Patent Publication Number: US-2023144703-A1

Title: Head-mounted display

Description:
TECHNICAL FIELD 
     The present invention relates to a head-mounted display that is wearable on a head of a user and displays an image within a visual field. 
     BACKGROUND ART 
     A wearable device such as a head-mounted display (hereinafter, also abbreviated as an HMD) is required to have not only display performance such as ensuring a good visual field and image visibility, but also a structure that is compact and has excellent wearability. 
     As a related patent literature in this technical field, PTL 1 is provided. PTL 1 discloses an optical device including a flat substrate that transmits light, an optical unit that couples light into the substrate by total internal reflection, and a plurality of partially reflection surfaces included in the substrate, in which the partially reflection surfaces are parallel to each other and are not parallel to any edge of the substrate. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-T-2003-536102 
     SUMMARY OF INVENTION 
     Technical Problem 
     An optical system of an HMD includes an image display unit including an illumination unit that transmits light emitted by a light source unit to a miniature display unit, and a projection unit that projects image light (virtual image) generated by the image display unit. When the HMD is displaced with respect to pupils of a user, part of a screen may be out of sight, and for this reason, an eye box is expanded by a pupil replication unit and a waveguide unit, and on the other hand, the expansion of the eye box causes problems such as an increase in the optical system size and a decrease in optical efficiency. 
     A miniature display disposed in the image display unit is generally an element having different screen aspect ratios in vertical and horizontal directions. When an image having a long aspect in the horizontal direction is displayed as a display screen, it is necessary to correspond to a screen long side direction of the image light from the miniature display in a horizontal plane direction of the projection unit. Due to this restriction, a width of a virtual image generation unit may be increased in a long side direction of the miniature display, and design of the HMD is deteriorated. 
     In PTL 1 described above, these problems are not taken into consideration in achieving both expansion of the eye box of the optical system and display of an image having a long side in the horizontal direction, and miniaturization and high efficiency of the HMD optical system. 
     An object of the invention is to provide an HMD that achieves both miniaturization and high efficiency of an optical system and expansion of an eye box. 
     Solution to Problem 
     An example of the invention is a head-mounted display that displays an image within a visual field of a user. The head-mounted display includes: an image display unit configured to generate an image to be displayed; a projection unit configured to project image light from the image display unit; an image rotation and replication unit configured to expand an eye box of projection light from the projection unit; and a waveguide unit configured to transmit image light from the image rotation and replication unit to a pupil of the user. The image rotation and replication unit includes an incidence surface, an emission surface, and at least two reflection surfaces, and an angle formed by the incidence surface and the emission surface is greater than 90°. 
     Advantageous Effect 
     According to the invention, it is possible to provide the HMD that achieves both miniaturization and high efficiency of an optical system and expansion of the eye box. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram of an HMD according to a first embodiment. 
         FIG.  2    is a block diagram of a virtual image generation unit according to the first embodiment. 
         FIG.  3    is a diagram showing a mode of use of the HMD according to the first embodiment. 
         FIG.  4    is a configuration diagram of a virtual image generation unit of the related art. 
         FIG.  5    is a configuration diagram of the virtual image generation unit including an image rotation and replication unit according to the first embodiment. 
         FIG.  6    is a configuration diagram of the image rotation and replication unit according to the first embodiment. 
         FIG.  7    is a configuration diagram of an image rotation and replication unit according to a second embodiment. 
         FIG.  8    is a configuration diagram of an image rotation and replication unit according to a third embodiment. 
         FIG.  9    is a configuration diagram of a virtual image generation unit including an image rotation and replication unit according to a fourth embodiment. 
         FIG.  10    is a configuration diagram of a virtual image generation unit according to a fifth embodiment. 
         FIG.  11    is a configuration diagram of a virtual image generation unit according to a sixth embodiment. 
         FIG.  12    is a configuration diagram of a virtual image generation unit according to a seventh embodiment. 
         FIG.  13    is a configuration diagram of an image rotation and replication unit according to the seventh embodiment. 
         FIG.  14    is a diagram showing an outline of a telecentric optical system according to an eighth embodiment. 
         FIG.  15    is a configuration diagram of a virtual image generation unit according to a ninth embodiment. 
         FIG.  16    is a configuration diagram of a virtual image generation unit according to a tenth embodiment. 
         FIG.  17    is a configuration diagram of a virtual image generation unit according to an eleventh embodiment. 
         FIG.  18    is a diagram showing an example of use of an HMD according to a twelfth embodiment. 
         FIG.  19    is a block diagram of the HMD according to the twelfth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a block diagram of an HMD according to a first embodiment. In  FIG.  1   , an HMD  1  includes a virtual image generation unit  101 , a control unit  102 , an image signal processing unit  103 , a power supply unit  104 , a storage unit  105 , a sensing unit  106 , a communication unit  107 , an audio processing unit  108 , an imaging unit  109 , and input-output units  91  to  93 . 
