Abstract:
A passive touch system includes a touch surface and at least one source of backlight illumination projecting backlighting across the touch surface. At least two image sensors are associated with the touch surface and acquire images of the touch surface from different locations. A digital signal processor is associated with each image sensor. The digital signal processors select pixel subsets of images acquired by the image sensors and process pixel data acquired by the selected pixel subsets to generate pointer characteristic data when a pointer exists in the acquired images. A master digital signal processor in communication with the digital signal processors triangulates the pointer characteristic data to determine the location of the pointer relative to the touch surface.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to touch systems and in particular to an illuminated bezel for a touch system and to a touch system incorporating the same. 
   BACKGROUND OF THE INVENTION 
   Touch systems are well known in the art and typically include a touch screen having a touch surface on which contacts are made using a pointer. Pointer contacts with the touch surface are detected and are used to generate output pointer position data representing areas of the touch surface where the contacts are made. There are basically two general types or touch systems available and they can be broadly classified as “active” touch systems and “passive” touch systems. 
   Active touch systems allow a user to generate pointer position data by contacting the touch surface with a special pointer that usually requires some form of on-board power source, typically batteries. The special pointer emits signals such as infrared light, visible light, ultrasonic frequencies, electromagnetic frequencies, etc. that activate the touch surface. 
   Passive touch systems allow a user to generate pointer position data by contacting the touch surface with a passive pointer and do not require the use of a special pointer in order to activate the touch surface. A passive pointer can be a finger, a cylinder of some material, or any suitable object that can be used to contact some predetermined area of interest on the touch surface. Since special active pointers are not necessary in passive touch systems, battery power levels and/or pointer damage, theft, or pointer misplacement are of no concern to users. 
   International PCT Application No. PCT/CA01/00980 filed on Jul. 5, 2001 and published under number WO 02/03316 on Jan. 10, 2002, assigned to the assignee of the present invention, discloses a camera-based touch system comprising a touch screen that includes a passive touch surface on which a computer-generated image is presented. A rectangular bezel or frame surrounds the touch surface and supports digital cameras at its corners. The digital cameras have overlapping fields of view that encompass and look along the touch surface. The digital cameras acquire images of the touch surface from different locations and generate image data. The image data is processed by digital signal processors to determine if a pointer exists in the captured image data. When it is determined that a pointer exists in the captured image data, the digital signal processors convey pointer characteristic data to a master controller, which in turn processes the pointer characteristic data to determine the location of the pointer relative to the touch surface using triangulation. The pointer location data is conveyed to a computer executing one or more application programs. The computer uses the pointer location data to update the computer-generated image that is presented on the touch surface. Pointer contacts on the touch surface can therefore be recorded as writing or drawing or used to control execution of the applications programs executed by the computer. 
   Although this touch system works extremely well, it has been found that when the digital camera frame rates are high, in less favorable light conditions the ability to determine the existence of a pointer in the captured image data is diminished. As a result, there exists a need to improve the lighting environment for the digital cameras to ensure high resolution irrespective of ambient lighting conditions. 
   The concept of providing an illumination source for a touch surface has been considered. For example, U.S. Pat. No. 4,144,449 to Funk et al. discloses a position detection apparatus for detecting the position of a passive object. The position detection apparatus includes a generally rectangular frame having an open interior. Fluorescent tube continuous light sources extend along three sides of the frame for illuminating the open interior of the frame. Linear image detectors are mounted at opposite corners of the fourth side of the frame. Aperture-defining devices are located between the linear image detectors and the open interior of the frame for configuring coincident fields of light from the open interior for the linear image detectors to view. Unfortunately, the light emitted by fluorescent tubes is limited to a very narrow frequency range within the visible light spectrum. This makes the position detection apparatus very susceptible to interference by ambient light. 
   It is therefore an object of the present invention to provide a novel illuminated bezel and a touch system incorporating the same. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention there is provided in a touch system including a touch surface and at least one optical sensor looking along the touch surface to acquire images of a pointer in proximity thereto, an illumination source to provide backlighting to said at least one optical sensor comprising: 
   at least one light source; and 
   a diffuser disposed between said at least one light source and said at least one optical sensor, said diffuser diffusing light projected by said at least one light source prior to said light being directed to said at least one optical sensor. 
   In one embodiment, the at least one light source includes a plurality of spaced discrete light sources. The discrete light sources are arranged in at least one row and are generally equally spaced. The light projected by the discrete light sources onto the diffuser is expanded so that the illumination source appears as a generally continuous illumination source to the at least one optical sensor. 
