# Patent

## Title
Systems and methods for sensing and tracking radiation blocking objects on a surface

## Abstract

Several systems for tracking one or more radiation blocking objects on a surface are disclosed. At least three radiation sensors are provided adjacent the surface and a plurality of radiation sensers are provided adjacent the surface. Radiation from at least some of the radiation sources can reach each of the radiation sensors. One or more radiation blocking objects on the surface attenuate radiation from one or more radiation sources from reaching each of the sensors. One or more polygons is calculated based on the attenuated radiation sources and the positions of the sensors. The position of the one or more radiation blocking objects is estimated and may be tracked based on the polygons.

## Background
This application is a continuation of patent application No. PCT/CA2011/000985 which was filed on Sep. 2, 2011 and is a non-provisional application of provisional application No. 61,379,585, which was filed on Sep. 2, 2010, and all of which are incorporated herein by reference.


FIELD
The described embodiments relate to systems, methods and sensors for sensing and tracking the position of one or more radiation blocking objects on a surface.
BACKGROUND
A variety of computer input and other devices require tracking of one or more objects such as fingers, styluses, pens or other objects as they are positioned on or moved across a surface. For example, computer monitors and other display screens may be fitted with a touchscreen that allows a user to provide inputs to a computer using a finger or a stylus, as they are moved across the display surface of the screen. Similarly, a whiteboard may be fitted with a pen positioning sensing system that tracks the position of one or more pens as they are moved across the writing surface of the whiteboard.
Existing systems suffer from a variety of deficiencies, including excessive complexity and cost, high computational overhead that affects both their accuracy and response time, and other deficiencies.
SUMMARY
The present invention provides various systems for detecting the presence and position of one or more radiation blocking objects as they radiation blocking objects are positioned on or moved across a surface. The surface may be any type of surface such as the display surface of a computer monitor or other display device, a writing surface such as a whiteboard, bulletin board, sheet of paper or wall or another surface such as a part of a toy or game.
Various embodiments according to a first aspect of the invention include a frame or housing with a plurality of radiation sources and radiation sensors mounted on it. The frame will typically, but not necessarily, be mounted to or be combined with a housing, frame or support of an underlying system such as a whiteboard, a display monitor, a bulletin board, a game, toy or other device. In some embodiments, the frame or housing may be combined with a display monitor to form a touchscreen. A controller activates some or all of the radiation sources sequentially. The radiation sources may be activated in a sweep fashion from one side of the frame to the other, or they may be activated in a different order. While each radiation source is activated, the radiation incident on some or all of the radiation sensors is measured.
A radiation blocking object present within the frame will typically block or attenuate one or more of the paths between some of the radiation sources and some of the radiation sensors. By successively measuring the attenuation of radiation from such blocking, the position of the radiation blocking object is estimated.
In embodiments according to another aspect of the invention, one or more diffusers are used to diffuse radiation emitted by the radiation sources. The diffusers may allow the position of a radiation blocking object to be estimated more accurately, particularly when the radiation blocking object blocks two or more of the paths between the radiation sources and a radiation sensor.
In some embodiments, radiation emitted by the radiation sources is modulated at a modulation frequency or with a modulation pattern. The sensors are sensitive to the modulation frequency or pattern and ignore radiation that is not modulated according to the frequency or pattern, reducing the effect of ambient and other spurious radiation in estimating the position of a radiation blocking object.
In one aspect, a system for sensing the position of one or more radiation blocking objects on a surface is provided. The surface is mounted to or within a frame, and in some embodiments, the surface and frame are generally rectangular. Radiation sources are provided on the frame and emit radiation across the surface. Radiation sensors are provided at two or more positions on the frame. Each sensor is positioned such that radiation from a plurality of the radiation sources may be incident on each sensor. Each sensor provides a radiation intensity level corresponding to the intensity of radiation incident on it to a controller. The controller is coupled to the radiation sources and sequentially activates the radiation sources. As each radiation source is activated, radiation from the radiation source may be incident on some or all of the radiation sensors. The controller samples the radiation intensity level from the radiation sensors. When a radiation blocking object is present on the surface, the radiation blocking object will typically block or attenuate radiation from one or more of the radiation sources. The controller identifies radiation sources for which the radiation intensity signal is attenuated compared to a baseline or threshold intensity level.
The controller estimates the position of the radiation blocking object based on the position of the attenuated radiation sources (i.e. radiation sources for which the radiation intensity level is attenuated due to the presence of a radiation blocking object) as measured from each radiation sensor. The controller first estimates an angular direction of the radiation blocking object relative to at least two of the radiation sensors. The angular directions are combined to estimate the position of the radiation blocking object on the surface relative to a reference position.
In some embodiments, the controller combines the radiation intensity level samples from a radiation source into a radiation intensity signal and identifies ranges of attenuated radiation sources. A center radiation source within the range is identified, and the angular position of the radiation blocking object relative to at least one of the radiation sensors is estimated based on the center radiation source in each radiation intensity signal.
In other embodiments, the relative attenuation of radiation intensity signals may be combined to estimate the position of the radiation blocking object. For example, if a range of radiation intensity levels corresponding to a range of radiation sources are attenuated by a radiation blocking object, a weighted average based on the relative attenuations and positions of each radiation source is used to refine the estimated angular position of the radiation blocking object relative to each radiation sensor. The refined estimate angular positions are combined to provide an estimated position of the radiation blocking object relative to the reference position.
In some embodiments, multiple radiation blocking objects on the surface may be sensed. The controller analyzes radiation intensity signals from each of the radiation sensors to identify attenuated radiation intensity levels corresponding to the presence of one or more radiation blocking objects. The maximum number of radiation blocking objects identified in any one radiation intensity signal is assumed to be the minimum number of radiation blocking objects present on the surface. The controller estimates an angular direction for each radiation blocking object apparently visible from each radiation sensor, relative to the sensors. The angular positions are combined to estimate the position of each radiation blocking object. The prior positions of radiation blocking objects, when such prior positions are known, may be used to select likely current positions of radiation blocking objects when the angular directions can lead to different estimates. For example, in some embodiments, two angular directions are identified relative to each of two radiation sensors. The angular directions can be represented as lines originating from each of the sensors. The lines intersect at four points, which may be considered in pairs to be potential positions of two radiation blocking objects. Previously known positions for one or both radiation blocking objects are used by calculating the shortest movement required from the previous positions of the radiation blocking objects to the potential current positions based on the intersections. The radiation blocking objects are deemed to be located at the potential position that requires the shortest movement. In other embodiments, other criteria may be used to resolve between different potential positions. For example, the trajectory of a radiation blocking object over a preceding time period, a distance or number of iterations of a sensing process may be used to estimate the current position of a radiation blocking object.
In some embodiments, angular positions of one or more radiation blocking objects relative to three or more radiation sensors are determined and the angular positions are combined to estimate the positions of the radiation blocking objects.
These and other aspects of the invention are described below in a description of the some example embodiments of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of the invention will now be described with reference to the drawings, in which:
FIG. 1 illustrates a first system according to the present invention;
FIGS. 2a and 2b illustrate radiation intensity signals according to the system of FIG. 1; and
FIG. 3 illustrates a radiation intensity signal according to another embodiment;
FIG. 4 illustrates a radiation intensity signal according to yet another embodiment;
FIGS. 5a and 5b illustrate another embodiment;
FIG. 6 illustrates another embodiment;
FIG. 7 illustrates yet a further embodiment;
FIG. 8 illustrates a method of identifying or estimating the positions of radiation blocking objects on a surface using the system of FIG. 7.
FIGS. 9a and 9b illustrate radiation intensity signals corresponding to one of the radiation blocking objects of FIG. 7;
FIGS. 10a and 10b illustrate radiation intensity signals corresponding to both of the radiation blocking objects of FIG. 7;
FIG. 11 illustrates the system of FIG. 7 with the radiation blocking objects in a different position;
FIGS. 12a and 12b illustrate radiation intensity signals corresponding to FIG. 11;
FIG. 13 illustrates the system of FIG. 7 with the radiation blocking objects in a different position;
FIGS. 14a and 14b illustrate radiation intensity signals corresponding to FIG. 13;
FIGS. 15a-15f illustrate another embodiment;
FIGS. 16a-16c and 17a-17c illustrates radiation intensity signals in the embodiment of FIGS. 15a and 15b; 
FIG. 18 illustrates a method of operating the embodiment of FIGS. 15a and 15b; 
FIG. 19 illustrates the embodiment of FIGS. 15a and 15b in a different configuration;
FIGS. 20a-20c illustrate radiation intensity signals corresponding to FIG. 19; and
FIG. 21 illustrates another embodiment.



