Patent Publication Number: US-2018039344-A1

Title: Coordinate detection apparatus, electronic blackboard, image display system, and coordinate detection method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application filed under 35 U.S.C. 111(a) claiming the benefit under 35 U.S.C. 120 and 365(c) of a PCT International Application No. PCT/JP2016/062505 filed on Apr. 20, 2016, which is based upon and claims the benefit of priority of Japanese Patent Application No. 2015-085954 filed on Apr. 20, 2015, and Japanese Patent Application No. 2016-083906 filed on Apr. 19, 2016, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coordinate detection apparatus, an electronic blackboard, an image display system, and a coordinate detection method. 
     2. Description of the Related Art 
     Electronic blackboards have been widely used that include a display of a liquid crystal panel or the like, a coordinate detection apparatus to detect coordinates on the display pointed by a user using a pointer such as an electronic pen, and a control unit to draw and display various images based on coordinate data output from the coordinate detection apparatus. 
     As a coordinate detection apparatus used for such an electronic blackboard, various methods of detecting coordinates have been proposed. Also, there has been demand for drawing various images by either of a light-emitting pointer equipped with a light emitting part, or a non-light-emitting pointer such as a finger. 
     To cope with such demand, a coordinate detection apparatus has been devised that detects coordinates of multiple pointers (see, for example, Patent document 1). Patent document 1 discloses a coordinate detection apparatus that causes propagation light rays having different wavelengths from each other to be incident on a light-guiding plate when a light-emitting pointer contacts the surface of the light-guiding plate, and extracts the propagation light rays having the different wavelengths by filters, respectively. Furthermore, the coordinate detection apparatus in Patent document 1 detects dispersion light of a light source emitted along the light-guiding plate dispersed by a non-light-emitting pointer (a finger) so as to detect the coordinates of the non-light-emitting pointer. Therefore, the coordinates of both the light-emitting pointer and the non-light-emitting pointer can be detected. 
     RELATED-ART DOCUMENTS 
     Patent Documents 
     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2013-175142 
     However, the coordinate detection apparatus disclosed in the Patent document 1 has a problem in that a light-guiding plate is required on the surface of the display. The necessity of a light-guiding plate on the surface of the display tends to generate a parallax between a contact position of the pointer and the display position on the image display unit (a difference that is generated between the position pointed by the user, and the drawing position of an image), to the extent of the thickness of the light-guiding plate. In addition, installing a coordinate detection apparatus that includes a light-guiding plate on an ordinary display tends to increase the manufacturing cost, which may increase the overall cost. 
     SUMMARY OF THE INVENTION 
     According to an embodiment, a coordinate detection apparatus for detecting coordinates of a light-emitting pointer and a non-light-emitting pointer on a display surface, the coordinate detection apparatus includes an imaging unit disposed on the display surface, and configured to capture an image of at least one of the light-emitting pointer and the non-light-emitting pointer in a predetermined range from the display surface; a detection unit disposed on the display surface, and configured to detect a quantity of light generated by the light-emitting pointer emitting light in the predetermined range so as to detect the coordinates of the light-emitting pointer, and to detect a quantity of light that is emitted on the display surface and is cut off by the non-light-emitting pointer in the predetermined range so as to detect the coordinates of the non-light-emitting pointer; and a light emitter unit configured to emit light having a wavelength different from that emitted by the light-emitting pointer in a predetermined range from the display surface. The imaging unit includes a first filter that transmits light having the wavelength output by the light-emitting pointer, and a second filter that transmits light having the wavelength emitted by the light emitter unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an example of a diagram illustrating an overview of a detection method of coordinates of a light-emitting pointer or coordinates of a non-light-emitting pointer; 
         FIG. 1B  is an example of a diagram illustrating an overview of a detection method of coordinates of a light-emitting pointer or coordinates of a non-light-emitting pointer; 
         FIG. 2  is an example of a perspective view illustrating an example of an appearance of an electronic blackboard; 
         FIG. 3  is an example of a configuration diagram illustrating a hardware configuration of an electronic blackboard; 
         FIG. 4  is an example of a hardware configuration diagram of a light-emitting pointer; 
         FIG. 5  is an example of a schematic view illustrating a configuration of a coordinate detection apparatus; 
         FIG. 6  is an example of a perspective view of a light reception/emission device; 
         FIG. 7  is an example of a diagram schematically illustrating a system configuration example including an electronic blackboard; 
         FIG. 8  is an example of a block diagram illustrating a control system of an electronic blackboard; 
         FIG. 9  is an example of a block diagram illustrating a hardware configuration of a coordinate detection apparatus of an electronic blackboard; 
         FIG. 10  is an example of a functional block diagram of a controller included in an electronic blackboard; 
         FIG. 11  is an example of a diagram illustrating calculation of a position of a light-emitting pointer having a light-emitting element installed; 
         FIG. 12A  is a diagram schematically illustrating a quantity of light sensed by a light reception sensor such as a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 12D  is a diagram schematically illustrating a quantity of light sensed by a light reception sensor such as a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 13  is an example of a flowchart illustrating steps by which a light reception/emission control circuit detects coordinates of a light-emitting pointer; 
         FIG. 14  is an example of a diagram illustrating detection of a position of a non-light-emitting pointer not having a light-emitting element installed, such as a finger; 
         FIG. 15A  is a diagram schematically illustrating a quantity of light sensed by a light reception sensor such as a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 15B  is a diagram schematically illustrating a quantity of light sensed by a light reception sensor such as a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 16  is an example of a flowchart illustrating steps by which a light reception/emission control circuit detects coordinates of a non-light-emitting pointer; 
         FIG. 17  is an example of a schematic perspective view of a light reception/emission device (second embodiment); 
         FIG. 18A  is a diagram schematically illustrating an example of a quantity of light detected by a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 18B  is a diagram schematically illustrating an example of a quantity of light detected by a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 18C  is a diagram schematically illustrating an example of a quantity of light detected by a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 18D  is a diagram schematically illustrating an example of a quantity of light detected by a light reception/emission device  300 - 1  or  300 - 2 ; 
         FIG. 19  is an example of a flowchart illustrating operational steps of a coordinate detection apparatus (second embodiment); 
         FIG. 20  is a diagram illustrating an example of a light-emitting pointer distant from a light reception sensor; 
         FIG. 21A  is an example of a diagram illustrating a quantity of light detected by a light reception sensor of a light reception/emission device in a state illustrated in  FIG. 20 ; 
         FIG. 21B  is an example of a diagram illustrating a quantity of light detected by a light reception sensor of a light reception/emission device in a state illustrated in  FIG. 20 ; 
         FIG. 22A  is a diagram illustrating an example of a light reception/emission device of a coordinate detection apparatus (third embodiment); 
         FIG. 22B  is a diagram illustrating an example of a light reception/emission device of a coordinate detection apparatus (third embodiment); 
         FIG. 22C  includes diagrams illustrating an example of a light reception/emission device of a coordinate detection apparatus (third embodiment); 
         FIG. 23  is an example of a flowchart illustrating operational steps of a coordinate detection apparatus (third embodiment); 
         FIG. 24  is an example of a schematic configuration diagram of a coordinate detection system that includes a projector and a coordinate detection apparatus; 
         FIGS. 25A-25D  are diagrams schematically illustrating an extension process of retroreflector plates integrated with a coordinate detection apparatus; 
         FIG. 26  is an example of a diagram schematically illustrating a system configuration example of a coordinate detection system; 
         FIG. 27  is an example of a schematic configuration diagram illustrating operational steps of a coordinate detection system in a case where a light-emitting pointer emits light; and 
         FIG. 28  is an example of a schematic configuration diagram illustrating operational steps of a coordinate detection system in a case where a non-light-emitting pointer cuts off infrared light. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the drawings. According to an embodiment, it is possible to provide a coordinate detection apparatus that can detect coordinates of a light-emitting pointer and a non-light-emitting pointer. 
     First Embodiment 
       FIGS. 1A and 1B  are examples of diagrams illustrating overviews of detection methods of coordinates of a light-emitting pointer, and coordinates of a non-light-emitting pointer  200 , respectively.  FIG. 1A  is a diagram illustrating a detection method of coordinates of the non-light-emitting pointer  200 . In principle, a coordinate detection apparatus  24  causes a light emitter  420  to constantly emit light, and infrared light emitted from the light emitter  420  is reflected by retroreflector plates and sensed by light reception sensors  410 . If the user brings the tip of the non-light-emitting pointer  200  (such as a finger) close to a display surface  22  of an image display unit  20 , the light reception sensors  410  at the upper left corner and the upper right corner detect the non-light-emitting pointer  200  as a dark spot, respectively. A light reception/emission control circuit  350  obtains angles θ 3  (a third position) and θ 4  (a fourth position) from the position of the dark spot in each of the light reception sensors  410 , to calculate coordinates (x 2 , y 2 ) of the non-light-emitting pointer  200  by the principle of triangulation. 
