Abstract:
A light pen which works in conjunction with an image sensor array to allow the sensor array to distinguish between a finger touch and touch by the light pen. The light pen is arranged to pass through a laser curtain or other light source undetected by the image sensor.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    This disclosure relates to a light pen. In particular, the disclosure relates to an active light pen which interacts with a surface location detection system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Active light pens generate a light signal in the infrared (IR) portion of the electromagnetic spectrum when a nib (i.e., pen tip) arrangement is in contact with a surface. 
         [0003]    When a plurality of IR detectors are arranged at spaced apart locations around a surface upon which a light pen is to be used (such as a wall-mounted whiteboard or a table-top collaboration space), the point at which the light signal is generated may be determined geometrically when the light signal is detected at each of the detectors. The pen tip may thus be tracked as it moves across the surface and the resulting track maybe captured and superposed on a projected image presented on the writing surface. Thus movements of the light pen (e.g., handwriting, diagrams, drawings, etc.) are translated into virtual pen marks in a projected image. 
         [0004]    Any pointing object, such as a conventional dry-wipe, white board pen or a finger tip may be detected by a different mechanism that also uses infrared light. In one known example an array of semiconductor diode laser sources, emitting coherent beams of light in the infrared, lays down a laser “curtain.”. Typically this curtain is in the near infrared range (i.e., with wavelengths between 0.7 micrometers and 1.3 micrometers (μm), which may be expressed as 0.7×10 −6  m to 1.3×10 −6  m). Again, spaced-apart IR sensors (whether the same as for detecting light pen movements or sensors dedicated to detect interruptions of the curtain) detect any deflection of the beams and interpret the overall pattern of disruption as a touch on the writing surface. 
         [0005]    In many cases the writing surfaces are uneven and it becomes necessary to offset the laser curtain by a distance (normal to the surface) known as the “engage distance.” 
         [0006]    As a result of the offset, writing upon the writing surface may become awkward—the curtain sensors may struggle to correctly distinguish between a finger touch and a light pen. 
         [0007]    Known solutions require communication between the pens and a projector (i.e., a device responsible for generating and displaying projected images). This communication uses light pulsing or a radio frequency (RF) signal to allow the pen to send information to the projector telling it if the pen down is activated or if the pen is just breaking the laser curtain. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect of present disclosure there is provided a system for determining a position on a surface, comprising:
       a light source arrangement generating at least one incident light beam;   a pointing device comprising: a body portion; a proximal portion at an extent of the pointing device configured to be proximal to the surface; and a light emitting unit for radiating light when actuated by contact between the proximal portion and the surface; and   a light sensor arrangement configured to sense reflected light from objects interrupting the light beam and the radiated light from the light emitting unit,   wherein output from the light sensor arrangement is processed to determine the position of detected objects on the surface; and wherein the proximal portion of the pointing device is made of an admissive material that admits a substantial portion of the incident light from the light source arrangement, thereby avoiding detection of the pointing device until the light emitting unit is actuated.       
 
         [0013]    In certain embodiments, the light source arrangement generates an IR laser curtain and the light sensor arrangement senses reflected IR light from the IR curtain. As such light is admitted and therefore not reflected by the pointing device, the pointing device remains effectively invisible to the sensor arrangement. 
         [0014]    As a result, the pointing device (e.g., a light pen) works in conjunction with the image sensor arrangement so that its presence is sensed only when the proximal portion (e.g., a pen head) is pressed to the writing surface. Otherwise, the light pen passes through the laser curtain or other light source arrangement undetected by the light sensor arrangement: a finger or a conventional stylus would by contrast be detected by virtue of the reflection of light from the laser curtain. 
         [0015]    Providing the light pen in combination with a touch tracking system significantly improves the subjective writing experience and substantially eliminates the deleterious effects of the presence of an engage distance. The tracking system only detects the pen when a pen surface touch is detected against the writing surface and not when it breaks the plane of the IR curtain generated by the tracking system. 
