Patent Publication Number: US-2016231565-A1

Title: Image display device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2013-223291, filed on Oct. 28, 2013, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to image display devices using the transmission-type screens. 
     2. Description of the Related Art 
     Display devices for vehicles called head-up displays are known. Head-up displays are display devices that display information over a landscape outside a vehicle by allowing light entering from outside the vehicle to pass through and reflecting, on a windshield or the like of the vehicle, an image projected from an optical unit arranged inside the vehicle. Head-up displays have received attention as display devices for vehicles in recent years since head-up displays allow a driver who is visually recognizing a view outside a vehicle to recognize information of an image projected from an optical unit almost without changing the line of sight or a focus. 
     Image display light projected from the optical unit once forms an image on a transmission-type screen, and the image formed on the screen is presented to the user. As such a transmission-type screen, a configuration is disclosed where two light diffusion plates are layered. 
     SUMMARY 
     The user recognizes the image via the transmission-type screen. Thus, the transmission-type screen is highly visible, desirably. 
     In this background, a purpose of the present invention is to provide transmission-type screens with enhanced visibility. 
     A transmission-type screen according to one embodiment of the present invention includes: two light diffusion plates that have respective flat surfaces and respective light diffusion surfaces that face the respective flat surfaces and diffuse and transmit incident light. The two light diffusion plates are layered such that the respective light diffusion surfaces face each other, and the light diffusion surfaces diffuse light that is incident on the respective light diffusion surfaces in substantially the same light distribution. 
     Another embodiment of the present invention relates to an image display device. This device includes: an image projection unit that projects image display light; an intermediate image formation unit that forms a real image that is based on the image display light projected from the image projection unit; and a projection mirror that reflects, toward a virtual image presenting surface, the image display light that has passed through the intermediate image formation unit. The intermediate image formation unit includes two light diffusion plates that have respective flat surfaces and respective light diffusion surfaces that face the respective flat surfaces and diffuse and transmit incident light. The two light diffusion plates are layered such that the respective light diffusion surfaces face each other, and the light diffusion surfaces diffuse light that is incident on the respective light diffusion surfaces in substantially the same light distribution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings that are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several figures, in which: 
         FIG. 1  is a diagram schematically illustrating a form of installation of a head-up display according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating the internal configuration of an optical unit; 
         FIG. 3  is a diagram schematically illustrating the internal configuration of an image projection unit; 
         FIG. 4  is a diagram illustrating an optical path of image display light that is projected on a windshield; 
         FIG. 5  is a diagram illustrating optical paths of image display light when a virtual image is presented for viewpoints of different height levels; 
         FIG. 6  is a diagram illustrating image display light that is distributed by an intermediate image formation unit; 
         FIG. 7  is a diagram illustrating the configuration of a diffusion screen; 
         FIGS. 8A and 8B  are diagrams illustrating the relationship between the angle of light incident on a light diffusion plate and the angle of light passing through the light diffusion plate; and 
         FIG. 9  is a graph illustrating the relationship between the angle of light incident on a light diffusion plate and a deviation angle of a principal ray emitted from the light diffusion plate. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     Described below is an explanation of the embodiments of the present invention with reference to figures. Specific numerical values and the like shown in the embodiments are shown merely for illustrative purposes to facilitate understanding of the invention and do not intend to limit the scope of the present invention, unless otherwise noted. In the subject specification and figures, elements having substantially the same functions and structures shall be denoted by the same reference numerals, and duplicative explanations will be omitted appropriately. Also, the illustration of elements that are not directly related to the present invention is omitted. 
     An explanation will be given using a head-up display  10 , which is installed and used inside a dashboard of a vehicle, as an example for an image display device according to an embodiment.  FIG. 1  is a diagram schematically illustrating a form of installation of the head-up display  10  according to the embodiment of the present invention. The head-up display  10  includes an optical unit  100  and a control device  50 .  FIG. 1  is a diagram illustrating a case where the optical unit  100  is arranged and used inside a left-side dashboard based on a travelling direction (leftward direction in  FIG. 1 ) of a vehicle. The following embodiment shows an example where the head-up display  10  is arranged for a driver of a left-hand drive vehicle. For a right-hand drive vehicle, the internal configuration of the optical unit  100  may be horizontally flipped based on a travelling direction of the vehicle. With reference to  FIG. 1 , an explanation will be given of the outline of the head-up display  10  in the following. 
