Abstract:
A pen tool comprises an elongate body, a tip adjacent one end the body, and a filtered reflector disposed on the body, the filtered reflector comprising a reflecting portion and at least one filtering element, the at least one filtering element configured to permit illumination emitted at a selected wavelength to pass therethrough and impinge on the reflecting portion and to permit illumination at the selected wavelength that is reflected by the reflecting portion to exit the filtered reflector.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/838,567 filed on Mar. 15, 2013, which claims the benefit of U.S. Provisional Application No. 61/618,695 filed on Mar. 31, 2012, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The subject application relates to an interactive input system and to a pen tool therefor. 
       BACKGROUND 
       [0003]    Interactive input systems that allow users to inject input into an application program using an active pointer (e.g. a pointer that emits light, sound or other signal), a passive pointer (e.g. a finger, cylinder or other object) or other suitable input device such as for example, a mouse or trackball, are well known. These interactive input systems include but are not limited to: touch systems comprising touch panels employing analog resistive or machine vision technology to register pointer input such as those disclosed in U.S. Pat. Nos. 5,448,263; 6,141,000; 6,337,681; 6,747,636; 6,803,906; 7,232,986; 7,236,162; and 7,274,356 and in U.S. Patent Application Publication No. 2004/0179001 assigned to SMART Technologies ULC of Calgary, Alberta, Canada, assignee of the subject application, the entire disclosures of which are incorporated by reference; touch systems comprising touch panels employing electromagnetic, capacitive, acoustic or other technologies to register pointer input; tablet and laptop personal computers (PCs); smartphones, personal digital assistants (PDAs) and other handheld devices; and other similar devices. 
         [0004]    Above-incorporated U.S. Pat. No. 6,803,906 to Morrison et al. discloses a touch system that employs machine vision to detect pointer interaction with a touch surface on which a computer-generated image is presented. A rectangular bezel or frame surrounds the touch surface and supports digital cameras at its corners. The digital cameras have overlapping fields of view that encompass and look generally across the touch surface. The digital cameras acquire images looking across the touch surface from different vantages and generate image data. Image data acquired by the digital cameras is processed by on-board digital signal processors to determine if a pointer exists in the captured image data. When it is determined that a pointer exists in the captured image data, the digital signal processors convey pointer characteristic data to a master controller, which in turn processes the pointer characteristic data to determine the location of the pointer in (x,y) coordinates relative to the touch surface using triangulation. The pointer coordinates are conveyed to a computer executing one or more application programs. The computer uses the pointer coordinates to update the computer-generated image that is presented on the touch surface. Pointer contacts on the touch surface can therefore be recorded as writing or drawing or used to control execution of application programs executed by the computer. 
         [0005]    U.S. Pat. No. 6,972,401 to Akitt et al. assigned to SMART Technologies ULC, the entire disclosure of which is incorporated herein by reference, discloses an illuminated bezel for use in a touch system such as that disclosed in above-incorporated U.S. Pat. No. 6,803,906 to Morrison et al. The illuminated bezel comprises infrared (IR) light emitting diodes (LEDs) that project infrared light onto diffusers. The diffusers in turn, diffuse the infrared light so that the intensity of backlighting provided over the touch surface by the illuminated bezel is generally even across the surfaces of the diffusers. As a result, the backlight illumination provided by the bezel appears generally continuous to the digital cameras. Although this illuminated bezel works very well, it adds cost to the touch system. 
         [0006]    U.S. Patent Application Publication No. 2011/0242060 to McGibney et al., filed on Apr. 1, 2010, and assigned to SMART Technologies ULC, the entire disclosure of which is incorporated herein by reference, discloses an interactive input system comprising at least one imaging assembly having a field of view looking into a region of interest and capturing image frames and processing structure in communication with the at least one imaging assembly. When a pointer exists in captured image frames, the processing structure demodulates the captured image frames to determine frequency components thereof and examines the frequency components to determine at least one attribute of the pointer. 
         [0007]    U.S. Patent Application Publication No. 2011/0242006 to Thompson et al., filed on Apr. 1, 2010, and assigned to SMART Technologies ULC, the entire disclosure of which is incorporated herein by reference, discloses a pen tool for use with a machine vision interactive input system comprising an elongate body and a tip arrangement at one end of the body, an end surface of the body at least partially about the tip arrangement carrying light reflective material that is visible to at least one imaging assembly of the interactive input system when the pen tool is angled. 
         [0008]    U.S. Pat. Nos. 7,202,860 and 7,414,617 to Ogawa disclose a coordinate input device that includes a pair of cameras positioned in an upper left position and an upper right position of a display screen of a monitor lying close to a plane extending from the display screen of the monitor and views both a side face of an object in contact with a position on the display screen and a predetermined desktop coordinate detection area to capture the image of the object within the field of view. The coordinate input device also includes a control circuit which calculates the coordinate value of a pointing tool, pointing to a position within a coordinate detection field, based on video signals output from the pair of cameras, and transfers the coordinate value to a program of a computer. 
         [0009]    U.S. Pat. No. 6,567,078 to Ogawa discloses a handwriting communication system, a handwriting input device and a handwriting display device used in the system, which can communicate by handwriting among a plurality of computers connected via a network. The communication system includes a handwriting input device which is provided at a transmitting side for inputting the handwriting into a transmitting side computer, and a handwriting display device which is provided at a receiving side for displaying the handwriting based on information transmitted from the transmitting side to a receiving side computer. The system transmits only a contiguous image around the handwritten portion, which reduces the communication volume compared to transmitting the whole image, and which makes the real time transmission and reception of handwriting trace possible. 
         [0010]    U.S. Pat. No. 6,441,362 to Ogawa discloses an optical digitizer for determining a position of a pointing object projecting a light and being disposed on a coordinate plane. In the optical digitizer, a detector is disposed on a periphery of the coordinate plane and has a view field covering the coordinate plane for receiving the light projected from the pointing object and for converting the received light into an electric signal. A processor is provided for processing the electric signal fed from the detector to compute coordinates representing the position of the pointing object. A collimator is disposed to limit the view field of the detector below a predetermined height relative to the coordinate plane such that through the limited view field the detector can receive only a parallel component of the light which is projected from the pointing object substantially in parallel to the coordinate plane. A shield is disposed to enclose the periphery of the coordinate plane to block a noise light other than the projected light from entering into the limited view field of the detector. 
         [0011]    Although the above references disclose a variety of interactive input systems, improvements are generally desired. It is therefore an object at least to provide a novel interactive input system and a novel pen tool therefor. 
       SUMMARY 
       [0012]    Accordingly, in one aspect there is provided a pen tool comprising an elongate body, a tip adjacent one end the body, and a filtered reflector disposed on the body, the filtered reflector comprising a reflecting portion and at least one filtering element, the at least one filtering element configured to permit illumination emitted at a selected wavelength to pass therethrough and impinge on the reflecting portion and to permit illumination at the selected wavelength that is reflected by the reflecting portion to exit the filtered reflector. 
         [0013]    In one embodiment, the filtered reflector is positioned adjacent the tip. The selected wavelength is within the infrared (IR) spectrum. The at least one filtering element is an optical bandpass filter having a peak wavelength corresponding to the selected wavelength. The peak wavelength is one of 780 nm, 830 nm, and 880 nm. 
         [0014]    According to another aspect there is provided an interactive input system comprising at least one imaging assembly having a field of view aimed into a region of interest and capturing image frames thereof, at least one light source configured to emit illumination into the region of interest at a selected wavelength, and processing structure configured to process the captured image frames to determine a location of at least one pointer in a first region of the captured image frames, to define a pointer analysis region in the captured image frames separate from the first region, and to identify the at least one pointer based on a calculated intensity of at least a portion of the pointer analysis region. 
         [0015]    According to another aspect there is provided a method of identifying at least one pointer brought into proximity with an interactive input system, the method comprising emitting illumination into a region of interest from at least one light source at a selected wavelength, capturing image frames of the region of interest, and processing the captured image frames to determine a location of at least one pointer in a first region of the captured image frames, to define a pointer analysis region in the captured image frames separate from the first region, and to identify the at least one pointer based on a calculated intensity of at least a portion of the pointer analysis region. 
