Patent Publication Number: US-9835864-B2

Title: Image display apparatus and method for displaying image

Description:
TECHNICAL FIELD 
     The present technique relates to an image display apparatus and a method for displaying an image and specifically relates, for example, to an image display apparatus such as a head-mounted display configured to lead an image displayed on a display element to an eye of an observer by a magnification optical system. 
     BACKGROUND ART 
     Recently, a head-mounted display (HMD) mounted to a head of a user has been known. Theoretically, the head-mounted display includes a configuration to magnify an image displayed on a small display element with a magnification optical system and to lead the image to an eye of an observer. That is, the head-mounted display is configured to optically magnify an image displayed on a display element and to make a user observe the image as a magnified virtual image. 
     For example, as illustrated in  FIG. 14( a ) , in such a head-mounted display, when a gap is generated between a center of an optical system and a position of an eye of an observer, a video image influences a conformable image observation. In this case, for example, according to a gap amount, a color shift is caused, contrast is decreased, or image distortion is caused. Note that in  FIG. 14( b ) , a case where there is no gap between a center of the optical system and a position of the eye of the observer is illustrated. 
     For example, in Patent Document 1, a head-mounted display in which an optical system is configured in a movable manner and a positional gap between an eye of an observer and the optical system can be corrected by a manual movement adjustment of the optical system by the observer has been proposed. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: JP 2009-116196 A 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the correction technique described in Patent Document 1, there has been no way for an observer to know an amount of a positional gap between an eye of the observer and an optical system and it has been difficult to perform movement adjustment of the optical system to an optimal position. 
     A purpose of the present technique is to make it possible to perform correction of a positional gap between an optical system and an eye of an observer appropriately. 
     Solutions to Problems 
     A concept of the present technique is an image display apparatus including a positional relationship detecting unit configured to detect a positional relationship between an optical system, which leads an image displayed on a display element to an eye of an observer, and the eye of the observer based on an eyeball position of the eye of the observer, and a positional relationship correcting unit configured to correct the positional relationship between the optical system and the eye of the observer based on the positional relationship detected by the positional relationship detecting unit. 
     In the present technique, by the positional relationship detecting unit, a positional relationship between an optical system, which leads an image displayed on a display element to an eye of an observer, and the eye of the observer is detected. The image displayed on the display element is lead to the eye of the observer by the optical system. Accordingly, the image displayed on the display element is optically magnified and is observed as a magnified virtual image by the observer. For example, as the optical system, there may be a first optical system to lead a left-eye image displayed on the display element to a left eye of the observer and a second optical system to lead a right-eye image displayed on the display element to a right eye of the observer. For example, an optical system to lead an image displayed on the display element to the eye of the observer may be further included. 
     For example, an eyeball position estimating unit to estimate an eyeball position of the eye of the observer may be further included and the positional relationship detecting unit may detect a positional relationship between the optical system and the eye of the observer based on the eyeball position estimated by the eyeball position estimating unit. The estimation of the eyeball position may be performed, for example, by application of a sclera reflection method, an electro-oculogram (EOG) method, a facial recognition technique, or the like. By the positional relationship detecting unit, a positional relationship between the optical system and the eye of the observer is detected based on the estimated eyeball position. 
     By the positional relationship correcting unit, a positional relationship between the optical system and the eye of the observer is corrected based on the positional relationship detected by the positional relationship detecting unit. For example, based on the positional relationship detected by the positional relationship detecting unit, the positional relationship correcting unit may shift a display position of the image displayed on the display element. Also, for example, the positional relationship correcting unit may move the optical system based on the positional relationship detected by the positional relationship detecting unit. 
     Also, for example, the positional relationship correcting unit may include a positional relationship presenting unit to present, to the observer, the positional relationship detected by the positional relationship detecting unit, and a control unit to control the positional relationship between the optical system and the eye of the observer according to operation by the observer. 
     For example, in this case, according to operation by the observer, the control unit may perform shift control of a display position of the image displayed on the display element. Also, for example, in this case, the control unit may perform movement control of the optical system according to operation by the observer. 
     Also, for example, in this case, the positional relationship presenting unit may display, on the display element, the positional relationship detected by the positional relationship detecting unit. In this case, for example, a gap between the optical system and the eye of the observer may be displayed as a gauge, a character, or the like. Also, in this case, for example, an operation method of a remote-control apparatus to reduce the gap may be displayed. Note that display of the positional relationship may be displayed on a display element which is different from the above-described display element. Also, presentation of the positional relationship may be performed by sound, vibration, or increasing/decreasing intensity of light emission. 
     In such a manner, in the present technique, based on a detection result of the positional relationship between the optical system and the eye of the observer, a positional gap between the optical system and the eye of the observer is corrected. Thus, it becomes possible to correct the positional gap between the optical system and the eye of the observer appropriately. 
     Note that in the present technique, for example, an inclination estimating unit to estimate an inclination of the optical system may be further included and the positional relationship detecting unit may detect a positional relationship between the optical system and the eye of the observer based on the estimated eyeball position and the estimated inclination. In this case, even in a case where the optical system is inclined, it becomes possible to detect a positional relationship between the optical system and the eye of the observer accurately and to correct a positional gap between the optical system and the eye of the observer appropriately. 
     Effects of the Invention 
     According to the present technique, a positional gap between an optical system and an eye of an observer can be corrected appropriately. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (binocular) as a first embodiment. 
         FIG. 2  is a view illustrating an example of a positional relationship between an optical system and an eyeball. 
         FIG. 3  is a view illustrating a different example of a positional relationship between the optical system and the eyeball. 
         FIG. 4  is a view schematically illustrating shift control of a display position of each of a left-eye image and a right-eye image. 
         FIG. 5  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (binocular) as a second embodiment. 
         FIG. 6  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (binocular) as a third embodiment. 
         FIG. 7( a )  to  FIG. 7( f )  are views illustrating an example of an OSD display. 
         FIG. 8  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (binocular) as a fourth embodiment. 
         FIG. 9  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (monocular) as a fifth embodiment. 
         FIG. 10  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (monocular) as a sixth embodiment. 
         FIG. 11  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (monocular) as a seventh embodiment. 
         FIG. 12  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (monocular) as an eighth embodiment. 
         FIG. 13( a )  to  FIG. 13( c )  are views illustrating an attachment example of an infrared sensor. 
         FIG. 14( a )  and  FIG. 14( b )  are views for describing a conventional head-mounted display. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     In the following, a mode for carrying out the invention (hereinafter, referred to as “embodiment”) will be described. Note that the description will be made in the following order. 
     1. First Embodiment 
     2. Second Embodiment 
     3. Third Embodiment 
     4. Fourth Embodiment 
     5. Fifth Embodiment 
     6. Sixth Embodiment 
     7. Seventh Embodiment 
     8. Eighth Embodiment 
     9. Modification 
     &lt;1. First Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Binocular)] 
       FIG. 1  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100  as the first embodiment. This configuration example is a binocular HMD. The HMD  100  includes a left-side spectacle lens unit  101 L and a right-side spectacle lens unit  101 R. The spectacle lens unit  101 L and the spectacle lens unit  101 R are connected integrally by a connection member  102 . 
     In each of the spectacle lens units  101 L and  101 R, a spectacle lens and a piece of holographic optical element (HOE) sheet are integrated with each other. The HOE sheet includes a function such as that of a half mirror to synthesize external light and display light and also includes a function of a recessed surface or a free-form surface to magnify a display image. 
     To the spectacle lens units  101 L and  101 R, infrared sensors  103 L and  103 R are respectively attached. In this case, the infrared sensor  103 L is attached to a center position in a horizontal direction of the spectacle lens unit  101 L (center position in horizontal direction of left-eye optical system). Also, the infrared sensor  103 R is attached to a center position in a horizontal direction of the spectacle lens unit  101 R (center position in horizontal direction of right-eye optical system). Also, to the spectacle lens unit  101 L, a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens units  101 L and  101 R is attached. 
     A sensor output from each of the infrared sensors  103 L and  103 R is used for estimation of an eyeball position by a sclera reflection method. The sclera reflection method is a method using a difference between reflectivity of a cornea (black part of eye) and that of a sclera (white part of eye). In this case, an infrared sensor performs scanning in a horizontal direction with a weak infrared ray emitted to a side of an eye of an observer and detects reflection light thereof. Since there is a large difference between intensity of reflection light from the cornea (black part of eye) and that from the sclera (white part of eye), an eyeball position of the observer can be estimated from the sensor outputs. 
     Also, the HMD  100  includes an image correcting unit  111 , display drivers  112 L and  112 R, displays  113 L and  113 R, an eyeball position estimating unit  114 , an inclination estimating unit  115 , and a display control unit  116 . 
     The display  113 L is included in the display element and displays a left-eye image. The display  113 L includes, for example, a liquid crystal display (LCD) and displays a left-eye image, which is included in a three-dimensional image, by being driven by the display driver  112 L. Also, the display  113 R is included in the display element and displays a right-eye image. The display  113 R includes, for example, a liquid crystal display (LCD) and displays a right-eye image, which is included in the three-dimensional image, by being driven by the display driver  112 R. 
     The eyeball position estimating unit  114  estimates eyeball positions of a left eye and a right eye of the observer based on the sensor outputs from the infrared sensors  103 L and  103 R. In this case, based on a level of the sensor output from the infrared sensor  103 L, the eyeball position estimating unit  114  sets an angle ωL, of when scanning of the cornea (black part of eye) is performed, as an estimation result of an eyeball position of the left eye. Also, in this case, based on a level of the sensor output from the infrared sensor  103 R, the eyeball position estimating unit  114  sets an angle ωR, of when the scanning of the cornea (black part of eye) is performed, as an estimation result of an eyeball position of the right eye. 
     Based on a sensor output from the gyro sensor  104 , the inclination estimating unit  115  estimates an inclination θ in the horizontal direction of the optical system including the spectacle lens units  101 L and  101 R. Based on estimation results of eyeball positions of the left eye and the right eye and an estimation result of an inclination the optical system, the display control unit  116  detects a positional relationship between the left-eye optical system and the left eye and a positional relationship between the right-eye optical system and the right eye. 
     That is, the display control unit  116  calculates a positional gap dL of the left-eye optical system relative to the left eye of the observer based on the angle ωL which is an estimation result of the eyeball position of the left eye and calculates a positional gap dR of the right-eye optical system relative to the right eye of the observer based on the angle ωR which is an estimation result of the eyeball position of the right eye. Then, the display control unit  116  calculates a distance between both eyes de based on the positional gaps dL and dR calculated in such a manner and an already-known distance between sensors ds of the infrared sensors  103 L and  103 R. 
       FIG. 2  is a view illustrating an example of the positional relationship between the optical system and the eyeball. In a case of this example, the distance between both eyes ds is expressed in the following equation (1).
 