       FIG. 1B  is a diagram illustrating a detection method of coordinates of a light-emitting pointer. A light-emitting pointer  100  can wirelessly communicate with the coordinate detection apparatus  24 , and if the user makes the tip of the light-emitting pointer  100  approach the display surface  22 , and further contact the display surface  22  (or press against it), transmits a signal representing that a pen pressure has been detected (referred to as a “writing detection signal”, below) to the coordinate detection apparatus  24 . The coordinate detection apparatus  24  exceptionally turns off the light emitter  420  until receiving a signal representing that the tip of the light-emitting pointer  100  is separated from the display surface  22  (referred to as a “writing completion signal”, below), or while receiving the writing detection signal. Besides, when the user makes the tip of the light-emitting pointer  100  contact the image display unit  20 , the tip of the light-emitting pointer  100  emits light. The light reception sensors  410  at the upper left corner and the upper right corner detect the light-emitting pointer  100  as a bright spot, respectively. The light reception/emission control circuit  350  obtains angles θ 1  (a first position) and θ 2  (a second position) from the position of the bright spot in each of the light reception sensors  410 , to calculate coordinates (x 1 , y 1 ) of the light-emitting pointer  100  by the principle of triangulation. 
     In this way, even if no light-guiding plate is provided in the image display unit  20 , the coordinate detection apparatus in the embodiment can distinguish and detect the coordinates of the light-emitting pointer  100  and the non-light-emitting pointer  200 . In addition, no necessity of a light-guiding plate can prevent a cost increase, and makes it possible to display an image with little or no parallax occurring. Note that even if the image display unit  20  is equipped with a light-guiding plate, the coordinate detection method in the embodiment can be applied effectively. 
     &lt;Terms&gt; 
     Information displayed on a display surface simply needs to be information that can be visually recognized, specifically, a character, a figure, a still picture, a video, and the like. Besides, the information may be provided by handwriting, or may be saved in advance. 
     Detecting coordinates means obtaining coordinates. A process for obtaining coordinates is not specifically limited, and may include sensing (detecting) by a sensor, calculation, and transformation during the course of the obtaining. 
     &lt;Configuration&gt; 
       FIG. 2  is a perspective view illustrating an example of an appearance of an electronic blackboard  10  in the embodiment. The electronic blackboard  10  includes an image display unit  20  as a display, a holding member  40  to hold this image display unit  20  at an appropriate height, and a device housing part  50  that houses a device or the like to control the image display unit  20 . Note that if the device can be contained in a bezel part  360  or the like, the device housing part  50  may be omitted. 
     The image display unit  20  includes a panel member such as a liquid crystal panel or a plasma panel. Also, a surface of the image display unit  20  on which an image is displayed is referred to as a display surface  22 , and the coordinate detection apparatus  24  that functions as a touch panel is placed in the surroundings of the display surface  22 . 
     Furthermore, on the electronic blackboard  10 , the user can write a character, a figure, and the like on the display surface  22  of the image display unit  20  by a light-emitting pointer  100 , which is a dedicated, pen-shaped input unit. The light-emitting pointer  100  is provided with a contact detection part  102  at a pen point  100 A, which will be described later, and if the pen point  100 A contacts the display surface  22  of the image display unit  20 , the light-emitting pointer  100  emits light, and at the same time, wirelessly transmits a writing detection signal. Accordingly, the coordinate detection apparatus  24  detects the coordinates of the light-emitting pointer  100 , and the image display unit  20  displays a dot at the detected coordinates. If the pen point  100 A is moved while contacting the display surface  22 , the electronic blackboard  10  displays an image such as a character and a figure, by connecting and drawing multiple dots. 
     Also, the back end switch  105  is installed at a pen bottom  100 B of the light-emitting pointer  100 , and if the pen bottom  100 B contacts the display surface  22  of the image display unit  20 , the light-emitting pointer  100  transmits an erasure detection signal to the image display unit  20 . In response to receiving an erasure detection signal transmitted from the light-emitting pointer  100 , the image display unit  20  erases an image, such as a character and a figure, written on the coordinates detected by the coordinate detection apparatus  24 , from the display surface  22  of the image display unit  20 . Note that as a display process for this erasure operation, a controller  60  as a display control unit, which will be described later, executes a process of setting the detected coordinate position to have the same color (for example, white) as set to the background of the image displayed on the display surface  22  of the image display unit  20 . 
     Furthermore, the light-emitting pointer  100  has a booster circuit embedded between a driving battery and the light-emitting element, to control the quantity of light emitted by the light-emitting element to be constant at all the time. If the remaining battery capacity becomes less than or equal to a default value, the light-emitting pointer  100  causes the light-emitting element to stop emitting the light. Also, the contact detection part  102  of the light-emitting pointer  100  detects a contact force on the display surface  22 , when the pen point  100 A contacts the display surface  22  of the image display unit  20 . For example, the contact detection part  102  may change the intensity of light of the light-emitting element, based on a detected value of the contact force. Depending on this intensity of the light output from the light-emitting element of the light-emitting pointer  100 , the controller  60  changes the thickness of lines of characters or figures written on the display surface  22  of the image display unit  20 . Alternatively, a detected value of a pressure-sensitive sensor may be wirelessly transmitted to the coordinate detection apparatus  24 . 
     The device housing part  50  houses various peripherals, for example, a controller, a printer, and a video disk device, as will be described later. Besides, an input unit  30 , which may be a keyboard to execute input operations, is installed on the upper surface of the device housing part  50 . 
       FIG. 3  is a configuration diagram illustrating a hardware configuration of the electronic blackboard  10 . As illustrated in  FIG. 3 , in addition to the image display unit  20 , the coordinate detection apparatus  24 , and the input unit  30  described above, the electronic blackboard  10  includes a CPU (Central Processing Unit)  501 , a ROM (Read-Only Memory)  502 , a RAM (Random Access Memory)  503 , an HDD (Hard Disk Drive)  504 , an HDC (Hard Disk Controller)  505 , a media drive  507 , an interface (I/F)  508 , a mouse  509 , a microphone  510 , a speaker  511 , a short-distance wireless communication unit  515 , a GPU (Graphics Processing Unit)  512 , and an extended bus line  520  that connects these units by address lines or data lines. 
     The CPU  501  controls overall operations of the electronic blackboard  10 . The ROM  502  stores a program used for operations of the CPU  501 , such as an IPL (Initial Program Loader). The RAM  503  is used as a work area of the CPU  501 . The HDD  504  stores various data items such as a program. The HDC  505  controls read and write of various data items on the HDD  504  under control of the CPU  501 . The media drive  507  controls read and write (storing) of data on a recording medium  506 , such as a flash memory. The interface  508  transmits data via a communication network, and connects a dongle used for preventing unauthorized use of software. The GPU  512  is connected to a ROM  513  that stores a program used for operations of the GPU  512 , and a RAM  514  that is used as a work area of the GPU  512 . The short-distance wireless communication unit  515  is a communication unit for wirelessly communicating mainly with the light-emitting pointer  100 . Specifically, communication is executed by a communication protocol such as Bluetooth (registered trademark), Bluetooth (registered trademark) Low Energy, and ZigBee (registered trademark). The extended bus line  520  is provided with an address bus, a data bus or the like for electrically connecting the components described above. 
       FIG. 4  illustrates an example of a hardware configuration diagram of the light-emitting pointer. The light-emitting pointer  100  includes a light-emitting element  110  that emits infrared light by an LED or the like installed at the pen point  100 A. The pen point  100 A is movable or deformable in the axis direction of the light-emitting pointer  100 , and a pen pressure generated when the pen point  100 A contacts the display surface  22  is detected by the contact detection part  102 . 
     The light-emitting pointer  100  also includes a wireless communication unit  103  that wirelessly communicates with the coordinate detection apparatus  24 , and the wireless communication unit  103  transmits a writing detection signal to the coordinate detection apparatus  24 . The pen bottom  100 B of the light-emitting pointer  100  is also movable or deformable in the axis direction, and if the pen bottom  100 B contacts the display surface  22 , the back end switch  105  detects the contact. 
     The light-emitting pointer  100  also includes a CPU  106  that controls the entire light-emitting pointer  100 , a RAM  107 , a ROM  108 , and an A/D converter  109 . The ROM  108  stores a program of the light-emitting pointer  100 , and the CPU  106  runs the program for the electronic pen, to provide functions as will be described in the following. Note that in addition to the illustrated components, the light-emitting pointer  100  includes generic components included in an information processing apparatus such as a microcomputer. Furthermore, the light-emitting pointer  100  may be implemented with other hardware components such as an ASIC and an FPGA. 