         [0016]    According to another aspect there is provided a non-transitory computer readable medium tangibly embodying a computer program for execution by a computer to perform a method for identifying at least one pointer brought into proximity with an interactive input system, the method comprising emitting illumination into a region of interest from at least one light source at a selected wavelength, capturing image frames of the region of interest, and processing the captured image frames to determine a location of at least one pointer in a first region of the captured image frames, to define a pointer analysis region in the captured image frames separate from the first region, and to identify the at least one pointer based on a calculated intensity of at least a portion of the pointer analysis region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Embodiments will now be described more fully with reference to the accompanying drawings in which: 
           [0018]      FIG. 1  is a schematic perspective view of an interactive input system; 
           [0019]      FIG. 2  is a schematic block diagram view of the interactive input system of  FIG. 1 ; 
           [0020]      FIG. 3  is a block diagram of an imaging assembly forming part of the interactive input system of  FIG. 1 ; 
           [0021]      FIG. 4  is a front perspective view of a housing assembly forming part of the imaging assembly of  FIG. 3 ; 
           [0022]      FIG. 5  is a block diagram of a master controller forming part of the interactive input system of  FIG. 1 ; 
           [0023]      FIG. 6   a  is a perspective view of a pen tool for use with the interactive input system of  FIG. 1 ; 
           [0024]      FIG. 6   b  is a side cross-sectional view of a portion of the pen tool of  FIG. 6   a;    
           [0025]      FIG. 7   a  is a perspective view of another pen tool for use with the interactive input system of  FIG. 1 ; 
           [0026]      FIG. 7   b  is a side cross-sectional view of a portion of the pen tool of  FIG. 7   a;    
           [0027]      FIG. 8  shows an image frame capture sequence used by the interactive input system of  FIG. 1 ; 
           [0028]      FIG. 9  is a flowchart showing steps of an image processing method; 
           [0029]      FIGS. 10A and 10B  are exemplary captured image frames; 
           [0030]      FIG. 11  shows another embodiment of an image frame capture sequence used by the interactive input system of  FIG. 1 ; 
           [0031]      FIG. 12  is a side cross-sectional view of a portion of another embodiment of a pen tool for use with the interactive input system of  FIG. 1 ; 
           [0032]      FIG. 13  is a perspective view of another embodiment of an interactive input system; 
           [0033]      FIG. 14  is a schematic plan view of an imaging assembly arrangement employed by the interactive input system of  FIG. 13 ; 
           [0034]      FIG. 15  shows an image frame capture sequence used by the interactive input system of  FIG. 13 ; 
           [0035]      FIG. 16  is a schematic side elevational view of another embodiment of an interactive input system; 
           [0036]      FIG. 17  is a schematic side elevational view of yet another embodiment of an interactive input system; 
           [0037]      FIG. 18  is a schematic top plan view of yet another embodiment of an interactive input system; 
           [0038]      FIG. 19   a  is a perspective view of a pen tool for use with the interactive input system of  FIG. 18 ; 
           [0039]      FIG. 19   b  is a side cross-sectional view of a portion of the pen tool of  FIG. 19   a;    
           [0040]      FIG. 19   c  is a side cross-sectional view of another portion of the pen tool of  FIG. 19   a;    
           [0041]      FIG. 20   a  is a perspective view of another pen tool for use with the interactive input system of  FIG. 18 ; 
           [0042]      FIG. 20   b  is a side cross-sectional view of a portion of the pen tool of  FIG. 20   a;    
           [0043]      FIG. 20   c  is a side cross-sectional view of another portion of the pen tool of  FIG. 20   a;    
           [0044]      FIG. 21  is a flowchart showing steps of an image processing method; 
           [0045]      FIG. 22  is a schematic view showing four operational phases of an illuminated bezel of the interactive input system of  FIG. 18 ; and 
           [0046]      FIG. 23  shows an image frame capture sequence used by the interactive input system of  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0047]    Turning now to  FIGS. 1 and 2 , an interactive input system that allows a user to inject input such as digital ink, mouse events etc. into an application program is shown and is generally identified by reference numeral  20 . In this embodiment, interactive input system  20  comprises an interactive board  22  mounted on a vertical support surface such as for example, a wall surface or the like or otherwise supported or suspended in an upright orientation. Interactive board  22  comprises a generally planar, rectangular interactive surface  24  that is surrounded about its periphery by a bezel  26 . An ultra-short throw projector (not shown) such as that sold by SMART Technologies ULC under the name SMART UX60 is also mounted on the support surface above the interactive board  22  and projects an image, such as for example a computer desktop, onto the interactive surface  24 . 
         [0048]    The interactive board  22  employs machine vision to detect one or more pointers brought into a region of interest in proximity with the interactive surface  24 . The interactive board  22  communicates with a general purpose computing device  28  executing one or more application programs via a universal serial bus (USB) cable  30  or other suitable wired or wireless connection. General purpose computing device  28  processes the output of the interactive board  22  and, if required, adjusts image data output to the projector so that the image presented on the interactive surface  24  reflects pointer activity. In this manner, the interactive board  22 , general purpose computing device  28  and projector allow pointer activity proximate to the interactive surface  24  to be recorded as writing or drawing or used to control execution of one or more application programs executed by the general purpose computing device  28 . 
         [0049]    The bezel  26  in this embodiment is mechanically fastened to the interactive surface  24  and comprises four bezel segments  40 ,  42 ,  44 ,  46 . Bezel segments  40  and  42  extend along opposite side edges of the interactive surface  24  while bezel segments  44  and  46  extend along the top and bottom edges of the interactive surface  24  respectively. In this embodiment, the inwardly facing surface of each bezel segment  40 ,  42 ,  44  and  46  comprises a single, longitudinally extending strip or band of retro-reflective material. To take best advantage of the properties of the retro-reflective material, the bezel segments  40 ,  42 ,  44  and  46  are oriented so that their inwardly facing surfaces extend in a plane generally normal to the plane of the interactive surface  24 . 
         [0050]    A tool tray  48  is affixed to the interactive board  22  adjacent the bezel segment  46  using suitable fasteners such as for example, screws, clips, adhesive etc. As can be seen, the tool tray  48  comprises a housing  48   a  having an upper surface  48   b  configured to define a plurality of receptacles or slots  48   c . The receptacles  48   c  are sized to receive one or more pen tools as will be described as well as an eraser tool that can be used to interact with the interactive surface  24 . Control buttons  48   d  are provided on the upper surface  48   b  of the housing  48   a  to enable a user to control operation of the interactive input system  20 . One end of the tool tray  48  is configured to receive a detachable tool tray accessory module  48   e  while the opposite end of the tool tray  48  is configured to receive a detachable communications module  48   f  for remote device communications. The housing  48   a  accommodates a master controller  50  (see  FIG. 5 ) as will be described. The tool tray  48  is described further in U.S. Patent Application Publication No. 2011/0169736 to Bolt et al., filed on Feb. 19, 2010, and assigned to SMART Technologies ULC, the relevant portions of the disclosure of which are incorporated herein by reference. 
         [0051]    As shown in  FIG. 2 , imaging assemblies  60  are accommodated by the bezel  26 , with each imaging assembly  60  being positioned adjacent a different corner of the bezel. The imaging assemblies  60  are oriented so that their fields of view overlap and look generally across the entire interactive surface  24 . In this manner, any pointer such as for example a user&#39;s finger, a cylinder or other suitable object, or a pen tool or eraser tool lifted from a receptacle  48   c  of the tool tray  48 , that is brought into proximity of the interactive surface  24  appears in the fields of view of the imaging assemblies  60 . A power adapter  62  provides the necessary operating power to the interactive board  22  when connected to a conventional AC mains power supply. 
         [0052]    Turning now to  FIG. 3 , components of one of the imaging assemblies  60  are shown. As can be seen, the imaging assembly  60  comprises a grey scale image sensor  70  such as that manufactured by Aptina (Micron) under Model No. MT9V034 having a resolution of 752×480 pixels, fitted with a two element, plastic lens (not shown) that provides the image sensor  70  with a field of view of approximately 104 degrees. In this manner, the other imaging assemblies  60  are within the field of view of the image sensor  70  thereby to ensure that the field of view of the image sensor  70  encompasses the entire interactive surface  24 . 
         [0053]    A digital signal processor (DSP)  72  such as that manufactured by Analog Devices under part number ADSP-BF522 Blackfin or other suitable processing device, communicates with the image sensor  70  over an image data bus  71  via a parallel port interface (PPI). A serial peripheral interface (SPI) flash memory  74  is connected to the DSP  72  via an SPI port and stores the firmware required for imaging assembly operation. Depending on the size of captured image frames as well as the processing requirements of the DSP  72 , the imaging assembly  60  may optionally comprise synchronous dynamic random access memory (SDRAM)  76  to store additional temporary data as shown by the dotted lines. The image sensor  70  also communicates with the DSP  72  via a two-wire interface (TWI) and a timer (TMR) interface. The control registers of the image sensor  70  are written from the DSP  72  via the TWI in order to configure parameters of the image sensor  70  such as the integration period for the image sensor  70 . 
         [0054]    In this embodiment, the image sensor  70  operates in snapshot mode. In the snapshot mode, the image sensor  70 , in response to an external trigger signal received from the DSP  72  via the TMR interface that has a duration set by a timer on the DSP  72 , enters an integration period during which an image frame is captured. Following the integration period after the generation of the trigger signal by the DSP  72  has ended, the image sensor  70  enters a readout period during which time the captured image frame is available. With the image sensor in the readout period, the DSP  72  reads the image frame data acquired by the image sensor  70  over the image data bus  71  via the PPI. The frame rate of the image sensor  70  in this embodiment is between about 900 and about 960 frames per second. The DSP  72  in turn processes image frames received from the image sensor  70  and provides pointer information to the master controller  50  at a reduced rate of approximately 100 points/sec. Those of skill in the art will however appreciate that other frame rates may be employed depending on the desired accuracy of pointer tracking and whether multi-touch and/or active pointer identification is employed. 
