Patent Publication Number: US-2011063253-A1

Title: Optical position detector and display device with position detection function

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an optical position detector that optically detects a position of a target object within a detection area and a display device with a position detection function including the optical position detector. 
     2. Related Art 
     In electronic apparatuses such as mobile phones, car navigation systems, personal computers, ticket-vending machines, and bank terminals, a display device with a position detection function in which a touch panel is arranged in front of an image forming device such as a liquid crystal device has been recently used. In the display device with a position detection function, input of information is performed with reference to an image displayed on the image forming device. Such a touch panel is configured as a position detector that detects a position of a target object within a detection area (for example, refer to U.S. Pat. No. 6,927,384). 
     A position detector described in U.S. Pat. No. 6,927,384 is of the optical type, in which a light guide plate is disposed on an input operation side with respect to a direct-view type display panel such as a liquid crystal panel, and a light source, a light-receiving element, and the like are arranged on the opposite side to the input operation side with respect to the light guide plate. A position detection light emitted from the light source is emitted to the input operation side via the light guide plate, and the position detection light reflected by a target object is received by the light-receiving element. 
     Here, the present inventor proposes, by applying the structure described in U.S. Pat. No. 6,927,384, an optical position detector schematically shown in  FIGS. 14A and 14B . In the optical position detector having the configuration, an intensity distribution of a position detection light is formed in an in-plane direction of a detection area  10 R, and the position detection light reflected by a target object Ob in the detection area  10 R is detected by alight detector  15 . For example, a light intensity distribution of the position detection light emitted from a light guide plate  13  to the detection area  10 R is different between when a position detection light L 2   a  is emitted from a position-detecting light source  12 A and when a position detection light L 2   b  is emitted from a position-detecting light source  12 B. Accordingly, when the received-light results by the light detector  15  are compared between when the position detection light L 2   a  is emitted and when the position detection light L 2   b  is emitted, a position of the target object Ob in a direction indicated by the arrows A can be detected. Moreover, when the received-light results by the light detector  15  are compared between when a position detection light L 2   c  is emitted from a position-detecting light source  12 C and when a position detection light L 2   d  is emitted from a position-detecting light source  12 D, a position of the target object Ob in a direction indicated by the arrows B can be detected. 
     However, the optical position detector having the configuration shown in  FIGS. 14A and 14B  has a problem in that, although position detection can be performed with high accuracy when the target object Ob is positioned at the central portion of the detection area  10 R, the detection accuracy is decreased when the target object Ob is positioned at an edge of the detection area  10 R. As a result of studying the problem, the present inventor has found that the decrease in detection accuracy when the target object Ob is positioned at an edge of the detection area  10 R is caused by the influence of sensitivity directivity of the light detector  15 . That is, the light detector  15  has high sensitivity in a predetermined angle range but has low sensitivity in an angular direction far away from its central optical axis. Therefore, not only the intensity distribution of the position detection light but also the sensitivity directivity of the light detector  15  greatly affect the detection result by the light detector  15 . It should be noted that the embodiment shown in  FIGS. 14A  and  14 B is a reference example of the invention but is not a related art. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide an optical position detector that can be less subject to sensitivity directivity of a light detector even when position detection is performed using an intensity distribution of a position detection light within a detection area, and a display device with a position detection function including the optical position detector. 
     A first aspect of the invention is directed to an optical position detector for optically detecting a position of a target object within a detection area, including: a position-detecting light source unit that emits a position detection light toward the detection area to form an intensity distribution of the position detection light in the detection area; a plurality of light detectors with their central optical axes directed to different areas from each other within the detection area; and a signal processing unit that detects a position of the target object based on the result of receiving, by the plurality of light detectors, the position detection light reflected by the target object in the detection area. 
     In the first aspect of the invention, the intensity distribution of the position detection light is formed in an in-plane direction of the detection area, and the position detection light reflected by the target object in the detection area is detected by the light detectors. In this case, the light detectors are used in plural numbers, and the plurality of light detectors have central optical axes directed to different areas from each other of the detection area. Accordingly, even when the light detector has sensitivity directivity, the detection area can be covered only with respective high-sensitivity angle ranges of the plurality of light detectors. Therefore, even when position detection is performed using the intensity distribution of the position detection light within the detection area, the position detection is less subject to the sensitivity directivity of the light detectors. 
     In the first aspect of the invention, it is preferable that the plurality of light detectors are arranged at a specified place adjacent to a side portion of the detection area with the central optical axes directed in different angular directions from each other. According to the configuration, the plurality of light detectors can be arranged in a narrow space around the detection area. 
     In this case, it is preferable that the plurality of light detectors include three or more light detectors with central optical axes equally angularly spaced. When the central optical axes are equally angularly spaced, the respective high-sensitivity angle ranges of the plurality of light detectors can be effectively utilized. 
     In the first aspect of the invention, the plurality of light detectors may be configured such that the plurality of light detectors are arranged in an identical side portion of the detection area at positions shifted in an extending direction of the side portion with the central optical axes directed in a direction parallel to each other. 
     In the first aspect of the invention, it is preferable that the light detector includes a light-receiving element including a light-receiving portion, and a directivity adjusting member that reduces a difference between an incident light amount on a central optical axis side of the light-receiving element and an incident light amount in an angular direction away from the central optical axis. With such a configuration, even when sensitivity directivity exists in a high-sensitivity angle range in the light detector, sensitivity can be equalized by the directivity adjusting member. Therefore, the position detection accuracy can be enhanced. 
     In the first aspect of the invention, as the directivity adjusting member, a light shielding member that causes a light incident opening to become narrower on the central optical axis side of the light-receiving element than in an angular direction away from the central optical axis can be used. With such a configuration, the sensitivity directivity within the high-sensitivity angle range in the light detector can be moderated with the simple configuration that the size of the light incident opening is increased or decreased. Therefore, the position detection accuracy can be enhanced. 
     In this case, it is preferable that the light shielding member includes as the light incident opening a slit that extends from the central optical axis side of the light-receiving element toward an angular direction side away from the central optical axis, and that a width dimension of the slit on the central optical axis side is narrower than a width dimension of the slit in an angular direction away from the central optical axis. When the light incident opening is formed as a slit, the sensitivity directivity within the high-sensitivity angle range can be properly eliminated in the light detector by changing the slit width continuously or stepwise. Therefore, the position detection accuracy can be enhanced. 
     In the first aspect of the invention, it is preferable that the signal processing unit includes a received-light intensity-determining section that determines absolute magnitude of received-light intensities by the plurality of light detectors or relative magnitude of received-light intensities by the plurality of light detectors, and a position detection section that detects a position of the target object based on, of received-light results by the plurality of light detectors, a received-light result by a light detector determined as having a higher received-light intensity in the determination result by the received-light intensity-determining section. With such a configuration, a position of the target object can be detected only using the received-light result by a light detector close to the target object of the plurality of light detectors. 
     A second aspect of the invention is directed to, for example, a display device with a position detection function including the optical position detector to which the first aspect of the invention is applied. In this case, the display device with a position detection function has an image forming device that forms an image in an area overlapping the detection area. 
     The display device with a position detection function according to the second aspect of the invention is used not only for various display devices such as projection type display devices but also for electronic apparatuses such as mobile phones, car navigation systems, personal computers, ticket-vending machines, and bank terminals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIGS. 1A and 1B  are explanatory views schematically showing the configuration of a display device with a position detection function to which the invention is applied. 
         FIGS. 2A to 2C  are explanatory views and a graph showing the basic configuration of an optical position detector to which the invention is applied. 
         FIG. 3  is a graph showing sensitivity directivity of a photodiode used for a light detector in the optical position detector to which the invention is applied. 
         FIG. 4  is an explanatory view showing the configuration of the light detector and a signal processing unit used for the optical position detector to which the invention is applied. 
         FIGS. 5A to 5D  are explanatory views of the light detector used for the optical position detector to which the invention is applied. 
         FIG. 6  is an explanatory view showing a modified example of the signal processing unit used for the optical position detector to which the invention is applied. 
         FIGS. 7A and 7B  are explanatory views of another position-detecting light source unit used for the optical position detector to which the invention is applied. 
         FIG. 8  is an explanatory view of another position-detecting light source unit used for the optical position detector to which the invention is applied. 
         FIG. 9  is an exploded perspective view of an optical position detector and a display device with a position detection function according to a first modified example of the invention. 
         FIG. 10  is an explanatory view showing the cross-sectional configuration of the optical position detector and the display device with a position detection function according to the first modified example of the invention. 
         FIG. 11  is an exploded perspective view of an optical position detector and a display device with a position detection function according to a second modified example of the invention. 
         FIGS. 12A and 12B  are an explanatory view showing a cross-sectional configuration and a graph, respectively, of the optical position detector and the display device with a position detection function according to the second modified example of the invention. 
         FIGS. 13A to 13C  are explanatory views of electronic apparatuses using the display device with a position detection function according to the invention. 
         FIGS. 14A and 14B  are explanatory views of an optical position detector according to a reference example of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the invention will be described in detail with reference to the accompanying drawings. In the following description, an in-plane direction within a detection area is defined as an XY plane in an XYZ orthogonal coordinate, while a direction orthogonal to the in-plane direction within the detection area is defined as a Z-axis direction. 
     Configuration of Optical Position Detector and Display Device with Position Detection Function
 
