Patent Publication Number: US-2013250380-A1

Title: Virtual image display apparatus

Description:
INCORPORATION BY REFERENCE 
     This application claims priority to Japanese Patent Application No. 2012-069203 filed on Mar. 26, 2012, in Japan, which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to virtual image display apparatuses. 
     2. Related Art 
     Virtual image display apparatuses are well known that guide image forming light outputted from electro-optical devices, such as a liquid crystal (LC) device, an organic electro luminescence (EL) device and the like, by using virtual image optical systems so as to make the guided light viewed by viewers (for example, see JP-A-2009-300480). Virtual image display apparatuses are used as a head-mounted display (HMD), for example, which is a head-worn type display apparatus and is widely used these days. 
     The virtual image display apparatus described in JP-A-2009-300480 includes a reflection-type volume hologram that selectively diffracts and reflects light of a specified wavelength band in a virtual image optical system. In this virtual image display apparatus, light outputted from an electro-optical device (image forming unit) is diffracted and reflected by a first reflection-type volume hologram to enter a light guide plate, and the light totally reflected inside the light guide plate is diffracted and reflected by a second reflection-type volume hologram to reach the eye of a viewer. 
     However, a wavelength range of the light that is diffracted and reflected by a reflection-type volume hologram is narrower with respect to the light having wavelength bands of red, green, blue and so on that is outputted by an electro-optical device. Accordingly, of image forming light outputted by the electro-optical device, light within a diffraction spectrum wavelength range of the reflection-type volume hologram reaches the eye of a viewer, but light outside of the diffraction spectrum wavelength range of the reflection-type volume hologram passes through the reflection-type volume hologram and does not reach the eye of the viewer. 
     As described above, with a virtual image display apparatus using a reflection-type volume hologram, because only part of the image forming light outputted from an electro-optical device is used for visual recognition by a viewer, an image (virtual image) viewed by the viewer is lower in luminance and visibility in comparison with those of an original image produced in the electro-optical device. For this reason, there has been a problem that the visibility of a virtual image viewed by the viewer is extremely reduced particularly in a case of a virtual image display apparatus such as a see-through type HMD in which an outside scene is transmitted and viewed by the viewer as a background. 
     SUMMARY 
     An advantage of some aspects of the invention is to solve at least part of the above problem, and the invention can be embodied in the following embodiments and application examples. 
     Application Example 1 
     A virtual image display apparatus according to application example 1 includes an organic EL device that outputs light of at least N kinds (N is an integer equal to or greater than 1) of wavelength bands, a light guide member, and a reflection-type volume hologram that is provided on a first face of the light guide member and diffracts and reflects light of a predetermined wavelength band of the light having entered. The organic EL device includes an optical resonance structure that causes light of each of the N kinds of wavelength bands to resonate. 
     According to this configuration, since the organic EL device included in the virtual image display apparatus has an optical resonance structure that causes light of each of N kinds of wavelength bands to resonate from among the light of at least N kinds of wavelength bands outputted from the organic EL device, this virtual image display apparatus outputs light with a spectrum having a stronger peak intensity and a narrower width in comparison with a case where an organic EL device without the optical resonance structure or a liquid crystal device is used. Accordingly, light intensity of light that enters the reflection-type volume hologram from the organic EL device equipped with the optical resonance structure is stronger in comparison with a case where an organic EL device without the optical resonance structure or a liquid crystal device is used. With this, luminance of the image (virtual image) viewed by the viewer is increased and visibility of the image can be enhanced in the virtual image display apparatus. 
     Application Example 2 
     In the virtual image display apparatus according to the above application example, it is preferable for the light of the N kinds of wavelength bands outputted by the organic EL device to be light that has not passed through a color filter. 
     According to this configuration, because, of the light outputted by the organic EL device, light other than the light of a predetermined wavelength band to be diffracted and reflected by the reflection-type volume hologram, is not used for displaying a virtual image, light other than the light of the predetermined wavelength band needed for displaying the virtual image, is substantially cut off even if the organic EL device is not equipped with a color filter. With this, since light outputted by the organic EL device can be used without the light passing through a color filter, luminance of the virtual image can be further enhanced. In addition, the organic EL device can be made thinner because color filters are not needed. 
     Application Example 3 
     In the virtual image display apparatus according to the above application examples, it is preferable for the reflection-type volume hologram to include a first reflection-type volume hologram into which light guided inside the light guide member enters, and which diffracts and reflects light of the predetermined wavelength band from among the light having entered and makes the diffracted and reflected light be outputted from the light guide member. 