         [0055]    Two strobe circuits  80  communicate with the DSP  72  via the TWI and via a general purpose input/output (GPIO) interface. The strobe circuits  80  also communicate with the image sensor  70  and receive power provided on LED power line  82  via the power adapter  62 . Each strobe circuit  80  drives a respective illumination source in the form of an infrared (IR) light emitting diode (LED)  84   a  and  84   b  that provides infrared backlighting over the interactive surface  24 . Further specifics concerning the strobe circuits  80  and their operation are described in U.S. Patent Application Publication No. 2011/0169727 to Akitt, filed on Feb. 19, 2010, and assigned to SMART Technologies ULC, the relevant portions of the disclosure of which are incorporated herein by reference. 
         [0056]    The DSP  72  also communicates with an RS-422 transceiver  86  via a serial port (SPORT0) and a non-maskable interrupt (NMI) port. The transceiver  86  communicates with the master controller  50  over a differential synchronous signal (DSS) communications link  88  and a synch line  90 . Power for the components of the imaging assembly  60  is provided on power line  92  by the power adapter  62 . DSP  72  may also optionally be connected to a USB connector  94  via a USB port as indicated by the dotted lines. The USB connector  94  can be used to connect the imaging assembly  60  to diagnostic equipment. 
         [0057]    The image sensor  70  and its associated lens as well as the IR LEDs  84   a  and  84   b  are mounted on a housing assembly  100  that is shown in  FIG. 4 . As can be seen, the housing assembly  100  comprises a polycarbonate housing body  102  having a front portion  104  and a rear portion  106  extending from the front portion. An imaging aperture  108  is centrally formed in the housing body  102  and accommodates an IR-pass/visible light blocking filter  110 . In this embodiment, the filter  110  has a wavelength range between about 810 nm and about 900 nm. The image sensor  70  and associated lens are positioned behind the filter  110  and oriented such that the field of view of the image sensor  70  looks through the filter  110  and generally across the interactive surface  24 . The rear portion  106  is shaped to surround the image sensor  70 . Two passages  112   a  and  112   b  are formed through the housing body  102 . Passages  112   a  and  112   b  are positioned on opposite sides of the filter  110  and are in general horizontal alignment with the image sensor  70 . 
         [0058]    Tubular passage  112   a  receives a light source socket  114   a  that is configured to receive IR LED  84   a . In this embodiment, IR LED  84   a  emits IR light having a peak wavelength of about 830 nm and is of the type such as that manufactured by Vishay under Model No. TSHG8400. Tubular passage  112   a  also receives an IR-bandpass filter  115   a . The filter  115   a  has an IR-bandpass wavelength range of about 830 nm±12 nm and is the type such as that manufactured by HB Optical Filters under Model No. NIR Narrow Bandpass Filter, 830 nm+/−12 nm. The light source socket  114   a  and associated IR LED  84   a  are positioned behind the IR-bandpass filter  115   a  and oriented such that IR illumination emitted by IR LED  84   a  passes through the IR-bandpass filter  115   a  and generally across the interactive surface  24 . 
         [0059]    Tubular passage  112   b  receives a light source socket  114   b  that is configured to receive IR LED  84   b . In this embodiment, IR LED  84   b  emits IR light having a peak wavelength of about 875 nm and is of the type such as that manufactured by Vishay under Model No. TSHA5203. Tubular passage  112   b  also receives an IR-bandpass filter  115   b . The filter  115   b  has an IR-bandpass wavelength range of about 880 nm±12 nm and is of the type such as that manufactured by HB Optical Filters under Model No. NIR Narrow Bandpass Filter, 880 nm+/−12 nm. The light source socket  114   b  and associated IR LED  84   b  are positioned behind the IR-bandpass filter  115   b  and oriented such that IR illumination emitted by IR LED  84   b  passes through the IR-bandpass filter  115   b  and generally across the interactive surface  24 . 
         [0060]    Mounting flanges  116  are provided on opposite sides of the rear portion  106  to facilitate connection of the housing assembly  100  to the bezel  26  via suitable fasteners. A label  118  formed of retro-reflective material overlies the front surface of the front portion  104 . Further specifics concerning the housing assembly and its method of manufacture are described in U.S. Patent Application Publication No. 2011/0170253 to Liu et al., filed on Feb. 19, 2010, and assigned to SMART Technologies ULC, the relevant portions of the disclosure of which are incorporated herein by reference. 
         [0061]    Components of the master controller  50  are shown in  FIG. 5 . As can be seen, master controller  50  comprises a DSP  200  such as that manufactured by Analog Devices under part number ADSP-BF522 Blackfin or other suitable processing device. A serial peripheral interface (SPI) flash memory  202  is connected to the DSP  200  via an SPI port and stores the firmware required for master controller operation. A synchronous dynamic random access memory (SDRAM)  204  that stores temporary data necessary for system operation is connected to the DSP  200  via an SDRAM port. The DSP  200  communicates with the general purpose computing device  28  over the USB cable  30  via a USB port. The DSP  200  communicates through its serial port (SPORT0) with the imaging assemblies  60  via an RS-422 transceiver  208  over the differential synchronous signal (DSS) communications link  88 . In this embodiment, as more than one imaging assembly  60  communicates with the master controller DSP  200  over the DSS communications link  88 , time division multiplexed (TDM) communications is employed. The DSP  200  also communicates with the imaging assemblies  60  via the RS-422 transceiver  208  over the camera synch line  90 . DSP  200  communicates with the tool tray accessory module  48   e  over an inter-integrated circuit (I 2 C) channel and communicates with the communications module  48   f  over universal asynchronous receiver/transmitter (UART), serial peripheral interface (SPI) and I 2 C channels. 
         [0062]    As will be appreciated, the architectures of the imaging assemblies  60  and master controller  50  are similar. By providing a similar architecture between each imaging assembly  60  and the master controller  50 , the same circuit board assembly and common components may be used for both thus reducing the part count and cost of the interactive input system  20 . Differing components are added to the circuit board assemblies during manufacture dependent upon whether the circuit board assembly is intended for use in an imaging assembly  60  or in the master controller  50 . For example, the master controller  50  may require a SDRAM  76  whereas the imaging assembly  60  may not. 
         [0063]    The general purpose computing device  28  in this embodiment is a personal computer or other suitable processing device comprising, for example, a processing unit comprising one or more processors, system memory (volatile and/or non-volatile memory), other non-removable or removable memory (e.g. a hard disk drive, RAM, ROM, EEPROM, CD-ROM, DVD, flash memory, etc.) and a system bus coupling the various computer components to the processing unit. The general purpose computing device  28  may also comprise a network connection to access shared or remote drives, one or more networked computers, or other networked devices. 
         [0064]      FIGS. 6   a  and  6   b  show a pen tool  220  for use with the interactive input system  20 . As can be seen, pen tool  220  has a main body  222  terminating in a generally conical tip  224 . A filtered reflector  226  is provided on the body  222  adjacent the tip  224 . Filtered reflector  226  comprises a reflective element  228  and a filtering element  230 . The reflective element  228  encircles a portion of the body  222  and is formed of a retro-reflective material such as for example retro-reflective tape. The filtering element  230  is positioned atop and circumscribes the reflective element  228 . The filtering element  230  is formed of the same material as the IR-bandpass filter  115   a  such that the filtering element  230  has an IR-bandpass wavelength range of about 830 nm±12 nm. 
         [0065]      FIGS. 7   a  and  7   b  show another pen tool  220 ′ for use with the interactive input system  20  that is similar to pen tool  220 . As can be seen, pen tool  220 ′ has a main body  222 ′ terminating in a generally conical tip  224 ′. A filtered reflector  226 ′ is provided on the body  222 ′ adjacent the tip  224 ′. Filtered reflector  226 ′ comprises a reflective element  228 ′ and a filtering element  230 ′. The reflective element  228 ′ encircles a portion of the body  222 ′ and is formed of a retro-reflective material such as for example retro-reflective tape. The filtering element  230 ′ is positioned atop and circumscribes the reflective element  228 ′. The filtering element  230 ′ is formed of the same material as the IR-bandpass filter  115   b  such that the filtering element  230 ′ has an IR-bandpass wavelength range of about 880 nm±12 nm. 
         [0066]    The differing filtering elements  230  and  230 ′ of the pen tools  220  and  220 ′ enable the interactive input system  20  to differentiate between the pen tools  220  and  220 ′ when the pen tools are brought into proximity with the interactive surface  24 , as will be described below. 
         [0067]    During operation, the DSP  200  of the master controller  50  outputs synchronization signals that are applied to the synch line  90  via the transceiver  208 . Each synchronization signal applied to the synch line  90  is received by the DSP  72  of each imaging assembly  60  via transceiver  86  and triggers a non-maskable interrupt (NMI) on the DSP  72 . In response to the non-maskable interrupt triggered by the synchronization signal, the DSP  72  of each imaging assembly  60  ensures that its local timers are within system tolerances and if not, corrects its local timers to match the master controller  50 . Using one local timer, the DSP  72  initiates a pulse sequence via the snapshot line that is used to condition the image sensor  70  to the snapshot mode and to control the integration period and frame rate of the image sensor  70  in the snapshot mode. The DSP  72  also initiates a second local timer that is used to provide output on the LED control line  174  so that the IR LEDs  84   a  and  84   b  are properly powered during the image frame capture cycle. In this embodiment, the pulse sequences and the outputs on the LED control line  174  are generated so that the image frame capture rate of each image sensor  70  is nine (9) times the desired image frame output rate. 