   In a preferred embodiment, the diffuser is generally transparent in a specified frequency range and generally opaque in a different specified frequency range. In one embodiment, the discrete light sources are infrared light emitting diodes and the diffuser is generally transparent in the infrared range and generally opaque in the visible range. 
   In an alternative embodiment, the at least one light source is a continuous light source. The continuous light source may project light in the infrared spectrum or project light in the visible spectrum. When the continuous light source projects light in the visible spectrum, the illumination source further includes a color filter either adjacent to or incorporated into the diffuser. 
   According to another aspect of the present invention there is provided a touch system comprising: 
   at least one optical sensor associated with a touch surface and having a field of view encompassing and looking across at least a portion of said touch surface; 
   at least one source of backlight illumination directing light into the field of view of said at least one optical sensor, said at least one illumination source including at least one light source and a diffusion medium to expand light projected by said at least one light source prior to said light being directed into said field of view; and 
   a pass filter associated with said at least one optical sensor to pass the light directed by said source of backlight illumination to said at least one optical sensor. 
   According to yet another aspect of the present invention there is provided a passive touch system comprising: 
   a touch surface; 
   at least one source of backlight illumination projecting backlighting across said touch surface; 
   at least two image sensors associated with said touch surface, said at least two image sensors acquiring images of said touch surface from different locations and having overlapping fields of view; 
   a pass filter associated with each of said image sensors generally to blind said image sensors except to said projected backlighting; 
   a digital signal processor associated with each image sensor, the digital signal processors associated with said at least two image sensors selecting pixel subsets of images acquired by said at least two image sensors and processing pixel data acquired by the selected pixel subsets to generate pointer characteristic data when a pointer exists in said acquired images; and 
   a master digital signal processor in communication with said digital signal processors, said master digital signal processor receiving pointer characteristic data from said digital signal processors and triangulating the pointer characteristic data to determine the location of said pointer relative to said touch surface. 
   According to yet another aspect of the present invention there is provided a touch system comprising: 
   at least two CMOS image sensors associated with a touch surface, said at least two CMOS image sensors acquiring images of said touch surface from different locations and having overlapping fields of view; 
   at least one source of backlight illumination projecting backlighting across said touch surface; 
   a pass filter associated with each of said image sensors generally to blind said image sensors except to said projected backlighting; and 
   at least one processor receiving and processing image data acquired by said at least two CMOS image sensors to detect the existence of a pointer in said images and to determine the location of said pointer relative to said touch surface. 
   According to still yet another aspect of the present invention there is provided a touch system comprising: 
   at least two optical recording devices associated with a touch surface, said at least two optical recording devices acquiring images of said touch surface from different locations and having overlapping fields of view; 
   at least one source of backlight illumination projecting backlighting across said touch surface; 
   a pass filter associated with said optical recording devices generally to blind said optical recording devices except to said projected backlighting; and 
   a processor receiving and processing image data acquired by said at least two optical recording devices to detect the existence of a pointer in said images and to determine the location of said pointer relative to said touch surface, wherein said processor includes first and second processing stages, said first processing stage processing pixel data from said at least two optical recording devices, said second processing stage processing image data from said first processing stage to determine the location of the pointer. 
   According to still yet another aspect of the present invention there is provided an imaging assembly comprising: 
   a substantially rectangular bezel to surround a display surface; 
   at least one optical sensor mounted on said bezel, said at least one optical sensor being oriented to have a field of view looking along said display surface; 
   at least one source of backlight illumination within said bezel projecting backlight illumination into said field of view; and 
   a pass filter associated with said at least one optical sensor to pass the light directed by said source of backlight illumination to said at least one optical sensor. 
   According to still yet another aspect of the present invention there is provided a method of detecting the position of a pointer relative to a touch surface comprising the steps of: 
   acquiring multiple images of a pointer relative to said touch surface; 
   selecting pixel subsets of said acquired images; 
   processing pixel data acquired by the pixel subsets to detect the existence of said pointer therein and to determine the location of the pointer relative to the touch surface using triangulation; and 
   during said acquiring providing backlight illumination across said touch surface and acquiring said images based on said backlight illumination. 