     The virtual image generation unit  101  enlarges and projects an image displayed on a miniature display unit (micro display) as a virtual image, and displays an image of augmented reality (AR) or mixed reality (MR) within a visual field of a wearer (user). 
     The control unit  102  integrally controls the entire HMD  1 . A function of the control unit  102  is implemented by an arithmetic device such as a central processing unit (CPU). The image signal processing unit  103  supplies an image signal for display to a display unit in the virtual image generation unit  101 . The power supply unit  104  supplies power to each unit of the HMD  1 . 
     The storage unit  105  stores information necessary for processing of each unit of the HMD  1  and information generated by each unit of the HMD  1 . The storage unit  105  stores programs and data executed by the CPU when the function of the control unit  102  is implemented by the CPU. The storage unit  105  includes, for example, a storage device such as a random access memory (RAM), a flash memory, a hard disk drive (HDD), or a solid state drive (SSD). 
     The sensing unit  106  is connected to various sensors via the input-output unit  91  which is a connector, and detects a posture (that is, a posture of the user, head orientation of the user), movement, an ambient temperature, and the like of the HMD  1  based on signals detected by the various sensors. For example, an inclination sensor, an acceleration sensor, a temperature sensor, a sensor of a global positioning system (GPS) that detects user position information, and the like are connected as the various sensors. 
     The communication unit  107  communicates with an external information processing device by short-range wireless communication, long-range wireless communication, or wired communication via the input-output unit  92  which is a connector. Specifically, the communication is performed by Bluetooth (registered trademark), Wi-Fi (registered trademark), a mobile communication network, a universal serial bus (USB, registered trademark), a high-definition multimedia interface (HDMI (registered trademark)), or the like. 
     The audio processing unit  108  is connected to an audio input-output device such as a microphone, an earphone, or a speaker via the input-output unit  93  which is a connector, and inputs or outputs an audio signal. The imaging unit  109  is, for example, a miniature camera or a miniature time of flight (TOF) sensor, and captures an image within a visual field direction of the user of the HMD  1 . 
       FIG.  2    is a block diagram of the virtual image generation unit  101  according to the present embodiment. The virtual image generation unit  101  includes an image display unit  120 , a projection unit  121 , an image rotation and replication unit  122 , and a waveguide unit  123 . The image display unit  120  is a processing unit that generates an image to be displayed, and irradiates a built-in miniature display unit (not shown) with light from a light source such as an LED or a laser. The miniature display unit is an element that displays an image, and uses a liquid crystal display, a digital micromirror device, an organic EL display, micro electro mechanical systems (MEMS), or the like. The projection unit  121  enlarges image light of the image display unit  120  and projects the enlarged image light as a virtual image. The image rotation and replication unit  122  performs image rotation and pupil replication for expanding an eye box. The waveguide unit  123  transmits image light from the projection unit  121  and the image rotation and replication unit  122  to a pupil  20  of a user. The image light is formed on the pupil  20 , and the user can visually recognize an image. 
       FIG.  3    is a diagram showing a mode of use of the HMD  1  according to the present embodiment.  FIG.  3    shows a state viewed down from above a head of a user  2 , and an X axis is a horizontal direction, a Y axis is a vertical direction, and a Z axis is a visual axis direction which is a visual line direction of the user  2 . In the following drawings, the directions of the X, Y, and Z axes are similarly defined. 
     The HMD  1  is mounted on the head of the user  2 , and transmits an image generated by the virtual image generation unit  101  to the pupil  20  of the user via the waveguide unit  123 . At this time, the user  2  can visually recognize the image (virtual image) in a state (see-through type) in which the outside world can be visually recognized in a part of an image display area  111  within a visual field.  FIG.  3    shows a configuration in which an image is displayed on one eye, and a configuration in which the image is displayed in both eyes may be used. The HMD  1  can also capture an image of a visual field range of the user  2  in the imaging unit  109  of  FIG.  1   . 
     Next,  FIG.  4    shows a configuration diagram of the virtual image generation unit  101  using the waveguide unit  123  of a mirror array type of the related art. In  FIG.  4   , (a) shows the virtual image generation unit  101  viewed from the Z axis direction, which is the visual axis direction, and (b) shows the virtual image generation unit  101  viewed from the Y axis direction, which is the vertical direction. The waveguide unit  123  is in a flat plate shape having two main parallel planes  171  and  172 , and includes at least two beam splitter mirror arrays  173 , which are partially reflection surfaces inside, in order to expand the eye box. A mirror array unit  126  having a reflecting film that reflects a part of the image light has a function of replicating an exit pupil of the projection unit  121  in the X axis direction. Preferably, the beam splitter mirror arrays  173  are substantially parallel to each other so that an angular deviation does not occur in reflected signal light. 