         [0068]    In response to the pulse sequence output on the snapshot line, the image sensor  70  of each imaging assembly  60  acquires image frames at the desired image frame rate. In this manner, image frames captured by the image sensor  70  of each imaging assembly can be referenced to the same point of time allowing the position of pointers brought into the fields of view of the image sensors  70  to be accurately triangulated. Each imaging assembly  60  has its own local oscillator (not shown) and synchronization signals are distributed so that a lower frequency synchronization signal for each imaging assembly  60  is used to keep image frame capture synchronized. By distributing the synchronization signals for the imaging assemblies  60 , rather than, transmitting a fast clock signal to each imaging assembly  60  from a central location, electromagnetic interference is reduced. 
         [0069]    During image frame capture by each imaging assembly  60 , one of IR LEDs  84   a  and  84   b  of the imaging assembly  60  is ON. As a result, the region of interest over the interactive surface  24  is flooded with infrared illumination. The infrared illumination has a peak wavelength of about 830 nm when IR LED  84   a  is ON and about 875 nm when IR LED  84   b  is ON Infrared illumination that impinges on the retro-reflective bands of bezel segments  40 ,  42 ,  44  and  46  and on the retro-reflective labels  118  of the housing assemblies  100  is returned to the imaging assembly  60 . Additionally, reflections of the illuminated retro-reflective bands of bezel segments  40 ,  42 ,  44  and  46  and the illuminated retro-reflective labels  118  appearing on the interactive surface  24  are visible to the image sensor  70 . As a result, in the absence of a pointer, the image sensor  70  of the imaging assembly  60  sees a bright band having a substantially even intensity over its length, together with any ambient light artifacts. When a pointer is brought into proximity with the interactive surface  24 , the pointer occludes infrared illumination. As a result, the image sensor  70  of the imaging assembly  60  sees a dark region that interrupts the bright band. 
         [0070]    If pen tool  220  is brought into proximity with the interactive surface  24  during image frame capture and the filtering element  230  has the same passband as the IR-bandpass filter associated with the IR LED that is ON, the image sensor  70  of the imaging assembly  60  will also see a bright region having a high intensity above the bright band corresponding to infrared illumination that impinges on the filtered reflector  226  of the pen tool  220  as a result of the infrared illumination being able to pass through the filtering element  230  and being reflected by the reflective element  228 . The intensity of the bright region will be greater than an intensity threshold. A reflection of the bright region appearing on the interactive surface  24  is also visible to the image sensor  70 , below the bright band. If filtering element  230  of the pen tool  220  does not have the same passband as the IR-bandpass filter associated with the IR LED that is ON, the image frame captured by the image sensor  70  of the imaging assembly  60  will not comprise a bright region having an intensity greater than the intensity threshold as a result of the infrared illumination not being able to pass through the filtering element  230 . By comparing the intensity of the bright region to the intensity threshold and by monitoring which IR LED is ON, the identity of the pen tool  220  can be determined. 
         [0071]    If pen tool  220 ′ is brought into proximity with the interactive surface  24  during image frame capture and the filtering element  230 ′ has the same passband as the IR-bandpass filter associated with the IR LED that is ON, the image sensor  70  of the imaging assembly  60  will also see a bright region having a high intensity above the bright band corresponding to infrared illumination that impinges on the filtered reflector  226 ′ of the pen tool  220 ′ as a result of the infrared illumination being able to pass through the filtering element  230 ′ and being reflected by the reflective element  228 ′. The intensity of the bright region will be greater than an intensity threshold. A reflection of the bright region appearing on the interactive surface  24  is also visible to the image sensor  70 , below the bright band. If filtering element  230 ′ of the pen tool  220 ′ does not have the same passband as the IR-bandpass filter associated with the IR LED that is ON, the image frame captured by the image sensor  70  of the imaging assembly  60  will not comprise a bright region having an intensity greater than the intensity threshold as a result of the infrared illumination not being able to pass through the filtering element  230 ′. By comparing the intensity of the bright region to the intensity threshold and by monitoring which IR LED is ON, the identity of the pen tool  220  can be determined. 
         [0072]    When the IR light sources  82   a  and  82   b  are OFF, no infrared illumination impinges on the retro-reflective bands of bezel segments  40 ,  42 ,  44  and  46  or on the retro-reflective labels  118  of the housing assemblies  100 . Consequently, the image sensor  70  of the imaging assembly  60  will not see the retro-reflective bands or the retro-reflective labels  118 . During this situation, if either pen tool  220  or pen tool  220 ′ is brought into proximity with the interactive surface  24 , no infrared illumination impinges on its filtered reflector and consequently the image sensor  70  of the imaging assembly  60  will not see a bright region corresponding to the filtered reflector. The imaging assembly  60  will however see artifacts resulting from ambient light on a dark background. The ambient light typically comprises light originating from the operating environment surrounding the interactive input system  20 , and infrared illumination emitted by the IR LEDs that is scattered off of objects proximate to the imaging assemblies  60 . 
         [0073]      FIG. 8  shows a portion of an image frame capture sequence  260  used by the interactive input system  20 . A background image frame (“Frame #1”) is initially captured by each of the imaging assemblies  60  with the IR LEDs  84   a  and  84   b  OFF. A first one of the imaging assemblies  60  is conditioned to capture an image frame (“Frame #2”) with its IR LED  84   a  ON and its IR LED  84   b  OFF and then to capture another image frame (“Frame #3”) with its IR LED  84   b  OFF and its IR LED  84   b  ON. The remaining three imaging assemblies  60  and their associated IR LEDs  84   a  and  84   b  are inactive when Frame #2 and Frame #3 are being captured. A second one of the imaging assemblies  60  is then conditioned to capture an image frame (“Frame #4”) with its IR LED  84   a  ON and its IR LED  84   b  OFF and then to capture another image frame (“Frame #5”) with its IR LED  84   b  OFF and its IR LED  84   b  ON. The remaining three imaging assemblies  60  and their associated IR LEDs  84   a  and  84   b  are inactive when Frame #4 and Frame #5 are being captured. A third one of the imaging assemblies  60  is then conditioned to capture an image frame (“Frame #6”) with its IR LED  84   a  ON and its IR LED  84   b  OFF and then to capture another image frame (“Frame #7”) with its IR LED  84   b  OFF and its IR LED  84   b  ON. The remaining three imaging assemblies  60  and their associated IR LEDs  84   a  and  84   b  are inactive when Frame #6 and Frame #7 are being captured. A fourth one of the imaging assemblies  60  is then conditioned to capture an image frame (“Frame #8”) with its IR LED  84   a  ON and its IR LED  84   b  OFF and then to capture another image frame (“Frame #9”) with IR LED  84   b  OFF and IR LED  84   b  ON. The remaining three imaging assemblies  60  and their associated IR LEDs  84   a  and  84   b  are inactive when Frame #8 and Frame #9 are being captured. As a result, the exposure of the image sensors  70  of the four (4) imaging assemblies  60  and the powering of the associated IR LEDs  84   a  and  84   b  are staggered to avoid any effects resulting from illumination of neighbouring IR LEDs. 
         [0074]    Once the sequence of image frames has been captured, the image frames in the sequence are processed according to an image frame processing method, which is shown in  FIG. 9  and is generally indicated by reference numeral  270 . To reduce the effects of ambient light, difference image frames are calculated. Each difference image frame is calculated by subtracting the background image frame (“Frame 1”) captured by a particular imaging assembly  60  from the other image frames captured by that particular imaging assembly  60 . In particular, the background image frame (“Frame 1”) captured by the first imaging assembly  60  is subtracted from the two image frames (“Frame 2” and “Frame 3”), the background image frame (“Frame 1”) captured by the second imaging assembly  60  is subtracted from the image frames (“Frame 4” and “Frame 5”), the background image frame (“Frame 1”) captured by the third imaging assembly  60  is subtracted from the two image frames (“Frame 6” and “Frame 7”) and the background image frame (“Frame 1”) captured by the fourth imaging assembly  60  is subtracted from the two image frames (“Frame 8” and “Frame 9”). As a result, eight difference image frames (“Difference Image Frame #2” to “Difference Image Frame #9”) are generated having ambient light removed (step  272 ). 
         [0075]    The difference image frames are then examined for values that represent the bezel and possibly one or more pointers (step  274 ). Methods for determining pointer location within image frames are described in U.S. Patent Application Publication No. 2009/0277697 to Bolt et al., filed on May 9, 2008, and assigned to SMART Technologies ULC, the relevant portions of the disclosure of which are incorporated herein by reference. As mentioned above, when a pointer exists in a captured image frame, the pointer occludes illumination and appears as a dark region interrupting the bright band. Thus, the bright bands in the difference image frames are analyzed to determine the locations of dark regions. 
         [0076]    Once the locations of dark regions representing one or more pointers in the difference image frames have been determined, one or more square-shaped pointer analysis regions are defined directly above the bright band and dark regions (step  276 ). In the event that pen tool  220  or pen tool  220 ′ appears in the captured image frames and the filtering element of the pen tool  220  or pen tool  220 ′ has the same passband as the IR-bandpass filter associated with the IR LED that is ON, the one or more square-shaped pointer analysis regions will comprise a bright region corresponding to infrared illumination that impinges on the filtered reflector of the pen tool  220  or pen tool  220 ′ and is reflected by the reflective element thereof. The intensity of the bright region is then calculated and compared to an intensity threshold (step  278 ). 