The drawings are illustrative only and are not drawn to scale. Various elements of some embodiments may not be shown for clarity. Similar and corresponding elements of the various embodiments are identified by similar reference numerals.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
Exemplary embodiments described herein provide details relating to systems and methods for determining the position of one or more radiation blocking objects in relation to various radiation sources and radiation sensors. In some embodiment, the radiation sources and sensors may be mounted in a frame. In some embodiments, the systems may include or be used with various underlying devices such as whiteboards, display monitors and other devices. In some embodiments, the systems may include or be used with an underlying surface such as a whiteboard, a wall, the surface of a display screen or any other generally planar surface. The radiation sources may emit radiation in the visible light spectrum or in other spectrums, such as the ultraviolet or infrared spectrums. The embodiments described herein are exemplary only and other implementations and configurations are also possible.
Reference is first made to FIG. 1, which illustrates a system 100 for sensing or estimating the position of a radiation blocking object 124. System 100 includes a pair of radiation sensors 102a, 102b, a controller 104 and a plurality of radiation sources 106 mounted on a frame or housing 108. Frame 108 has a top side 110, bottom side 112, left side 114 and a right side 116. In this embodiment, radiation sources 106 are mounted on the left, bottom and right sides of frame 108. Radiation sensor 102a is mounted at the top left corner of the frame 108 and radiation sensor 102b is mounted at the top right corner of the frame 108.
Frame 108 surrounds a surface 128. In various embodiments, the surface 128 may be the surface of a display screen, a writing surface or another surface. In this embodiment, frame 108 provides a bezel at the edges of the surface 128. Radiation sources 106 and radiation sensors 102 are mounted within the bezel. In some embodiments, the frame may only partially surround or enclose the surface, for example, the frame may not enclose the top edge of the surface if no radiation sensors or sources are mounted adjacent the top edge. In other embodiments, the frame may support but not enclose the surface. For example, the frame may provide a support for the surface, radiation sensors and radiation sources, but may not have a bezel or other element that surrounds the surface. In other embodiments, the frame may itself provide some or all of the surface. For example, the frame may have solid surface between its edges and radiation blocking objects may be positioned on the solid surface when system 100 is in use. Typically, as in these examples, the surface will be mounted to the frame.
The top left corner of frame 108 is cut away in FIG. 1 to reveal radiation sensor 102a and several radiation sources 106. The bottom right corner of frame 108 is also cut away to reveal some of the radiation sources 106. Each radiation source 106, in this embodiment, is a LED that emits radiation in the infra-red spectrum. In other embodiments, the radiation sources may be various types of sources that emit radiation in other spectrums, including the visible light spectrum and the UV spectrum. Radiation sources 106 are mounted on frame 108 such that radiation from the radiation sources reaches one or both of the radiation sensors 102. In this embodiment, radiation sources are equally spaced along the left, bottom and right sides of frame 108. In this embodiment, frame 108 is rectangular with square corners. The sides of frame 108 are parallel to the axes of an x-y plane. In some embodiments, the radiation sources may not be equally spaced. In some embodiments, the frame may have a non-rectangular shape.
Controller 104 includes a processor 120, which may any type of device or component capable of operating system 100, including a hardware component, a software component or a component including both hardware and software or firmware or both. For example, processor 120 may be a microprocessor, microcontroller, gate array or any type of data processing or computing device. The processor can be programmed or configured to operate system 100 and its components and to communicate with external devices. Controller 104 may also includes a memory 121, which may be accessed by processor 120. Processor 120 controls the operation of controller 104 and system 100. Instructions may be recorded in the memory 121, and may be loaded into the processor to configure the processor to perform control, data processing, data transformation and communication operations for controlling the operation of the controller 104 and the system 100 as described below. Controller 104 is coupled to each radiation source 106. Only some of these connections are illustrated in FIG. 1. Controller 104 is capable of activating each radiation source 106 independently so that when one radiation source is activated or on (i.e. emitting radiation) the remaining radiation sources are not activated or off (i.e. not emitting radiation).
In this embodiment, each radiation sensor 102 is a PIN photodiode that is capable of sensing radiation emitted by the radiation sources 106 on the two opposing sides of frame 108. Radiation sensor 102a senses radiation emitted by the radiation sources 106 on the bottom and right sides of frame 108. Radiation sensor 102b senses radiation emitted by the radiation sources 106 on the bottom and left sides of frame 108. Each radiation sensor 102 is coupled to controller 104 and provides a radiation intensity level to the controller corresponding to the intensity of radiation falling on the radiation sensor 102 at any particular time. The radiation intensity level has a relatively high value when the corresponding radiation sensor 102 is receiving radiation from a radiation source 106 and a relatively low value when the corresponding radiation sensor 102 is not receiving radiation from a radiation source 106. A series of radiation intensity levels corresponding to the radiation sources 106 may be combined into a radiation intensity signal that can be used to estimate the position of the radiation blocking object 124. This is explained below.
In other embodiments each radiation sensor may be any device that is responsive to the radiation emitted by the radiation sources and capable of providing a radiation intensity level corresponding to radiation incident on the sensor. For example, a light sensitive element such as a photosensor, photodiode, photocell, a solar cell or a photovoltaic cell may be used to provide radiation intensity levels. The radiation sensor may provide the output radiation intensity level in any format compatible with the controller 104, including a digital or analog format.
Controller 104 is programmed with the dimensions of frame 108, the position of each radiation source 106 and the positions of each radiation sensor 102. In this example, controller 104 is programmed with the following information:

    
    
        Sensors 102a and 102b are separated by a distance d. Radiation sensor 102a is at the (0,0) position on the x-y plane and radiation sensor 102b is at the (d,0) position on the x-plane.
        For each radiation source on the bottom or right side of the frame 108, the angle between the left side of the frame (or a line parallel to the left side of the frame, depending on the position of the radiation sensor 102a) and the path between radiation sensor 102a and the radiation source, or a value corresponding to the angle.
        For each radiation source on the left or bottom side of the frame 108, the angle between the right side of the frame (or a line parallel to the right side of the frame, depending on the position of the radiation sensor 102b) and the path between radiation sensor 102b and the radiation source, or a value corresponding to the angle.
    
    


Under the control of controller 104, system 100 is operable to estimate the physical position P124a(x124a, y124a) of radiation blocking object 124. In FIG. 1, radiation blocking object 124 is illustrated as a stylus. The tip of the stylus is in contact with the surface 128, at point P124, which corresponds to the physical position P124a discussed here and the pixel position P124d discussed below.
In operation, controller 104 sequentially activates the radiation sources 106. While a radiation source 106 is activated, controller 104 samples the output from one or both of the radiation sensors 102 to obtain a radiation intensity level corresponding to the intensity of radiation incident on each radiation sensor 102. Typically, the path between the radiation source and each radiation sensor will be blocked, partially blocked (ie. partially attenuated) or clear. In some embodiments, while a radiation source 106 is activated, the controller may only check the radiation intensity level for a radiation sensor 102 if there is a direct path between the radiation source 106 and the radiation sensor 102. For example, there is a direct path between radiation sensor 102a and the radiation sources 106 on the bottom side 112 and the right side 116 of frame 108. Similarly, there is a direct path between radiation sources 106 on the left side 114 and the bottom side 112 of the frame 108 and radiation source 102b. In other embodiments, the controller 104 may check the radiation intensity level at a radiation sensor 102 even when the activated radiation source 106 does not have a direct path to the radiation sensor.
Instructions for performing this process are recorded in memory 121. Processor 120 accesses the instructions in memory 121 an executes the instructions to perform the process described above and those described below. Processor 120 may also record data in memory 121 during the performance of this process.
In other embodiments, the specific placement of the radiation sources and radiation sensors and the shape of the frame (which need not be rectangular and may have another shape) will effect which radiation sources have a direct path to which radiation sensors.
Returning to the present embodiment, when radiation source 106a is activated, controller 104 need not sample radiation sensor 102a to obtain a radiation intensity level because there is no direct path between radiation source 106a and radiation sensor 102a that is not obstructed by other radiation sources 106. Controller 104 does sample the radiation intensity level provided by radiation sensor 102b, which will have a relatively high value indicating that the path between radiation source 106a and radiation sensor 102b is clear, or not blocked.
When radiation source 106c is activated, controller 104 samples both radiation sensors 102a and 102b. The radiation intensity level from radiation sensor 102a is relatively high, indicating that the path between radiation source 106c and radiation sensor 102a is clear. The radiation intensity level from radiation sensor 102b is relatively low, indicating that the path between radiation source 106c and radiation sensor 102b is blocked, in this example, by radiation blocking object 124.
When radiation source 106e is activated, the radiation intensity levels from radiation sensors 102a and 102b respectively indicate that the paths between radiation source 106e and radiation sensors 102a and 102b are clear.
When radiation source 106f is activated, controller 104 samples the radiation intensity level from radiation source 102a which indicates that the path between radiation source 106f and radiation sensor 102a is blocked by radiation blocking object 124. Controller 104 samples the radiation intensity level from radiation sensor 102b, which indicates that the path between radiation source 106f and radiation sensor 102a is clear.
As controller 104 sequentially activates the radiation sources and samples the radiation intensity levels corresponding to each radiation source 106, controller 104 records the outcomes as follows:













Path to Radiation
Path to Radiation


Radiation source
Sensor 102a
Sensor 102b







. . .
—
. . .


106a
—
Clear


. . .
. . .
. . .


106c
Clear
Blocked


. . .
. . .
. . .


106e
Clear
Clear


. . .
. . .
. . .


106f
Blocked
—


. . .
. . .
—









Reference is made to FIGS. 2a and 2b. FIG. 2a illustrates a radiation intensity signal 122a corresponding to the radiation intensity levels obtained by controller 104 from radiation sensor 102a. FIG. 2b illustrates a radiation intensity signal 122b corresponding to the radiation intensity levels obtained by controller 104 from radiation sensor 102b. Each radiation intensity signal comprises the output of radiation sensor 102b as the radiation sources, including radiations sources 106a, 106b, 106c and 106d are sequentially activated and then deactivated. While any one radiation source is on, the remaining radiation sources are off.
Using the radiation intensity signals 122a and 122b, controller 104 can estimate the physical position of radiation blocking object 124. Controller 104 assumes that the radiation block object 124 is located in the blocked path for each radiation sensor. In this example, the position P124a(x124a, y124a) for the radiation blocking object 124 can be estimated:




  
    
      
        
          x
          
            124
            ⁢
            
              
            
            ⁢
            a
          
        
        =
        
          
            
              d
              ·
              tan
            
            ⁢
            
              
            
            ⁢
            
              θ
              f
            
          
          
            
              tan
              ⁢
              
                
              
              ⁢
              
                θ
                f
              
            
            +
            
              tan
              ⁢
              
                
              
              ⁢
              
                φ
                c
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  
  
    
      
        
          y
          
            124
            ⁢
            
              
            
            ⁢
            a
          
        
        =
        
          
            
              x
              
                124
                ⁢
                
                  
                
                ⁢
                a
              
            
            ·
            tan
          
          ⁢
          
            
          
          ⁢
          
            θ
            f
          
        
      
    
    
      
        (
        2
        )
      
    
  




