Patent Publication Number: US-2022236567-A1

Title: Image display apparatus and display apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to an image display apparatus and a display apparatus, and in particular to a display apparatus that can be used as, for example, a head-mounted display (HMD), and an image display apparatus that is suitable for use in the display apparatus. 
     BACKGROUND ART 
     A Maxwellian viewing retinal projection display that displays thereon an image by the image (a pencil of light) being directly projected onto the retina of an observer, that is, a Maxwellian viewing retinal projection head-mounted display (hereinafter abbreviated as a “retinal projection HMD” in some cases) in particular is well known. Here, there is a need to align the light convergence with the pupil in the use of such a retinal projection HMD. However, the diameter of a human pupil is in a very narrow range of between 2 mm in a bright environment and 7 mm in a dark environment. Thus, when the retinal projection HMD fails to follow the movement of an eyeball (primarily, the rotation of an eyeball), an image (a pencil of light) deviates from the pupil of an observer. This results in being unable to continuously observe the image properly. 
     A technology used to move an ocular optical system in a retinal projection HMD is known, the technology being disclosed in, for example, Japanese Patent Laid-Open No. 2009-294605. Specifically, as illustrated in, for example, FIG. 3, a scanning display apparatus disclosed in Japanese Patent Laid-Open No. 2009-294605 includes 
     a light source 101, 
     a scanning unit 104 that scans a pencil of light from the light source 101, 
     a scanning optical system 105 that collects the pencil of light from the scanning unit 104, and 
     an ocular optical system 106 that guides the pencil of light from the scanning optical system 105 to an exit pupil with which the eye of an observer is aligned, 
     the pencil of light that travels from the scanning optical system 105 to the ocular optical system 106 being telecentric, 
     the ocular optical system 106 including a first reflecting surface 106c off which the pencil of light from the scanning optical system 105 is reflected to be directed to the exit pupil, 
     the scanning display apparatus including a first mechanism 110 that enables the ocular optical system 106 to move with respect to the scanning optical system 105 and the scanning unit 104 in parallel with a direction in which the pencil of light travels from the scanning optical system 105 to the ocular optical system 106. 
     Further, in addition to being horizontally moved, the exit pupil 107 can also be vertically moved using the optical system illustrated in FIG. 10 of Japanese Patent Application Laid-Open No. 2009-294605. 
     In the scanning display apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-294605, a primary portion of the scanning optical system 105 and the first reflecting surface 106c are arranged in an imaginary plane (an x axis represents the horizontal direction and a y axis represents the vertical direction) that is orthogonal to a pupillary axis of an observer (that is represented by a z axis). Further, the first reflecting surface 106c bends the pencil of light from the scanning optical system 105 ninety degrees and causes the pencil of light to enter the eye of the observer. The first reflecting surface 106c moves in an xy plane. Further, according to the movement of the first reflecting surface 106c, a light-collecting optical system 102 that is a movable optical system is moved in its optical-axis direction to move the position of an image point for the pencil of light from the light source unit 101. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2009-294605 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     Note that, in the scanning display apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-294605 described above, the first reflecting surface 106c bends the pencil of light from the scanning optical system 105 ninety degrees and causes the pencil of light to enter the eye of an observer. Thus, there is an increase in the thickness of the ocular optical system 106 when the first reflecting surface 106c is orthogonally projected onto an xz plane. This results in difficulty in making the ocular optical system 106 smaller. Such difficulty in making the ocular optical system 106 smaller is a great obstacle to the practical use of a head-mounted display including a design of the head-mounted display. Further, it is difficult to make the first reflecting surface 106c that is a movable portion smaller. Thus, in reality, it is difficult to cause the first reflecting surface 106c to follow the high-speed movement of an eyeball, and it is also difficult to reduce power consumption for moving the ocular optical system 106. 
     Thus, it is an object of the present disclosure to provide an image display apparatus that can be made smaller and lighter, and an image display apparatus that is suitable for use in the display apparatus. 
     Solution to Problem 
     In order to achieve the object described above, an image display apparatus according to a first aspect or a second aspect of the present disclosure includes 
     an image forming apparatus, 
     an optical element that is arranged in front of a face of an observer, and 
     a movement control apparatus, in which 
     when a region situated on a side of an ear of the observer, as viewed from the optical element, is referred to as a region situated behind the optical element, the image forming apparatus is arranged in the region situated behind the optical element, and 
     an image exiting the image forming apparatus obliquely enters the optical element from the region situated behind the optical element, and is reflected off the optical element to reach a pupil of the observer. 
     Further, in the image display apparatus according to the first aspect of the present disclosure, when there is a change in a position of the pupil of the observer, the optical element is moved using the movement control apparatus, and a position of the image exiting the image forming apparatus is controlled using the movement control apparatus. Furthermore, in the image display apparatus according to the second aspect of the present disclosure, when there is a change in a position of the pupil of the observer, the image forming apparatus is moved using the movement control apparatus. 
     In order to achieve the object described above, a display apparatus of the present disclosure includes 
     a frame that is worn by an observer, and 
     an image display apparatus that is attached to the frame, the image display apparatus being the image display apparatus according to the first aspect of the present disclosure, or the image display apparatus according to the second aspect of the present disclosure, or a combination of the image display apparatus according to the first aspect of the present disclosure and the image display apparatus according to the second aspect of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  schematically illustrates a display apparatus of a first embodiment, as viewed from the front. 
         FIGS. 2A and 2B  are a conceptual diagram of the display apparatus of the first embodiment, as viewed from above an observer, and a diagram used to describe a relationship between an x axis and an X axis, respectively. 
         FIGS. 3A and 3B  are conceptual diagrams of an optical element and the like, as viewed from above, that are used to describe an operation of an image display apparatus of the first embodiment. 
         FIGS. 4A and 4B  are conceptual diagrams of the optical element and the like, as viewed from above, that are subsequent to  FIG. 3B  and used to describe the operation of the image display apparatus of the first embodiment. 
         FIGS. 5A and 5B  are conceptual diagrams of the optical element and the like, as viewed from above, that are used to describe the operation of the image display apparatus of the first embodiment. 
         FIGS. 6A and 6B  are conceptual diagrams of the optical element and the like, as viewed from above, that are subsequent to  FIG. 5B  and used to describe the operation of the image display apparatus of the first embodiment. 
         FIG. 7  schematically illustrates the display apparatus of a second embodiment, as viewed from the front. 
         FIGS. 8A and 8B  are conceptual diagrams of the optical element and the like, as viewed from the lateral direction, that are used to describe an operation of the image display apparatus of the second embodiment. 
         FIGS. 9A and 9B  are conceptual diagrams of the optical element and the like, as viewed from the lateral direction, that are subsequent to  FIG. 8B  and used to describe the operation of the image display apparatus of the second embodiment. 
         FIG. 10  schematically illustrates a modification of the display apparatus of the second embodiment, as viewed from the front. 
         FIG. 11  schematically illustrates the display apparatus of a third embodiment, as viewed from the front. 
         FIGS. 12A and 12B  are conceptual diagrams of the optical element and the like, as viewed from above, that are used to describe an operation of the image display apparatus of the third embodiment. 
         FIG. 13  schematically illustrates the display apparatus of a fourth embodiment, as viewed from the front. 
         FIGS. 14A and 14B  are conceptual diagrams of the optical element and the like, as viewed from the lateral direction, that are used to describe an operation of the image display apparatus of the fourth embodiment. 
         FIGS. 15A and 15B  are conceptual diagrams of the optical element and the like, as viewed from the lateral direction, that are used to describe operations of the image display apparatuses of a sixth embodiment and a seventh embodiment. 
         FIGS. 16A to 16C  are principle diagrams used to describe a light-collecting member that is included in the image forming apparatus of the seventh embodiment. 
         FIG. 17  is a conceptual diagram of the image display apparatus included in the display apparatus of an eighth embodiment. 
         FIG. 18  is a conceptual diagram of the display apparatus of a ninth embodiment, as viewed from above the observer. 
         FIG. 19  is a conceptual diagram of a modification of the display apparatus of the first embodiment, as viewed from above the observer. 
         FIGS. 20A and 20B  are conceptual diagrams of modifications of the image forming apparatus. 
         FIG. 21  is a schematic cross-sectional view illustrating an enlarged portion of a hologram diffraction grating. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     The present disclosure is described below on the basis of embodiments with reference to the drawings. However, the present disclosure is not limited to the embodiments, and various numerical values and materials in the embodiments are merely illustrative. Note that the description is made in the following order. 
     1. General Descriptions of Image Display Apparatuses According to First and Second Aspects of Present Disclosure, and Display Apparatus of Present Disclosure 
     2. First Embodiment (Image Display Apparatus According to First Aspect of Present Disclosure, and Display Apparatus of Present Disclosure) 
     3. Second Embodiment (Modification of First Embodiment) 
     4. Third Embodiment (Image Display Apparatus According to Second Aspect of Present Disclosure, and Display Apparatus of Present Disclosure) 
     5. Fourth Embodiment (Modification of Third Embodiment) 
     6. Fifth Embodiment (Combination of First and Fourth Embodiments) 
     7. Sixth Embodiment (Modification of Fifth Embodiment) 
     8. Seventh Embodiment (Modification of Sixth Embodiment) 
     9. Eighth Embodiment (Another Modification of Sixth Embodiment) 
     10. Ninth Embodiment (Modification of First to Eighth Embodiments) 
     11. Others 
     &lt;General Descriptions of Image Display Apparatuses According to First and Second Aspects of Present Disclosure, and Display Apparatus of Present Disclosure&gt; 
     In an image display apparatus according to a first aspect of the present disclosure, or the image display apparatus according to the first aspect of the present disclosure, which is included in a display apparatus of the present disclosure (they may be hereinafter collectively referred to as an “image display apparatus or the like according to the first aspect of the present disclosure”), an optical element may be moved, using a movement control apparatus, in a direction corresponding to a change in the position of the pupil of an observer in parallel with a horizontal direction. Note that, for convenience, such a movement of an optical element may be referred to as a “horizontal movement of an optical element”. 
     Further, in the image display apparatus or the like according to the first aspect of the present disclosure including such a favorable configuration, an image forming apparatus may be moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil of the observer in parallel with a vertical direction. Note that, for convenience, such a movement of a movement control apparatus may be referred to as a “vertical movement of a movement control apparatus”. Further, in this case, 
     the image forming apparatus may include a 4f optical system through which an image exiting the image forming apparatus passes, and 
     the 4f optical system may be moved, using the movement control apparatus, in the direction corresponding to the change in the position of the pupil of the observer in parallel with the vertical direction. 
     Further, the image forming apparatus may include a reflecting mirror off which the image exiting the image forming apparatus is reflected, and 
     a light reflection angle of the reflecting mirror may be changed using the movement control apparatus. 
     Further, in the image display apparatus or the like according to the first aspect of the present disclosure including the favorable configuration described above, the optical element may be moved, using the movement control apparatus, in the direction corresponding to the change in the position of the pupil of the observer in parallel with the vertical direction. Note that, for convenience, such a movement of an optical element may be referred to as a “vertical movement of an optical element”. 
     Here, primarily due to the rotation of an eyeball of the observer, the position of the pupil is horizontally or vertically changed, or is horizontally and vertically changed. 
     In the image display apparatus or the like according to the first aspect of the present disclosure including the favorable configurations described above, the optical element may include a reflective hologram diffraction grating. In this case, the reflective hologram diffraction grating may include a light-collecting function. In other words, the reflective hologram diffraction grating may include a function as a concave mirror. Further, the optical element may include the concave mirror. Furthermore, in the favorable configurations described above, a pencil of light exiting the image forming apparatus may enter the optical element in a telecentric state. This makes it possible to reduce various aberrations, and to prevent the size of an image from being changed due to the movement of the optical element. Further, in the image display apparatus or the like according to the first aspect of the present disclosure including the favorable configurations described above, the optical element may include a lens that has a positive optical power, and a flat reflecting mirror, the lens being a lens through which an image exiting the image forming apparatus passes. 
     Further, in the image display apparatus or the like according to the first aspect of the present disclosure, the optical element may be rotated, using the movement control apparatus, according to a change in the position of the pupil of the observer in parallel with the vertical direction. Further, in the image display apparatus or the like according to the first aspect of the present disclosure, an angle of arrangement of the optical element may also be changed, using the movement control apparatus, according to a change in the position of the pupil of the observer in parallel with the vertical direction. Note that, in these cases, it is sufficient if the optical element is horizontally moved using the movement control apparatus when there is a change in the position of the pupil of the observer in parallel with the horizontal direction. 
     In an image display apparatus according to a second aspect of the present disclosure, or the image display apparatus according to the second aspect of the present disclosure, which is included in the display apparatus of the present disclosure (they may be hereinafter collectively referred to as an “image display apparatus or the like according to the second aspect of the present disclosure”), an optical element may include a reflective hologram diffraction grating. In this case, the reflective hologram diffraction grating may include a light-collecting function. Further, a pencil of light exiting the image forming apparatus may enter the optical element in a telecentric state. This makes it possible to reduce various aberrations, and to prevent the size of an image from being changed due to the movement of the optical element. 
     Further, in the image display apparatuses or the like according to the first and second aspects of the present disclosure including the favorable configurations described above, the image forming apparatus may include a dispersion compensation element. 
     The display apparatus of the present disclosure including the favorable configurations described above may include the image display apparatus for a right eye and the image display apparatus for a left eye. 
     Further, the display apparatus of the present disclosure including the various favorable configurations described above may be worn on the head of the observer. In other words, the display apparatus of the present disclosure may be a head-mounted display (HMD), that is, a Maxwellian viewing retinal projection HMD in particular. 
     The movement control apparatus includes a drive mechanism, and, in some cases, the movement control apparatus further includes a position control circuit that controls the position of an image exiting the image forming apparatus. The drive mechanism includes, for example, a drive apparatus and a slide bar. The position of an image exiting the image forming apparatus is controlled using the movement control apparatus. Specifically, for example, it is sufficient if the position of an image exiting the image forming apparatus (that is, the position of an image formed by the image forming apparatus or a region of an image exiting a scanning mechanism described later) is moved on the basis of a control signal from the position control circuit included in the movement control apparatus. When the display apparatus includes the image display apparatus for a right eye and the image display apparatus for a left eye, an amount of movement of the position of an image exiting the image forming apparatus, and an amount of a change in a region onto which the image exiting the image forming apparatus is projected depend on, for example, an amount of parallax, or the depth of a three-dimensional stereoscopic image to be displayed. 
     The reflective hologram diffraction grating may have a well-known configuration and structure. The image display apparatus can be a semi-transmissive (see-through) image display apparatus by using a hologram diffraction grating, and this makes it possible to see outside through the optical element. When the optical element includes a concave mirror, a flat reflecting mirror, or a Fresnel reflecting mirror, the concave mirror, the flat reflecting mirror, or the Fresnel reflecting mirror can be obtained by a light reflective film off which light of a specific wavelength is reflected being formed on a light reflective surface of a transparent member (that may be referred to as a “base portion” for convenience) that is included in the concave mirror, the flat reflecting mirror, or the Fresnel reflecting mirror. This makes it possible to see outside through the concave mirror, the flat reflecting mirror, or the Fresnel reflecting mirror. 
       FIG. 21  is a schematic cross-sectional view illustrating an enlarged portion of a hologram diffraction grating, where an interference pattern that has an angle of inclination (a slant angle) φ is formed in the hologram diffraction grating. The angle of inclination φ refers to an angle formed by an interference pattern and the surface of a hologram diffraction grating. The interference pattern is formed from the interior of the hologram diffraction grating to the surface of the hologram diffraction grating. The interference pattern satisfies the Bragg condition. The Bragg condition refers to a condition that satisfies Formula (A) indicated below. In Formula (A), m is a positive integer, λ is a wavelength, d is a pitch of a grating plane (spacing of an imaginary plane including an interference pattern in a normal direction), and Θ is a complement of an angle at which light enters an interference pattern. Further, when light enters a hologram diffraction grating at an angle of incidence ψ, a relationship between Θ, the angle of inclination φ, and the angle of incidence ψ is represented by Formula (B) indicated below. 
         m·λ= 2· d ·sin(Θ)  (A)
 