   The present invention provides advantages in that the illuminated bezel provides good backlighting for the optical sensors allowing the optical sensors to detect the presence of a pointer in close proximity to the touch surface in a wide range of ambient lighting conditions. This of course increases the resolution of the touch system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic diagram of a camera-based touch system; 
       FIG. 2  is a front elevation view of a touch screen forming part of the touch system of  FIG. 1  including an illuminated bezel in accordance with the present invention; 
       FIG. 3  is a cross-sectional view of a side fame assembly forming part of the illuminated bezel of  FIG. 2 ; 
       FIG. 4  is a perspective view of the side frame assembly of  FIG. 3 ; 
       FIG. 5  shows the radiation pattern of a discrete light source forming part of the illuminated bezel of  FIG. 2 ; 
       FIG. 6   a  is a perspective view of a portion of a continuous illumination source including a row of discrete light sources and a diffuser forming part of the illuminated bezel of  FIG. 2 ; 
       FIG. 6   b  is a front elevation view of the diffuser of  FIG. 6   a  showing illumination spots projected thereon by the discrete light sources; 
       FIG. 7  is a schematic diagram of a digital camera forming part of the touch screen of  FIG. 2 ; 
       FIG. 8  is a schematic diagram of a master controller forming part of the touch system of  FIG. 1 ; 
       FIG. 9  is a front elevation view of the touch screen of  FIG. 2  showing the illumination sources and the fields of view of the digital cameras; 
       FIGS. 10   a  and  10   b  show backlight illumination projected by a continuous illumination source as seen by a digital camera with and without a diffuser; 
       FIGS. 11   a  to  11   c  are front elevation views of a diffuser showing illumination spots projected thereon at different discrete light source spacings; 
       FIG. 12  is a front elevation view of a diffuser showing an illumination spot projected thereon at an increased discrete light source throw; 
       FIG. 13   a  is a perspective view showing an alternative discrete light source orientation for an illumination source; 
       FIG. 13   b  is a front elevation view of a diffuser showing an illumination spot projected thereon by the discrete light source of  FIG. 13   a;    
       FIG. 14   a  is a side elevation view showing an alternative diffuser profile for an illumination source; 
       FIG. 14   b  is a front elevation view of the diffuser of  FIG. 14   a  showing an illumination spot projected thereon by a discrete light source; and 
       FIGS. 15   a  and  15   b  are front elevation views of alternative discrete light source arrangements for an illumination source. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates generally to a touch system including at least one optical sensor having a field of view encompassing a touch surface. At least one source of backlight illumination directs light towards the at least one optical sensor to enable pointer contacts with the touch surface to be clearly detected by the at least one optical sensor in a variety of ambient lighting conditions. Preferred embodiments of the present invention will now be described. 
   Turning now to  FIG. 1 , a camera-based touch system in accordance with the present invention is shown and is generally identified by reference numeral  50 . Camera-based touch system  50  is similar to that disclosed in International PCT Application Serial No. WO 02/03316, assigned to SMART Technologies Inc., assignee of the present invention, the content of which is incorporated herein by reference. As can be seen, touch system  50  includes a touch screen  52  coupled to a digital signal processor (DSP) based master controller  54 . Master controller  54  is also coupled to a computer  56 . Computer  56  executes one or more application programs and provides computer-generated image output to the touch screen  52 . The touch screen  52 , master controller  54  and computer  56  form a closed-loop so that pointer contacts with the touch screen  52  can be recorded as writing or drawing or used to control execution of application programs executed by the computer  56 . 
     FIG. 2  better illustrates the touch screen  52 . Touch screen  52  in the present embodiment includes a high-resolution display device such as a plasma display  58 , the front surface of which defines a touch surface  60 . The touch surface  60  is bordered by an illuminated bezel or frame  62  coupled to the display device. Illuminated bezel  62  includes elongate side frame assemblies  64  that are coupled to the sides of the plasma display  58 . Each side frame assembly  64  accommodates a generally continuous illumination source  66  (see  FIG. 3 ) as will be described. The ends of the side frame assemblies  64  are joined by corner pieces  68  that house DSP-based CMOS digital cameras  70  (see  FIG. 7 ). Each digital camera  70  is mounted within its respective corner piece  68  so that its field of view encompasses and looks across the entire touch surface  60 . 
   One of the side frame assemblies  64  is shown in  FIGS. 3 and 4 . As can be seen, each side frame assembly  64  includes an extrusion  64   a  that snaps onto a side of the plasma display  58 . The extrusion  64   a  has an open face  64   b  directed towards the touch surface  60  and defines a housing  64   c  to accommodate the generally continuous illumination source  66 . 
