Abstract:
A touch indication device comprises a light guide structure for passing light of a light source via total internal reflection. A touch surface disturbs the total internal reflection when touched, causing light to leave the light guide structure. A light enhancement layer converts the light leaving the light guide and sends the converted light to a light detector. The light enhancement layer amplifies the intensity and/or changes the color of the light leaving the light guide. The detector determines the position of the touch on the touch surface based on the light from the light enhancement layer.

Description:
FIELD OF THE INVENTION 
   The invention relates to the field of data input methods and apparatus generally. 
   BACKGROUND OF THE INVENTION 
   Input devices based on touch screens provide a convenient method for inputting commands in a manner that is easily reconfigured. In such devices, a simulated button pattern is displayed on the screen and the user selects a “button” by touching the screen over the button image. 
   One particular type of touch screen makes use of frustrated total internal reflection in a light guide to detect when the screen has been touched. This type of touch screen is constructed from a touch panel, an image generator and an imaging system. The image generator creates an image of simulated buttons or other objects to touch. This image is visible through the touch panel. The touch panel includes an optically transparent layer, or light guide, having an index of refraction greater than that of the surrounding air. A light source generates a light signal that is reflected between first and second sides of the optically transparent layer. The imaging system records an image of a second side of the touch screen. When a user touches a first side of the touch screen, a portion of the total-internal reflected light is reflected towards the second side of the touch panel at an angle less than the critical angle and escapes from the second side of the touch panel. This creates a bright spot on the surface of the second side of the touch panel and the location of this bright spot is recorded by the imaging system. A simulated button push is generated based on the location of the detected bright spot. 
   One example of this type of touch screen is described in the International Application Published Under The Patent Cooperation Treaty (PCT) by the applicant “Konin-Klijke Philips Electronics N.V.”, having International Publication Number WO 2005/029394 A2 and published on 31 Mar. 2005 (hereinafter referred to as “Philips”) and entitled “Light Guide Touch Screen”. As illustrated in  FIG. 1 , Philips includes a light guide  102  arranged adjacent to a screen  101 . The light guide  102  has a light source  108  arranged to emit light  110  into the light guide  102 . The optical matching between the light guide  102  and its surroundings is adapted such that the light  110  of the light source  108  is normally confined within the light guide  102  by means of total internal reflection. However, a user establishing physical contact with the light guide  102  perturbs the state of total internal reflection, and some of the light  110  is extracted from the light guide  102 . In the display device, light detecting means  103  is arranged to detect the light  110  and relate this detection to an input position where the user contact occurred. 
   Another example of this type of touch screen is described in U.S. Patent Application Publication US 2004/0108990 A1 to Lieberman et al. (hereinafter referred to as “Lieberman”) which shows a similar touch screen making use of frustrated total internal reflection to determine an input position where user contact to the screen occurred. As illustrated in  FIG. 2 , Lieberman employs a transparent data entry-object engagement surface  290 , exhibiting total internal reflection. A planar beam of light, designated by reference numeral  299 , is emitted by an illuminator  294  and coupled to an edge  295  of the surface  290  through which a beam  292  passes by total internal reflection. The presence of an object, such as a data entry object  296  in contact with the surface  290 , causes light from the beam  292  to be scattered into a scattered beam  297  due to frustrated total internal reflection and inter alia to pass through the transparent data entry object engagement surface  290  so as to be detected by a camera  298 . 
   A problem with these “frustrated total internal reflection” based touch screens is that it can be difficult to detect a “bright spot” over the noise created by the ambient light. In a bright environment, ambient light can pass through the touch screen to the light detector causing “noise” which makes the “bright spot” seem relatively dim. 
   As mentioned above, these touch screens are often used with an image generator which creates an image of simulated buttons or other objects to touch. The image generator often requires lighting and this lighting can also cause “noise”, making it more difficult to detect the “bright spot”. 
   One solution to these problems is to use a brighter light source for the light guide, however, this requires greater electrical power which is undesirable for portable devices. 
   It would be desirable to provide a “frustrated total internal reflection” based touch indication screen that uses less electrical power and works well even in bright ambient light environments. 
   SUMMARY OF THE INVENTION 
   These and other objects are provided by the present invention which provides a “frustrated total internal reflection” based touch indication device. The present invention incorporates a light-enhancing layer to provide a “frustrated total internal reflection” based touch indication device that can use less power and can work well even in bright ambient light environments. 
