Patent Publication Number: US-2018052566-A1

Title: Coordinate input apparatus and coordinate position calculating method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119(a) to Japanese Patent Application No. 2016-160930 filed on Aug. 19, 2016, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a coordinate input apparatus and a coordinate position calculating method. 
     Description of the Related Art 
     In the past, an apparatus has been provided which includes light source units around the periphery of a coordinate input area, obtains the information of an emitted light interception point of a pointer (e.g., a human finger or a stylus pen), and calculates by triangulation the information of an insertion position of the pointer in the coordinate input area. 
     For example, there is an electronic whiteboard system including fluorescent lamps around the periphery of the coordinate input area as the light source units. The electronic whiteboard system displays, on a display thereof, a touch point at a known coordinate position to prompt a user to touch the touch point with the pointer to perform calibration. 
     SUMMARY 
     In one embodiment of this invention, there is provided an improved coordinate input apparatus that includes, for example, an imaging circuit, a retroreflecting member, an illuminating unit, a light emitting device, and circuitry. The imaging circuit includes an image sensor, and is disposed on a baseline parallel to one side of a peripheral portion of a two-dimensional coordinate input area including a calibration coordinate position virtually defined for calibration, with an optical axis of the imaging circuit forming a predetermined installation angle with the baseline. The retroreflecting member is disposed along the peripheral portion of the coordinate input area. The illuminating unit is disposed at a same position as a position of the imaging circuit to emit light traveling through the coordinate input area toward the retroreflecting member. The light emitting device is disposed around the peripheral portion of the coordinate input area on a straight line passing through an optical center of the imaging circuit and the calibration coordinate position. The circuitry controls turn-on and turn-off of the illuminating unit and the light emitting device, respectively. When a specific two-dimensional coordinate position in the coordinate input area is pointed by a pointer, the circuitry detects an imaging position of an image of the pointer on the image sensor of the imaging circuit, calculates the specific two-dimensional coordinate position in the coordinate input area by triangulation based on the imaging position and the installation angle, and calibrates the installation angle of the imaging circuit based on the imaging position and the calibration coordinate position. 
     In one embodiment of this invention, there is provided an improved coordinate input apparatus that includes, for example, an imaging circuit, a retroreflecting member, an illuminating unit, an interceptor, and circuitry. The imaging circuit includes an image sensor, and is disposed on a baseline parallel to one side of a peripheral portion of a two-dimensional coordinate input area including a calibration coordinate position virtually defined for calibration, with an optical axis of the imaging circuit forming a predetermined installation angle with the baseline. The retroreflecting member is disposed along the peripheral portion of the coordinate input area. The illuminating unit is disposed at a same position as a position of the imaging circuit to emit light traveling through the coordinate input area toward the retroreflecting member. The interceptor is disposed around the peripheral portion of the coordinate input area to intercept a position on the retroreflecting member intersecting a straight line passing through an optical center of the imaging circuit and the calibration coordinate position. The circuitry controls turn-on and turn-off of the illuminating unit and the interceptor, respectively. When a specific two-dimensional coordinate position in the coordinate input area is pointed by a pointer, the circuitry detects an imaging position of an image of the pointer on the image sensor of the imaging circuit, calculates the specific two-dimensional coordinate position in the coordinate input area by triangulation based on the imaging position and the installation angle, and calibrates the installation angle of the imaging circuit based on the imaging position and the calibration coordinate position. 
     In one embodiment of this invention, there is provided an improved coordinate position calculating method executed by a coordinate input apparatus that includes, for example, an imaging circuit, a retroreflecting member, an illuminating unit, and a light emitting device. The imaging circuit includes an image sensor, and is disposed on a baseline parallel to one side of a peripheral portion of a two-dimensional coordinate input area including a calibration coordinate position virtually defined for calibration, with an optical axis of the imaging circuit forming a predetermined installation angle with the baseline. The retroreflecting member is disposed along the peripheral portion of the coordinate input area. The illuminating unit is disposed at a same position as a position of the imaging circuit to emit light traveling through the coordinate input area toward the retroreflecting member. The light emitting device is disposed around the peripheral portion of the coordinate input area on a straight line passing through an optical center of the imaging circuit and the calibration coordinate position. The coordinate position calculating method includes controlling turn-on and turn-off of the illuminating unit, controlling turn-on and turn-off of the light emitting device, detecting, when a specific two-dimensional coordinate position in the coordinate input area is pointed by a pointer, an imaging position of an image of the pointer on the image sensor of the imaging circuit, calculating the specific two-dimensional coordinate position in the coordinate input area by triangulation based on the imaging position and the installation angle, and calibrating the installation angle of the imaging circuit based on the imaging position and the calibration coordinate position. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram illustrating a configuration of a coordinate input apparatus of a first embodiment of the present invention; 
         FIG. 2  is a functional block diagram of the coordinate input apparatus of the first embodiment; 
         FIG. 3  is a hardware configuration diagram of the coordinate input apparatus of the first embodiment; 
         FIG. 4  is a flowchart illustrating a coordinate position calculation process of the first embodiment; 
         FIGS. 5A and 5B  are diagrams illustrating the coordinate position calculation process of the first embodiment; 
         FIG. 6  is a flowchart illustrating a first type of installation angle calibration process of the first embodiment; 
         FIGS. 7A and 7B  are diagrams illustrating the first type of installation angle calibration process of the first embodiment; 
         FIG. 8  is a flowchart illustrating a second type of installation angle calibration process of the first embodiment; 
         FIG. 9  is a schematic diagram illustrating a configuration of a coordinate input apparatus of a second embodiment of the present invention; 
         FIG. 10  is a functional block diagram of the coordinate input apparatus of the second embodiment; 
         FIG. 11  is a flowchart illustrating a first type of installation angle calibration process of the second embodiment; and 
         FIG. 12  is a flowchart illustrating a second type of installation angle calibration process of the second embodiment. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     Referring now to the accompanying drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of the present invention will be described. Redundant description of identical or corresponding parts will be omitted where appropriate. 