     The contact detection part  102  includes a high polymer pressure film or the like, and a pen pressure detected by the contact detection part  102  is transmitted to the A/D converter  109 . The A/D converter  109  converts an analog signal representing a pen pressure into a digital signal representing the pen pressure information. The CPU  106  compares a pen pressure with a threshold, and can detect that the pen point  100 A has contacted the display surface  22  (the CPU  106  generates a writing detection signal), and has been separated from the display surface  22  (in this case, the CPU  106  generates a writing completion signal). The CPU  106  causes the light-emitting element  110  to emit light if the pen point  100 A contacts the display surface  22 , and causes the light-emitting element  110  to turn off the light if the pen point  100 A is separated from the display surface  22 . This can reduce power consumption. Note that to “turn off” includes to make the quantity of light so weak that the light reception sensor  410  cannot detect the light. 
     Alternatively, the light-emitting element  110  may be always turned on. In this case, a sensor such as an acceleration sensor may be installed for estimating a use state of the user so that the CPU  106  determines whether the user is using the light-emitting pointer  100  based on the output of the sensor, and if not using, turns off the light. 
     If the pen bottom  100 B comes into contact with the display surface  22 , the back end switch  105  is turned on, and the CPU  106  detects a back-end-on signal. On the other hand, if the pen bottom  100 B becomes separated from the display surface  22 , the back end switch  105  is turned off, and the CPU  106  detects a back-end-off signal. 
     Furthermore, it is preferable for the light-emitting pointer  100  to store attribute information including a specific ID and the like in the ROM  108  or the like. Accordingly, even if there are multiple light-emitting pointers  100 , the coordinate detection apparatus  24  can identify a light-emitting pointer  100  that corresponds to a writing detection signal. For example, depending on the light-emitting pointer  100  being used, the electronic blackboard  10  can change the color, thickness, line type, and the like of a character, a figure, and the like. 
     The wireless communication unit  103  executes communication by a communication protocol such as Bluetooth (registered trademark), Bluetooth (registered trademark) Low Energy, and ZigBee (registered trademark). Other than these, the wireless communication unit  103  may execute communication by infrared light, wireless LAN, ultrasonic wave, visible light communication, and the like. The wireless communication unit  103  can transmit a writing detection signal, a writing completion signal, a back-end-on signal, a back-end-off signal, the ID, and the like to the coordinate detection apparatus  24 . Note that information transmitted from the light-emitting pointer  100  to the coordinate detection apparatus  24  is not limited to these signals. 
       FIG. 5  is an example of a schematic view illustrating a configuration of the coordinate detection apparatus, and  FIG. 6  is an example of a perspective view of a light reception/emission device  300 . As illustrated in  FIG. 5 , the coordinate detection apparatus  24 , which is a touch panel, has light reception/emission devices  300  (designated as  300 - 1 ,  300 - 2 ,  300 - 3 , and  300 - 4  if necessary to distinguish, below) disposed, for example, at the upper left corner, the upper right corner, the lower left corner, and the lower right corner of the display surface  22  of the image display unit  20 . The light reception/emission device  300  includes a light reception sensor  410  that detects infrared light, and a light emitter  420  that emits infrared light along the display surface  22  of the image display unit  20 . The infrared light along the display surface  22  is infrared light emitted nearly parallel with the display surface  22 , and just needs to be emitted in a predetermined range from the display surface  22 . Besides, the light emitted from the light emitter  420  does not need to be infrared light, for example, may be visible light or ultraviolet light. However, visible light tends to be noticed by the user when turned on and off, and ultraviolet light has a chemical effect. Therefore, infrared light (including near-infrared light) is preferable. 
     Also, at least two light reception/emission devices  300  may be provided, having one side of the display surface  22  interposed. Providing the two devices enables to detect the coordinates of the light-emitting pointer  100  and the non-light-emitting pointers  200 . Besides, as will be described later in a second embodiment, the coordinates of the light-emitting pointer  100  and the non-light-emitting pointer  200  may be detected at the same time. Note that if there are two light reception/emission devices  300 , it is possible to detect the coordinates of two or more light-emitting pointers at the same time, and to detect the coordinates of two or more non-light-emitting pointers  200  at the same time. Meanwhile, providing three or more light reception/emission devices  300  makes it possible to increase the number of light-emitting pointers and non-light-emitting pointers  200  whose coordinates can be detected at the same time. For example, even if the coordinates of a light-emitting pointer  100  cannot be detected due to a position not visible from certain two light reception/emission devices  300 , other two light reception/emission devices  300  may be able to detect the light-emitting pointer  100 , and to detect the coordinates of the light-emitting pointer  100 . 
     Also, on four sides of the display surface  22  of the image display unit  20 , retroreflector plates  320  (designated as  320 - 1 ,  320 - 2 ,  320 - 3 , and  320 - 4  if necessary to distinguish, below) are placed. Light emitted from the light reception/emission device  300  is incident on the retroreflector plate  320 , and the retroreflector plate  320  reflects the light in the incident direction. In other words, the retroreflector plate  320  is a plate-shaped reflector that reflects light. On the periphery on the surface side of the display surface  22  of the image display unit  20 , a bezel part  360  is placed to hide the retroreflector plates  320 . In other words, the retroreflector plate  320  is placed between the display surface  22  in a direction normal to the display surface  22 , and the bezel part  360 , and the inner frame  360   a  of the bezel part protrudes more inward toward the display surface  22  than the retroreflector plate  320 . This bezel part  360  provided in the image display unit  20  prevents the retroreflector plates  320  from being smeared by touches of the user&#39;s fingers and the like. 
     The light emitter  420  of the light reception/emission device  300 - 1  emits infrared light along the display surface of the image display unit  20  toward a range entirely covering the rightward retroreflector plate  320 - 2  and the downward retroreflector plate  320 - 3 . 
     Also, the light emitter  420  of the light reception/emission device  300 - 2  placed at the upper right corner emits infrared light along the display surface  22  of the image display unit  20 . The light emitter  420  of the light reception/emission device  300 - 2  emits the infrared light toward a range entirely covering the leftward retroreflector plate  320 - 4  and the downward retroreflector plate  320 - 3 . 
     This is the same for the light reception/emission device  300 - 3  and the light reception/emission device  300 - 4 . The light emitter  420  of the light reception/emission device  300 - 3  emits the infrared light toward a range entirely covering the leftward retroreflector plate  320 - 4  and the upward retroreflector plate  320 - 1 . The light emitter  420  of the light reception/emission device  300 - 4  emits the infrared light toward a range entirely covering the upward retroreflector plate  320 - 1  and the rightward retroreflector plate  320 - 2 . 
     If nothing contacts the display surface  22  of the image display unit  20 , the infrared light emitted from the light reception/emission device  300 - 1  is retroreflected by the retroreflector plate  320 - 2  and  320 - 3 ; the infrared light emitted from the light reception/emission device  300 - 2  is retroreflected by the retroreflector plate  320 - 3  and  320 - 4 ; the infrared light emitted from the light reception/emission device  300 - 3  is retroreflected by the retroreflector plate  320 - 4  and  320 - 1 ; and, the infrared light emitted from the light reception/emission device  300 - 4  is retroreflected by the retroreflector plate  320 - 1  and  320 - 2 . 
     Reflected light reflected by each of the retroreflector plates  320  is detected by the light reception sensor  410  of the corresponding light reception/emission device  300 . 
     As described above, the light-emitting pointer  100  includes the light-emitting element  110 , which is a light-emitting element. If the pen point  100 A of the light-emitting pointer  100  contacts the display surface  22  of the image display unit  20 , the light-emitting element  110  of the pen point  100 A of the light-emitting pointer  100  outputs infrared light. 
     As illustrated in  FIG. 6 , the light reception/emission device  300  includes the light reception sensor  410  and the light emitter  420 . The light reception sensor  410  includes an image sensor  411 , such as a CCD (charge-coupled device) image sensor or a CMOS (Complementary Metal Oxide Semiconductor) image sensor, and an imaging forming lens  412 . The light emitter  420  emits infrared light by an infrared LED (Light emitting diode), an infrared laser, or the like. The light reception/emission device  300  is connected to the light reception/emission control circuit  350 , and the light reception/emission control circuit  350  controls driving the light reception sensor  410  and the light emitter  420 . 
     This infrared light is incident on the light reception sensor  410  of the light reception/emission device  300 . The image sensor  411  includes elements constituting multiple pixels arranged in a horizontal direction with respect to the display surface  22 , and the infrared light emitted by the light-emitting element  110  is detected by one or more elements of the image sensor  411 . In other words, the quantity of light of an element that has sensed the infrared light becomes remarkably large. The position of such an element corresponds to the angles θ 1  and θ 2 . 