Overall Configuration of Display Device with Position Detection Function
 
       FIGS. 1A and 1B  are explanatory views schematically showing the configuration of a display device with a position detection function to which the invention is applied, in which  FIG. 1A  is an explanatory view schematically showing the main part of the display device with a position detection function as viewed obliquely from above; and  FIG. 1B  is an explanatory view schematically showing the same as viewed from the side. 
     The display device with a position detection function  100  shown in  FIGS. 1A and 1B  includes an optical position detector  10  and an image forming device  200 . The optical position detector  10  detects, when a target object Ob such as a finger approaches a detection area  10 R based on an image displayed by the image forming device  200 , a planar position (X-coordinate position and Y-coordinate position) of the target object Ob. 
     As will be described in detail later, the optical position detector  10  has a position-detecting light source unit  11  including a plurality of position-detecting light sources  12  each emitting a position detection light formed of an infrared light, and a light detector  15  having light-receiving portions  151  each directed to the detection area  10 R. In the embodiment, the position-detecting light source unit  11  also includes a light guide plate  13  disposed parallel to the XY plane. The light detector  15  includes a light-receiving element such as a photodiode or a phototransistor. 
     In the embodiment, the image forming device  200  is of the projection type and has a screen member  220  disposed on a front side (input operation side) of the light guide plate  13  in an overlapped manner and an image projector  250  that projects a display light in an enlarged manner on one surface  220   s  side of the screen member  220 . The image forming device  200  has an image display area  20 R on the screen member  220 . On the one surface  220   s  side where the image projector  250  is positioned with respect to the screen member  220 , the detection area  10 R of the optical position detector  10  is positioned, while on the other surface  220   t  side of the screen member  220 , the position-detecting light source unit  11  including the light guide plate  13  and the position-detecting light sources  12  is arranged. In the embodiment, the image display area  20 R is an area substantially overlapping the detection area  10 R. 
     In the embodiment, various kinds of those described below can be used for the screen member  220 . Any of those is formed of a material capable of transmitting an infrared light. First, as the screen member  220 , a white screen formed of a cloth having a surface coated with white paint or an embossed white vinyl material can be used. Moreover, as the screen member  220 , a silver screen having high silver color for increasing the light reflectance can be used. Further, as the screen member  220 , a pearl screen having a resinated cloth surface for increasing the light reflectance or a beads screen having a surface coated with fine glass powders for increasing the light reflectance can be used. The screen member  220  is configured as a manual hanging screen with a light-receiving element or a motorized screen with a light-receiving element. 
     Although  FIGS. 1A and 1B  show the example in which the image projector  250  is arranged in front of the screen member  220 , the image projector  250  sometimes obliquely provides a display light toward the screen member  220  as shown by dashed-dotted lines in  FIG. 1B . 
     Basic Configuration of Optical Position Detector  10   
       FIGS. 2A to 2C  are explanatory views and a graph showing the basic configuration of the optical position detector  10  to which the invention is applied, in which  FIG. 2A  is an explanatory view schematically showing the cross-sectional configuration of the optical position detector  10 ;  FIG. 2B  is an explanatory view showing the configuration of the light guide plate  13  and the like used for the optical position detector; and  FIG. 2C  is a graph showing the attenuation condition of a position-detecting infrared light within the light guide plate  13 . In  FIGS. 2A to 2C , the Z-axis direction is the vertical direction. 
     As shown in  FIGS. 2A and 2B , in the optical position detector  10  of the embodiment, the position-detecting light source unit  11  includes the light guide plate  13  having substantially a rectangular planner shape. At a side edge surface  13   m  of the light guide plate  13 , side portions  13   k  and  13   l  corresponding to long sides face each other in a Y-axis direction, and side portions  13   i  and  13   j  corresponding to short sides face each other in an X-axis direction. According to the shape of the light guide plate  13 , the optical position detector  10  has four position-detecting light sources  12 A to  12 D (the position-detecting light sources  12  shown in  FIGS. 1A and 1B ) emitting position detection lights L 2   a  to L 2   d . The light guide plate  13  includes at the side edge surface  13   m  four light incident portions  13   a  to  13   d  on which the position detection lights L 2   a  to L 2   d  are incident. The light guide plate  13  includes at one surface (upper surface in the drawing) a light exiting surface  13   s  from which the position detection lights L 2   a  to L 2   d  propagating through the inside of the light guide plate  13  exit. The light exiting surface  13   s  and the side edge surface  13   m  are orthogonal to each other. The optical position detector  10  includes the light detector  15  (light detectors  15 A,  15 B, and  15 C) having the light-receiving portion  151  directed to the detection area  10 R. 
     In the embodiment, both the four position-detecting light sources  12 A to  12 D and the four light incident portions  13   a  to  13   d  are disposed at corner portions  13   e ,  13   f ,  13   g , and  13   h  of the light guide plate  13 . The position-detecting light sources  12 A to  12 D are arranged so as to face the light incident portions  13   a  to  13   d  and preferably arranged so as to be in close contact with the light incident portions  13   a  to  13   d.    
     The light guide plate  13  is formed of a transparent resin plate such as polycarbonate or acrylic resin. In the light guide plate  13 , a surface asperity structure, a prism structure, a scattering layer (all not shown), and the like are disposed on the light exiting surface  13   s  or a back surface  13   t  on the opposite side of the light exiting surface  13   s . With such a light scattering structure, the light incident from the light incident portions  13   a  to  13   d  to propagate through the inside of the light guide plate is gradually deflected as it progresses in its propagation direction and exits from the light exiting surface  13   s . On the light exiting side of the light guide plate  13 , an optical sheet such as a prism sheet or a light scatter is sometimes arranged as necessary for equalizing the position detection lights L 2   a  to L 2   d.    
     The position-detecting light sources  12 A to  12 D each include a light-emitting element such as a light-emitting diode (LED) and emit as diverging light the position detection lights L 2   a  to L 2   d  formed of an infrared light in response to a drive signal output from a drive circuit (not shown). The kinds of the position detection lights L 2   a  to L 2   d  are not particularly limited, but the position detection lights L 2   a  to L 2   d  may be different in wavelength distribution from a visible light or may be different in light-emitting mode therefrom by adding modulation such as flashing. The position detection lights L 2   a  to L 2   d  preferably have a wavelength band in which the light is effectively reflected by the target object Ob such as a finger or a touch pen. Accordingly, when the target object Ob is a human body such as a finger, it is desirable that the position detection light is an infrared ray (especially a near infrared ray close to the visible light range, for example, close to a wavelength of 850 nm) that has a high reflectance for the surface of a human body, or has a wavelength of 950 nm. 
     The position-detecting light sources  12 A to  12 D are disposed essentially in plural numbers and configured so as to emit the position detection lights L 2   a  to L 2   d  from different positions from one another. Of the four position-detecting light sources  12 A to  12 D, the position-detecting light sources at diagonal positions are paired to form a first light source, and the other two position-detecting light sources are paired to form a second light source. Of the four position-detecting light sources  12 A to  12 D, the neighboring two position-detecting light sources are paired to form a first light source pair, and the other two position-detecting light sources are paired to form a second light source pair, in some cases. 
     The detection area  10 R is a planar area where the position detection lights L 2   a  to L 2   d  are emitted to a viewing side (operation side) and the reflected light can be generated by the target object Ob. In the embodiment, the planar shape of the detection area  10 R is a rectangle in which the light detector  15  (the light detectors  15 A,  15 B, and  15 C) is arranged substantially at the central portion, in a length direction, of one side portion of the four side portions. In the detection area  10 R, the inner angle of the corner portion formed by adjacent sides is 90 degrees, which is the same angle as the inner angle of the corner portions  13   e  to  13   h  of the light guide plate  13 . 
     In the thus configured display device with a position detection function  100 , the position detection light L 2   a  and the position detection light L 2   b  exit from the light exiting surface  13   s  while counterpropagating through the inside of the light guide plate  13  in a direction indicated by the arrows A. The position detection light L 2   c  and the position detection light L 2   d  exit from the light exiting surface  13   s  while counterpropagating in a direction (direction indicated by the arrows B) crossing the direction indicated by the arrows A. Accordingly, the light amount of the position detection light L 2   a  exiting from the light guide plate  13  toward the detection area  10 R has an intensity distribution that linearly declines with the distance from the position-detecting light source  12 A as indicated by a solid line in  FIG. 2C . The light amount of the position detection light L 2   b  exiting toward the detection area  10 R has an intensity distribution that linearly declines with the distance from the position-detecting light source  12 B as indicated by a dotted line in  FIG. 2C . 
     Basic Principle for Detecting XY Coordinates 
     A method of obtaining XY coordinates of the target object Ob based on the detection by the light detector  15  will be described. Various obtaining methods of the position information are conceivable. For example, one example is a method of determining the ratio of attenuation coefficients based on the ratio of the detected light amounts of two position detection lights and determining the propagation distances of both the position detection lights based on the ratio of the attenuation coefficients to determine a position coordinate in a direction connecting the two corresponding light sources. Another example is a method of determining the difference of the detected light amount between two position detection lights and determining, based on the difference, a position coordinate in a direction connecting the two corresponding light sources. Any of the methods includes a method of using the output value from the light detector  15  as it is for calculation and a method of using, for calculation, the time until the voltage across terminals of a capacitor reaches a predetermined voltage by causing the capacitor to store charge or discharge via the light detector  15 . Any of the cases uses the nature described below. 
     In the display device with a position detection function  100 , the position detection lights L 2   a  to L 2   d  emitted from the position-detecting light sources  12 A to  12 D respectively enter the inside of the light guide plate  13  from the light incident portions  13   a  to  13   d  and gradually exit from the light exiting surface  13   s  while propagating through the inside of the light guide plate  13 . As a result, the position detection lights L 2   a  to L 2   d  exit from the light exiting surface  13   s  in a planar manner. 
     For example, the position detection light L 2   a  gradually exits from the light exiting surface  13   s  while propagating through the inside of the light guide plate  13  from the light incident portion  13   a  toward the light incident portion  13   b . Similarly, the position detection lights L 2   c  and L 2   d  gradually exit from the light exiting surface  13   s  while propagating through the inside of the light guide plate  13 . Accordingly, when the target object Ob such as a finger is arranged in the detection area  10 R, the position detection lights L 2   a  to L 2   d  are reflected by the target object Ob, and portion of the reflected light is detected by the light detector  15 . 
     In this case, it is considered that the light amount of the position detection light L 2   a  emitted to the detection area  10 R linearly declines with the distance from the position-detecting light source  12 A as indicated by the solid line in  FIG. 2C , and that the light amount of the position detection light L 2   b  emitted to the detection area  10 R linearly declines with the distance from the position-detecting light source  12 B as indicated by the dotted line in  FIG. 2C . 
     When the control amount (for example, current amount), conversion coefficient, and emitting light amount of the position-detecting light source  12 A are Ia, k, and Ea respectively, and the control amount (current amount), conversion coefficient, and emitting light amount of the position-detecting light source  12 B are Ib, k, and Eb respectively, the following equations are given. 
     