       FIG. 1  schematically illustrates a configuration of a coordinate input apparatus  200  according to a first embodiment of the present invention. 
     As illustrated in  FIG. 1 , the coordinate input apparatus  200  of the first embodiment includes a display unit  40  with a rectangular display screen  42 , two imaging units  20 , i.e., a first imaging unit  20 A and a second imaging unit  20 B, a computer  10 , and a retroreflecting member  34 . The retroreflecting member  34  is disposed in a roughly U-shape around a peripheral portion of the display screen  42  excluding an upper side of the display screen  42  to surround a rectangular two-dimensional coordinate input area defined on the rectangular display screen  42 . The computer  10 , the first imaging unit  20 A, the second imaging unit  20 B, and the display unit  40  are mutually communicably connected by wire or radio. Herein, the retroreflecting member  34  is a reflecting member that reflects incident light toward the optical path of the incident light. Examples of such a reflecting member include a member formed of an array of multiple conical corner cubes. 
     The display unit  40  is a display, preferably a flat panel display. The display unit  40  displays an image output from the computer  10 . 
     The first imaging unit  20 A is a digital camera including an image forming optical system  24 A and an image sensor  26 A illustrated in  FIG. 5A . Similarly, the second imaging unit  20 B is a digital camera including an image forming optical system  24 B and an image sensor  26 B illustrated in  FIG. 5B . Each of the image sensors  26 A and  26 B is preferably a line sensor having charge-coupled devices (CCDs) or complementary metal oxide semiconductors (CMOSs) arranged in a line. Herein, the first imaging unit  20 A is fixed with an optical axis a 1  of the image forming optical system  24 A extending substantially parallel to a surface of the display screen  42  (i.e., a plane of the coordinate input area), and the optical axis a 1  and a baseline B forming a predetermined installation angle α 1  such that the angle of field of the image forming optical system  24 A covers the entire coordinate input area. Similarly, the second imaging unit  20 B is fixed with an optical axis a 2  of the image forming optical system  24 B extending substantially parallel to the surface of the display screen  42  (i.e., the plane of the coordinate input area), and the optical axis a 2  and the baseline B forming a predetermined installation angle α 2  such that the angle of field of the image forming optical system  24 B covers the entire coordinate input area. The first imaging unit  20 A and the second imaging unit  20 B are spaced from each other by a predetermined distance L. 
     The coordinate input apparatus  200  of the first embodiment further includes two illuminating units  38 , i.e., a first illuminating unit  38 A and a second illuminating unit  38 B, to emit probe light traveling through the coordinate input area defined on the display screen  42 . As illustrated in  FIG. 1 , in the first embodiment, the first illuminating unit  38 A is disposed at the same position as that of the first imaging unit  20 A, and the second illuminating unit  38 B is disposed at the same position as that of the second imaging unit  20 B. Each of the first illuminating unit  38 A and the second illuminating unit  38 B radially emits the probe light to cover the entire coordinate input area. Examples of the illuminating units  38  include highly directional light emitting diode (LED) lamps. The first illuminating unit  38 A is not necessarily required to be at exactly the same position as that of the first imaging unit  20 A, as long as the first illuminating unit  38 A is capable of radially emitting the probe light to cover the entire coordinate input area. Similarly, the second illuminating unit  38 B is not necessarily required to be at exactly the same position as that of the second imaging unit  20 B, as long as the second illuminating unit  38 B is capable of radially emitting the probe light to cover the entire coordinate input area. 
     The coordinate input apparatus  200  of the first embodiment further includes two light emitting devices  36 , i.e., a first light emitting device  36 A and a second light emitting device  36 B, disposed around the peripheral portion of the coordinate input area defined on the display screen  42 . The light emitting devices  36  are preferably LED devices. 
