then determined using the weighted average
 
 X   p =( W   a   X   a   +W   b   X   b )/( W   a   +W   b )  (2)
 
where the weights W a  and W b  are normalized signal differences for beam  178  and beam  179 , respectively. The signal difference used is the difference between a baseline, or expected, light value and the actual detected light value. Such difference indicates that an object is touching the screen, blocking a portion of the expected light. The weights W a  and W b  are normalized because the detection signal of a touch occurring near the row of emitters is different from a touch occurring near the row of receivers, as described hereinbelow with reference to  FIGS. 144-151 . A touch screen design is tested to determine different signal strength and attenuation patterns as an object crosses a beam at various portions along the length of the beam. Different scenarios are tested, e.g., a scenario for objects near the beam&#39;s emitter, a scenario for objects near the beam&#39;s receiver, and a scenario for objects in the middle of the screen. When a touch is detected, the detection pattern of detecting receivers is analyzed to select an appropriate scenario, and the signals are normalized according to the selected scenario. Calibration and further normalization of the weights is described hereinbelow. A similar weighted average is used to determine the y-coordinate Y P .
 
     If the pointer  900  is detected by more than two emitter-receiver pairs, then the above weighted average is generalized to
 
 X   P =Σ( W   n   X   n )/(Σ W   n ),  (3)
 
where the weights W n  are normalized signal differences, and the X n  are weight positions.
 
     In one embodiment of the present invention, where the pointer  900  is a small object, the largest signal difference is used in conjunction with the two closest signals to calculate the position. This compensates for the fact that the signal differences for small objects are small, and noise thus becomes a dominant error factor. Use of the two closest signals reduces error due to noise. In another embodiment of the present invention, only the two largest signal differences are used. 
     Reference is made to  FIG. 139 , which is an illustration of a light guide frame for the configuration of  FIGS. 136 and 137 , in accordance with an embodiment of the present invention. Shown in  FIG. 139  are four edges of a light guide frame, with optical emitter lenses  511  and optical receiver lenses  512 . It is noted that the inner edges of the frame are not completely covered by beams  182 . As such, in some embodiments of the present invention only an inner touch area  993 , indicated by the dashed rectangle, is used. 
     To reduce error due to signal noise, the final coordinate is determined as the output of a temporal filter, using the spatially filtered current coordinate value, determined as above, and a previous coordinate value. The higher the filter weight given to the current x-coordinate, the closer the output will be to that value, and the less will be the impact of the filter. Generally, use of substantially equal weights for both coordinate values results in a strong filter. In one embodiment of the present invention, the temporal filter is a low-pass filter, but other filters are also contemplated by the present invention. In accordance with an embodiment of the present invention, different pre-designated filter weight coefficients may be used in different cases. In an alternative embodiment, the filter weight coefficients are calculated as needed. 
     Choice of appropriate filter coefficients is based on scanning frequency, the speed at which a touch object is moving across the screen, whether the object motion is along a straight line or not, and the size of the touch object. 
     Generally, the higher the scanning frequency, the nearer the current coordinate value is to the previous coordinate value, and a stronger filter is used. Scanning frequency is used to estimate the speed and direction of movement of an object. Based on the scanning frequency, a threshold distance is assigned to two input values, the threshold indicating fast movement. If the difference between the current and previous coordinate values is greater than the threshold distance, a weaker filter is used so that the output coordinate not lag considerably behind the actual touch location. It has been found by experiment that the filter
 
output_ val=  1/10*previous_ val+  9/10*current_ val   (4)
 
provides good results in this case. In addition, the lag value, described hereinbelow, is reset to equal the output value in this case.
 
     If the difference between the current and previous coordinate values is less than the threshold distance, then a lag value is determined. The lag value indicates speed and direction along an axis. In has been found by experiment that the value
 
lag=⅚*lag+⅙*current_ val   (5)
 
provides good results in this case. The filter weight coefficients are selected based on the difference between the lag value and the current coordinate value. Generally, the greater this difference, which indicates either fast motion or sudden change in direction, the weaker the filter.
 
     For example, if the touch object is stationary, the lag value eventually is approximately equal to the current coordinate value. In such case, signal noise may cause small differences in the spatially calculated touch position, which in turn may cause a disturbing jitter effect; i.e., the touch screen would show the object jittering. Use of a strong temporal filter substantially dampens such jittering. 
     If the touch object is moving fast or makes a sudden change in direction, a strong temporal filter may create a perceptible lag between the actual touch location and the displayed touch location. In the case of a person writing with a stylus, the written line may lag behind the stylus. In such cases, use of a weak temporal filter reduces such lagging. 
     When the touch object covers a relatively large screen area, such as a finger or other blunt object touching the screen, the lag between the actual finger motion and the displayed trace of the motion is less perceptible, because the finger covers the area of the lag. In such case, a different temporal filter is used. 
     The type of object, finger vs. stylus, being used may be inferred by knowing expected user behavior; e.g., a user interface intended for finger touch assumes a finger being used. The type of object may also be inferred by the shadowed area created by the object. The size of the touch area as determined based on shadowed emitter signals, is therefore also a factor used in selecting temporal filter weight coefficients. 
     Reference is made to  FIG. 140 , which is a simplified flowchart of a method for touch detection for a light-based touch screen, in accordance with an embodiment of the present invention. At operation  1021 , a current coordinate value is received, based on a spatial filter that processes signals from multiple emitter-receiver pairs. A threshold distance is provided, based on a scan frequency. At operation  1022 , the difference between the current coordinate value and a previous coordinate value is compared to the threshold distance. If the difference is less than or equal to the threshold distance, then at operation  1023  a new lag value is calculated, as in EQ. (5). At operation  1024  temporal filter weight coefficients are determined based on the difference between the current coordinate value and the lag value. At operation  1025 , the temporal filter is applied to calculate an output coordinate value, as in EQ. (4). 
     If, at operation  1022 , the difference between the current coordinate value and previous coordinate value is greater than the threshold distance, then weak filter weight coefficients are selected at operation  1026 . At operation  1027 , the temporal filter is applied to calculate an output coordinate value, as in EQ. (4). At operation  1028  the lag value is set to the output coordinate value. 
     Embodiments of the present invention provide a method and apparatus for detecting a multi-touch operation whereby two touches occur simultaneously at two corners of a touch screen. An example of such a multi-touch is a rotation gesture, shown in  FIGS. 141-143 , whereby a user places two fingers  900  on a screen  800  and turns them around an axis. As pointed out hereinabove with reference to  FIGS. 15 and 16 , it is difficult for a light-based system to discriminate between a top-left &amp; bottom-right touch vs. a bottom-left &amp; top-right touch. Use of shift-aligned emitters and receivers enables such discrimination, as described hereinbelow. 
     In accordance with an embodiment of the present invention, data from receivers along a first axis is used to determine a touch location along two axes. Reference is made to  FIGS. 144-147  which are illustrations of a finger  900  touch event at various locations on a touch screen, and corresponding  FIGS. 148-151 , which are respective bar charts of light saturation during the touch events, in accordance with an embodiment of the present invention.  FIG. 144  shows a touch located near a row of emitters, between two emitters.  FIG. 145  shows a touch located near a row of receivers, blocking a receiver.  FIG. 146  shows a touch located near a row of emitters, blocking an emitter.  FIG. 147  shows a touch located near a row of receivers, between two receivers. 
       FIGS. 148-151  each include two bar charts; namely, an upper chart showing light saturation at receivers along an x-axis, and a lower chart showing light saturation at receivers along a y-axis. Each row of receivers is shift-aligned with an opposite row of emitters. As such, each emitter is detected by two receivers. Correspondingly,  FIGS. 148-151  show two bars for each emitter, one bar per receiver. 
       FIGS. 148-151  exhibit four distinct detection patterns.  FIG. 148  shows an absence of light detected primarily by one receiver from its two respective emitters. The absence of light is moderate.  FIG. 149  shows an absence of light detected primarily by one receiver from its two respective emitters. The absence of light is large.  FIG. 150  shows two adjacent receivers detecting a large absence of expected light from the blocked emitter. Both receivers detect some light from neighboring elements.  FIG. 151  shows two adjacent receivers detecting a moderate absence of expected light from the blocked emitter. Both receivers detect some light from neighboring emitters. TABLE III summarizes these different patterns. 
     