   Each generally continuous illumination source  66  includes a row of discrete light sources  66   a  mounted on the forward surface of a printed circuit board  66   b  and a diffuser  66   c  covering the open face  64   b  of the extrusion  64   a . The top and bottom edges of the printed circuit board  66   b  are received by channels  64   d  formed within the extrusion  64   a  to maintain the printed circuit board  66   b  in an orientation generally orthogonal to the plane of the touch surface  60 . In the present embodiment, the discrete light sources  66   a  are in the for of infrared light emitting diodes (IR LEDs) aimed at the diffuser  66   c . The spacing between each IR LED  66   a  is equal and is in the range of from about 1 to 2 inches. The IR LEDs  66   a  are oriented generally perpendicular to the plane of the diffuser  66   c  and are spaced from the diffuser  66   c  by approximately 0.8 inches.  FIG. 5  shows the radiation pattern of each IR LED  66   a  and as can be seen, the half power field of view is approximately 120°. 
   The printed circuit board and IR LED arrangement is made in strips of fixed length, in this case twelve (12) inch strips. A feed through power terminal  66   d  is provided on the rearward side of the printed circuit board  66   b  and is coupled to each IR LED  66   a  on the strip. By providing the printed circuit board and IR LED arrangement in strips, illuminated bezels  62  for a wide variety of touch screen sizes can easily be constructed by populating the extrusions  64   a  with the appropriate numbers of strips and attaching power lines to the feed through terminals  66   d.    
   Each diffuser  66   c  is formed of plastic that is semi-transparent or transparent (i.e. generally transparent) within a specified frequency range, in this case the infrared range, but substantially opaque in the visible light spectrum. As a result, the diffuser  66   c  obscures the internal components of the illuminated bezel  62  from view making the illuminated bezel more aesthetic. The diffuser  66   c  acts to diffuse or expand light emitted by the IR LEDs  66   a  so that the illumination sources  66  are seen by the digital cameras  70  as generally continuous illumination sources. In the present embodiment, the spacing between adjacent IR LEDs  66   a , the throw of the IR LEDs  66   a  and the distance between the IR LEDs  66   a  and the diffusers  66   c  is such that the illumination spots  72  projected onto the diffusers  66   c  by the IR LEDs  66   a  partially overlap at the diffusers  66   c  and remain within the boundaries of the diffusers as shown in  FIGS. 6   a  and  6   b . The slight curved shape of the diffusers  66   c  results in the illumination spots  72  taking on a generally elliptical shape. 
   One of the digital cameras  70  within a corner piece  68  is shown in  FIG. 7 . As can be seen, each digital camera  70  includes a two-dimensional CMOS image sensor and associated lens assembly  80 , a first-in-first-out (FIFO) buffer  82  coupled to the image sensor and lens assembly  80  by a data bus and a digital signal processor (DSP)  84  coupled to the FIFO  82  by a data bus and to the image sensor and lens assembly  80  by a control bus. A boot EPROM  86  and a power supply subsystem  88  are also included. In the present embodiment, the CMOS camera image sensor is configured for a 20×640 pixel subarray that can be operated to capture image flames at rates in excess of 200 frames per second since arbitrary pixel rows can be selected. Also, since the pixel rows can be arbitrarily selected, the pixel subarray can be exposed for a greater duration for a given digital camera frame rate allowing for good operation in dark rooms as well as well lit rooms. 
   The DSP  84  provides control information to the image sensor and lens assembly  80  via the control bus. The control information allows the DSP  84  to control parameters of the image sensor and lens assembly  80  such as exposure, gain, array configuration, reset and initialization. The DSP  84  also provides clock signals to the image sensor and lens assembly  80  to control the frame rate of the image sensor and lens assembly  80 . 
   An infrared pass filter  89  is provided on the digital camera image sensor and lens assembly  80  to blind the digital camera  70  to frequencies of light other than the light broadcasted by the illuminated bezel  62 . 
   Master controller  54  is best illustrated in  FIG. 8  and includes a DSP  90 , a boot EPROM  92 , a serial line driver  94  and a power supply subsystem  95 . The DSP  90  communicates with the DSPs  84  of the digital cameras  70  over a data bus via a serial port  96  and communicates with the computer  56  over a data bus via a serial port  98  and the serial line driver  94 . 