     The eye box formed by the virtual image generation unit  101  is preferably expanded in a two-dimensional direction from the viewpoint of image visibility. Since the waveguide unit  123  expands the eye box only in the horizontal direction, an optical engine needs to input image light having a large light beam diameter in the vertical direction. Therefore, it is necessary to reduce an F value of an optical system of the image display unit  120  in the vertical direction, and portions of the image display unit  120  and the projection unit  121  having a dimension A in (a) of  FIG.  4    are greater and the virtual image generation unit  101  is greater. Due to the characteristics of an HMD as a device that is wearable for use, a weight and appearance design are also important factors in use, and are important points for increasing the commercial value. 
     In addition, there is a problem in a case of coping with a user wearing vision correction glasses. That is, in the configuration of the related art of  FIG.  4   , since the projection unit  121  and the image display unit  120  are closer to a pupil side than the waveguide unit  123  as shown in (b) of  FIG.  4   , it is necessary to move the projection unit  121  and the image display unit  120  to the outside of a side surface of the face of the user in order to prevent a corner portion connecting the front of the vision correction glasses and a hinge from interfering with the projection unit  121  and the image display unit  120 . Therefore, it is necessary to lengthen the horizontal direction of the waveguide unit  123  in order to provide an image emitted from the waveguide unit  123  to the pupil  20  of the user. The eye box is reduced when a distance from the projection unit to the pupil of the user increases. Therefore, it is necessary to expand the eye box in the vertical direction according to the reduction, and the dimension A of the image display unit  120  and the projection unit  121  is further increased. 
     As shown in (b) of  FIG.  4   , a miniature display unit  125  disposed in the image display unit  120  is generally an element having different screen aspect ratios in vertical and horizontal directions. When an image having a long aspect in the horizontal direction is displayed as a screen to be displayed from the waveguide unit  123 , it is necessary to correspond to a screen long side direction of the image light incident from the miniature display on the projection unit in a horizontal plane (XZ plane) direction of the projection unit  121 . Due to this restriction, a portion having a dimension B is increased in a long side direction of the miniature display, the size of the image display unit  120  is increased and the design of the HMD is impaired. 
     As described above, in the HMD, there is a problem in achieving both miniaturization and high luminance for expansion of the eye box and display of an aspect image having a long side in the horizontal direction. Hereinafter, solutions thereof are described. 
       FIG.  5    is a configuration diagram of the virtual image generation unit according to the present embodiment. In  FIG.  5   , the components same as those in  FIG.  4    are denoted by the same reference numerals, and the description thereof is omitted. In  FIG.  5   , (a) and (b) respectively show a case where the virtual image generation unit  101  is viewed from the Z axis direction and a case where the virtual image generation unit  101  is viewed from the Y axis direction, as in  FIG.  4   . In the present embodiment, the above-described problem is solved by the image rotation and replication unit  122 . 
     As described above, the eye box formed by the virtual image generation unit  101  is preferably expanded in the two-dimensional direction from the viewpoint of image visibility. In order to two-dimensionally expand the eye box, the eye box is expanded in the vertical direction by the image rotation and replication unit  122 . The image rotation and replication unit  122  includes an emission reflective surface group including at least two emission reflective surfaces that are partially reflection surfaces that reflect image light toward the waveguide unit  123 . As shown in (a) of  FIG.  5   , the present embodiment shows an example in which emission reflective surfaces  131  to  134  are provided as the emission reflective surface group. Preferably, the emission reflective surface groups  131  to  134  are substantially parallel to each other.  FIG.  5    shows a configuration in which the number of emission reflective surfaces is four, but the number of emission reflective surfaces is not necessarily limited thereto. With this configuration, a beam diameter of the image light from the projection unit  121  can be reduced. Therefore, the F value of the optical system can be increased, and sizes of portions of the image display unit  120  and the projection unit  121  corresponding to the dimension A can be reduced. 
     Further, in order to reduce the size of the portion having the dimension B, the miniature display unit  125  is rotated by 90° with respect to the projection unit  121  as shown in (b) of  FIG.  5   . In this case, in the horizontal plane (XZ plane) of the projection unit  121 , a screen short side direction of the image light corresponds to a direction of the light incident from the miniature display unit  125  to the projection unit  121 , so that the dimension B of the image display unit  120  and the projection unit  121  can be reduced, the size of the virtual image generation unit  101  can be reduced, and the design of the virtual image generation unit  101  can be improved. However, in this case, a problem occurs that a visible image becomes a screen having a long aspect in the vertical direction. Therefore, by providing an incidence reflective surface  130  that reflects the image light from the projection unit  121  to the inside in the image rotation and replication unit  122 , and setting predetermined twist angles between a normal line of the incidence reflective surface  130  and normal lines of the emission reflective surface groups  131  to  134 , a screen visually recognized from the waveguide unit  123  can be rotated by a predetermined angle. For example, when the miniature display unit  125  is rotated by 90°, the predetermined twist angle between the normal line of the incidence reflective surface and the normal line of the emission reflective surface group is 90°. The image light emitted from the image rotation and replication unit  122  is incident on the waveguide unit  123  via a coupling unit  124  so as to be propagated to the waveguide unit by total reflection. 