       
         
           
               
             
               
                 TABLE III 
               
             
            
               
                   
               
               
                 Patterns of touch detection based on proximity to and 
               
               
                 alignment with emitters and receivers 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 No. of Receivers 
                 Amount of 
               
               
                 Pattern No. 
                   
                 Detecting the 
                 Expected Light 
               
               
                 FIGS. 
                 Touch Location 
                 Touch 
                 that is Blocked 
               
               
                   
               
               
                 1 
                 Near a row of 
                 1 
                 Moderate 
               
               
                 FIG. 142 
                 emitters, between 
               
               
                 FIG. 145 
                 two emitters 
               
               
                 2 
                 Near a row of 
                 1 
                 Large 
               
               
                 FIG. 143 
                 receivers, blocking 
               
               
                 FIG. 147 
                 a receiver 
               
               
                 3 
                 Near a row of 
                 2 
                 Large 
               
               
                 FIG. 144 
                 emitters, blocking 
               
               
                 FIG. 148 
                 an emitter 
               
               
                 4 
                 Near a row of 
                 2 
                 Moderate 
               
               
                 FIG. 145 
                 receivers, between 
               
               
                 FIG. 149 
                 two receivers 
               
               
                   
               
            
           
         
       
     
     According to an embodiment of the present invention, determination of location of a multi-touch is based on the patterns indicated in TABLE III. Thus, referring back to  FIG. 142 , four detection points are shown along two rows of receivers. Detections D 1 -D 4  detect touch points  971  in upper-right &amp; lower-left corners of the screen. Based on whether the detection pattern of each point is of type 1 or 3, or of type 2 or 4, the detection patterns determine whether the corresponding touch is closer to the emitters, or closer to the receivers. Each touch has two independent indicators; namely, the X-axis detectors, and the Y-axis detectors. Thus, for detection points  971  in  FIG. 142 , detections D 1  and D 3  are of types 2 or 4, and detections D 2  and D 4  are of types 1 or 3. In distinction, for detection points  971  in  FIG. 142 , detections D 2  and D 4  are of types 2 or 4, and detections D 1  and D 3  are of types 1 or 3. 
     In addition to evaluation of detection points independently, the various detection patterns may be ranked, to determine which touch point is closer to the emitters or to the receivers. 
     Moreover, when a rotate gesture is performed, from touch points  971  to touch points  972 , movement of detections discriminates whether the gesture glides away from the emitters and toward the receivers, or vice versa. In particular, subsequent detections are compared, and discrimination is based on whether each detection pattern is becoming more like type 1 or 3, or more like type 2 or 4. 
     Reference is made to  FIG. 152 , which is a simplified flowchart of a method for determining the locations of simultaneous, diagonally opposed touches, in accordance with an embodiment of the present invention. At operation  1031 , two x-coordinates and two y-coordinates are detected, such as x-coordinates D 1  and D 2 , and y-coordinates D 3  and D 4 , shown in  FIGS. 142 and 143 . At operation  1032  the detected x-coordinates are analyzed to identify a pattern of detection from among those listed in TABLE I. At operation  1033  the detected x-coordinates are ranked according to touches that occurred closer to or farther from a designated screen edge, based on the pattern detected at operation  1032  and based on the “Touch Location” column of TABLE III. The y-coordinates represent distances from the designated edge. At operation  1034 , each ranked x-coordinate is paired with a corresponding y-coordinate. Operations  1035 - 1037  are performed for the y-coordinates, similar to operations  1032 - 1034  performed for the x-coordinates. At operation  1038 , the two sets of results are compared. 
     Reference is made to  FIG. 153 , which is a simplified flowchart of a method for discriminating between clockwise and counter-clockwise gestures, in accordance with an embodiment of the present invention. At operation  1041 , two glide gestures are detected along an x-axis. Each glide gesture is detected as a series of connected touch locations. Thus, with reference to  FIGS. 142 and 143 , a first glide gesture is detected as a connected series of touch locations beginning at x-coordinate D 1 , and a second concurrent glide gesture is detected as a connected series of touch locations beginning at x-coordinate D 2 . At operation  1042 , the x-glide detections are analyzed to determine the types of detections that occurred in each series, from among the patterns listed in TABLE III. 
     At operation  1043 , the x-glide detections are ranked according to touches that occurred closer to or farther from a designated screen edge, based on the patterns of detections determined at operation  1042 , and based on the “Touch Location” column of TABLE III. Operation  1043  relates to series of connected touch detections over a time interval. Each series generally includes touch detections of patterns 1 and 3, or of patterns 2 and 4, listed in TABLE III, depending on whether the glide was closer to or further away from the designated edge. In addition to analyzing the individual detections that comprise a glide, the series of touch detections is also analyzed to determine if the glide is moving closer to or farther from the designated edge, based on comparison of intensities of detections over time. E.g., in one series of detections having multiple pattern 1 detections, if the amount of blocked light increases over time, then it is inferred that the glide is moving toward the receivers, otherwise the glide is moving toward the emitters. 
     The y-coordinates represent distances from a designated edge, such as the edge of emitters. At operation  1044  each ranked x-axis glide is paired with a corresponding y-axis glide. Operations  1045 - 1047  are performed for the y-axis glide, similar to operations  1042 - 1044  performed for the x-axis glide. At operation  1048  the two sets of results are compared. At step  1049  a discrimination is made as to whether the rotation gesture is clockwise or counter-clockwise. 
       FIGS. 63 and 79  show alignments of emitters and receivers whereby right and left halves of each beam overlap neighboring beams, as shown in  FIGS. 70 and 82 . Three beams are shown in these figures; namely, beams  167 ,  168  and  169 . The left half of beam  167  overlaps the right half of beam  168 , and the right half of beam  167  overlaps the left half of beam  169 . As such, a touch at any location within beam  167  is detected by two beams. The two detecting beams have different detection gradients along the widths of the beams, as shown by light detection areas  910 - 912  in the figures. 
     The gradient of light attenuation is substantially linear across the width of the beam. As such, a weighted average of the different detection signals is used to calculate a position along one axis using EQS. (2) and (3) above. EQ. (2) extends to a number, n, of samples. E.g., if a finger at the center of beam a blocks 40% of the expected signal of beam a, and blocks none of the expected signal of beam b, then W a  and W b  are 0.4 and 0, respectively, and the location X P  is calculated as
 
 X   P =(0.4* X   a +0* X   b )/(0.4+0)= X   a .
 
The same value of X P  is obtained for a stylus at the screen position which, due to its being narrower than the finger, blocks only 20% of the expected signal of beam a.
 
     Similarly, if a finger between the centers of beams a and b blocks similar amounts of expected light from both beams, say 30%, then X P  is calculated as
 
 X   P =(0.3 *X   a +0.3* X   b )/(0.3+0.3)=½( X   a   +X   b ),
 
which is the midpoint between X a  and X b .
 