       Θ=90°−(φ+ψ)  (B)
 
     The optimization of the angle of inclination (the slant angle) φ and the pitch (d) makes it possible to provide a light-collecting function to the hologram diffraction grating. Examples of a material of the hologram diffraction grating include a photopolymer material. It is sufficient if the material and a basic structure of the hologram diffraction grating are similar to a material and a structure of a conventional hologram diffraction grating. An interference pattern is formed from the interior of the hologram diffraction grating to the surface of the hologram diffraction grating, and it is sufficient if a method for forming such an interference pattern itself is similar to a conventional formation method. 
     The optical element may be attached to a base material, or the optical element may be formed on the surface of the base material. Examples of a material of the base material or the base portion include a plastic material or glass. Specifically, when the base material or the base portion is made of a transparent plastic material, examples of the transparent plastic material include polyethylene terephthalate, polyethylene naphthalate, polycarbonate, cellulose esters such as cellulose acetate, fluorine polymers such as a copolymer of polyvinylidene fluoride or polytetrafluoroethylene and hexafluoropropylene, polyether such as polyoxymethylene, polyacetal, polystyrene, polyethylene, polypropylene, polyolefins such as a metylpentene polymer, polyimide such as polyamide-imide or polyetherimide, polyamide, polyethersulfone, polyphenylene sulfide, polyvinylidene fluoride, tetraacetylcellulose, brominated phenoxy, polyarylate, and polysulfone. When the base material or the base portion is made of glass, examples of the glass include transparent glass such as soda-lime glass and a plate of white glass. A hard coat layer that includes an organic-inorganic mixture layer, or an antireflective film made of a fluororesin may be formed on an outer surface of the base material or the base portion. The base material is attached to a front portion that is included in a frame. The base material may be attached to the front portion through the drive mechanism, or the base material may be attached directly to the front portion. 
     The hologram diffraction grating is an optical element that makes it possible to appropriately select a path of light other than specularly reflected light (light of which an angle of incidence and an angle of reflection are different from each other), which is different from an ordinary reflective optical element. This makes it possible to arrange the optical element along the shape of the face of an observer with a high degree of freedom, and the hologram diffraction grating is a space-saving optical element suitably arranged in front of the eyes of the observer. Further, the hologram diffraction grating is a very thin optical element. This provides an advantage in making an apparatus smaller and lighter. Further, the use of a hologram diffraction grating makes it possible to display a virtual image without affecting a pencil of light entering from the outside world. This is based on two features that are the wavelength selectivity and the angular selectivity of a hologram diffraction grating. The wavelength selectivity of a hologram diffraction grating is a property of being able to change the direction of light of a specific wavelength by diffracting the light. If a light-emitting diode (LED) or a semiconductor laser element of which a wavelength band is relatively narrow is used as a light source included in the image forming apparatus and if the hologram diffraction grating is designed to only diffract light of a specified wavelength from the light source, light from the outside world will be hardly affected by the hologram diffraction grating, since a wavelength band of the light from the light source is only a portion of a wavelength band of the light from the outside world. Further, the angular selectivity of a hologram diffraction grating is a property of only diffracting light entering from a specific angle. The hologram diffraction grating only diffracts light entering from a region situated behind the optical element, on the basis of the angular selectivity. Thus, light from the outside world that enters the pupil of an observer is not affected by the hologram diffraction grating. As described above, properties suitable for a see-through head-mounted display can be obtained on the basis of the wavelength selectivity and the angular selectivity. 
     It is assumed that a pupillary axis of an observer is represented by the z axis, a horizontal axis that is orthogonal to the z axis is represented by the x axis, a vertical axis that is orthogonal to the z axis and the x axis is represented by the y axis, a horizontal axis that intersects the x axis in the xz plane to form an acute angle θ 0  (including 0 degrees) with the x axis is represented by an X axis, a vertical axis that is orthogonal to the X axis in the xz plane is represented by a Z axis, and a vertical axis that is orthogonal to the X axis and the Z axis is represented by a Y axis. The Y axis may form an acute angle  110  with a yz plane. Note that the pupillary axis is defined by a line that passes through the center of an entrance pupil of an eyeball and is orthogonal to a corneal surface (for reference: http://www.visionsociety.jp/vision/koumokuPDF/04lecture/L1989.01.01.pdf). In the image display apparatus or the like according to the first aspect of the present disclosure, the optical element is moved using the movement control apparatus when there is a change in the position of the pupil of an observer, and it is assumed that the optical element moves in an XY plane. θ 0  may exhibit a value indicated below, although the value is not limited thereto. 
       −5 (degrees)≤θ 0 ≤5 (degrees)
 