       
      
       Ea=k·Ia  
      
     
     
       
      
       Eb=k·Ib  
      
     
     When the attenuation coefficient and detected light amount of the position detection light L 2   a  are fa and Ga respectively, and the attenuation coefficient and detected light amount of the position detection light L 2   b  are fb and Gb respectively, the following equations are given. 
     
       
      
       Ga=fa·Ea=fa·k·Ia  
      
     
     
       
      
       Gb=fb·Eb=fb·k·Ib  
      
     
     Accordingly, if Ga/Gb, which is the ratio of the detected light amounts of both the position detection lights, can be detected in the light detector  15 , the following equation is given. 
         Ga/Gb =( fa·Ea )/( fb·Eb )=( fa/fb )·( Ia/Ib )
 
     Therefore, when the values corresponding to the ratio Ea/Eb of the emitting light amounts and the ratio Ia/Ib of the control amounts are known, the ratio fa/fb of the attenuation coefficients can be known. If there is a linear relation between the ratio of the attenuation coefficients and the ratio of the propagation distances of both the position detection lights, the position information of the target object Ob can be obtained by previously setting the linear relation. 
     As a method of determining the ratio fa/fb of the attenuation coefficients, for example, the position-detecting light source  12 A and the position-detecting light source  12 B are caused to flash out of phase (for example, drive signals having a rectangular waveform or a sine waveform are operated so as to be 180 degrees of phase difference with each other at a frequency at which a phase difference caused by the difference of propagation distance is negligible), and the waveform of the detected light amount is analyzed. More realistically, for example, one control amount Ia is fixed (Ia=Im), the other control amount Ib is controlled so that the waveform to be detected cannot be observed, that is, so that the ratio Ga/Gb of the detected light amounts becomes 1, and the ratio fa/fb of the attenuation coefficients is derived from the control amount Ib=Im·(fa/fb) at this time. 
     Moreover, the control amounts may be controlled so that the sum of both the control amounts is always constant, that is, so as to satisfy the following equation. 
     
       
      
       Im=Ia+Ib  
      
     
     In this case, the following equation is given. 
         Ib=Im·fa /( fa+fb ) 
     Therefore, assuming that fa/(fa+fb)=α, the ratio of attenuation coefficients is determined by the following equation. 
         fa/fb =α/(1−α)
 