   The master controller  54  and each digital camera  70  follow a communication protocol that enables bi-directional communications via a common serial cable similar to a universal serial bus (USB). The transmission bandwidth is divided into thirty-two (32) 16-bit channels. Of the thirty-two channels, six (6) channels are assigned to each of the DSPs  84  in the digital cameras  70  and to the DSP  90  in the master controller  54  and the remaining two (2) channels are unused. The master controller  54  monitors the twenty-four (24) channels assigned to the DSPs  84  while the DSPs  84  monitor the six (6) channels assigned to the DSP  90  of the master controller  54 . Communications between the master controller  54  and the digital cameras  70  are performed as background processes in response to interrupts. 
   The operation of the touch system  50  will now be described. Each digital camera  70  acquires images looking along the touch surface  60  within the field of view of its image sensor and lens assembly  80  at a desired frame rate and processes each acquired image to determine if a pointer is in the acquired image. If a pointer is in the acquired image, the image is further processed to determine characteristics of the pointer contacting or hovering above the touch surface  60 . Pointer information packets (PIPs) including pointer characteristics, status and/or diagnostic information are then generated by the digital cameras  70  and ,the PIPs are queued for transmission to the master controller  54 . The digital cameras  70  also receive and respond to command PIPs generated by the master controller  54 . 
   The master controller  54  polls the digital cameras  70  for PIPs. If the PIPs include pointer characteristic information, the master controller  54  triangulates pointer characteristics in the PIPs to determine the position of the pointer relative to the touch surface  60  in Cartesian rectangular coordinates. The master controller  54  in turn transmits calculated pointer position data, status and/or diagnostic information to the computer  56 . In this manner, the pointer position data transmitted to the computer  56  can be recorded as writing or drawing or can be used to control execution of application programs executed by the computer  56 . The computer  56  also updates the computer-generated image output conveyed to the plasma display  58  so that information presented on the touch surface  60  reflects the pointer activity. 
   The master controller  54  also receives commands from the computer  56  and responds accordingly as well as generates and conveys command PIPs to the digital cameras  70 . Specifics concerning the processing of acquired images and the triangulation of pointer characteristics in PIPs are described in PCT Application No. WO 02/03316 and therefore will not be described further herein. 
   To provide adequate backlighting for the digital cameras  70 , the IR LEDs  66   a  within each side frame assembly  64  are powered and project infrared light onto the diffusers  66   c . The diffusers  66   c  in turn, diffuse and hence, expand the illumination spots  72  so that the intensity of light passing through the diffusers into the region encompassed by the illuminated bezel  62  is generally even across the surfaces of the diffusers  66   c . As a result, the illumination sources  66  appear as generally continuous illumination sources to the digital cameras  70 . Since the digital cameras  70  include infrared pass filters  89 , the digital cameras  70  are effectively blind to the background and only see the infrared light broadcast by the illuminated bezel  62 . This backlight illumination in conjunction with the pass filters  89  allow the digital cameras  70  to capture distinct images of a pointer in proximity to the touch surface  60  since the pointer occludes some of the backlight illumination. As a result, this helps to binarize the images captured by the digital cameras  70 .  FIG. 9  shows the field of views of the digital cameras  70  and as can be seen in this arrangement each digital camera  70  receives backlight illumination directly from two illumination sources  66 .  FIG. 10   a  shows a continuous illumination source of backlight illumination as seen by one of the digital cameras  70 . For contrast,  FIG. 10   b  shows the continuous illumination source of backlight illumination as seen be one of the digital cameras  70  with the diffuser  66   c  removed. 
   In the preferred embodiment, the spacing between the IR LEDs  66   a  is such that the illumination spots  72  projected onto the diffusers  66   c  partially overlap as shown in  FIGS. 6   a  and  6   b . The optical properties of the diffusers  66   c  are such that the diffusers  66   c  expand the illumination spots  72  so that light passing through the diffusers has a generally even intensity over the entire surfaces of the diffusers  66   c . As will be appreciated, alternative arrangements are possible. The IR LEDs  66   a  can be spaced so that the illumination spots  72  projected onto the diffusers  64   a  significantly overlap as shown in  FIG. 11   a , abut as shown in  FIG. 11   b  or are spaced apart as shown in  FIG. 11   c . In the case where the illumination spots  72  are spaced apart, if the optical properties of the diffusers  66   c  are such that the illumination spots  72  cannot be adequately expanded, the digital cameras  70  will see the illumination sources  66  as being discontinuous or discrete. 