     An image exiting the image forming apparatus obliquely enters the optical element from a region situated behind the optical element, and exits the optical element. When a pencil of light exiting a center of an image formed by the image forming apparatus (referred to as an “image-center pencil of light”) enters the optical element (referred to as an entering pencil of light), and exits the optical element (referred to as an exiting pencil of light), an angle θ 1  formed by the entering pencil of light and the exiting pencil of light may exhibit a value indicated below, although the value is not limited thereto. 
       45 (degrees)≤θ 1 ≤80 (degrees)
 
     In the display apparatus of the present disclosure, the frame includes a front portion that is arranged in front of an observer, two temple portions that are respectively rotatably attached to two ends of the front portion through respective hinges, and a nose pad. A temple-tip covering portion is attached to a tip of each temple portion. An assembly of the frame (including a rim portion) and the nose pad has a structure substantially the same as ordinary glasses. The nose pad may also have a well-known configuration and structure. Further, the front portion and the two temple portions may be integrated. In other words, the frame generally has substantially the same structure as ordinary glasses when the entirety of the display apparatus of the present disclosure is viewed. A material of the frame including the nose pad may be the same as the material of ordinary glasses, such as metal, an alloy, plastics, and a combination thereof. 
     The movement control apparatus includes a drive mechanism. For example, the drive mechanism includes a first drive apparatus and a first slide bar, and includes a second drive apparatus and a second slide bar. For example, it is sufficient if the horizontally extending first slide bar is slidably attached to the first drive apparatus, the first drive apparatus is fixed to an upper portion or a lower portion of the front portion, and the optical element (specifically, the base body) is fixed to the first slide bar. Then, the first drive apparatus is driven to slide the first slide bar with respect to the first drive apparatus, and this makes it possible to horizontally move the optical element fixed to the first slide bar. Likewise, it is sufficient if the vertically extending second slide bar is slidably attached to the second drive apparatus, the second drive apparatus is fixed to a portion situated on an ear side of the front portion, and the optical element (specifically, the base body) is fixed to the second slide bar. Then, the second drive apparatus is driven to slide the second slide bar with respect to the second drive apparatus, and this makes it possible to vertically move the optical element fixed to the second slide bar. Further, it is sufficient if the horizontally extending second slide bar is slidably attached to the second drive apparatus, the second drive apparatus is fixed to the portion situated on the ear side of the front portion, and the entirety or a portion of the image forming apparatus is fixed to the second slide bar. Then, the second drive apparatus is driven to slide the second slide bar with respect to the second drive apparatus, and this makes it possible to horizontally move the entirety or a portion of the image forming apparatus fixed to the second slide bar. Likewise, it is sufficient if the vertically extending second slide bar is slidably attached to the second drive apparatus, the second drive apparatus is fixed to the portion situated on the ear side of the front portion, and the entirety or a portion of the image forming apparatus is fixed to the second slide bar. Then, the second drive apparatus is driven to slide the second slide bar with respect to the second drive apparatus, and this makes it possible to vertically move the entirety or a portion of the image forming apparatus fixed to the second slide bar. Examples of a combination of the drive apparatus and the slide bar include a combination of a motor and a rack-and-pinion mechanism, and a combination of a motor and a ball-screw mechanism. Further, the drive mechanism may include a linear actuator. 
     It is favorable that a pupil position detecting mechanism used to detect the position of a pupil of an observer be attached to the front portion. The pupil position detecting mechanism may include, for example, a light-emitting section that emits infrared light, and a light-receiving section or an imaging device that receives the infrared light reflected off the pupil of the observer. Further, the pupil position detecting mechanism may include an imaging device that images the pupil of the observer. 
     In the image display apparatuses or the like according to the first and second aspects of the present disclosure including the various favorable configurations described above, the image forming apparatus may include a plurality of pixels arranged in a two-dimensional matrix. For convenience, the image forming apparatus having such a configuration is referred to as an “image forming apparatus having a first configuration”. 
     Examples of the image forming apparatus having the first configuration include an image forming apparatus that includes a reflective spatial light modulating apparatus and a light source, an image forming apparatus that includes a transmissive spatial light modulating apparatus and a light source, and an image forming apparatus that includes a light-emitting element such as an organic electroluminescence (EL), an inorganic EL, a light-emitting diode (LED), and a semiconductor laser element. In particular, it is favorable that the image forming apparatus having the first configuration be the image forming apparatus including an organic-EL light-emitting element (an organic-EL display apparatus), or the image forming apparatus including a reflective spatial light modulating apparatus and a light source. Examples of the spatial light modulating apparatus include a light bulb such as a transmissive or reflective liquid crystal display apparatus of, for example, liquid crystal on silicon (LCOS); and a digital micromirror device (DMD). Examples of the light source include a light-emitting element. Further, the reflective spatial light modulating apparatus may include a liquid crystal display apparatus and a polarization beam splitter in which a portion of light from the light source is reflected off the polarization beam splitter to be guided to the liquid crystal display apparatus, and a portion of light reflected off the liquid crystal display apparatus passes through the polarization beam splitter to be guided to the optical element. A red-light-emitting element, a green-light-emitting element, a blue-light-emitting element, and a white-light-emitting element may be used as the light-emitting elements included in the light source. Further, red light, green light, and blue light that are respectively emitted by the red-light-emitting element, the green-light-emitting element, and the blue-light-emitting element may be mixed and the brightness may be made uniform using a light pipe to obtain white light. Examples of the light-emitting element include a semiconductor laser element, a solid-state laser, and an LED. It is sufficient if the number of pixels is determined on the basis of the specifications necessary for the image display apparatus. Examples of a specific value of the number of pixels include 320×240, 432×240, 640×480, 1024×768, and 1920×1080. In the image forming apparatus having the first configuration, a diaphragm may be arranged at a front focal position of a light-collecting member (described later) (a focal point on the side of the image forming apparatus). 
     Further, in the image display apparatuses or the like according to the first and second aspects of the present disclosure including the favorable configurations described above, the image forming apparatus may include a light source, and a scanning mechanism that scans light emitted by the light source to form an image. For convenience, such an image forming apparatus is referred to as an “image forming apparatus having a second configuration”. 
     Examples of the light source included in the image forming apparatus having the second configuration include a light-emitting element. Specifically, a red-light-emitting element, a green-light-emitting element, a blue-light-emitting element, and a white-light-emitting element may be used as the light-emitting elements. Further, red light, green light, and blue light that are respectively emitted by the red-light-emitting element, the green-light-emitting element, and the blue-light-emitting element may be mixed and the brightness may be made uniform using a light pipe to obtain white light. Examples of the light-emitting element include a semiconductor laser element, a solid-state laser, and an LED. It is sufficient if the number of pixels (virtual pixels) in the image forming apparatus having the second configuration is also determined on the basis of the specifications necessary for the image display apparatus. Examples of a specific value of the number of pixels (virtual pixels) include 320×240, 432×240, 640×480, 1024×768, and 1920×1080. Further, when a color image is displayed and the light source includes a red-light-emitting element, a green-light-emitting element, and a blue-light-emitting element, colors may be combined using, for example, an X-prism. A microelectromechanical systems (MEMS) mirror or a galvanometer mirror that horizontally scans and vertically scans light emitted by the light source may be used as the scanning mechanism, the MEMS mirror including, for example, a two-dimensionally rotatable micromirror. In the image forming apparatus having the second configuration, the MEMS mirror or the galvanometer mirror may be arranged at a front focal position of the light-collecting member (described later) (a focal point on the side of the image forming apparatus). 
     For example, in the image forming apparatus having the first configuration or the image forming apparatus having the second configuration, light is formed into parallel light by the light-collecting member (an optical system that forms exiting light into parallel light), and the parallel light enters the optical element. Such a formation of light into parallel light makes it possible to cause an image to enter the optical element in a telecentric state. Specifically, for example, it is sufficient if a light exiting portion of the image forming apparatus is placed at a point (a position) corresponding to a focal length of the light-collecting member, in order to generate parallel light. Examples of the light-collecting member include an optical system that has a positive optical power as a whole and in which a convex lens, a concave lens, a freeform prism, or a hologram lens is used alone, or a combination thereof is used. A light blocking portion that includes an opening may be arranged near the light-collecting member between the light-collecting member and the optical element, in order to prevent undesired light from exiting the light-collecting member and from entering the optical element. 
     In the display apparatus of the present disclosure including the various favorable configurations described above, a signal used to display an image on the image forming apparatus may be received from the outside (the outside of a system of the display apparatus). In such a configuration, information and data regarding an image to be displayed on the image forming apparatus are recorded, held, or saved in, for example, a so-called cloud computer or server. When the image display apparatus includes a communication mechanism such as a phone line, an optical line, a cellular phone, or a smartphone, or when the image display apparatus and the communication mechanism are used in combination, various information and data can be communicated and exchanged between the cloud computer or server and the image display apparatus, and a signal based on various information and data, that is, the signal used to display an image on the image forming apparatus can be received. Further, the signal used to display an image on the image forming apparatus may be stored in the image display apparatus. The image displayed on the image forming apparatus includes various information and various data. The image display apparatus in the form of a wearable device may include a camera (an image-capturing apparatus). An image captured by the camera may be transmitted to a cloud computer or a server through the communication mechanism, various information and data that correspond to the image captured by the camera may be searched for in the cloud computer or the server, various information and data that are obtained by the search may be transmitted to the image display apparatus through the communication mechanism, and an image may be displayed on the image forming apparatus on the basis of the various information and data being obtained by the search. 
     For example, the display apparatus of the present disclosure including the various configurations described above can be used to display, for example, various information in various sites on the Internet; to display various descriptions, a symbol, a sign, a mark, an emblem, a design, and the like that are used at the time of, for example, driving, an operation, a maintenance, and disassembling of an observation target such as various apparatuses; to display various descriptions, a symbol, a sign, a mark, an emblem, a design, and the like regarding an observation target such as a person and a product; to display a moving image and a still image; to display subtitles for, for example, a movie; to display an explanatory text and closed captions regarding a video in synchronization with the video; and to display various descriptions regarding an observation target in a play, Kabuki, Noh, Kyogen, opera, a concert, a ballet, various theaters, an amusement park, a museum, a tourist spot, a resort, tourist information services, and the like, as well as, for example, an explanatory text and closed captions that are used to describe, for example, details, the progress, and the background thereof. With respect to the play, the Kabuki, the Noh, the Kyogen, the opera, the concert, the ballet, the various theaters, the amusement park, the museum, the tourist spot, the resort, the tourist information services, and the like, it is sufficient if a text related to an observation target is displayed in the form of an image on the image forming apparatus at an appropriate timing. Specifically, for example, according to the progress of, for example, a movie, or according to the progress of, for example, a play, an image control signal is transmitted to the image forming apparatus by an operation performed by an operator, or under the control of, for example, a computer, on the basis of a specified schedule and the allotment of time, and an image is displayed on the image forming apparatus. Further, various descriptions regarding an observation target such as various apparatuses, a person, and a product are displayed. An image of the observation target such as various apparatuses, a person, and a product is captured by a camera, and details of the captured image are analyzed by the image forming apparatus. This makes it possible to display, on the image forming apparatus, pre-created various descriptions regarding the observation target such as various apparatuses, a person, and a product. 
     First Embodiment 
     The image display apparatus and the display apparatus of the present disclosure are described below on the basis of embodiments. An outline of each embodiment is given in Table 1 indicated below. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Direction 
                   