     A control device  50  is provided with a central processing unit (CPU) (not shown) and generates an image signal used for display on the optical unit  100 . The control device  50  is also provided with an external input interface (not shown). An image signal output from an external device (not shown) such as a navigation device, a media reproduction device, or the like is input to the control device  50 , and the control device  50  is also capable of outputting the image signal to the optical unit  100  after performing a predetermined process on the signal that has been input. 
     The optical unit  100  generates image display light that is displayed as a virtual image  450  on a windshield  610  based on the image signal generated by the control device  50 . Therefore, the optical unit  100  is provided with an image projection unit  210 , an intermediate mirror  350 , an intermediate image formation unit  360 , and a projection mirror  400  inside a housing  110 . 
     The image projection unit  210  houses a light source, an image display element, various optical lenses, and the like. The image projection unit  210  generates image display light based on the image signal output from the control device  50  and projects the image display light. In the present embodiment, a case where a liquid crystal on silicon (LCOS), which is a reflection type liquid crystal display panel, is used as an image display element is illustrated for example. However, a digital micromirror device (DMD) may be used as the image display element. In that case, the DMD is assumed to be formed by an optical system and a drive circuit according to a display element to which the DMD is applied. 
     The image display light projected by the image projection unit  210  is reflected by the intermediate mirror  350 . The image display light reflected by the intermediate mirror  350  forms an image in the intermediate image formation unit  360 . The image display light related to a real image formed in the intermediate image formation unit  360  is transmitted through the intermediate image formation unit  360  and projected on the projection mirror  400 . 
     The projection mirror  400  is a concave mirror, and the image display light transmitted through the intermediate image formation unit  360  is enlarged and projected on the windshield  610  by the projection mirror  400 . The optical path of the image display light projected on the windshield  610  is changed to be directed toward the user by the windshield  610 . A user E, who is the driver, recognizes the image display light reflected by the windshield  610  as a virtual image  450  in front of the windshield  610  in the direction of the line of sight. 
       FIG. 2  is a diagram illustrating the internal configuration of the optical unit  100  according to the embodiment of the present invention. With reference to  FIG. 2 , an explanation will be given of the internal configuration of the optical unit  100  in the following. 
     As described above, the optical unit  100  is provided with an image projection unit  210 , an intermediate mirror  350 , an intermediate image formation unit  360 , and a projection mirror  400  on the inside of a housing  110 . The image projection unit  210  is provided with three different types of light sources each generating red light, green light, or blue light. The details will follow. The light sources can be realized using light emitting diodes (LED) or semiconductor laser light sources. In the present embodiment, a case where LEDs are used as the light sources will be explained. 
     The light sources generate heat during use. Therefore, the optical unit  100  is provided with a heat sink for cooling the light sources. There are three types of light sources. Thus, in order to cool these light sources, the optical unit  100  is provided with a heat sink  120   a  that is connected to a red light source, a heat sink  120   b  (not shown) that is connected to a green light source, and a heat sink  120   c  that is connected to a blue light source on the outside of the housing  110 . 
     The housing  110  is a die case made of aluminum. The heat sink  120   b  and the heat sink  120   c  for cooling the blue light source and the green light source, respectively, are formed integrally with the housing  110 . On the other hand, the heat sink  120   a  for cooling the red light source is installed at a place that is spatially apart from the heat sink  120   b  and the heat sink  120   c  and is externally attached separately from the housing  110 . Therefore, heat generated by the red light source is transferred to the heat sink  120   a  via a heat pipe  25 . 