     Location calculation in a system of aligned emitters and receivers differs in several aspects from location calculation in a system of shift-aligned emitters and receivers. In a system of aligned emitters and receivers, beams are aligned with the coordinate system used for specifying the touch location. In this case, the touch location is calculated along a first axis without regard for the touch location along the second axis. By contrast, in a shift-aligned system the primary beam coordinate, e.g., X a  for beam a, is determined based on an assumed touch coordinate on the second axis, Y initial . 
     Further, in a system of aligned emitters and receivers the attenuation and signal strength pattern generated by an object crossing the beam is substantially the same at all locations along the length of the beam. As described hereinabove with reference to  FIGS. 76 and 107 , as an object moves across the width of a beam, it generates substantially similar signal patterns whether it crosses the beam near the beam&#39;s emitter, detector or in mid-screen. Therefore, an initial normalizing of weights, W a , W b , . . . , W n , based on the detection pattern is required in shift-aligned systems, and is not required in aligned systems. 
     When a light-blocking object is placed at the center of a beam, such as beam  167  in  FIGS. 70 and 82 , a portion of the neighboring beam is blocked. E.g., 40% of beam  167  is blocked and 5% of beam  168  is blocked. However, the signals include both random noise and also noise caused by the alternating facets that may account for signal fluctuations. A technique is required to determine whether the touch is in fact at the center of beam  167 , or slightly offset from the center. 
     In accordance with an embodiment of the present invention, multiple samples of each signal are taken, and combined to filter out signal noise. Additionally, the neighboring beams  168  and  169  are configured by their respective optical elements to overlap around the center of beam  167 , as seen in  FIGS. 72 and 106  where all three signals detect touches around the center of the middle signal. In cases where the main detection signal is concentrated in one beam, detection signals from both left and right neighboring beams are used to fine tune the touch location calculation. Specifically, filtered signals of neighboring beams  168  and  169  are used to determine an offset from the center of beam  167 . 
     In embodiments with optical elements with three-way lenses that create light beams along two sets of axes, similar calculations are performed on the diagonal detection beams to determine locations on the second axis system. As described hereinabove, touch objects typically block a larger portion of the diagonal signals than of the orthogonal signals. 
     The spatial and temporal filters described hereinabove with reference to shift-aligned emitter-receiver arrangements are applied in aligned emitter-receiver arrangements as well. 
     Calibration of Touch Screen Components 
     Reference is made to  FIG. 154 , which is a simplified flowchart of a method of calibration and touch detection for a light-based touch screen, in accordance with an embodiment of the present invention. In general, each emitter/receiver pair signal differs significantly from signals of other pairs, due to mechanical and component tolerances. Calibration of individual emitters and receivers is performed to ensure that all signal levels are within a pre-designated range that has an acceptable signal-to-noise ratio. 
     In accordance with an embodiment of the present invention, calibration is performed by individually setting (i) pulse durations, and (ii) pulse strengths, namely, emitter currents. For reasons of power consumption, a large current and a short pulse duration is preferred. When a signal is below the pre-designated range, pulse duration and/or pulse strength is increased. When a signal is above the pre-designated range, pulse duration and/or pulse strength is decreased. 
     As shown in  FIG. 154 , calibration (operation  1051 ) is performed at boot up (operation  1050 ), and is performed when a signal is detected outside the pre-designated range (operation  1055 ). Calibration is only performed when no touch is detected (operation  1053 ), and when all signals on the same axis are stable (operation  1054 ); i.e., signal differences are within a noise level over a time duration. 
     Reference signal values for each emitter/receiver pair are used as a basis of comparison to recognize a touch, and to compute a weighted average of touch coordinates over a neighborhood. The reference signal value for an emitter/receiver pair is a normal signal level. Reference signal values are collected at boot up, and updated when a change, such as a change in ambient light or a mechanical change, is detected. In general, as shown in  FIG. 154 , reference signal values are updated (operation  1056 ) when signals are stable (operation  1054 ); i.e., when signal variations are within an expected range for some number, N, of samples over time. 
     A touch inside the touch area of a screen may slightly bend the screen surface, causing reflections that influence detected signal values at photo diodes outside of the touch area. Such bending is more pronounced when the touch object is fine or pointed, such as a stylus. In order to account for such bending, when a touch is detected (operation  1053 ), all stable signals (operation  1058 ) outside the touch area undergo a reference update (operation  1059 ). When no touch is present and all signals are stable (operation  1054 ), but a signal along an axis differs from the reference value by more than the expected noise level (operation  1055 ), the emitters are calibrated (operation  1051 ). Recalibration and updating of reference values require stable signals in order to avoid influence of temporary signal values, such as signal values due to mechanical stress by bending or twisting of the screen frame. 
     To further avoid error due to noise, if the result of an emitter/receiver pair differs from a previous result by more than an expected noise level, a new measurement is performed, and both results are compared to the previous result, to get a best match. If the final value is within the expected noise level, a counter is incremented. Otherwise, the counter is cleared. The counter is subsequently used to determine if a signal is stable or unstable, when updating reference values and when recalibrating. 
     After each complete scan, signals are normalized with their respective reference values. If the normalized signals are not below a touch threshold, then a check is made if a recalibration or an update of reference values is necessary. If a normalized signal is below the touch threshold, then a touch is detected (operation  1053 ). 
     To reduce risk of a false alarm touch detection, due to a sudden disturbance, the threshold for detecting an initial point of contact with the screen, such as when a finger first touches the screen, is stricter than the threshold for detecting movement of a point of contact, such as gliding of a finger along the screen while touching the screen. I.e., a higher signal difference is required to detect an initial touch, vis-à-vis the difference required to detect movement of an object along the screen surface. Furthermore, an initial contact is processed as pending until a rescan verifies that the touch is valid and that the location of the touch remains at approximately the same position. 
     To determine the size of a touch object (operation  1057 ), the range of blocked signals and their amplitudes are measured. For large objects, there is a wait for detecting an initial point of contact with the screen, until the touch has settled, since the touch of a large object is generally detected when the object is near the screen before it has actually touched the screen. Additionally, when a large object approaches the screen in a direction not perpendicular to the touch area, the subsequent location moves slightly from a first contact location. 
     However, objects with small contact areas, such as a pen or a stylus, are typically placed directly at the intended screen location. As such, in some embodiments of the present invention, the wait for detecting an initial contact of a fine object is shortened or skipped entirely. 
     It has been found advantageous to limit the size of objects that generate a touch, in order to prevent detection of a constant touch when a device with a touch screen is stored in a pouch or in a pocket. 
     At operation  1053 , it is also necessary to distinguish between signals representing a valid touch, and signals arising from mechanical effects. In this regard, reference is made to  FIG. 155 , which is a picture showing the difference between signals generated by a touch, and signals generated by a mechanical effect, in accordance with an embodiment of the present invention. Each of the four graphs in  FIG. 155  shows detection beams 1-10 during a scan along one screen axis. As seen in  FIG. 155 , signal gradients discriminate between a valid touch and a mechanical effect. 
     Reference is made to  FIG. 156 , which is a simplified diagram of a control circuit for setting pulse strength when calibrating a light-based touch screen, in accordance with an embodiment of the present invention. Reference is also made to  FIG. 157 , which is a plot of calibration pulses for pulse strengths ranging from a minimum current to a maximum current, for calibrating a light-based touch screen in accordance with an embodiment of the present invention.  FIG. 157  shows plots for six different pulse durations (PULSETIME1-PULSETIME 6), and sixteen pulse strength levels (1-16) for each plot. 
     The control circuit of  FIG. 156  includes 4 transistors with respective variable resistors R 1 , R 2 , R 3  and R 4 . The values of the resistors control the signal levels and the ratio between their values controls gradients of the pulse curves shown in  FIG. 156 . 