     In the first embodiment, the first light emitting device  36 A and the second light emitting device  36 B are installed at respective predetermined positions around the peripheral portion of the coordinate input area. Specifically, as illustrated in  FIG. 1 , the first light emitting device  36 A is installed at a position E 1  on a straight line passing through the optical center of the first imaging unit  20 A and a calibration coordinate position P′ (x, y). Further, the second light emitting device  36 B is installed at a position E 2  on a straight line passing through the optical center of the second imaging unit  20 B and the calibration coordinate position P′ (x, y). 
     The computer  10  is an information processor that controls the light emission of the first light emitting device  36 A and the second light emitting device  36 B, executes calculation by triangulation based on the installation angles α 1  and α 2  and camera output signals from the first imaging unit  20 A and the second imaging unit  20 B, and calculates and outputs a two-dimensional coordinate position in the defined coordinate input area on the display screen  42  pointed by a given pointer  50 , such as a stylus pen or a human finger. The computer  10  may be a dedicated built-in computer integrated with the coordinate input apparatus  200 , or may be a personal computer. 
     In the first embodiment, the coordinate input apparatus  200  functions as an electronic whiteboard when displaying, on the display screen  42  of the display unit  40 , the trajectory of the two-dimensional coordinate position calculated by the computer  10  as a drawn line. The coordinate input apparatus  200  further has a function of automatically calibrating the installation angle α 1  of the first imaging unit  20 A and the installation angle α 2  of the second imaging unit  20 B. 
       FIG. 2  illustrates functional blocks of the coordinate input apparatus  200  of the first embodiment. The computer  10  forming the coordinate input apparatus  200  includes an illumination control unit  11  that controls the first illuminating unit  38 A and the second illuminating unit  38 B, a light emission control unit  12  that controls the light emission of the first light emitting device  36 A and the second light emitting device  36 B, an imaging position detecting unit  13 , a coordinate position calculating unit  14 , a coordinate position output unit  15 , an installation angle calibrating unit  16 , each of which, or which are collectively, referred to as circuitry. The computer  10  further includes a storage area  18 , which may be implemented by any desired memory that operates under control of the circuitry. 
     The light emission control unit  12  controls turn-on and turn-off of the first light emitting device  36 A and the second light emitting device  36 B. In the first embodiment, each of the installation positions of the first light emitting device  36 A and the second light emitting device  36 B is assigned with an installation position identifier (ID). The light emission control unit  12  is capable of individually controlling turn-on and turn-off of the first light emitting device  36 A and turn-on and turn-off of the second light emitting device  36 B based on the installation position ID. 
     If the pointer  50  is inserted in the coordinate input area, the imaging position detecting unit  13  detects respective imaging positions of the image of the pointer  50  on the image sensor  26 A of the first imaging unit  20 A and the image sensor  26 B of the second imaging unit  20 B. 
     The coordinate position calculating unit  14  calculates the two-dimensional coordinate position of the pointer  50  by triangulation based on the installation angles α 1  and α 2  and the respective imaging positions of the image of the pointer  50  on the first imaging unit  20 A and the second imaging unit  20 B. 
     The coordinate position output unit  15  outputs the two-dimensional coordinate position of the pointer  50  calculated by the coordinate position calculating unit  14  to a specified output destination, such as an external device or an application. 
     The installation angle calibrating unit  16  calibrates the installation angle α 1  of the first imaging unit  20 A and the installation angle α 2  of the second imaging unit  20 B. 
     The storage area  18  is provided by an auxiliary storage device  105  of the computer  10  illustrated in  FIG. 3 . The storage area  18  stores the installation angle α 1  of the first imaging unit  20 A, the installation angle α 2  of the second imaging unit  20 B, and the calibration coordinate position P′ (x, y). The storage area  18  may also store a point image for displaying a point icon at the calibration coordinate position P′ (x, y). 
     Herein, the calibration coordinate position P′ (x, y) refers to a two-dimensional coordinate position virtually defined on the coordinate input area for calibration. A given position on the coordinate input area corresponding to the display screen  42  is previously defined as the calibration coordinate position P′ (x, y). 
     The installation position E 1  of the first light emitting device  36 A on the straight line passing through the optical center of the first imaging unit  20 A and the calibration coordinate position P′ (x, y), as illustrated in  FIG. 1 , is assigned with an installation position ID IDE 1  to indicate a first turn-off position. Similarly, the installation position E 2  of the second light emitting device  36 B on the straight line passing through the optical center of the second imaging unit  20 B and the calibration coordinate position P′ (x, y) is assigned with an installation position ID IDE 2  to indicate a second turn-off position. 
     In the coordinate input apparatus  200  of the first embodiment with the above-described functional configuration, the computer  10  executes a predetermined program to cause the coordinate input apparatus  200  to function as the respective units described above. 
     A hardware configuration of devices forming the coordinate input apparatus  200  of the first embodiment will now be described based on  FIG. 3 . 