         [0077]    For a particular difference image frame, if the intensity of the bright region that is within the pointer analysis region is above the intensity threshold, the dark region is determined to be caused by one of the pen tools  220  and  220 ′ and the pen tool can be identified (step  280 ). For example, if the intensity of the bright region that is within the pointer analysis region is above the intensity threshold in Difference Image Frame #2, pen tool  220  is identified, as it is known that Difference Image Frame #2 is calculated using Frame #2, which is captured when IR LED  84   a  is ON. Difference Frame #3 is calculated using Frame #3 (captured when IR LED  84   b  is ON). As such pen tool  220  is not identifiable in Difference Image Frame #3 since the illumination emitted by IR LED  84   b  is filtered out by the filtering element  230  of pen tool  220 . 
         [0078]    Once the identity of the pen tool  220  or pen tool  220 ′ is determined, the identity may be used to assign an attribute such as for example a pen color (red, green, black, blue, yellow, etc.) or a pen function (mouse, eraser, passive pointer) to the pen tool  220  or pen tool  220 ′. In the event the pen tool  220  or pen tool  220 ′ is assigned the pen function of a mouse, the pen tool  220  or pen tool  220 ′ may be further assigned a sub-attribute such as for example a right mouse click, a left mouse click, a single mouse click, or a double mouse click. The pen tool  220  or pen tool  220 ′ may alternatively be associated with a particular user. 
         [0079]    Turning now to  FIGS. 10A and 10B , exemplary difference image frames are shown. The difference image frames are associated with image frames captured in the event pen tool  220  and pen tool  220 ′ are in proximity with the interactive surface  24  with IR LED  84   a  ON and IR LED  84   b  OFF ( FIG. 10A ) and IR LED  84   a  OFF and IR LED  84   b  ON ( FIG. 10B ). As can be seen, the difference image frames comprise a direct image of pen tool  220  and pen tool  220 ′ as well as a reflected image of pen tool  220  and pen tool  220 ′appearing on the interactive surface  24 . Only the direct image of each pen tool  220  and  220 ′ is used for processing. 
         [0080]    As can be seen in  FIG. 10A , the filtered reflector  226  of pen tool  220  is illuminated as the illumination emitted by IR LED  84   a  passes through the filtering element  230  and is reflected by the reflective element  228  back through the filtering element  230  and towards the imaging assembly  60 . The filtered reflector  226 ′ of pen tool  220 ′ is not illuminated as the illumination emitted by IR LED  84   a  is blocked by the filtering element  230 ′. 
         [0081]    As can be seen in  FIG. 10B , the filtered reflector  226  of pen tool  220  is not illuminated as the illumination emitted by IR LED  84   b  is blocked by the filtering element  230 . The filtered reflector  226 ′ of pen tool  220 ′ is illuminated as the illumination emitted by IR LED  84   b  passes through the filtering element  230 ′ and is reflected by the reflective element  228 ′ back through the filtering element  230 ′ and towards the imaging assembly  60 . 
         [0082]    As will be appreciated, the image frame capture sequence is not limited to that described above. In other embodiments, different image frame capture sequences may be used. For example, in another embodiment, first and second ones of the imaging assemblies  60  are configured to capture image frames generally simultaneously while third and fourth ones of the imaging assemblies  60  are inactive, and vice versa. An exemplary image frame capture sequence for this embodiment is shown in  FIG. 11  and is generally indicated using reference numeral  360 . A background image frame (“Frame #1”) is initially captured by each of the imaging assemblies  60  with all IR LEDs  84   a  and  84   b  OFF. First and second ones of the imaging assemblies  60  are then conditioned to capture an image frame (“Frame 2”) with their IR LEDs  84   a  ON and their IR LEDs  84   b  OFF and then to capture another image frame (“Frame 3”) with their IR LEDs  84   a  OFF and their IR LEDs  84   b  ON. The other two imaging assemblies and their associated IR LEDs  84   a  and  84   b  are inactive when Frame #2 and Frame #3 are being captured. Third and fourth ones of the imaging assemblies  60  are then conditioned to capture an image frame (“Frame 4”) with their IR LEDs  84   a  ON and their IR LEDs  84   b  OFF and then to capture another image frame (“Frame 5”) with their IR LEDs  84   a  OFF and their IR LEDs  84   b  ON. The other two imaging assemblies and their associated IR LEDs  84   a  and  84   b  are inactive when Frame #4 and Frame #5 are being captured. As a result, the exposure of the image sensors  70  of the first and second imaging assemblies  60  and the powering of the associated IR LEDs  84   a  and  84   b  are opposite those of the third and fourth imaging assemblies  60  to avoid any potential effects resulting from illumination of opposing IR LEDs and to reduce the time of the image frame capture sequence, thereby increasing the overall system processing speed. In this embodiment, the master controller  50  operates at a rate of 160 points/second and the image sensors operate at a frame rate of 960 frames per second. 
         [0083]    Once the sequence of image frames has been captured, the image frames are processed according to an image frame processing method similar to image frame processing method  270  described above. 
         [0084]      FIG. 12  shows another embodiment of a pen tool generally indicated using reference numeral  320 . Pen tool  320  is similar to pen tool  220  described above, and comprises a filtered reflector  326  adjacent the generally conical tip  324  of the pen tool body  322 . Similar to pen tool  220 , the filtered reflector  326  comprises a reflective element  328  and a filtering element  330 . The reflective element  328  encircles a portion of the body and is made of a retro-reflective material such as for example retro-reflective tape. The filtering element  330  is positioned atop and circumscribes an upper portion of the reflective element  328 . In this embodiment, the lower portion of the reflective element  328  is not covered by the filtering element  330 . A transparent protective layer  332  is positioned atop and circumscribes the filtering element  330  and the reflective element  328 . 
         [0085]    Since the lower portion of the reflective element  328  is not covered by the filtering element  330 , IR illumination emitted by any of the IR LEDs is reflected by the lower portion of the reflective element  328 , enabling the pen tool  320  to be identified in captured image frames and distinguished from other types of pointers such as for example a user&#39;s finger. The identity of the pen tool  320  is determined in a manner similar to that described above as the upper portion of the filtered reflector  326  will only reflect IR illumination that has a wavelength in the bandpass range of the filtering element  330 . 
         [0086]    Although IR-bandpass filters having wavelengths of about 830 nm±12 nm and about 880 nm±12 nm are described above, those skilled in the art will appreciate that other bandpass filters with different peak wavelengths such as 780 nm, 810 nm and 850 may be used. Alternatively, quantum dot filters may be used. 
         [0087]    Although the interactive input system  20  is described as comprising two IR LEDs associated with each imaging assembly  60 , those skilled in the art will appreciate that more IR LEDs may be used. For example, in another embodiment each imaging assembly  60  comprises three (3) IR LEDs, each having a different peak wavelength and a corresponding IR filter. In this embodiment, three (3) different pen tools are identifiable provided each one of the pen tools has a filtering element associated with one of the IR LEDs and its filter. 
         [0088]    Pen tools  220  and  220 ′ described above are not only for use with the interactive input system  20  described above, and may alternatively be used with other interactive input systems employing machine vision. For example,  FIGS. 13 and 14  show another embodiment of an interactive input system in the form of a touch table, and which is generally referred to using reference numeral  400 . Interactive input system  400  is similar to that described in U.S. Patent Application Publication No. 2011/0006981 to Chtchetinine et al., filed on Jul. 10, 2009, and assigned to SMART Technologies, ULC, the relevant portions of the disclosure of which are incorporated herein by reference. Interactive input system  400  comprises six (6) imaging assemblies  470   a  to  470   f  positioned about the periphery of an input area  462 , and which look generally across the input area  462 . An illuminated bezel  472  surrounds the periphery of the input area  462  and generally overlies the imaging assemblies  470   a  to  470   f . The illuminated bezel  472  provides backlight illumination into the input area  462 . To detect a pointer, processing structure of interactive input system  400  utilizes a weight matrix method as disclosed in PCT Application No. PCT/CA2010/001085 to Morrison et al., filed on Jan. 13, 2011, and assigned to SMART Technologies, ULC, the relevant portions of the disclosure of which are incorporated herein by reference. 
         [0089]    Each imaging assembly  470   a  to  470   f  comprises a pair of IR LEDs  474   a  and  474   a ′ to  474   f  and  474   f ′, respectively, that is configured to flood the input area  462  with infrared illumination. In this embodiment, the imaging assemblies  470   a  to  470   f  are grouped into four (4) imaging assembly banks, namely, a first imaging assembly bank  480   a  comprising imaging assemblies  470   a  and  470   e , a second imaging assembly bank  480   b  comprising imaging assemblies  470   b  and  470   f , a third imaging assembly bank  480   c  comprising imaging assembly  470   c , and a fourth imaging assembly bank  480   d  comprising imaging assembly  470   d . The imaging assemblies within each bank capture image frames simultaneously. The IR LEDs associated with the imaging assemblies of each bank flood the input area  462  with infrared illumination simultaneously. 