In the embodiment of FIG. 1, the resolution with which the position of radiation blocking object 124 can be estimated depends on a number of factors, including the spacing between the radiation sources 106. By placing the radiation sources close to one another, a greater resolution may be achieved.
In equations (1) and (2) above, the tan of angles θf and φc are used to calculate the position of point P124. In system 100, the tan of the angles θ between the left side 114 and the path to the radiation sources 106 visible to radiation detector 102a, and the tan of the angles φ between the right side 116 and path to the radiation sources 106 visible to radiation sensor 102b are recorded in a data storage location accessible to the controller 104. This allows equations (1) and (2) to be calculated without requiring the tan of each angle θf and φc to be calculated, thereby allowing the position of P124 to be calculated more rapidly. In other embodiments, the angles themselves may be recorded or another value corresponding to the angles may be recorded. In some embodiments multiple values corresponding to the angular relationship between each of the radiation sources, the radiation sensors and reference lines (such as lines parallel to the right and left edges of the frame) may be recorded.
System 100 may be operated in different manners, depending on the programming of controller 104.
In another embodiment, system 100 may be operated to refine the estimated positions P124a of a radiation blocking object on the surface 128. Reference is made to FIGS. 1 and 3. Depending on the distance between the radiation sources, the dimensions of the radiation blocking object and the distance between the radiation blocking object and a radiation sensor, the path between several radiation sources and the radiation sensor may be blocked by the radiation blocking object. For example, if radiation sources 106b, 106c and 106d are sufficiently close together, then radiation blocking object 124 may at least partially block the path between two or all three of the radiation sources and radiation sensor 102b, thereby attenuating the radiation intensity level for all three radiation sources, particularly if the radiation blocking object is close to radiation sensor 102b. In some embodiments, controller 104 determines the center radiation source in a range of radiation sources whose path to a particular radiation sensor is blocked. Optionally, the controller 104 may treat a radiation source as blocked only if its radiation intensity level is below some threshold level, providing a mechanism for including or excluding slightly attenuated radiation sources at the edges of a range of attenuated radiation sources. In this example, the center radiation source would be radiation source 106c. The controller then estimates the position of the radiation blocking object based on the angle θ or φ between the center radiation source and the relevant side of the frame 108 (in this case, angle φ, relative to the right side 116). In other embodiments, the controller may use the middle angle θ or φ (depending on the relevant radiation sensor) among the range of angles for the radiation sources that are blocked. If a different value corresponding to each angle relating the radiation sources to the radiation sensors is recorded in the controller 104, such as the tan of each angle, then the recorded value may be used after determining the center radiation source or angle.
The estimated position P124a(x124a, y124a) is a physical position, measured in the same units as dimension d that separates radiation sensors 102a and 102b. 
Controller 104 is coupled to an interface 148, which in this embodiment is a universal serial bus port.
In other embodiments, the interface may be any type of communication interface. For example, interface 148 may be an analog interface or a digital data interface such as a serial data port or a parallel data port. In embodiments where the interface is an analog interface, the controller may provide analog signals (such as a current signal or a voltage signal) corresponding to the value of x124a and y124a. In an embodiment where the interface is a digital interface, the controller may be configured to convert the physical positions x124a and y124a into corresponding digital positions x124d and y124d relative to the sensors 102a and 102b. The controller may be configured to provide the digital positions x124d and y124d at the interface.
In the present embodiment, the surface 128 is the surface of a LCD display screen. The LCD display screen has a resolution of X horizontal pixels by Y vertical pixels. For example, in some embodiments, the screen may have a resolution of 1280×1024 pixels or 1920×1080 pixels. In other embodiments a display screen may have any other standard or non-standard pixel resolution. Controller 104 converts the physical position a corresponding pixel position P124d(x124d, y124d). Controller 104 may be configured to do so using a variety of techniques, including the use of lookup tables that provide the horizontal and vertical pixel positions corresponding the horizontal and vertical physical positions, using a formula to convert between the physical and pixel positions or using any other method. Controller 104 provides the digital position P124d at the interface 148.
Reference is made to FIGS. 1 and 4. In another embodiment, the controller 104 is configured or programmed differently to estimate the position P124a of the radiation blocking object 124 in a different manner. In this embodiment, the intensity signals 122 are used to more precisely estimate the angular position of the radiation blocking object 124 relative to each radiation sensor 102 and a side of the frame 108.
FIG. 4 illustrates a portion of a radiation intensity signal 122b when controller 104 is configured according to this embodiment. In this embodiment, the controller 104 establishes a baseline intensity level for each radiation source in combination with each radiation sensor. For each radiation source, controller 104 samples the radiation intensity level from radiation sensor 102b while the radiation source is on, and in the absence of a radiation blocking object to generate a baseline intensity level 126. The baseline intensity levels for radiation source 106a and 106b-106d are shown.
In this embodiment, during startup of system, the baseline intensity level is initially determined for each radiation source, with respect to each radiation sensor from which the radiation source is visible (i.e. if there is a direct path between the radiation source and the radiation sensor). An initial set of samples of the intensity signal are discarded while the system is starting up. For a selected time period following this initial start-up period, the radiation intensity level is sampled while the radiation source is on. The radiation intensity level is recorded and an average intensity level is determined for the radiation source at each radiation sensor. For example, if each radiation source is activated 50 times per second, the baseline intensity level may be calculated using the first 25 samples for each radiation source, at each radiation sensor, representing half of a second. In other embodiments, the baseline intensity level may be calculated over more or fewer samples, or for a longer period or shorter period. The baseline intensity level for each radiation sensor inherently takes into account for ambient and other conditions affecting the amount of radiation that reaches the radiation sensor when a particular radiation source is switched on. Such other conditions include the amount of radiation emitted by each radiation source, the physical distance between the radiation source and the radiation sensor and may also include the manner in which system 100 is used.
The baseline intensity level calculated for each radiation source 106, with respect to each radiation sensor 102, may be updated over time. For example, a moving average of some of the radiation intensity readings over a recent time period may be calculated to refine the baseline level as ambient and other conditions change. Some radiation intensity readings may not be used to calculate the updated baseline intensity level. For example, every tenth or twentieth radiation intensity reading may be used to calculate the moving average for each baseline intensity level. This reduces the amount of data that must be stored to calculate a baseline intensity level corresponding to a longer time period and also reduces the computation time required in the controller to address this task. Typically, the baseline intensity level will be calculated for a recent period from a part of a second to a few seconds or tens of seconds. When the path between a radiation source 106 and a radiation sensor 102 is blocked the radiation intensity level for that source at that sensor will be significantly reduced, although ambient radiation and some radiation may still reach the radiation sensor around the radiation blocking object. The controller may exclude radiation intensity levels below a certain threshold compared to the current baseline intensity level when refining the baseline intensity as is further described below. Various other methods for calculating a baseline intensity level for each radiation source at each radiation sensor may also be used. In some embodiments, one baseline intensity level may be calculated for a group or all of the radiation sensors. In other embodiments a pre-determined intensity level may be used as the baseline intensity level for some or all of the radiation sources.
In this embodiment, each time a radiation source 106 is activated, the radiation intensity level from each radiation sensor 102 from which the radiation source is visible is sampled and compared to the existing baseline intensity level for that radiation source at that radiation sensor. If the current intensity level is more than some threshold below the baseline intensity level, the percentage difference from the baseline level is calculated. For example, the threshold may be 90% of the baseline intensity level. If the current intensity level is greater than 90% of the baseline level, the current intensity level may be used to further refine the baseline level, or it may be discarded. If it is less than 90% of the baseline level, the processor assumes that the path between the radiation source 106 and the radiation sensor 102 is at least partially blocked. In other embodiments, other threshold levels may be used.
The controller successively activates the radiation sources in a cyclic process. After each cycle of switching on the radiation sources 106 and measuring the radiation intensity level from each radiation sensor for the radiation sources, the controller estimates the position of the radiation blocking object.
FIG. 4 illustrates the attenuation of several radiation sources 106 relative to their respective baseline levels 126. The current intensity level for radiation source 106a, as measured at radiation sensor 102 is greater than 90% of the baseline intensity level 126a, so it is ignored for the purpose of estimating the position of the radiation blocking object 124, although the current intensity level may be used to refine the baseline level for radiation source 106a as measured at radiation sensor 102b. Similarly, the current intensity level for radiation source 106b is greater than 90% of baseline intensity level 126b, so it is ignored for the purpose of estimating the position of the radiation blocking element, but may be used to refine the baseline level, which would then be slightly higher.
The current intensity levels for radiation sources a106c and 160d are below 90% of their respective baseline intensity levels 126c and 126d. The current intensity level for radiation source 106c is at 53% of baseline intensity level 126c. The current intensity level for radiation source 106d is at 31% of the baseline intensity level 126d. Controller 104 normalizes these deviations to a total of 100%: the relative attenuation of radiation from radiation source 106c represents 63% of the total attenuation (31%/84%=63%); and the relative attenuation of radiation from radiation source 106d represents 37% of the total attenuation.
The angle φ between the right side 116 and a line 132 between radiation source 102b and radiation blocking object 124 is then estimated as follows. The angle φc for radiation source 106c is 44°. The angle φd (not shown) corresponding to radiation source 106d is 42°. In this embodiment, rather than recording the angles themselves, the tan of each angle is recorded. The tan of the angle φ124 between the left side of the frame 108 and the path between radiation sensor 102b and radiation blocking object 124 can then be estimated as follows:




  
    
      
        
          Tan
          ⁡
          
            (
            
              φ
              124
            
            )
          
        
        =
        
        ⁢
        
          
            0.63
            ·
            
              tan
              ⁡
              
                (
                
                  44
                  ⁢
                  °
                
                )
              
            
          
          +
          
            0.37
            ·
            
              tan
              ⁡
              
                (
                
                  42
                  ⁢
                  °
                
                )
              
            
          
        
      
    
  
  
    
      
        =
        
        ⁢
        
          0.9415
          .
        
      
    
  




Angle φ124 is 43.27°

In an embodiment in which the angles themselves are recorded, angle φ124 may be estimated as follows:




  
    
      
        
          φ
          124
        
        =
        
        ⁢
        
          
            
              0.63
              ·
              44
            
            ⁢
            °
          
          +
          
            
              0.37
              ·
              42
            
            ⁢
            °
          
        
      
    
  
  
    
      
        =
        
        ⁢
        
          43.26
          ⁢
          
            °
            .
          