     As illustrated in  FIG. 3 , the computer  10 , which is an information processor forming the coordinate input apparatus  200  of the first embodiment, includes a processor  102 , a read only memory (ROM)  103 , a random access memory (RAM)  104 , the auxiliary storage device  105 , an image output interface (I/F)  106 , a device control I/F  107 , and an imaging unit I/F  108 . The processor  102  controls the operation of the entire coordinate input apparatus  200 . The ROM  103  stores a boot program and a firmware program, for example. The RAM  104  provides an area for deploying programs for execution. The auxiliary storage device  105  stores the program for causing the coordinate input apparatus  200  to function as the above-described units, an operating system (OS), and a variety of data. The image output I/F  106  connects the computer  10  to the display unit  40 . The device control I/F  107  connects the computer  10  to the first light emitting device  36 A and the second light emitting device  36 B. The imaging unit I/F  108  connects the computer  10  to the first imaging unit  20 A and the second imaging unit  20 B. 
     A coordinate position calculation process executed by the coordinate input apparatus  200  will now be described based on the flowchart of  FIG. 4 . 
     At step S 401 , the illumination control unit  11  first turns on the first illuminating unit  38 A and the second illuminating unit  38 B to emit the probe light therefrom. The probe light emitted from the first illuminating unit  38 A and the second illuminating unit  38 B travels parallel to the display screen  42 , and is reflected by the retroreflecting member  34  disposed around the peripheral portion of the display screen  42 . The reflected probe light is then received by the first imaging unit  20 A and the second imaging unit  20 B. If the pointer  50  is inserted in the coordinate input area in this state, the image of the pointer  50  is formed on each of the image sensor  26 A of the first imaging unit  20 A and the image sensor  26 B of the second imaging unit  20 B as a dark spot. 
     At step S 402 , the imaging position detecting unit  13  detects an imaging position p 1  of the image of the pointer  50  on the image sensor  26 A of the first imaging unit  20 A, as illustrated in  FIG. 5A . 
       FIG. 5A  schematically illustrates a state in which the image of the pointer  50  inserted at an insertion position P (x, y) is formed on the image sensor  26 A of the first imaging unit  20 A via the image forming optical system  24 A. 
     If the pointer  50  is not inserted in the coordinate input area, the light intensity distribution on the image sensor  26 A is substantially uniform. If the pointer  50  is inserted in the coordinate input area, and if the image of the pointer  50  is formed on the image sensor  26 A, the light intensity is reduced at the imaging position p 1  on the image sensor  26 A to form the dark spot, which appears at a peak point in a light intensity waveform of the camera output signal from the image sensor  26 A. Based on the peak point in the light intensity waveform of the camera output signal from the image sensor  26 A of the first imaging unit  20 A, i.e., a point corresponding to the dark spot, the imaging position detecting unit  13  detects the imaging position p 1  of the image of the pointer  50 . 
     At step S 403 , the imaging position detecting unit  13  detects an imaging position p 2  of the image of the pointer  50  on the image sensor  26 B of the second imaging unit  20 B, as illustrated in  FIG. 5B . 
       FIG. 5B  schematically illustrates a state in which the image of the pointer  50  inserted at the insertion position P (x, y) is formed on the image sensor  26 B of the second imaging unit  20 B via the image forming optical system  24 B. Similarly as in the detection of the imaging position p 1 , the imaging position detecting unit  13  detects the imaging position p 2  of the image of the pointer  50  based on a peak point in a light intensity waveform of the camera output signal from the image sensor  26 B of the second imaging unit  20 B, i.e., a point corresponding to the dark spot. 
     At step S 404 , the coordinate position calculating unit  14  reads the installation angle α 1  of the first imaging unit  20 A and the installation angle α 2  of the second imaging unit  20 B from the storage area  18 . 
     At step S 405 , the coordinate position calculating unit  14  calculates the insertion position P (x, y) of the pointer  50 , i.e., the two-dimensional coordinate position of the pointer  50  inserted in the coordinate input area, by triangulation based on the imaging position p 1  and the installation angle α 1  of the first imaging unit  20 A and the imaging position p 2  and the installation angle α 2  of the second imaging unit  20 B. 