 de=ds−dL−dR   (1)
 
     Note that in an example in  FIG. 2 , a case where the inclination θ of the optical system is zero is illustrated. However, there may be a case where the inclination θ of the optical system is not zero. In such a case, the display control unit  116  calculates the distance between both eyes de based on the positional gaps dL and dR calculated in such a manner, the already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ. 
       FIG. 3  is a view illustrating an example of a positional relationship between the optical system and the eyeball in such a case. In a case of this example, the distance between both eyes de is expressed in the following equation (2). In such a manner, by taking the inclination θ into consideration, the distance between both eyes de can be acquired accurately. 
     
       
         
           
             
               
                 
                   
                     
                       
                         de 
                         = 
                           
                         ⁢ 
                         
                           
                             
                               ds 
                               / 
                               cos 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θ 
                           
                           - 
                           
                             
                               dL 
                               / 
                               cos 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θ 
                           
                           + 
                           
                             
                               dR 
                               / 
                               cos 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θ 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               ( 
                               
                                 ds 
                                 - 
                                 dL 
                                 + 
                                 dR 
                               
                               ) 
                             
                             / 
                             cos 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           θ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The display control unit  116  performs shift control of the display positions of the left-eye image and the right-eye image in directions opposite to each other in the horizontal direction based on the distance between both eyes de. That is, the display control unit  116  automatically corrects a positional gap between the optical system and the eyeball position of the observer by operating the images and moving the optical system electronically. 
     The display control unit  116  shifts the display positions of the left-eye image and the right-eye image according to a difference s (de−ds·cos θ) between the distance between both eyes de and the distance between sensors ds·cos θ in which the inclination θ is taken into consideration.  FIG. 4  is a view schematically illustrating shift control of the display positions of the left-eye image and the right-eye image. In a case where S=0, as illustrated in  FIG. 4( a ) , a shift amount in the horizontal direction of each of the left-eye image and the right-eye image is zero. 
     Also, in a case where S&gt;0, as illustrated in  FIG. 4( b ) , a shift amount in the horizontal direction of each of the left-eye image and the right-eye image is s/2 and the display positions thereof are made away from each other. Also, in a case where S&lt;0, as illustrated in  FIG. 4( c ) , a shift amount in the horizontal direction of each of the left-eye image and the right-eye image is |s|/2 and the display positions thereof are made closer to each other. 
     Referring back to  FIG. 1 , under control by the display control unit  116 , the image correcting unit  111  performs, on left-eye image data and right-eye image data to display a three-dimensional (3D) image, image correction processing to shift a display position. The image correcting unit  111  supplies the corrected left-eye image data to the display driver  112 L and supplies the corrected right-eye image data to the display driver  112 R. 
     [Operation During Correction of Positional Gap] 
     An operation during correction of a positional gap between the optical system and the eye of the observer in the HMD  100  illustrated in  FIG. 1  will be described. During the correction of a positional gap, for example, left-eye image data and right-eye image data for a test are used. 
     The left-eye image data is supplied to the display driver  112 L through the image correcting unit  111 . By the display driver  112 L, the display  113 L is driven and a left-eye image is displayed on the display  113 L. Similarly, the right-eye image data is supplied to the display driver  112 R through the image correcting unit  111 . By the display driver  112 R, the display  113 R is driven and a right-eye image is displayed on the display  113 R. 
     Light from the left-eye image displayed on the display  113 L is reflected by the spectacle lens unit  101 L and reaches the left eye of the observer. Accordingly, the left-eye image displayed on the display  113 L is optically magnified by the spectacle lens unit  101 L and is observed as a magnified virtual image by the left eye of the observer. Similarly, light from the right-eye image displayed on the display  113 R is reflected by the spectacle lens unit  101 R and reaches the right eye of the observer. Accordingly, the right-eye image displayed on the display  113 R is optically magnified by the spectacle lens unit  101 R and is observed as a magnified virtual image by the right eye of the observer. In such a manner, the left-eye image and the right-eye image are respectively observed by the left eye and the right eye of the observer, and thus, a magnified three-dimensional (3D) image can be perceived by the observer. 
     In such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 L attached to a center position in the horizontal direction of the spectacle lens unit  101 L (center position in horizontal direction of left-eye optical system) and scanning in the horizontal direction is performed. Also, in such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 R attached to a center position in the horizontal direction of the spectacle lens unit  101 R (center position in horizontal direction of right-eye optical system) and scanning in the horizontal direction is performed. 
     A sensor output from each of the infrared sensors  103 L and  103 R is supplied to the eyeball position estimating unit  114 . In the eyeball position estimating unit  114 , eyeball positions of the left eye and the right eye of the observer are estimated based on the sensor outputs from the infrared sensors  103 L and  103 R. From the eyeball position estimating unit  114 , the angles ωL and ωR, of when the scanning of the cornea (black part of eye) is performed, are output respectively as estimation results of the eyeball positions of the left eye and the right eye (see  FIG. 2  and  FIG. 3 ). 
     Also, a sensor output from the gyro sensor  104  attached to the spectacle lens unit  101 L is supplied to the inclination estimating unit  115 . In the inclination estimating unit  115 , an inclination θ in the horizontal direction of the optical system including the spectacle lens units  101 L and  101 R is estimated based on the sensor output from the gyro sensor  104  (see  FIG. 3 ). 
     The angles ωL and ωR which are the estimation results by the eyeball position estimating unit  114  are supplied to the display control unit  116 . Also, the inclination θ which is the estimation result by the inclination estimating unit  115  is supplied to the display control unit  116 . In the display control unit  116 , the positional gap dL of the left-eye optical system relative to the left eye of the observer and the positional gap dR of the right-eye optical system relative to the right eye of the observer are calculated based on the estimation results of the eyeball positions of the left eye and the right eye and the estimation result of the inclination of the optical system (see  FIG. 2  and  FIG. 3 ). 
     In the display control unit  116 , the distance between both eyes de is calculated based on the positional gaps dL and dR, the already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ (see equation (2)). In this case, since the inclination θ is taken into consideration, the distance between both eyes de is calculated with high accuracy. 
     Then, by the display control unit  116 , the image correcting unit  111  is controlled and shift control of the display positions of the left-eye image and the right-eye image respectively displayed on the displays  113 L and  113 R is performed. In this case, according to a difference s (de−ds·cos θ) between the distance between both eyes de and the distance between sensors ds·cos θ in which the inclination θ is taken into consideration, control is performed in such a manner that the difference s becomes zero. Accordingly, a center in the horizontal direction of the left-eye image is made coincident with the left eye of the observer and a center in the horizontal direction of the right-eye image is made coincident with the right eye of the observer, whereby the positional gap between the optical system and the eye of the observer is corrected. 
     As described above, in the HMD  100  illustrated in  FIG. 1 , based on the detection result of the positional relationship between the optical system and the eye of the observer, shift control of the display positions of the left-eye image and the right-eye image is performed and the positional gap between the optical system and the eye of the observer is corrected electronically. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;2. Second Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Binocular)] 