     Accordingly, the position information of the target object Ob in the arrow A direction can be obtained by driving the position-detecting light source  12 A and the position-detecting light source  12 B out of phase with each other. The position information of the target object Ob in the arrow B direction can be obtained by driving the position-detecting light source  12 C and the position-detecting light source  12 D out of phase with each other. Therefore, the position coordinates of the target object Ob on the XY plane can be obtained by sequentially performing the detection operation in the direction A and the direction B in a control system. 
     As described above, for obtaining the planar position information of the target object Ob within the detection area  10 R based on the light amount ratio of the position detection lights detected by the light detector  15 , a configuration can be employed in which, for example, a microprocessing unit (MPU) is used as a signal processing unit by which processing is performed according to the execution of predetermined software (operation program). Also a configuration can be employed in which processing is performed with a signal processing unit using hardware such as a logic circuit. Such a signal processing unit may be incorporated as a part of the display device with a position detection function  100  or may be configured in an electronic apparatus on which the display device with a position detection function  100  is mounted. 
     Detecting Method of Embodiment 
     For detecting the X-coordinate position of the target object Ob in the detection area  10 R in the optical position detector  10  of the embodiment, the position-detecting light sources  12 A and  12 D are driven in phase, the position-detecting light sources  12 B and  12 C are driven in phase, and the position-detecting light sources  12 A and  12 D and the position-detecting light sources  12 B and  12 C are driven out of phase. That is, a first period during which the position-detecting light sources  12 A and  12 D are turned on, and the position-detecting light sources  12 B and  12 C are turned off, to form an intensity distribution having a high exiting intensity in one direction of the X-axis direction, and a second period during which the position-detecting light sources  12 B and  12 C are turned on, and the position-detecting light sources  12 A and  12 D are turned off, to form an intensity distribution having a high exiting intensity in the other direction of the X-axis direction, are alternately set. Accordingly, by using the ratio or difference of the detected value of the light detector  15  between the first period and the second period in a later-described signal processing unit, the X coordinate of the target object Ob in the detection area  10 R can be detected. 
     For detecting the Y-coordinate position of the target object Ob in the detection area  10 R, the position-detecting light sources  12 A and  12 C are driven in phase, the position-detecting light sources  12 B and  12 D are driven in phase, and the position-detecting light sources  12 A and  12 C and the position-detecting light sources  12 B and  12 D are driven out of phase. That is, a first period during which the position-detecting light sources  12 A and  12 C are turned on, and the position-detecting light sources  12 B and  12 D are turned off, to form an intensity distribution having a high exiting intensity in one direction of the Y-axis direction, and a second period during which the position-detecting light sources  12 B and  12 D are turned on, and the position-detecting light sources  12 A and  12 C are turned off, to form an intensity distribution having a high exiting intensity in the other direction of the Y-axis direction, are alternately set. Accordingly, by using the ratio or difference of the detected value of the light detector  15  between the first period and the second period in a position detection section of the signal processing unit, the Y coordinate of the target object Ob in the detection area  10 R can be detected. 
     Here, the detection of the Z coordinate may be performed by causing the four position-detecting light sources  12 A to  12 D to be turned on simultaneously and forming an intensity distribution of the position detection light in the Z-axis direction. 
     Detailed Configuration of Optical Position Detector 
       FIG. 3  is a graph showing the sensitivity directivity of the photodiode used for the light detector  15  in the optical position detector  10  to which the invention is applied.  FIG. 4  is an explanatory view showing the configuration of the light detector  15  and the signal processing unit used for the optical position detector and the display device with a position detection function to which the invention is applied. 
     The light detector  15  shown in  FIGS. 1A to 2B  includes photodiodes as light-receiving elements. The photodiode has sensitivity directivity shown in  FIG. 3 .  FIG. 3  shows the relation between an angle Φ formed relative to the central optical axis of the light detector  15  and a sensitivity f(Φ), where the sensitivity f(Φ) on the side of the central optical axis (front) of the light detector  15  is 1. As shown in  FIG. 3 , the sensitivity f(Φ) of the light detector  15  is maximum on the side of the central optical axis (front); as the angle Φ formed relative to the central optical axis of the light detector  15  increases, the sensitivity f(Φ) decreases; and the sensitivity f(Φ) is 0 in an angular direction of 90° relative to the central optical axis. In this case, in an angle range where the angle Φ formed relative to the central optical axis of the light detector  15  is 30° or less on one side (angle range of 60° with the central optical axis as a center), the sensitivity f(Φ) is 0.87 or more. In such a high-sensitivity angle range, the position detection described with reference to  FIGS. 2A to 2C  can be performed accurately. On the other hand, when the angle Φ formed relative to the central optical axis of the light detector  15  exceeds 30° on one side, the sensitivity f(Φ) decreases. In such an angle range, detection error increases. 
     In the embodiment, therefore, the plurality of light detectors with their central optical axes directed to different areas of the detection area  10 R are disposed as the light detector  15  as shown in  FIGS. 1A and 1B ,  2 A and  2 B, and  4 , and a high-sensitivity angle range of each of the plurality of light detectors  15  is used for position detection. In the embodiment, the three light detectors  15 A,  15 B, and  15 C are used as the plurality of light detectors  15 . In  FIG. 4 , the central optical axes of the light detectors  15 A,  15 B, and  15 C are indicated by a dashed-dotted line L 151 A, a solid line L 151 B, and a dashed-two dotted line L 151 C, respectively. 
     In this case, the three light detectors  15 A,  15 B, and  15 C are arranged at a specified place (specified place adjacent to the side portion  13   l  of the light guide plate  13 ) adjacent to the side portion of the detection area  10 R with their central optical axes directed to different angular directions from one another. The three light detectors  15 A,  15 B, and  15 C are arranged so that the central optical axes are equally angularly spaced. In the embodiment, the angle formed by the central optical axes of the neighboring light detectors  15 A,  15 B, and  15 C is 60° or substantially 60°. Also in the embodiment, in view of the sensitivity directivity described with reference to  FIG. 3 , the high-sensitivity angle range (angle range in which the sensitivity f(Φ) is high) of 60° with the central optical axis as a center is used in each of the light detectors  15 A,  15 B, and  15 C. That is, the high-sensitivity angle ranges of the light detectors  15 A,  15 B, and  15 C are indicated by αA, αB, and αC, respectively, in  FIG. 4 . Substantially the entire detection area  10 R is covered with the high-sensitivity angle ranges αA, αB, and αC of the light detectors  15 A,  15 B, and  15 C. In the example shown in  FIG. 4 , although portions of the detection area  10 R are not covered with the high-sensitivity angle ranges αA, αB, and αC of the light detectors  15 A,  15 B, and  15 C, input operation is not performed in the portions, causing no trouble. For covering the entire detection area  10 R with the high-sensitivity angle ranges αA, αB, and αC of the light detectors  15 A,  15 B, and  15 C, the light detectors  15 A,  15 B, and  15 C are arranged away from the detection area  10 R. 
     Detailed Configuration of Light Detector  15   
       FIGS. 5A to 5D  are explanatory views of the light detector  15  used for the optical position detector and the display device with a position detection function to which the invention is applied, in which  FIG. 5A  is a perspective view of the light detector  15 ;  FIG. 5B  is an exploded perspective view of the light detector  15  as viewed from an upper perspective;  FIG. 5C  is an exploded perspective view of the light detector  15  as viewed from a lower perspective; and  FIG. 5D  is an elevation view of the light detector  15 . 
     In the embodiment, based on the sensitivity directivity shown in  FIG. 3 , position detection is performed based on the result of light received in the high-sensitivity angle ranges αA, αB, and αC of the light detector  15 . As will be understood from  FIG. 3 , however, the sensitivity f(Φ) varies in a range of from 1 to 0.87 even in the high-sensitivity angle ranges αA, αB, and αC (range of 30° on one side). 
     In the embodiment, therefore, as shown in  FIGS. 5A to 5D , the light detector  15  includes a light-receiving element  150  (photodiode) including the light-receiving portion  151  and a directivity adjusting member  155  that reduces the difference between an incident light amount on a central optical axis L 150  side of the light-receiving element  150  and an incident light amount in an angular direction away from the central optical axis. In the embodiment, the directivity adjusting member  155  includes a first light shielding member  156  formed of a black resin molded article and a second light shielding member  157  formed of a black resin molded article interposing the light-receiving element  150  between the first light shielding member  156  and the second light shielding member  157 . The directivity adjusting member  155  (the first light shielding member  156  and the second light shielding member  157 ) causes a light incident opening to become narrower on the central optical axis L 150  side of the light-receiving element  150  than in an angular direction away from the central optical axis L 150 . 
     More specifically, in the directivity adjusting member  155  as shown in  FIGS. 5A and 5D , the first light shielding member  156  and the second light shielding member  157  form as a light incident opening a slit  158  that extends from the central optical axis L 150  side of the light-receiving element  150  to both sides in a circumferential direction. A width dimension Ga of the slit  158  on the central optical axis L 150  side is narrower than a width dimension Gb in an angular direction away from the central optical axis L 150 . 
     For configuring the directivity adjusting member  155  having such a configuration, the first light shielding member  156  includes a substantially rectangular parallelepiped base  156   a  that holds two lead wires  150   a  and  150   b  of the light-receiving element  150  and a half-disc-shaped light shielding portion  156   b  that protrudes forward from the front surface of the base  156   a . Similarly to the first light shielding member  156 , the second light shielding member  157  includes a substantially rectangular parallelepiped base  157   a  and a half-disc-shaped light shielding portion  157   b  that protrudes forward from the front surface of the base  157   a.    
     In this case, the light-receiving element  150  protrudes at the base  156   a  of the first light shielding member  156  on the surface side overlapping the second light shielding member  157 . In the second light shielding member  157 , a recess  157   e  is formed at a portion overlapping the light-receiving element  150 , and the recess  157   e  is open at its front. Therefore, the first light shielding member  156  and the second light shielding member  157  can be overlapped so that the bases  156   a  and  157   a  overlap, and the light-receiving portion  151  of the light-receiving element  150  is open to the outside via the recess  157   e  in a state where the first light shielding member  156  and the second light shielding member  157  are overlapped. 