                 Image 
                 Direction of movement 
                   
               
               
                   
                 of change 
                 Optical 
                 forming 
                 of image 
                 Embodiment 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Embodiment 
                 in pupil 
                 element 
                 apparatus 
                 Horizontal 
                 Vertical 
                 for reference 
               
               
                   
               
               
                 1st 
                 Horizontal 
                 Moved 
                 Fixed 
                 ◯ 
                   
                   
               
               
                 2nd 
                 Vertical 
                 Moved 
                 Fixed 
                   
                 ◯ 
                   
               
               
                 3rd 
                 Horizontal 
                 Fixed 
                 Entirely 
                   
                   
                   
               
               
                   
                   
                   
                 moved 
                   
                   
                   
               
               
                 4th 
                 Vertical 
                 Fixed 
                 Entirely 
                   
                   
                   
               
               
                   
                   
                   
                 moved 
                   
                   
                   
               
               
                 5th 
                 Horizontal 
                 Moved 
                   
                 ◯ 
                   
                 1st 
               
               
                   
                 Vertical 
                   
                 Entirely 
                   
                   
                 4th 
               
               
                   
                   
                   
                 moved 
                   
                   
                   
               
               
                 6th/7th 
                 Horizontal 
                 Moved 
                   
                 ◯ 
                   
                 1st 
               
               
                   
                 Vertical 
                   
                 Movement 
                   
                 ◯ 
                 5th 
               
               
                   
                   
                   
                 of lens 
                   
                   
                   
               
               
                   
                   
                   
                 system 
                   
                   
                   
               
               
                 8th 
                 Horizontal 
                   
                   
                 ◯ 
                   
                 1st 
               
               
                   
                 Vertical 
                   
                 Rotation of 
                   
                 ◯ 
                 5th 
               
               
                   
                   
                   
                 flat mirror 
               
               
                   
               
            
           
         
       
     
     A first embodiment relates to the image display apparatus according to the first aspect of the present disclosure and the display apparatus of the present disclosure.  FIG. 1  schematically illustrates the display apparatus of the first embodiment, as viewed from the front, and  FIG. 2A  is a conceptual diagram of the display apparatus, as viewed from above an observer. Further,  FIGS. 3A, 3B, 4A, 4B, 5A, 5B, 6A , and  6 B are conceptual diagrams of an optical element and the like, as viewed from above, that are used to describe an operation of the image display apparatus of the first embodiment. 
     The display apparatus of the first embodiment or a third embodiment described later includes 
     a frame  10  that is worn by an observer  80 , and 
     the image display apparatus attached to the frame  10 , and 
     the image display apparatus is an image display apparatus  20  of the first embodiment described below or the third embodiment described later. Further, the display apparatus of the first embodiment or the third embodiment described later includes an image display apparatus  20 R for a right eye and an image display apparatus  20 L for a left eye, and is worn on the head of the observer  80 . The display apparatus of the first embodiment or the third embodiment described later is a head-mounted display (HMD), and a Maxwellian viewing retinal projection HMD in particular. 
     Further, the image display apparatus  20  of the first embodiment or the third embodiment described later includes 
     an image forming apparatus  30 , 
     an optical element  40  that is arranged in front of a face of the observer  80 , and 
     a movement control apparatus, in which 
     when a region  85  situated on the side of the ear of the observer  80 , as viewed from the optical element  40 , is referred to as a region situated behind the optical element, 
     the image forming apparatus  30  is arranged in the region situated behind the optical element, and 
     an image exiting the image forming apparatus  30  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach a pupil  82  of the observer  80 . 
     Further, when there is a change in the position of the pupil  82  of the observer  80  (specifically, when there is a change in the position of the pupil  82  primarily due to a rotational movement of an eyeball  81  of the observer  80 ), the optical element  40  is moved using the movement control apparatus, and the position of an image exiting the image forming apparatus  30  is controlled using the movement control apparatus. 
     In the image display apparatus  20  of the first embodiment or the third embodiment described later, the frame  10  includes a front portion  11  that is arranged in front of the observer  80 , two temple portions  12  that are respectively rotatably attached to two ends of the front portion  11  through respective hinges (not illustrated), and a nose pad (not illustrated). A temple-tip covering portion (not illustrated) is attached to a tip of each temple portion  12 . An assembly of the frame  10  and the nose pad has a structure substantially the same as ordinary glasses. Note that the front portion  11  and the two temple portions  12  may be integrated. 
     In the image display apparatus  20  of the first embodiment or the third embodiment described later, the optical element  40  includes a reflective hologram diffraction grating. The reflective hologram diffraction grating includes a light-collecting function. In other words, the reflective hologram diffraction grating includes a function as a concave mirror. The image display apparatus  20  can be a semi-transmissive (see-through) image display apparatus by using a hologram diffraction grating as the optical element  40 , and this makes it possible to see outside through the optical element  40 . The optical element  40  is attached to a base material  41  made of a plastic material or glass (specifically, bonded to the base material  41 ). Further, a pencil of light exiting the image forming apparatus  30  enters the optical element  40  in a telecentric state. Furthermore, the image forming apparatus  30  includes a light-collecting member  34  that has a positive optical power and through which an image exiting the image forming apparatus  30  passes. 
     The movement control apparatus includes a drive mechanism, and further includes a position control circuit (not illustrated) that controls the position of an image exiting the image forming apparatus. 
     In other words, in a movement control apparatus  50 A of the first embodiment, the drive mechanism specifically includes a first drive apparatus  51  and a first slide bar  52 . Further, the position of an image exiting the image forming apparatus is controlled using the movement control apparatus  50 A. Specifically, the position of an image exiting the image forming apparatus  30  is moved on the basis of a control signal from the position control circuit. 
     Specifically, it is sufficient if the horizontally extending first slide bar  52  is slidably attached to the first drive apparatus  51 , the first drive apparatus  51  is fixed to an upper portion or a lower portion of the front portion  11  (to the upper portion in the illustrated example), and the optical element  40  is fixed to the first slide bar  52  through an attachment member  53 . Here, the optical element  40  is bonded to the base body  41 , and it is sufficient if the base body  41  is fixed to the attachment member  53 . Then, the first drive apparatus  51  is driven to slide the first slide bar  52  with respect to the first drive apparatus  51 , and this makes it possible to horizontally move the optical element  40  fixed to the first slide bar  52 . Examples of a combination of the first drive apparatus  51  and the first slide bar  52 , or a combination of a drive apparatus and a slide bar described below include a combination of a motor and a rack-and-pinion mechanism, and a combination of a motor and a ball-screw mechanism. Further, the drive mechanism may include a linear actuator. 
     The image forming apparatus  30  of the first embodiment or the third embodiment described later is the image forming apparatus having the second configuration, and the image forming apparatus having the second configuration includes, but not limited to, a light source  31 R, a light source  31 G, a light source  31 B, and a scanning mechanism  32  that scans pieces of light respectively emitted by the light source  31 R, the light source  31 G, and the light source  31 B to form an image. 
     Examples of the light source  31 R,  31 G,  31 B included in the image forming apparatus  30  include a light-emitting element. Specifically, a red-light-emitting element  31 R, a green-light-emitting element  31 G, and a blue-light-emitting element  31 B may be used as the light-emitting elements. Examples of the light-emitting element include a semiconductor laser element, a solid-state laser, and an LED. For example, the scanning mechanism  32  is a MEMS mirror that includes a two-dimensionally rotatable micromirror, and horizontally scans and vertically scans light emitted by the light sources  31 R,  31 G, and  31 B. In the image forming apparatus  30 , the MEMS mirror is arranged at a front focal position of the light-collecting member  34  (a focal point on the side of the image forming apparatus). The light source  31 R, the light source  31 G, the light source  31 B, and the scanning mechanism  32  are accommodated in a housing  33 , and the housing  33  is attached to, for example, the temple portion  12 . 
     A pupil position detecting mechanism  90  used to detect the position of the pupil  82  of the observer  80  is attached to the lower portion of the front portion  11 . The pupil position detecting mechanism  90  includes, for example, a light-emitting section that emits infrared light, and a light-receiving section or an imaging device that receives the infrared light reflected off the pupil  82  of the observer  80 . Further, the pupil position detecting mechanism  90  may include an imaging device that images the pupil  82  of the observer  80 . 
     In the image forming apparatus  30  of the first embodiment or the third embodiment described later, light is formed into parallel light by the light-collecting member  34  (an optical system that forms exiting light into parallel light), and the parallel light enters the optical element  40 . Such a formation of light into parallel light enables an image to enter the optical element  40  in a telecentric state. Examples of the light-collecting member  34  used to generate parallel light include an optical system that has a positive optical power as a whole and in which a convex lens, a concave lens, a freeform prism, or a hologram lens is used alone, or a combination thereof is used. Note that the figures illustrate the light-collecting member  34  in the form of a single lens. However, the light-collecting member  34  may include a plurality of lenses or a combination of a lens and a prism. 
     In the first embodiment, the optical element  40  is moved using the movement control apparatus  50 A when there is a change in the position of the pupil  82  of the observer  80 , and it is assumed that the optical element  40  moves in the XY plane. Further, θ 0  described above (refer to  FIG. 2B ) may exhibit a value indicated below, although the value is not limited thereto. 
       −5 (degrees)≤θ 0 ≤5 (degrees), and
 