     The coordinate detection apparatus  24  converts the position of the element that has sensed the light into coordinates by using the formula of triangulation, to detect the coordinates of the light-emitting pointer  100  having the light-emitting element  110  installed. The formula of triangulation will be described later. 
     Besides, if the non-light-emitting pointers  200 , such as the user&#39;s finger, contacts the display surface  22  of the image display unit  20 , infrared light emitted from the light emitter  420  of the light reception/emission device  300  is cut off at the contact spot of the non-light-emitting pointer  200  on the display surface  22 . Therefore, one or more elements whose quantity of light becomes remarkably small is identified in the image sensor  411 . The position of such an element corresponds to the angles θ 3  and θ 4 . 
     The coordinate detection apparatus  24  detects the coordinates of the non-light-emitting pointer  200  by converting the position of the element at which the infrared light is cut off into coordinates, by using the formula of triangulation. 
     The formula of triangulation will be described later. 
     &lt;System Configuration Example&gt; 
       FIG. 7  is a diagram schematically illustrating a system configuration example including the electronic blackboard  10 , and  FIG. 8  is an example of a block diagram illustrating a control system of the electronic blackboard  10 . The electronic blackboard  10  is connected to a user PC (Personal Computer)  90  and a network  204 . The image display unit  20  is controlled by the controller  60 . The controller  60  is provided with a USB socket  72  to which a USB (Universal Serial Bus) cable  70  is connected, and an input socket  82  to which a VGA (Video Graphics Array) cable  80  is connected. In the example illustrated in  FIG. 7 , although the input socket  82  is connected to the VGA cable, the input socket  82  may be configured to be connectable with an input cable of other standards, such as HDMI (registered trademark) (High-Definition Multimedia Interface) and DisplayPort. 
     The user PC  90  includes a storage  94  that may be constituted with a magnetic disk drive. The storage  94  stores various contents and programs such as application software for displaying the contents. Accordingly, the user selects a desired content among the contents stored in the storage  94 , to display the content on a monitor  92 . 
     Therefore, when image data displayed on the monitor  92  of the user PC  90  is transferred via the USB cable  70  and the VGA cable  80 , the controller  60  displays the same image as the image data displayed on the monitor  92 , on a user PC screen  28  of the image display unit  20 . The user PC screen  28  is taken from the user PC  90  via the interface  508  of the electronic blackboard  10 . 
     The controller  60  is also connected to the network  204  such as the Internet or a LAN (Local Area Network) via a communication line  201 , such as an optical fiber, and a network socket  202 . 
     Besides, the controller  60  displays screen operation parts  26  to be pressed by the user when performing an input operation on the image display unit  20 . 
       FIG. 9  is an example of a block diagram illustrating a hardware configuration of the coordinate detection apparatus of the electronic blackboard  10 . The coordinate detection apparatus  24  includes a touch panel driver unit  250 , the light reception/emission control circuit  350 , a pen signal receiver unit  210 , and the light reception/emission devices  300 . 
     If the pen point  100 A of the light-emitting pointer  100  contacts the display surface  22 , a writing detection signal is received by the pen signal receiver unit  210 . If the pen bottom  100 B of the light-emitting pointer  100  contacts the display surface  22  of the image display unit  20 , an erasure detection signal is received by the pen signal receiver unit  210 . The pen signal receiver unit  210  is virtually the same as the short-distance wireless communication unit  515 , and the pen signal receiver unit  210  is a functional name of the short-distance wireless communication unit  515  when receiving a signal from the light-emitting pointer  100 . Note that the pen signal receiver unit  210  may transmit a signal to the light-emitting pointer  100 . 
     As a detection unit, the light reception/emission control circuit  350  detects the coordinates of the light-emitting pointer  100 , and detects the coordinates of the non-light-emitting pointer  200 . The light reception/emission control circuit  350  calculates the coordinates of the light-emitting pointer  100  and the non-light-emitting pointer  200  based on the quantities of light detected by the light reception sensors  410  of the light reception/emission devices  300 , and also controls light emission of the light emitters  420 . The coordinates calculated by the light reception/emission control circuit  350  are input into the touch panel driver unit  250  as a coordinate position signal. 
     Also, the touch panel driver unit  250  converts a coordinate position signal, a writing detection signal, a writing completion signal, a back-end-on signal, a back-end-off signal, or an erasure detection signal input from the pen signal receiver unit  210  and the light reception/emission control circuit  350 , into a predetermined event signal, to transmit the event signal to the controller  60 . 
     &lt;Functions of Controller  60 &gt; 
     The controller  60  is a control unit of the electronic blackboard  10 , having the hardware configuration illustrated in  FIG. 3 . The controller  60  provides functions implemented by software using these hardware components. For example, the CPU  501  runs a program loaded from the HDD  504  into the RAM  503 , to implement the following functions. 
       FIG. 10  is an example of a functional block diagram of the controller  60  included in the electronic blackboard  10 . The controller  60  of the electronic blackboard  10  includes a controller operation system unit  220  and an application unit  230 . 
     The controller operation system unit  220  is the main control unit that manages and executes control processes executed on the controller  60 . The controller operation system unit  220  is implemented by an OS, device drivers, and the like, to execute control and communication with the coordinate detection apparatus  24 , the image display unit  20 , and the user PC  90 . 
     The application unit  230  executes a process of generating an image to be displayed on the display surface  22  of the image display unit  20 , a process of displaying on the user PC screen  28 , and the like. Also, the application unit  230  includes an event signal determination unit  231 , a video input processing unit  232 , an image drawing processing unit  234 , a screen erasure processing unit  236 , and a screen operation processing unit  238 . 
     The event signal determination unit  231  monitors an event signal input from the controller operation system unit  220 , and executes a process depending on the input event signal. The video input processing unit  232  executes a process for displaying an image input from the user PC  90  on the user PC screen  28  of the display surface  22  of the image display unit  20 . 
     The image drawing processing unit  234  generates a handwriting image based on data of a coordinate position input from the coordinate detection apparatus  24  via the event signal determination unit  231 . The image drawing processing unit  234  also superimposes the handwriting image on an image that has been already displayed, and displays the superimposed image on the display surface  22  of the image display unit  20 . 
     The screen erasure processing unit  236  generates an image in the same background color as an image currently displayed, based on information of a coordinate position input from the coordinate detection apparatus  24  via the event signal determination unit  231 , and superimposes the graphics of the background color on the image currently displayed, to display the superimposed image on the display surface  22  of the image display unit  20 . 
     Thus, the handwriting image displayed on the image display unit  20  is superimposed on the image of the background color, and the handwriting image is seemingly erased from the display surface  22  of the image display unit  20 . 
     The screen operation processing unit  238  converts information (a signal) of a coordinate position input from the coordinate detection apparatus  24  into a pointing device signal, such as a mouse event, and executes a process corresponding to turning on and off operations of the screen operation parts  26  displayed on the display surface  22  of the image display unit  20 . 
     &lt;Calculation of Coordinates&gt; 
     &lt;&lt;Coordinate Detection of Light-Emitting Pointer  100 &gt;&gt; 
     Next, coordinate detection of the light-emitting pointer  100  will be described by using  FIGS. 11, 12A, and 12B .  FIG. 11  is an example of a diagram illustrating calculation of the position of a light-emitting pointer having a light-emitting element installed.  FIGS. 12A and 12B  are diagrams schematically illustrating quantities of light sensed by the light reception/emission device  300 - 1  and the light reception sensor of  300 - 2 , respectively. 
     The light-emitting element  110  is placed at the pen point  100 A of the light-emitting pointer  100 , and if the pen point  100 A contacts the display surface  22  of the image display unit  20 , infrared light is emitted from the light-emitting element  110 . This infrared light is reflected by the retroreflector plates  320 , and is incident on the light reception sensors  410  of the light reception/emission devices  300 . However, if the pen point  100 A contacts the display surface  22  of the image display unit  20 , the light reception/emission devices  300  turn off the infrared light. Note that to “turn off” includes to make the quantity of light so weak that the light reception sensor  410  cannot detect the light. Therefore, at the position where the light-emitting pointer  100  contacts the display surface  22  of the image display unit  20 , a bright spot appears on which the quantity of infrared light is larger. 
     As illustrated in  FIGS. 12A and 12B , when the infrared light is emitted from the light-emitting element  110  of the light-emitting pointer  100 , the quantity of light of an element that has captured the light-emitting pointer  100  becomes remarkably large. 