         [0090]      FIG. 15  shows a portion of the image frame capture sequence  460  used by the interactive input system  400 . A background image frame (“Frame #1”) is initially captured by each of the imaging assemblies  470   a  to  470   f  in each of the imaging assembly banks  480   a  to  480   d  with all IR LEDs OFF and with the illuminated bezel  472  OFF. A second image frame (“Frame #2”) is captured by each of the imaging assemblies  470   a  to  470   f  in each of the imaging assembly banks  480   a  to  480   d  with all IR LEDs OFF and with the illuminated bezel  472  ON. Frame #1 and Frame #2 captured by each imaging assembly bank  480   a  to  480   d  are used to determine the location of a pen tool using triangulation. Each of the imaging assembly banks  480   a  and  480   b  is then conditioned to capture an image frame (“Frame #3) with IR LEDs  474   a ,  474   e ,  474   f ,  474   b  ON and IR LEDs  474   a ′,  474   e ′,  474   f ′,  474   b ′ OFF and then to capture another image frame (“Frame #4) with IR LEDs  474   a ,  474   e ,  474   f ,  474   b  OFF and IR LEDs  474   a ′,  474   e ′,  474   f ′,  474   b ′ ON. Imaging assembly banks  480   c  and  480   d  and their associated IR LEDs are inactive when Frame #3 and Frame #4 are being captured. Each of the imaging assembly banks  480   c  and  480   d  is then conditioned to capture an image frame (“Frame #5) with IR LEDs  474   c  and  474   d  ON and IR LEDs  474   c ′ and  474   d ′ OFF and then to capture another image frame (“Frame #6) with IR LEDs  474   c  and  474   d  OFF and IR LEDs  474   c ′ and  474   d ′ ON. Imaging assembly banks  480   a  and  480   b  and their associated IR LEDs are inactive when Frame #5 and Frame #6 are being captured. As a result, the exposure of the image sensors of the imaging assemblies  470   a  to  470   f  of the four (4) imaging assembly banks  480   a  to  480   d  and the powering of the associated IR LEDs  474   a  to  474   f  and  474   a ′ to  474   f  are staggered to avoid any potential effects resulting from illumination of opposing IR LEDs. To reduce the effects ambient light may have on pointer discrimination, each background image frame (“Frame 1”) is subtracted from the illuminated image frames (“Frame #2” to “Frame #6”) captured by the same imaging assembly  60  as described previously. 
         [0091]    Once the sequence of image frames has been captured, the image frames are processed according to an image frame processing method similar to image frame processing method  270  described above to determine the location and identity of any pen tool brought into proximity with the input area  462 . In this embodiment, each background image frame (“Frame #1”) is subtracted from the first image frame (“Frame #2”) captured by the same imaging assembly so as to yield a difference image frame (“Difference Image Frame #2”) for each imaging assembly. Each Difference Image Frame #2 is processed to determine the location of a pen tool using triangulation. Each background image frame (“Frame #1”) is subtracted from the remaining image frames (“Frame #3” to “Frame #6) captured by the same imaging assembly. As a result, four difference image frames (“Difference Image Frame #3” to “Difference Image Frame #6”) are generated for each imaging assembly having ambient light removed. The difference image frames (“Difference Image Frame #3” to “Difference Image Frame #6”) are processed to determine one or more pointer analysis regions to determine the identify of any pen tool brought into proximity with the input area  462 , similar to that described above. 
         [0092]    Although it is described above that each imaging assembly comprises a pair of associated IR LEDs, those skilled in the art will appreciate that the entire interactive input system may utilize only a single pair of IR LEDs in addition to the illuminated bezel. In this embodiment, the image frame capture sequence comprises four (4) image frames. The first image frame of each sequence is captured with the illuminated bezel  472  OFF and with the IR LEDs OFF, so as to obtain a background image frame. The second image frame of each sequence is captured with the illuminated bezel  472  ON and with the IR LEDs OFF, so as to obtain a preliminary illuminated image frame. The first two image frames in the sequence are used to determine the location of a pen tool, using triangulation. The next image frame is captured with the illuminated bezel  472  OFF, a first one of the IR LEDs ON, and a second one of the IR LEDs OFF. The final image frame is captured with the illuminated bezel OFF, the first one of the IR LEDs OFF, and the second one of the IR LEDs ON. The image frames are then processed similar to that described above to detect the location of a pen tool and to identify the pen tool. 
         [0093]    Pen tool  220  and pen tool  220 ′ may also be used with other interactive input systems. For example,  FIG. 16  shows another embodiment of an interactive input system  600  comprising an assembly  622  surrounding a display surface of a front projection system. The front projection system utilizes a projector  698  that projects images on the display surface. Imaging assemblies  660  positioned at the bottom corners of the assembly  622  look across the display surface. Each imaging assembly  660  is generally similar to imaging assembly  60  described above and with reference to  FIGS. 1 to 11 , and comprises an image sensor (not shown) and a set of IR LEDs (not shown) mounted on a housing assembly (not shown). A DSP unit (not shown) receives image frames captured by the imaging assemblies  660  and carries out the image frame processing method described above. 
         [0094]      FIG. 17  shows another embodiment of an interactive input system using a front projection system. Interactive input system  700  comprises a single imaging assembly  760  positioned in proximity to a projector  798  and configured for viewing a display surface. Imaging assembly  760  is generally similar to imaging assembly  60  described above and with reference to  FIGS. 1 to 11 , and comprises an image sensor and a set of IR LEDs mounted on a housing assembly. A DSP unit receives image frames captured by the imaging assembly  760  and carries out the image frame processing method described above. 
         [0095]      FIG. 18  shows another embodiment of an interactive input system in the form of a touch table, and which is generally referred to using reference numeral  800 . Interactive input system  800  is similar to that described in above-mentioned U.S. Patent Application Publication No. 2011/0006981, the relevant portions of the disclosure of which are incorporated herein by reference. Interactive input system  800  comprises twelve (12) imaging assemblies  870   a  to  870   l  positioned about the periphery of the input area  862 , and which look generally across an input area  862 . An illuminated bezel (not shown) surrounds the periphery of the input area  862  and generally overlies the imaging assemblies  870   a  to  870   l . The illuminated bezel provides backlight illumination into the input area  862 . Interactive input system  800  operates in pointer detection and pointer identification modes, as will be described below. 
         [0096]    In this embodiment, a set of IR LEDs  874   a  to  874   d  is positioned adjacent each of the four (4) corner imaging assemblies  870   a  to  870   d . Each set of IR LEDs  874   a  to  874   d  comprises three (3) IR LEDS. In particular, the set of IR LEDs  874   a  comprises IR LEDs  874   a - 1 ,  874   a - 2  and  874   a - 3 , the set of IR LEDs  874   b  comprises IR LEDs  874   b - 1 ,  874   b - 2  and  874   b - 3 , the set of IR LEDs  874   c  comprises IR LEDs  874   c - 1 ,  874   c - 2  and  874   c - 3 ; and the set of IR LEDs  874   c  comprises IR LEDs  874   d - 1 ,  874   d - 2  and  874   d - 3 . In this embodiment, IR LEDs  874   a - 1 ,  874   b - 1 ,  874   c - 1  and  874   c - 1  emit infrared illumination at a wavelength of 780 nm, IR LEDs  874   a - 2 ,  874   b - 2 ,  874   c - 2  and  874   c - 2  emit infrared illumination at a wavelength of 850 nm, and IR LEDs  874   a - 3 ,  874   b - 3 ,  874   c - 3  and  874   c - 3  emit infrared illumination at a wavelength of 940 nm. The IR LEDs of each set of IR LEDs  874   a  to  874   d  are configured to flood the input area  862  with infrared illumination. 
         [0097]      FIGS. 19   a  to  19   c  show a first type of pen tool  920  for use with the interactive input system  800 . As can be seen, pen tool  920  is similar to pen tool  220  shown in  FIGS. 6   a  and  6   b , with the addition of an eraser end  940 . In particular, pen tool  920  has a main body  222  terminating in a generally conical tip  224 . A filtered reflector  226  is provided on the main body  222  adjacent the tip  224 . Filtered reflector  226  comprises a reflective element  228  and a filtering element  230 . Reflective element  228  encircles a portion of the main body  222 . Filtering element  230  is positioned atop and circumscribes reflective element  228 . An eraser end  940  is positioned at the end of the main body  222  opposite that of conical tip  224 . A filtered reflector  942  is positioned on the main body  222  at the eraser end  940  and comprises a reflective element  944  and a filtering element  946 . The reflective element  944  encircles a portion of the main body  222  and is formed of a retro-reflective material such as for example retro-reflective tape. The filtering element  946  is positioned atop and circumscribes the reflective element  944 . 
         [0098]      FIGS. 20   a  to  20   c  show a second type of pen tool  920 ′ for use with the interactive input system  800  that is similar to pen tool  920 . As can be seen, pen tool  920 ′ has a main body  222 ′ terminating in a generally conical tip  224 ′. A filtered reflector  226 ′ is provided on the main body  222 ′ adjacent the tip  224 ′. Filtered reflector  226 ′ comprises a reflective element  228 ′ and filtering elements  230   a ′ and  230   b ′. Reflective element  228 ′ encircles a portion of the main body  222 ′. Filtering element  230   a ′ is positioned atop and circumscribes a lower portion of reflective element  228 ′ and filtering element  230   b ′ is positioned atop and circumscribes an upper portion of reflective element  228 ′. The filtering elements  230   a ′ and  230   b ′ have different bandpass wavelength ranges. An eraser end  940 ′ is positioned at the end of the main body  222 ′ opposite that of conical tip  224 ′. A filtered reflector  942 ′ is positioned on the main body  222 ′ at the eraser end  940 ′ and comprises a reflective element  944 ′ and a filtering element  946 ′. The reflective element  944 ′ encircles a portion of the main body  222 ′ and is formed of a retro-reflective material such as retro-reflective tape. The filtering element  946 ′ is positioned atop and circumscribes the reflective element  944 ′. 
         [0099]    In this embodiment, interactive input system  800  is able to identify four (4) different pen tools, namely two (2) pen tools  920  of the first type (Black and Green) and two (2) pen tools  920 ′ of the second type (Red and Blue). Each first type of pen tool  920  has a particular filtering element  230  used to identify the pen tool. Each second type of pen tool  920 ′ has particular filtering elements  230   a ′ and  230   b ′ used to identify pen tool. All of the pen tools have a filtering element  944 ,  944 ′ positioned adjacent the eraser end  940 ,  940 ′ used to detect when the pen tools are being used as an eraser. The four different pen tools  920  and  920 ′ and the bandpass wavelength ranges of their corresponding filtering elements are shown in Table 1 below: 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Filtering element(s) 
               