     Reference is made to  FIG. 158 , which is a simplified pulse diagram and a corresponding output signal graph, for calibrating a light-based touch screen, in accordance with an embodiment of the present invention. The simplified pulse diagram is at the left in  FIG. 158 , and shows different pulse durations, t 0 , . . . , t N , that are managed by a control circuit when calibrating the touch screen. As shown in  FIG. 158 , multiple gradations are used to control duration of a pulse, and multiple gradations are used to control the pulse current. The corresponding output signal graph is at the right in  FIG. 158 . 
     As shown in  FIG. 158 , different pulse durations result in different rise times and different amplitudes. Signal peaks occur close to the time when the analog-to-digital (A/D) sampler closes its sample and hold circuit. In order to obtain a maximum output signal, the emitter pulse duration is controlled so as to end at or near the end of the A/D sampling window. Since the A/D sampling time is fixed, the timing, t d , between the start of A/D sampling and the pulse activation time is an important factor. 
     Assembly of Touch Screen Components 
     As described hereinabove, a minimum of tolerances are required when aligning optical guides that focus on respective light emitters and light receivers, in order to achieve accurate precision on a light-based touch screen. A small misalignment can severely degrade accuracy of touch detection by altering the light beam. It is difficult to accurately place a surface mounted receiver and transmitter such that they are properly aligned with respective light guides. 
     Because of this difficulty, in an embodiment of the present invention, a light guide and transmitter or receiver are combined into a single module or optical element, as described above with reference to  FIGS. 115-118 . 
     In some instances it may be of advantage not to combine an emitter or a receiver into an optical element, e.g., in order to use standard emitter and receiver components. In such instances precision placement of components is critical. 
     In some embodiments of the present invention, the optical lens that includes the feather pattern is part of a frame that fits over the screen.  FIG. 46  shows a cross-section of such a frame  455 , which is separate from LED  200 . 
     Reference is made to  FIG. 159 , which is an illustration showing how a capillary effect is used to increase accuracy of positioning a component, such as an emitter or a receiver, on a substrate, inter alia a printed circuit board or an optical component, in accordance with an embodiment of the present invention. Shown in  FIG. 159  is an emitter a receiver  398  that is to be aligned with an optical component or temporary guide  513 . Optical component or temporary guide  513  is fixed to a printed circuit board  763  by guide pins  764 . Solder pads  765  are placed at an offset from component solder pads  766 . Printed circuit board  763  is then inserted into a heat oven for soldering. 
     Reference is made to  FIG. 160 , which is an illustration showing the printed circuit board  763  of  FIG. 159 , after having passed through a heat oven, in accordance with an embodiment of the present invention. As shown in  FIG. 160 , component  398  has been sucked into place by the capillary effect of the solder, guided by a notch  768  and a cavity  769  in optical component or temporary guide  513 . When a temporary guide is used, it may be reused for subsequent soldering. 
     The process described with reference to  FIGS. 159 and 160  is suitable for use in mass production of electronic devices. 
     ASIC Controller for Light-Based Touch Screens 
     Aspects of the present invention relate to design and use of a programmable state machine for novel light-based touch screen ASIC controllers that execute a scanning program on a series of emitters and detectors. The scanning program determines scan sequence, current levels and pulse widths. The controller includes integrated LED drivers for LED current control, integrated receiver drivers for photo detector current measurement, and an integrated A/D convertor to enable communication between the controller and a host processor using a standard bus interface, such as a Serial Peripheral Interface (SPI). 
     In accordance with the present invention, a program is loaded onto the controller, e.g., over SPI. Thereafter, scanning execution runs independently from the host processor, optimizing overall system power consumption. When the scan data are ready, the controller issues an interrupt to the host processor via an INT pin. 
     Reference is made to  FIG. 161 , which is a simplified illustration of a light-based touch screen  800  and an ASIC controller therefor, in accordance with an embodiment of the present invention. 
     Reference is made to  FIG. 162 , which is a circuit diagram of a chip package  731  for a controller of a light-based touch screen, in accordance with an embodiment of the present invention. 
     As shown in  FIG. 162 , chip package  731  includes emitter driver circuitry  740  for selectively activating a plurality of photoemitters  200  that are outside of the chip package, and signal conducting pins  732  for connecting photoemitters  200  to emitter driver circuitry  740 . Emitter driver circuitry  740  is described in applicants&#39; co-pending patent application U.S. Ser. No. 12/371,609 entitled LIGHT-BASED TOUCH SCREEN filed on Feb. 15, 2009, the contents of which are hereby incorporated by reference. Inter alia, reference is made to paragraphs [0073], paragraphs [0087]-[0091] and  FIG. 11  of this application as published in U.S. Publication No. 2009/0189878 A1 on Jul. 30, 2009. 
     Emitter driver circuitry  740  includes circuitry  742  for configuring individual photoemitter pulse durations and pulse currents for each emitter-detector pair via a programmable current source. Circuitry  742  is described in applicants&#39; co-pending patent application U.S. Ser. No. 13/052,511 entitled LIGHT-BASED TOUCH SCREEN WITH SHIFT-ALIGNED EMITTER AND RECEIVER LENSES filed on Mar. 21, 2011, the contents of which are hereby incorporated by reference. Inter alia, reference is made to paragraphs [0343]-[0358] and FIGS. 99-101 of this application as published in U.S. Publication No. 2011/0163998 on Jul. 7, 2011. 
     Chip package  731  includes detector driver circuitry  750  for selectively activating a plurality of photo detectors  300  that are outside of the chip package, and signal conducting pins  733  for connecting photo detectors  300  to detector driver circuitry  750 . Detector driver circuitry  750  includes circuitry  755  for filtering current received from photo detectors  300  by performing a continuous feedback bandpass filter, and circuitry  756  for digitizing the bandpass filtered current. Circuitry  755  is described inter alia at paragraphs [0076], paragraphs [107]-[0163] and FIGS. 14-23B of the above-referenced U.S. Publication No. 2009/0189878 A1. Chip package  731  also includes detector signal processing circuitry  753  for generating detection signals representing measured amounts of light detected on photo detectors  300 . 
     Chip package  731  further includes I/O pins  736  for communicating with a host processor  772 . Chip package  731  further includes controller circuitry  759  for controlling emitter driver circuitry  740  and detector driver circuitry  750 . Controller circuitry  759  communicates with host processor  772  using a bus standard for a Serial Peripheral Interface (SPI)  775 . Chip package  731  further includes a chip select (CS) pin  737  for coordinating operation of controller circuitry  759  with at least one additional controller  774  for the light-based touch screen. 
     The controller shown in  FIG. 162  packages all of the above mentioned elements within chip package  731 , (i) thereby enabling automatic execution of an entire scan sequence, such as 52 emitter-receiver pairs, and (ii) thereby storing the detection signals in a register array located in controller circuitry  759 , for subsequent analysis by host processor  772 . This register array provides storage for at least 52, 12-bit receiver results. Additional registers in controller circuitry  759  are provided for configuring individual pulse durations and pulse currents for individual emitter-receiver pairs. In order to support 52 unique emitter-receiver pairs, at least 104 registers are provided; namely, 52 registers for configuring individual pulse durations, and 52 registers for configuring individual pulse currents. 
     Reference is made to  FIG. 163 , which is a circuit diagram for six rows of photo emitters with 4 or 5 photo emitters in each row, for connection to pins  732  of chip package  731 , in accordance with an embodiment of the present invention. The 11 lines LED_ROW 1 , . . . , LED_ROW 6  and LED_COL 1 , . . . , LED_COL 5  provide two-dimensional addressing for 26 photo emitters, although the photo emitters are physically arranged around two edges of the touch screen, as shown in  FIG. 150 . TABLE IV shows LED multiplex mapping from photo emitter LEDs to LED_ROW and LED_COL pins. More generally, an LED matrix may include an m×n array of LEDs supported by m+n I/O pins on the controller. 
     As such, an LED is accessed by selection of a row and a column I/O pin. The controller includes push-pull drivers for selecting rows and columns. It will be appreciated by those skilled in the art that the row and column coordinates of the LEDs are unrelated to the physical placement of the LEDs and the push-pull drivers. In particular, the LEDs do no need to be physically positioned in a rectangular matrix. 
     In an alternative embodiment of the controller of the present invention, current source drivers are used instead of push-pull drivers. In another embodiment of the controller of the present invention, some of the push-pull drivers are combined with current source drivers, and others of the push-pull drivers are combined with current sink drivers. 
     