     specifically, θ 0 =0 degrees. In this case, the x axis and the X axis described above coincide with each other, the y axis and the Y axis described above coincide with each other, and the z axis and the Z axis described above coincide with each other. An image exiting the image forming apparatus  30  obliquely enters the optical element  40  from a region situated behind the optical element, and the angle θ 1  (refer to  FIG. 3 ) formed by a pencil of light that enters the optical element  40  and a pencil of light that exits the optical element  40  may exhibit a value indicated below, although the value is not limited thereto. 
       45 (degrees)≤θ 1 ≤80 (degrees), and
 
     specifically, θ 0 =55 degrees. 
     Further, in the image display apparatus  20  of the first embodiment, the optical element  40  is moved, using the movement control apparatus  50 A, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ). In other words, the optical element  40  is horizontally moved using the movement control apparatus  50 A. 
     The movements of the optical element and the like are described below with reference to  FIGS. 3A, 3B, 4A, 4B, 5A, 5B, 6A, and 6B . Note that, in  FIGS. 3A, 3B, 4B, 5A, 5B, 6B, 8A, 9B, 12B, 14B, and 15B , a largest region for an image that can be output by the image forming apparatus is indicated by a dot-dash line, a region for an image that is output by the image forming apparatus is indicated by a dotted line, and an image-center pencil of light is indicated by a solid line LC, LC′ or LC″. 
     As illustrated in  FIG. 3A , light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach a center CP of the pupil  82  of the observer  80 . Note that the light (the image) exiting the scanning mechanism  32  is indicated by a solid line and a dotted line. The pencil of light LC indicated by the solid line represents a pencil of light exiting a center of an image formed by the image forming apparatus, and the pencil of light LC is referred to as an image-center pencil of light, as described above. Further, a center point of the optical element  40  is represented by “O”. An image-center pencil of light LC enters the center point O of the optical element  40 . Here, the center point of the optical element  40  is an intersection of an optical axis of the optical element  40  and the optical element  40 . 
     Next, it is assumed that a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ) is caused, as illustrated in  FIG. 3B . In the illustrated example, the change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction is a rotation of the eyeball  81  of the observer  80  in a direction of a nose of the observer  80 . Consequently, there is a deviation of the light convergence from the center CP of the pupil. Note that, in the figures, the nose of the observer  80  is denoted by reference numeral  83 . 
     Thus, as illustrated in  FIG. 4A , the optical element  40  is horizontally moved using the movement control apparatus  50 A (in an X-axis direction and in a direction toward the nose of the observer  80  in the illustrated example). In this state, light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  is reflected off the optical element  40 , but the image-center pencil of light LC does not reach the center CP of the pupil  82  of the observer  80 . In order to cause the image-center pencil of light to reach the center CP of the pupil  82  of the observer  80 , there is a need to move the image-center pencil of light LC to the position of an image-center pencil of light LC′. 
     Thus, the position of an image exiting the image forming apparatus  30  is controlled using the movement control apparatus  50 A. Specifically, the position of an image exiting the image forming apparatus  30  is moved on the basis of a control signal from the position control circuit included in the movement control apparatus  50 A. In other words, an image formed by the image forming apparatus is moved on the basis of a control signal from the position control circuit included in the movement control apparatus  50 A such that the image-center pencil of light LC′ enters the center point O of the optical element  40 , as illustrated in  FIG. 4B . Accordingly, the image-center pencil of light LC coincides with the image-center pencil of light LC′. More specifically, the scanning mechanism  32  is designed such that the scanning mechanism  32  can scan a region larger in size than an image to be displayed. As described above, a largest region for an image that can be output by the image forming apparatus is indicated by a dot-dash line. Further, a region for an image that is output by the image forming apparatus is indicated by a dotted line. Then, it is sufficient if a portion of a region that can be scanned by the scanning mechanism  32  is used to form an image. In other words, it is sufficient if a region of an image exiting the scanning mechanism  32  is moved such that the image-center pencil of light LC′ enters the center point O of the optical element  40 . Accordingly, light (an image) that is formed into parallel light by the light-collecting member  34  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 . 
     In the examples illustrated in  FIGS. 5A, 5B, 6A, and 6B , the change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction is a rotation of the eyeball  81  of the observer in a direction of an ear of the observer. Except for this point, descriptions related to FIGS.  5 A,  5 B,  6 A, and  6 B are similar to the descriptions related to  FIGS. 3A, 3B, 4A, and 4B . Thus, detailed descriptions thereof are omitted. 
     The image display apparatus of the first embodiment is different from conventional technologies in that the image forming apparatus is arranged in a region situated behind the optical element. An image exiting the image forming apparatus obliquely enters the optical element from the region situated behind the optical element, and is reflected off the optical element to reach the pupil of an observer. Further, when there is a change in the position of the pupil of the observer, the optical element is moved using the movement control apparatus, and the position of the image exiting the image forming apparatus is controlled using the movement control apparatus. This makes it possible to make the image display apparatus smaller and lighter. Since the image display apparatus can be made smaller and lighter, the image display apparatus can follow a quick movement of the pupil of an observer, energy (power consumption) necessary to move the optical element can be reduced, and the image display apparatus can be designed with a high degree of freedom. 
     Second Embodiment 
     A second embodiment is a modification of the first embodiment.  FIG. 7  schematically illustrates the display apparatus of the second embodiment, as viewed from the front, and  FIGS. 8A, 8B, 9A, and 9B  are conceptual diagrams of the optical element and the like, as viewed from the lateral direction, that are used to describe an operation of the image display apparatus of the second embodiment. 
     In the image display apparatus of the second embodiment, the optical element  40  is moved, using a movement control apparatus  50 B, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ). In other words, the optical element  40  is vertically moved using the movement control apparatus  50 B. 
     As in the case of the first embodiment, the movement control apparatus  50 B includes a drive mechanism, and further includes a position control circuit (not illustrated) that controls the position of an image exiting the image forming apparatus. Specifically, the drive mechanism includes a second drive apparatus  54  and a second slide bar  55 . Further, the position of an image exiting the image forming apparatus is controlled using the movement control apparatus  50 B. Specifically, the position of an image exiting the image forming apparatus  30  is moved on the basis of a control signal from the position control circuit. 
     More specifically, it is sufficient if the vertically extending second slide bar  55  is slidably attached to the second drive apparatus  54 , the second drive apparatus  54  is fixed to a portion on an ear side of the front portion  11  (specifically, to the temple portion  12 ), and the optical element  40  (specifically, the base body  41 ) is fixed to the second slide bar  55  through an attachment member  56 . Then, the second drive apparatus  54  is driven to slide the second slide bar  55  with respect to the second drive apparatus  54 , and this makes it possible to vertically move the optical element  40  fixed to the second slide bar  55 . 
     The movements of the optical element and the like are described below with reference to  FIGS. 8A, 8B, 9A, and 9B . 
     As illustrated in  FIG. 8A , light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 . The image-center pencil of light LC enters the center point O of the optical element  40 . 
     Next, it is assumed that a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ) is caused, as illustrated in  FIG. 8B . In the illustrated example, the change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction is a rotation of the eyeball  81  of the observer  80  in a direction of the top of the head of the observer  80 . Consequently, there is a deviation of the light convergence from the center CP of the pupil. 
     Thus, as illustrated in  FIG. 9A , the optical element  40  is vertically moved using the movement control apparatus  50 B (in a Y-axis direction and in a direction toward the top of the head of the observer  80  in the illustrated example). In this state, light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  is reflected off the optical element  40 , but the image-center pencil of light LC does not reach the center CP of the pupil  82  of the observer  80 . In order to cause the image-center pencil of light to reach the center CP of the pupil  82  of the observer  80 , there is a need to move the image-center pencil of light LC to the position of the image-center pencil of light LC′. 
     Thus, the position of an image exiting the image forming apparatus  30  is controlled using the movement control apparatus  50 B. Specifically, the position of an image exiting the image forming apparatus  30  is moved on the basis of a control signal from the position control circuit included in the movement control apparatus  50 B. In other words, an image formed by the image forming apparatus is moved on the basis of a control signal from the position control circuit included in the movement control apparatus  50 B such that the image-center pencil of light LC′ enters the center point O of the optical element  40 , as illustrated in  FIG. 9B . Accordingly, the image-center pencil of light LC coincides with the image-center pencil of light LC′. More specifically, as in the case the descriptions in the first embodiment, the scanning mechanism  32  is designed such that the scanning mechanism  32  can scan a region larger in size than an image to be displayed. Then, it is sufficient if a portion of a region that can be scanned by the scanning mechanism  32  is used to form an image. In other words, it is sufficient if a region of an image exiting the scanning mechanism  32  is moved such that the image-center pencil of light LC′ enters the center point O of the optical element  40 . Accordingly, light (an image) that is formed into parallel light by the light-collecting member  34  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 . 
     Except that there is a difference in movement direction, the same applies to the case in which the position of the pupil  82  of the observer  80  is changed downward in parallel with the vertical direction. 
       FIG. 10  schematically illustrates a modification of the display apparatus of the second embodiment, as viewed from the front. In this modification, the optical element  40  is moved, using the movement control apparatuses  50 A and  50 B, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction that is caused due to the rotation of the eyeball  81  of the observer  80 ). In other words, the optical element  40  is horizontally and vertically moved using the movement control apparatuses  50 A and  50 B. With respect to the display apparatus illustrated in  FIG. 10 , it is sufficient if the movement control apparatus  50 A described in the first embodiment and the movement control apparatus  50 B described in the second embodiment are used in combination. The first drive apparatus  51  in the movement control apparatus  50 A described in the first embodiment is fixed to a first front portion  11 A. The first front portion  11 A is surrounded by a second front portion  11 B, the second drive apparatus  54  in the movement control apparatus  50 B described in the second embodiment is fixed to the first front portion  11 B, and the attachment member  56  is fixed to the second front portion  11 B. 
     Third Embodiment 
     A third embodiment relates to the image display apparatus according to the second aspect of the present disclosure, and the display apparatus of the present disclosure including the image display apparatus according to the second aspect of the present disclosure.  FIG. 11  schematically illustrates the display apparatus of the third embodiment, as viewed from the front, and  FIGS. 12A and 12B  are conceptual diagrams of the optical element and the like, as viewed from above, that are used to describe an operation of the image display apparatus of the third embodiment. 
     In the image display apparatus  20  of the third embodiment, when there is a change in the position of the pupil  82  of the observer  80 , the image forming apparatus  30  is moved, using a movement control apparatus  50 C, in a direction corresponding to the change (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ). Further, the display apparatus of the third embodiment includes the image display apparatus  20  of the third embodiment. 
     Specifically, the entirety of the image display apparatus  30  is moved, using the movement control apparatus  50 C, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction (that is, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ). In other words, the entirety of the image display apparatus  30  is horizontally moved using the movement control apparatus  50 C. 
     The movement control apparatus  50 C includes a drive mechanism. The drive mechanism includes a first drive apparatus  61  and a first slide bar  62 . Specifically, it is sufficient if the horizontally extending first slide bar  62  is slidably attached to the first drive apparatus  61 , the first drive apparatus  61  is fixed to a portion on the ear side of the front portion  11  (or to the temple portion  12 ), and the image display apparatus  30  is fixed to the first slide bar  62  through an attachment member  63 . Then, the first drive apparatus  61  is driven to slide the first slide bar  62  with respect to the first drive apparatus  61 , and this makes it possible to horizontally move the image display apparatus  30  fixed to the first slide bar  62 . 
     The base body  41  to which the optical element  40  is bonded is fitted into the front portion  11 . 
     The movements of the image forming apparatus and the like are described below with reference to  FIGS. 3A, 3B, 12A , and  12 B. 
     As illustrated in  FIG. 3A , light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 . The image-center pencil of light LC enters the center point O of the optical element  40 . 
     Next, it is assumed that a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ) is caused, as illustrated in  FIG. 3B . In the illustrated example, the change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction is a rotation of the eyeball  81  of the observer  80  in a direction of the nose of the observer  80 . Consequently, there is a deviation of the light convergence from the center CP of the pupil. 
     Thus, as illustrated in  FIG. 12A , the image display apparatus  30  is moved, using the movement control apparatus  50 C, in a direction corresponding to the horizontal direction (in the X-axis direction and in the direction toward the nose of the observer  80  in the illustrated example). It is sufficient if the image-center pencil of light LC is moved to the position of the image-center pencil of light LC′, in order to cause the image-center pencil of light to reach the center CP of the pupil  82  of the observer  80 . Light (an image) that is formed into parallel light by the light-collecting member  34  is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 .  FIG. 12B  illustrates this state. Note that the movement of the image forming apparatus  30  is a movement of the entirety of the image forming apparatus  30 , that is, a movement of the light sources  31 R,  31 G, and  31 B, the scanning mechanism  32 , the housing  33 , and the light-collecting member  34 . As described above, the position of an image exiting the image forming apparatus  30  is controlled using the movement control apparatus  50 C. However, there is no need to move the position of an image exiting the image forming apparatus  30  on the basis of a control signal since the entirety of the image forming apparatus  30  is moved, which is different from the first and second embodiments. 
