Abstract:
The coordinate-position input device comprises a substantially flat surface for specifying a desired position using a pointing tool. A reflecting member is provided around this surface. A light emitter emits a flux of light of desired thickness over the entire surface towards the reflecting member. The light flux is such that it is thicker near the reflecting member and becomes thinner and thinner as it goes away from the reflecting member. That is, the light flux diverges as it approaches the reflecting member.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device which optically detects the coordinates of a specified position, thereby it can be used to draw images, write text on electronic boards or the like. More particularly, this invention relates to a device in which a probe beam is emitted towards a reflecting member, probe beam reflected from the reflecting member are received, and the coordinates of the specified position is detected based on the intensity of the received light. 
     BACKGROUND OF THE INVENTION 
     As a conventional device which optically detects the coordinates of a specified position and inputs the detected coordinates to some other device (hereafter referred to as coordinate-position input device), there is one comprising a light emitter that emits a probe beam (irradiation light), a reflector that reflects the probe beam emitted by the light emitter, and a light receiver that receives and converges the probe beam reflected by the reflector. The radiated light is a flux of collimated light beam which is parallel but at a certain height from a coordinate input surface (a whiteboard or a blackboard). 
     FIG. 9 schematically shows the conventional coordinate-position input device  200 . FIG. 10 schematically shows the light receiver when viewed from the vertical direction with respect to the coordinate input surface  202 . A light reception/emission section  203  is provided in this coordinate-position input device  200 . This light reception/emission section  203  comprises a light emitter that emits irradiation light (probe beam) along the coordinate input surface  202 , and a light receiver  220  that receives the reflected light. The light emitter is not specifically shown in these figures. A recursive reflector  204  is provided on the three sides of the coordinate input surface  202 . This recursive reflector  204  comprises a reflection plate that reflects an incoming probe beam recursively to the direction from which light came in. The light emitter comprises a light-emitting element that emits irradiation light, and a cylindrical lens that converges or diffuses the irradiation light emitted by the light-emitting element in a prescribed direction of its travel. The functions of this light emitter will be explained in detail later. The light receiver  220  comprises a light receiving lens  221  that receives and converges the reflection light, and a photoreceptor  222  that detects the intensity of the received light converged by the light receiving lens  221 . When a position on the coordinate input surface  202  is specified by pointing that position with a pointing tool, the coordinate-position input device  200  detects the specified position in terms of its coordinates. The specified position is detected base on detection of the direction θ in which the light is blocked due to the invasion of the pointing tool in the light flux. By the way, the pointing tool may be a pen, finger or the like. 
     Structure of the light emitter will now be explained in detail here. FIG. 11A shows the conventional light emitter when viewed from the direction parallel to the coordinate input surface  202  and also from the direction perpendicular to the direction of travel of the irradiation light. In this light emitter, a positional relation between the light-emitting element  211  and the cylindrical lens  212  is so adjusted that the irradiation light travels parallel to the coordinate input surface  202 . Precisely, the light-emitting element  211  is disposed at one focal point of the cylindrical lens  212 . In other words, the light-emitting element  211  is so positioned that the light coming out of the cylindrical lens  212  is parallel to the optical axis of the light emitted from the light-emitting element  211 . The reason why the light-emitting element  211  is positioned in this manner is as follows. That is, by positioning the light-emitting element  211  in this manner, if the collimated light beam is blocked by the pointing tool at some place, then the shadow of the pointing tool proceeds as it is without changing its shape because of the property of the collimated light beam. In other words, a sharp shadow of the pointing took fall on the photoreceptor. Conventionally, as shown in FIG. 11B, it was considered that, because the shadow of the pointing tool is sharp it appears as a dark spot on the photoreceptor, whereby the position of the pointing tool can be detected with highest precision. 
     On the other hand, in another conventional method, the cylindrical lens  212  and the light-emitting element  211  are so adjusted that the light coming out of the cylindrical lens  212  is not parallel but becomes narrower (that, is the light converges) as it reaches the recursive reflector  204  as shown in FIG.  12 A. This method will be called as the method of converging the light to differentiate it from the above-explained method of parallel light. When the light is converging, since the irradiation light is not collimated light, due to diffraction or the like, the shadow of the pointing tool is not sharp. Accordingly, as shown in FIG. 12B, the negative peak of the intensity of the, corresponding to the position of the pointing tool is not very distinct, furthermore, the peak is not sharp. Because of these facts the precision in detection of the position of the pointing tool degrades. 