     As illustrated in  FIG. 5A , s 10  represents the distance from the center of the image sensor  26 A of the first imaging unit  20 A to the center of the imaging position p 1 , and f represents the focal length of the image forming optical system  24 A of the first imaging unit  20 A. Further, θ 10  represents the angle formed by a line segment connecting the imaging position p 1  and the insertion position P (x, y) and the center line of the image sensor  26 A, i.e., the optical axis a 1  of the image forming optical system  24 A. The angle θ 10  is calculated from equation (1) given below: 
       θ10=tan −1 ( s 10/ f )  (1)
 
     Further, an angle β 10  formed by the line segment connecting the imaging position p 1  and the insertion position P (x, y) and the baseline B is calculated from equation (2) given below with the installation angle α 1  of the first imaging unit  20 A: 
       β10=α1−θ10  (2)
 
     Similarly, as illustrated in  FIG. 5B , s 20  represents the distance from the center of the image sensor  26 B of the second imaging unit  20 B to the center of the imaging position p 2 , and f represents the focal length of the image forming optical system  24 B of the second imaging unit  20 B. Further, θ 20  represents the angle formed by a line segment connecting the imaging position p 2  and the insertion position P (x, y) and the center line of the image sensor  26 B, i.e., the optical axis a 2  of the image forming optical system  24 B. The angle θ 20  is expressed by equation (3) given below: 
       θ20=tan −1 ( s 20/ f )  (3)
 
     Further, an angle β 20  formed by the line segment connecting the imaging position p 2  and the insertion position P (x, y) and the baseline B is calculated from equation (4) given below with the installation angle α 2  of the second imaging unit  20 B: 
       β20=α 2 −θ 20   (4)
 
     Further, two-dimensional coordinates (x, y) of the insertion position P (x, y) of the pointer  50  are calculated from equations (5) and (6) given below by the principle of triangulation with the angles β 10  and β 20  calculated by the above-described procedures and the distance L in  FIG. 1  between the center of the image forming optical system  24 A of the first imaging unit  20 A and the center of the image forming optical system  24 B of the second imaging unit  20 B: 
         x=L ·tan β20/(tan β10+tan β20)  (5)
 
         y=x  tan β10  (6)
 
     In the first embodiment, the coordinate input apparatus  200  repeats the above-described execution of steps S 401  to S 405  at predetermined time intervals, and the coordinate position output unit  15  outputs the two-dimensional coordinates (x, y) calculated during the execution of the steps to the specified output destination. For example, if rendering software for the electronic whiteboard is specified as the output destination of the two-dimensional coordinates (x, y), the trajectory of the pointer  50  is displayed as a drawn line overlaid on contents displayed on the display screen  42 . 
     Following the above description of the coordinate position calculation process executed by the coordinate input apparatus  200  of the first embodiment, an installation angle calibration process executed by the coordinate input apparatus  200  will now be described. 
     The installation angle α 1  of the first imaging unit  20 A and the installation angle α 2  of the second imaging unit  20 B may deviate from the values thereof stored in the storage area  18  for various reasons. If such deviation is left uncalibrated, the coordinate position calculating unit  14  will eventually fail to calculate the correct insertion position P (x, y) of the pointer  50 . In the first embodiment, therefore, the coordinate input apparatus  200  executes the installation angle calibration process of calibrating the installation angle α 1  of the first imaging unit  20 A and the installation angle α 2  of the second imaging unit  20 B stored in the storage area  18 . The installation angle calibration process has two types. 
     A first type of installation angle calibration process will now be described based on the flowchart of  FIG. 6 . 
     At step S 501 , the illumination control unit  11  first turns off the first illuminating unit  38 A and the second illuminating unit  38 B to stop the emission of the probe light. 
     At step S 502 , the light emission control unit  12  simultaneously turns on the first light emitting device  36 A and the second light emitting device  36 B. In this case, the respective images of the turned-on first light emitting device  36 A and second light emitting device  36 B are formed on each of the image sensor  26 A of the first imaging unit  20 A and the image sensor  26 B of the second imaging unit  20 B as bright spots. 
     At step S 503 , the imaging position detecting unit  13  performs the following procedure to detect an imaging position e 1 A of two imaging positions e 1 A and e 2 A on the image sensor  26 A of the first imaging unit  20 A as the imaging position of the image of the first light emitting device  36 A, as illustrated in  FIG. 7A . 
       FIG. 7A  schematically illustrates a state in which the respective images of the first light emitting device  36 A and the second light emitting device  36 B are formed on the image sensor  26 A of the first imaging unit  20 A via the image forming optical system  24 A. The imaging position detecting unit  13  first performs a procedure similar to that for detecting the imaging position p 1  of the image of the pointer  50  inserted in the coordinate input area, to thereby detect the imaging position e 1 A of the image of the first light emitting device  36 A and the imaging position e 2 A of the image of the second light emitting device  36 B each based on the peak point in the light intensity waveform of the camera output signal from the image sensor  26 A of the first imaging unit  20 A, i.e., the point corresponding to the bright spot. 
     In this case, the imaging position e 1 A of the image of the first light emitting device  36 A is constantly in front of the imaging position e 2 A of the image of the second light emitting device  36 B in the direction of arrow d, as illustrated in  FIG. 7A . Based on this relationship between the imaging positions e 1 A and e 2 A, the imaging position detecting unit  13  detects the imaging position e 1 A of the two imaging positions e 1 A and e 2 A, which is located in front of the imaging position e 2 A in the direction of arrow d, as the imaging position of the image of the first light emitting device  36 A. 
     At step S 504 , the installation angle calibrating unit  16  calculates the latest installation angle α 1  of the first imaging unit  20 A based on the imaging position e 1 A and the calibration coordinate position P′ (x, y). 