     As described above, by adopting a configuration using the image rotation and replication unit  122 , it is possible to provide the virtual image generation unit  101  that is compact and has good design while two-dimensionally expanding the eye box. A direction of the pupil replication of the waveguide unit and the image rotation and replication unit is not necessarily the directions shown in (b) of  FIG.  5   , and may be any direction as long as the eye box is expanded two-dimensionally. 
     As a further advantage of the configuration of the present embodiment, there is an effect that the size of the device can be reduced in order to make the HMD wearable even by a user wearing vision correction glasses. That is, as shown in (b) of  FIG.  5   , the image light can be incident on the waveguide unit  123  from a side opposite to a side where the pupil  20  of the user is located via the coupling unit  124 . Accordingly, the image rotation and replication unit  122 , the projection unit  121 , and the image display unit  120  can be disposed on the side opposite to the side where the pupil is located with respect to the waveguide unit  123  as compared with the related art. Therefore, in response to the problem in the case of coping with a user wearing vision correction glasses having the configuration of the related art, a horizontal dimension of the waveguide unit  123  can be made shorter than that of the configuration of the related art without interfering with the vision correction glasses of the user. Accordingly, it is possible to reduce an expansion amount of the eye box, and thus it is possible to reduce manufacturing costs by reducing the size of the image rotation and replication unit (particularly, in the vertical direction) and reducing the size of the emission reflective surface. 
     As described above, by making an emission direction of the image light from a projection lens different from an incidence direction of the image light to the waveguide unit and making it possible to input the image light from the outside of the waveguide unit by using the image rotation and replication unit  122 , it is possible to achieve a configuration capable of coping with a user wearing vision correction glasses, achieving reduction in size, and achieving reduction in manufacturing costs. 
     A specific configuration of the image rotation and replication unit  122  in the present embodiment will be described with reference to  FIG.  6   . In  FIG.  6   , (a) is an external view of the image rotation and replication unit  122 , (b) is a schematic view of the image rotation and replication unit  122  as viewed from a direction parallel to the emission reflective surfaces  131  to  134  and an emission surface  135 , and (c) is an external view of the image rotation and replication unit  122  as viewed from a direction parallel to the emission surface  135  and an incidence surface  136 . For explanation, the incidence reflective surface  130  is hatched. 
     When the image light incident from the projection unit  121  and reflected by the incidence reflective surface  130  is incident on the emission surface  135  of the image rotation and replication unit  122  before being reflected by the emission reflective surfaces  131  to  134 , stray light is generated due to internal total reflection. Due to a geometric configuration, in particular, the emission surface  135  side has a distance from the incidence reflective surface  130  to the emission reflective surface, and thus stray light due to internal reflection is easily generated on the emission surface  135  side. 
     Therefore, in order to prevent a component of image light having a predetermined angle of view range from being reflected by the incidence reflective surface  130  toward the emission surface  135  side as shown by a dotted line in (b) of  FIG.  6    and to achieve a geometric configuration that does not affect screen rotation, it is necessary that an angle θ 1  formed by the incidence surface  136  and the emission surface  135  is 90° or more as shown in (c) of  FIG.  6   , and an angle θ 2  formed by the emission reflective surfaces  131  to  134  and a normal line of the emission surface  135  is 45° or more as shown in (b) of  FIG.  6   . The angles θ 1  and θ 2  satisfy θ 2 ×2=θ 1 , so that an image light propagation angle in the image rotation and replication unit  122  can be adjusted to prevent the generation of stray light inside without affecting the rotation of an image from the waveguide unit  123 , offset of the angle of view, and the like. 
     From the viewpoint of preventing luminance unevenness of a screen, spacings L 1  to L 3  between the respective reflection surfaces of the emission reflective surfaces  131  to  134  are preferably less than an outer diameter of a projection lens constituting the projection unit  121 . Accordingly, the image light is reproduced seamlessly, and luminance unevenness can be prevented. Further, by making the spacings L 1  to L 3  less than a diameter of the exit pupil formed by the projection unit  121 , the reproduced image light has no seams, and the luminance unevenness can be prevented. 
     As described above, according to the present embodiment, it is possible to provide the HMD that achieves both miniaturization and high efficiency of an optical system and expansion of an eye box. 