     According to this configuration, the first reflection-type volume hologram that diffracts and reflects light of the predetermined wavelength band from among the light guided inside the light guide member, and makes the diffracted and reflected light be outputted toward a viewer, is included in the configuration, thereby making it possible to provide a virtual image display apparatus capable of giving an excellent visibility. 
     Application Example 4 
     In the virtual image display apparatus according to the above application examples, it is preferable for the reflection-type volume hologram to include a second reflection-type volume hologram into which light having been outputted from the organic EL device enters, and which diffracts and reflects light of the predetermined wavelength band from among the light having entered, and guides the diffracted and reflected light inside the light guide member. 
     According to this configuration, the second reflection-type volume hologram that diffracts and reflects light of a predetermined wavelength band from among the light outputted from the organic EL device so as to guide the diffracted and reflected light inside the light guide member, is included in the configuration, thereby making it possible to provide a virtual image display apparatus capable of giving an excellent visibility. 
     Application Example 5 
     In the virtual image display apparatus according to the above application examples, it is preferable for the light of N kinds of wavelength bands outputted by the organic EL device to include light of a red wavelength band, light of a green wavelength band and light of a blue wavelength band. 
     According to this configuration, light that is outputted by the organic EL device included in the virtual image display apparatus, includes light of the red wavelength band, light of the green wavelength band and light of the blue wavelength band, and utilization efficiency of light of each of these wavelength bands is enhanced, thereby making it possible for the virtual image display apparatus to display a full-color virtual image with a higher luminance. 
     Application Example 6 
     In the virtual image display apparatus according to the above application examples, it is preferable for the light of the predetermined wavelength band that is diffracted and reflected by the reflection-type volume hologram to correspond to a wavelength band that is caused to resonate in the optical resonance structure. 
     According to this configuration, because the light of a predetermined wavelength band that is diffracted and reflected by the reflection-type volume hologram corresponds to a wavelength band that is caused to resonate in the optical resonance structure, the amount of light that is diffracted and reflected to reach the eye of a viewer is increased whereas the amount of light that is not diffracted and reflected but passes through is reduced; in other words, utilization efficiency of light in the virtual image display apparatus is enhanced. This makes it possible to further raise the luminance of an image viewed by the viewer and enhance the visibility thereof in the virtual image display apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a schematic diagram illustrating a general configuration of a virtual image display apparatus according to a first embodiment of the invention. 
         FIG. 2  is an equivalent circuit diagram illustrating an electric configuration of an organic EL device according to the first embodiment. 
         FIG. 3  is a schematic plan view illustrating the configuration of the organic EL device according to the first embodiment. 
         FIG. 4  is a schematic cross-sectional view illustrating the organic EL device according to the first embodiment. 
         FIGS. 5A through 5C  are diagrams for explaining utilization efficiencies of light given by a reflection-type volume hologram. 
         FIG. 6  is a schematic cross-sectional view illustrating an organic EL device according to a second embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, embodiments in which the invention is embodied will be described with reference to the drawings. The drawings used in the following explanation are appropriately enlarged or reduced so that portions of the drawings to be mentioned can be easily recognized. Note that constituent elements other than those needed in the explanation may be omitted in the drawings in some case. 
     It is to be noted that, in the following descriptions of the embodiments, in the case where, for example, an expression “on a substrate” is given in the description, the expression can have the following meanings; that is, something is “placed in contact with the surface of a substrate”, something is “placed with another something therebetween”, or “a part of something is placed in contact with the surface of a substrate while the other part of it is placed with another something therebetween”. 
     First Embodiment 
     Virtual Image Display Apparatus 
     First, a virtual image display apparatus according to a first embodiment of the invention will be described with reference to the drawings.  FIG. 1  is a schematic diagram illustrating a general configuration of the virtual image display apparatus according to the first embodiment. The virtual image display apparatus according to the first embodiment is a head-mounted display (HMD) that is worn on the head of a viewer and displays an image (virtual image), and in which an organic EL device as an electro-optical device that outputs image forming light, which is light to form an image, is provided. 
     As shown in  FIG. 1 , a virtual image display apparatus  100  includes an organic EL device  1 , a collimator  110 , a light guide member  120 , a reflection-type volume hologram  132  as the first reflection-type volume hologram, and a reflection-type volume hologram  130  as the second reflection-type volume hologram. 