     An explanation will now be given regarding the optical system of the head-up display  10  with reference to  FIG. 3  and  FIG. 4 .  FIG. 3  is a diagram schematically illustrating the internal configuration of the image projection unit  210  along with the optical path of the image display light.  FIG. 4  is a diagram illustrating the optical path of the image display light that is projected on the windshield  610  via the intermediate mirror  350 , the intermediate image formation unit  360 , and the projection mirror  400 . 
     With reference to  FIG. 3 , an explanation will be given of the internal configuration of the image projection unit  210 . The image projection unit  210  is provided with illumination unit  230   a ,  230   b , and  230   c  (hereinafter, also referred to as illumination units  230  generically), a dichroic cross prism  244 , a reflection mirror  236 , a field lens  237 , a polarization beam splitter  238 , a retardation plate  239 , an analyzer  241 , and a projection lens group  242 . In  FIG. 3 , the descriptions regarding the internal configuration of the first illumination unit  230   a  and the internal configuration of the third illumination unit  230   c  are omitted, and only the internal configuration of the second illumination unit  230   b  is shown. However, the illumination units  230  have the same configuration. 
     The illumination units  230  are each provided with a light source  231 , a collimate lens  232 , an ultraviolet-infrared ray (UV-IR) cut filter  233 , a polarizer  234 , and a fly-eye lens  235 . The light source  231  consists of a light-emitting diode that emits light of any one of a red color, a green color, and a blue color. The first illumination unit  230   a  has a light-emitting diode that emits red light as a light source. The second illumination unit  230   b  has a light-emitting diode that emits green light as the light source  231 . The third illumination unit  230   c  has a light-emitting diode that emits blue light as a light source. 
     The light source  231  is attached to a light-source attachment portion  243 . The light-source attachment portion  243  is combined thermally with a heat sink (not shown) and releases heat that is generated along with the emission of light by the light source  231 . Light emitted by the light source  231  is changed to parallel light by the collimate lens  232 . The UV-IR cut filter  233  absorbs and removes ultraviolet light and infrared light from the parallel light passed through the collimate lens  232 . The polarizer  234  changes light that has passed through the UV-IR cut filter  233  to P-polarized light without disturbance. The fly-eye lens  235  then adjusts the brightness of light that has passed through the polarizer  234  to be uniform. 
     Light that has passed through respective fly-eye lenses  235  of the illumination units  230  enter the dichroic cross prism  244  from different directions. Red light, green light, and blue light that have entered the dichroic cross prism  244  become white light in which the three colors are combined and travel to the reflection mirror  236 . The reflection mirror  236  changes the optical path of white light that has been synthesized by the dichroic cross prism  244  by 90 degrees. Light reflected by the reflection mirror  236  is collected by the field lens  237 . The light collected by the field lens  237  is radiated to the image display element  240  via the polarization beam splitter  238  and the retardation plate  239 , which transmit P-polarized light. 
     The image display element  240  is provided with a color filter of a red color, a green color, or a blue color for each pixel. The light radiated to the image display element  240  is changed to a color that corresponds to each pixel and modulated by a liquid crystal composition provided on the image display element  240 . The light then becomes S-polarized image display light and emitted toward the polarization beam splitter  238 . The emitted S-polarized light is reflected by the polarization beam splitter  238  and enters the projection lens group  242  after changing the optical path and passing through the analyzer  241 . The image display light transmitted through the projection lens group  242  exits the image projection unit  210  and enters the intermediate mirror  350 . 
     With reference to  FIG. 4 , an explanation will be given regarding the optical path of the image display light that is projected on the windshield  610  via the intermediate image formation unit  360  and the projection mirror  400  from the intermediate mirror  350 . The optical path of the image display light emitted from the projection lens group  242  of the image projection unit  210  is changed to an optical path that is traveling to the projection mirror  400  by the intermediate mirror  350 . In the meantime, a real image based on the image display light reflected by the intermediate mirror  350  is formed in the intermediate image formation unit  360 . 
     The intermediate image formation unit  360  has a diffusion screen  362  and a concave lens  364 . The diffusion screen  362  controls a light distribution angle ψ of the image display light traveling to the projection mirror  400  as well as forming a real image based on the image display light passing through the intermediate image formation unit  360 . The concave lens  364  controls the direction of a principal ray of the image display light traveling to the projection mirror  400  and adjusts an angle θ formed by image display light before passing through the intermediate image formation unit  360  and image display light after passing through the intermediate image formation unit  360 . 