     Specifically, the brightness detected by the image sensor  411  is large on the element that has captured the light-emitting pointer  100 . The light reception/emission control circuit  350  identifies an element in an image obtained by the light reception sensor  410  (a first imaging unit) of the light reception/emission device  300 - 2 , at which the brightness greater than or equal to a threshold Th 1  is obtained, and then, converts the position of this element in the horizontal direction into an angle θ 1  (threshold Th 1  is an example of a first threshold). For example, it is possible to obtain the angle θ 1  by obtaining the sequence number of the element that has sensed the quantity of light greater than or equal to the threshold Th 1  (greater than or equal to the first threshold), counting from an edge in the horizontal direction of the light reception sensor  410 . Similarly, the angle θ 2  is obtained from the position of an element in the light reception sensor  410  (a second imaging unit) of the light reception/emission device  300 - 2  on which the quantity of light becomes remarkably large. 
     Note that the light reception/emission control circuit  350  may identify multiple elements that have sensed respective quantities of light of the threshold Th 1  or greater. This is because if the user uses multiple light-emitting pointers  100  at the same time, it is necessary to detect the coordinates of the multiple light-emitting pointers  100 . 
     By converting the position of an element corresponding to a bright spot viewed from the light reception/emission devices  300 - 1  and  300 - 2  into the angles θ, the light reception/emission control circuit  350  can calculate the coordinates (x 1 , y 1 ) of the bright spot by the following formulas. 
     In the formulas, θ 1  represents an angle formed by the straight line connecting the bright spot and the light reception/emission device  300 - 1 , and the upper side of the display surface  22 ; θ 2  represents an angle formed by the straight line connecting the bright spot and the light reception/emission device  300 - 2 , and the upper side of the display surface  22 ; and W represents the interval between the light reception/emission device  300 - 1  and the light reception/emission device  300 - 2 . 
     The coordinates (x 1 , y 1 ) of the light-emitting pointer  100  are calculated as follows: 
         x 1= w  tan θ 2 /(tan θ 1 +tan θ 2 )
 
         y 1= w  tan θ 1 ·tan θ 2 /(tan θ 1 +tan θ 2 )
 
     In this way, by taking the geometric arrangement of the light reception/emission devices  300  into consideration, it is possible to calculate the coordinates of a point pointed by the light-emitting pointer  100 . 
     Note that if the user uses multiple light-emitting pointers  100 , the light reception/emission control circuit  350  assigns an identifier (for example, a consecutive number starting from one) to each bright spot on the light reception sensor  410 . The light reception/emission devices  300 - 1  and  300 - 2  recognize the same identifier for the same light-emitting pointer  100 , and thereby, the light reception/emission control circuit  350  can detect the coordinates of each of the multiple light-emitting pointers  100 , by using the angle θ obtained from the bright spot of the same identifier. 
     Also, by installing transmitters on light-emitting pointers  100  that generate different identification signals from each other, respectively, the coordinate detection apparatus  24  that receives these identification signals can identify the respective light-emitting pointers  100 . 
       FIG. 13  is an example of a flowchart illustrating steps by which the light reception/emission control circuit  350  detects the coordinates of the light-emitting pointer  100 . The process illustrated in  FIG. 13  is executed repeatedly and periodically to detect the coordinates if a bright spot is detected. 
     First, suppose that the light-emitting element  110  emits light (S 1 ). The light-emitting element  110  does not emit light at all the time, but emits light only when contacting the display surface  22 . This can prevent the light-emitting pointer  100  from being detected before contacting the display surface  22 , and can avoid drawing an image such as a character and a figure at a position that is not intended by the user. 
     In response to the light emission of the light-emitting element  110 , the light reception/emission control circuit  350  causes the light emitter  420  to turn off the infrared light (S 2 ). Turning off the infrared light makes it easier for the light reception/emission control circuit  350  to detect a bright spot that is generated by the light-emitting element  110  of the light-emitting pointer  100 . Besides, it is possible to avoid detecting a part of the user&#39;s body as a dark spot, and drawing an image such as a character and a figure at a position that is not intended by the user. 
     The light reception sensor  410  of the light reception/emission device  300 - 1  obtains an image (S 3 ), namely, captures an image along the display surface  22 . The light reception sensor  410  of the light reception/emission device  300 - 2  also obtains an image (S 3 ). Note that “to capture an image along the display surface  22 ” means to capture an image in a predetermined range from the display surface  22 . Although it is preferable not to capture the display surface  22  in the image, the display surface  22  may be captured in the image if the light reception sensor  410  does not detect visible light. Besides, the maximum of the predetermined range is the imaging range of the light reception sensor  410 . A range narrower than the imaging range obtained by trimming or the like may be used as the predetermined range. This is because it is just necessary to capture an image of the light-emitting pointer  100  and the non-light-emitting pointer  200  near the display surface  22 . Specifically, a range may be several cm to 10 cm from the display surface  22 . 
     Next, the light reception/emission control circuit  350  identifies the position of the bright spot captured by the light reception sensor  410  of the light reception/emission device  300 - 1  (S 4 ). The light reception/emission control circuit  350  also identifies the position of the bright spot captured by the light reception sensor  410  of the light reception/emission device  300 - 2  (S 4 ). 
     Next, the light reception/emission control circuit  350  calculates an angle θ 1  (S 5 ), namely, converts the position of an element corresponding to the bright spot into the angle θ 1 . The light reception/emission control circuit  350  calculates an angle θ 2  (S 5 ), namely, converts the position of an element corresponding to the bright spot into the angle θ 2 . 
     The light reception/emission control circuit  350  calculates the coordinates of the light-emitting pointer  100  by using the angles θ 1  and θ 2  (S 6 ). 
     &lt;&lt;Coordinate Detection of Non-Light-Emitting Pointer  200 &gt;’ 
     Next, coordinate detection of the non-light-emitting pointer  200  will be described by using  FIGS. 14, 15A and 15B .  FIG. 14  is an example of a diagram illustrating detection of the position of the non-light-emitting pointers  200 , such as a finger, that does not have a light-emitting element installed.  FIGS. 15A and 15B  are diagrams schematically illustrating quantities of light sensed by the light reception sensors of the light reception/emission devices  300 - 1  and  300 - 2 , respectively. 
     The non-light-emitting pointer  200 , such as the user&#39;s finger, is detected by using infrared light cut off by the non-light-emitting pointer  200 . Infrared light emitted from the light emitters  420  of the light reception/emission devices  300 - 1  and  300 - 2  is captured by the light reception sensors  410  of the light reception/emission devices  300 - 1  and  300 - 2  in a state where the infrared light is cut off by the non-light-emitting pointer  200 . From the two light emitters  420  of the light reception/emission devices  300 - 1  and  300 - 2 , the infrared light is emitted at all the time, which is incident on the retroreflector plates  320 - 2 ,  320 - 3 , and  320 - 4  placed on the right, left, and lower sides of the display surface  22  of the image display unit  20 , to be retroreflected and to return. If the non-light-emitting pointers  200 , such as the user&#39;s finger, cuts off a path of this infrared light, the infrared light is cut off, and as illustrated in  FIGS. 15A and 15B , a dark spot appears at which the quantity of light decreases remarkably. 
     The light reception/emission control circuit  350  identifies an element in the image sensor  411 , at which the brightness lower than or equal to a threshold Th 2  (threshold Th 2  is an example of a second threshold) is obtained. The light reception/emission control circuit  350  converts the position of this element in the horizontal direction with respect to the display surface  22  into an angle θ 3 . For example, it is possible to obtain the angle θ 3  by obtaining the sequence number of the element that has sensed the quantity of light less than or equal to the threshold Th 2  (less than or equal to the second threshold), counting from an edge in the horizontal direction of the light reception sensor  410 . Similarly, the angle θ 4  is obtained from the position of an element in the image sensor  411  of the light reception/emission device  300 - 2  on which the quantity of light is less than or equal to the threshold Th 2 . 
     Note that the light reception/emission control circuit  350  may identify multiple elements in the horizontal direction that have sensed quantities of light of the threshold Th or less. This is because if the user makes two non-light-emitting pointers  200  contact the display surface  22 , it is necessary to detect the coordinates of the two non-light-emitting pointers  200 . Furthermore, three or more non-light-emitting pointers  200  may be used at the same time, or multiple persons may perform handwriting by non-light-emitting pointers  200  at the same time. 
     In the following formulas, θ 3  represents an angle formed by the straight line connecting a dark spot and the light reception/emission device  300 - 1 , and a horizontal line; θ 4  represents an angle formed by the straight line connecting the bright spot and the light reception/emission device  300 - 2 , and the horizontal line; and W represents the interval between the light reception/emission device  300 - 1  and the light reception/emission device  300 - 2 . The coordinates (x 2 , y 2 ) of the non-light-emitting pointer  200  are calculated as follow: 
         x 2= w  tan θ 4 /(tan θ 3 +tan θ 4 )
 
         y 2= w  tan θ 3 ·tan θ 4 /(tan θ 3 +tan θ 4 )
 
       FIG. 16  is an example of a flowchart illustrating steps by which the light reception/emission control circuit  350  detects the coordinates of the non-light-emitting pointer  200 . The process illustrated in  FIG. 16  is executed repeatedly and periodically to detect the coordinates if a dark spot is detected. 