       
         
           
               
             
               
                 TABLE IV 
               
             
            
               
                   
               
               
                 LED multiplex mapping to LED_ROW and LED_COL pins 
               
            
           
           
               
               
               
            
               
                 LED 
                 LED_ROW pin enabled 
                 LED_COL pin enabled 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 1 
                 1 
               
               
                 2 
                 2 
                 1 
               
               
                 3 
                 3 
                 1 
               
               
                 4 
                 4 
                 1 
               
               
                 5 
                 5 
                 1 
               
               
                 6 
                 6 
                 1 
               
               
                 7 
                 1 
                 2 
               
               
                 8 
                 2 
                 2 
               
               
                 9 
                 3 
                 2 
               
               
                 10 
                 4 
                 2 
               
               
                 11 
                 5 
                 2 
               
               
                 12 
                 6 
                 2 
               
               
                 13 
                 1 
                 3 
               
               
                 14 
                 2 
                 3 
               
               
                 15 
                 3 
                 3 
               
               
                 16 
                 4 
                 3 
               
               
                 17 
                 5 
                 3 
               
               
                 18 
                 6 
                 3 
               
               
                 19 
                 1 
                 4 
               
               
                 20 
                 2 
                 4 
               
               
                 21 
                 3 
                 4 
               
               
                 22 
                 4 
                 4 
               
               
                 23 
                 5 
                 4 
               
               
                 24 
                 6 
                 4 
               
               
                 25 
                 1 
                 5 
               
               
                 26 
                 2 
                 5 
               
               
                   
               
            
           
         
       
     
     Advantages of having a dedicated controller for emitters and receivers in a light-based touch screen are power savings and performance. In conventional systems, a conventional chip, such as the MSP430 chip manufactured by Texas Instruments of Dallas, Tex., controls emitters and receivers. Regarding power savings, conventional chips do not provide access to all of the power consuming chip elements. Moreover, with conventional chips it is not possible to power on and off external elements in sync with the emitters. For example, with a conventional chip the amplifier unit connected to the receivers and the analog-to-digital convertor (ADC) for digitizing receiver light detection current, cannot be turned on and off in sync with activation of the emitters. In conventional systems, these elements are left powered on throughout an entire scan sequence. In distinction, the dedicated controller of the present invention is able to power these elements on and off at a resolution of microseconds, in sync with emitter activation. This and other such selective activation of controller blocks, reduce the total power consumption of the touch system considerably. In fact, power consumption for the amplifier, the ADC and other controller blocks is reduced to the extent that their collective power consumption is negligible as compared to photoemitter activation power. As such, system power consumption is nearly the same as the power consumption for activating the photoemitters. 
     When the dedicated controller of the present invention scans a series of emitter-receiver pairs, an LED driver supplies an amount of current to an LED in accordance with settings in LED current control registers and LED pulse length control registers. TABLE V shows the power consumption of the dedicated controller, for 50 emitter-receiver pairs at 100 Hz with a power source of 2.7V. Pulse durations and pulse currents are set via circuitry  742  using configuration registers. Current consumption is calculated as
 
100 Hz×50 activation pairs×pulse duration (μs)×pulse current (A)==current consumption (μA) from the battery.
 
Power consumption is calculated as
 
current consumption (μA)*voltage (V)=power (mW).
 
     
       
         
           
               
             
               
                 TABLE V 
               
             
            
               
                   
               
               
                 Photometer power consumption for 50 emitter-receiver pairs 
               
               
                 at 100 Hz with 2.7 V power source 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pulse 
                 Pulse 
                 Current 
                 Power 
               
               
                   
                 duration (μs) 
                 current (A) 
                 consumption (μA) 
                 (mW) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 0.125 
                 0.05 
                 31.25 
                 0.084375 
               
               
                   
                 0.25 
                 0.05 
                 62.5 
                 0.16875 
               
               
                   
                 0.5 
                 0.05 
                 125 
                 0.3375 
               
               
                   
                 1 
                 0.05 
                 250 
                 0.675 
               
               
                   
                 2 
                 0.05 
                 500 
                 1.35 
               
               
                   
                 4 
                 0.05 
                 1000 
                 2.7 
               
               
                   
                 0.125 
                 0.1 
                 62.5 
                 0.1685 
               
               
                   
                 0.25 
                 0.1 
                 125 
                 0.3375 
               
               
                   
                 0.5 
                 0.1 
                 250 
                 0.675 
               
               
                   
                 1 
                 0.1 
                 500 
                 1.35 
               
               
                   
                 2 
                 0.1 
                 1000 
                 2.7 
               
               
                   
                 4 
                 0.1 
                 2000 
                 5.4 
               
               
                   
                 0.125 
                 0.2 
                 125 
                 0.3375 
               
               
                   
                 0.25 
                 0.2 
                 250 
                 0.675 
               
               
                   
                 0.5 
                 0.2 
                 500 
                 1.35 
               
               
                   
                 1 
                 0.2 
                 1000 
                 2.7 
               
               
                   
                 2 
                 0.2 
                 2000 
                 5.4 
               
               
                   
                 4 
                 0.2 
                 4000 
                 10.8 
               
               
                   
                 0.125 
                 0.4 
                 250 
                 0.675 
               
               
                   
                 0.25 
                 0.4 
                 500 
                 1.35 
               
               
                   
                 0.5 
                 0.4 
                 1000 
                 2.7 
               
               
                   
                 1 
                 0.4 
                 2000 
                 5.4 
               
               
                   
                 2 
                 0.4 
                 4000 
                 10.8 
               
               
                   
                 4 
                 0.4 
                 8000 
                 21.6 
               
               
                   
                   
               
            
           
         
       