     Second Embodiment 
       FIG.  7    is a configuration diagram of an image rotation and replication unit according to a second embodiment. In  FIG.  7   , the components same as those in  FIG.  6    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  7    is different from  FIG.  6    in that the incidence reflective surface  130  and a part of the emission reflective surface  134  closest to the incidence reflective surface  130  intersect with each other. When avoiding stray light due to internal reflection, the image light is a path as shown by a dotted line as described above, so that the distances between the incidence reflective surface  130  and the emission reflective surfaces  131  to  134  of the image rotation and replication unit  122  can be reduced and the size can be reduced without affecting the substantial image quality even if the incidence reflective surface  130  and a part of the emission reflective surface  134  intersect with each other. 
     Therefore, according to the present embodiment, it is possible to provide the HMD that achieves both miniaturization and high efficiency of an optical system and expansion of an eye box. 
     Third Embodiment 
       FIG.  8    is a configuration diagram of an image rotation and replication unit according to a third embodiment. In  FIG.  8   , the components same as those in  FIG.  6    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  8    is different from  FIG.  6    in that the incidence reflective surface  130  and the emission reflective surfaces  131  to  134  are not integrated and are separated into a first prism  122   a  having the incidence reflective surface  130  and a second prism  122   b  having the emission reflective surfaces  131  to  134 . As shown in  FIG.  6   , in the manufacture of a prism in which the incidence reflective surface  130  and the emission reflective surfaces  131  to  134  are integrated, steps of joining and cutting a plurality of reflection surfaces are complicated, and manufacturing costs are increased. In contrast, as in the present embodiment, the incidence reflective surface  130  and the emission reflective surfaces  131  to  134  are separated from each other, and the first and second prisms having a shape close to a simple triangular prism are manufactured, respectively, so that the costs can be reduced. 
     Therefore, according to the present embodiment, it is possible to achieve both miniaturization and high efficiency of an optical system and expansion of an eye box, and to provide the HMD with lower costs. 
     Fourth Embodiment 
       FIG.  9    is a configuration diagram of a virtual image generation unit according to a fourth embodiment. In  FIG.  9   , the components same as those in  FIG.  5    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  9    is different from  FIG.  5    in the configuration of the waveguide unit  123 . 
     In  FIG.  9   , the waveguide unit  123  in the present embodiment has a waveguide unit configuration using a diffraction grating, a volume hologram, a beam splitter array (BSA), or the like, and thus can be an HMD having see-through properties. 
     In (a) of  FIG.  9   , a volume hologram is used for the waveguide unit  123 . That is, this is an example of a case where image light propagating in the waveguide unit is diffracted by a volume hologram  140  instead of a mirror array as a unit that outputs the image light to the pupil  20  of a user. In (b) of  FIG.  9   , the waveguide unit  123  uses a surface digging-type diffraction grating. This is an example of a case where a surface digging-type diffraction grating  141  is provided as a unit that inputs image light to the waveguide unit and a surface digging-type diffraction grating  142  is used as a unit that propagates the captured light in a parallel plate by total reflection and outputs the image light to the pupil  20  of a user. 
     Therefore, according to the present embodiment, it is possible to provide the HMD having the effect of the first embodiment and the see-through properties. 
     Fifth Embodiment 
       FIG.  10    is a configuration diagram of a virtual image generation unit according to a fifth embodiment. In  FIG.  10   , the components same as those in  FIG.  5    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  10    is different from  FIG.  5    in that the specific configuration of the image display unit  120  is described. 
     In  FIG.  10   , the image display unit  120  includes the miniature display unit  125  and an illumination optical system  169  that transmits light emitted by a light source unit to the miniature display unit  125 . 
     The illumination optical system  169  includes a light source unit  150  of green (G) and a light source unit  151  of red (R) and blue (B) as light source units. The light from each light source is substantially collimated by condenser lenses  152  and  153 . The substantially collimated light from the light sources of respective colors is combined by a color combining unit  154 . 
     Here, an example in which a wedge-shaped dichroic mirror is used as the color combining unit  154  is shown. The dichroic mirror combines substantially collimated light of an R light, a B light, and a G light and emits the combined light. At this time, optical axes of the respective colors do not necessarily have to completely coincide with each other, and the optical axes may be slightly shifted so that intensity distributions substantially coincide with each other on a predetermined surface. 
     The color-combined light is incident on a microlens array  155  serving as a virtual secondary light source. The microlens array  155  is illuminated with the substantially collimated light flux emitted from the color combining unit  154 . By using the microlens array  155 , light can be collected only in a predetermined range of the miniature display unit  125 . A luminance distribution of the illumination light on the miniature display unit  125  can be made uniform. 
     A bending mirror  156  has a function of bending an optical path from the microlens array  155  to the miniature display unit  125 . That is, by inserting the bending mirror  156 , a length of the dimension A of the image display unit  120  can be reduced. A condensing lens as a condensing optical member  157  forms a cell image of the microlens array  155  on the miniature display unit  125 . 