     Except that there is a difference in movement direction, the same applies to the case in which the position of the pupil  82  of the observer  80  is changed in a direction toward the ear of the observer  80 . 
     The image display apparatus of the third embodiment is different from conventional technologies in that the image forming apparatus is arranged in a region situated behind the optical element. An image exiting the image forming apparatus obliquely enters the optical element from the region situated behind the optical element, and is reflected off the optical element to reach the pupil of an observer. Further, when there is a change in the position of the pupil of the observer, the image forming apparatus is moved using the movement control apparatus. This makes it possible to make the image display apparatus smaller and lighter. Since the image display apparatus can be made smaller and lighter, the image display apparatus can follow a quick movement of the pupil of an observer, energy (power consumption) necessary to move the optical element can be reduced, and the image display apparatus can be designed with a high degree of freedom. Further, the movement control apparatus is less noticeable when the image display apparatus is viewed from the front, since the image forming apparatus is arranged in the region situated behind the optical element. 
     Fourth Embodiment 
     A fourth embodiment is a modification of the third embodiment.  FIG. 13  schematically illustrates the display apparatus of the fourth embodiment, as viewed from the front, and  FIGS. 14A and 14B  are conceptual diagrams of the optical element and the like, as viewed from the lateral direction, that are used to describe an operation of the image display apparatus of the fourth embodiment. 
     In the image display apparatus  20  of the fourth embodiment, the entirety of the image forming apparatus  30  is moved, using a movement control apparatus  50 D, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction (that is, a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ). In other words, the entirety of the image forming apparatus  30  is vertically moved using the movement control apparatus  50 D. 
     The movement control apparatus  50 D includes a drive mechanism. The drive mechanism includes a second drive apparatus  64  and a second slide bar  65 . Specifically, it is sufficient if the vertically extending second slide bar  65  is slidably attached to the second drive apparatus  64 , the second drive apparatus  64  is fixed to a portion on the ear side of the front portion  11  (or to the temple portion  12 ), and the image display apparatus  30  is fixed to the second slide bar  65  through an attachment member  66 . Then, the second drive apparatus  64  is driven to slide the second slide bar  65  with respect to the second drive apparatus  64 , and this makes it possible to vertically move the image display apparatus  30  fixed to the second slide bar  65 . 
     The movements of the image forming apparatus and the like are described below with reference to  FIGS. 8A, 8B, 14A , and  14 B. 
     As illustrated in  FIG. 8A , light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 . The image-center pencil of light LC enters the center point O of the optical element  40 . 
     Next, it is assumed that a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ) is caused, as illustrated in  FIG. 8B . In the illustrated example, the change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction is a rotation of the eyeball  81  of the observer  80  in a direction of the top of the head of the observer  80 . Consequently, there is a deviation of the light convergence from the center CP of the pupil. 
     Thus, as illustrated in  FIG. 14A , the image display apparatus  30  is moved, using the movement control apparatus  50 D, in a direction corresponding to the vertical direction (in the Y-axis direction and in the direction toward the top of the head of the observer  80  in the illustrated example). It is sufficient if the image-center pencil of light LC is moved to the position of the image-center pencil of light LC′, in order to cause the image-center pencil of light to reach the center CP of the pupil  82  of the observer  80 . Light (an image) that is formed into parallel light by the light-collecting member  34  is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 .  FIG. 14B  illustrates this state. Note that, as in the case of the third embodiment, the movement of the image forming apparatus  30  is a movement of the entirety of the image forming apparatus  30 , that is, a movement of the light sources  31 R,  31 G, and  31 B, the scanning mechanism  32 , the housing  33 , and the light-collecting member  34 . As described above, the position of an image exiting the image forming apparatus  30  is controlled using the movement control apparatus  50 D. However, as in the case of the third embodiment, there is no need to move the position of an image exiting the image forming apparatus  30  on the basis of a control signal since the entirety of the image forming apparatus  30  is moved. 
     In a modification of the image display apparatus  20  of the fourth embodiment, the image forming apparatus  30  is moved, using the movement control apparatuses, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction that is caused due to the rotation of the eyeball  81  of the observer  80 ). In other words, the image forming apparatus  30  is horizontally and vertically moved using the movement control apparatuses. Specifically, it is sufficient if the movement control apparatus  50 C and the movement control apparatus  50 D are used in combination. 
     Fifth Embodiment 
     A fifth embodiment is a combination of the first and fourth embodiments. In the image display apparatus of the fifth embodiment, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction that is caused due to the rotation of the eyeball  81  of the observer  80 ), the optical element  40  is moved using the movement control apparatus  50 A and the image forming apparatus  30  is moved using the movement control apparatus  50 D. In other words, the optical element  40  is horizontally moved using the movement control apparatus  50 A, and the image forming apparatus  30  is vertically moved using the movement control apparatus  50 D. Except for the points described above, the configurations and the structures of the image display apparatus and the display apparatus of the fifth embodiment are similar to the configurations and the structures of the image display apparatus and the display apparatus of the first embodiment, and to the configurations and the structures of the image display apparatus and the display apparatus of the fourth embodiment. Thus, detailed descriptions thereof are omitted. 
     Sixth Embodiment 
     A sixth embodiment is a modification of the fifth embodiment.  FIGS. 15A and 15B  are conceptual diagrams of the optical element and the like, as viewed from the lateral direction, that are used to describe an operation of the image display apparatus of the sixth embodiment. 
     As illustrated in  FIG. 14B , in the image display apparatus of the fourth embodiment, light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 . However, the image-center pencil of light LC enters a point shifted from the center point O of the optical element  40 . This may result in an increased aberration. 
     The movements of the image forming apparatus and the like are described below with reference to  FIGS. 8A, 8B, 15A , and  15 B. 
     As illustrated in  FIG. 8A , light (an image) that exits the scanning mechanism  32  and is formed into parallel light by the light-collecting member  34  obliquely enters the optical element  40  from the region situated behind the optical element, and is reflected off the optical element  40  to reach the center CP of the pupil  82  of the observer  80 . The image-center pencil of light LC enters the center point O of the optical element  40 . 
     Next, it is assumed that a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction that is caused primarily due to the rotation of the eyeball  81  of the observer  80 ) is caused, as illustrated in  FIGS. 8B and 15A . In the illustrated examples, the change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction is a rotation of the eyeball  81  of the observer  80  in a direction of the top of the head of the observer  80  (in an upward direction). Consequently, there is a deviation of the light convergence from the center CP of the pupil. The image-center pencil of light in this state is indicated by the solid line LC, where the image-center pencil of light LC enters the center point O of the optical element  40 . 
     Thus, as illustrated in  FIGS. 15A and 15B , the image-center pencil of light LC (refer to  FIG. 15A ) is rotated centered at the center point O of the optical element  40  (refer to the image-center pencil of light LC′ in  FIG. 15A ), and is moved (refer to an image-center pencil of light LC″ in  FIG. 15B ). Consequently, the image-center pencil of light LC″ obtained by the rotation and movement enters the center point O of the optical element  40 , and also reaches the center CP of the pupil. When the image-center pencil of light LC enters the light-collecting member  34  along an optical axis of the light-collecting member  34  (the optical axis of the light-collecting member  34  in this case is referred to as an “optical axis η” for convenience), it is sufficient if the light-collecting member  34  is moved using the movement control apparatus in a direction orthogonal to the optical axis η, in order to achieve the state described above. The optical axis of the light-collecting member  34  after the movement of the light-collecting member  34  is referred to as an “optical axis η” for convenience. Accordingly, the image-center pencil of light LC′ that is parallel to the optical axis η′ enters the light-collecting member  34 , but the image-center pencil of light LC′ enters a point shifted from the center point O of the optical element  40 , as illustrated in  FIG. 15A . Thus, an image formed by the image forming apparatus is moved on the basis of a control signal from the position control circuit included in the movement control apparatus, such that the image-center pencil of light LC′ enters the center point O of the optical element  40 . Accordingly, the image-center pencil of light LC″ enters the center point O of the optical element  40 , as illustrated in  FIG. 15B . 
     As in the case of the first embodiment, the movement control apparatus includes a drive mechanism, and a position control circuit that controls the position of an image exiting the image forming apparatus. Specifically, the drive mechanism includes the first drive apparatus  51  and the first slide bar  52 . Further, the position of an image exiting the image forming apparatus is controlled using the movement control apparatus. Specifically, as in the case of the first embodiment, the position of an image exiting the image forming apparatus  30  is moved in a direction corresponding to the X-axis direction, on the basis of a control signal from the position control circuit. 
     As in the case of the fourth embodiment, the drive mechanism further includes the second drive apparatus  64  and the second slide bar  65 . The vertically extending second slide bar  65  is slidably attached to the second drive apparatus  64 , the second drive apparatus  64  is fixed to a portion on the ear side of the front portion  11  (or to the temple portion  12 ), and the light-collecting member  34  is fixed to the second slide bar  65  through the attachment member  66 . The light sources  31 R,  31 G, and  31 B, and the scanning mechanism  32  are fixed, that is, immobile, which is different from the fourth embodiment. Then, the second drive apparatus  64  is driven to slide the second slide bar  65  with respect to the second drive apparatus  64 , and this makes it possible to move the light-collecting member  34  fixed to the second slide bar  65  in a direction orthogonal to the optical axis of the light-collecting member  34  (that is, a direction corresponding to the Y-axis direction). Further, the position of an image exiting the image forming apparatus  30  is moved in a direction corresponding to the Y-axis direction, on the basis of a control signal from the position control circuit. 
     When the optical element  40  is moved, using the movement control apparatuses  50 A and  50 B, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction (that is, when the optical element  40  is horizontally and vertically moved using the movement control apparatuses  50 A and  50 B), as in the case of the modification of the display apparatus of the second embodiment illustrated in  FIG. 10 , this may result in there being room for improvement in the design of the display apparatus when the display apparatus is viewed from the front. Further, when the image display apparatus  30  is moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction (that is, when the image display apparatus  30  is horizontally moved using the movement control apparatus), as in the case of the third embodiment, this may result in an increased aberration due to the horizontal movement of the image display apparatus  30 . 
     In the fifth embodiment or the sixth embodiment, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction, the optical element  40  is moved and the image forming apparatus  30  or a portion of the image forming apparatus  30  is moved using the respective movement control apparatuses. In other words, using the respective movement control apparatuses, the optical element  40  is horizontally moved and the image forming apparatus  30  or a portion of the image forming apparatus  30  is vertically moved. When the display apparatus is viewed from the front, the movement control apparatus  50 A for the optical element  40  is noticeable, whereas the movement control apparatus for the image forming apparatus  30  is not noticeable. Thus, the display apparatus has a more excellent design. 
     In general, an aberration generated in an eccentric optical system due to the movements of an optical component and an optical apparatus is larger in an eccentric direction. In other words, an aberration generated due to the movement of the optical element  40  in an XZ plane (a horizontal movement of the optical element  40 ) is larger than an aberration generated due to the movement of the optical element  40  in a YZ plane. An aberration generated due to the horizontal movement of the image forming apparatus  30  is larger than an aberration generated due to the vertical movement of the image forming apparatus  30 . Further, a distance from the pupil  82  of the observer  80  to the image forming apparatus  30  is longer than a distance from the pupil  82  of the observer  80  to the optical element  40 . Thus, when an impact of an aberration generated due to the movement of the optical element  40  in the XZ plane (the horizontal movement of the optical element  40 ) is compared with an impact of an aberration generated due to a horizontal movement of the image forming apparatus  30 , the impact of an aberration is larger in the latter case. Thus, when the optical element  40  is moved in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction, and when a portion of the image forming apparatus  30  is moved in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction, this makes it possible to more certainly suppress impacts of aberrations due to these movements. 
     Further, it is sufficient if a portion of the image forming apparatus  30  is moved in the sixth embodiment or a seventh embodiment described later. This makes it possible to reduce the amount of movement, to follow the movement of a pupil more quickly, and to reduce energy consumption (power consumption). 
     Seventh Embodiment 
     A seventh embodiment is a modification of the sixth embodiment.  FIGS. 16A to 16C  are principle diagrams used to describe a light-collecting member included in the image forming apparatus of the seventh embodiment. In the image display apparatus of the seventh embodiment, according to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction and the vertical direction that is caused due to the rotation of the eyeball  81  of the observer  80 ), the optical element  40  is horizontally moved using the movement control apparatus  50 A and an image is vertically moved using a 4f optical system described below. 