       FIG. 5  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100 A as the second embodiment. This configuration example is a binocular HMD. In  FIG. 5 , the same sign is assigned to a part corresponding to  FIG. 1  and detail description of the part will be omitted arbitrarily. 
     The HMD  100 A includes a left-side spectacle lens unit  101 L and a right-side spectacle lens unit  101 R. The spectacle lens unit  101 L and the spectacle lens unit  101 R are connected to each other by an optical system adjusting mechanism  122 . The optical system adjusting mechanism  122  performs expansion and contraction of a distance between right and left optical systems, that is, a distance between the spectacle lens unit  101 L which is included in a left-eye optical system and the spectacle lens unit  101 R which is included in a right-eye optical system. 
     To the spectacle lens units  101 L and  101 R, infrared sensors  103 L and  103 R are respectively attached. Also, to the spectacle lens unit  101 L, a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens units  101 L and  101 R is attached. A sensor output from each of the infrared sensors  103 L and  103 R is used for estimation of an eyeball position by a sclera reflection method. 
     Also, the HMD  100 A includes display drivers  112 L and  112 R, displays  113 L and  113 R, an eyeball position estimating unit  114 , an inclination estimating unit  115 , a display control unit  116 A, and a unit for correcting a position of an optical system  117 . The display  113 L is driven by the display driver  112 L and displays a left-eye image which is included in a three-dimensional image. Also, the display  113 R is driven by the display driver  112 R and displays a right-eye image which is included in the three-dimensional image. 
     Based on sensor outputs from the infrared sensors  103 L and  103 R, the eyeball position estimating unit  114  estimates eyeball positions of a left eye and a right eye of an observer and outputs angles ωL and ωR, of when scanning of a cornea (black part of eye) is performed, as estimation results of the eyeball positions (see  FIG. 2  and  FIG. 3 ). Based on a sensor output from the gyro sensor  104 , the inclination estimating unit  115  outputs, as an estimation result, an inclination θ in a horizontal direction of the optical system including the spectacle lens units  101 L and  101 R (see  FIG. 3 ). 
     The display control unit  116 A calculates a positional gap dL of the left-eye optical system relative to the left eye of the observer based on the angle ωL and calculates a positional gap dR of the right-eye optical system relative to the right eye of the observer based on the angle ωR. The display control unit  116 A calculates a distance between both eyes de based on the positional gaps dL and dR, an already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ (see equation (2)). 
     Then, according to a difference s (de−ds·cos θ) between the distance between both eyes de and a distance between sensors ds·cos θ in which the inclination θ is taken into consideration, the display control unit  116 A performs shift control of positions of the left-eye optical system and the right-eye optical system in directions opposite to each other in the horizontal direction. That is, the display control unit  116 A automatically corrects a positional gap between the optical system and the eyeball position of the observer by mechanically moving the optical system. The shift control is similar to the above-described shift control of the display positions of the left-eye image and the right-eye image (see  FIG. 4 ). 
     The unit for correcting a position of an optical system  117  corrects a position of the optical system under control by the display control unit  116 A. That is, the unit for correcting a position of an optical system  117  adjusts expansion and contraction of the optical system adjusting mechanism  122  and adjusts a position in the horizontal direction of each of the spectacle lens unit  101 L and the spectacle lens unit  101 R. Also, along with this adjustment, the unit for correcting a position of an optical system  117  also adjusts positions of the displays  113 L and  113 R. 
     [Operation During Correction of Positional Gap] 
     An operation during correction of a positional gap between the optical system and the eye of the observer in the HMD  100 A illustrated in  FIG. 5  will be described. During the correction of a positional gap, for example, left-eye image data and right-eye image data for a test are used. 
     The left-eye image data is supplied to the display driver  112 L. By the display driver  112 L, the display  113 L is driven and a left-eye image is displayed on the display  113 L. Similarly, the right-eye image data is supplied to the display driver  112 R. By the display driver  112 R, the display  113 R is driven and a right-eye image is displayed on the display  113 R. 
     Light from the left-eye image displayed on the display  113 L is reflected by the spectacle lens unit  101 L and reaches the left eye of the observer. Accordingly, the left-eye image displayed on the display  113 L is optically magnified by the spectacle lens unit  101 L and is observed as a magnified virtual image by the left eye of the observer. Similarly, light from the right-eye image displayed on the display  113 R is reflected by the spectacle lens unit  101 R and reaches the right eye of the observer. Accordingly, the right-eye image displayed on the display  113 R is optically magnified by the spectacle lens unit  101 R and is observed as a magnified virtual image by the right eye of the observer. In such a manner, the left-eye image and the right-eye image are respectively observed by the left eye and the right eye of the observer, and thus, a magnified three-dimensional (3D) image can be perceived by the observer. 
     In such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 L attached to a center position in the horizontal direction of the spectacle lens unit  101 L (center position in horizontal direction of left-eye optical system) and scanning in the horizontal direction is performed. Also, in such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 R attached to a center position in the horizontal direction of the spectacle lens unit  101 R (center position in horizontal direction of right-eye optical system) and scanning in the horizontal direction is performed. 
     A sensor output from each of the infrared sensors  103 L and  103 R is supplied to the eyeball position estimating unit  114 . In the eyeball position estimating unit  114 , eyeball positions of the left eye and the right eye of the observer are estimated based on the sensor outputs from the infrared sensors  103 L and  103 R. From the eyeball position estimating unit  114 , the angles ωL and ωR, of when the scanning of the cornea (black part of eye) is performed, are output respectively as estimation results of the eyeball positions of the left eye and the right eye (see  FIG. 2  and  FIG. 3 ). 
     Also, a sensor output from the gyro sensor  104  attached to the spectacle lens unit  101 L is supplied to the inclination estimating unit  115 . In the inclination estimating unit  115 , an inclination θ in the horizontal direction of the optical system including the spectacle lens units  101 L and  101 R is estimated based on the sensor output from the gyro sensor  104  (see  FIG. 3 ). 
     The angles ωL and ωR which are the estimation results by the eyeball position estimating unit  114  are supplied to the display control unit  116 A. Also, the inclination θ which is the estimation result by the inclination estimating unit  115  is supplied to the display control unit  116 A. In the display control unit  116 A, the positional gap dL of the left-eye optical system relative to the left eye of the observer and the positional gap dR of the right-eye optical system relative to the right eye of the observer are calculated based on the estimation results of the eyeball positions of the left eye and the right eye and the estimation result of the inclination of the optical system (see  FIG. 2  and  FIG. 3 ). 
     In the display control unit  116 A, the distance between both eyes de is calculated based on the positional gaps dL and dR, the already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ (see equation (2)). In this case, since the inclination θ is taken into consideration, the distance between both eyes de is calculated with high accuracy. 
     Then, by the display control unit  116 A, the unit for correcting a position of an optical system  117  is controlled and shift control of positions of the left-eye optical system and the right-eye optical system, that is, positions of the spectacle lens units  101 L and  101 R and positions of the displays  113 L and  113 R is performed. In this case, according to a difference s (de−ds·cos θ) between the distance between both eyes de and the distance between sensors ds·cos θ in which the inclination θ is taken into consideration, control is performed in such a manner that the difference s becomes zero. Accordingly, a center in the horizontal direction of the left-eye optical system is made coincident with the left eye of the observer and a center in the horizontal direction of the right-eye optical system is made coincident with the right eye of the observer, whereby the positional gap between the optical system and the eye of the observer is corrected. 