     The light emitter  420  emits infrared light (S 1 ). 
     The non-light-emitting pointer  200  cuts off the infrared light (S 2 ). 
     The light reception sensor  410  of the light reception/emission device  300 - 1  obtains an image (S 3 ), namely, captures an image along the display surface  22 . The light reception sensor  410  of the light reception/emission device  300 - 2  also obtains an image (S 3 ). 
     Next, the light reception/emission control circuit  350  identifies the position of the dark spot captured by the light reception sensor  410  of the light reception/emission device  300 - 1  (S 4 ). The light reception/emission control circuit  350  also identifies the position of the dark spot captured by the light reception sensor  410  of the light reception/emission device  300 - 2  (S 4 ). 
     Next, the light reception/emission control circuit  350  calculates an angle θ 3  (S 5 ), namely, converts the position of an element corresponding to the dark spot into the angle θ 3 . The light reception/emission control circuit  350  calculates an angle θ 4  (S 5 ), namely, converts the position of an element corresponding to the dark spot into the angle θ 4 . 
     The light reception/emission control circuit  350  calculates the coordinates of the non-light-emitting pointer  200  by using the angles θ 3  and (S 6 ). 
     As described above, according to the embodiment using the coordinate detection apparatus  24  or the coordinate detection method executed by the coordinate detection apparatus  24 , it is possible to distinguish and detect a light-emitting pointer  100  equipped with a light-emitting element, and a non-light-emitting pointer  200  such as the user&#39;s finger. In addition, since a light-guiding plate is not required necessarily for the display surface  22 , a parallax is not generated, or is hardly generated. Furthermore, since the light reception/emission devices  300  are placed at the four corners of the image display unit  20 , space saving is achieved. 
     Note that in the embodiment, it has been described that when the light-emitting pointer  100  contacts the display surface  22 , the light emitter  420  is turned off; however, even if the light-emitting pointer  100  contacts the display surface  22 , the light emitter  420  may emit the infrared light. The light reception/emission control circuit  350  can detect a light-emitting pointer  100  if the quantity of light is greater than or equal to a threshold, and can detect a non-light-emitting pointer  200  if the quantity of light is less than or equal to the threshold. Therefore, the coordinates of the light-emitting pointer  100  and the non-light-emitting pointer  200  can be detected at the same time. 
     However, when the light-emitting pointer  100  emits light while the light emitter  420  emits infrared light, there is a possibility that the infrared light of the light-emitting pointer  100  may become obscured in the infrared light emitted by the light emitter  420 . Also, if the light-emitting pointer  100  is located distant from the light reception/emission device  300 , it may be difficult for the light reception sensor  410  to detect the light-emitting pointer  100 . Furthermore, it may be difficult for a person in charge at a manufacturer to appropriately specify the threshold for detecting infrared light of the light-emitting pointer  100  while the light emitter  420  emits infrared light. Therefore, in the embodiment, it is preferable that when the light-emitting pointer  100  contacts the display surface  22 , the light emitter  420  is turned off. This makes it easier to detect the light-emitting pointer  100  even if the light-emitting pointer  100  is located distant from the light reception/emission device  300 , and also makes it easier to specify the threshold. Besides, it is possible to avoid detecting a part of the user&#39;s body as a dark spot, and drawing an image such as a character and a figure at a position that is not intended by the user, while the user performs handwriting by using the light-emitting pointer  100 . 
     Second Embodiment 
     In this embodiment, an electronic blackboard  10  will be described in which the wavelength of infrared light from a light-emitting element  110  of a light-emitting pointer  100  is differentiated from the wavelength of infrared light from a light emitter  420  installed in a light reception/emission device  300 , so that a light reception/emission control circuit  350  can distinguish and detect both at the same time. 
       FIG. 17  is a schematic perspective view of the light reception/emission device  300  in the embodiment. In the embodiment, two types of band-pass filters to transmit light of specific wavelength ranges are placed in a light reception sensor  410  installed in the light reception/emission device  300 . 
     Accordingly, the light reception/emission device  300  can distinguish and detect infrared light emitted by the light-emitting element  110  of the light-emitting pointer  100 , and infrared light emitted by the light emitter  420  installed in the light reception/emission device  300 . For example, suppose that the wavelength of the infrared light emitted by the light-emitting pointer  100  is 800 nm, and the wavelength of the infrared light emitted by the light emitter  420  of the light reception/emission device  300  is 900 nm. Since the light reception sensor  410  of the light reception/emission device  300  is configured to include an image sensor  411  and an imaging forming lens  412 , it is possible to place the different band-pass filters attached to the imaging forming lens  412 . For example, a first band-pass filter  421  (a first filter) that cuts off infrared light of 850 nm or longer, and transmits infrared light below 850 nm; and a second band-pass filter  422  (a second filter) that transmits infrared light of 850 nm or longer, and cuts off infrared light below 850 nm, are placed in the imaging forming lens  412 . With such a configuration, infrared light below  850  nm that has been transmitted by the first band-pass filter  421  is incident on a first region  431  of the image sensor  411 , and infrared light of 850 nm or longer that has been transmitted by the second band-pass filter  422  is incident on a second region  432  of the image sensor  411 . 
     In other words, the infrared light from the light-emitting pointer  100  that has been transmitted by the first band-pass filter  421  is incident on the first region  431 , and the light reception/emission device  300  can detect only the light-emitting pointer  100 . Similarly, the infrared light emitted from the light emitter  420  of the light reception/emission device  300  that has reflected by the retroreflector plates  320  and has been transmitted by the second band-pass filter  422  is incident on the second region  432 , and the light reception/emission device  300  can detect only the non-light-emitting pointer  200 , such as the user&#39;s finger. Configured as such, the light reception/emission device  300  can distinguish and detect the light-emitting pointer  100  and the non-light-emitting pointer  200 . 
     Therefore, in the first region  431 , only the infrared light from the light-emitting pointer  100  is detected, and the infrared light emitted from the light emitter  420  is not detected. Therefore, in the embodiment, when the light-emitting pointer  100  contacts the display surface  22 , the light reception/emission control circuit  350  does not need to turn off the light emitter  420  of the light reception/emission device  300 . By not turning off the light emitter  420 , the light reception/emission control circuit  350  can detect the light-emitting pointer  100  and the non-light-emitting pointer  200  at the same time. 
       FIGS. 18A-18D  are diagrams schematically illustrating examples of quantities of light detected by the light reception/emission devices  300 - 1  and  300 - 2 .  FIG. 18A  illustrates an angle θ 1  of the light-emitting pointer  100  detected by the light reception/emission device  300 - 1  in the first region  431 , and  FIG. 18B  illustrates an angle θ 2  of the light-emitting pointer  100  detected by the light reception/emission device  300 - 2  in the first region  431 .  FIG. 18C  illustrates an angle θ 3  of the non-light-emitting pointer  200  detected by the light reception/emission device  300 - 1  in the second region  432 , and  FIG. 18D  illustrates an angle θ 4  of the non-light-emitting pointer  200  detected by the light reception/emission device  300 - 2  in the second region  432 . 
     In this way, the light reception/emission device  300 - 1  can detect the light-emitting pointer  100  and the non-light-emitting pointer  200  in the first region  431  and the second region  432  at the same time, and the light reception/emission device  300 - 2  can detect the light-emitting pointer  100  and the non-light-emitting pointer  200  in the first region  431  and the second region  432  at the same time. 
       FIG. 19  is an example of a flowchart illustrating operational steps of the coordinate detection apparatus  24 . 
     The light emitter  420  emits the infrared light (S 1 ). The non-light-emitting pointer  200  cuts off the infrared light (S 2 ). Also, suppose that the light-emitting element  110  emits light (S 3 ). Steps S 1 -S 3  may be executed in a different sequence. 
     The light reception sensor  410  of the light reception/emission device  300 - 1  obtains an image (S 4 ), namely, captures an image along the display surface  22 . The light reception sensor  410  of the light reception/emission device  300 - 2  also obtains an image (S 4 ). 
     Next, the light reception/emission control circuit  350  identifies the position of the bright spot captured by the light reception sensor  410  of the light reception/emission device  300 - 1  (S 5 ). The light reception/emission control circuit  350  also identifies the position of the bright spot captured by the light reception sensor  410  of the light reception/emission device  300 - 2  (S 5 ). 