         [0016]    According to a second aspect of present invention there is provided a pointing device for indicating a position on a surface to a surface tracking system, the surface tracking system including a light source arrangement and a light sensor arrangement, the pointing device comprising:
       a body portion;   a proximal portion at an extent of the pointing device configured to be proximal to the surface; and   a light emitting unit for radiating light when actuated by contact between the proximal portion and the surface;   wherein the proximal portion is made of an admissive material that admits a substantial portion of the incident light from the light source arrangement       
 
         [0021]    It is an aim of certain embodiments of the present disclosure to solve, mitigate or obviate, at least partly, at least one of the problems and/or disadvantages associated with the prior art. Certain embodiments aim to provide at least one of the advantages described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Various embodiments of the present disclosure will now be described with reference to the accompanying drawings, in which: 
           [0023]      FIGS. 1A and 1B  illustrate a conventional pen or stylus interrupting a laser curtain, respectively at a distance normal to, and in contact with, a writing surface; 
           [0024]      FIGS. 2A and 2B  illustrate certain structural features of a pointing device in accordance with an aspect of the present disclosure when in contact with a surface and at an offset distance from the surface; and 
           [0025]      FIGS. 3A and 3B  illustrate a pointing device in accordance with an aspect of the present disclosure when a light emitting unit is either inactive or actuated by contact with a surface. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    The present disclosure relates to an active light pen which interacts with a surface location detection system. 
         [0027]    In certain embodiments, the surface location detection system comprises a laser curtain touch tracking system that radiates a near-infrared light (IR) curtain over a surface (typically a board such as a whiteboard). When an object passes through the IR curtain, it will reflect a proportion of the light back to a light sensor arrangement. The output from the light sensor arrangement is processed to determine the position of detected objects on the surface. In certain arrangements, this operation is effected by a processor included in the surface location detection system: this processor may be integral to the light sensor arrangement or a distinct element of the detection system. In certain alternative arrangements, the operation is effected in a separate entity (for example in a projector used to project an image over the surface including graphical elements mapped to the detected object or a PC controlling the tracking system and/or a projector system. 
         [0028]      FIGS. 1A and 1B  illustrate a conventional pen or stylus  120  interacting with such a surface location detection system. In this case, the surface location detection system uses a laser curtain generated by a source  110 . Light reflected from the stylus  120  is detected by an image sensor  130  of the surface location detection system. 
         [0029]    In  FIG. 1A , the stylus is offset at a distance normal to a writing surface  140  yet still interrupting the laser curtain. 
         [0030]    In  FIG. 1B , the stylus is in contact with the writing surface  140 , yet again interrupting the laser curtain. 
         [0031]    In both cases, the image sensor detects an IR signature caused by the interrupting stylus  120 . 
         [0032]      FIGS. 2A and 2B  illustrate certain structural features of a pointing device  200  in accordance with an aspect of the present disclosure at an offset distance from a surface  240  and when in contact with the surface. 
         [0033]    A common problem with laser curtain systems is that the IR curtain needs to be offset from the board surface to allow for surface irregularities. The distance between the board surface and the IR curtain is referred to as the “engage distance.” The presence of an engage distance can however lead to writing quality issues. The natural movements of the pen lifting the pen tip and then depressing once more may not be detected as the pen may not be lifted sufficiently above the engage distance for this movement to be correctly interpreted. The writer is therefore forced to exaggerate certain writing actions to achieve a desired output—this can detract from the effective use of the tracking system. 
         [0034]    In  FIG. 2A , a laser curtain  250  is offset from the surface  240  by an engage distance  230 . The pointing device  200  is offset at a distance normal to a surface  240 : as this distance is less than the engage distance  230  the pointing device  200  still interrupts the laser curtain. 
         [0035]    In  FIG. 2B , the pointing device  200  is in contact with the surface  240  yet again interrupting the laser curtain. 
         [0036]    The proximal portion of the pointing device  200  interrupts the laser curtain in both cases. The proximal portion is shown to comprise both a pen tip section  210  and a proximal housing section  220 . The pointing device  200  in  FIG. 2B  extends through the laser curtain  250  where the proximal housing section  220  extends from the tip in contact with the surface  240  to a distance normal to the surface exceeding the engage distance. 