   In general terms, one embodiment of the invention is a touch indication device, or more specifically a touch screen data input device, comprising a light guide structure for passing light of a light source via total internal reflection. A touch surface, which disturbs the total internal reflection when touched, causes light to leave the light guide structure. A light enhancement layer converts the light leaving the light guide and sends the converted light to a light detector. The light enhancement layer amplifies the intensity and/or changes the color of the light leaving the light guide. The light enhancement layer can be comprised of phosphor. The thickness and/or composition of the phosphor layer can vary with position to output phosphor-converted light having position information. A CPU receives data from the detector and controls the display screen. The CPU also generates and modifies simulated buttons and other objects as touch targets displayed on a display screen. The detector determines the position of the touch on the touch surface based on the light from the light enhancement layer. 
   The light guide can be made from ITO glass in which case the touch surface of the light guide disturbs the total internal reflection when touched by a touch object which displaces the ambient air adjacent to the touch surface thereby reducing the difference in indices of refraction at the touch surface interface and disturbing the total internal reflection. 
   The light guide can also be made of a deformable plastic in which case the touch surface of the light guide deforms the light guide inwardly when touched, causing an indentation in the light guide so that the light scatters from the indentation, thereby disturbing the total internal reflection. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further preferred features of the invention will now be described for the sake of example only with reference to the following figures, in which: 
       FIG. 1  illustrates a light guide touch screen making use of the frustrated total internal reflection of the prior art. 
       FIG. 2  illustrates another light guide touch screen making use of the frustrated total internal reflection of the prior art. 
       FIG. 3  is a side view of a touch screen of the present invention making use of frustrated total internal reflection and utilizing a light enhancement layer. 
       FIG. 4  is a diagrammatic top view of one embodiment of a phosphor layer having a spatial thickness variation used with the invention of  FIG. 3 . 
       FIG. 5  is a diagrammatic top view of one embodiment of a phosphor layer having both a continuous spatial thickness variation and a spatial phosphor composition variation used with the invention of  FIG. 3 . 
   

   DETAILED DESCRIPTION 
     FIG. 3  is a side view of a touch screen  300 . More generally, the touch screen  300  is a touch indication device. A light source  301  emits light  303  into a light guide  305 . The optical matching between the light guide  305  and its surroundings is adapted such that the light rays  303  of the light source  301  are normally confined within the light guide  305  by means of total internal reflection. However, when a user touches the touch surface  309  with a touch-object  325 , which can be a stylus, finger, or other object, the state of the total internal reflection is perturbed, and perturbed light  307  is extracted from the light guide  305  through a light extraction surface  311  of the light guide  305 . 
   A phosphor layer  313  is deposited on a display screen  321 . The display screen  321  is shown between the phosphor layer  313  and the light guide  305 . Alternatively the phosphor layer  313  can be between the display screen  321  and light guide  305 . The phosphor layer  313  enhances the efficiency of the perturbed light  307  and outputs phosphor-converted light  315 . 
   Directly adjacent to the light extraction surface  311  can be a layer of ambient air  323  to provide a favorable critical angle for the total internal reflection of the light rays  303 . 
   A light detection means  317  is arranged to detect the phosphor-converted light  315  and relate this detection to an input position  319  where the user contact occurred. The light detection means  317  can include one or more light detectors  331 , such as photodetectors. 
   The display screen  321  can be a mask with images of simulated buttons or other objects to touch and also possibly other labels printed thereon. Alternatively, the display screen  321  can be a LCD or other programmable display which is controlled by the CPU  329  so that the simulated buttons or other objects to touch and other labels can change during operation. The display screen  321  can be illuminated by its own light source to increase the visibility of the touch objects or labels. This light source can be in front of or in back of the display screen  321 . The display screen  321  is transparent to the perturbed light  307  so that the perturbed light  307  can reach the light detection means  317 . The display screen  321  is also made transparent to the phosphor-converted light  315  when the display screen  321  is positioned between the phosphor layer  313  and the light detection means  317 . 
   In other embodiments the display screen  321  is adjacent the touch surface  309  and in these embodiments the display screen  321  need not be transparent to the perturbed light  307  or the phosphor-converted light  315 . 
   The principal behind the disturbed total internal reflection of the present invention is now considered in more detail, again with reference to  FIG. 3 . The light source  301  shines the light rays  303  into the light guide  305  through an edge of the light guide  305 . The plurality of light rays  303  that strike the touch surface  309  and the light extraction surface  311  at angles to the surface normal greater than the critical angle are totally internally reflected from the touch surface  309  and the light extraction surface  311 . 