     Holes  156   s  and  157   s  in communication with each other are formed in one end portions of the bases  156   a  and  157   a , respectively, and holes  156   t  and  157   t  in communication with each other are formed in the other end portions of the bases  156   a  and  157   a , respectively. Therefore, by threaded engagement at the holes  156   s  and  157   s  and threaded engagement at the holes  156   t  and  157   t , the first light shielding member  156  and the second light shielding member  157  can be coupled. Moreover, the bases  156   a  and  157   a  may be adhesively fixed to each other to couple the first light shielding member  156  with the second light shielding member  157 . Here, the forming region of the hole  156   s  is a recess  156   r  in the base  156   a.    
     In the thus configured directivity adjusting member  155 , when the first light shielding member  156  and the second light shielding member  157  are coupled, the slit  158  is formed in an angle range of about 180° between the light shielding portion  156   b  and the light shielding portion  157   b . The light-receiving portion  151  of the light-receiving element  150  is positioned at the back of the slit  158 . In this case, since the light shielding portion  156   b  of the first light shielding member  156  has a constant thickness, an inner surface  156   c  on the side where the slit  158  is positioned in the light shielding portion  156   b  is a surface parallel to the central optical axis L 150  of the light-receiving element  150 . On the other hand, in the light shielding portion  157   b  of the second light shielding member  157 , an outer surface on the opposite side to the side where the slit  158  is positioned is a surface parallel to the central optical axis L 150  of the light-receiving element  150 , but an inner surface  157   c  on the side where the slit  158  is positioned is a tapered surface. Therefore, a thickness ta of the light shielding portion  157   b  on the central optical axis L 150  side is greater than a thickness tb in an angular direction away from the central optical axis L 150 . Therefore, the width dimension Ga of the slit  158  on the central optical axis side is narrower than the width dimension Gb in the angular direction away from the central optical axis. In this case, the width dimension of the slit  158  continuously expands from the side where the central optical axis L 150  is positioned to both the end portions in the circumferential direction, which cancels out variations in the sensitivity f(Φ) in the light detector  15 . For example, the width dimension of the slit  158  at each angular position is the inverse of the sensitivity f(Φ). For the width dimension of the slit  158 , a configuration may be employed in which the width dimension expands stepwise from the side where the central optical axis L 150  is positioned toward both end portions in the circumferential direction. Moreover, the light detector  15  is used in a range of 30° on one side, but the slit  158  is formed in a range of 90° on one side. 
     In the thus configured light detector  15 , the sensitivity f(Φ) varies in the range of from 1 to 0.87 in an angle range of 30° on one side with the light-receiving element  150  alone. However, when the light-receiving element  150  is combined with the directivity adjusting member  155 , the light incident opening is narrower on the central optical axis L 150  side of the light-receiving element  150  than in an angular direction away from the central optical axis L 150 . Therefore, the sensitivity f(Φ) in a range of 30° on one side is constant. That is, in the light detector  15 , the sensitivity f(Φ) is adjusted to the sensitivity (=0.87) at an angle of 30° shown in  FIG. 3  in the entire range of 30° on one side. Accordingly, in the light detectors  15 A,  15 B, and  15 C shown in  FIG. 4 , the sensitivity f(Φ) is equal in the entire high-sensitivity angle ranges αA, αB, and αC each of which is the range of 30° on one side. 
     A recess  156   e  is formed in the bottom of the first light shielding member  156  of the embodiment. At an end portion of the recess  156   e , a groove-like through hole  156   g  penetrating through the base  156   a  is formed. In the surface overlapping the second light shielding member  157  in the base  156   a , a portion where the groove-like through hole  156   g  is open serves as a recess  156   f . At a portion overlapping the recess  156   f  in the second light shielding member  157 , a recess  157   f  is formed. Further, the recess  157   f  is communicated with the recess  157   e  into which the light-receiving element  150  is accommodated. Therefore, in the directivity adjusting member  155  shown in  FIGS. 5A to 5D , when the light-receiving element  150  is used in a state of being surface-mounted on a flexible wiring board (not shown), the flexible wiring board can be extended to the outside through the recesses  156   f  and  157   f  and the groove-like through hole  156   g.    
     Specific Configuration of Signal Processing Unit 
     Again in  FIG. 4 , the detection results by the light detectors  15 A,  15 B, and  15 C are output to a signal processing unit  450  in the embodiment. In the embodiment, the signal processing unit  450  includes a received-light intensity-determining section  451  that determines absolute magnitude of received-light intensities by the three light detectors  15 A,  15 B, and  15 C, and three position detection sections  152 A,  152 B, and  152 C that detect a position of the target object Ob in the detection area  10 R based on the received-light result by the three light detectors  15 A,  15 B, and  15 C. In this case, the received-light intensity-determining section  451  compares between the received-light intensities by the three light detectors  15 A,  15 B, and  15 C and a predetermined threshold value, and outputs only a received-light intensity equal to or greater than the threshold value among the received-light intensities by the light detectors  15 A,  15 B, and  15 C to the position detection sections  152 A,  152 B, and  152 C. Therefore, any of the three light detectors  15 A,  15 B, and  15 C detects the position of the target object Ob based only on the received-light result by a light detector determined as having a higher received-light intensity in the determination result by the received-light intensity-determining section  451  and outputs the detection result. 
     In the embodiment, the signal processing unit  450  also includes a coordinate comparing section  453 . The coordinate comparing section  453  compares the position detection results (XY coordinates) output from the position detection sections  152 A,  152 B, and  152 C and outputs the position detection results (XY coordinates) output from the position detection sections  152 A,  152 B, and  152 C when the detected positions are away. On the other hand, when there are coordinates close to each other in the position detection results (XY coordinates) output from the position detection sections  152 A,  152 B, and  152 C, the coordinate comparing section  453  assumes the coordinates as the same coordinates and outputs, for example, values obtained by averaging the coordinates close to each other. 
     Operation 
     In the optical position detector  10  of the embodiment, the position-detecting light sources  12 A and  12 D and the position-detecting light sources  12 B and  12 C are driven out of phase, and the ratio or difference between the detected values by the light detector  15  during the driving is used to detect the X coordinate of the target object Ob in the detection area  10 R. Moreover, the position-detecting light sources  12 A and  12 C and the position-detecting light sources  12 B and  12 D are driven out of phase, and the ratio or difference between the detected values by the light detector  15  during the driving is used to detect the Y coordinate of the target object Ob in the detection area  10 R. At the time of such a detection, the signal processing unit  450  shown in  FIG. 4  can improve the position detection accuracy and perform multi-position detection as will be described below. 
     For example, it is assumed that the target object Ob is present at a position indicated as a point Ob 1  in  FIG. 4 . In this case, the position detection light reflected by the target object Ob at the position indicated as the point Ob 1  is received by the three light detectors  15 A,  15 B, and  15 C. Here, the target object Ob is within the high-sensitivity angle range αA of the light detector  15 A but is out of the high-sensitivity angle ranges αB and αC of the light detectors  15 B and  15 C. Therefore, received-light intensities by the three light detectors  15 A,  15 B, and  15 C are in the following relation. 
       light detector  15 B&lt;light detector  15 C&lt;threshold value&lt;light detector  15 A 
     Accordingly, the received-light intensity-determining section  451  outputs only the detection result of the light detector  15 A among the detection results of the light detectors  15 A,  15 B, and  15 C to the position detection section  152 A. Accordingly, only coordinates detected by the position detection section  152 A are output. 
     It is assumed that the target object Ob is present at a position indicated as a point Ob 2  in  FIG. 4 . In this case, the position detection light reflected by the target object Ob at the position indicated as the point Ob 2  is received by the three light detectors  15 A,  15 B, and  15 C, but the target object Ob is within the high-sensitivity angle range αB of the light detector  15 B. Therefore, received-light intensities by the three light detectors  15 A,  15 B, and  15 C are in the following relation. 
       light detector  15 C&lt;light detector  15 A&lt;threshold value&lt;light detector  15 B 
     Accordingly, the received-light intensity-determining section  451  outputs only the detection result of the light detector  15 B among the detection results of the light detectors  15 A,  15 B, and  15 C to the position detection section  152 B. Accordingly, only coordinates detected by the position detection section  152 B are output. 
     On the other hand, it is assumed that the target object Ob is present at a position indicated as a point Ob 3  in  FIG. 4 . In this case, the target object Ob at the position indicated as the point Ob 3  is within both the high-sensitivity angle ranges αA and αB of the light detector  15 A and the light detector  15 B. Therefore, received-light intensities by the three light detectors  15 A,  15 B, and  15 C are in the following relation. 
       light detector  15 C&lt;threshold value&lt;light detector  15 B&lt;light detector  15 A 
     Accordingly, the received-light intensity-determining section  451  outputs the detection results of the light detectors  15 A and  15 B among the detection results of the light detectors  15 A,  15 B, and  15 C to the position detection sections  152 A and  152 B. Accordingly, coordinates are output from both the position detection sections  152 A and  152 B. In this case, the coordinate comparing section  453  compares the position detection results (XY coordinates) output from the position detection sections  152 A and  152 B. In this case, the coordinate comparing section  453  assumes the detection results as the position of one target object Ob because the detected positions are close to each other and outputs average values of the position detection results (XY coordinates) output from the position detection sections  152 A and  152 B. 
     Also in the embodiment, it is assumed that the target object Ob is present at both the position indicated as the point Ob 1  and the position indicated as the point Ob 2  in  FIG. 4 . In this case, the target object Ob at the position indicated as the point Ob 1  is present in the high-sensitivity angle range αA of the light detector  15 A, while the target object Ob at the position indicated as the point Ob 2  is present in the high-sensitivity angle range αB of the light detector  15 B. Therefore, received-light intensities by the three light detectors  15 A,  15 B, and  15 C are in the following relation. 
       light detector  15 C&lt;threshold value&lt;light detector  15 B≅light detector  15 A
 