     An experiment was conducted as follows. FIG. 13A to FIG. 13B show detection characteristics of the coordinate-position input device observed in this experiment. The distance between the pointing tool and the coordinate input surface is plotted along the horizontal axis. The degree of detection precision (sensitivity of the photoreceptor) is plotted along the vertical axis. The lower the value of this degree of detection precision, the higher is the precision. The method of converging light explained with respect to FIG. 12A was employed in this experiment. Further, two pointing tools, one with a diameter of 5 mm and the other with a diameter of 20 mm were used to specify a position. FIG. 13A shows the detection characteristics when the direction  0  of the pointing tool is zero degree. FIG. 13B shows the detection characteristics when the direction θ of the pointing tool is 20 degrees. Finally, FIG. 13C shows the detection characteristics when the direction θ of the pointing tool is 40 degrees. Each line in these plots shows the detection characteristics corresponding to the distance between the pointing tool and the light receiving lens  221 . FIG. 14A to FIG. 14C correspond to FIG. 13A to FIG. 13C with the difference that the method of parallel light explained with respect to FIG. 11A was employed. 
     When the method of parallel light is employed, it is clear from FIG. 14A to FIG. 14C that, the precision of detection of the specified position increases as the distance between the pointing tool and the coordinate input surface decreases. Further, it is apparent that, the distance between the pointing tool and the light receiving lens  221 , the direction θ, or the size of the pointing tool does not make any difference. In other words, if the method of parallel light is employed in the coordinate-position input device, then the specified position can be detected at a high precision. Furthermore, the detection characteristics depend only on the distance between the pointing tool and the coordinate input surface and does not depend on any other parameter. 
     On the contrary, when the method of converging light is employed, it is clear from FIG. 13A to FIG. 13C that, the detection precision is not uniform because it is affected by the distance between the pointing tool and the light receiving lens  221 , the direction θ, or the size of the pointing tool. Particularly, the detection precision is low even if the pointing tool is brought very close (of the order of 1.0 mm) to the coordinate input surface. 
     As can be seen from the results of the experiments, the detection precision in the conventional coordinate-position input device is improved by employing the method of parallel light. 
     However, the conventional method has following problems. Consider that a whiteboard is used as a coordinate input surface, and some image is actually drawn with a pointing tool (for example, a pen) on the coordinate input surface. It is important that the precision in detection of the specified position is high when the pointing tool is touching the coordinate input surface. In other words, whether the coordinate-position input device is good or bad is determined in many cases according to how high the detection precision of input coordinates on the coordinate input surface is. However, as can be seen from FIG. 13A to FIG. 14C, the precession in detection of the specified position in the conventional coordinate-position input device is quite low when the distance between the pointing tool and the coordinate input surface is zero. Thus, there was a requirement of a coordinate-position input device with still higher detection precision as compared to the conventional one. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a coordinate-position input device with improved precision in detection of a coordinate input position. 
     According to the coordinate-position input device of one aspect of this invention, the probe beam spreads along the coordinate input surface rather than being parallel or converging. Thus, when viewed from the side of the light receiver, the light is focused on it because the light is recursively reflected by the recursive reflector. Accordingly, precision in detection of a specified position when the pointing tool touches the coordinate input surface is improved. 
     According to the coordinate-position input device of another aspect of this invention, the coordinate-position input device has a light-emitting element that emits a probe beam to detect a coordinate input position and a refractive lens that refracts the probe beam emitted by the light-emitting element to be a beam flux to travel in a prescribed direction. Further, the light-emitting element is disposed at a location closer to the refractive lens than the focal point of the refractive lens. Thus, when viewed from the side of the light receiver, the light is focused on it because the light is recursively reflected by the recursive reflector. Accordingly, precision in detection of a specified position when the pointing tool touches the coordinate input surface is improved. 
     Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows the structure of a coordinate-position input device according to this invention; 
     FIG.  2 A and FIG. 2B show the structure of a corner cube reflector; 
     FIG.  3 A and FIG. 3B show the structure and operation of the light emitter according to the present invention; 
     FIG. 4 schematically shows an internal structure of the light receiver when viewed from the direction perpendicular to the coordinate input surface; 
     FIG. 5 shows a relation between reflected light and the intensity of the light received at the photoreceptor; 
     FIG. 6 shows a relation between a specified position B, distance w between the light reception/emission sections, and angles θR and θL of the specified position B from the top edge of the coordinate input surface; 
     FIG. 7A to FIG. 7C show detection characteristics of the coordinate-position input device; 
     FIG. 8 shows an external view when the coordinate-position input device according to this invention is applied to a display board system; 
     FIG. 9 schematically shows the conventional coordinate-position input device; 
     FIG. 10 schematically shows the conventional light receiver when viewed from the direction perpendicular to the coordinate input surface; 
     FIG. 11A shows the conventional light emitter when viewed from the direction parallel to the coordinate input surface and the direction at right angles to the direction of travel of the irradiation light, 
     FIG. 11B shows a graph of the intensity of the received light; 
     FIG. 12A shows the conventional light emitter when viewed from the direction parallel to the coordinate input surface and the direction at right angles to the direction of travel of the irradiation light, 
     FIG. 12B shows a graph of the intensity of the received light; 
     FIG. 13A to FIG. 13C show detection characteristics of the conventional coordinate-position input device at three different angles; and 
     FIG. 14A to FIG. 14C show detection characteristics of the conventional coordinate-position input device at three different angles. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of this invention is explained in detail below with reference to the drawings. FIG. 1 schematically shows the coordinate-position input device according to the present invention. The coordinate-position input device  100  comprises a coordinate input surface  102  on which a position is specified (position A in the figure) with the pointing tool  101 , a left-side light reception/emission section  103 L that emits irradiation light along the coordinate input surface  102 , and a right-side light reception/emission section  103 R that emits irradiation light in a like manner. The pointing tool may be a pen or finger. The coordinate-position input device  100  further comprises a rectangular recursive reflector  104  that is disposed along three sides of the device. This recursive reflector  104  recursively reflects the irradiation light that has been emitted by the light reception/emission section  103 L or the light reception/emission section  103 R in a direction from where the light came in. The light reception/emission section  103 L and the light reception/emission section  103 R does not only emit light but also receive the light reflected by the recursive reflector  104 . Further, the light reception/emission section  103 L and the light reception/emission section  103 R detect the intensity of the received light. The light reception/emission section  103 L or the light reception/emission section  103 R will collectively be referred to as the light reception/emission section  103  as necessary in the following explanation. 
     The surface of the recursive reflection section  104  is covered with a material that recursively reflects light. An example of this material is the corner cube reflector. FIG. 2A is a perspective view of the corner cube reflector and FIG. 2B is a cross-sectional view taken along the line passing through the top and the center of the circle of the bottom face. The corner cube reflector is conical in shape. Its internal surface is aluminum-evaporated, so that reflection efficiency is enhanced. As shown in these figures, the cone angle in this corner cube reflector 90 degrees. Therefore, incident light is recursively reflected. 
     The structure of the light reception/emission section  103  will be explained here. The light reception/emission section  103  comprises a light emitter  110  that emits irradiation light, and a light receiver  120  that receives the light. FIG. 3A shows the light emitter  110  when viewed from the direction orthogonal to the direction of travel of the irradiation light within a plane parallel to the coordinate input surface  102  (that is, along the y axis). FIG. 3B shows the light emitter  110  when viewed from the direction of travel of the irradiation light (that is, along the x axis). The light emitter  110  comprises a light-emitting element  111  formed with a laser diode or a pin point LED that emits irradiation light, cylindrical lenses  112   a  to  112   c  that deflect the irradiation light emitted by the light-emitting element  111  to a prescribed direction, and a slit  113 . A half mirror  114  reflects the irradiation light passing through the slit  113  toward the recursive reflection section  104 . 
     The light emitted by the light-emitting element  111  is converged by the cylindrical lens  112 a. This converged light is substantially collimated and made slightly wider in the xy plane and with a slight amount of spread in the negative direction of the z axis (see FIG.  3 A). The irradiation light is then converged in the y-axial direction through the two cylindrical lenses  112 b and  112 c, and is focused to the location of the slit  113  (see FIG.  3 B). The slit  113  has a fine and slit-like hole extending along the x axis. Accordingly, when the light passes through this slit  113 , it spreads in a fan shape along the y axis. Thus, this slit  113  functions what is called a linear light source. The irradiation light spreads in a fan shape from this linear light source toward the coordinate input surface  102 , and travels substantially parallel to the surface while the beam has some thickness and is slightly spreading in the direction perpendicular to the coordinate input surface  102  (that is, along the z axis). Thus, the irradiation light is substantially parallel to the coordinate input surface  102  and spreads in the direction perpendicular to the coordinate input surface  102 . 