        
      
    
  




The estimates of angle φ124 differ due to the non-linearity between an angle and its tangent.
An angle θ124 is calculated for the angle between left side 114 and the line between radiation sensor 102a and the radiation blocking object 124. The two calculated angles φ124 and θ124 are used to estimate the position (xb, yb) of the radiation blocking object 124.
In this manner, controller 104 may use the attenuation of two or more radiation sources as measured at one of the radiation sensors to estimate the angular position of radiation blocking object relative to the left or right side of the frame 108 and one of the radiation sensors 102 by normalizing the relative attenuations of the different radiation sources and then calculating a weighted average of the angle of those sources from the relevant side of the frame and the radiation sensor.
This embodiment may allow the position of the radiation blocking object 124 to be estimated more accurately than the first embodiment by allowing angles θ and φ to be estimated between the specific angles at which the radiation sources 106 are positioned.
In some embodiments, it may be desirable to create a baseline range of intensity for each radiation source to account for ambient radiation. For example, in some embodiments, ambient radiation may be sensed by a radiation sensor, with the result that the radiation intensity level provided by a radiation sensor may measure both radiation from a radiation source and from ambient radiation. Controller 104 may be configured to determine the radiation intensity level at each radiation sensor 102 while all of the radiation sources 106 are switched off, thereby establishing an ambient radiation level for each radiation sensor 102. Each ambient radiation level may be an average of a group of samples, it may be a moving average of recently obtained samples or may be calculated in another manner. In some cases, the amount of ambient radiation incident on a radiation sensor may vary over time. It may be desirable to periodically sample ambient radiation at each radiation sensor to update the ambient radiation level. In some embodiments, it may be desirable to obtain an ambient radiation level for each radiation sensor with all of the radiation sources off immediately before (or after) obtaining a radiation intensity level with a radiation source turned on.
The ambient radiation level may be used to scale or otherwise adjust the radiation intensity level to remove or reduce the effect of ambient radiation on the estimated positions of a radiation blocking object. For example, the ambient radiation level (or an amount based on the ambient radiation level) may be subtracted from both the baseline intensity level 126 and the measured radiation intensity level for each radiation source before analyzing a radiation intensity signal and estimating the position of radiation blocking object.
System 100 may be used in various configurations to identify the position of various types of radiation blocking objects 124. For example, system 100 may be used with a whiteboard or other display surface. Frame 108 may be attached to the edge or frame of the whiteboard, or may also be the frame of the whiteboard. The radiation blocking object 124 may be a pen used to write on the whiteboard and as the pen is moved about the surface of the whiteboard, its position is estimated by controller 104. Controller 104 may be coupled to (or may be part of) a whiteboard system for recording estimates of the pen's position. By recording successive estimates of the pen's position, information on the whiteboard may be recreated in an electronic form and may be recorded for subsequent use, and it may be displayed or printed. The whiteboard system may include software to calculate the path of movement of the pen between estimated positions and to smooth the calculated path.
As the pen is used to write on the whiteboard, the ink on the whiteboard may change the amount of ambient light reflected on to a radiation sensor 102 and could also change the amount of radiation propagating from a radiation source 106 to a radiation sensor 102, thereby affecting the level of the radiation intensity measured for some or all of the radiation sources 106. In such embodiments, periodically updating the baseline intensity level for some or all of the radiation sources may improve the accuracy of estimates of the position of a radiation blocking object.
In other embodiments, system 100 may be used with a display monitor or screen to form a touchscreen. Frame 108 may be mounted to the display monitor or may be part of the display monitor's housing. The radiation blocking object 124 in this case may be a finger, and as a person moves their finger onto or off of the display monitor, the presence of the finger is detected and its position on the display screen is estimated by controller 104. Controller 104 may be coupled to (or may be part of) a touch screen system (which would also include the display monitor) and may provide estimates of the finger's position to the touch screen system. As a finger is moved about on the display screen, successive estimates of the finger's position can be recorded in the touch screen system to provide an electronic record of the finger's movement and the estimated positions can be displayed on the display monitor. The touch screen system may include software or other components to calculate the path of movement of the finger between its successive estimated positions and to smooth the calculated path. Such a touch screen system, in combination with system 100, would effectively allow a user to write or draw on the display monitor, or to manipulate objects displayed on the display monitor, using the person's finger.
In a touch screen system, the radiation sources 106 and radiation sensors 102 may be located relatively close to the display screen and the amount of radiation incident on the radiation sensors may vary as the information displayed on the display screen changes. In such embodiments, it may also be beneficial to update the baseline intensity level for some or all of the radiation sources.
Reference is next made to FIGS. 5a and 5b. FIG. 5a illustrates another system 500 for estimating the position of a radiation blocking object 524. FIG. 5b illustrates the bottom right corner of system 500 in greater detail. System 500 is largely similar to system 100 and corresponding elements are identified with corresponding reference numerals. System 500 includes diffusers 530 mounted adjacent to the radiation sources 506. Diffusers 530 diffuse radiation emitted by the radiation sources, thereby smoothing the amount of radiation apparently emitted along the left, bottom and right sides of the frame 508 by the radiation sources, as viewed from the radiation sensor 502. In this embodiment, the angular position of the radiation blocking object 524 relative to the left and right sides of the frame and the radiation sensors is estimated as described above in relation to system 100. The inventors have found that diffusing the radiation emitted by radiation sources 506 can provide a more accurate estimate of the radiation blocking object's position.
Various materials are suitable for use as diffusers 530, including slightly clouded or translucent plastics or other materials that diffuse but do not excessively scatter radiation from the radiation sources such that it cannot accurately be measured by the radiation sensors 102. In some embodiments, optical grade diffusers which diffuse, but do not substantially block the radiation passing through the diffuser, may be used effectively, including diffraction gratings, lenticular diffusers and lenticular diffraction gratings may be used for the diffusers 530. FIG. 5b illustrates a continuous lenticular diffuser 530b installed on the bottom side 512 of frame 508 and a continuous lenticular diffuser 530r installed on the right side 516 of frame 508.
FIG. 6 illustrates a portion of another embodiment 600, corresponding to the portion of system 500 illustrated in FIG. 5b. In system 600, individual diffusers 630 are installed adjacent each radiation source 506.
In some embodiments of the invention, the controller may vary the intensity of radiation emitted by some or all of the radiation sources. This may be done to vary the measured intensity level for a radiation source at the radiation sensors, to overcome the effect of ambient light, to reduce power consumption by the system, or for other reasons.
In the embodiments described above the frame is rectangular and the radiation sensors are mounted in two corners of the frame. In other embodiments, the frame may have a different shape. For example, the present invention may be used with a bulletin board or other object that has any regular or irregular shape and the frame may be shaped and sized to fit on or over the underlying object. Sensors may be positioned at various places on the frame, including along the sides (which may be straight or curved) of the frame. In each case, the position of each sensor and of the radiation sources visible from the sensor are used geometrically to identify the presence and position of a radiation blocking object.
In some embodiments with rectangular or other frame shapes, additional sensors may be used. For example, additional sensors could be added at the bottom left and right corners of system 100 (FIG. 1) and 500 (FIG. 5a). In some embodiments, additional radiation sources could be added along the top side 110 of the frame. In some embodiments, additional information about the position of the radiation blocking object 124 or 524 from the additional sensors may be combined to provide a more accurate estimate of the position of the radiation blocking object.
In some embodiments, with rectangular or other frame shapes, sensors may be placed along the sides of the frame. The positioning of radiation sensor and radiation sources may depend on the portion of an underlying system (such as a whiteboard, display monitor or other system) in which a radiation blocking object is to be detected.
In various embodiments, a system according to the present invention may include a bezel (which may be part of the frame) that conceals some or all of the components of the system including the radiation sources, the radiation sensors and diffusers. In some embodiments, the bezel or the frame or both may be painted with radiation absorbing paint or otherwise adapted to reduce the amount of radiation that is reflected toward the radiation sensors from the bezel or the frame or both.
In some embodiments, an optical filter may be placed between some or all of the radiation sensors and some or all of the radiation sources. For example, an optical filter could be installed around the radiation sensors to reduce the amount of ambient and other undesirable radiation that is incident on the radiation sensors.
Reference is next made to FIG. 7, which illustrates a system 700 for simultaneously tracking the position of two or more radiation blocking objects. System 700 is a touchscreen that operates as both an input device and an output device for a connected computer or other external system.
System 700 is similar in construction to systems 100 and 500, and corresponding components are identified by similar reference numerals. System 700 may be used as an electronic whiteboard system or an LCD touch screen.
System 700 includes a pair of radiation sensors 702a, 702b, a controller 704, a plurality of radiation sources 706 mounted on a frame 708 and an LCD display screen. Sources 706 are mounted on the left side 714, bottom side 712 and right side 716 of the frame 708. Frame 708 also has a top side 710. Radiation sensor 702a is mounted at the top left corner of frame 708 and radiation sensor 702b is mounted at the top right corner of the frame 708. Radiation sensors 702a and 702b are separated by a distance d. Controller 704 is coupled to radiation sensors 702 and radiation sources 706. Controller 704 controls the radiation sources and receives radiation intensity levels from the radiation sensors as described above in relation to system 100.
The sides of frame 708 are parallel to the axes of an x-y plane. A pair of radiation blocking objects 724a and 724b are positioned such that each of the radiation blocking objects 724 obstructs the straight line path between at least one of the radiation sources 706 and the radiation sensors 702.
The LCD display screen is mounted within frame 708 and has a display surface 728. The line of sight paths along which radiation from the radiation sources 706 to the radiation sensors 702 pass above the display surface, and are generally parallel to the display surface. The LCD display screen has a resolution of X horizontal pixels by Y vertical pixels. For example, in some embodiments the LCD display screen may have a resolution of 1280×1024 pixels or 1920×1080 pixels. Many other pixel resolutions are possible for various display panels. In various embodiments, any type of display panel may be used in place of an LCD panel. Typically, frame 708 will be mounted to the display panel, or will also form part of the housing of the display panel.
System 700 may optionally include diffusers, such as the diffusers 530 and 630 illustrated in FIGS. 5 and 6.
System 700 will typically include several input/output interfaces. In the present embodiment, controller 704 is coupled to a computing device through an interface 748 to transmit the position of radiation blocking objects to the computing device. For example, interface 748 may be a serial interface such as a USB interface or a parallel interface. The LCD display is coupled to the computing device to receive video signals, which are displayed on the display 728, through a video signal interface (not shown).
Reference is next made to FIG. 8, which illustrates a method 800 for identifying or estimating the positions of radiation blocking objects 724a and 724b. In this embodiment, method 800 is performed by controller 704. Prior to the start of method 800, no radiation blocking object is positioned on the display surface 728.
Method 800 begins in step 802, in which a first radiation blocking object 724a is initially positioned on the display surface 728. Instructions for performing method 800 are recorded in memory 721. Controller 720 accesses the stored instructions and executes the instructions to perform the method.
Method 800 will be explained by way of example. For the purposes of the example, the first radiation blocking object is initially placed on the display surface in the position shown in FIG. 7. In this step, radiation blocking object 724b is not placed on the display surface 728.
Reference is made to FIGS. 9a and 9b, which illustrate radiation intensity signals 722a and 722b after radiation blocking object 724a has been placed on the display surface 728.
Radiation intensity signal 722a illustrates that radiation intensity levels from radiation sources 706i-706k are attenuated at radiation sensor 702a. Radiation intensity signal 722b illustrates that radiation intensity levels from radiation source 706a-406c are attenuated at radiation sensor 702b. 
Controller 704 uses radiation intensity signal 722a and 722b as described above in relation to system 100 to estimate the physical position P724a(xaa, yaa) of radiation blocking object 724a. Position P724a(xaa, yaa) is a physical (or analog) position calculated relative to positions of the sensors 702 and based on angles (θa, φa).
Controller 704 maintains a touch table, in which the last known position of each radiation blocked object that has been detected on the surface 728 is recorded. Typically, the touch table may be a set of variables or part of a database that is stored in memory 721. In the present embodiment, the touch table includes two slots, A and B, for recording the last known positions of up to two radiation blocking objects. In other embodiments, the touch table may include more than two slots, or may include a variable number of slots.
Controller 704 records the physical position P724a of the first radiation blocking object 724a in slot A in the touch table:












Slot
X Position
Y Position







A
xaa
yaa


B
—
—









Physical position P724a(xaa, yaa) corresponds to a pixel (or digital) position P724d(Xad, yad) on the LCD display 728. Controller 704 converts the physical position P724a to the corresponding pixel position P724d, and provides the pixel position P724d at interface 748.
Method 800 then proceeds to step 804. In step 804, Controller 704 operates radiation sources 706 and sensors 702 to sequentially obtain radiation intensity levels associated with radiation sources 706 from each radiation sensor 702. The radiation intensity levels from each radiation sensor are combined into a radiation intensity signal 722. Controller 704 analyzes each radiation intensity signal 722 to determine the number of radiation blocking objects that are represented in each of the radiation intensity signals.
In this embodiment, up to two radiation blocking objects may be placed on surface 728.
Reference is made to FIGS. 10a and 10b, which illustrate example radiation intensity signals 722a and 722b when two radiation blocking objects 724a and 724b are placed on surface 728. Each of the radiation intensity signals 722a and 722b has two distinct ranges of radiation intensity levels that are attenuated at each of the radiation sensors 702. (A radiation source for which the radiation intensity level is attenuated may be referred to as an attenuated radiation source.) Each range of attenuated radiation intensity levels corresponds to a separate radiation blocking object 724. The ranges of attenuated radiation intensity levels are separated by at least one radiation source that is not attenuated. For example, and referring also to FIG. 7, in radiation intensity signal 722a, radiation intensity levels for radiation sources 706i-706k and 706p-706r are attenuated at radiation sensor 702a. The attenuation of radiation sources 706i-706k corresponds to radiation blocking object 724a. The attenuation of radiation sources 706p-706r corresponds to radiation blocking object 724b. Controller 704 is configured to identify the two distinct ranges of attenuated radiation sources by identifying at least one radiation source between the ranges that is not attenuated. In some situations, a range of attenuated radiation sources may consist of a single attenuated radiation source.
Reference is made to FIGS. 11, 12a and 12b, which illustrate another condition in which two radiation blocks have been placed on surface 728. In FIG. 11, radiation blocking objects 724a and 724b are positioned such that angles θa and θb are separated by a relatively small angle.
FIG. 12a is a radiation intensity signal 722a which illustrates that the radiation blocking objects 724a and 724b have an overlapping effect on the attenuation of radiation intensity levels from one or more radiation sources. Radiation blocking object 724a appears to attenuate radiation from radiation sources 706i-706l. Radiation blocking object 724b appears to attenuate radiation from radiation sources 706l-706o. Controller 704 is configured to distinguish the two ranges of attenuated radiation signals by identifying two distinct minima in the radiation intensity signal, separated by at least one radiation intensity value that is higher than either of the minima. For example, in FIG. 12a, radiation intensity levels for radiation sources 706j and 706n are local minima. These local minima are separated by several radiation intensity levels that are higher than either minima. In various embodiments, the controller 704 may be configured to identify two distinct ranges in various ways. In some cases, a range of attenuation radiation intensity levels may have only a single attenuated radiation source. For example, in some embodiments, the controller 104 may be configured to identify at least one radiation intensity level between local minima radiation intensity levels that exceed the minima by some predetermined about or ratio. In some embodiments, the controller may be configured to require at least two (or a high number) radiation intensity values between local minima.
FIG. 12b illustrates a radiation intensity signal 722b corresponding to the positions of radiation blocking object 724a and 724b in FIG. 11. Radiation intensity signal 722b includes two distinct regions of the attenuated radiation intensity levels at radiation sources 706a-706c and 706g to 706i. The two ranges are separated by one or more radiation intensity levels that are not attenuated. Controller 704 is configured to distinguish the two ranges of attenuated radiation intensity levels as described above in relation to FIGS. 10a and 10b. 
Controller 704 is thus configured to identify to ranges of attenuated radiation sources in each of the radiation intensity signals illustrated in FIGS. 10a, 10b, 12a and 12b. 
Reference is next made to FIGS. 13, 14a and 14b, which illustrate another condition in which two radiation blocking objects 724a and 724b have been placed on the surface 728.
In FIG. 13, the two radiation blocking objects 724a and 724b are generally collinear with radiation sensor 702a. Radiation emitted by radiation source 706j, which is also generally collinear with the radiation blocking objects, is at least partially blocked by radiation blocking object 724b from reaching radiation sensor 702a. Radiation blocking object 724a may block additional radiation from radiation source 706j from reaching radiation sensor 702a, but radiation blocking object 724a is at least partially in the shadow of radiation blocking object 724b. 
FIG. 14a illustrates a radiation intensity signal 722a corresponding to the positions of the radiation blocking object 724a and 724b in FIG. 13. The radiation intensity levels for radiation sources 706i-706k are attenuated by radiation blocking objects 724a and 724b. Radiation intensity signal 722a is similar to a radiation intensity signal resulting from a single radiation blocking object on the surface 728. Controller 704 analyzes radiation intensity signal 722a and is able to identify only one apparent radiation blocking object.
FIG. 14b illustrates a radiation intensity signal 722b corresponding to the positions of radiation blocking object 724a and 724b in FIG. 13. Radiation intensity signal 722b includes two distinct regions of the attenuated radiation intensity levels at radiation sources 706a-706c and 706d to 706f. The two ranges are separated by one or more radiation intensity levels that are not attenuated. Controller 704 is configured to distinguish the two ranges of attenuated radiation intensity levels as described above.
Controller 704 thus determines whether each of radiation intensity signals 722a and 722b obtained in this step 804 appears to contain zero, one or two ranges of attenuated radiation sources.
Method 800 continues from step 804 depending on the number of radiation blocking objects identified in the radiation intensity signals as follows:

    
    
        if each of the radiation intensity signals 722 contains one range of attenuated radiation sources (as illustrated in FIGS. 9a and 9b), then method 800 proceeds to step 806;
        if both of the radiation intensity signals 722 contains two ranges of attenuated radiation sources (as illustrated in FIGS. 10a and 10b and in FIGS. 12a and 12b), then method 800 proceeds to step 808;
        if either one of the radiation intensity signals 722 contains two ranges of attenuated radiation sources and the other radiation intensity signal contains one range of attenuated radiation sources (as illustrated in FIGS. 14a and 14b), then method 800 proceeds to step 810; and
        if both of the radiation intensity signals 722 contains zero ranges of attenuated radiation sources, method 800 proceeds to step 820; and
        otherwise, method 800 returns to step 804;
    
    


In step 806, controller 704 determines the position of a radiation blocking object 724 on the display surface 728. Controller 704 calculates an angle θ and an angle φ corresponding to the weighted average attenuation in the respective ranges of attenuated intensity levels in each of the radiation intensity signals 722a and 722b. A radiation blocking object 724 is deemed to be located at the intersection point of a pair of lines 746 and 732 corresponding to the angles θ and φ and the positions of the radiation sensors 702a and 702b, as described above.
If only one position corresponding to one radiation blocking object is recorded in the touch table, the intersection point is deemed to be the new physical position of the radiation blocking object. The new position is recorded in the touch table in place of the previously recorded position. The controller converts the physical position of the radiation blocking object into a corresponding pixel position, which is then provided at the interface 748.
If two positions corresponding to two radiation blocking objects are recorded in the touch table, the controller determines which of the previously recorded positions is closest to the intersection point. The intersection point is deemed to be the new position of the radiation blocking object corresponding to the closest previously recorded position, which is replaced in the touch table with the position of the intersection point. The controller converts the physical position of the radiation blocking object into a corresponding pixel position, which is then provided at the interface 748.
The further previously recorded position is deleted from the touch table.
Method 800 then returns to step 804.
Reference is additionally made to FIGS. 7, 10a and 10b. 
In step 808, controller 704 determines various points at which the radiation blocking objects 724 may be positioned based on the two ranges of attenuated radiation sources identified in each of the radiation intensity signals 722a and 722b in step 804.
For example, In radiation intensity signal 722a, radiation sources 706i-406k and 706p-406r are attenuated at radiation sensor 702a. The two ranges of attenuated radiation sources are separated by at least one radiation source that is not attenuated.
Controller 704 analyzes each group of attenuated sensors independently and calculates an angle θa based on a weighted averaging of the attenuation of sources 706i-706k, as described in relation to angle θ124 above. Angle θa defines a line 746a that extends through the position of radiation sensor 702a. 
Controller 704 also calculates an angle θb based on the attenuation of sources 706p-706r. Angle θp defines a line 746b that extends through the position of sensor 702a. 
In radiation intensity signal 722b, radiation sources 706a-706c and 706g-706i are attenuated at radiation sensor 702b. Controller 704 calculates an angle φa based on the attenuation of sources 706a-706c and an angle φb based on the attenuation of source 706g-706i. Angle φa defines a line 732a that passes through the position of sensor 702b. Angle φb defines a line 732b that passes through the position of sensor 702b. 
Line 746a intersects with lines 732a and 732b at points 734 and 736. Line 746b intersects with line 732a and 732b and points 738 and 740. The four intersection points are shown in the following table:














Line 732a
Line 732b




















Line 746a
734
736



Line 746b
738
740










The four points 734-740 may be considered in two pairs. The radiation blocking objects 724a and 724b may be either at points 734 and 740 or at points 736 and 738.