     As described above, in the HMD  100 A illustrated in  FIG. 5 , based on a detection result of the positional relationship between the optical system and the eye of the observer, shift control of the positions of the left-eye optical system and the right-eye optical system is performed and the positional gap between the optical system and the eye of the observer is corrected mechanically. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;3. Third Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Binocular)] 
       FIG. 6  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100 B as the third embodiment. This configuration example is a binocular HMD. In  FIG. 6 , the same sign is assigned to a part corresponding to  FIG. 1  and detail description of the part will be omitted arbitrarily. The HMD  100 B includes a left-side spectacle lens unit  101 L and a right-side spectacle lens unit  101 R. The spectacle lens unit  101 L and the spectacle lens unit  101 R are connected integrally by a connection member  102 . 
     To the spectacle lens units  101 L and  101 R, infrared sensors  103 L and  103 R are respectively attached. Also, to the spectacle lens unit  101 L, a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens units  101 L and  101 R is attached. A sensor output from each of the infrared sensors  103 L and  103 R is used for estimation of an eyeball position by a sclera reflection method. 
     Also, the HMD  100 B includes display drivers  112 L and  112 R, displays  113 L and  113 R, an eyeball position estimating unit  114 , an inclination estimating unit  115 , a display control unit  116 B, an image correcting unit  111 , and an OSD superimposing unit  118 . The display  113 L is driven by the display driver  112 L and displays a left-eye image which is included in a three-dimensional image. Also, the display  113 R is driven by the display driver  112 R and displays a right-eye image which is included in the three-dimensional image. 
     Based on sensor outputs from the infrared sensors  103 L and  103 R, the eyeball position estimating unit  114  estimates eyeball positions of a left eye and a right eye of an observer and outputs angles ωL and ωR, of when scanning of a cornea (black part of eye) is performed, as estimation results of the eyeball positions (see  FIG. 2  and  FIG. 3 ). Based on a sensor output from the gyro sensor  104 , the inclination estimating unit  115  outputs, as an estimation result, an inclination θ in a horizontal direction of the optical system including the spectacle lens units  101 L and  101 R (see  FIG. 3 ). 
     The display control unit  116 B calculates a positional gap dL of the left-eye optical system relative to the left eye of the observer based on the angle ωL and calculates a positional gap dR of the right-eye optical system relative to the right eye of the observer based on the angle ωR. The display control unit  116 B calculates a distance between both eyes de based on the positional gaps dL and dR, an already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ (see equation (2)). 
     Then, according to a difference s (de−ds·cos θ) between the distance between both eyes de and a distance between sensors ds·cos θ in which the inclination θ is taken into consideration, the display control unit  116 B performs OSD display of the difference s or OSD display to prompt a user to perform operation of correcting the difference s.  FIG. 7( a )  to  FIG. 7( f )  are views illustrating an example of OSD display.  FIG. 7( a )  is an example of displaying a gauge indicating the difference s (amount of positional gap) and the gauge is displayed on a left end side of a screen. 
       FIG. 7( b )  is also an example of displaying a gauge indicating the difference s (amount of positional gap) and the gauge is displayed on an upper end side of the screen.  FIG. 7( c )  is also an example of displaying a gauge indicating the difference s (amount of positional gap) and the gauge is displayed at a center of the screen. By displaying the gauge at the center of the screen, an observer focuses on the gauge itself and deviation of a line of sight can be prevented.  FIG. 7( d )  and  FIG. 7( e )  are examples of displaying a character indicating the difference s (amount of positional gap). Also,  FIG. 7( f )  is a view illustrating an example of the OSD display to prompt a user to perform operation of correcting the difference s. As indicated by an arrow P, an operation button (place to be operated) of a remote-control apparatus to be operated by the user is illustrated. Note that it is also considered that the display of the gauge, the display of the character, the display of the operation button of the remote-control apparatus may be arbitrarily combined and displayed. 
     Also, based on user operation acquired in a user operation acquiring unit  119 , the display control unit  116 B shifts display positions of the left-eye image and the right-eye image. Note that in this case, as described above, a user performs correction operation in such a manner that the difference s becomes zero based on the OSD display of the difference s or the OSD display to prompt the user to perform operation of correcting the difference s. 
     Under control by the display control unit  116 B, the image correcting unit  111  performs, on left-eye image data and right-eye image data to display a three-dimensional (3D) image, image correction processing to shift a display position. Through the OSD superimposing unit  118 , the image correcting unit  111  supplies the corrected left-eye image data to the display driver  112 L and supplies the corrected right-eye image data to the display driver  112 R. The OSD superimposing unit  118  superimposes a display signal for the OSD display, which signal is output from the display control unit  116 B, on the left-eye image data and the right-eye image data. 
     [Operation During Correction of Positional Gap] 
     An operation during correction of a positional gap between the optical system and the eye of the observer in the HMD  100 B illustrated in  FIG. 6  will be described. During the correction of a positional gap, for example, left-eye image data and right-eye image data for a test are used. 
     The left-eye image data is supplied to the display driver  112 L through the image correcting unit  111 . By the display driver  112 L, the display  113 L is driven and a left-eye image is displayed on the display  113 L. Similarly, the right-eye image data is supplied to the display driver  112 R through the image correcting unit  111 . By the display driver  112 R, the display  113 R is driven and a right-eye image is displayed on the display  113 R. 
     Light from the left-eye image displayed on the display  113 L is reflected by the spectacle lens unit  101 L and reaches the left eye of the observer. Accordingly, the left-eye image displayed on the display  113 L is optically magnified by the spectacle lens unit  101 L and is observed as a magnified virtual image by the left eye of the observer. Similarly, light from the right-eye image displayed on the display  113 R is reflected by the spectacle lens unit  101 R and reaches the right eye of the observer. Accordingly, the right-eye image displayed on the display  113 R is optically magnified by the spectacle lens unit  101 R and is observed as a magnified virtual image by the right eye of the observer. In such a manner, the left-eye image and the right-eye image are respectively observed by the left eye and the right eye of the observer, and thus, a magnified three-dimensional (3D) image can be perceived by the observer. 
     In such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 L attached to a center position in the horizontal direction of the spectacle lens unit  101 L (center position in horizontal direction of left-eye optical system) and scanning in the horizontal direction is performed. Also, in such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 R attached to a center position in the horizontal direction of the spectacle lens unit  101 R (center position in horizontal direction of right-eye optical system) and scanning in the horizontal direction is performed. 
     A sensor output from each of the infrared sensors  103 L and  103 R is supplied to the eyeball position estimating unit  114 . In the eyeball position estimating unit  114 , eyeball positions of the left eye and the right eye of the observer are estimated based on the sensor outputs from the infrared sensors  103 L and  103 R. From the eyeball position estimating unit  114 , the angles ωL and ωR, of when the scanning of the cornea (black part of eye) is performed, are output respectively as estimation results of the eyeball positions of the left eye and the right eye (see  FIG. 2  and  FIG. 3 ). 
     Also, a sensor output from the gyro sensor  104  attached to the spectacle lens unit  101 L is supplied to the inclination estimating unit  115 . In the inclination estimating unit  115 , an inclination θ in the horizontal direction of the optical system including the spectacle lens units  101 L and  101 R is estimated based on the sensor output from the gyro sensor  104  (see  FIG. 3 ). 
     The angles ωL and ωR which are the estimation results by the eyeball position estimating unit  114  are supplied to the display control unit  116 B. Also, the inclination θ which is the estimation result by the inclination estimating unit  115  is supplied to the display control unit  116 B. In the display control unit  116 B, the positional gap dL of the left-eye optical system relative to the left eye of the observer and the positional gap dR of the right-eye optical system relative to the right eye of the observer are calculated based on the estimation results of the eyeball positions of the left eye and the right eye and the estimation result of the inclination of the optical system (see  FIG. 2  and  FIG. 3 ). 