     The image display light transmitted through the intermediate image formation unit  360  is reflected by the projection mirror  400  and projected on the windshield  610 . The optical path of the image display light projected on the windshield  610  is changed to be directed toward the user by the windshield  610 . Thereby, as described above, the user is able to visually recognize a virtual image based on the image display light in the forward direction via the windshield  610 . Therefore, the windshield  610  functions as a virtual image presenting surface. 
     A configuration such as the one described above allows for the user to visually recognize a virtual image, which is based on an image signal output from the control device  50 , over the real landscape via the windshield  610 . 
     With reference to  FIG. 5  and  FIG. 6 , functions of the intermediate image formation unit  360  according to the present embodiment will be described in detail.  FIG. 5  is a diagram illustrating optical paths of image display light when a virtual image  450  is presented for viewpoints E and E 2  of different height levels.  FIG. 6  is a diagram illustrating image display light that is distributed by the intermediate image formation unit  360  and shows, in an enlarged view, optical paths between the intermediate image formation unit  360  and the projection mirror  400  that are shown in  FIG. 5 . 
     As shown in  FIG. 5 , the viewpoints E 1  and E 2  of the user, who is the driver, change in the vertical direction depending on the height or the seating position of the driver. Even when a viewpoint of the user changes, the entirety of the virtual image  450  from an upper end portion  451  to a lower end portion  452  can be visually recognized preferably. Further, instead of presenting the virtual image  450  right in front of a line-of-sight direction C 1  or C 2  in which the user looks in the forward direction of the vehicle, presenting the virtual image  450  at a position that is shifted in the vertical direction allows the user to refer to the virtual image  450  by slightly shifting the direction of the line of sight when necessary, thus ensuring the user-friendliness. 
     In the present embodiment, by combining the diffusion screen  362  and the concave lens  364  as the intermediate image formation unit  360 , the direction of a principal ray and the light distribution angle of the image display light that has passed through the intermediate image formation unit  360  are controlled, and the visibility of the virtual image  450  is increased. In particular, by providing the concave lens  364  eccentrically in the vertical direction, the presentation position of the virtual image  450  can be shifted in the vertical direction, and the virtual image  450  can be presented at an easily viewable position. In the present embodiment, a configuration is shown for a case where the virtual image  450  is presented downward with respect to the line-of-sight directions C 1  and C 2 . However, by changing the state of eccentricity of the concave lens  364 , the virtual image  450  may be presented at a different position. 
     First, differences in a path of image display light according to differences between the viewpoint E 1  and the viewpoint E 2  are described in detail with reference to  FIG. 5 . The first viewpoint E 1  is an upper limit position that allows the entirety of the virtual image  450  to be visually recognized, and the second viewpoint E 2  is a lower limit position that allows the entirety of the virtual image  450  to be visually recognized. Therefore, the user is able to visually recognize the entirety of the virtual image  450  as long as the user&#39;s viewpoint is in a range between the first viewpoint E 1  and the second viewpoint E 2 . 
     In  FIG. 5 , light A 1  and light A 2  that are shown by solid lines represent light rays for presenting the user the upper end portion  451  of the virtual image  450 , and light that is emitted from an upper end portion  371  of a real image  370  formed in the intermediate image formation unit  360  is reflected on the projection mirror  400  and the windshield  610  and reaches the user&#39;s viewpoints E 1  and E 2 . The light A 1  that is traveling to the first viewpoint E 1  is reflected at a first reflection position  401  of the projection mirror  400 , and the light A 2  that is traveling to the second viewpoint E 2  is reflected at a second reflection position  402  of the projection mirror  400 . In an optical system shown in the present embodiment, a configuration is employed where image display light is reflected on the projection mirror  400  and the windshield  610 . Thus, a real image that is vertically flipped is formed in the intermediate image formation unit  360 . 