     Accordingly, the received-light intensity-determining section  451  outputs the detection results of the light detectors  15 A and  15 B among the detection results of the light detectors  15 A,  15 B, and  15 C to the position detection sections  152 A and  152 B. Accordingly, coordinates are output from both the position detection sections  152 A and  152 B. In this case, the coordinate comparing section  453  outputs the respective position detection results (XY coordinates) output from the position detection sections  152 A and  152 B because the detected positions are away from each other. Accordingly, when the target object Ob is present at both the position indicated as the point Ob 1  and the position indicated as the point Ob 2  in  FIG. 4 , the coordinates of the point Ob 1  and the coordinates of the point Ob 2  are output. 
     Modified Example of Signal Processing Unit  450   
       FIG. 6  is an explanatory view showing a modified example of the signal processing unit  450  used for the optical position detector  10  to which the invention is applied. In the signal processing unit  450  shown in  FIG. 4 , the position of the target object Ob is detected based on the received-light result by the light detector determined as having a received-light intensity greater than the threshold value in the determination result by the received-light intensity-determining section  451 . On the other hand, in the signal processing unit  450  shown in  FIG. 6 , the received-light intensity-determining section  451  outputs a relatively highest received-light intensity among the received-light intensities by the three light detectors  15 A,  15 B, and  15 C to a position detection section  452 . Other configurations are the same as those described with reference to  FIG. 4 . 
     In the case of such a configuration, when the target object Ob is present at the position indicated as the point Ob 3  in  FIG. 4 , received-light intensities by the three light detectors  15 A,  15 B, and  15 C are in the following relation. 
       light detector  15 C&lt;light detector  15 B&lt;light detector  15 A 
     Accordingly, the received-light intensity-determining section  451  outputs only the detection result of the light detector  15 A among the detection results of the light detectors  15 A,  15 B, and  15 C to the position detection section  452 . Accordingly, only coordinates detected based on the received-light result by the light detector  15 A are output from the position detection section  452 . 
     Main Effect of Embodiment 
     As described above, in the optical position detector  10  and the display device with a position detection function  100  of the embodiment, when the position detection lights L 2   a  to L 2   d  exit from the light exiting surface  13   s  of the light guide plate  13  and is reflected by the target object Ob arranged on the light exiting side of the light guide plate  13 , the reflected light is detected by the light detector  15 . In this case, since the intensity of the position detection lights L 2   a  to L 2   d  and the distance from the position-detecting light sources  12 A to  12 D in the detection area  10 R have a certain correlativity, the XY coordinates of the target object Ob can be detected based on the received-light intensity obtained through the light detector  15 . According to such a detection system, since it is sufficient to form the light intensity distribution of the position detection light on the one surface  220   s  side of the screen member  220 , the light guide plate  13  is not necessarily arranged on the front side of the screen member  220 . Therefore, the detection system is suitable for configuring the display device with a position detection function  100  of the type that displays an image on the screen member  220 . 
     In the embodiment, the three light detectors  15  (the light detectors  15 A,  15 B, and  15 C) are used, and the three light detectors  15  have central optical axes directed to different areas from one another of the detection area  10 R. Accordingly, even when the light detector  15  has sensitivity directivity, the detection area  10 R can be covered only with the respective high-sensitivity angle ranges of the three light detectors  15 . Therefore, even when position detection is performed utilizing the intensity distribution of the position detection light within the detection area  10 R, the position detection is less subject to the sensitivity directivity of the light detector  15 . Accordingly, the position detection can be performed with high accuracy. 
     In the embodiment, the three light detectors  15 A,  15 B, and  15 C are arranged at the specified place adjacent to the side portion of the detection area  10 R with their central optical axes directed to different angular directions from one another. Therefore, the plurality of light detectors  15 A,  15 B, and  15 C can be arranged within the narrow space around the detection area  10 R. Since the three light detectors  15 A,  15 B, and  15 C are arranged such that the central optical axes are equally angularly spaced, the high-sensitivity angle ranges of the three light detectors  15 A,  15 B, and  15 C can be effectively utilized. 
     Further, the light detector  15  includes the directivity adjusting member  155  that reduces the difference between the incident light amount on the central optical axis side of the light-receiving element  150  and the incident light amount in an angular direction away from the central optical axis. Therefore, even if sensitivity directivity exists within the high-sensitivity angle range utilized by the light detector  15 , since the sensitivity directivity can be moderated by the directivity adjusting member  155 , the position detection accuracy can be enhanced. 
     The directivity adjusting member  155  includes two light shielding members (the first light shielding member  156  and the second light shielding member  157 ) that cause the light incident opening to become narrower on the central optical axis side of the light-receiving element  150  than in an angular direction away from the central optical axis. Therefore, with such a simple configuration that the size of the light incident opening is increased or decreased, since the sensitivity directivity within the angle range utilized as the high-sensitivity angle range in the light detector  15  can be moderated, the position detection accuracy can be enhanced. Further, the first light shielding member  156  and the second light shielding member  157  include as the light incident opening the slit  158  that extends from the central optical axis side of the light-receiving element  150  toward the angular direction side away from the central optical axis. Therefore, since the sensitivity directivity within the high-sensitivity angle range utilized in the light detector  15  can be effectively canceled out by changing the slit width continuously or stepwise, the position detection accuracy can be enhanced. 
     Configuration of Another Position-Detecting Light Source Unit 
       FIGS. 7A and 7B  are explanatory views of another position-detecting light source unit  11  used for the optical position detector  10  to which the invention is applied. In the above-described embodiment, the light guide plate  13  is used as the position-detecting light source unit  11 . As shown in  FIGS. 7A and 7B , however, a position-detecting light source unit  11  having a configuration without a light guide plate may be employed in which, on a back surface side of a screen member  210 , a substrate  120  on which the plurality of position-detecting light sources  12  are arranged at positions facing the detection area  10 R in the Z-axis direction is disposed. 
     Also in such a configuration, in the case of detecting the X-coordinate position of the target object Ob, when only the position-detecting light sources  12  on one side away in the X-direction are turned on among the plurality of position-detecting light sources  12 , the intensity distribution of the position detection light can be formed. In the case of detecting the Y-coordinate position of the target object Ob, when only the position-detecting light sources  12  on one side away in the Y-direction are turned on among the plurality of position-detecting light sources  12 , the intensity distribution of the position detection light can be formed. 
     Another Layout Example of Light Detector  15   