         [0037]    In certain embodiments, the light pen is constructed so that the tracking system “sees” (i.e., detects reflected IR radiation from) the pen when a pen surface touch is detected on (or at) the surface and not when the pen breaks the plane of the IR curtain. 
         [0038]    When a light pen breaks the IR curtain, IR light having a characteristic signature is reflected back to the system, the IR signature resembling that of a finger activating the curtain. Certain embodiments allow the tracking system to distinguish the IR signatures of a conventional pointing object (such as a finger or a conventional whiteboard marker) from the light pen by providing a mechanism where the tracking system may ignore the light pen while it was approaching the surface (but not yet touching it). 
         [0039]    Certain embodiments of the present disclosure add a near-infrared absorbent material to the pen, which allows it to reflect minimal amounts of IR (which in turn leaves the pen practically undetectable to the tracking system until a pen surface touch event is registered). The light absorbing pen will pass through the IR curtain undetected and will radiate IR onto the surface (and the IR sensors) only when a pen surface touch event is detected, allowing it to be tracked by the system. 
         [0040]    A light pen in accordance with certain embodiments is constructed of a material that does not allow the near-infrared to be reflected back to the image sensor. In certain embodiments, the light pen is alternatively constructed of a material that allows light (or at least the IR portion) to pass through or be redirected (i.e., an IR-transparent or IR-translucent material). 
         [0041]      FIGS. 3A and 3B  illustrate a pointing device  320  in accordance with an aspect of the present disclosure when a light emitting unit at the portion of the pointing device  320  proximal to a surface  340  is either inactive or actuated by contact with the surface. 
         [0042]    As for  FIGS. 1A and 1B ,  FIGS. 3A and 3B  illustrate a surface location detection system that includes a laser curtain generated by a source  310  to track the pointing device  320 . Light reflected from the pointing device  320  is detected by an image sensor  330  of the surface location detection system. 
         [0043]    In  FIG. 3A , the pointing device  320  (i.e., an active light pen) is offset at a distance normal to the surface  340  yet still interrupting the laser curtain. As the pointing device  320  has at least a proximal portion constructed of a material that admits light (here shown as fully absorbed), the interruption of the laser curtain does not result in an IR signature that can be detected by the image sensor  330 . 
         [0044]    In  FIG. 3B , the stylus is in contact with the writing surface  140 , yet again interrupting the laser curtain. Here, the light emitting unit of the pointing device  320  is actuated by contact with the surface  340  and the image sensor  330  detects the IR signature caused by the activated light emitting unit. 
         [0045]    In certain embodiments the entire front of the pen is designed to absorb (or pass) as much near-infrared light as possible. The two major parts that make up the front of the light absorbent pen is the front housing and the tip (for example, the proximal housing section  220  and the pen tip section  210  respectively of  FIGS. 2A and 2B ). 
         [0046]    In certain embodiments, the front housing is made of a near-infrared absorbent material such as, but not limited to, EPDM (ethylene propylene diene terpolymer), vinyl, plastic dyes, films, or paint that covers the majority of the surface area that will come in contact with the plane of the IR curtain. In certain embodiments, the front housing material is also wrapped around to the very front of the pen to limit IR reflection when the pen is not at a perpendicular angle from the white board. 
         [0047]    Typically, the IR-absorbent material is too soft to be used as a practical pen tip (which may be subject to considerable impact). To provide a more satisfactory pen tip, the pen tip is manufactured, in certain embodiments, in a more robust material that would nevertheless allow the IR to pass through as much as possible minimizing the amount of IR reflected back to the image sensor. The use of such IR-transparent material provides a degree of design freedom for the pen manufacturer. 
         [0048]    By ensuring that the front housing and pen tip extend to encompass substantially all of the area breaking the plane of the IR curtain, the IR signature of the light pen is significantly reduced (e.g. to power levels more than 50% lower than a conventional stylus, and typically in the range of 90% to 95% lower), preventing the image sensor tracking system from detecting the signature. 
         [0049]    Lastly, while the accompanying claims set out particular combinations of features described herein, the scope of the present invention is not limited to the particular combinations hereafter claimed, but instead extends to encompass any combination of features or embodiments herein disclosed irrespective of whether or not that particular combination has been specifically enumerated in the accompanying claims at this time.