     The organic EL device  1  outputs light of at least N kinds of wavelength bands (N is an integer equal to or greater than 1). The light of N kinds of wavelength bands includes, for example, light of a red (R) wavelength band, light of a green (G) wavelength band, and light of a blue (B) wavelength band. The organic EL device  1  is capable of forming a full-color image with the light of these R, G and B wavelength bands. Further, the organic EL device  1  is equipped with an optical resonance structure that causes the light of R, G and B wavelength bands to resonate respectively. The configuration of the organic EL device  1  will be described in detail later. 
     The collimator  110  is interposed between the organic EL device  1  and the light guide member  120 . The collimator  110  has a function to convert the light of R, G and B wavelength bands to collimated beams of light. The collimator  110  is configured of a collimator lens or the like. The light of R, G and B wavelength bands, which has been converted to the collimated beams of light by the collimator  110 , enters the light guide member  120 . 
     The light guide member  120  has a function that totally reflects the collimated beams of light of R, G and B wavelength bands entering via the collimator  110  and guides them within the guide member. The light guide member  120  is constituted with a material that is formed in a predetermined shape; the material in this case is, for example, a resin having an excellent characteristic of transparency such as an acrylic resin, polycarbonate resin, polystyrene resin or the like, or glass. 
     The light guide member  120  extends from one end  120   a  to the other end  120   b  in a direction that intersects with a direction of the light entering via the collimator  110 , and is formed in a thin plate shape, in which a first optical face  121  as a first face arranged on the collimator  110  side and a second optical face  122  opposed to the first optical face  121  are provided as the main faces. In the first optical face  121  of the light guide member  120 , a light incidence port through which light enters is provided on the side of the one end  120   a , while on the side of the other end  120   b , a light output port through which light is outputted is provided. 
     On the second optical face  122  of the light guide member  120 , a reflection-type volume hologram  132  is provided at a position opposed to the light incidence port on the side of the one end  120   a , and a reflection-type volume hologram  130  is provided at a position opposed to the light output port on the side of the other end  120   b.    
     In order for the collimated beams of light of R, G and B wavelength bands that enter via the collimator  110  to be totally reflected inside the light guide member  120 , the reflection-type volume hologram  132  diffracts and reflects the collimated beams of light of a predetermined wavelength band of each color wavelength band. The reflection-type volume hologram  130  diffracts and reflects the collimated beams of light of a predetermined wavelength band of each color wavelength band toward the eye of a viewer  200  from among the light of R, G and B wavelength bands having been totally reflected and guided inside the light wave guide member  120 . With this, an image (virtual image) formed by the image forming light outputted from the organic EL device  1  can be viewed by the viewer. 
     The reflection-type volume holograms  130  and  132  have a diffraction structure including interference fringes corresponding to each of the N kinds of wavelength bands. The reflection-type volume holograms  130  and  132  according to this embodiment have a diffraction structure corresponding to each of the R, G and B wavelength bands that is made to resonate in the optical resonance structure of the organic EL device  1 , and selectively diffract and reflect the light of R, G and B wavelength bands. Note that the half width of the light that is diffracted and reflected by the reflection-type volume holograms  130 ,  132  is smaller than that of the light that is outputted from the organic EL device  1 ; details of this will be given later. 
     As the reflection-type volume holograms  130  and  132 , a known structure can be used. The reflection-type volume holograms  130 ,  132  may have a structure in which interference fringes corresponding to each of the R, G and B wavelength bands are laminated in three layers, or may have a structure in which interference fringes corresponding to each of the R, G and B wavelength bands are formed being overlapped with each other in the same layer. 
     Organic EL Device 
     Next, a configuration of the organic EL device according to the first embodiment will be described with reference to the drawings.  FIG. 2  is an equivalent circuit diagram illustrating an electric configuration of the organic EL device according to the first embodiment.  FIG. 3  is a schematic plan view illustrating the configuration of the organic EL device according to the first embodiment.  FIG. 4  is a schematic cross-sectional view illustrating the organic EL device according to the first embodiment. 
     As shown in  FIG. 2 , an organic EL device  1  is an active-matrix type organic EL device using thin film transistors (hereinafter, called TFTs) as switching elements. The organic EL device  1  includes a substrate  10 , scanning lines  16  disposed on the substrate  10 , signal lines  17  extending in a direction that intersects with the scanning lines  16 , and power lines  18  extending in parallel to the signal lines  17 . 