     Specifically, in the image display apparatus  20  of the seventh embodiment, the image forming apparatus  30  includes a 4f optical system through which an image exiting the image forming apparatus  30  passes, and the 4f optical system is moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction. More specifically, the 4f optical system includes a first lens  34 A, a second lens  34 B, and a third lens  34 C, and the scanning mechanism  32  is situated at a focal position of the first lens  34 A on the side of the scanning mechanism. Further, a focal position of the second lens  34 B on the side of the optical element, and a focal position of the third lens  34 C on the side of the scanning mechanism coincide. Note that, in the figure, the optical axis of a lens is indicated by a dot-dash line. Then, the first lens  34 A and the second lens  34 B are moved in a direction orthogonal to their optical axes. The light sources  31 R,  31 G, and  31 B, the scanning mechanism  32 , and the third lens  34 C are fixed and immobile. The amount of movement of the lenses  34 A and  34 B can be half the amount of movement of the light-collecting member  34  in the sixth embodiment by using the 4f optical system having such a configuration. 
     Eighth Embodiment 
     An eighth embodiment is another modification of the sixth embodiment.  FIG. 17  is a conceptual diagram of the image display apparatus included in the display apparatus of the eighth embodiment. In the image display apparatus of the eighth embodiment, according to a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction that is caused due to the rotation of the eyeball  81  of the observer  80 ), an image entering the optical element  40  is vertically moved using a reflecting mirror  91 . In other words, in the eighth embodiment, the movement control apparatus includes the reflecting mirror  91  off which an image exiting the image forming apparatus  30  is reflected, and the reflecting mirror  91  is rotated about a rotational axis  92  using the movement control apparatus to change the light reflection angle of the reflecting mirror  91 . This results in being able to cause an image exiting the image forming apparatus  30  to reach the pupil  82  of the observer  80  with certainty. The reflecting mirror  91  is arranged between the light-collecting member  34  and the optical element  40 . Note that it is sufficient if the optical element  40  is horizontally moved on the basis of the first embodiment, according to a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction (specifically, a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction that is caused due to the rotation of the eyeball  81  of the observer  80 ). 
     Ninth Embodiment 
     A ninth embodiment is a modification of the first to eighth embodiments.  FIG. 18  is a conceptual diagram of the display apparatus of the ninth embodiment, as viewed from above the observer, and the image forming apparatus  30  includes a dispersion compensation element  37  in the ninth embodiment, as illustrated in  FIG. 18 . Here, the dispersion compensating element  37  is arranged between the light sources  31 R,  31 G, and  31 B, and the scanning mechanism  32 . The dispersion compensating element  37  is a type of correction lens, and is an element that corrects for a chromatic aberration that occurs in the optical element  40 . Specifically, the adjustment of the beam shape (the beam diameter, the beam divergence angle) of light exiting the dispersion compensating element  37  makes it possible to correct for a chromatic aberration. 
     The present disclosure has been described above on the basis of the favorable embodiments. However, the present disclosure is not limited to these embodiments. The configurations and the structures of the display apparatus (the head-mounted display), the image display apparatus, and the image forming apparatus described in the embodiments are merely illustrative, and modifications may be made thereto as appropriate. 
     The optical element  40  may include a concave mirror, instead of a reflective hologram diffraction grating. In this case, a light reflective film off which light of a specific wavelength is reflected is formed on a light reflective surface of a transparent member (a base portion) that is included in the concave mirror, or in a flat reflecting mirror or a Fresnel reflecting mirror that will be described later. This makes it possible to see outside through the optical element. Further, the optical element may include a lens that has a positive optical power, and the flat reflecting mirror, the lens being a lens through which an image exiting the image forming apparatus  30  passes. Further, the optical element may include the Fresnel reflecting mirror. Further, the optical element may be rotated, using the movement control apparatus, according to a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction. Note that it is sufficient if the optical element  40  is horizontally moved using the movement control apparatus when there is a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction. Further, the angle of arrangement of the optical element  40  may be changed, using the movement control apparatus, according to a change in the position of the pupil  82  of the observer  80  in parallel with the vertical direction. Note that it is sufficient if the optical element  40  is horizontally moved using the movement control apparatus when there is a change in the position of the pupil  82  of the observer  80  in parallel with the horizontal direction. 
       FIG. 19  is a conceptual diagram of a modification of the display apparatus of the first embodiment, as viewed from above the observer, and θ 0  may be unequal to 0 degrees, as illustrated in  FIG. 19 . 
     The image forming apparatus  30  may be the image forming apparatus having the first configuration. This image forming apparatus  30  in a conceptual diagram of  FIG. 20A  includes a plurality of pixels arranged in a two-dimensional matrix. Specifically, the entirety of the image forming apparatus  30  is accommodated in the housing  33  (indicated by a dot-dash line in  FIG. 20A ). The housing  33  includes an opening (not illustrated), and light exits an optical system (a parallel-light output optical system, or a collimating optical system)  35 D through the opening. A reflective spatial light modulating apparatus includes a liquid crystal display apparatus (LCD)  35 C of LCOS that serves as a light bulb. The reflective spatial light modulating apparatus further includes a polarization beam splitter  35 B in which a portion of light from a light source  35 A is reflected off the polarization beam splitter  35 B to be guided to the liquid crystal display apparatus  35 C, and a portion of light reflected off the liquid crystal display apparatus  35 C passes through the polarization beam splitter  35 B to be guided to the optical system  35 D. The liquid crystal display apparatus  35 C includes a plurality of (for example, 640×480) pixels (liquid crystal cells or liquid crystal display elements) arranged in a two-dimensional matrix. The polarization beam splitter  35 B has a well-known configuration and structure. Unpolarized light emitted by the light source  35 A impinges on the polarization beam splitter  35 B. A p-polarization component passes through the polarization beam splitter  35 B, and exits the system. On the other hand, an s-polarization component is reflected off the polarization beam splitter  35 B, and enters the liquid crystal display apparatus  35 C. Further, the s-polarization component is internally reflected off the liquid crystal display apparatus  35 C, and exits the liquid crystal display apparatus  35 C. Here, from among light exiting the liquid crystal display apparatus  35 C, light exiting a pixel used to display “white” includes a large amount of p-polarization component, and light exiting a pixel used to display “black” includes a large amount of s-polarization component. Thus, from among light that exits the liquid crystal display apparatus  35 C and impinges on the polarization beam splitter  35 B, the p-polarization component passes through the polarization beam splitter  35 B to be guided to the optical system  35 D. On the other hand, the s-polarization component is reflected off the polarization beam splitter  35 B to be returned to the light source  35 A. The optical system  35 D includes, for example, a convex lens, and the image forming apparatus  30  (more specifically, the liquid crystal display apparatus  35 C) is arranged at a point (a position) corresponding to a focal length of the optical system  35 D, in order to generate parallel light. An image exiting the image forming apparatus  30  reaches the pupil  82  of the observer  80  through the optical element  40 . 
     Further,  FIG. 20B  illustrates a conceptual diagram of the image forming apparatus  30  of the image forming apparatus having the first configuration, and the image forming apparatus  30  includes an organic EL display apparatus  36 A. An image exiting the organic EL display apparatus  36 A passes through a first convex lens  36 B that is included in a lens system. The image further passes through a second convex lens  36 D that is included in the lens system to become parallel light, and is headed for the optical element  40 . A front focal position of the second convex lens  36 D coincides with a back focal position of the first convex lens  36 B. Further, a diaphragm  36 C is arranged at the back focal position of the first convex lens  36 B (the front focal position of the second convex lens  36 D). The entirety of the image forming apparatus  30  is accommodated in the housing  33 . The organic EL display apparatus  36 A includes a plurality of (for example, 640×480) pixels (organic EL elements) arranged in a two-dimensional matrix. 
     Note that the present disclosure may also take the following configurations. 
     [A01]&lt;&lt;Image Display Apparatus: First Aspect&gt;&gt; 
     An image display apparatus, including: 
     an image forming apparatus; 
     an optical element that is arranged in front of a face of an observer; and 
     a movement control apparatus, in which 
     when a region situated on a side of an ear of the observer, as viewed from the optical element, is referred to as a region situated behind the optical element, the image forming apparatus is arranged in the region situated behind the optical element, 
     an image exiting the image forming apparatus obliquely enters the optical element from the region situated behind the optical element, and is reflected off the optical element to reach a pupil of the observer, and 
     when there is a change in a position of the pupil of the observer, the optical element is moved using the movement control apparatus, and a position of the image exiting the image forming apparatus is controlled using the movement control apparatus. 
     [A02] The image display apparatus according to [A01], in which 
     the optical element is moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil of the observer in parallel with a horizontal direction. 
     [A03] The image display apparatus according to [A01] or [A02], in which 
     the image forming apparatus is moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil of the observer in parallel with a vertical direction. 
     [A04] The image display apparatus according to [A03], in which 
     the image forming apparatus includes a 4f optical system through which the image exiting the image forming apparatus passes, and 
     the 4f optical system is moved, using the movement control apparatus, in the direction corresponding to the change in the position of the pupil of the observer in parallel with the vertical direction. 
     [A05] The image display apparatus according to [A03], in which 
     the image forming apparatus includes a reflecting mirror off which the image exiting the image forming apparatus is reflected, and 
     a light reflection angle of the reflecting mirror is changed using the movement control apparatus. 
     [A06] The image display apparatus according to [A01] or [A02], in which 
     the optical element is moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil of the observer in parallel with a vertical direction. 
     [A07] The image display apparatus according to any one of [A01] to [A06], in which 
     the optical element includes a reflective hologram diffraction grating. 
     [A08] The image display apparatus according to [A07], in which 
     the reflective hologram diffraction grating includes a light-collecting function. 
     [A09] The image display apparatus according to any one of [A01] to [A06], in which 
     the optical element includes a concave mirror. 
     [A10] The image display apparatus according to any one of [A01] to [A09], in which 
     a pencil of light exiting the image forming apparatus enters the optical element in a telecentric state. 
     [A11] The image display apparatus according to any one of [A01] to [A06], in which 
     the optical element includes a lens that has a positive optical power, and a flat reflecting mirror, the lens being a lens through which the image exiting the image forming apparatus passes. 
     [A12] The image display apparatus according to [A11], in which 
     a pencil of light exiting the image forming apparatus enters the lens in a telecentric state. 
     [A13] The image display apparatus according to [A01] or [A02], in which 
     the optical element is rotated, using the movement control apparatus, according to a change in the position of the pupil of the observer in parallel with a vertical direction. 
     [A14] The image display apparatus according to [A01] or [A02], in which 
     an angle of arrangement of the optical element is changed, using the movement control apparatus, according to a change in the position of the pupil of the observer in parallel with a vertical direction. 
     [A15] The image display apparatus according to [A13] or [A14], in which 
     according to a change in the position of the pupil of the observer in parallel with a horizontal direction, the optical element is horizontally moved using the movement control apparatus. 
     [A16] The image display apparatus according to any one of [A01] to [A15], in which 
     the image forming apparatus includes a dispersion compensation element. 
     [B01]&lt;&lt;Image Display Apparatus: Second Aspect&gt;&gt; 
     An image display apparatus, including: 
     an image forming apparatus; 
     an optical element that is arranged in front of a face of an observer; and 
     a movement control apparatus, in which 
     when a region situated on a side of an ear of the observer, as viewed from the optical element, is referred to as a region situated behind the optical element, the image forming apparatus is arranged in the region situated behind the optical element, 
     an image exiting the image forming apparatus obliquely enters the optical element from the region situated behind the optical element, and is reflected off the optical element to reach a pupil of the observer, and 
     when there is a change in a position of the pupil of the observer, the image forming apparatus is moved using the movement control apparatus. 
     [B02] The image display apparatus according to [B01], in which 
     the optical element includes a reflective hologram diffraction grating. 
     [B03] The image display apparatus according to [B02], in which 
     the reflective hologram diffraction grating includes a light-collecting function. 
     [B04] The image display apparatus according to [B01], in which 
     the optical element includes a concave mirror. 
     [B05] The image display apparatus according to [B01], in which 
     the optical element includes a lens that has a positive optical power, and a flat reflecting mirror, the lens being a lens through which the image exiting the image forming apparatus passes. 
     [B06] The image display apparatus according to any one of [B01] to [B05], in which 
     the optical element is moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil of the observer in parallel with a vertical direction. 
     [B07] The image display apparatus according to any one of [B01] to [B05], in which 
     the image forming apparatus includes a reflecting mirror off which the image exiting the image forming apparatus is reflected, and 
     a light reflection angle of the reflecting mirror is changed using the movement control apparatus. 
     [B08] The image display apparatus according to any one of [B01] to [B05], in which 
     the image forming apparatus includes a 4f optical system through which the image exiting the image forming apparatus passes, and 
     the 4f optical system is moved, using the movement control apparatus, in a direction corresponding to a change in the position of the pupil of the observer in parallel with a vertical direction. 
     [B09] The image display apparatus according to any one of [B01] to [B08], in which 
     a pencil of light exiting the image forming apparatus enters the optical element in a telecentric state. 
     [B10] The image display apparatus according to any one of [B01] to [B09], in which 
     the image forming apparatus includes a dispersion compensation element. 
     [C01]&lt;&lt;Display Apparatus&gt;&gt; 
     A display apparatus, including: 
     a frame that is worn by an observer; and 
     an image display apparatus that is attached to the frame, 
     the image display apparatus including
         an image forming apparatus,   an optical element that is arranged in front of a face of the observer, and   a movement control apparatus, in which       