       FIG. 8  is an explanatory view showing another layout of the light detector  15  in the optical position detector  10  to which the invention is applied. In the above-described embodiment, the three light detectors  15 A,  15 B, and  15 C are arranged at the specified place adjacent to the side portion of the detection area  10 R with their central optical axes directed to different angular directions from one another. In the example, however, as shown in  FIG. 8 , the three light detectors  15 A,  15 B, and  15 C are arranged in an area adjacent to one side portion (side portion along the side portion  13   l  of the light guide plate  13 ) of the detection area  10 R at positions shifted in the extending direction of the side portion with their central optical axes directed in a direction parallel to one another. More specifically, the three light detectors  15 A,  15 B, and  15 C are spaced equally in the long side direction of the detection area  10 R (the image display area  20 R), and the central optical axes of the light detectors  15 A,  15 B, and  15 C extend in a direction orthogonal to the side portions  13   k  and  13   l  of the light guide plate  13 . Also in such a configuration, the entire or substantially entire detection area  10 R can be covered with the high-sensitivity areas of the three light detectors  15 A,  15 B, and  15 C. Accordingly, the position detection accuracy is high. 
     Other Embodiments 
     In the above-described embodiment, the optical position detector  10  using the three light detectors  15  (the light detectors  15 A,  15 B, and  15 C) is illustrated. However, two light detectors  15  or four or more light detectors  15  may be used. 
     In the above-described embodiment, the invention is applied to the landscape-oriented screen member  220 . However, the invention may be applied to the case where a portrait-oriented screen member  220  is used. In the above-described embodiment, the position-detecting light source unit  11  is arranged on the other surface  220   t  side of the screen member  220 . However, the position-detecting light source unit  11  may be arranged on the one surface  220   s  side of the screen member  220 . 
     In the above-described embodiment, the invention is applied to a screen unit used for a projection type display device. However, the invention may be applied to a screen unit used for an electronic blackboard. 
     Modified Examples of Display Device with Position Detection Function  100   
     In the above-described embodiment, the display device with a position detection function  100  is applied to a projection type display device or an electronic blackboard. However, as shown in  FIGS. 9 to 12B , the display device with a position detection function  100  can be used for electronic apparatuses which will be described later with reference to  FIGS. 13A to 13C  by employing a direct-view type display device as the image forming device  200 . 
     First Modified Example of Display Device with Position Detection Function  100   
       FIGS. 9 and 10  are an exploded perspective view and an explanatory view showing a cross-sectional configuration, respectively, of the optical position detector  10  and the display device with a position detection function  100  according to a first modified example of the invention. In the display device with a position detection function  100  of the first modified example, since the optical position detector  10  has the same configuration as that of the above-described embodiment, the common parts are denoted by the same reference numerals and signs, and the description thereof is omitted. 
     The display device with a position detection function  100  shown in  FIGS. 9 and 10  includes the optical position detector  10  and the image forming device  200 . The optical position detector  10  includes the position-detecting light source  12  that emits a position detection light, the light guide plate  13 , and the light detector  15  (the light detectors  15 A,  15 B, and  15 C) with the light-receiving portion  151  directed to the detection area  10 R. The image forming device  200  is a direct-view type display device  208  such as an organic electroluminescence device or a plasma display device and is disposed on the opposite side to the input operation side with respect to the optical position detector  10 . The direct-view type display device  208  includes the image display area  20 R in an area overlapping the light guide plate  13  in plan view. The image display area  20 R overlaps the detection area  10 R in plan view. 
     Second Modified Example of Display Device with Position Detection Function  100   
       FIGS. 11 to 12B  are explanatory views and a graph of the optical position detector  10  and the display device with a position detection function  100  according to a second modified example of the invention, in which  FIG. 11  is an exploded perspective view of the optical position detector  10  and the display device with a position detection function  100 ; and  FIGS. 12A and 12B  are an explanatory view showing the cross-sectional configuration of the same and a graph, respectively. In the display device with a position detection function  100  of the second modified example, the optical position detector  10  has the same configuration as that of the above-described embodiment, the common parts are denoted by the same reference numerals and signs, and the description thereof is omitted. 
     The display device with a position detection function  100  shown in  FIGS. 11 and 12A  includes the optical position detector  10  and the image forming device  200 . The optical position detector  10  includes the position-detecting light source  12  that emits a position detection light, the light guide plate  13 , and the light detector  15  (the light detectors  15 A,  15 B, and  15 C) with the light-receiving portion  151  directed to the detection area  10 R. The image forming device  200  includes a liquid crystal device  209  that is a direct-view type display device and a light-transmissive cover member  30 . The liquid crystal device  209  includes the image display area  20 R in an area overlapping the light guide plate  13  in plan view. The image display area  20 R overlaps the detection area  10 R in plan view. 
     In the display device with a position detection function  100  of the second modified example, an optical sheet  16  for equalizing the position detection lights L 2   a  to L 2   d  is arranged as necessary on the light exiting side of the light guide plate  13 . In the second modified example, as the optical sheet  16 , a first prism sheet  161  facing the light exiting surface  13   s  of the light guide plate  13 , a second prism sheet  162  facing the first prism sheet  161  on the opposite side to the side where the light guide plate  13  is positioned, and a light scatter  163  facing the second prism sheet  162  on the opposite side to the side where the light guide plate  13  is positioned are used. A rectangular frame-shaped light shielding sheet  17  is arranged around the optical sheet  16  on the opposite side to the side where the light guide plate  13  is positioned with respect to the optical sheet  16 . The light shielding sheet  17  prevents the position detection lights L 2   a  to L 2   d  emitted from the position-detecting light sources  12 A to  12 D from leaking. 
     The liquid crystal device  209  (the image forming device  200 ) includes a liquid crystal panel  209   a  on the opposite side to the side where the light guide plate  13  is positioned with respect to the optical sheet  16  (the first prism sheet  161 , the second prism sheet  162 , and the light scatter  163 ). In the second modified example, the liquid crystal panel  209   a  is a transmissive liquid crystal panel having a structure in which two light-transmissive substrates  21  and  22  are bonded together with a sealing material  23  and liquid crystals  24  are filled between the substrates. In the second modified example, the liquid crystal panel  209   a  is an active matrix type liquid crystal panel in which light-transmissive pixel electrodes, data lines, scanning lines, pixel switching elements (all not shown) are formed on one of the two light-transmissive substrates  21  and  22 , and light-transmissive common electrodes (not shown) are formed on the other substrate. Pixel electrodes and a common electrode are sometimes formed on an identical substrate. In the liquid crystal panel  209   a , when a scanning signal is output to each of pixels via the scanning line, and an image signal is output via the data line, the orientation of the liquid crystals  24  are controlled in each of the plurality of pixels. As a result, an image is formed on the image display area  20 R. 
     In the liquid crystal panel  209   a , on one light-transmissive substrate  21 , a substrate extended portion  21   t  extending peripherally from the outline of the other light-transmissive substrate  22  is disposed. On a surface of the substrate extended portion  21   t , electronic components  25  constituting a drive circuit and the like are mounted. A wiring member  26  such as a flexible wiring board or a flexible printed circuit (FPC) is connected to the substrate extended portion  21   t . Here, only the wiring member  26  may be mounted on the substrate extended portion  21   t . As necessary, a polarizer (not shown) is arranged on the outer surface sides of the light-transmissive substrates  21  and  22 . 