     
     Regarding performance, the time required to complete a scan of all emitter-receiver pairs around the screen is critical, especially for fast stylus tracing. Reference is made to  FIG. 164 , which is a simplified illustration of a touch screen surrounded by emitters  200  and receivers  300 , in accordance with an embodiment of the present invention. Emitters  200  are scanned in a scan sequence; e.g., emitters  200  may be scanned in the numbered order 1-16 shown in  FIG. 164 . Touch points  900  correspond to touches made by a person writing his signature in a fast scrawl using a fine-point stylus. Three locations are indicated for touch points  900 . At a time t1, when emitter  1  is activated, the stylus is located at a location a. At a time t2, when emitter  16  is activated, the stylus is located at a location b, due to the quick movement as the user signs his name. However, the detected location on the screen at time t2 is a location c, different than location b; because at time t2, when emitter  16  is activated, the stylus has moved from its location at time t1. Such time lag between x-coordinate detection and y-coordinate detection produces errors in detecting touch positions of the stylus on the screen. These errors are most pronounced with fast stylus writing. As such, it is desirable to complete an entire scan sequence as fast as possible. 
     The dedicated controller of the present invention completes a scan sequence faster than conventional chips. The dedicated controller of the present invention includes register arrays that store necessary parameters to execute an entire scan sequence automatically. The dedicated controller further includes a register array for storing filtered, digital results for a scan sequence. In distinction, with conventional chips not all registers are available, and configuration data in registers is not automatically parsed. Thus, during a scan sequence using conventional chips, some cycles are required for configuring further emitter activations and for reading results. 
     In accordance with an embodiment of the present invention, for configurations where the number of emitters and receivers is larger than what may be supported by a single dedicated controller, multiple controllers are used. The multiple controllers are each configured prior to executing a scan, and then a scan is executed by each controller in rapid succession. For this embodiment, after configuring registers in all controllers, a host selects a first controller chip, using the chip-select (CS) pin shown in  FIG. 162 , and activates that chip. When the scan sequence on that chip is completed, the chip sends an interrupt to the host. The host then selects a second controller chip using its CS pin, and runs the second chip&#39;s scan sequence. After all of the controller chips have completed their respective scans, the host reads the results from each chip and calculates touch locations. 
     In this regard, reference is made to  FIG. 165 , which is a simplified application diagram illustrating a touch screen configured with two controllers, indicated as Device  1  and Device  2 , in accordance with an embodiment of the present invention. Shown in  FIG. 165  is touch screen  800  surrounded with LEDs and shift-aligned PDs. Twenty-six LEDs, LED 1 -LED 26 , are connected along a first screen edge to LED pins from Device  1 , and additional LEDS, LED 1 -LED CR , along this edge are connected to LED pins from Device  2 . Along the opposite edge, PDs are shift-aligned with the LEDs. PDs that detect light from the Device  1  LEDS are connected to Device  1  PD pins, and PDs that detect light from Device  2  LEDs are connected to Device  2  PD pins. The dashed lines connecting each LED to two PDs show how light from each LED is detected by two PDs. Each PD detects light from two LEDs. 
     As shown in  FIG. 165 , PD 27  of Device  1  detects light from LED 26  of Device  1  and also from LED 1  of Device  2 . As such, PD 27  is connected to the PD 27  pin of Device  1  and also to the PD 1  pin of Device  2 . When detecting light from LED 26  of Device  1 , PD 27  is sampled over the PD 27  pin of Device  1  and its result is stored on Device  1 ; and when detecting light from LED 1  of Device  2 , PD 27  is sampled over the PD 1  pin of Device  2  and its result is stored on Device  2 . As such, each controller coordinates LED activation with respective PD activation. The host processor calculates touch locations along the Device  1 -Device  2  border by interpolating the PD results from the two devices. 
     Reference is made to  FIG. 166 , which is a graph showing performance of a scan sequence using a conventional chip vs. performance of a scan using a dedicated controller of the present invention. The duration of each complete screen scan is longer with the conventional chip than with the dedicated controller. The dedicated controller can be powered down between scan sequences, providing further power savings, especially since the stretches of time between scan sequences may be larger with use of the dedicated controller than with use of a conventional chip. To connect touch points of multiple scans, the host processor may use spline interpolation or such other predictive coding algorithms, to generate smooth lines that match the user&#39;s pen strokes. Of significance is that each touch point is very accurate, when using a dedicated controller of the present invention. 
     Moreover, it is apparent from  FIG. 166  that a host using a dedicated controller of the present invention may increase the scan frequency beyond the limits possible when using a conventional chip. E.g., a host can scan 50 emitter receiver pairs at 1000 Hz, using a controller of the present invention. In distinction, touch screens using convention chips typically operate at frequencies of 100 Hz or less. The high sampling rate corresponding to 1000 Hz enables accurate touch location calculation over time. In turn, this enables temporal filtering of touch coordinates that substantially eliminates the jitter effect described above when a stylus remains stationary, while substantially reducing the lag time described above between a stylus location and a line representing the stylus&#39; path along the screen. 
     Such high sampling rates on the order of 50 emitter-receiver pairs at 1000 Hz cannot be achieved if individual LEDs require configuration prior to activation. The dedicated controller of the present invention achieves such high sampling rates by providing the registers and the circuitry to automatically activate an entire scan sequence. 
     A further advantage of completing multiple scan sequences in a short time is disambiguation of touch signals. The problem of ambiguous signals is described above with reference to  FIGS. 15 and 16 . As explained above, the same detection pattern of photo detectors is received for two concurrent touches along a screen diagonal, as illustrated in  FIGS. 15 and 16 . When placing two fingers on the screen, there is an inherent delay between the first and second touches. Completing multiple scan sequences in a very short time allows the system to determine the first touch, which is unambiguous. Then, assuming that the first touch is maintained when the second touch is detected, the second touch location is easily resolved. E.g., if it is determined that one touch is in the upper left corner and the touch detection pattern is as shown in  FIGS. 15 and 16 , then the second touch location must be at the lower right corner of the screen. 
     Thus it will be appreciated by those skilled in the art that a dedicated controller in accordance with the present invention is power-efficient, highly accurate and enables highs sampling rates. The host configures the controller for low power, corresponding to 100 Hz or less, or for high frequency scanning, such as 500 Hz-1000 Hz. 
     Determination of which configuration is appropriate is based inter alia on the area of the touch screen covered by a touch pointer, since jitter and lag are less prominent for a touch covering a relative large area, such as a finger touch, than for a touch covering a relatively small area, such as a stylus touch. Based on the area covered by the pointer, as determined by the size of the shadowed area of light-based touch screen signals, the host determines whether a finger or a stylus is being used, and configures an appropriate scan rate based on the trade-off between power and accuracy. 
     In accordance with an embodiment of the present invention, the dedicated controller includes scan range registers for selectively activating LEDs, and current control and pulse duration registers for specifying an amount of current and a duration, for each activation. The scan range registers designate a first LED and a first PD to be activated along each screen edge, the number of LEDs to be activated along each edge, and the step factor between activated LEDs. A step factor of 0 indicates that at each step the next LED is activated, and a step factor of 1 indicates that every other LED is activated. Thus, to activate only odd or only even LEDs, a step factor of 1 is used. Step factors of 2 or more may be used for steps of 2 or more LEDs, respectively. An additional register configures the number of PDs that are activated with each LED. A value of 0 indicates that each LED is activated with a single corresponding PD, and a value of 1 indicates that each LED is activated with two PDs. The number of PDs activated with each LED may be as many PD that are available around the touch screen. 
     To save power, it is advantageous to have a low resolution scan mode for detecting an initial touch location. The host may run in this mode, for example, when no touch is detected. When a touch is detected, the host switches to a high resolution scan mode, in order to calculate a precise touch location, as described above with reference to  FIG. 136 . In terms of controller scan sequence registers, every emitter is activated, i.e., step=0, with one receiver. The scan sequence of  FIG. 136( d )  differs from that of  FIG. 136( e )  in the initial PD used in the sequence on each screen edge. Specifically, the first PD, namely, PD0, is used in  FIG. 136( d ) , and the second PD, namely, PD1, is used in  FIG. 136( e ) . The initial PD to be used along each screen edge is configured by a register. 
     When each LED is activated with more than one PD, the LED is activated separately for each of the PDs. Each such separate activation has respective current control and pulse duration registers. 
     The controller of the present invention automatically controls a mux to direct current to desired LEDs. The LED mux control is set by the scan control registers. The controller automatically synchronizes the correct PD receivers when the drivers pulse the LEDS. Twelve-bit ADC receiver information is stored in PD data registers. Upon completion of scanning, the controller issues an interrupt to the host processor, and automatically enters standby mode. The host then reads receiver data for the entire scan sequence over the SPI interface. 
     In some touch screen configurations, emitters are shift-aligned with receivers, with emitters being detected by more than one receiver and being activated one or more times for each detecting receiver. For example, an emitter may be activated three times in rapid succession, and with each activation a different receiver is activated. Moreover, a receiver is further activated during the interval between emitter activations to determine an ambient light intensity. 
     In other touch screen configurations, emitters and receivers are aligned, but each emitter is detected by more than one receiver, and each emitter is activated separately for each detecting receiver. Emitter-receiver activation patterns are described in applicants&#39; co-pending patent application U.S. Ser. No. 12/667,692 entitled SCANNING OF A TOUCH SCREEN filed on Jan. 5, 2010, the contents of which are hereby incorporated by reference. Inter alia, reference is made to paragraphs [0029], [0030], [0033] and [0034] of this application as published in U.S. Publication No. 2011/0043485 on Feb. 24, 2011. 
     Reference is made to  FIG. 167 , which is a simplified illustration of a touch screen  800  having a shift-aligned arrangement of emitters and receivers, in accordance with an embodiment of the present invention. Shown in  FIG. 167  are emitters  204 - 208  along the south edge of screen  800 , shift-aligned receivers  306 - 311  along the north edge of screen  800 , emitters  209 - 211  along the east edge of screen  800 , and shift-aligned receivers  312 - 315  along the west edge of screen  800 . It is noted that each edge of receivers has one or more receivers than the number of emitters along the opposite edge, in order to detect touches in the corners of screen  800 . A beam  174  depicts activation of emitter  204  and detection by receiver  306 . TABLE VI lists an activation sequence in terms of emitter-receiver pairs. 
     