     In the display control unit  116 B, the distance between both eyes de is calculated based on the positional gaps dL and dR, the already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ (see equation (2)). In this case, since the inclination θ is taken into consideration, the distance between both eyes de is calculated with high accuracy. 
     Then, in the display control unit  116 B, the difference s (de−ds·cos θ) between the distance between both eyes de and the distance between sensors ds·cos θ in which the inclination e is taken into consideration is calculated. From the display control unit  116 B, a display signal for the OSD display of the difference s or for the OSD display to prompt a user to perform operation of correcting the difference s is output. The display signal is supplied to the OSD superimposing unit  118  and is superimposed on the left-eye image data and the right-eye image data. Accordingly, the OSD display is performed on the displays  113 L and  113 R. 
     By the observer, correction operation to make the difference s into zero is performed based on the OSD display. The user operation acquiring unit  119  acquires the user operation and transmits the acquired user operation to the display control unit  116 B. Based on the correction operation, by the display control unit  116 B, the image correcting unit  111  is controlled and shift control of the display positions of the left-eye image and the right-eye image respectively displayed on the displays  113 L and  113 R is performed. Accordingly, a center in the horizontal direction of the left-eye image is made coincident with the left eye of the observer and a center in the horizontal direction of the right-eye image is made coincident with the right eye of the observer, whereby the positional gap between the optical system and the eye of the observer is corrected. 
     As described above, in the HMD  100 B illustrated in FIG.  6 , based on a detection result of the positional relationship between the optical system and the eye of the observer, OSD display of the positional gap between the optical system and the eye of the observer is performed. Based on the OSD display, correction operation is performed by the observer to eliminate the positional gap. Then, according to the correction operation, shift control of the display positions of the left-eye image and the right-eye image is performed and the positional gap between the optical system and the eye of the observer is electronically corrected. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;4. Fourth Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Binocular)] 
       FIG. 8  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100 C as the fourth embodiment. This configuration example is a binocular HMD. In  FIG. 8 , the same sign is assigned to a part corresponding to  FIG. 2  or  FIG. 6  and detail description of the part will be omitted arbitrarily. The HMD  100 C includes a left-side spectacle lens unit  101 L and a right-side spectacle lens unit  101 R. The spectacle lens unit  101 L and the spectacle lens unit  101 R are connected to each other by an optical system adjusting mechanism  122 . 
     To the spectacle lens units  101 L and  101 R, infrared sensors  103 L and  103 R are respectively attached. Also, to the spectacle lens unit  101 L, a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens units  101 L and  101 R is attached. A sensor output from each of the infrared sensors  103 L and  103 R is used for estimation of an eyeball position by a sclera reflection method. 
     Also, the HMD  100 C includes display drivers  112 L and  112 R, displays  113 L and  113 R, an eyeball position estimating unit  114 , an inclination estimating unit  115 , a display control unit  116 C, a unit for correcting a position of an optical system  117 , an OSD superimposing unit  118 , and a user operation acquiring unit  119 . The display  113 L is driven by the display driver  112 L and displays a left-eye image which is included in a three-dimensional image. Also, the display  113 R is driven by the display driver  112 R and displays a right-eye image which is included in the three-dimensional image. 
     Based on sensor outputs from the infrared sensors  103 L and  103 R, the eyeball position estimating unit  114  estimates eyeball positions of a left eye and a right eye of an observer and outputs angles ωL and ωR, of when scanning of a cornea (black part of eye) is performed, as estimation results of the eyeball positions (see  FIG. 2  and  FIG. 3 ). Based on a sensor output from the gyro sensor  104 , the inclination estimating unit  115  outputs, as an estimation result, an inclination θ in a horizontal direction of the optical system including the spectacle lens units  101 L and  101 R (see  FIG. 3 ). 
     The display control unit  116 C calculates a positional gap dL of the left-eye optical system relative to the left eye of the observer based on the angle ωL and calculates a positional gap dR of the right-eye optical system relative to the right eye of the observer based on the angle ωR. The display control unit  116 C calculates a distance between both eyes de based on the positional gaps dL and dR, an already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ (see equation (2)). 
     According to a difference s (de−ds·cos θ) between the distance between both eyes de and the distance between sensors ds·cos θ in which the inclination θ is taken into consideration, the display control unit  116 C performs OSD display of the difference s or OSD display to prompt a user to perform operation of correcting the difference s. 
     Also, the display control unit  116 C shifts positions of the left-eye optical system and the right-eye optical system based on user operation acquired in the user operation acquiring unit  119 . Note that in this case, as described above, a user performs correction operation in such a manner that the difference s becomes zero based on the OSD display of the difference s or the OSD display to prompt the user to perform operation of correcting the difference s. 
     The unit for correcting a position of an optical system  117  corrects a position of the optical system under control by the display control unit  116 C. That is, the unit for correcting a position of an optical system  117  adjusts expansion and contraction of the optical system adjusting mechanism  122  and adjusts a position in the horizontal direction of each of the spectacle lens unit  101 L and the spectacle lens unit  101 R. Also, along with this adjustment, the unit for correcting a position of an optical system  117  also adjusts positions of the displays  113 L and  113 R. The OSD superimposing unit  118  superimposes a display signal for the OSD display, which signal is output from the display control unit  116 C, on the left-eye image data and the right-eye image data. 
     [Operation During Correction of Positional Gap] 
     An operation during correction of a positional gap between the optical system and the eye of the observer in the HMD  100 C illustrated in  FIG. 8  will be described. During the correction of a positional gap, for example, left-eye image data and right-eye image data for a test are used. 
     The left-eye image data is supplied to the display driver  112 L. By the display driver  112 L, the display  113 L is driven and a left-eye image is displayed on the display  113 L. Similarly, the right-eye image data is supplied to the display driver  112 R. By the display driver  112 R, the display  113 R is driven and a right-eye image is displayed on the display  113 R. 
     Light from the left-eye image displayed on the display  113 L is reflected by the spectacle lens unit  101 L and reaches the left eye of the observer. Accordingly, the left-eye image displayed on the display  113 L is optically magnified by the spectacle lens unit  101 L and is observed as a magnified virtual image by the left eye of the observer. Similarly, light from the right-eye image displayed on the display  113 R is reflected by the spectacle lens unit  101 R and reaches the right eye of the observer. Accordingly, the right-eye image displayed on the display  113 R is optically magnified by the spectacle lens unit  101 R and is observed as a magnified virtual image by the right eye of the observer. In such a manner, the left-eye image and the right-eye image are respectively observed by the left eye and the right eye of the observer, and thus, a magnified three-dimensional (3D) image can be perceived by the observer. 
     In such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 L attached to a center position in the horizontal direction of the spectacle lens unit  101 L (center position in horizontal direction of left-eye optical system) and scanning in the horizontal direction is performed. Also, in such an image display state, a weak infrared ray is emitted to a side of the eye of the observer from the infrared sensor  103 R attached to a center position in the horizontal direction of the spectacle lens unit  101 R (center position in horizontal direction of right-eye optical system) and scanning in the horizontal direction is performed. 
     A sensor output from each of the infrared sensors  103 L and  103 R is supplied to the eyeball position estimating unit  114 . In the eyeball position estimating unit  114 , eyeball positions of the left eye and the right eye of the observer are estimated based on the sensor outputs from the infrared sensors  103 L and  103 R. From the eyeball position estimating unit  114 , the angles ωL and ωR, of when the scanning of the cornea (black part of eye) is performed, are output respectively as estimation results of the eyeball positions of the left eye and the right eye (see  FIG. 2  and  FIG. 3 ). 
     Also, a sensor output from the gyro sensor  104  attached to the spectacle lens unit  101 L is supplied to the inclination estimating unit  115 . In the inclination estimating unit  115 , an inclination θ in the horizontal direction of the optical system including the spectacle lens units  101 L and  101 R is estimated based on the sensor output from the gyro sensor  104  (see  FIG. 3 ). 