     A procedure of calculating the latest installation angle α 1  will now be described with reference to  FIG. 7A . 
     Herein, s 11  represents the distance from the center of the image sensor  26 A of the first imaging unit  20 A to the center of the imaging position e 1 A, and f represents the focal length of the image forming optical system  24 A of the first imaging unit  20 A. Further, θ 11  represents the angle formed by a line segment connecting the imaging position e 1 A and the two-dimensional coordinate position of the first light emitting device  36 A and the center line of the image sensor  26 A, i.e., the optical axis a 1  of the image forming optical system  24 A. 
     The angle θ 11  is calculated from equation (7) given below: 
       θ11=tan −1 ( s 11/ f )  (7)
 
     Further, an angle β 11  formed by the line segment connecting the imaging position e 1 A and the two-dimensional coordinate position of the first light emitting device  36 A and the baseline B passing through the optical center of the image forming optical system  24 A of the first imaging unit  20 A is calculated from equation (8) given below with the calibration coordinate position P′ (x, y) located on the line segment: 
       tan β11= y/x   (8)
 
     Further, the latest installation angle α 1  of the first imaging unit  20 A is calculated from equation (9) given below with the angles θ 11  and β 11  calculated by the above-described procedures: 
       α1=β11+θ11  (9)
 
     At step S 505 , the imaging position detecting unit  13  performs a procedure similar to the above-described procedure to detect an imaging position e 2 B of two imaging positions e 1 B and e 2 B on the image sensor  26 B of the second imaging unit  20 B as the imaging position of the image of the second light emitting device  36 B. 
     That is, when the respective images of the first light emitting device  36 A and the second light emitting device  36 B are formed on the image sensor  26 B of the second imaging unit  20 B via the image forming optical system  24 B, the imaging position e 2 B of the image of the second light emitting device  36 B is constantly in front of the imaging position e 1 B of the image of the first light emitting device  36 A in the direction of arrow d, as illustrated in  FIG. 7B . Based on this relationship between the imaging positions e 1 B and e 2 B, the imaging position detecting unit  13  detects the imaging position e 2 B of the two imaging positions e 1 B and e 2 B, which is located in front of the imaging position e 1 B in the direction of arrow d, as the imaging position of the image of the second light emitting device  36 B. 
     At step S 506 , the installation angle calibrating unit  16  performs a procedure similar to the above-described procedure to calculate the latest installation angle α 2  of the second imaging unit  20 B based on the imaging position e 2 B and the calibration coordinate position P′ (x, y). 
     That is, as illustrated in  FIG. 7B , s 21  represents the distance from the center of the image sensor  26 B of the second imaging unit  20 B to the center of the imaging position e 2 B, and f represents the focal length of the image forming optical system  24 B of the second imaging unit  20 B. Further, θ 21  represents the angle formed by a line segment connecting the imaging position e 2 B and the two-dimensional coordinate position of the second light emitting device  36 B and the center line of the image sensor  26 B, i.e., the optical axis a 2  of the image forming optical system  24 B. The angle θ 21  is calculated from equation (10) given below: 
       θ21=tan −1 ( s 21/ f )  (10)
 
     Further, an angle β 21  formed by the line segment connecting the imaging position e 2 B and the two-dimensional coordinate position of the second light emitting device  36 B and the baseline B passing through the optical center of the image forming optical system  24 B of the second imaging unit  20 B is calculated from equation (11) given below with the calibration coordinate position P′ (x, y) on the line segment and the maximum value xmax of the x-coordinates in the coordinate input area: 
       tan β 21 = y /( x max− x )  (11)
 
     Further, the latest installation angle α 1  of the second imaging unit  20 B is calculated from equation (12) given below with the angles θ 21  and β 21  calculated by the above-described procedures: 
       α2=β21+θ21  (12)
 
     Finally, at step S 507 , the installation angle calibrating unit  16  discards the installation angles α 1  and α 2  currently stored in the storage area  18 , and newly registers in the storage area  18  the latest installation angles α 1  and α 2  calculated at steps S 504  and S 506 , respectively. 
     According to the above-described first type of installation angle calibration process executed by the coordinate input apparatus  200  of the first embodiment, there is no need for synchronization of the exposure of the first imaging unit  20 A and the second imaging unit  20 B and the light emission of the first light emitting device  36 A and the second light emitting device  36 B. 
     A second type of installation angle calibration process executed by the coordinate input apparatus  200  of the first embodiment will now be described based on the flowchart of  FIG. 8 . 
     At step S 601 , the illumination control unit  11  first turns off the first illuminating unit  38 A and the second illuminating unit  38 B to stop the emission of the probe light. 
     At step S 602 , the light emission control unit  12  selectively turns on only the first light emitting device  36 A installed at the position E 1 . In this step, the second light emitting device  36 B installed at the position E 2  is turned off. Consequently, only the image of the turned-on first light emitting device  36 A is formed on the image sensor  26 A of the first imaging unit  20 A as a bright spot. 