     When liquid crystal on silicon (LCOS, registered trademark) or the like is used for the miniature display unit  125 , an optical path to the image display unit  120  and the projection unit  121  is separated by a polarization splitting element  158 .  FIG.  10    shows an example in which a polarization beam splitter (PBS) is used as the polarization splitting element  158 . The projection unit  121  is a projection optical system including a plurality of lenses, and projects image light from the miniature display unit  125  as infinity or a virtual image by changing an angle according to an angle of view. The image light from the projection unit  121  is incident on the waveguide unit  123  via the image rotation and replication unit  122 , and a user can visually recognize an image in a state where see-through properties are secured. 
     As described above, according to the present embodiment, it is possible to improve the image quality while reducing the dimension A of the image display unit  120  in the vertical direction. 
     Sixth Embodiment 
       FIG.  11    is a configuration diagram of a virtual image generation unit according to a sixth embodiment. In  FIG.  11   , the components same as those in  FIG.  10    are denoted by the same reference numerals, and the description thereof will be omitted.  FIG.  11    is different from  FIG.  10    in that a polarization filter  160 , a quarter-wave plate  161 , a polarization filter  162 , and a diffusion plate  163  are added. 
     When liquid crystal on silicon (LCOS, registered trademark) or the like is used for the miniature display unit  125  and an optical path to the image display unit  120  and the projection unit  121  is separated by the polarization splitting element  158 , as shown in  FIG.  11   , the polarization filter  162  is disposed to extract only a necessary polarization component in advance in order to illuminate with predetermined polarized light. The polarization filter  162  also has an advantage in terms of measures against stray light and contrast. 
     The polarization filter  160  and the quarter-wave plate  161  prevent stray light due to return light from the projection unit  121  and the waveguide unit  123 . 
     In order to increase the efficiency and luminance of an optical system, it is effective to use a microlens array in an image display unit. In this regard, the present inventors have found that when a microlens array is used for the image display unit, a conjugate image of the microlens array is also formed in an exit pupil of a projection lens. That is, when an image is visually recognized by the optical system, a conjugate image of the light source unit  150  replicated by the microlens array  155  is formed on an emission surface of the microlens array  155 . The emission surface of the microlens array  155  and the exit pupil of the projection unit  121  have a substantially conjugate positional relation. Therefore, at an exit pupil position of the projection unit  121 , a conjugate image of a lens cell emission surface of the microlens array  155  and a further conjugate image of the conjugate image of the light source unit  150  formed on the emission surface of the microlens array  155  are formed. Therefore, when a user views an image through the waveguide unit  123 , a conjugate image of a microlens cell and a conjugate image of a light source appear to be superimposed in front of the image, which causes a problem that image visibility is deteriorated. 
     The image rotation and replication unit  122  and the waveguide unit  123  have a function of replicating the exit pupil of the projection unit  121  in order to expand the eye box, and the conjugate images may be repeatedly superimposed and become inconspicuous if the number of replications is large. In contrast, when the waveguide unit  123  of a beam splitter mirror array type is used, the number of replications is reduced in principle as compared with other methods, and image visibility is greatly deteriorated by the conjugate image. 
     Therefore, in the present embodiment, the diffusion plate  163  is added between the microlens array  155  and the miniature display unit  125  to prevent the conjugate images of a periodic microlens array (lens cell) and a light source replicated by the waveguide unit  123 . Accordingly, it is possible to blur only the conjugate images of the microlens cell and the light source to make the conjugate images inconspicuous without affecting the resolution of an image (virtual image) which is an enlarged image of the miniature display unit  125 . 
     Here, the diffusion plate  163  is disposed at a position close to the condensing lens  157  away from the microlens array  155 . In the present example, the diffusion plate  163  is disposed right behind the condensing lens  157  (upper side in  FIG.  11   ), but it may be disposed on the front side of the condensing lens  157  (lower side in  FIG.  11   ). By disposing the diffusion plate  163  at a position close to the condensing lens  157 , it is possible to reduce a diffusion angle of the diffusion plate  163 , and it is possible to blur only the conjugate images of the microlens cell and the light source while preventing the decrease in efficiency due to the insertion of the diffusion plate  163 . 
     In consideration of the influence of the diffusion plate  163  on polarization, the polarization filter  162  is disposed right behind the diffusion plate  163 . At this time, for example, the diffusion plate  163  and the polarization filter  162  are attached to the polarization splitting element  158  and are integrated with each other. 
     Meanwhile, it is also possible to integrate the diffusion plate  163  and the condensing lens  157 . At this time, instead of the diffusion plate  163 , a surface of the condensing lens  157  may be a sand-blasting surface to which a diffusion function is added. 
     As described above, according to the present embodiment, by using the diffusion plate that eliminates the conjugate image seen through the waveguide unit, an illumination optical system of the image display unit can be used as a Kohler illumination system using a microlens array, and high efficiency and high luminance can be implemented. 