     The angles ωL and ωR which are the estimation results by the eyeball position estimating unit  114  are supplied to the display control unit  116 C. Also, the inclination θ which is the estimation result by the inclination estimating unit  115  is supplied to the display control unit  116 C. In the display control unit  116 C, the positional gap dL of the left-eye optical system relative to the left eye of the observer and the positional gap dR of the right-eye optical system relative to the right eye of the observer are calculated based on the estimation results of the eyeball positions of the left eye and the right eye and the estimation result of the inclination of the optical system (see  FIG. 2  and  FIG. 3 ). 
     In the display control unit  116 C, the distance between both eyes de is calculated based on the positional gaps dL and dR, the already-known distance between sensors ds of the infrared sensors  103 L and  103 R, and the inclination θ (see equation (2)). In this case, since the inclination θ is taken into consideration, the distance between both eyes de is calculated with high accuracy. 
     Then, in the display control unit  116 C, the difference s (de−ds·cos θ) between the distance between both eyes de and the distance between sensors ds·cos θ in which the inclination e is taken into consideration is calculated. From the display control unit  116 C, a display signal for the OSD display of the difference s or for the OSD display to prompt a user to perform operation of correcting the difference s is output. The display signal is supplied to the OSD superimposing unit  118  and is superimposed on the left-eye image data and the right-eye image data. Accordingly, the OSD display is performed on the displays  113 L and  113 R. 
     By the observer, correction operation to make the difference s into zero is performed based on the OSD display. The user operation acquiring unit  119  acquires the user operation and transmits the acquired user operation to the display control unit  116 C. Based on the correction operation, by the display control unit  116 C, the unit for correcting a position of an optical system  117  is controlled and shift control of positions of the left-eye optical system and the right-eye optical system, that is, positions of the spectacle lens units  101 L and  101 R and positions of the displays  101 L and  101 R is performed. Accordingly, a center in the horizontal direction of the left-eye optical system is made coincident with the left eye of the observer and a center in the horizontal direction of the right-eye optical system is made coincident with the right eye of the observer, whereby the positional gap between the optical system and the eye of the observer is corrected. 
     As described above, in the HMD  100 C illustrated in  FIG. 8 , based on a detection result of the positional relationship between the optical system and the eye of the observer, OSD display of the positional gap between the optical system and the eye of the observer is performed. Based on the OSD display, correction operation is performed by the observer to eliminate the positional gap. Then, according to the correction operation, shift control of the left-eye optical system and the right-eye optical system is performed and the positional gap between the optical system and the eye of the observer is mechanically corrected. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;5. Fifth Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Monocular)] 
       FIG. 9  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100 D as the fifth embodiment. This configuration example is a monocular HMD. In  FIG. 9 , the same sign is assigned to a part corresponding to  FIG. 1  and detail description of the part will be omitted arbitrarily. 
     Since the HMD  100 D is a monocular HMD, the HMD  100 D includes one spectacle lens unit  101  while the HMD  100  illustrated in  FIG. 1  includes the two spectacle lens units  101 L and  101 R. To the spectacle lens unit  101 , an infrared sensor  103  is attached. The infrared sensor  103  functions similarly to the infrared sensors  103 L and  103 R in the HMD  100  illustrated in  FIG. 1  and transmits a sensor output therefrom to an eyeball position estimating unit  114 . Also, to the spectacle lens unit  101 , a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens unit  101  is attached. The gyro sensor  104  transmits a sensor output therefrom to an inclination estimating unit  105 . 
     Also, since the HMD  100 D is a monocular HMD, the HMD  100 D includes a display driver  112  and a display  113  of one system. The display  113  is driven by the display driver  112  and displays an image. Light from the image displayed on the display  113  is reflected by the spectacle lens unit  101  and reaches an eye of an observer (left eye or right eye). Accordingly, the image displayed on the display  113  is optically magnified by the spectacle lens unit  101  and is observed as a magnified virtual image by the eye of the observer. 
     Similarly to the eyeball position estimating unit  114  in the HMD  100  in  FIG. 1 , based on the sensor output from the infrared sensor  103 , the eyeball position estimating unit  114  estimates an eyeball position of the eye of the observer and outputs an angle ω, of when scanning of a cornea (black part of eye) is performed, as an estimation result of the eyeball position (see  FIG. 2  and  FIG. 3 ). Also, similarly to the inclination estimating unit  115  in the HMD  100  in  FIG. 1 , based on the sensor output from the gyro sensor  104 , the inclination estimating unit  115  estimates an inclination θ in a horizontal direction of the optical system including the spectacle lens unit  101  (see  FIG. 3 ). 
     The display control unit  116 D calculates a positional gap d of the optical system relative to the eye of the observer based on the estimation result of the eyeball position and the estimation result of the inclination of the optical system. With reference to  FIG. 3 , the positional gap d corresponds to dL/cos θ or dR/cos θ. Based on the positional gap d, the display control unit  116 D controls an image correcting unit  111  in such a manner that a value of the positional gap d becomes zero and performs shift control, in the horizontal direction, of a display position of the image displayed on the display  113 . Accordingly, a center in the horizontal direction of the image is made coincident with the eye of the observer and the positional gap between the optical system and the eye of the observer is corrected. 
     Detail description of the other parts of the HMD  100 D illustrated in  FIG. 9  is omitted. However, the other parts are configured and operate similarly to those of the HMD  100  illustrated in  FIG. 1 . As described above, in the HMD  100 D illustrated in  FIG. 9 , based on a detection result of the positional relationship between the optical system and the eye of the observer, shift control of the display position of the image is performed and the positional gap between the optical system and the eye of the observer is corrected electronically. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;6. Sixth Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Monocular)] 
       FIG. 10  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100 E as the sixth embodiment. This configuration example is a monocular HMD. In  FIG. 10 , the same sign is assigned to a part corresponding to  FIG. 5  or  FIG. 9  and detail description of the part will be omitted arbitrarily. 
     Since the HMD  100 E is a monocular HMD, the HMD  100 E includes one spectacle lens unit  101  while the HMD  100 A illustrated in  FIG. 5  includes two spectacle lens units  101 L and  101 R. To the spectacle lens unit  101 , an infrared sensor  103  is attached. The infrared sensor  103  functions similarly to the infrared sensors  103 L and  103 R in the HMD  100 A illustrated in  FIG. 5  and transmits a sensor output therefrom to an eyeball position estimating unit  114 . Also, to the spectacle lens unit  101 , a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens unit  101  is attached. The gyro sensor  104  transmits a sensor output therefrom to an inclination estimating unit  105 . 
     Also, since the HMD  100 E is a monocular HMD, the HMD  100 E includes a display driver  112  and a display  113  of one system. The display  113  is driven by the display driver  112  and displays an image. Light from the image displayed on the display  113  is reflected by the spectacle lens unit  101  and reaches an eye of an observer (left eye or right eye). Accordingly, the image displayed on the display  113  is optically magnified by the spectacle lens unit  101  and is observed as a magnified virtual image by the eye of the observer. 
     Similarly to the eyeball position estimating unit  114  in the HMD  100 A in  FIG. 5 , based on the sensor output from the infrared sensor  103 , the eyeball position estimating unit  114  estimates an eyeball position of the eye of the observer and outputs an angle ω, of when scanning of a cornea (black part of eye) is performed, as an estimation result of the eyeball position (see  FIG. 2  and  FIG. 3 ). Also, similarly to the inclination estimating unit  115  in the HMD  100 A in  FIG. 5 , based on the sensor output from the gyro sensor  104 , the inclination estimating unit  115  estimates an inclination θ in a horizontal direction of the optical system including the spectacle lens unit  101  (see  FIG. 3 ). 