     For detecting the planar position of the target object Ob in this case, the position detection lights L 2   a  to L 2   d  have to be emitted to the viewing side where operation is performed by the target object Ob. Therefore, the liquid crystal panel  209   a  is arranged closer to the viewing side (operation side) than the light guide plate  13  and the optical sheet  16 . Accordingly, in the liquid crystal panel  209   a , the image display area  20 R is configured to be able to transmit the position detection lights L 2   a  to L 2   d . When the liquid crystal panel  209   a  is arranged on the opposite side to the viewing side of the light guide plate  13 , the image display area  20 R does not have to be configured so as to transmit the position detection lights L 2   a  to L 2   d . Instead, the image display area  20 R has to be configured so as to be visible from the viewing side through the light guide plate  13 . 
     The liquid crystal device  209  includes an illumination unit  40  for illuminating the liquid crystal panel  209   a . In the second modified example, the illumination unit  40  is arranged between the light guide plate  13  and a reflector  14  on the opposite side to the side where the liquid crystal panel  209   a  is positioned with respect to the light guide plate  13 . The illumination unit  40  includes illuminating light sources  41  and an illuminating light guide plate  43  that causes the illumination light emitted from the illuminating light source  41  to propagate therethrough and exit therefrom. The illuminating light guide plate  43  has a rectangular planar shape. The illuminating light source  41  includes, for example, a light-emitting element such as a light-emitting diode (LED), and emits, for example, a white illumination light L 4  in response to a drive signal output from the drive circuit (not shown). In the second modified example, the illuminating light sources  41  are arranged in plural numbers along a side portion  43   a  of the illuminating light guide plate  43 . 
     The illuminating light guide plate  43  is provided with an inclined surface  43   g  at a surface portion (outer peripheral portion of a light exiting surface  43   s  on the side portion  43   a  side) on a light exiting side adjacent to the side portion  43   a . The illuminating light guide plate  43  gradually increases in thickness toward the side portion  43   a . With the light incident structure having the inclined surface  43   g , the height of the side portion  43   a  is adjusted to the height of the light exiting surface of the illuminating light source  41  while suppressing an increase in thickness of a portion where the light exiting surface  43   s  is disposed. 
     In the illumination unit  40 , the illumination light emitted from the illuminating light source  41  is incident from the side portion  43   a  of the illuminating light guide plate  43  to the inside of the illuminating light guide plate  43 , thereafter propagates through the inside of the illuminating light guide plate  43  toward an outer edge portion  43   b  on the opposite side, and exits from the light exiting surface  43   s  as one surface. In this case, the illuminating light guide plate  43  has a light guide structure in which the light amount ratio of the exiting light from the light exiting surface  43   s  to the internal propagation light monotonously increases from the side portion  43   a  side toward the outer edge portion  43   b  on the opposite side. Such a light guide structure can be realized by, for example, gradually increasing the area of a refracting surface having microscopic asperities for light deflection or light scattering, or the forming density of a printed scattering layer, formed on the light exiting surface  43   s  or a back surface  43   t  of the illuminating light guide plate  43  toward the internal propagation direction. By providing the light guide structure, the illumination light L 4  incident from the side portion  43   a  exits from the light exiting surface  43   s  substantially equally. 
     In the second modified example, the illuminating light guide plate  43  is arranged on the opposite side to the viewing side of the liquid crystal panel  209   a  so as to overlap the image display area  20 R of the liquid crystal panel  209   a  in a planar manner and functions as the so-called backlight. However, the illuminating light guide plate  43  may be configured so as to function as the so-called frontlight by arranging it on the viewing side of the liquid crystal panel  209   a . In the second modified example, the illuminating light guide plate  43  is arranged between the light guide plate  13  and the reflector  14 . However, the illuminating light guide plate  43  may be arranged between the optical sheet  16  and the light guide plate  13 . The illuminating light guide plate  43  and the light guide plate  13  may be configured as a common light guide plate. Also in the second modified example, the optical sheet  16  is shared between the position detection lights L 2   a  to L 2   d  and the illumination light L 4 . However, a dedicated optical sheet different from the optical sheet  16  may be arranged on the light exiting side of the illuminating light guide plate  43 . This reason is as follows. In many cases, a light scatter exhibiting a sufficient light scattering effect is used in the illuminating light guide plate  43  for equalizing the planar brightness of the illumination light L 4  exiting from the light exiting surface  43   s . In the position-detecting light guide plate  13 , however, the large scattering of the position detection lights L 2   a  to L 2   d  exiting from the light exiting surface  13   s  hinders position detection. Therefore, since a light scatter does not have to be disposed, or a light scatter exhibiting a relatively mild light scattering effect has to be used, a light scatter dedicated for the illuminating light guide plate  43  is preferably used. However, an optical sheet having a light condensing effect, such as a prism sheet (the first prism sheet  161  or the second prism sheet  162 ), may be shared. 
     Example of Mounting on Electronic Apparatuses 
     With reference to  FIGS. 13A to 13C , electronic apparatuses to which the display device with a position detection function  100  described with reference to  FIGS. 9 to 12B  is applied will be described.  FIGS. 13A to 13C  are explanatory views of electronic apparatus using the display device with a position detection function according to the invention.  FIG. 13A  shows the configuration of a mobile personal computer including the display device with a position detection function  100 . The personal computer  2000  includes the display device with a position detection function  100  as a display unit and a main body  2010 . The main body  2010  is provided with a power switch  2001  and a keyboard  2002 .  FIG. 13B  shows the configuration of a mobile phone including the display device with a position detection function  100 . The mobile phone  3000  includes a plurality of operation buttons  3001 , scroll buttons  3002 , and the display device with a position detection function  100  as a display unit. An image displayed on the display device with a position detection function  100  is scrolled by operating the scroll buttons  3002 .  FIG. 13C  shows the configuration of a personal digital assistant (PDA) to which the display device with a position detection function  100  is applied. The PDA  4000  includes a plurality of operation buttons  4001 , a power switch  4002 , and the display device with a position detection function  100  as a display unit. When the power switch  4002  is operated, various types of information such as an address book and a schedule note are displayed on the display device with a position detection function  100 . 
     Examples of electronic apparatuses to which the display device with a position detection function  100  is applied include not only the electronic apparatuses shown in  FIGS. 13A to 13C  but also digital still cameras, liquid crystal televisions, viewfinder type or direct-view monitor type video tape recorders, car navigation systems, pagers, electronic notebooks, calculators, word processors, workstations, videophones, POS terminals, and bank terminals. The above-described display device with a position detection function  100  is applicable as a display unit of the various kinds of electronic apparatuses. 
     The entire disclosure of Japanese Patent Application No. 2009-215380, filed Sep. 17, 2009 is expressly incorporated by reference herein.