     Seventh Embodiment 
       FIG.  12    is a configuration diagram of a virtual image generation unit according to a seventh embodiment. In  FIG.  12   , the components same as those in  FIG.  11    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  12    is different from  FIG.  11    in that a wire grid film  159  is disposed as the polarization splitting element  158 . A configuration is shown in which the wire grid film  159  is attached to an optically transparent substrate  164  so as not to be distorted. With this configuration, costs can be reduced by using the wire grid film as the polarization splitting element. 
     Since the diffusion plate  163  of the image display unit  120  diffuses image light, the diffusion plate  163  has an effect of reducing the F value of an optical system in addition to eliminating a conjugate image. Therefore, by adopting a configuration in which the F value of the projection unit  121  is also reduced, the exit pupil is expanded and the eye box is expanded. In this case, as shown in  FIG.  13   , the emission reflective surface  131  of the image rotation and replication unit  122  may be configured to be one surface. Accordingly, the number of emission reflective surfaces can be reduced, and manufacturing costs of the image rotation and replication unit can be reduced. 
     Eighth Embodiment 
       FIG.  14    is a diagram showing an outline of a telecentric optical system according to an eighth embodiment. Here, as the configuration of the image display unit  120 , a condensing optical member is disposed between the microlens array  155  and the miniature display unit  125 . Here, the single condensing lens  157  is used as the condensing optical member. The bending mirror  156  is disposed between the condensing lens  157  and the microlens array  155 . 
     In addition, when the miniature display unit  125  is a reflective liquid crystal display, it is necessary to dispose the polarization splitting element  158  such as a polarization beam splitter or a wire grid film between the condensing lens  157  and the miniature display unit  125 . Here, as the polarization splitting element  158 , a configuration is shown in which the wire grid film  159  is attached to the optically transparent substrate  164 . 
     In order to form a telecentric optical system, it is necessary to set a distance between the microlens array  155  and the condensing lens  157  and a distance between the condensing lens  157  and the miniature display unit  125  to a focal length f of the lens. Therefore, in a general telecentric optical system, reflection surfaces of the polarization splitting element  158  and the bending mirror  156  need to have substantially the same area due to geometric symmetry. 
     Ninth Embodiment 
       FIG.  15    is a configuration diagram of a virtual image generation unit according to a ninth embodiment. In  FIG.  15   , the components same as those in  FIG.  11    are denoted by the same reference numerals, and the description thereof will be omitted.  FIG.  15    is different from  FIG.  11    in that a reflection surface area of the bending mirror  156  is less than a reflection surface area of the polarization splitting element  158 . 
     That is, it may be difficult for the waveguide unit  123  to capture all the image light of the exit pupil of the projection unit  121  inside, and in many cases, the waveguide unit  123  captures a part of the image light of the exit pupil. In view of these, the efficiency does not decrease even if an effective area of the bending mirror  156  is less than an effective area of the reflection surface of the polarization splitting element  158 . 
     Therefore, according to the present embodiment, there is an effect that the size can be reduced by making the area of the reflection surface of the bending mirror  156  less than the area of the reflection surface of the polarization splitting element  158 . 
     Tenth Embodiment 
       FIG.  16    is a configuration diagram of a virtual image generation unit according to a tenth embodiment. In  FIG.  16   , the components same as those in  FIG.  11    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  16    is different from  FIG.  11    in that a display  125 D of a digital micromirror device type is used as the miniature display unit  125 . The display  125 D of a digital micromirror device type does not require separation of polarized light and can improve light utilization efficiency. 
     In  FIG.  16   , illumination light from a light source unit is incident on a condensing lens  157   a  divided into two pieces, reflected by the bending mirror  156 , and incident on a condensing lens  157   b.  A light flux that passes through the condensing lenses  157   a  and  157   b  passes through an inclined surface of a total reflection prism  165 , and is applied to the display  125 D of a digital micromirror device type. The light flux reflected by a micromirror surface of the display  125 D of a digital micromirror device type is incident on the total reflection prism  165  again at a different angle, and is totally reflected by the inclined surface. The totally reflected image light is incident on the waveguide unit  123  through the projection unit  121  and the image rotation and replication unit  122 , and displays an image within a visual field of a user. Since it is necessary to obliquely apply illumination light to the display  125 D of a digital micromirror device type and a reflection angle of the bending mirror  156  can be reduced, the dimension A of the image display unit  120  in the vertical direction can be reduced. 
     When the display  125 D of a digital micromirror device type is used as the miniature display unit  125 , an optical axis of light of a part to be displayed in black (hereinafter, referred to as OFF light) in an image is inclined by the digital micromirror device, and the light travels toward the projection unit. When the projection unit captures the OFF light, a display screen does not become a proper black color, which causes a decrease in screen contrast and generation of stray light. Therefore, by providing a light shielding unit  170  that shields the OFF light between the projection unit and the image rotation and replication unit or on the incident surface of the image rotation and replication unit so that the OFF light does not enter the image rotation and replication unit, the decrease in contrast and the generation of stray light can be prevented. 