     FIG. 4 schematically shows an internal structure of the light reception section when viewed from the direction perpendicular to the coordinate input surface. Detection of reflection light within a two-dimensional plane parallel to the coordinate input surface  102  will be explained here to make the explanation simple. The light receiver  120  comprises a light receiving lens  121  that focuses light reflected by the recursive reflector  104  and a photoreceptor  122  formed with a photosensor or the like that detects the intensity of the received light. FIG. 4 also shows the light-emitting element  111  and the half mirror  114 . The light-emitting element  111  is located above the half mirror  114  (the location indicated by z&gt;0 in the coordinate system of the figure), therefore, the light-emitting element  111  is indicated here by a dot. The irradiation light irradiated from the light-emitting element  111  in the direction Lr is reflected by the recursive reflector  104 , passes through the light-receiving lens  121 , and reaches a position Lr&#39; on the photoreceptor  122 . Further, the irradiation light travelling along direction Ls is reflected by the recursive reflector  104 , and reaches a position Ls&#39; on the photoreceptor  122 . 
     The reflected light, that is the light emitted from the light-emitting element  111  and reflected by the recursive reflector  104 , returns along the same path, but falls on a different positions on the photoreceptor  122  due to the light-receiving lens  121 . Accordingly, when the pointing tool  101  is inserted at a certain position on the coordinate input surface  102  and blocks the irradiation light, the reflection light does not reach the point on the photoreceptor  122  corresponding to its reflecting direction. Thus, by referring to a distribution of the light intensity on the photoreceptor  122 , in which direction the irradiation light has been blocked can be determined. 
     FIG. 5 shows a relation between reflected light and the intensity of received beams by the photoreceptor. The photoreceptor  122  is located on the surface of the focal point of the light-receiving lens  121 . The intensity of the light received by the photoreceptor  122  is substantially constant in a portion where there is no pointing tool on the coordinate input surface  102 . Consider that the pointing tool  101  is inserted in the coordinate input surface  102  at a position B (that is, the position B is specified with the pointing tool). Since the light will be blocked by the pointing tool, an area with low intensity of the received light beam (dark point) will appear in the position Dn on the photoreceptor  122  corresponding to the point B. This position Dn corresponds to the illuminating angle (incidence angle) On of the blocked light one to one. Therefore, if the position Dn of the dark point on the photoreceptor  122  is obtained, the angle θn can be determined. Assuming the distance between the light-receiving lens  121  and the photoreceptor  122  to be f, θn can be obtained as follows: 
     
       
         θ n= arctan ( Dn/f )  (1) 
       
     
     FIG. 6 shows a relation between a specified position B, distance w between the light reception/emission sections, and angles θR and θL of the specified position B from the top edge of the coordinate input surface. Let us represent θn and Dn corresponding to the light reception/emission section  103 L as θnL and DnL, and represent θn and Dn corresponding the light reception/emission section  103 R as θnR and DnR. 
     In general, the direction of a reference line to measure θn does not match the direction of a straight line connecting between the light reception/emission sections  103  of the coordinate input surface  100 . That is, the directions θL and θR indicating the pointed position B based on the reference line connecting between the light reception/emission sections  103  in FIG. 6 are not the same as the directions θnL and θnR. However, θL and θR are correlated one-to-one through simple conversion using θnL and θnR although explanation of the difference is omitted here. Coordinates (x, y) of the specified position B can be obtained through the equation (2) from the obtained θL and θR, and distance w. 
     
       
           x=w× tan θR/(tan θ L+ tan θR) 
       
     
     
       
           y=w× tan θ L ×tan θ R/ (tan θ L +tan θ R )  (2) 
       
     
     As explained above, x and y can be calculated from DnL and DnR. Thus, by measuring the positions DnL and DnR of the dark point on the photoreceptor  122  based on the geometrical arrangement of the light reception/emission sections  103 , the coordinates of the position B specified using the pointing tool  101  can be detected. 