     On the other hand, light B 1  and light B 2  that are shown by broken lines represent light rays for presenting the user the lower end portion  452  of the virtual image  450 , and light that is emitted from a lower end portion  372  of the real image  370  formed in the intermediate image formation unit  360  is reflected on the projection mirror  400  and the windshield  610  and reaches the viewpoints E 1  and E 2 . The light B 1  that is traveling to the first viewpoint E 1  is reflected at a third reflection position  403  of the projection mirror  400 , and the light B 2  that is traveling to the second viewpoint E 2  is reflected at a fourth reflection position  404  of the projection mirror  400 . 
     Then, image display light that is distributed in the vertical direction by the intermediate image formation unit  360  will be described in detail with reference to  FIG. 6 .  FIG. 6  shows, in an enlarged view, the optical paths between the intermediate image formation unit  360  and the projection mirror  400  that are shown in  FIG. 5 . Light A that forms an image as the upper end portion  371  of the real image  370  enters the concave lens  364 , changes the direction in the upward direction (y direction) by an angle θ 1 , and becomes transmitted based on a direction that is perpendicular to the diffusion screen  362 . Then, the light A forms an image as a real image and becomes diffused on the diffusion screen  362  and travels to the projection mirror  400  as image display light having a light distribution angle ψ 1 . As a result, the light A that enters the intermediate image formation unit  360  becomes image display light that is distributed between light A 1  traveling to the first reflection position  401  and light A 2  traveling to the second reflection position  402 , centering around a principal ray A 0 . 
     Similarly, light B that forms an image as the lower end portion  372  of the real image  370  enters the concave lens  364 , changes the direction in the upward direction (y direction) by an angle θ 2 , and becomes transmitted. Then, the light A forms an image as a real image and becomes diffused on the diffusion screen  362  and travels to the projection mirror  400  as image display light having a light distribution angle ψ 2 . As a result, the light B that enters the intermediate image formation unit  360  becomes image display light that is distributed between light B 1  traveling to the third reflection position  403  and light B 2  traveling to the fourth reflection position  404 , centering around a principal ray B 0 . 
     The concave lens  364  according to the present embodiment is provided eccentrically in the vertical direction (the vertical direction in  FIG. 4 ) based on the z direction. More specifically, the position of an optical axis of the concave lens  364  is located below the center position of the diffusion screen  362 . Therefore, the angle θ 2  of the principal ray B 0  emitted from the lower end portion  372 , which is far away from the optical axis of the concave lens  364 , is larger than the angle θ 1  of the principal ray A 0  emitted from the upper end portion  371 , which is close to the optical axis of the concave lens  364 . The concave lens  364  according to the present embodiment is formed such that the optical axis of the concave lens  364  is not included in a concave surface thereof. Thus, the principal rays A 0  and B 0  are both emitted in a tilted manner toward the upward direction (y direction). 
     Then, with reference to  FIG. 7 , a description will be made in detail regarding the diffusion screen  362  in the present embodiment.  FIG. 7  is a diagram illustrating the configuration of the diffusion screen  362 . The diffusion screen  362 , which is a transmission-type screen, is provided with two light diffusion plates  363   a  and  363   b . The two light diffusion plates  363   a  and  363   b  are layered in such a direction that light diffusion surfaces  367   a  and  367   b  on which diffusion beads  369   a  and  369   b  are respectively provided face each other. In the diffusion screen  362 , the degree of difference is reduced between an incident angle θ in  of light entering the diffusion screen  362  and an emission angle θ out  of a principal ray of diffused light emitted from the diffusion screen  362  by arranging the light diffusion surfaces  367   a  and  367   b  to face each other. Thereby, the diffusion screen  362  properly adjusts the emission angle θ out  of the principal ray of the diffused light and presents an image that is highly visible to the user. 