     Also, the light reception/emission control circuit  350  identifies the position of the dark spot captured by the light reception sensor  410  of the light reception/emission device  300 - 1  (S 6 ). The light reception/emission control circuit  350  also identifies the position of the dark spot captured by the light reception sensor  410  of the light reception/emission device  300 - 2  (S 6 ). Steps S 5  and S 6  may be executed in a different sequence. 
     Next, the light reception/emission control circuit  350  calculates an angle θ 1  (S 7 ). The light reception/emission control circuit  350  also calculates an angle θ 2  (S 7 ). 
     Furthermore, the light reception/emission control circuit  350  calculates an angle θ 3  (S 8 ). The light reception/emission control circuit  350  also calculates an angle θ 4  (S 8 ). Steps S 7  and S 8  may be executed in a different sequence. 
     The light reception/emission control circuit  350  calculates the coordinates of the light-emitting pointer  100  by using the angles θ 1  and θ 2 , and calculates the coordinates of the non-light-emitting pointer  200  by using the angles θ 3  and θ 4  (S 9 ). 
     In this way, the light reception/emission control circuit  350  can detect the coordinates of the light-emitting pointer  100  and the non-light-emitting pointer  200  at the same time. In addition to such an effect, according to the embodiment, even if a contact position of the light-emitting pointer  100  on the display surface  22  of the image display unit  20  is extremely distant from or extremely close to the light reception/emission device  300 , it is easy for the light reception/emission device  300  to detect the light-emitting pointer  100 . 
       FIG. 20  is a diagram illustrating an example of a light-emitting pointer distant from a light reception sensor.  FIGS. 21A and 21B  are examples of diagrams illustrating quantities of light detected by the light reception sensors of the light reception/emission devices in a state illustrated in  FIG. 20 . 
     If the position of the light-emitting pointer  100  is far from the light reception/emission device  300 - 1 , the quantity of light detected by the light reception sensor  410  of the light reception/emission device  300 - 1  is very small as illustrated in  FIG. 21A . On the other hand, since the position of the light-emitting pointer  100  is close to the light reception/emission device  300 - 2 , the quantity of light detected is very large as illustrated in  FIG. 21B . 
     However, even in a state where there is a long distance between the light reception/emission device  300 - 1  and the light-emitting pointer  100 , and the infrared light from the light-emitting pointer  100  is barely detected as illustrated in  FIG. 21A , the light reception/emission device  300 - 1  can cut off infrared light other than that coming from the light-emitting-pointer  100 , which makes it easier to detect the light-emitting pointer  100 . Therefore, even if the light-emitting pointer  100  is on the edge of the display surface  22  as in  FIG. 20 , it is easy to detect the light-emitting pointer  100 . 
     Third Embodiment 
     In this embodiment, an electronic blackboard  10  will be described that can detect multiple light-emitting pointers  100  at the same time. 
       FIGS. 22A-22C  illustrate an example of a light reception/emission device of a coordinate detection apparatus in the embodiment where  FIG. 22A  is a perspective view of a light reception/emission device  300 ;  FIG. 22B  is a schematic view illustrating a light reception state of an image sensor; and  FIG. 22C  includes diagrams illustrating quantities of light detected by light reception sensors  410 . 
     Multiple light-emitting pointers  100  in the embodiment will be described. The light-emitting pointers  100  can output infrared light in frequency ranges different from each other. For example, suppose that there are three light-emitting pointers  100  (designated as  100 - 1 ,  100 - 2 , and  100 - 3  to be distinguished below) that output infrared having wavelengths of λ 1  nm, λ 2  nm, and λ 3  nm, respectively. Also, suppose that the wavelength of infrared light output by the light emitter  420  is λ 4  nm. λ 1  to λ 4  are different from each other. 
     Meanwhile, four types of band-pass filters  441 ,  442 ,  443 , and  444  that exclusively transmit wavelength ranges for λ 1  nm, λ 2  nm, λ 3  nm, and λ 4  nm are placed in the imaging forming lens  412  of the light reception sensor  410  in the light reception/emission device  300 . Therefore, four regions  451 ,  452 ,  453 , and  454  corresponding to these band-pass filters  441 ,  442 ,  443 , and  444  are formed on the image sensor  411 . 
     Only infrared light of the wavelength λ 1  is incident on the region  451 ; only infrared light of the wavelength λ 2  is incident on the region  452 ; only infrared light of the wavelength λ 3  is incident on the region  453 ; and only infrared light of the wavelength λ 4  is incident on the region  454 . 
     Accordingly, as illustrated in  FIG. 22C , the light reception/emission control circuit  350  can separate and detect the infrared light of each of the light-emitting pointers  100 - 1  to  100 - 3 , and the non-light-emitting pointer  200 . Besides, by using θ α1  and θ β1  detected in the region  451 , θ α2  and detected in the region  452 , θ α3  and θ β3  detected in the region  453 , and θ α4  and θ β4  detected in the region  454 , and using the formula of triangulation, the light reception/emission control circuit  350  can measure the coordinates of the light-emitting pointers  100 - 1  to  100 - 3 , and the non-light-emitting pointer  200 . Therefore, the electronic blackboard  10  in the embodiment can detect the coordinates of three light-emitting pointers  100 , and the non-light-emitting pointer  200  at the same time. 
       FIG. 23  is an example of a flowchart illustrating operational steps of the coordinate detection apparatus  24 . In the description of  FIG. 23 , differences from  FIG. 19  will be mainly described. First, Steps S 1  and S 2  are the same as in  FIG. 19 . Suppose that at Step S 3 , three light-emitting elements  110  emit light, respectively. The light reception sensors  410  obtain respective images at Step S 4 . 
     At Step S 5 , the light reception/emission control circuit  350  identifies three bright spot positions captured by the light reception sensor  410  of the light reception/emission device  300 - 1  (S 5 ). The light reception/emission control circuit  350  also identifies three bright spot positions captured by the light reception sensor  410  of the light reception/emission device  300 - 2  (S 5 ). In other words, the light reception/emission control circuit  350  identifies the bright spot positions in the regions  451 - 453 , respectively. 
     Besides, the light reception/emission control circuit  350  identifies a dark spot position captured by the light reception sensor  410  of the light reception/emission device  300 - 1  (S 6 ). The light reception/emission control circuit  350  also identifies a dark spot position captured by the light reception sensor  410  of the light reception/emission device  300 - 2  (S 6 ). In other words, the light reception/emission control circuit  350  identifies a dark spot position in the region  454 . 
     Next, the light reception/emission control circuit  350  calculates angles θ α1  to θ ≢3  (S 7 ). The light reception/emission control circuit  350  also calculates angles θ β1  to θ β3  (S 7 ). 
     Also, the light reception/emission control circuit  350  calculates an angle θ α4  (S 8 ). The light reception/emission control circuit  350  also calculates an angle θ β4  (S 8 ). 
     The light reception/emission control circuit  350  calculates the coordinates of the three light-emitting pointers  100 , and calculates the coordinates of the single non-light-emitting pointer  200  (S 9 ). 
     Similar to the second embodiment, in the present embodiment, even if the light-emitting pointer  100  contacts the display surface  22 , the light reception/emission control circuit  350  does not turn off the light emitter  420 . Therefore, the light reception/emission control circuit  350  can measure the coordinates of the three light-emitting pointers  100 - 1  to  100 - 3 , and the non-light-emitting pointer  200  at the same time. 
     Note that depending on the extent of the regions  451 - 454 , the sensitivity of the image sensor  411 , and the like, the number of the light-emitting pointers  100  (three in  FIG. 22A-22C ) and the number of band-pass filters (four in  FIG. 22A-22C ) can be changed, and is not limited to the illustrated example. 
     Fourth Embodiment 
     In this embodiment, a coordinate detection apparatus  24  will be described that can detect the coordinates of a light-emitting pointer  100  and a non-light-emitting pointer  200  on the projection surface of a projector. 
       FIG. 24  illustrates a schematic configuration diagram of an image display system  400  that includes a projector  500  and a coordinate detection apparatus  24 . The projector  500  projects an image  67  on a screen  66 . Since the image  67  is displayed by being projected, the projection surface corresponds to a display surface  22  in the embodiment. The coordinate detection apparatus  24  is installed on the upper side of the projected image  67 . The light reception/emission device  300  is placed at the right and left edges of the coordinate detection apparatus  24 . 
     The coordinate detection apparatus  24  is attached to the screen  66  or a wall by an attachment method suitable for the screen  66  or the wall. For example, the magnetic force from magnets may be used for attaching to a metal wall, or suction cups may be used for attaching to a flat wall. Alternatively, the user may affix Velcro (registered trademark) on the wall, to attach the coordinate detection apparatus  24  by Velcro. If attaching to the screen  66 , a binder or the like may be used for attaching the coordinate detection apparatus  24  so that the screen  66  is interposed. 