   Although a particular IR LED throw, distance between the IR LEDs  66   a  and diffusers  66   c , and angular orientation of the IR LEDs  66   a  with respect to the diffusers  66   c  have been disclosed, those of skill in the art will appreciate that the IR LED throw, distance between the IR LEDs  66   a  and the diffusers  66   c , and the angular orientation of the IR LEDs  66   a  with respect to the diffusers  66   c  may be altered to suit the particular environment. An increase in IR LED throw or distance between the IR LEDs  66   a  and the diffusers  66   c  will result in expanded illumination spots  72  projected onto the diffusers  66   c  as shown in  FIG. 12 . 
   Changes in the angular orientation of the IR LEDs  66   a  with respect to the diffusers  66   c  determines the geometry of the illumination spots  72  as shown in  FIGS. 13   a  and  13   b . In this example, the angular orientation of the IR LED  66   a  results in an elongate illumination spot being projected onto the diffuser  66   c . The profile geometry of the diffusers  66   c  will also alter the profiles of the illumination spots  72  as shown in  FIGS. 14   a  and  14   b . In this example, the diffuser profile geometry results in circular illumination spots being projected onto the diffuser  66   c  rather than elliptical illumination spots as shown in  FIGS. 6   a  and  6   b.    
   Although the IR LEDs  66   a  have been described as being equally spaced along the lengths of the printed circuit boards  66   b , those of skill in the art will appreciate that the spacing between the IR LEDs need not be equal along the lengths of the printed circuit boards: For example, the spacing between the IR LEDs  66   a  may be non-linear and correspond to the resolution of the digital cameras  70 . In addition, although each illumination source  66  is described as including a single row of IR LEDs  66   a , it will be appreciated by those of skill in the art that an array of IR LEDs  66   a  including stacked rows or other two-dimensional arrays of IR LEDs may be provided in each illumination source  66  to enhance the backlight illumination provided to the digital cameras  70 . In the stacked row IR LED arrangement, the rows of IR LEDs  66   a  can be aligned as shown in  FIG. 15   a  or staggered as shown in  FIG. 15   b.    
   Although the diffusers  66   c  are described as being formed of plastic that is generally transparent in the IR range and generally opaque in the visible range, those of skill in the art will appreciate that the diffusers  66   c  may be formed of other suitable materials and/or have alternative optical properties. For example, the diffusers  66   c  may be formed of a polymer impregnated with a suitable material to aid in light diffusion. Furthermore, the diffusers may also be designed to act as polarizers to polarize the light emitted by the illumination sources  66 . The diffusers  66   c  can also be modified to control the backlight illumination as seen by the digital cameras  70 . For example, the diffusers  66   c  may be provided with horizontal slits therein defining apertures to limit the vertical backlight illumination as seen by the digital cameras. In this case, backlight illumination projected by the illumination sources  66  is effectively cropped to remove top and bottom fringe effects thereby to provide a more continuous source of backlight illumination. 
   Rather than using discrete light sources, continuous light sources in conjunction with colour filters incorporated into the diffusers or in close proximity thereto to block unwanted frequencies can be used to provide the desired backlight illumination for the digital cameras  70 . For example, the IR LEDs  66   a  can be replaced with electroluminescent wire extending around the illuminated bezel  62  within the side frame assemblies  64 . As is known, electroluminescent wire when powered casts continuous light in the visible range in one of eight frequencies. Of course other continuous sources of IR illumination can be used. As will be appreciated, when non-infrared light sources are used in the illumination sources  62 , the filters  89  of the digital cameras  70  are selected to pass the appropriate frequencies of light broadcast by the illuminated bezel  62  and blind the digital cameras  70  to the background. 
   Although the touch system  50  has been described as including a plasma display  58  to present images on the touch surface  60 , those of skill in the art will appreciate that this is not required. The touch screen  52  may be a rear or front projection display device or virtually any surface on which a computer generated image is projected. Alternatively, the touch system  50  may be a writeboard where images are not projected thereon. 
   Also, although the touch system  50  is described as including a master controller  54  separate from the digital cameras  70 , if desired one of the digital cameras  70  can be conditioned to function as both a camera and the master controller and poll the other digital cameras for PIPs. In this case, it is preferred that the digital camera functioning as the master controller includes a faster DSP  84  than the remaining digital cameras. 
   Furthermore, although the touch system  50  has been described as including four digital cameras  70 , each mounted adjacent a corner of the illuminated bezel  62 , those of skill in the art will appreciate that other image sensing arrangements can be used. The touch system  50  may include basically any number of optical sensors to acquire images along the touch surface  60  and one or more illumination sources  66  to provide the desired backlight illumination. 
   Although preferred embodiments of the present invention have been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.