     A positional relation between the light-emitting element  111  and the cylindrical lens  112   a  in order to improve detection precision of the position of the blocked point (dark point) on the photoreceptor  122  will be explained below. In the conventional art explained in FIG.  11  and FIG. 12, the light-emitting element  111  is disposed at a location (at the focal point of the cylindrical lens) so that the irradiation light is parallel to the coordinate input surface  102 . In the coordinate-position input device  100  according to this invention, the light-emitting element  111  is disposed at a location closer to the cylindrical lens  112 a than the focal point of this lens. Accordingly, the irradiation light passing through the cylindrical lens  112 a is travelling while keeping the substantially parallel relation with the coordinate input surface  102  yet gradually spreading, as shown in FIG.  3 A. 
     Detection characteristics of the coordinate-position input device  100  having the light emitter  110  that emits a light in a spreading manner (in contrast to parallel or converging light) were measured in an experiment. FIG. 7A to FIG. 7C show the results of this experiment. A comparison will be made between FIG. 7A to FIG. 7C, FIG. 13A to FIG. 13C, and FIG. 14A to FIG.  14 C. The detection characteristics for various parameters (the size of the pointing tool, direction to which the rod blocks the beam, and distance between the light-receiving lens  121  and the pointing tool) exhibit a wide range of variations as compared to the case where the irradiation light is parallel to the coordinate input surface  102  (see FIG.  14 ). However, the detection characteristics exhibit a narrow range of variations as compared to the case where the irradiation light is getting converged (see FIG.  13 ). Thus, it can be said that the irradiation light as parallel light is more excellent from the viewpoint of setting of a threshold value for the photoreceptor  122  and setting of a detection range. 
     However, when the pointing tool touches the coordinate input surface  102  (the distance between the pointing tool and the coordinate input surface  102  is zero), in the coordinate-position input device  100  in which the light from the light emitter  110  spreads, the detection precision for any parameter is excellent. 
     The reason why the detection precision is improved may be explained as follows. Spreading of the irradiation light indicates that the light is being converged (being focused) to the light receiver  120  when viewed from the side of the light receiver  120  because the irradiation light is recursively reflected by the recursive reflector  104 . Accordingly, it is considered that the detection precision of the blocked point is improved due to the higher light intensity. 
     In an actual case, the recursive reflector  104  does not properly reflect the light. For example, if there is a faulty point at some part of the recursive reflector  104  that fails to recursively reflect the irradiation light, precision in detection of coordinates may be reduced because the precision is largely affected by this faulty point when the irradiation light is not wide enough. However, when the irradiation light is spread like in the coordinate-position input device  100 , the area of the recursive reflection section  104  which the irradiation light strikes is widened. Accordingly, there is an effect that the failed reflection occurring at some part of the recursive reflection section  104  can be compensated for with the remaining part of the widened area. Thus, the detection precision can be improved. 
     Although the cylindrical lenses are used in the light emission section for convenience in explanation, the lenses are not limited by these lenses, but various types of lenses may be used. The focal point in this case means the position at which the lens forms the sharpest image. In other words, the focal point indicates the position at which the lens focuses the light at its peak in intensity. 
     FIG. 8 shows an external view when the coordinate-position input device  100  according to this invention is applied to a display board system  300 . The display board system  300  has a large-sized display unit (e.g., a plasma display) as shown in the figure, and the coordinate-position input device  100  of this invention is provided on the front surface of the display unit. When the coordinate-position input device  100  is applied in such a large-sized display unit, the detection precision of a coordinate input position is further improved. 
     As explained above, according to one aspect of this invention, the probe beam is substantially parallel to the coordinate input surface and spreads in the direction perpendicular to the coordinate input surface. Accordingly, it is possible to provide a coordinate-position input device with improved precision in detection of the specified position. 
     According to another aspect of this invention, the coordinate-position input device has a light-emitting element that emits a probe beam to detect a coordinate input position and a refractive lens that refracts the probe beam emitted by the light-emitting element to be a beam flux to travel in a prescribed direction. Further, the light-emitting element is disposed at a location closer to the refractive lens than the focal point of the refractive lens. Accordingly, it is possible to provide a coordinate-position input device with improved precision in detection of the specified position. 
     The present document incorporates by reference the entire contents of Japanese priority documents, 11-258077 filed in Japan on Sep. 10, 1999. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.