     The display control unit  116 E calculates a positional gap d of the optical system relative to the eye of the observer based on the estimation result of the eyeball position and the estimation result of the inclination of the optical system. With reference to  FIG. 3 , the positional gap d corresponds to dL/cos θ or dR/cos θ. Based on the positional gap d, the display control unit  116 E controls a unit for correcting a position of an optical system  117  in such a manner that a value of the positional gap d becomes zero and performs shift control, in the horizontal direction, of a position of the spectacle lens unit  101 , that is, of the optical system. Also, the display control unit  116 E also controls a position of the display  113  along with the shift control of the spectacle lens unit  101 . Accordingly, a center in the horizontal direction of the image is made coincident with the eye of the observer and the positional gap between the optical system and the eye of the observer is corrected. 
     Detail description of the other parts of the HMD  100 E illustrated in  FIG. 10  is omitted. However, the other parts are configured and operate similarly to those of the HMD  100 A illustrated in  FIG. 5 . As described, in the HMD  100 E illustrated in  FIG. 10 , based on a detection result of the positional relationship between the optical system and the eye of the observer, shift control of the optical system is performed and the positional gap between the optical system and the eye of the observer is mechanically corrected. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;7. Seventh Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Monocular)] 
       FIG. 11  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100 F as the seventh embodiment. This configuration example is a monocular HMD. In  FIG. 11 , the same sign is assigned to apart corresponding to  FIG. 6  or  FIG. 9  and detail description of the part will be omitted arbitrarily. 
     Since the HMD  100 F is a monocular HMD, the HMD  100 F includes one spectacle lens unit  101  while the HMD  100 B illustrated in  FIG. 6  includes two spectacle lens units  101 L and  101 R. To the spectacle lens unit  101 , an infrared sensor  103  is attached. The infrared sensor  103  functions similarly to the infrared sensors  103 L and  103 R in the HMD  100 B illustrated in  FIG. 6  and transmits a sensor output therefrom to an eyeball position estimating unit  114 . Also, to the spectacle lens unit  101 , a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens unit  101  is attached. The gyro sensor  104  transmits a sensor output therefrom to an inclination estimating unit  105 . 
     Also, since the HMD  100 F is a monocular HMD, the HMD  100 F includes a display driver  112  and a display  113  of one system. The display  113  is driven by the display driver  112  and displays an image. Light from the image displayed on the display  113  is reflected by the spectacle lens unit  101  and reaches an eye of an observer (left eye or right eye). Accordingly, the image displayed on the display  113  is optically magnified by the spectacle lens unit  101  and is observed as a magnified virtual image by the eye of the observer. 
     Similarly to the eyeball position estimating unit  114  in the HMD  100 B in  FIG. 6 , based on the sensor output from the infrared sensor  103 , the eyeball position estimating unit  114  estimates an eyeball position of the eye of the observer and outputs an angle ω, of when scanning of a cornea (black part of eye) is performed, as an estimation result of the eyeball position (see  FIG. 2  and  FIG. 3 ). Also, similarly to the inclination estimating unit  115  in the HMD  100 B in  FIG. 6 , based on the sensor output from the gyro sensor  104 , the inclination estimating unit  115  estimates an inclination θ in a horizontal direction of the optical system including the spectacle lens unit  101  (see  FIG. 3 ). 
     The display control unit  116 F calculates a positional gap d of the optical system relative to the eye of the observer based on the estimation result of the eyeball position and the estimation result of the inclination of the optical system. With reference to  FIG. 3 , the positional gap d corresponds to dL/cos θ or dR/cos θ. The display control unit  116 F outputs a display signal for OSD display of the positional gap d or OSD display to prompt a user to perform operation of correcting the positional gap d and supplies the display signal to an OSD superimposing unit  118 . Accordingly, the OSD display is performed on the displays  113 . 
     By an observer, correction operation to make the positional gap d into zero is performed based on the OSD display. The user operation acquiring unit  119  acquires the user operation and transmits the acquired user operation to the display control unit  116 F. Based on the correction operation, the display control unit  116 F performs shift control of a display position of the image displayed on the display  113 . Accordingly, a center in the horizontal direction of the image is made coincident with the eye of the observer and the positional gap between the optical system and the eye of the observer is corrected. 
     As described above, in the HMD  100 F illustrated in  FIG. 11 , based on a detection result of the positional relationship between the optical system and the eye of the observer, OSD display of the positional gap between the optical system and the eye of the observer is performed. Based on the OSD display, correction operation is performed by the observer to eliminate the positional gap. Then, according to the correction operation, shift control of the display position of the image is performed and the positional gap between the optical system and the eye of the observer is electronically corrected. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;8. Eighth Embodiment&gt; 
     [Configuration Example of Head-Mounted Display (Monocular)] 
       FIG. 12  is a view illustrating a schematic configuration example of an optical transmission head-mounted display (HMD)  100 G as the eighth embodiment. This configuration example is a monocular HMD. In  FIG. 12 , the same sign is assigned to apart corresponding to  FIG. 8  or  FIG. 9  and detail description of the part will be omitted arbitrarily. 
     Since the HMD  100 G is a monocular HMD, the HMD  100 G includes one spectacle lens unit  101  while the HMD  100 C illustrated in  FIG. 8  includes two spectacle lens units  101 L and  101 R. To the spectacle lens unit  101 , an infrared sensor  103  is attached. The infrared sensor  103  functions similarly to the infrared sensors  103 L and  103 R in the HMD  100 C illustrated in  FIG. 8  and transmits a sensor output therefrom to an eyeball position estimating unit  114 . Also, to the spectacle lens unit  101 , a gyro sensor  104  to detect an inclination of an optical system including the spectacle lens unit  101  is attached. The gyro sensor  104  transmits a sensor output therefrom to an inclination estimating unit  105 . 
     Also, since the HMD  100 G is a monocular HMD, the HMD  100 G includes a display driver  112  and a display  113  of one system. The display  113  is driven by the display driver  112  and displays an image. Light from the image displayed on the display  113  is reflected by the spectacle lens unit  101  and reaches an eye of an observer (left eye or right eye). Accordingly, the image displayed on the display  113  is optically magnified by the spectacle lens unit  101  and is observed as a magnified virtual image by the eye of the observer. 
     Similarly to the eyeball position estimating unit  114  in the HMD  100 C in  FIG. 8 , based on the sensor output from the infrared sensor  103 , the eyeball position estimating unit  114  estimates an eyeball position of the eye of the observer and outputs an angle ω, of when scanning of a cornea (black part of eye) is performed, as an estimation result of the eyeball position (see  FIG. 2  and  FIG. 3 ). Also, similarly to the inclination estimating unit  115  in the HMD  100 C in  FIG. 8 , based on the sensor output from the gyro sensor  104 , the inclination estimating unit  115  estimates an inclination θ in a horizontal direction of the optical system including the spectacle lens unit  101  (see  FIG. 3 ). 
     The display control unit  116 G calculates a positional gap d of the optical system relative to the eye of the observer based on the estimation result of the eyeball position and the estimation result of the inclination of the optical system. With reference to  FIG. 3 , the positional gap d corresponds to dL/cos θ or dR/cos θ. The display control unit  116 G outputs a display signal for OSD display of the positional gap d or OSD display to prompt a user to perform operation of correcting the positional gap d and supplies the display signal to an OSD superimposing unit  118 . Accordingly, the OSD display is performed on the displays  113 . 
     By an observer, correction operation to make the positional gap d into zero is performed based on the OSD display. The user operation acquiring unit  119  acquires the user operation and transmits the acquired user operation to the display control unit  116 G. Based on the correction operation, the display control unit  116 G controls a unit for correcting a position of an optical system  117  and performs shift control of a position of the optical system, that is, a position of the spectacle lens unit  101  and a position of the display  103 . Accordingly, a center in the horizontal direction of the optical system is made coincident with the eye of the observer and the positional gap between the optical system and the eye of the observer is corrected. 
     As described above, in the HMD  100 G illustrated in  FIG. 12 , based on a detection result of the positional relationship between the optical system and the eye of the observer, OSD display of the positional gap between the optical system and the eye of the observer is performed. Based on the OSD display, correction operation is performed by the observer to eliminate the positional gap. Then, according to the correction operation, shift control of the optical system is performed and the positional gap between the optical system and the eye of the observer is mechanically corrected. Thus, correction of the positional gap between the optical system and the eye of the observer can be performed appropriately. 