               
                 Pen Tool 
                 Pen Tool 
                 Filtering 
                 bandpass wavelength 
               
               
                 ID 
                 Type 
                 Element 
                 range (±12 nm) 
               
               
                   
               
             
             
               
                 Black 
                 pen tool 920 
                 230 
                 940 nm 
               
               
                 Red 
                 pen tool 920′ 
                 230a′ and 230b′ 
                 940 nm and 850 nm 
               
               
                 Green 
                 pen tool 920 
                 230 
                 850 nm 
               
               
                 Blue 
                 pen tool 920′ 
                 230a′ and 230b′ 
                 940 nm and 780 nm 
               
               
                 Eraser 
                 pen tool 920 
                 944 and 944′ 
                 780 nm 
               
               
                   
                 and 920′ 
               
               
                   
               
             
          
         
       
     
         [0100]    As mentioned previously, the interactive input system  800  operates in pointer detection and pointer identification modes. A flowchart of the method of operation of the interactive input system  800  is shown in  FIG. 21  and is generally identified by reference numeral  1000 . In the pointer detection mode, the interactive input system  800  uses the twelve imaging assemblies  870   a  to  870   l  (step  1002 ). During operation in the pointer detection mode, processing structure of interactive input system  800  utilizes the weight matrix method disclosed in above-incorporated PCT Application No. PCT/CA2010/001085 to Morrison et al. 
         [0101]    During operation in the pointer detection mode, a pointer detection image frame capture sequence is performed using the twelve imaging assemblies  870   a  to  870   l . Generally, the pointer detection image frame capture sequence comprises eight (8) stages Stage #1 to Stage #8. During the odd numbered stages, that is, Stages #1, #3, #5 and #7, the illuminated bezel and imaging assemblies operate in four phases. The four phases of illuminated bezel illumination are shown in  FIG. 22 . As can be seen, in phase 0 the west side of the illuminated bezel is OFF, while the remaining sides are ON. In phase 1 the north side of the illuminated bezel is OFF, while the remaining sides are ON. In phase 2 the east side of the illuminated bezel is OFF, while the remaining sides are ON. In phase 3 the south side of the illuminated bezel is OFF, while the remaining sides are ON. 
         [0102]    Table 2 below shows the imaging assemblies that are on during each of the four phases. As will be appreciated, in Table 2, “ON” is used to indicate that an imaging assembly is capturing an image frame whereas “OFF” is used to indicate that an imaging assembly is not used to capture an image frame. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Imaging 
                   
                   
                   
                   
               
               
                   
                 Assembly 
                 Phase 0 
                 Phase 1 
                 Phase 2 
                 Phase 3 
               
               
                   
                   
               
             
             
               
                   
                 870a 
                 ON 
                 OFF 
                 OFF 
                 OFF 
               
               
                   
                 870b 
                 OFF 
                 ON 
                 OFF 
                 OFF 
               
               
                   
                 870c 
                 OFF 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                 870d 
                 OFF 
                 OFF 
                 OFF 
                 ON 
               
               
                   
                 870e 
                 OFF 
                 ON 
                 OFF 
                 OFF 
               
               
                   
                 870f 
                 OFF 
                 ON 
                 OFF 
                 OFF 
               
               
                   
                 870g 
                 OFF 
                 OFF 
                 OFF 
                 ON 
               
               
                   
                 870h 
                 OFF 
                 OFF 
                 OFF 
                 ON 
               
               
                   
                 870i 
                 ON 
                 OFF 
                 OFF 
                 OFF 
               
               
                   
                 870j 
                 OFF 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                 870k 
                 ON 
                 OFF 
                 OFF 
                 OFF 
               
               
                   
                 870l 
                 OFF 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                   
               
             
          
         
       
     
         [0103]    During the even numbered stages, that is, Stages #2, #4, #6 and #8, the illuminated bezel is off and the imaging assemblies operate in four phases, similar to that shown in Table 2 above. 
         [0104]    Once the image frames have been captured, the image frames are processed according to an image frame processing method. The image frames captured during Stages #2, #4, #6 and #8 are summed together and the resultant image frame is used as a background image frame. To reduce the effects of ambient light, difference image frames are calculated by subtracting the background image frame from the image frames captured during Stages #1, #3, #5 and #7. The difference image frames are then examined for values that represent the bezel and possibly one or more pointers. Methods for determining pointer location within image frames are described in U.S. Patent Application Publication No. 2009/0277697 to Bolt et al., filed on May 9, 2008, and assigned to SMART Technologies ULC, the relevant portions of the disclosure of which are incorporated herein by reference. As mentioned above, when a pointer exists in a captured image frame, the pointer occludes illumination and appears as a dark region interrupting a bright band. Thus, the bright bands in the difference image frames are analyzed to determine the locations of dark regions. 
         [0105]    A check is performed to determine if a new pointer is detected (step  1004 ) by determining for example, if the number of detected pointers in the current pointer detection image frame capture sequence has increased as compared to the previous pointer detection image frame capture sequence and/or the location of one or more pointers has changed by more than a threshold amount over the previous and current pointer detection image frame capture sequences. If no new pointer has been detected, the interactive input system  800  continues to operate in the pointer detection mode (step  1002 ). If a new pointer is detected at step  1004 , the interactive input system  800  is conditioned to operate in the pointer identification mode (step  1006 ). During operation in the pointer identification mode, a pointer identification image frame capture sequence is performed by the two corner imaging assemblies  870   a  to  870   d  that are closest to the new pointer to identify the new pointer. The remaining imaging assemblies capture image frames according to the pointer detection image frame capture sequence described above (step  1008 ). 
         [0106]      FIG. 23  shows a portion of an exemplary pointer identification image frame capture sequence  1060  used by the two closest corner imaging assemblies during operation of the interactive input system  800  in the pointer identification mode. In this example, the two corner imaging assemblies used to identify the new pointer are imaging assemblies  870   a  and  870   b . The corner imaging assemblies  870   a  and  870   b  remain idle until Stage #1 of the pointer detection image frame capture sequence is complete. Once Stage #1 is complete, an image frame is captured by the imaging assemblies  870   a  and  870   b  with IR LEDs  874   a - 1  and  874   b - 1  ON (“Image Frame G”) and with the illuminated bezel OFF. The corner imaging assemblies  870   a  and  870   b  then remain idle until Stages #2 and #3 of the pointer detection image frame capture sequence are complete. Once Stages #2 and #3 are complete, an image frame is captured by the imaging assemblies  870   a  and  870   b  with all IR LEDs OFF (“Background Image Frame”). The corner imaging assemblies  870   a  and  870   b  then remain idle until Stages #4 and #5 of the pointer detection image frame capture sequence are complete. Once Stages #4 and #5 are complete, an image frame is captured by the imaging assemblies  870   a  and  870   b  with IR LEDs  874   a - 2  and  874   b - 2  ON (“Image Frame B”) and with the illuminated bezel OFF. The corner imaging assemblies  870   a  and  870   b  then remain idle until Stages #6 and #7 of the pointer detection image frame capture sequence are complete. Once Stages #6 and #7 are complete, an image frame is captured by the imaging assemblies  870   a  and  870   b  with IR LEDs  874   a - 3  and  874   b - 3  ON (“Image Frame R”) and with the illuminated bezel OFF. Stage #8 of the pointer detection image frame capture sequence is then performed. 
         [0107]    Once the sequence of image frames has been captured, the Background Image Frame is subtracted from Image Frame G, Image Frame B and Image Frame R resulting in Difference Image Frame G, Difference Image Frame B and Difference Image Frame R, respectively. The three Difference Image Frames R, G and B are processed to determine the identity of any pen tool brought into proximity with the input area  862  (step  1010 ). 
         [0108]    In this embodiment, the three Difference Image Frames R, G and B are processed to define one or more pointer analysis regions and to calculate an intensity signal corresponding to the presence of the new pointer. The intensity signals are calculated according to Equations (1) to (3) below: 
         [0000]        R   pen =Σ x=X 0 −WX 1 +W   R ( x )  (1)
 
         [0000]        G   pen =Σ x=X 0 −WX 1 +W   G ( X )  (2)
 
         [0000]        B   pen =Σ x=X 0 −WX 1 +W   B ( x )  (3)
 
         [0000]    wherein the pointer analysis region is defined between columns X 0 &lt;x&lt;X 1 , and W is a predefined widening factor. 
         [0109]    The maximum value of the intensity signals R pen , G pen  and B pen  is determined and compared to an intensity threshold. If the maximum value is below the intensity threshold, it is determined that the new pointer is not a pen tool and the interactive input system  800  reverts back to operation in the pointer detection mode using all twelve imaging assemblies. If the maximum value is above the intensity threshold, it is determined that the new pointer is a pen tool and the intensity signals are normalized according to Equations (4) to (7) below so that the maximum value of the intensity signals is set to unity: 
         [0000]        m =max( R   pen   ,G   pen   ,B   pen )  (4)
 
         [0000]        R   n   =R   pen   /m   (5)
 
         [0000]        G   n   =G   pen   /m   (6)
 
         [0000]        B   n   =B   pen   /m   (7)
 
         [0110]    The normalized intensity signals R n , G n  and B n  are compared to respective threshold values R t , G t  and B t  to identify the pen tool. Table 3 shows the criteria for identifying the pen tools of Table 1: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Pen Tool ID 
                 R n  &gt; R t ? 
                 G n  &gt; G t   
                 B n  &gt; B t   
               
               
                   
                   
               
             
             
               
                   
                 Black 
                 YES 
                 NO 
                 NO 
               
               
                   
                 Red 
                 YES 
                 YES 
                 NO 
               
               
                   