     Eleventh Embodiment 
       FIG.  17    is a configuration diagram of a virtual image generation unit according to an eleventh embodiment. In  FIG.  17   , the components same as those in  FIG.  10    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  17    is different from  FIG.  10    in that a self-luminous display  1250  is used as the miniature display unit  125 . Accordingly, an illumination optical system is no longer needed and the size is greatly reduced. As the self-luminous display, an organic EL display or a micro LED display may be used. 
     Twelfth Embodiment 
     In a twelfth embodiment, an application example of the HMD described in each embodiment will be described.  FIG.  18    is a diagram showing an example of use of an HMD according to the present embodiment. 
     In  FIG.  18   , within a visual field of the user  2 , content is displayed in an image (virtual image) display area  111  from the HMD  1 . For example, a work procedure manual  201  or a drawing  202  for inspection, assembly, and the like of industrial equipment are displayed. Since the image display area  111  is limited, when the work procedure manual  201  and the drawing  202  are displayed at the same time, the content becomes small and the visibility becomes poor. Thus, the visibility can be improved by performing head tracking in which head orientation of the user  2  is detected by an acceleration sensor and by changing a display content according to the head orientation. That is, in  FIG.  18   , the work procedure manual  201  is displayed in the image display area  111  in a state where the user  2  faces the left, but when the user faces the right, the drawing  202  is displayed in the image display area  111 . The work procedure manual  201  and the drawing  202  can be displayed as if there is a virtual image display area  112  in which the work procedure manual  201  and the drawing  202  can be visually recognized in a wide visual field. 
     Accordingly, since the visibility is improved, and the user  2  can execute the work while visually recognizing a work target (device, tool, or the like) and a work instruction at the same time, the work can be performed more reliably and errors can be reduced. 
       FIG.  19    is a block diagram of the HMD according to the present embodiment. In  FIG.  19   , the components same as those in  FIG.  1    are denoted by the same reference numerals, and the description thereof is omitted.  FIG.  19    is different from  FIG.  1    in that a head tracking function is added in particular. That is, an image signal processing unit  103 A of the HMD  1  is provided with a head tracking unit  103 H. The head tracking unit  103 H detects the head orientation of the user  2  based on information of an acceleration sensor  106 H of a sensing unit  106 A, and changes a display content according to the head orientation. 
     The HMD is used indoors or outdoors. Therefore, it is necessary to adjust the luminance of a display image according to the brightness of the surrounding environment. For example, an illuminance sensor  106 M may be mounted on the sensing unit  106 A, and the luminance of an image displayed by the image signal processing unit  103 A may be adjusted according to the output of the illuminance sensor. 
     The embodiments according to the invention have been described above, but the invention is not limited to the above-described embodiments, and includes various modifications. For example, functional configurations of the HMD and the virtual image generation unit described above are classified according to main processing contents to facilitate understanding. The invention is not limited by the method or name of classification of the constituent elements. The configurations of the HMD and the virtual image generation unit can be further classified into more constituent elements according to the processing contents. It is also possible to perform classification such that one constituent element executes more processes. 
     The invention can be applied not only to an HMD but also to other image (virtual image) display devices having the configuration of the virtual image generation unit described in each embodiment. 
     A part of the configuration of one embodiment can be replaced with the configuration of another embodiment. The configuration of another embodiment can also be added to the configuration of one embodiment. A part of the configurations of the embodiments can be added to, deleted from, or replaced with another configuration. 
     REFERENCE SIGN LIST 
       1  head-mounted display (HMD) 
       101  virtual image generation unit 
       102  control unit 
       103  image signal processing unit 
       104  power supply unit 
       105  storage unit 
       106  sensing unit 
       107  communication unit 
       108  audio processing unit 
       109  imaging unit 
       91  to  93  input-output unit 
       111  image display area 
       112  virtual image display area 
       120  image display unit 
       121  projection unit 
       122  image rotation and replication unit 
       123  waveguide unit 
       125  miniature display unit 
       130  incidence reflective surface 
       131  to  134  emission reflective surface 
       135  emission surface 
       136  incidence surface 
       140  volume hologram 
       141 ,  142  surface digging-type diffraction grating 
       150 ,  151  light source unit 
       154  color combining unit 
       155  microlens array 
       156  bending mirror 
       157  condensing lens (condensing optical member) 
       158  polarization splitting element 
       159  wire grid film 
       160 ,  162  polarization filter 
       161  quarter-wave plate 
       163  diffusion plate 
       169  illumination optical system 
       170  light shielding unit