Method 800 then proceeds to decision step 812.
In step 810, controller 704 identifies various points at which the radiation blocking objects 724a and 724b may be positioned based on the two ranges of attenuated radiation sources in one of the radiation intensity signals 722 and the single range of attenuated radiation sources in the other radiation intensity signal.
Reference is additionally to made to FIGS. 13, 14a and 14b. 
For the radiation intensity signal 722 having two attenuated ranges of radiation sources, each range is analyzed separately to determine two angles θa and θb or φa and φb, as described in relation to step 808. For example, radiation intensity signal 722b in FIG. 14b has two distinct ranges of attenuated radiation sources and the two angles φa and φb illustrated in FIG. 13 are calculated as described above. Two corresponding lines extending from the corresponding radiation sensor 702 are also calculated. In this example, lines 732a and 732b are calculated.
For the radiation intensity signal having only one range of attenuated radiation sources 706, only one corresponding angle θ or φ can be calculated. In this example, radiation intensity signal 722a (FIG. 14a) has only one range of attenuated radiation sources 706i-706k. A corresponding angle θa and line 746a are calculated.
Angle θa is duplicated as angle θb, and line 746a is duplicated as line 746b. 
Controller 704 then calculates points 734-740 based on the intersections of lines 746a and 746b with lines 732a and 732b, as described in step 808.
Method 800 then proceeds to step 812.
In step 812, controller 704 determines the number of position recorded in the touch table. If only one position is recorded in the touch table, then method 800 proceeds to step 812. If two positions are recorded in the touch table, then method 800 proceeds to step 814.
Step 814 is performed if the position of one radiation blocking is recorded in the touch table, and one additional radiation blocking object is newly identified, based on at least one of the radiation intensity signals 722a (FIG. 9a) or 722b (FIG. 9b) or both having two ranges of attenuated radiation sources.
Controller 704 determines which points 734 and 740 or 736 and 738 correspond to the two radiation blocking objects 724a and 724b. 
Controller 704 determines which point 734-440 is closest to the position recorded in the touch table. In this example, the physical position P1a of radiation blocking object 724a was recorded in slot A of the touch table in step 806. The closest point (among points 734-740) to the previously known position P1a is deemed to be the current position P1a of the first radiation blocking object 724a. Position P1a will correspond to one point in one of the pairs of points (734 and 740 or 736 and 738). The other point in the same pair is deemed to be the position P2a (xba, yba) of the second radiation blocking object 724b. For example, in the example illustrated in FIG. 7, the last known position P1a for radiation blocking object 724a is closest to position 734. Radiation blocking object 724a is deemed to be positioned at point 734, and the position P2a of the second radiation blocking object 724b is deemed to be point 740.
Controller 704 updates the touch table with the position P1a of the first radiation blocking object 724a in slot A of the touch table and records the position P2a of the second radiation blocking object 724b in slot B of the touch table:












Slot
X Position
Y Position







A
xaa
yaa


B
xba
yba









Controller 704 converts the physical positions P1a and P1a of the radiation blocking objects 724a and 724b into corresponding pixel positions P1d and P2d and provides them at interface 748 to a coupled computing device.
Method 800 then returns to step 804.
In step 816, the positions of the first and second radiation blocking object 724 are tracked as they are moved on the display surface 728.
Method 800 reaches step 816 when the touch table has previously been updated with the positions of two radiation blocking object 724 (in either step 814 or 816). The positions of the two radiation blocking objects is updated in the touch table and their respective positions are reported at interface 748.
Controller 704 analyzes each possible combination of movements from the last recorded positions P1a and P2a in the touch table. In this embodiment, the four possible combinations are as follows:

    
    
        Combination 1: Radiation blocking object 724a moved to position 734; and Radiation blocking object 724b moved to position 740.
        Combination 2: Radiation blocking object 724a moved to position 740; and Radiation blocking object 724b moved to position 734.
        Combination 3: Radiation blocking object 724a moved to position 736; and Radiation blocking object 724b moved to position 738.
        Combination 4: Radiation blocking object 724a moved to position 738; and Radiation blocking object 724b moved to position 736.

For each combination, controller 704 is configured to calculate the total distance that the two radiation blocking objects 724 would move. For example, for combination 3, the first radiation blocking object 724a would move from position P1a to position 736 and the second radiation blocking object 724b would move from position P2a to position 738. The distance that each radiation blocking object may be calculated using standard geometric techniques.

    
    


For each combination, the distances that each radiation blocking object would move are summed together. In this example, each combination results in the following total distances:

    
    
        Combination 1: 0.2 mm
        Combination 2: 82.4 mm
        Combination 3: 46.5 mm
        Combination 4: 85.3 mm
    
    


Controller 704 is configured to deem the radiation blocking objects to have moved in accordance with the combination that requires the shortest total movement of the two radiation blocking objects. In the present example, this is combination 1. Radiation blocking object 724a is deemed to have moved to point 734. Radiation blocking object 724b is deemed to have moved to point 740. Controller 704 updates the touch table with the new position of each radiation blocking object. Controller 704 converts the new physical positions P1a and P2a of the radiation blocking objects 724a and 724b into corresponding pixel positions P1d and P2d and provides them at interface 748 to a coupled computing device.
Method 800 then returns to Step 804.
Method 800 reaches step 820 if both radiation blocking objects have been removed from the display surface 728. The controller deletes all recorded positions in the touch table and may optionally provide an indication at interface 748 that no radiation blocking objects have been detected on the display surface 728.
Using method 800, controller 704 provides successive positions of one or two radiation blocking objects as they are positioned on the display surface 728 and moved about the display surface 728. The method terminates when no radiation blocking object is identified on the display surface.
In system 700 and method 800, the positions of radiation blocking objects are recorded in the touch table as physical positions and the distances between various points are calculated in physical dimensions. In other embodiments, the positions may be recorded and distances may be calculated in pixel dimensions.
Reference is next made to FIGS. 15a and 15b, which illustrate another system 1500 for simultaneously tracking the position of multiple radiation blocking objects. System 1500 is similar in construction to systems 100, 500 and 700 and corresponding components are identified by similar reference numerals. As with the systems described above, system 1500 may be used as an electronic whiteboard system or a touchscreen system.
System 1500 includes three radiation sensors 1502a, 1502b and 1502c, a controller 1504, a plurality of radiation sources 1506 mounted on a frame 1508 and an LCD display screen. Sources 1506 are mounted on the left side 1514, bottom side 1512 and right side 1516 of the frame 1508. Frame 1508 also has a top side 1510. Radiation sensor 1502a is mounted at the top left corner of frame 1508. Radiation sensor 1502b is mounted at the top right corner of the frame 1508. Radiation sensor 1502c is mounted between radiation sensors 1502a and 1502b on the top side 1520 of the frame. Radiation sensors 1502a and 1502b are separated by a distance d1. Radiation sensors 1502a and 1502c are separated by a distance d2. Controller 1504 is coupled to radiation sensors 1502 and radiation sources 1506. Controller 1504 controls the radiation sources and receives radiation intensity levels from the radiation sensors as described above in relation to system 100.
The sides of frame 1508 are parallel to the axes of an x-y plane. In FIG. 15a, a radiation blocking object 1524a is positioned such that the radiation blocking object 1524 obstructs the straight line path between at least one of the radiation sources 1506 and the radiation sensors 1502. FIG. 15b additionally illustrates a section radiation blocking objects 1524b that is also positioned to obstruct the straight line path between at least one of the radiation sources 1506 and each of the radiation sensor 1502.
The LCD display screen is mounted within frame 1508 and has a display surface 1528. The line of sight paths along which radiation from the radiation sources 1506 to the radiation sensors 1502 pass above the display surface, and are generally parallel to the display surface. Like LCD display screen of system 700 (FIG. 7), the LCD display screen of system 1500 has a resolution of X horizontal pixels by Y vertical pixels. Typically, frame 1508 will be mounted to the display screen, or will also form part of the housing of the display screen.
System 1500 may optionally include diffusers, such as the diffusers 530 and 630 illustrated in FIGS. 5 and 6.
System 1500 will typically include several input/output interfaces, similar to system 700 (FIG. 7).
As radiation blocking objects 1524 are moved about the surface 1528, the radiation blocking objects will block the path between one or more of the radiation sources 1506 and each of the radiation sensors 1502.
Referring also to FIGS. 17a, 17b and 17c, radiation intensity signals 1522a, 1522b and 1522c corresponding to each of the radiation sensors 1502a, 1502b and 1502c respectively are illustrated when the radiation blocking objects 1524a and 1524b are in the positions illustrated in FIG. 15b. 
Radiation intensity signal 1522a has two ranges of radiation sources with attenuated radiation intensity levels. Attenuated radiation sources 1506j-1506l correspond to radiation blocking object 1524a. Attenuated radiation sources 1506q-1506s correspond to radiation blocking object 1524b. 
Radiation intensity signal 1522b also has two ranges of radiation sources with attenuated radiation intensity levels. Attenuated radiation source 1506a-1506c correspond to radiation blocking object 1524a. Attenuated radiation source 1506h-1506j correspond to radiation blocking object 1524b. 
Radiation intensity signal 1522c also has two ranges of radiation sources with attenuated radiation intensity levels. Attenuated radiation source 1506a-1506c correspond to radiation blocking object 1524a. Attenuated radiation source 1506h-1506j correspond to radiation blocking object 1524b. 
Controller 1504 is configured to analyze the radiation intensity signal for each radiation sensor to identify zero, one or more ranges of attenuated radiation sources as described above in relation to system 700 (FIGS. 7-14). Controller 1504 is also configured to calculate an angle corresponding to each range of attenuated radiation sources in radiation intensity signal 1522. In the example illustrated in FIG. 15b, controller 1504 calculates angles θa, θb, φa, φb, αa and αb corresponding to the weighted average attenuation in the respective ranges in each of the radiation intensity signals 1522a, 1522b and 1522c. The six calculated angles correspond to lines 1546a, 1546b, 1532a, 1532b, 1550a and 1550b. 
Reference is next made to FIG. 18, which illustrates a method 1800 for estimating or identifying the positions of radiation blocking objects 1524a and 1524b. Method 1800 is performed under the control of controller 1504. Instructions for performing method 1800 are recorded in memory 1521. Processor 1520 accesses memory 1521 of obtain the stored instructions and executes the instructions to perform the method.
Method 1800 will be explained by way of example. Prior to the start of method 1800, no radiation blocking object is positioned on the display surface 1528.
Method 1800 begins in step 1802 in which a first radiation blocking object 1524a is placed on the display surface 1528, as is illustrated in FIG. 15a. FIGS. 16a, 16b and 16c illustrates radiation intensity signals 1522a, 1522b and 1522c corresponding to each of the radiation sensors 1502a, 1502b and 1502c respectively when the radiation blocking object 1524a is in the position illustrated in FIG. 15a. 
Processor 1504 calculates angles θa, φa and αa corresponding to the weighted average attenuation in the respective ranges in each of the radiation intensity signals 1522a, 1522b and 1522c. The calculated angles correspond to lines 1546a, 1532a, 1532b and 1550a. Typically, lines 1546a, 1532a and 1550a will not intersect. In the event that the three lines intersect, the point of intersection is calculated based on the positions of the sensors 1502 and the angles (θa, φa, αa) and treated as the physical position P1524aa(xaa, yaa) of the radiation blocking object 1524a. 
Reference is made to FIG. 15c, which illustrates lines 1546a, 1532a and 1550a near radiation blocking object 1524a in an expanded view. Typically, lines 1546a, 1532a and 1550a will not intersect, as is illustrated. In the present embodiment, controller 1504 is configured to calculate a circle 1554 that is inscribed within the triangle 1552 formed by lines 1546a, 1532a and 1550a. The center 1534 of this circle is calculated as the physical position P1524aa of radiation blocking object 1524a. 
In various embodiments, the physical position P1524aa of radiation blocking object 1524a may be estimated in various manners. Reference is made to FIG. 15d which illustrates an alternative method of estimating the physical position P1524aa of radiation blocking object 1524a. In this method, the point 1534 is calculated as the point at which the sum of the lengths of the lines 1554, 1556 and 1558, which extend respectively between the point and each of lines 1532a, 1546a and 1550a is minimized. In other embodiments, the physical position P1524aa may be calculated as the point at which the sum of the squares of the respective lengths of lines 1554, 1556 and 1558 is minimized. In other embodiments, the physical position P1524aa may be calculated as the center of a circle that circumscribes triangle 1552. Various other geometric approaches may be used to determine the physical position P1524aa of the radiation blocking object 1524.
To complete step 1802, controller 1504 records the physical position P1524aa in a slot in a touch table recorded in memory 1521, converts the physical position to digital position P1524d and provides the digital position at an interface 1548.
Method 1800 then proceeds to step 1804, which is similar in some regards to step 804 of method 800 (FIG. 8). In step 1804, controller 1504 operates system 1500 to sequentially obtain radiation intensity signals 1522a, 1522b and 1522c from each of the sensor 1502a, 1502b and 1502c. Controller 1504 analyzes each radiation intensity signal to determine the number of radiation blocking object represented in each radiation intensity signal 1522. As described above in relation to FIGS. 15a and 16a-16c, when one radiation blocking object 1524a is present on screen 1528 typically, each radiation intensity signal 1522 will have one range of attenuated radiation sources. FIGS. 15b and 17a-17c, when two radiation blocking objects 1524a, 1524b are present on the screen 1528, each of the radiation intensity signals may have two ranges of attenuated radiation sources.
Reference is next made to FIGS. 19 and 20a, 20b and 20c. FIG. 19 illustrates system 1500 in a condition where two radiation blocking objects 1524a, 1524b are collinear or almost collinear with radiation sensor 1502b. FIGS. 20a-20c illustrates corresponding radiation intensity signals 1522a-1522c. Radiation intensity signal 1522a and 1522c each have two ranges of attenuated radiation intensity levels, similar FIGS. 17a and 17c. However, radiation intensity signal 1522b has only one range of attenuated radiation levels corresponding to radiation sources 1506a, 1506b and 1506c. Both radiation blocking objects 1524a and 1524b are blocking radiation emitted by these three radiation sources from reaching radiation sensor 1502b. 
Controller 1504 is configured to determine if each of the radiation intensity signals 1522 obtained in step 1804 appears to contain zero, one or two ranges of attenuated radiation intensity signals, as described in relation to step 804 of method 800 (FIG. 8).
In general, the number of radiation blocking objects will correspond the highest number of ranges of attenuated radiation sources identified in any one of radiation intensity signals 1522.
Method 1800 continues from step 1804 depending on the number of radiation blocking objects identified in the radiation intensity signals as follows:














Number of




radiation



Next
blocking


Condition
step
objects







Each radiation intensity signal contains zero ranges of
1806
Zero


attenuated radiation sources.


Each radiation intensity signal contains one range of
1808
One


attenuated radiation sources.


Two radiation intensity signals contain one range of
1810


attenuated radiation sources and the other radiation


intensity signal contains zero ranges of attenuated


radiation sources.


Each radiation intensity signal contains two ranges of
1814
Two


attenuated radiation sources.


Two radiation intensity signals contain two ranges of
1816


attenuated radiation sources and the other radiation


intensity signal contains one range of attenuated


radiation sources.


Two radiation intensity signals contain two ranges of
1818


attenuated radiation sources and the other radiation


intensity signal contains zero ranges of attenuated


radiation sources.


One radiation intensity signal contain two ranges of
1820


radiation sources and at least one of the other two


radiation intensity signals contains one range of


attenuated radiation sources.


Otherwise.
1804









Method 1800 reaches step 1806 if no radiation blocking objects appear to be present on the screen 1528. Controller 1504 clears the touch table and method 1800 ends.
In step 1808, controller 1504 determines the physical position P1a of a radiation blocking object 1524 in the manner described above in relation to step 1802.
Method 1800 then proceeds to step 1812, in which controller 1504 updates the touch table as described above in relation to step 806 of method 800 (FIG. 8). If only one physical position corresponding to one radiation blocking object is recorded in the touch table, the newly calculated physical position P1a is recorded as the physical position of the radiation blocking object.
If two positions corresponding to two radiation blocking objects are recorded in the touch table, the controller determines which of the previously recorded positions is closest to the newly calculated physical position P1a, which is then replaced with the newly calculated physical position. The other previously recorded position is deleted from the touch table.
Controller 1504 also converts the newly calculated physical position P1a into a corresponding digital or pixel position P1d, which is then provided at interface 1548.
Method 1800 then returns to step 1804.
In step 1810, only two of the three radiation sensors 1502 have radiation intensity signals 1522 that contain a range of attenuated radiation intensity levels. Controller 1504 estimates the physical position of a radiation blocking object in the manner described above in relation to step 806 of method 800 (FIG. 8). Controller 1504 calculates two angles (of the three angles θa, φa and αa illustrated in FIG. 15a) corresponding to the two radiation intensity signals that have attenuated radiation sources. The controller then calculates the intersection point of the two lines (of the three lines 1546a, 1532a and 1550a) corresponding to the two calculated angles. This intersection point is the physical position P1a of the radiation blocking object.
Method 1800 then proceeds to step 1812.
Reference is made to FIGS. 15b, 17a-17c and 18. In step 1814, each of the three radiation intensity signals 1522a, 1522b, 1522c contains two ranges of attenuated radiation intensity levels. As described above, controller 1504 is configured to calculates angles θa, θb, φa, φb, αa and αb and corresponding lines 1546a, 1546b, 1532a, 1532b, 1550a and 1550b. 
Controller 1504 determines the likely positions of two radiation blocking object using these six lines. By selecting one line corresponding to each radiation sensor 1502, the following triangles may be defined:

    
    
        A: Lines 1546a, 1532a, 1550a; 
        B: Lines 1546a, 1532a, 1550b; 
        C: Lines 1546a, 1532b, 1550a; 
        D: Lines 1546a, 1532b, 1550b; 
        E: Lines 1546b, 1532a, 1550a; 
        F: Lines 1546b, 1532a, 1550b; 
        G: Lines 1546b, 1532b, 1550a; and
        H: Lines 1546b, 1532b, 1550b. 
    
    