       
         
           
               
             
               
                 TABLE VI 
               
             
            
               
                   
               
               
                 Activation sequence of emitter-receiver pairs 
               
            
           
           
               
               
               
            
               
                 Activation No. 
                 Emitter 
                 Receiver 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 204 
                 306 
               
               
                 2 
                 204 
                 307 
               
               
                 3 
                 205 
                 307 
               
               
                 4 
                 205 
                 308 
               
               
                 5 
                 206 
                 308 
               
               
                 6 
                 206 
                 309 
               
               
                 7 
                 207 
                 309 
               
               
                 8 
                 207 
                 310 
               
               
                 9 
                 208 
                 310 
               
               
                 10 
                 208 
                 311 
               
               
                 11 
                 209 
                 312 
               
               
                 12 
                 209 
                 313 
               
               
                 13 
                 210 
                 313 
               
               
                 14 
                 210 
                 314 
               
               
                 15 
                 211 
                 314 
               
               
                 16 
                 211 
                 315 
               
               
                   
               
            
           
         
       
     
     Activation no. 10,  208 - 311 , is the last activation along the horizontal dimension of screen  800 . Activation no. 11 is the first activation along the vertical dimension of screen  800 . Such turning of a corner alters the activation pattern along screen edges. Specifically, the activation pattern along a screen edge is of the form AA-AB-BB-BC-CC-CD . . . , where the first letter of each pair designates an emitter and the second letter designates a receiver. Thus in AA-AB a same emitter is activated with two receivers, and in AB-BB two emitters are activated with a same receiver. When turning a corner, as at activation no. 11, the pattern is reset. The active emitter,  209 , is not detected by the previously activated receiver  311 , since emitter  209  and receiver  311  are not situated along opposite screen edges. Instead, emitter  209  is detected by receiver  312 , thus starting a new AA-AB-BB-BC . . . activation pattern along the vertical screen dimension. The controller handles a pattern reset based on the scan sequence registers, which indicate when a scan along a screen edge is complete. 
     Reference is made to  FIG. 168 , which is a simplified diagram of a touch screen  800  having alternating emitters and receivers along each screen edge, in accordance with an embodiment of the present invention. As shown in  FIG. 165 , each emitter is situated between two receivers, resulting in n emitters and n+1 receivers along a given edge, for some number n.  FIG. 165  shows touch screen  800  surrounded by ten emitters  204 - 213  and fourteen receivers  306 - 319 . As described above with reference to  FIG. 164 , each emitter is paired with two receivers. The dotted arrows  174  and  175  in  FIG. 168  indicate two activations of emitter  204 ; namely, an activation detected by receiver  316 , and another activation detected by receiver  315 . 
     In accordance with an embodiment of the present invention, when an activation sequence arrives at the end of a sequence of emitters along a screen edge, the activation pattern is restarted when activating emitters along an adjacent edge. In accordance with another embodiment of the present invention, the angle of orientation of each emitter with a detecting receiver is substantially 45° from the normal to the edge along which the emitter is arranged. In such case, a receiver along an adjacent edge is operative to detect light from an emitter near a screen corner. As such, the activation pattern is not restarted, but instead continues as a series of activated emitters turn a corner. Alternatively, the controller may restart the activation pattern when turning a corner by use of registers to store the index of the last LED to be activated by the controller along each screen dimension. 
     In accordance with an embodiment of the present invention, the controller is a simple state machine and does not include a processor core, such as an ARM core. As such, costs of controllers of the present invention are low. A light-based touch screen using a controller of the present invention costs less than a comparable capacitive touch screen, since a capacitive touch screen requires a processor core in order to integrate a large number of signals and calculate a touch location. In order to achieve a quick response time, a capacitive touch screen uses a dedicated processor core to calculate a touch location, instead of offloading this calculation to a host processor. In turn, this increases the bill of materials for capacitive touch screens. In distinction, light-based touch screens of the present invention use two neighboring receiver values to calculate a touch location along an axis, which enables the host to calculate a touch location and, consequently, enables use of a low-cost controller. 
     In accordance with an embodiment of the present invention, multiple controllers may be operative to control touch screen  800 . As mentioned above, chip package  731  includes a chip select (CS) pin  737  for coordinating operation of scanning controller circuitry  759  with at least one additional controller  774  for the light-based touch screen. 
     In accordance with embodiments of the present invention, the controller supports activation sequences for the touch screen of configuration no. 6 described hereinabove. In a first embodiment, emitters are positioned along two screen edges, directly opposite respective receivers along the remaining two screen edges, as shown in  FIG. 63 . Each emitter sends a two-pitch wide light beam to its respective receiver. An optical element, such as element  530  described hereinabove with reference to  FIG. 64 , interleaves this wide beam with neighboring wide beams, to generate two sets of overlapping wide beams that cover the screen; e.g., the set including every second beam covers the screen.  FIG. 69  shows a contiguous area covered by beams  168  and  169  generated by respective emitters  201  and  202 , with emitter  200  between them. 
     Two activation sequences are provided; namely, an activation sequence for low-resolution detection when no touch is detected, and an activation sequence for high resolution detection for tracing one or more detected touches. In low-resolution detection every second emitter-receiver pair is activated along one screen edge. For a rectangular screen, the shorter edge is used. In order to distribute use of components uniformly, odd and even sets of emitter-receiver pairs are activated alternately. Thus in low-resolution detection each emitter is configured to be activated with one receiver, and the step factor is 1; i.e., every second emitter is activated. In high resolution detection mode each emitter is configured to be activated with one receiver, and the step factor is 0; i.e., every emitter is activated. The scan in this mode activates emitters along both emitter-lined screen edges. 
     In an alternative embodiment, emitters and receivers are alternated along screen edges, as shown in  FIG. 79 . Each emitter sends a two-pitch wide beam to its respective receiver. An optical element, such as element  530  described hereinabove with reference to  FIG. 64 , interleaves this wide beam with neighboring wide beams, to generate two sets of overlapping wide light beams that cover the screen; e.g., the set including every second beam covers the screen.  FIG. 78  shows a contiguous area covered by beams  168  and  169  generated by respective emitters  201  and  202 , with receiver  300  between them. 
     In this embodiment three activation sequences are provided; namely, an activation sequence for low-resolution detection using detection on one axis, an activation sequence for high resolution detection using detection on two axes, and an activation sequence for high resolution detection using detection in four axes. In low-resolution detection every second emitter-receiver pair is activated along one screen edge. For a rectangular screen, the shorter edge is used. In order to distribute use of components uniformly, odd and even sets of beams are activated alternately. However, because neighboring beams are aimed in opposite directions, the emitters are connected to the ASIC LED connectors in such a way that the index of emitters is configured to increment along a single screen edge. Thus the step factor is 0; i.e., every second beam is activated, and the activation series ends at the last emitter along the active edge. In an alternative embodiment the emitters are connected to the ASIC LED connectors such that the index of emitters is configured to increment together with the series of beams. In this case the step factor is 1; i.e., every second beam is activated. 
     In high resolution detection mode using beams along two axes, each emitter is configured to be activated with one respective receiver, the step factor is 0, and the activation series covers all emitters. 