   The critical angle is measured from a normal to the surface and is determined by Snell&#39;s Law, which determines the refraction of light at a boundary between two media through which light passes: 
   A sin X=B sin Y; where A=index of refraction of the first material through which light passes (in this case, the material of the light guide  305 ), B=index of refraction of the second material through which light passes (in this case, the layer of ambient air  327  adjacent to the touch surface  309 ), Y=angle between a normal to the touch surface  309  and a light ray  303  after refracting through touch surface  309 , and X=angle between a normal to the touch surface  309  and a light ray  303  before refracting at the touch surface. The critical angle for total internal reflection is the value of X where Y=90 degrees. If A=1.5 (the approximate index of refraction for glass) and B=1.0 (the approximate index of refraction for air), then the critical angle is approximately 41.8 degrees. The same analysis applies at the light extraction surface  311 . 
   When the touch-object  325  displaces the ambient air adjacent to the touch surface  309 , total internal reflection is interrupted. This phenomenon occurs because air has an index of refraction significantly lower than that of the material of the light guide  305  or that for the touch-object  325 . Where there is not a significant difference in indices of refraction at the two sides of the touch surface  309 , total internal reflection does not occur at that point. The layer of air adjacent to the touch surface  309  necessary for total internal reflection is very thin. Thus, paper- or cloth-covered objects (and, in some cases, fingers with very dry skin) might not effectively displace the layer, whereas most fingers or rubbery objects likely would displace the layer and thus interrupt total internal reflection. Some of the light rays  303  that experience the disruption of total internal reflection are scattered or reflected back through the light extraction surface  311 , whereupon they refract at various angles as perturbed light  307 . The perturbed light  307  passes through the ambient air  323  and to the phosphor layer  313 . 
   The light guide  305  can be made from a rigid material such as ITO (Indium Tin Oxide) glass. 
   Rather than using ITO glass, the light guide  305  can be molded from an elastomeric material such as an untinted, diffused, optical grade silicone rubber, such as 150-OU which can be supplied by Tory Rubber Company, a division of Dow Corning. Also, Kurabe Industrial Co. Ltd produces bendable Elastomer light strips. 
   A combination of a rigid material such as ITO glass and a flexible material such as silicone rubber can be used to form the light guide  305 . 
   When a deformable light guide  305  is used, a different mechanism can be used for disturbing total internal reflection. The touch-object  325  can deform the light guide  305  inwardly, causing an indentation in the light guide  305  so that the light  303  scatters from the indentation. This results in positional dependent converted light  315  which can be used to determine the input position  319  where the user contact occurred as described with reference to  FIG. 3  above. 
   In one embodiment the phosphor layer  313  is made from yellow phosphor. The light source  301  can be a blue LED generating blue light as the light rays  303 . The yellow phosphor enhances the efficiency of the blue perturbed light  307  passing through it. The yellow phosphor layer  313  partially converts the blue perturbed light  307  to converted yellow light. The converted yellow light mixes with the unconverted blue light to produce white light. This conversion increases the brightness of the perturbed light  307  in the range of approximately 2.5 to 6 times. The exact brightness increase depends on the phosphor efficiency and thickness of the phosphor layer. Also, different thickness of the phosphor layer  313  will produce different colors. 
   In another embodiment, the light  303  which the light source  301  emits into the light guide  305  can be UV light and the phosphor layer  313  converts the UV light to white light. In this embodiment a UV inhibitor layer should be placed between the touch surface  309  and the observer to prevent UV light from harming the observer&#39;s eyes. 
   Rather than using the phosphor layer  313 , other methods can be used to amplify the light. For example, amplifiers can be used or other materials can be used. 
   “OLEDs” using Organic types of phosphor can also be used to enhance or amplify the light. Also, quantum dots phosphor can be used. 
   Various types or phosphor can be used depending on the color of the light that is desired for illumination or backlighting. Red phosphors such as CaS, SrS, CaSrS, ZnS, ZnSe, ZnSeS or green phosphors such as SrTg, BaGa2S4 can be used. Also, a mixture of these or other phosphors can be used to produce different color hues. 
   Therefore, the layer  313  can generally be described as a light enhancement layer. 
     FIG. 4  is a diagrammatic top view of one embodiment of the phosphor layer  313 . In this embodiment the thickness of the phosphor layer  313  varies with position. The phosphor layer is shown divided into sixteen discrete sections  401 , each section having a different thickness. In this example the thicknesses are shown to vary from 10 to 25 units, wherein the units are scalable to the desired dimensions depending on the particular touch screen  300  and type of phosphor used. Of course the number, area, and thickness of the sections can be varied as would be understood by one skilled in the art. 