     The diffusion screen  362  is provided with a first light diffusion plate  363   a  and a second light diffusion plate  363   b . The first light diffusion plate  363   a  has a first base member  366   a  and a plurality of first diffusion beads  369   a . The first base member  366   a  has a first light diffusion surface  367   a  and a first flat surface  368   a  that face each other. Similarly, the second light diffusion plate  363   b  has a second base member  366   b  and a plurality of second diffusion beads  369   b , and the second base member  366   b  has a second light diffusion surface  367   b  and a second flat surface  368   b  that face each other. 
     The first base member  366   a  and the second base member  366   b  (hereinafter, also referred to as base members  366 , generically) are flat plates formed of transparent resin materials or the like. Flexible transparent films may be used as the base members  366 . The first diffusion beads  369   a  and the second diffusion beads  369   b  (hereinafter, also referred to as diffusion beads  369 , generically) are highly-transparent optical beads, and the diameter thereof is 10 micrometers or less. The diffusion beads  369  are applied on the first light diffusion surface  367   a  and the second light diffusion surface  367   b  (hereinafter, also referred to as light diffusion surfaces  367 , generically) in a thickness of 10 to 15 micrometers. 
     In the present embodiment, light diffusion plates  363  having the same light diffusion capability are used as the first light diffusion plate  363   a  and the second light diffusion plate  363   b . Therefore, the first light diffusion plate  363   a  and the second light diffusion plate  363   b  that are provided to face each other diffuse light in the same light distribution. Having an identical light distribution means having optical properties that allow the intensity distribution of transmitted light to be almost identical when light having a specific intensity distribution becomes incident. 
     Then, with reference to  FIGS. 8A and 8B  and  FIG. 9 , an explanation will be given regarding the optical properties of the light diffusion plates  363  forming the diffusion screen  362 .  FIGS. 8A and 8B  are diagrams illustrating the relationship between an incident angle θ in  of light incident on a light diffusion plate  363  and emission angles θ out1  and θ out2  of light passing through the light diffusion plate  363 .  FIG. 8A  shows a case where light becomes incident on a light diffusion surface  367 , which is a bead surface, and  FIG. 8B  shows a case where light becomes incident on a flat surface  368 , which is not a bead surface. When letting light to enter through the light diffusion surface  367 , which is a bead surface, the angle of a principal ray changes to an emission angle θ out1 , which is larger by Δθ 1  with respect to the incident angle θ in . On the other hand, when letting light to enter through the flat surface  368 , which is not a bead surface, the angle of a principal ray changes to an emission angle θ out2 , which is smaller by Δθ 2  with respect to the incident angle θ in . This is because the way of change in angle is different in a case where the angle of light changes when entering a diffusion bead  369 , which forms a spherical surface, and in a case where the angle of light changes when being emitted from the diffusion bead  369 . 
       FIG. 9  is a graph illustrating the relationship between the angle of light incident on a light diffusion plate  363  and a deviation angle of a principal ray emitted from the light diffusion plate  363 . A straight line shown by (a) shows a deviation angle Δθ 1  occurring when light enters the light diffusion surface  367 , which is a bead surface, and corresponds to a case where light enters as shown in  FIG. 8A . On the other hand, a straight line shown by (b) shows a deviation angle Δθ 2  occurring when light enters through the flat surface  368 , which is not a bead surface, and corresponds to a case where light enters as shown in  FIG. 8B . In  FIG. 9 , with regard to whether a deviation angle Δθ is positive or negative, the deviation angle Δθ has a positive value when an emission angle θ out  is larger than an incident angle θ in  and has a negative value when the emission angle θ out  is smaller than the incident angle θ in , and a relationship, θ out =θ in +Δθ, is satisfied. 
     As shown in  FIG. 9 , the value of the deviation angle Δθ is found to become larger as the incident angle θ in  becomes larger in both the case of (a) where light enters the light diffusion surface  367  and the case of (b) where light enters the flat surface  368 . In the case of (a) where light enters the light diffusion surface  367  and in the case of (b) where light enters the flat surface  368 , the size of the deviation angle Δθ with respect to the incident angle θ in  is found to be almost the same although the positive or negative sign of the deviation angle Δθ is different. 