     At step S 603 , the imaging position detecting unit  13  detects the imaging position e 1 A of the image of the first light emitting device  36 A on the image sensor  26 A of the first imaging unit  20 A. More specifically, the imaging position detecting unit  13  detects the imaging position e 1 A of the image of the first light emitting device  36 A based on the peak point in the light intensity waveform of the camera output signal from the image sensor  26 A of the first imaging unit  20 A, i.e., the point corresponding to the bright spot. 
     At step S 604 , the installation angle calibrating unit  16  calculates the latest installation angle α 1  of the first imaging unit  20 A by the same procedure as that of step S 504  in  FIG. 6  based on the imaging position e 1 A and the calibration coordinate position P′ (x, y). 
     At step S 605 , the light emission control unit  12  selectively turns on only the second light emitting device  36 B installed at the position E 2 . In this step, the first light emitting device  36 A installed at the position E 1  is turned off. Consequently, only the image of the turned-on second light emitting device  36 B is formed on the image sensor  26 B of the second imaging unit  20 B as a bright spot. 
     At step S 606 , the imaging position detecting unit  13  detects the imaging position e 2 B of the image of the second light emitting device  36 B on the image sensor  26 B of the second imaging unit  20 B by the same procedure as that of step S 505  in  FIG. 6 . 
     At step S 607 , the installation angle calibrating unit  16  calculates the latest installation angle α 2  of the second imaging unit  20 B by the same procedure as that of step S 506  in  FIG. 6  based on the imaging position e 2 B and the calibration coordinate position P′ (x, y). 
     Finally, at step S 608 , the installation angle calibrating unit  16  discards the installation angles α 1  and α 2  currently stored in the storage area  18 , and newly registers in the storage area  18  the latest installation angles α 1  and α 2  calculated at steps S 604  and S 607 , respectively. 
     According to the above-described second type of installation angle calibration process, the imaging position (i.e., the bright spot) detected by the imaging position detecting unit  13  is directly used as the imaging position of the image of the target light emitting device. 
     As described above, according to the first embodiment, the installation angle α 1  of the first imaging unit  20 A and the installation angle α 2  of the second imaging unit  20 B are automatically calibrated without involvement of a user. 
     Following the above description of the first embodiment of the present invention, a second embodiment of the present invention will now be described. The following description will focus on differences from the first embodiment, with description of parts in common with those of the first embodiment omitted. 
       FIG. 9  schematically illustrates a configuration of a coordinate input apparatus  300  according to the second embodiment of the present invention. As illustrated in  FIG. 9 , the coordinate input apparatus  300  of the second embodiment is different from the coordinate input apparatus  200  of the first embodiment in including two interceptors  39 , i.e., a first interceptor  39 A and a second interceptor  39 B, each of which is capable of intercepting a portion of the retroreflecting member  34 . 
     In the second embodiment, each of the interceptors  39  is a member installed at a position on the retroreflecting member  34  to prevent the probe light from being reflected to the first imaging unit  20 A and the second imaging unit  20 B, and may be a mechanical shutter including a rotary plate or an optical shutter including an optical device such as a polarizing plate and liquid crystal. 
     In the second embodiment, the first interceptor  39 A and the second interceptor  39 B are installed at respective predetermined positions around the peripheral portion of the coordinate input area. Specifically, as illustrated in  FIG. 9 , the first interceptor  39 A is installed to be capable of intercepting the position E 1 , which is on the straight line passing through the optical center of the first imaging unit  20 A and the calibration coordinate position P′ (x, y) and is around the peripheral portion of the coordinate input area. Further, the second interceptor  39 B is installed to be capable of intercepting the position E 2 , which is on the straight line passing through the optical center of the second imaging unit  20 B and the calibration coordinate position P′ (x, y) and is around the peripheral portion of the coordinate input area. 
       FIG. 10  illustrates functional blocks of the coordinate input apparatus  300  of the second embodiment. As illustrated in  FIG. 10 , a computer  10 B forming the coordinate input apparatus  300  of the second embodiment is different from the foregoing computer  10  forming the coordinate input apparatus  200  of the first embodiment in including an interceptor control unit  19  that controls the first interceptor  39 A and the second interceptor  39 B in place of the light emission control unit  12  of the coordinate input apparatus  200 . 
     A coordinate position calculation process executed by the coordinate input apparatus  300  of the second embodiment is the same as that of the first embodiment described above with reference to  FIG. 4 , and thus a description thereof will be omitted here. The following description will be given of an installation angle calibration process executed by the coordinate input apparatus  300 . 
     Similarly as in the first embodiment, the installation angle calibration process of the second embodiment has two types. A first type of installation angle calibration process will first be described based on the flowchart of  FIG. 11 . 
     At step S 701 , the illumination control unit  11  first turns on the first illuminating unit  38 A and the second illuminating unit  38 B to emit the probe light therefrom. 