Typically, each radiation blocking object will be positioned within one of these triangles. Controller 1504 analyzes the triangles to determine which triangles are most likely to contain the radiation blocking objects. Referring to FIGS. 15e and 15f, triangles A and E are illustrated, on different scales from one another. Triangle A is illustrated substantially expanded relative to FIG. 15b. 
In the present embodiment, controller 1504 is configured to determine the area of each triangle A-H. The triangles with the smallest area are deemed to be the triangles in which the radiation blocking objects are positioned. In this example, triangle A and triangle H have the smallest area. Controller 1504 is configured to identify a point within the triangles having the smallest area, which may be done in the manner described above in relation to step 1802. In FIG. 15b, point 1534 is identified as the center of a circle inscribed in triangle A and point 1540 is identified as the center of a circle inscribed in triangle H (not shown in enlarged form). These points 1534 and 1540 are estimates of the physical locations P1a and P2a of the radiation blocking objects.
The triangles A-H may be compared in different manners to select triangles that are most likely to contain the physical locations of the radiation blocking objects. For example, in some embodiments, the triangles with the smallest perimeter may be selected. In other embodiments, other geometric techniques may be used to select triangles in which the radiation blocking objects are likely to be positioned. For example, in a situation where two triangles cannot be clearly distinguished as being smaller than the remaining triangles, some triangles may be selected based on their distance (or the distance of a selected point within the triangle) from currently recorded positions in the touch table.
In some embodiments, the size, perimeter or some other characteristic of each triangle may be combined with the position of the triangle from one (or more) currently recorded position in the touch table to select triangles. For example, in some embodiments, the perimeter of each triangle may be summed with the distance of the triangle (or a selected point within the triangle) from the closest currently recorded position in the touch table. The triangles with the lowest sum may be selected.
In other embodiments, the area of each triangle may be summed with the square of the distance between the triangle (or a selected point within the triangle) to the closest currently recorded position in the touch table. The triangles having the smallest sums may be selected.
In the event that two of the lines 1546a, 1546b, 1532a, 1532b, 1550a and 1550b are parallel, any combination of lines containing the parallel lines will not form a triangle and that combination is ignored.
When a pair of triangles have been selected, method 1800 proceeds to step 1822.
In step 1816, two of the radiation intensity signals 1522 have a pair of ranges of attenuated radiation intensity levels while the third has a single range of attenuated radiation intensity levels. Step 1816 is similar to step 1814, except that only combinations of lines that can be identified from the radiation intensity signals are used to identify triangles. For example, in FIG. 19, only line 1532a may be identified based on radiation intensity signal 1522b (FIG. 20b). As a result, only triangles A, B, E and F described in step 1814 may be identified and analyzed.
The controller 1504 selects two triangles from the identifiable triangles and identifies physical positions P1a and P2a within those triangles.
Alternatively, the calculated angle and line for the radiation intensity signal having only one range of attenuated radiation levels (φa and 1532a in FIG. 19) are duplicated (as φb and 1532b in this example) and the triangles are identified and analyzed as in step 1814.
Method 1800 then proceeds to step 1822.
In step 1818, two of the radiation intensity signals 1522 have a pair of ranges of attenuated radiation intensity levels while the third does not have any range of attenuated radiation intensity levels. Controller 1504 ignores the radiation intensity signal without a range of attenuated radiation sources and identifies physical locations P1a and P2a of the radiation blocking objects in the manner described above in relation to step 808 of method 800 (FIG. 8).
Method 1800 then proceeds to step 1822.
In step 1820, one of the radiation intensity signals 1522 has a pair of ranges of attenuated radiation intensity levels and at least one of the other radiation intensity signals has a single range of attenuated radiation intensity levels.
If one of the other radiation intensity signals contains a range of attenuated radiation intensity levels, then the two radiation intensity signals have one and two ranges of attenuated radiation intensity levels are used to calculates positions P1a and P2a as described above in relation to step 810 of method 800 (FIG. 8).
If both of the other radiation intensity signals contains a range of attenuated radiation intensity levels, one of the radiation intensity signals is selected. In the present embodiment, the radiation intensity signal is selected based on a preference order of selecting the radiation intensity signal corresponding to radiation sensor 1502a, 1502b and then 1502c. This order is used to ensure that, if possible, radiation intensity signals for radiation sensors 1502a and 1502b will be used, thereby providing a maximum separation between the vantage point of the sensor used to estimate the positions of the radiation blocking objects. The radiation intensity signal with two ranges of attenuated radiation levels and the selected radiation intensity signal are used to calculate physical positions P1a and P2a the manner described above in relation to step 810 of method 800 (FIG. 8).
In other embodiments, other techniques may be used to select between radiation intensity signals that include a single range of attenuated radiation intensity levels. For example, one of the radiation intensity signals may be chosen randomly or semi-randomly. One of the radiation intensity signals may be chosen if it was used in the previous or a recent calculation of positions P1a or P2a. In some embodiments, a radiation intensity signal may be selected based on a currently recorded position in one or two slots in the touch table. For example, if a currently recorded position is near the top edge 1510 and between radiation sensors 1502b and 1502c, then it may be preferable to estimate the position of a radiation blocking object using the radiation intensity signals for those sensors. In some embodiments, a different pair of radiation intensity signals may be used to calculate positions P1a and P2a. 
Method 1800 then proceeds to step 1822.
In step 1822, method 1800 proceeds to step 1824 if two positions are currently recorded in the touch table, or to step 1826 if one position is recorded in the touch table.
In step 1824, controller 1504 updates the touch table, converts the physical positions into pixel positions P1d and P2d and reports the pixel positions at interface 1548, in the manner described above in relation to step 816 of method 800 (FIG. 8).
In step 1826, controller 1504 updates the touch table, converts the physical positions into pixel positions P1d and P2d and reports the pixel positions at interface 1548, in the manner described above in relation to step 814 of method 800 (FIG. 8).
Using method 1800, system 1500 is able to successively estimate the position of one or two radiation blocking objects on screen 1528.
Reference is next made to FIG. 21, which illustrates a system 2100 according to another embodiment of the invention. System 1500 is similar in construction to systems 100, 500, 700 and 1500 and corresponding components are identified by similar reference numerals. As with the systems described above, system 2100 may be used as an electronic whiteboard system or a touchscreen system.
In system 2100, four radiation sensors 2102a, 2102b, 2102c and 2102d are provided along the top edge 2110 of from 2108. Each radiation sensor is operated as described in relation to system 1500 to provide a radiation intensity signal.
System 2100 is used in a method analogous to method 1800 to estimate the positions of one or two radiation blocking objects on screen 2128.
The number of radiation blocking objects present on screen 2128 is assumed to be equal to the maximum number of ranges of attenuated radiation levels in any one of the radiation intensity signals.
If only one radiation blocking object is assumed to be present, then two or three radiation intensity signals that have one range of attenuated radiation intensity levels are selected. The radiation intensity signals may be selected based on a preferential order or using other techniques as described above in relation to step 1820 of method 1800 (FIG. 18). In this embodiment, a preferential order of radiation sensors in the order 2102a, 2102b, 2102c and then 2102d is used to select two three radiation intensity signals. If two signals are selected, then they are used to estimate the position of the radiation blocking object as described in step 1810 of method 1800. If three radiation intensity signals are selected, they are used to estimate the position of the radiation blocking object as described in step 1808 of method 1800.
If two radiation blocking objects are assumed to be present, then two or three of the radiation intensity signals are selected as described above. Radiation intensity signals having two ranges of attenuated radiation intensity levels are preferentially selected over radiation intensity signal having one or zero attenuated radiation intensity levels. Depending on the number of attenuated radiation intensity levels in the two or three radiation intensity signals, they selected radiation intensity signals are used as described above in steps 1814, 1816, 1818 or 1820 to estimate the positions of the radiation blocking objects.
The present invention has been described here by way of example only. Various modification and variations may be made to these exemplary embodiments without departing from the spirit and scope of the invention.

## Claims

1. A method of estimating the position of a radiation blocking object on a surface, the method comprising:
providing at least three radiation sensors adjacent to one or more edges of the surface including a first radiation sensor, a second radiation sensor and a third radiation sensor;
providing a plurality of radiation sources adjacent to edges of the surface, wherein:
radiation emitted by at least some of the radiation sources passes across the surface and is directly incident on the first sensor;
radiation emitted by at least some of the radiation sources passes across the surface and is directly incident on the second sensor; and
radiation emitted by at least some of the radiation sources passes across the surface and is directly incident on the third sensor;

establishing a first series of first baseline levels, wherein each first baseline level corresponds to a level of radiation incident on the first sensor when a corresponding one of the radiation sources is activated in the absence of a radiation blocking object;
establishing a second series of second baseline levels, wherein each second baseline level corresponds to a level of radiation incident on the second sensor when a corresponding one of the radiation sources is activated in the absence of a radiation blocking object;
establishing a third series of third baseline levels, wherein each third baseline level corresponds to a level of radiation incident on the third sensor when a corresponding one of the radiation sources is activated in the absence of a radiation blocking object;
assembling a first radiation intensity signal corresponding to the first radiation sensor, wherein the first radiation intensity signal corresponds to a series of first radiation intensity levels, each of the first radiation intensity level corresponding to the intensity of radiation emitted by one of the radiation sources incident on the first radiation sensor in the presence of a radiation blocking object;
assembling a second radiation intensity signal corresponding to the second radiation sensor, wherein the second radiation intensity signal corresponds to a series of second radiation intensity levels, each of the second radiation intensity level corresponding to the intensity of radiation emitted by one of the radiation sources incident on the second radiation sensor in the presence of a radiation blocking object;
assembling a third radiation intensity signal corresponding to the third radiation sensor, wherein the third radiation intensity signal corresponds to a series of third radiation intensity levels, each of the third radiation intensity level corresponding to the intensity of radiation emitted by one of the radiation sources incident on the third radiation sensor in the presence of a radiation blocking object;
identifying a first group of one or more attenuated radiation sources in the first radiation intensity signal by, for at least some of the radiation sources, comparing a corresponding first radiation intensity level with a corresponding first baseline level;
identifying a second group of one or more attenuated radiation sources in the second radiation intensity signal by, for at least some of the radiation sources, comparing a corresponding first radiation intensity level with a corresponding first baseline level;
identifying a third group of one or more attenuated radiation sources in the third radiation intensity signal by, for at least some of the radiation sources, comparing a corresponding first radiation intensity level with a corresponding first baseline level; and
estimating the position of the radiation blocking object based on the position of the first group of attenuated radiation sources relative to first radiation sensor, the position of the second group of attenuated radiation sources relative to the second radiation sensor, and the position of the third group of attenuated radiation sources relative to the third radiation sensor.

2. The method of claim 1 wherein each radiation intensity signal corresponding to a radiation sensor is assembled by sequentially sampling a radiation intensity level from the radiation sensor while at least some of the radiation sources are sequentially activated.

3. The method of claim 1 wherein the radiation intensity signals are assembled contemporaneously.

4. The method of claim 1 wherein at least one of the radiation sources is activated separately at different intensities to generate a radiation intensity signal corresponding to the first radiation sensor and a radiation intensity signal corresponding to the second radiation sensor.

5. The method of claim 1 wherein the radiation intensity signals are assembled sequentially.

6. The method of claim 5 wherein a first radiation intensity signal corresponding to the first radiation sensor is assembled and then a second radiation intensity signal corresponding to the second radiation sensor.

7. The method of claim 1 wherein estimating the position of the radiation blocking object includes:
determining a polygon corresponding to the position of the first group of attenuated radiation sources relative to the first radiation sensor, the position of the second group of attenuated radiation sources relative to the second radiation sensor, and the position of the third group of attenuated radiation sources relative to the third radiation sensor; and
estimating the position of the radiation blocking object based on the polygon.

8. The method of claim 7 wherein estimating the position of the radiation blocking object based on the polygon includes identifying a point relative to the polygon.

9. The method of claim 8 wherein the identified point is selected from the group consisting of: a point at the center of a circle inscribed within the polygon; a point at the center of a circle that circumscribes the polygon; and a point at which the sum of the shortest distance from the point to the sides of the polygon is minimized.

10. The method of claim 1 wherein:
a radiation source is only included in the first group if the radiation intensity level for the radiation source is below the corresponding first baseline level by a first threshold;
a radiation source is only included in the second group if the radiation intensity level for the radiation source is below the corresponding second baseline level by a second threshold; and
a radiation source is only included in the third group if the radiation intensity level for the radiation source is below the corresponding third baseline level by a third threshold.

11. The method of claim 1 wherein the estimating the position of the radiation blocking object includes:
identifying a first center radiation source based on the first group;
identifying a second center radiation source based on the second group;
identifying a third center radiation source based on the third group; and
estimating the position of the radiation blocking object based on the first center radiation source, the second center radiation source and the third center radiation source.

12. The method of claim 1 wherein estimating the position of the radiation blocking object includes:
determining a first line based on the first group of attenuated radiation sources and the position of the first radiation sensor;
determining a second line based on the second group of attenuated radiation sources and the position of the second radiation sensor;
determining a third line based on the third group of attenuated radiation sources and the position of the third radiation sensor; and
estimating the position of the position of the radiation blocking object based on the first, second and third lines.

13. The method of claim 1 wherein estimating the position of the radiation blocking object includes:
determining a polygon corresponding to the first line, the second line and third line; and
estimating the position of the radiation blocking object based on the polygon.

14. The method of claim 12 wherein determining the first line includes looking up a first angle in a lookup table, determining the second line includes looking up a second angle in a lookup table and determining the third line includes looking up a third angle in a lookup table.

15. The method of claim 12 wherein determining the first line includes calculating a first angle, determining the second line includes calculating a second angle and determining the third line includes calculating a third angle.

16. The method of claim 12 wherein:
determining the first line includes calculating a weighted average based on the attenuation of radiation sources in the first group;
determining the second line includes calculating a weighted average based on the attenuation of radiation sources in the second group; and
determining the third line includes calculating a weighted average based on the attenuation of radiation sources in the third group.