     According to optical properties shown in  FIG. 9 , if only one light diffusion plate  363  is used as a diffusion screen, the angle of a principal ray of emitted light changes when light enters the diffusion screen at an angle. In that case, even when the direction of the principal ray is properly controlled by the concave lens  364 , the direction of the principal ray changes due to the light diffusion plate  363 , which is used independently, and the visibility of image display light that is presented to the user may be lowered. 
     Meanwhile, in the diffusion screen  362  according to the present embodiment, the first light diffusion plate  363   a  and the second light diffusion plate  363   b , which exhibit the same light distribution, are combined such that the respective light diffusion surfaces  367   a  and  367   b  face each other. Therefore, even when a deviation angle occurs between an incident angle and an emission angle in each of the first light diffusion plate  363   a  and the second light diffusion plate  363   b , a deviation angle caused by the first light diffusion plate  363   a  can be corrected by a deviation angle caused by the second light diffusion plate  363   b . This is because, as shown in  FIG. 7 , light that enters the first flat surface  368   a  of the first light diffusion plate  363   a  is emitted from the light diffusion surface  367   a  at an emission angle that is smaller than its incident angle by a deviation angle Δθ, and the light that then directly enters the second light diffusion surface  367   b  of the second light diffusion plate  363   b  is emitted from the second flat surface  368   b  at an emission angle that is larger than its incident angle by the deviation angle Δθ. 
     A description will be given in the following regarding effects that are achieved by the intermediate image formation unit  360  in the present embodiment. 
     The intermediate image formation unit  360  in the present embodiment has a diffusion screen  362  that controls the light distribution angle of a principal ray such that image display light is realized that has predetermined light distribution angles ψ 1  and ψ 2  with respect to principal rays A 0  and B 0 , respectively. Therefore, a virtual image with a certain level of brightness can be presented even when the line-of-sight position is moved as long as the line-of-sight position is moved within a predetermined range. Also, by selecting, as the diffusion screen  362 , a diffusion screen having characteristics where light distribution angles ψ 1  and ψ 2  fall within a range from the first reflection position  401  to the second reflection position  402  of the projection mirror  400  or a range from the third reflection position  403  to the fourth reflection position  404 , the image display light can be utilized highly efficiently. If the light distribution angles are narrower than these reflection position ranges, the range of a viewpoint where the virtual image  450  is able to be presented in a bright manner becomes narrow. On the other hand, if the light distribution angles are wider than these reflection position ranges, the proportion of image display light that is not reflected by the projection mirror  400  increases, and the virtual image  450  presented to the user thus becomes dark. As described, by properly controlling the light distribution angles ψ 1  and ψ 2 , the virtual image  450  can be presented to the user in a bright manner with high efficiency, and the visibility of the virtual image  450  can be increased. 
     The intermediate image formation unit  360  has a concave lens  364  that controls the respective directions of the principal rays A 0  and B 0  that have passed through the intermediate image formation unit  360 . By providing the concave lens  364  as the intermediate image formation unit  360 , the virtual image  450  that is presented to the user can be further enlarged even when a distance D between the intermediate image formation unit  360  and the projection mirror  400  has to be shortened. Therefore, by providing the concave lens  364 , a larger virtual image  450  can be presented while the size of the optical unit  100  is reduced, and the visibility of the virtual image  450  can be increased. 
     In the intermediate image formation unit  360 , the concave lens  364  is provided eccentrically in the vertical direction. Thereby, instead of presenting the virtual image  450  right in front of the user&#39;s line-of-sight direction, the virtual image  450  can be presented at a position that is shifted slightly in the vertical direction. This is because an angular difference can be provided between light for presenting an upper end portion  451  of the virtual image  450  and light for presenting a lower end portion  452  of the virtual image  450 . By shifting the virtual image  450  in the vertical direction, the virtual image  450  can be presented at a position that can be easily viewed by the user, and the visibility of the virtual image  450  can be increased. By using a concave lens that is eccentrically provided in the vertical direction, the optical unit  100  can be further downsized. 