     At step S 702 , the interceptor control unit  19  controls the first interceptor  39 A and the second interceptor  39 B to simultaneously intercept the positions E 1  and E 2  on the retroreflecting member  34 . In this case, respective images of the first interceptor  39 A and the second interceptor  39 B not reflecting the probe light are formed on each of the image sensor  26 A of the first imaging unit  20 A and the image sensor  26 B of the second imaging unit  20 B as dark spots. 
     At step S 703 , the imaging position detecting unit  13  detects the imaging position e 1 A of the two imaging positions e 1 A and e 2 A on the image sensor  26 A of the first imaging unit  20 A as the imaging position of the image of the first interceptor  39 A by the same procedure as that of the first embodiment. 
     At step S 704 , the installation angle calibrating unit  16  calculates the latest installation angle α 1  of the first imaging unit  20 A by the same procedure as that of the first embodiment based on the imaging position e 1 A and the calibration coordinate position P′ (x, y). 
     At step S 705 , the imaging position detecting unit  13  detects the imaging position e 2 B of the two imaging positions e 1 B and e 2 B on the image sensor  26 B of the second imaging unit  20 B as the imaging position of the image of the second interceptor  39 B by the same procedure as that of the first embodiment. 
     At step S 706 , the installation angle calibrating unit  16  calculates the latest installation angle α 2  of the second imaging unit  20 B by the same procedure as that of the first embodiment based on the imaging position e 2 B and the calibration coordinate position P′ (x, y). 
     Finally, at step S 707 , the installation angle calibrating unit  16  discards the installation angles α 1  and α 2  currently stored in the storage area  18 , and newly registers in the storage area  18  the latest installation angles α 1  and α 2  calculated at steps S 704  and S 706 , respectively. 
     According to the above-described first type of installation angle calibration process executed by the coordinate input apparatus  300  of the second embodiment, there is no need for synchronization of the exposure of the first imaging unit  20 A and the second imaging unit  20 B and the control of the first interceptor  39 A and the second interceptor  39 B. 
     A second type of installation angle calibration process executed by the coordinate input apparatus  300  of the second embodiment will now be described based on the flowchart of  FIG. 12 . 
     At step S 801 , the illumination control unit  11  first turns on the first illuminating unit  38 A and the second illuminating unit  38 B to emit the probe light therefrom. 
     At step S 802 , the interceptor control unit  19  controls the first interceptor  39 A to selectively intercept only the position E 1  on the retroreflecting member  34 . In this step, the second interceptor  39 B does not intercept the position E 2  on the retroreflecting member  34 . Consequently, only the image of the first interceptor  39 A not reflecting the probe light is formed on the image sensor  26 A of the first imaging unit  20 A as a dark spot. 
     At step S 803 , the imaging position detecting unit  13  detects the imaging position e 1 A of the image of the first interceptor  39 A on the image sensor  26 A of the first imaging unit  20 A by the same procedure as that of the first embodiment. 
     At step S 804 , the installation angle calibrating unit  16  calculates the latest installation angle α 1  of the first imaging unit  20 A by the same procedure as that of the first embodiment based on the imaging position e 1 A and the calibration coordinate position P′ (x, y). 
     At step S 805 , the interceptor control unit  19  controls the second interceptor  39 B to selectively intercept only the position E 2  on the retroreflecting member  34 . In this step, the first interceptor  39 A does not intercept the position E 1  on the retroreflecting member  34 . Consequently, only the image of the second interceptor  39 B not reflecting the probe light is formed on the image sensor  26 B of the second imaging unit  20 B as a dark spot. 
     At step S 806 , the imaging position detecting unit  13  detects the imaging position e 2 B of the image of the second interceptor  39 B on the image sensor  26 B of the second imaging unit  20 B by the same procedure as that of the first embodiment. 
     At step S 807 , the installation angle calibrating unit  16  calculates the latest installation angle α 2  of the second imaging unit  20 B by the same procedure as that of the first embodiment based on the imaging position e 2 B and the calibration coordinate position P′ (x, y). 
     Finally, at step S 808 , the installation angle calibrating unit  16  discards the installation angles α 1  and α 2  currently stored in the storage area  18 , and newly registers in the storage area  18  the latest installation angles α 1  and α 2  calculated at steps S 804  and S 807 , respectively. 
     The respective functions of each of the foregoing embodiments may be implemented by a program described in C, C++, C#, or Java (registered trademark), for example. Further, the program of each of the embodiments may be distributed as stored in a recording medium, such as a hard disk device, a compact disc-ROM (CD-ROM), a magneto-optical (MO) disc, a digital versatile disk (DVD), a flexible disk, an electrically erasable programmable ROM (EEPROM), or an erasable programmable ROM (EPROM), for example, or may be transmitted via a network in a format readable by another apparatus. 
     The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention. Further, the above-described steps are not limited to the order disclosed herein. 
     Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), digital signal processor (DSP), field programmable gate array (FPGA), and conventional circuit components arranged to perform the recited functions.