     Similarly, the retroreflector plates  320  are attached on the left, right, and lower sides of the image  67 . The method for attaching the retroreflector plates  320  may be the same as the method for the coordinate detection apparatus  24 . Alternatively, the coordinate detection apparatus  24  and the retroreflector plate  320  may be provided as an integrated device. 
       FIG. 25  includes diagrams schematically illustrating an extension process of the retroreflector plates  320  integrated with the coordinate detection apparatus  24 . The retroreflector plates  320  are first attached to the wall in a folded state such that the single retroreflector plate  320 - 2  comes upward, and the two retroreflector plates  320 - 3  and  320 - 4  come downward with respect to the coordinate detection apparatus  24 . As illustrated in  FIG. 25( b ) , a revolving part  601  rotatably supports the two downward retroreflector plates  320 - 3  and  320 - 4 , and the user can rotate the two retroreflector plates  320 - 3  and  320 - 4  clockwise by 90 degrees. 
     Next, as illustrated in  FIG. 25( c ) , among the two retroreflector plates  320 - 3  and  320 - 4 , the user rotates the retroreflector plate  320 - 3  clockwise by 90 degrees. Next, as illustrated in  FIG. 25( d ) , the user rotates the retroreflector plate  320 - 2  clockwise by 270 degrees. This arrangement enables to retroreflect infrared light emitted by the light reception/emission devices  300 - 1  and  300 - 2 . 
     In this way, having the coordinate detection apparatus  24  and the retroreflector plates  320  attached appropriately, the coordinate detection apparatus  24  can detect coordinates of the light-emitting pointer  100  and the non-light-emitting pointer  200  as in the first to third embodiments. 
     Note that three retroreflector plates  320  (or one or more retroreflector plates  320 ) may be provided independently. In this case, the user attaches the three retroreflector plates  320  so as to surround the image  67 . 
     Referring back to  FIG. 24 , description will continue. A user PC  90  is connected to the coordinate detection apparatus  24 , and the user PC  90  is connected to the projector  500  via a VGA cable or the like. The user PC  90  is also connected to the coordinate detection apparatus  24 . The coordinate detection apparatus  24  and the user PC  90  may be connected wirelessly, and the user PC  90  and the projector  500  may be connected wirelessly. 
     Predetermined application software runs on the user PC  90 , to provide an API (Application Program Interface) to the coordinate detection apparatus  24 . In other words, the coordinate detection apparatus  24  outputs a coordinate position signal to this API so that the application software can obtain the coordinates of the light-emitting pointer  100  and the non-light-emitting pointer  200 . Therefore, it is possible to draw an image such as a character and a figure from a trajectory of coordinates. 
     The application software displays an image such as a character and a figure on a monitor  92  of the user PC  90 . Since this image is transmitted to the projector  500  via a VGA cable or the like, the projector  500  can project an image such as a character and a figure on the screen  66  or the wall. Note that application software may have a function to display electronic data and the like used as a meeting material, and the application software can superimpose an image such as a character and a figure on the meeting material, and can display the superimposed image on the monitor  92 . Therefore, the projector  500  can also project an image having such data superimposed. Note that although PowerPoint (registered trademark) is known as such application software, the application software simply needs to have the above API. 
       FIG. 26  is a diagram schematically illustrating a system configuration example of the image display system in the embodiment.  FIG. 26  will be described mainly focusing on differences from  FIG. 9 . The controller  60  in  FIG. 9  is replaced with the user PC  90  in  FIG. 26 . In addition, the user PC  90  outputs an image to the projector  500 , not to the image display unit  20 . 
     In addition to the components in  FIG. 9 , the coordinate detection apparatus  24  in  FIG. 26  includes a communication unit  270 . Similarly, the user PC  90  includes a communication unit  56 , and the coordinate detection apparatus  24  can transmit a coordinate position signal to the user PC  90 . The communication unit  270  may be a wireless communication device using, for example, Bluetooth (registered trademark), ZigBee (registered trademark), or wireless LAN, or may be a USB host or the like to be capable of priority connection. Note that the user PC  90  includes a CPU  51 , a RAM  52 , a ROM  53 , an HDD  54 , and a VGA I/F  55 , as generic components of an information processing apparatus. The user PC  90  provides the same functions as the controller  60  illustrated in  FIG. 10 , by causing the CPU  51  to run a program stored in the HDD  54 . 
       FIG. 27  is an example of a schematic configuration diagram illustrating operational steps of the image display system when the light-emitting pointer  100  emits light. 
     S 1 : First, the light reception/emission control circuit  350  continues to keep the light emitter  420  turned on until receiving a writing detection signal or after receiving the writing completion signal. 
     S 2 : In response to a contact on the display surface  22  of the light-emitting pointer  100  performed by the user, the light-emitting element  110  of the light-emitting pointer  100  emits light. 
     S 3 : Having detected the contact on the display surface  22  by the contact detection part  102 , the light-emitting pointer  100  transmits a writing detection signal to the coordinate detection apparatus. 
     S 4 : The pen signal receiver unit  210  of the coordinate detection apparatus  24  receives the writing detection signal, and the light reception/emission control circuit  350  causes the light emitter  420  to stop emitting the infrared light (to turn off the light). 
     S 5 : The light reception sensor  410  of the light reception/emission device  300  obtains an image. The light reception sensor  410  of the light reception/emission device  300  periodically captures an image, and obtains the image after the infrared has been turned off. 
     S 6 : The light reception/emission control circuit  350  calculates the coordinates of the light-emitting pointer  100 , as described in the first embodiment. 
     S 7 : The communication unit  270  of the coordinate detection apparatus  24  transmits a coordinate position signal to the user PC  90 . 
     S 8 : The communication unit  56  of the user PC  90  receives the coordinate position signal, and the application software draws an image such as a character and a figure, using the coordinate position signal. The application software also executes a process of superimposing an image such as a character and a figure on electronic data used as a meeting material. 
     S 9 : the user PC  90  transmits the image, having the image such as a character and a figure superimposed on the electronic data, to the projector  500 . 
     S 10 : Accordingly, the projector  500  projects the image. 
       FIG. 28  is an example of a schematic configuration diagram illustrating operational steps of the image display system if the non-light-emitting pointer  200  cuts off the infrared light.  FIG. 28  will be described mainly focusing on differences from  FIG. 27 . First, Step S 1  is the same as in  FIG. 27 . 
     S 2 : In  FIG. 28 , the user cuts off infrared light by the non-light-emitting pointer  200 . The event that the infrared light has been cut off is not specifically indicated to the coordinate detection apparatus. Subsequent Steps S 5 -S 10  may be the same as in  FIG. 27 . 
     In this way, the coordinate detection apparatus  24  and the retroreflector plates  320  in the embodiment can be transported by the user, so as to be attached to a suitable wall or a screen. Therefore, there is an advantage that the electronic blackboard  10  does not occupy a space in an office or the like. Besides, the coordinate detection apparatus  24  can be installed in a location where users hold a meeting. 
     Alternatively, although the configuration example in  FIGS. 24 and 25  has the two light reception/emission devices  300  integrated, the light reception/emission devices  300 - 1  and  300 - 2  may be attached independently. In other words, the user may attach the light reception/emission devices  300 - 1  and  300 - 2  separately to a wall or the like. This means that the user can discretionarily determine the interval between the light reception/emission devices  300 - 1  and  300 - 2 . Therefore, it is possible to install the coordinate detection apparatus  24  in accordance with the size of an image  67  to be projected by the projector  500 . 
     Other Embodiments 
     As above, most preferable embodiments of the present invention have been described. Note that the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
     For example, the retroreflector plate  320  does not need to be an object that reflects light, and may be a light emitter, for example, a plate-shaped light emitter that uniformly emits light, such as an LED, an organic EL, and fluorescent light. Besides, the retroreflector plates  320  may consist of light-guiding plates, and in this case, infrared light emitted by the light reception/emission device  300  is guided by the light-guiding plate to the surroundings of the display surface  22 . 
     Also, the configuration examples illustrated in  FIGS. 9, 10  and the like in the above embodiments include components that are divided depending on main functions in order to make the processes on the electronic blackboard  10  easily understandable. However, the present invention is not limited by a division of processing units and/or names of components. A process of the electronic blackboard  10  may be further divided into more processing units depending on content of the process. Besides, a processing unit may be further divided to include more processes. 
     Note that the light reception sensor  410  is an example of an imaging unit; the light reception/emission control circuit  350  is an example of a detection unit; the pen signal receiver unit  210  is an example of a signal receiver unit; the light emitter  420  is an example of a light emitter unit; and the controller  60  is an example of a display control unit.