                 Green 
                 NO 
                 YES 
                 NO 
               
               
                   
                 Blue 
                 YES 
                 NO 
                 YES 
               
               
                   
                 Eraser 
                 NO 
                 NO 
                 YES 
               
               
                   
                   
               
             
          
         
       
     
         [0111]    Once the new pen tool is identified (in step  1010 ), the method returns to step  1002  wherein the interactive input system  800  operates in the pointer detection mode. 
         [0112]    In another embodiment, a tool tray similar to that described above may be used with interactive input system  800 . In this embodiment, the tool tray is used to support one or more of the pen tools  920  and  920 ′. When operating in the pointer identification mode, image frames captured by imaging assemblies  870   a  and  870   b  include images of the tool tray and any pen tools supported thereon. As such, the interactive input system  800  is able to determine when a pen tool is removed from the tool tray, is able to identify the removed pen tool, and can assume that the next detected pen tool brought into proximity with the input area  862  is the removed pen tool. 
         [0113]    Although interactive input system  800  is described as comprising four sets of three infrared LEDs positioned adjacent to respective imaging assemblies  870   a  to  870   d , those skilled in the art will appreciate that variations are available. For example, in another embodiment four sets of two infrared LEDs may be positioned adjacent the respective imaging assemblies  870   a  to  870   d . It will be appreciated that in this embodiment two different pen tools  920  and  920 ′ of the first and second types may be identified. To identify additional pen tools, more infrared LEDs may be used. For example, in another embodiment, four sets of four infrared LEDs may be positioned adjacent to respective imaging assemblies  870   a  to  870   d.    
         [0114]    Although during operation in the pointer identification mode the interactive input system  800  is described as using the two corner imaging assemblies closest to the new pointer, those skilled in the art will appreciate that alternative are available. For example, in another embodiment the interactive input system  800  may use all four corner imaging assemblies for new pointer identification. In another embodiment, in the event the new pointer is within a threshold distance from one of the corner imaging assemblies, that imaging assembly is not used for pointer identification and thus the next closest corner imaging assembly is used in its place. In another embodiment, in the event that the new pointer and another pointer are in proximity within the input area  862  and the new pointer is occluded and cannot be seen by one of the corner imaging assemblies, that imaging assembly is not used for pointer identification but rather the next closest corner imaging assembly is used in its place. 
         [0115]    Pen tools  920  and  920 ′ are not only for use with interactive input system  800  described above. For example, in another embodiment, an interactive input system similar to interactive input system  20  may be used. In this embodiment, rather than the infrared LEDs being positioned on the housing of the imaging assemblies, as is the case with interactive input system  20 , the infrared LEDs are positioned adjacent to the imaging assemblies, similar to that of interactive input system  800 . In this embodiment, three infrared LEDs are positioned adjacent each imaging assembly. Each one of the three infrared LEDs emits infrared illumination at a particular wavelength, which in this embodiment is 780 nm, 850 nm and 940 nm. As will be appreciated, in this embodiment, the interactive input system is able to track multiple pen tools brought into proximity with the input area but is not able to assign a unique ID to each pen tool. 
         [0116]    The two different pen tools  920  and  920 ′ and their corresponding filtering element(s) used in this embodiment are shown in Table 4: 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 Filtering element(s) 
               
               
                 Pen Tool 
                 Pen Tool 
                 Filtering 
                 bandpass wavelength 
               
               
                 ID 
                 Type 
                 Element 
                 range (±12 nm) 
               
               
                   
               
             
             
               
                 Black 
                 pen tool 920 
                 230 
                 940 nm 
               
               
                 Red 
                 pen tool 920′ 
                 230a′ and 230b′ 
                 940 nm and 850 nm 
               
               
                 Eraser 
                 pen tool 920 
                 944 and 944′ 
                 780 nm 
               
               
                   
                 and 920′ 
               
               
                   
               
             
          
         
       
     
         [0117]    An image frame capture sequence is performed by the imaging assemblies of the interactive input system, similar to image frame capture sequence  1060  described above. Generally, three image frames R, G and B are captured by each imaging assembly. Each image frame R, G and B corresponds to an image frame captured when a respective IR LED is ON. Difference image frames R, G and B are calculated as described above. Intensity signals R(x), G(x) and B(x) are calculated and compared to the intensity signals R b (x), G b (x) and B b (x) of the corresponding background image frame to determine if a pointer has been brought into proximity with the interactive surface. In this embodiment, if the intensity signal R(x), G(x) and B(x) is less than 75% of the respective intensity signal R b (x), G b (x) and B b (x) of the corresponding background image frame, it is determined that a pointer has been brought into proximity with the interactive surface. For example, if the intensity signal R(x)&lt;0.75R b (x), it is determined than a pointer has been brought into proximity with the interactive surface. For calculation, it is assumed that the pointer is not a pen tool. 
         [0118]    To test if the pointer is a pen tool, the intensity signal R(x) is used for calculating predicted intensity signals G p (x) and B p (x) for the intensity signals G(x) and B(x). This is because both pen tools in Table 3 would appear in the image frame R captured when the IR LED emitting a wavelength of 940 nm is ON. The predicted intensity signals G p (x) and B p (x) are calculated according to Equations (8) and (9) below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       G 
                       p 
                     
                      
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         G 
                         b 
                       
                        
                       
                         ( 
                         x 
                         ) 
                       
                     
                      
                     
                       
                         R 
                          
                         
                           ( 
                           x 
                           ) 
                         
                       
                       
                         
                           R 
                           b 
                         
                          
                         
                           ( 
                           x 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       B 
                       p 
                     
                      
                     
                       ( 
                       x 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         B 
                         b 
                       
                        
                       
                         ( 
                         x 
                         ) 
                       
                     
                      
                     
                       
                         R 
                          
                         
                           ( 
                           x 
                           ) 
                         
                       
                       
                         
                           R 
                           b 
                         
                          
                         
                           ( 
                           x 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0119]    The predicted intensity signals G p (x) and B p (x) are subtracted from the intensity signals G(x) and B(x) to calculate residual intensity signals and the residual intensity signals are summed according to Equations (10) to (12) below: 
         [0000]        R   pen =0  (10)
 
         [0000]        G   pen =Σ x=X     0     −WX     1     +W   [G ( x )− G   p ( x )]  (11)
 
         [0000]        B   pen =Σ x=X     0     −WX     1     +W   [B ( x )− B   p ( x )  (12)
 
         [0000]    wherein the pointer analysis region is defined between columns X 0 &lt;x&lt;X 1 , and W is a predefined widening factor. 
         [0120]    The residual intensity signals R pen , G pen  and B pen  represent the signal coming from the reflective element of the pen tool with the signal from the retro-reflective bezel removed. 
         [0121]    The above calculations are repeated to test if the pointer is the eraser end of the pen tool. As will be appreciated, in this case, the intensity signal B(x) is used for calculating predicted intensity signals G p (x) and R p (x) for the intensity signals G(x) and R(x). This is because the eraser in Table 3 would appear in image frame B captured when the IR LED emitting a wavelength of 780 nm is ON. The intensity signals R pen  and G pen  are calculated similar to that described above, wherein the intensity signal B pen  is set to zero. 
         [0122]    The residual intensity signals R pen , G pen  and B pen  calculated are interpreted to determine if the pointer is a pen tool or the eraser end of the pen tool similar to that described above with reference to Equations (4) to (7) and Table 3. Table 5 below shows the criteria for identifying each pen tool of Table 4: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Pen Tool ID 
                 R n  &gt; R t ? 
                 G n  &gt; G t   
                 B n  &gt; B t   
               
               
                   
                   
               
             
             
               
                   
                 Black 
                 YES 
                 NO 
                 NO 
               
               
                   
                 Red 
                 YES 
                 YES 
                 NO 
               
               
                   
                 Eraser 
                 NO 
                 NO 
                 YES 
               
               
                   
                   
               
             
          
         
       
     
         [0123]    In another embodiment, image frames captured by imaging assemblies of interactive input system  110  may include images of the tool tray and any pen tools supported thereon. As such, the interactive input system is able to determine when a pen tool is removed from the tool tray, is able to identify the removed pen tool, and can assume that the next detected pen tool brought into proximity with the interactive surface is the removed pen tool. 
         [0124]    Although in embodiments described above difference image frames are obtained by subtracting background image frames from illuminated image frames, where the background image frames and the illuminated image frames are captured successively, in other embodiments, the difference image frames may be obtained using an alternative approach. For example, the difference image frames may be obtained by dividing the background image frames by the illuminated image frames, or vice versa. In still other embodiments, non-successive image frames may be used for obtaining the difference image frames. 
         [0125]    Although in embodiments described above the pointer analysis region is described as being square shaped, those skilled in the art will appreciate that the pointer analysis region may be another shape such as for example rectangular, circular, etc. Also, although in the embodiments described above, the light sources emit infrared illumination, in other embodiments, illumination of other wavelengths may alternatively be emitted. 
         [0126]    Although in embodiments described above, IR-bandpass filters having wavelengths of about 830 nm±12 nm and about 880 nm±12 nm are employed, those skilled in the art will appreciate that high pass filters may be used. For example, in another embodiment a high pass filter having a passband above about 750 nm may be associated with each located pointer. 
         [0127]    Although in embodiments described above a single pointer analysis region is associated with each located pointer, in other embodiments, multiple pointer analysis regions may be associated with each located pointer. 
         [0128]    Although preferred embodiments have been described, those of skill in the art will appreciate that variations and modifications may be made with departing from the scope thereof as defined by the appended claims.