   By varying the thickness of the phosphor layer  313  with position, the phosphor-converted light  315  will have a color and brightness that corresponds to one of the discrete sections  401  which in turn corresponds to a position where the touch-object  325  touches the touch surface  309 . Thus the phosphor-converted light contains position information. 
   Rather than using the phosphor layer discrete spatial thickness variation of  FIG. 4 , a continuous spatial thickness variation can be used as illustrated in  FIG. 5 . In  FIG. 5  the phosphor thickness (“T”) of the phosphor layer  313  varies continuously with increasing “y”. The phosphor layer  313  is made up of two different color producing components, illustrated as “A” and “B”, and the relative ratio of these components vary inversely to each other with increasing “x”. Thus, each position of the phosphor layer  313  has a unique color/intensity combination. From the unique color/intensity combination it can be determined at which position the touch-object  325  touched the touch surface  309 . 
   In  FIG. 3  the phosphor layer  313  is shown deposited on the display screen  321 . The phosphor layer  313  can also be deposited on a separate plate rather than directly on the display screen  321 . The phosphor layer  313  can even be deposited directly on the light extraction surface  311  of the light guide  305 , so long as it has an index of refraction less than that of the light guide so as not to frustrate the total internal reflection of the light rays  303  in the light guide  305 . In general, the phosphor layer  313  can be deposited in any way so long as it is between the light guide  305  and the light detecting means  317  and so as to achieve the objects of the invention. 
   Various processes can be used to deposit the phosphor. 
   Electrophoretic deposition processes can be used to deposit the phosphor layer  313  having the desired thickness within a desired precision. U.S. Pat. No. 6,576,488 to Collins et al. and U.S. Pat. No. 6,864,110 to Summers et al. both provide examples of electrophoretic deposition processes that can be adopted to achieve the desired thickness within a desired precision. 
   U.S. Pat. No. 6,869,753 to Chua et al. describes a screen printing process that can be used to deposit the phosphor layer  313  with less precision but also with less expense. 
   US Patent Publication US2004/0196318 by Su et al. describes another relatively inexpensive phosphor coating method using inkjet “printing” that can be adopted to deposit the phosphor layer  313 . US Patent Publication US 2004/0166234 to Chua et al. also describes a method that that can be adopted to deposit the phosphor layer  313 . 
   The light detection means  317  can include one or more conventional type of light detector  331 . For example, the light detector  331  can be an image sensor array of a conventional digital camera. The image sensor array can be a complementary metal oxide semiconductor (CMOS) or charge coupled device (CCD), for example. The light detector  331  is made up of many photosites or pixels, each acquiring a portion of the image. The image can be that of the phosphor layer  313 , or in general, the light coming from any applicable type of light enhancement layer. It can be pre-determined which part of the phosphor layer  313  each of the image sensor array pixels corresponds to and thus it can be determined which part of the light guide  305  touch surface  309  has been touched by the touch-object  325 . 
   The light detection means  317  can also include an interface  333  for communicating with the CPU  329 . The CPU  329  receives instructions based on which part of the light guide  305  touch surface  309  has been touched by the touch-object  325 . The CPU  329  can also generate and modify the simulated buttons, other objects to touch and other labels displayed on the display screen  321  depending on where the touch surface  306  has been touched. 
   The variation of the thickness of the phosphor layer  313  with position and/or the variation of the ratios of different phosphor components with position results in position-dependent color and intensity output values of the phosphor-converted light  315 . In other words, the phosphor-converted light  315  has position information. The light detection means  317  when in the form of a sensor array, can use this position information to better distinguish between adjacent input positions  319 . 
   The light detection means  317  can also be a single sensor or photodetector. Since each input position  319  corresponds to the discrete sections  401  of  FIG. 4  or continuous phosphor variations of  FIG. 5 , by measuring light intensity the single sensor can determine the input position  319 . 
   The light detection means  317  can also be a color sensor. The continuous phosphor composition and thickness variations of  FIG. 5 , allow determination of the input position  319  based upon the measurement of light intensity and color by the color sensor. Also, measuring the of light intensity and color by the color sensor can allow for the determination of the input position  319  when the phosphor layer  313  has the discrete sections  401  of  FIG. 4 . 
   A light focusing or guiding means can be positioned between the phosphor layer  313  and the light detection means  317  to optimize the amount of light received by the light detector  331 . 
   The present invention may be embodied in other forms without departing from its spirit and scope. The embodiments described above are therefore illustrative and not restrictive, since the scope of the invention is determined by the appended claims rather then by the foregoing description, and all changes that fall within the meaning and range of equivalency of the claims are to be embraced within their scope.