     when a region situated on a side of an ear of the observer, as viewed from the optical element, is referred to as a region situated behind the optical element, the image forming apparatus is arranged in the region situated behind the optical element, 
     an image exiting the image forming apparatus obliquely enters the optical element from the region situated behind the optical element, and is reflected off the optical element to reach a pupil of the observer, and 
     when there is a change in a position of the pupil of the observer, the optical element, or the image forming apparatus, or the optical element and the image forming apparatus are moved using the movement control apparatus. 
     [C02] The display apparatus according to [C01], including: 
     the image display apparatus for a right eye; and 
     the image display apparatus for a left eye. 
     REFERENCE SIGNS LIST 
     
         
           10  frame 
           11  front portion 
           11 A first front portion 
           11 B second front portion 
           12  temple portion 
           20  image display apparatus 
           20 R image display apparatus for right eye 
           20 L image display apparatus for left eye 
           30  image forming apparatus 
           31 R,  31 G,  31 B light source 
           32  scanning mechanism 
           33  housing 
           34  light-collecting member 
           34 A,  34 B,  34 C lens 
           35 A light source 
           35 B polarization beam splitter 
           35 C liquid crystal display apparatus 
           35 D collimating optical system 
           36 A organic EL display apparatus 
           36 B first convex lens 
           36 C diaphragm 
           36 D second convex lens 
           37  dispersion compensation element 
           40  optical element 
           41  base body 
           50 A,  50 B,  50 C,  50 D movement control apparatus 
           51 ,  54 ,  61 ,  64  drive apparatus 
           52 ,  55 ,  62 ,  65  slide bar 
           53 ,  56 ,  63 ,  66  attachment member 
           80  observer 
           81  eyeball of observer 
           82  pupil of observer 
           83  nose of observer 
           85  region situated on side of ear of observer 
           90  pupil position detecting mechanism 
           91  reflecting mirror 
           92  rotational axis of reflecting mirror 
         LC, LC′, LC″ image-center pencil of light 
         CP center of pupil of observer 
         O center point of optical element