     &lt;9. Modification&gt; 
     Note that in the above-described embodiments, correction accuracy is improved by taking the inclination θ of the optical system into consideration. However, a simplified configuration in which the inclination θ of the optical system is not taken into consideration may also be considered. In such a case, components such as the gyro sensor  104  and the inclination estimating unit  115  are not necessary. 
     Also, in the above-described embodiments, an example in which a gy sensor  104  estimates the inclination θ of the whole optical system based on the sensor output from the gy sensor  104  attached to the spectacle lens unit  101 L has been described. However, a configuration in which an inclination of the optical system on each of the left-eye side and the right-eye side is detected may also be considered. In such a case, a gyro sensor is attached not only to the spectacle lens unit  101 L but also to the spectacle lens unit  101 R. 
     Also, in the above-described embodiments, an example in which the positional gap in the horizontal direction between the optical system and the eye of the observer is corrected by performing shift control, in the horizontal direction, of the left-eye image and the right-eye image respectively displayed on the displays  113 L and  113 R has been described. Although detail description is omitted, it may also be considered to correct a positional gap between the optical system and the eye of the observer in a vertical direction in a similar manner. 
     Also, in the above-described embodiments, an example in which one infrared sensor is attached to each spectacle lens unit and scanning operation is performed with a weak infrared ray emitted to a side of the observer has been described. However, for example, as illustrated in  FIG. 13( a ) , a configuration in which a plurality of infrared sensors IRS is attached to a spectacle lens unit GL and a position of the eye of the observer is determined comprehensively based on a sensor output from each of the infrared sensors IRS may also be considered. In such a case, when there are a great number of the infrared sensors IRS, it becomes unnecessary to perform the scanning operation with a weak infrared ray emitted from each of the infrared sensors IRS to the side of the observer. 
     Also, in the above-described embodiment, an example in which an infrared sensor is attached to each spectacle lens unit has been described. However, as illustrated in  FIG. 13( b ) , by using a reflection sheet RS, it is also possible to install the infrared sensor IRS to a place which is away from the spectacle lens unit GL for a certain degree. Note that even in this case, as illustrated in  FIG. 13( c ) , a configuration in which a plurality of infrared sensors IRS is arranged and a position of the eye of the observer is determined comprehensibly based on a sensor output from each of the infrared sensors IRS may also be considered. 
     Also, in the above-described embodiments, an example in which a display position of an image is shifted in the horizontal direction in the image correcting unit  111  has been described. To be exact, it may be further considered to perform correction of a position by a shift, rotation, magnification, contraction or the like in a vertical direction. 
     Also, in the above-described embodiments, estimation of an eyeball position has been performed by application of the sclera reflection method. However, application of a different technique such as an electro-oculogram (EOG) method or a facial recognition technique may also be considered. 
     Also, in the above-described embodiments, an example in which OSD display of a positional relationship (such as difference s or positional gap d) detected by the positional relationship detecting unit is displayed on a display for displaying an image has been described. However, the positional relationship may be displayed on a display element which is different from the display for displaying an image. Also, along with or instead of these, the positional relationship may be presented by sound, vibration, or increasing/decreasing intensity of light emission. 
     For example, in a case of using the vibration, a vibrator is installed and vibration becomes stronger as an amount of the positional gap becomes larger. Also, for example, in a case of using the sound, a gap amount or an operation method is reproduced or a volume becomes larger as a gap amount becomes larger. 
     Also, in the above-described embodiments, an example in which the present technique is applied to an HMD has been described. However, the present technique is not limited to the above and may be widely applied to a different image display apparatus such as a finder of a camera or a pair of electronic binoculars. 
     Also, the present technique may include the following configurations. 
     (1) An image display apparatus including: a positional relationship detecting unit configured to detect a positional relationship between an optical system, which leads an image displayed on a display element to an eye of an observer, and the eye of the observer based on an eyeball position of the eye of the observer; and a positional relationship correcting unit configured to correct the positional relationship between the optical system and the eye of the observer based on the positional relationship detected by the positional relationship detecting unit. 
     (2) The image display apparatus according to ( 1 ), wherein the positional relationship correcting unit shifts a display position of the image displayed on the display element based on the positional relationship detected by the positional relationship detecting unit. 
     (3) The image display apparatus according to ( 1 ), wherein the positional relationship correcting unit moves the optical system based on the positional relationship detected by the positional relationship detecting unit. 
     (4) The image display apparatus according to ( 1 ), wherein the positional relationship correcting unit includes a positional relationship presenting unit configured to present, to the observer, the positional relationship detected by the positional relationship detecting unit, and a control unit configured to control the positional relationship between the optical system and the eye of the observer according to operation by the observer. 
     (5) The image display apparatus according to ( 4 ), wherein the control unit performs shift control of a display position of the image displayed on the display element according to the operation by the observer. 
     (6) The image display apparatus according to claim  4 ), wherein the control unit performs movement control of the optical system according to the operation by the observer. 
     (7) The image display apparatus according to any of (4) to (6), wherein the positional relationship presenting unit displays, on the display element, the positional relationship detected by the positional relationship detecting unit. 
     (8) The image display apparatus according to any of (1) to (7), further including an inclination estimating unit configured to estimate an inclination of the optical system, wherein the positional relationship detecting unit detects the positional relationship between the optical system and the eye of the observer based on the estimated eyeball position and the estimated inclination. 
     (9) The image display apparatus according to any of (1) to (8), further including an optical system configured to lead, to the eye of the observer, the image displayed on the display element. 
     (10) The image display apparatus according to (9), wherein as the optical system, there are a first optical system configured to lead, to a left eye of the observer, a left-eye image displayed on the display element and a second optical system configured to lead, to a right eye of the observer, a right-eye image displayed on the display element. 
     (11) The image display apparatus according to (9) or (10), further including an eyeball position estimating unit configured to estimate an eyeball position of the eye of the observer, wherein the positional relationship detecting unit detects the positional relationship between the optical system and the eye of the observer based on the eyeball position estimated by the eyeball position estimating unit. 
     (12) A method for displaying an image including: a positional relationship detection step of detecting a positional relationship between an optical system, which leads an image displayed on a display element to an eye of an observer, and the eye of the observer; and a positional relationship correction step of correcting the positional relationship between the optical system and the eye of the observer based on the detected positional relationship. 
     REFERENCE SIGNS LIST 
     
         
           100 ,  100 A to  100 G head-mounted display 
           101 ,  101 L,  101 R spectacle lens unit 
           102  connection member 
           103 ,  103 L,  103 R infrared sensor 
           104  gyro sensor 
           111  image correcting unit 
           112 ,  112 L,  112 R display driver 
           113 ,  113 L,  113 R display 
           114  eyeball position estimating unit 
           115  inclination estimating unit 
           116 ,  116 A to  116 G display control unit 
           117  unit for correcting a position of an optical system 
           118  OSD superimposing unit 
           119  user operation acquiring unit 
           122  optical system adjusting mechanism