     In the intermediate image formation unit  360 , two light diffusion plates  363   a  and  363   b  on which light diffusion surfaces  367   a  and  367   b , which are bead surfaces, are layered respectively to face each other are used as the diffusion screen  362 . Thereby, even when light becomes incident on the diffusion screen  362  at an angle in order to present image display light with an angular difference to the user, changes in the direction of a principal ray caused before and after passing through the diffusion screen  362  can be reduced. Therefore, image display light with a maintained angular difference that is caused by the concave lens  364  can be presented to the user, and the visibility of a virtual image  450  can be increased. 
     Further, in the diffusion screen  362 , the respective light diffusion surfaces  367   a  and  367   b  of the two light diffusion plates  363   a  and  363   b  are layered to face each other. A configuration where the flat surfaces  368   a  and  368   b  are layered to face each other is a possible configuration of the diffusion screen  362 . In this case, a distance between the first light diffusion surface  367   a  and the second light diffusion surface  367   b  on which image display light forms an image is large. As a result, image display light forms an image on each of the first light diffusion surface  367   a  and the second light diffusion surface  367   b , resulting in the generation of a double image on the diffusion screen  362 ; thus, visibility to the user is lowered. In the present embodiment, by closely arranging the first light diffusion surface  367   a  and the second light diffusion surface  367   b , the generation of a double image can be prevented, and the visibility of a virtual image  450  can be increased. 
     The present invention has been described by referring to each of the above-described embodiments. However, the present invention is not limited to the above-described embodiments only, and those resulting from any combination of them as appropriate or substitution are also within the scope of the present invention. 
     In the above-described embodiment, a case is shown where a concave lens  364  is arranged in front of a diffusion screen  362  as an intermediate image formation unit  360 , i.e., a case is shown where image display light that pas passed through the concave lens  364  enters the diffusion screen  362 . As another exemplary variation, the diffusion screen  362  and the concave lens  364  may be arranged reversely. In this case, optical elements are arrayed in the order of an intermediate mirror  350 , a diffusion screen  362 , a concave lens  364 , and a projection mirror  400  between the intermediate mirror  350  and the projection mirror  400 . Even when the direction of the intermediate image formation unit  360  is reversed, a virtual image  450  with high visibility can be presented by controlling the light distribution angle of image display light by the diffusion screen  362  and by controlling the direction of a principal ray by the concave lens  364 . 
     In the above-described embodiment, the direction of a principal ray of image display light is controlled by using the concave lens  364  as the intermediate image formation unit  360 . In an exemplary variation, the intermediate image formation unit  360  may be provided with only the diffusion screen  362  without providing the concave lens  364 . In this exemplary variation, the direction of a principal ray of image display light is adjusted by a projection lens group  242  provided in an image projection unit  210 . Also in this case, changes in the direction of a principal ray before and after passing through the diffusion screen  362  can be suppressed. Thus, image display light with a maintained direction of a principal ray that is determined by the projection lens group  242  can be presented. This allows a highly-visible virtual image  450  to be presented. 
     In the above-described embodiment, a light diffusion plate that has a bead surface on which a plurality of diffusion beads  369  are provided is used as a light diffusion plate  363  that forms the diffusion screen  362 . Alternatively, a light diffusion plate on which diffusion beads are not used may be used in an exemplary variation. For example, while using light diffusion plates with respective light diffusion surfaces on which microlens arrays are formed, respective microlens arrays of two light diffusion plates are layered to face each other. Also in this case, while reducing changes in the direction of a principal ray before and after passing by providing the light diffusion surfaces such that the light diffusion surfaces face each other, the generation of a double image can be prevented. Thus, the visibility of a virtual image  450  can be increased. As another exemplary variation, a base member that is provided with light diffusivity by performing surface roughening on a light diffusion surface may be used as a light diffusion plate. 
     In the above-described embodiment, an explanation is made regarding a diffusion screen  362  that is used in an intermediate image formation unit  360  used for a head-up display. As an exemplary variation, the above-described diffusion screen  362  may be used as a screen for rear projection television. Since changes in the direction of a principal ray before and after passing through the diffusion screen  362  can be suppressed, a highly-visible image can be provided.