     In high resolution detection mode using beams along four axes, multiple activations are executed. A first activation activates beams along the horizontal and vertical axes. The initial emitter index matches the initial receiver index, and the emitter index increments together with the receiver index. A second activation series activates a first set of diagonal beams. In this case, the initial emitter and receiver indices define endpoints of one of the diagonal beams from the initial emitter. The emitter index then increments together with the receiver index around the screen. A third activation series activates a second set of diagonal beams. In this case, the initial emitter and receiver indices define endpoints of the second diagonal beam from the initial emitter. 
     Resilient Touch Surfaces 
     Reference is made to  FIG. 169 , which is a simplified illustration of a touch surface with a flexible compressible layer on top of the surface, in accordance with an embodiment of the present invention. Light beams that cross above the surface to provide touch detection are directed through the compressible layer.  FIG. 169  shows emitters  200  and receivers  300  on a PCB  700 , and a resilient flexible layer  650  situated above a display  642  and bonded to an outer edge of a light guide. The light guide has two units, namely, an upper section  463  and a lower section  464 . Generally layer  650  is transparent, to enable viewing of display  462 . 
     Reference is made to  FIG. 170 , which is a magnified view of the touch surface of  FIG. 169 , in accordance with an embodiment of the present invention. As shown in  FIG. 170 , light beam  100  travels from emitter  200  through light guide units  463  and  464 , and into flexible layer  650 . Light beam  100  is detected at the opposite edge of the surface by a respective receiver  300 , shown in  FIG. 169 . 
     Reference is made to  FIG. 171 , which is a simplified illustration of an object pressing down on layer  650  of the touch surface of  FIG. 169 , and creating an impression thereon, in accordance with an embodiment of the present invention. As shown in  FIG. 171 , a user presses his finger  900  on layer  650  and disrupts or frustrates any beam  100  crossing the location of the impression before the beam can reach receiver  300 . Such disruption or frustration of beam  100  has two measurable effects; namely, (i) a diminished detection signal at a corresponding receiver or receivers  300  at which the beam was directed, and (ii) increased detection signals at others of the receivers that receive the frustrated beams. Moreover, the greater the impression, the greater is the disruption. As such, the amount of missing expected light at some receivers and the pattern of increased detection at other receivers indicates the amount of pressure exerted by finger  900  on layer  650 . 
     When layer  650  is formed as a single gel-like body, a deep impression, created by a large amount of downward pressure, has a wider radius than a shallow impression. In turn, the amount of light detected at the receivers indicates the width of the radius, which determines the amount of downward force applied by finger  900 . In general, the pattern of blocked and frustrated beams created by an impression into a transmissive body, as in embodiments of the present invention, is more substantial than the frustrated total internal reflection of light transmitted into a rigid body when an object teaches the surface of the transmissive body but does not form an impression therein. 
     Reference is made to  FIG. 172 , which is a simplified illustration of an alternative touch surface with a flexible compressible layer on top of the surface, in accordance with an embodiment of the present invention. In the touch surface of  FIG. 172 , a flexible layer  650  is flush with an upper edge of light guide unit  463 , and is suspended above the surface of a display  642 , to form an air gap  843 .  FIG. 172  shows two light beams emitted from an emitter  200 . A first light beam  100  travels through layer  650 , and a second light beam  101  travels across air gap  843 . 
     Reference is made to  FIG. 173 , which is a simplified illustration of an object pressing down on layer  650  of the touch surface of  FIG. 172 , and creating an impression thereon, in accordance with an embodiment of the present invention. A user pressing his finger  900  into layer  650  from above bends the layer and disturbs beam  100 . In addition, the bent layer extends into air gap  843  and blocks beam  101 . 
     In an alternative embodiment of the present invention, layer  650  is a thin elastic membrane, and only beams inside of air gap  843  are used for touch detection. In this alternative embodiment, light is not sent through the membrane, and the membrane may wrap the device. 
     In some embodiments of the present invention, a thin transparent elastic membrane is placed inside a frame that snaps on to and snaps off of a touch surface. In one embodiment, a handset for a police or fire department includes a light-based touch surface as described above, which is generally used without an elastic upper layer. However, when a policeman or fireman encounters a harsh environment, where water or debris may hit the surface and interfere with touch detections on the surface, the policeman or fireman snaps on the transparent elastic layer. The elastic layer protects the surface and prevents water and debris from reaching the light beams and causing false touch detections. Touches performed through the elastic layer are detected at a coarser resolution than touches performed without the elastic layer, because of the tapering of the elastic layer when it is pressed onto the surface by a pointer object. Moreover, often in harsh environments the policeman or fireman is wearing gloves, which also reduces the resolution of the touch since the surface area of a gloved finger is larger than that of a bare finger. For these reasons, in accordance with an embodiment of the present invention, a handset of this type provides a high-resolution user interface for use without the elastic membrane, and a low-resolution user interface for use with the elastic membrane. One difference between a high-resolution and a low-resolution user interface is the size and density of buttons presented on the display; namely, a low resolution user interface uses larger buttons that are spaced farther apart, and a high resolution user interface uses smaller buttons that are spaced closer together. A low resolution user interface provides an opportunity to reduce the scan rate, and to reduce the number of emitters and receivers used when scanning a surface, vis-à-vis a high resolution user interface, since lower touch precision is required. In some embodiments of the present invention, the snap-on frame includes an RFID chip, or such other identifier, whereby the handset detects when the elastic layer is snapped on or off and automatically toggles the low-resolution/high-resolution user interface accordingly. 
     Reference is made to  FIG. 174 , which is a simplified illustration of another alternative touch surface with a flexible compressible layer on top of the surface, in accordance with an embodiment of the present invention.  FIG. 174  shows the edges of a light guide unit  463  extending above a flexible layer  650 . A first light beam  100  travels above layer  650  and is interrupted when an object touches layer  650 . A second light beam  101  travels through layer  650 , and is interrupted or frustrated only when an object exerts downward pressure on layer  650 , forming an impression thereon. As such, the touch surface of  FIG. 174  provides at least two levels of touch detection; namely, detection of an initial touch, and detection of a touch with pressure. 
     It will thus be appreciated by those skilled in the art that embodiments of the present invention provide several advantages for handset and display manufacturers. A first advantage is having light-based touch surfaces without raised bezels around the screen, as shown in  FIGS. 169-172 . This advantage is also achieved using an elastic sheet suspended above the display, as described above. A second advantage is having light-based touch surfaces that operate in environments of water droplets, dust and dirt. The water, dust and dirt settle on top of layer  650 , but do not generate a touch signal, since water, dust and dirt do not create impressions in layer  650 . A third advantage is having light-based touch surfaces that provide tactile sensations to a user pressing a finger or stylus into the surface, by using an upper layer of semi-hard gel that cradles an object pressed upon it. The semi-hard layer transmits haptic feedback from the device to the user&#39;s finger or stylus. A semi-hard material transmits more compelling haptic sensations to the user than do the rigid plastic and glass surfaces used in prior art touch surface devices. 
     Touch surfaces in accordance with the present invention may be manufactured by performing a double injection mold of the light guide, referred to as “overmolding”, with a soft material such as inter alia silicon, optically clear adhesive, or a bladder filled with a liquid. Overmolding mates the light guide and the soft material in a single process or tool, and reduces cost as compared with manufacturing a light guide and a flexible layer in two separate processes. 
     Light guides in accordance with the present invention may be made inter alia of polycarbonate or a cyclic olefin copolymer (COC) having a high glass transition temperature. COC has better optical properties than polycarbonate, better chemical resistance, better flow in the mold properties, and a lower shrinkage values, reducing the risk of sink marks. Thus COC provides flexibility in light guide design as well as high yield. 
     The present invention has broad application to electronic devices with touch sensitive screens, including small-size, mid-size and large-size screens. Such devices include inter alia computers, home entertainment systems, car entertainment systems, security systems, PDAs, cell phones, electronic games and toys, digital photo frames, digital musical instruments, e-book readers, TVs and GPS navigators. 
     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 as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.