     A data line driving circuit  14  having a shift register, a level shifter, a video line, and an analog switch is connected with the signal lines  17 . Meanwhile, with the scanning lines  16 , a scanning line driving circuit  15  having a shift register and a level shifter is connected. 
     Regions of sub-pixels  2  are defined by the scanning lines  16  and the signal lines  17 . The sub-pixels  2  are a minimum display unit of the organic EL device  1 , and arranged in matrix form along an extension direction of the scanning lines  16  and an extension direction of the signal lines  17 , for example. Each of the sub-pixels  2  includes a switching TFT  11 , a driving TFT 12 , a retention capacitor  13 , an anode  24 , a cathode  32  and an organic function layer  30 . 
     The organic function layer  30  is configured of a hole transport layer, a light emitting layer and an electron transport layer, which are laminated in series, for example. The anode  24 , the cathode  32  and the organic function layer  30  constitute an organic electro luminescence element (organic EL element)  8 . The organic EL element  8  emits light through recombining holes injected from the hole transport layer and electrons injected from the electron transport layer in the light emitting layer. 
     In the organic EL device  1 , when the scanning line  16  is driven and the switching TFT  11  is turned on, an image signal supplied through the signal line  17  is retained by the retention capacitor  13 , and a conductive state between the source and the drain of the driving TFT  12  is determined in accordance with the state of the retention capacitor  13 . Upon being electrically connected with the power line  18  through the driving TFT  12 , a driving electric current flows from the power line  18  to the anode  24 , and further flows to the cathode  32  through the organic function layer  30 . 
     The amount of the driving electric current depends upon the conductive state of the source and the drain of the driving TFT 12 . The light emitting layer of the organic function layer  30  emits light with luminance in proportion to the amount of the electric current that flows between the anode  24  and the cathode  32 . In other words, in the case where a light emitting state of the organic EL element  8  is controlled by the driving TFT  12 , one of the source and the drain of the driving TFT  12  is electrically connected with the power line  18 , and the other one of the source and the drain of the driving TFT  12  is electrically connected with the organic EL element  8 . 
     As shown in  FIG. 3 , the organic EL device  1  has a light emitting area  4  formed in an approximately rectangular planar shape on the substrate  10 . The light emitting area  4  is an area that substantially contributes to the light emission in the organic EL device  1 . The organic EL device  1  may have a dummy area that does not substantially contribute to the light emission in the periphery of the light emitting area  4 . The sub-pixels  2  are arranged in matrix form in the light emitting area  4 . The sub-pixel  2  has an approximately rectangular planar shape, for example. The four corners of the rectangle-shaped sub-pixel  2  may be formed in a round shape. 
     The organic EL device  1  according to this embodiment includes sub-pixels  2 R that output light of a red (R) wavelength band, sub-pixels  2 G that output light of a green (G) wavelength band, and sub-pixels  2 B that output light of a blue (B) wavelength band (hereinafter, also referred to simply as “sub-pixels  2 ” if corresponding colors are not needed to be distinguished). Organic EL elements  8 R,  8 G and  8 B are provided corresponding to the sub-pixels  2 R,  2 G and  2 B, respectively (hereinafter, also referred to simply as “organic EL elements  8 ”, like in the sub-pixels  2 , if corresponding colors are not needed to be distinguished). 
     In the periphery of the light emitting area  4 , the two scanning line driving circuits  15  and an inspection circuit  19  are disposed. The inspection circuit  19  is a circuit to inspect an operational status of the organic EL device  1 . Cathode wiring  33  is disposed on the circumference of the substrate  10 . Further, a flexible substrate  20  is provided at one side of the substrate  10 . The flexible substrate  20  includes a driving IC  21  connected with each wiring. 
     In the organic EL device  1  according to this embodiment, a basic unit in forming an image is configured of a group of the sub-pixels  2 R,  2 G and  2 B; by appropriately changing the luminance of each of the sub-pixels  2 R,  2 G and  2 B at each basic unit, various kinds of colors of light can be outputted. Through this, the organic EL device  1  can display a full-color image or emit full-color light. 
     As shown in  FIG. 4 , the organic EL device  1  includes a reflection layer  22 , a protection layer  26 , the anodes  24 , partition walls  28 , the organic function layers  30 , the cathode  32 , a sealing layer  44 , and a color filter substrate  40  on the substrate  10 . The organic EL device  1  is a top-emission type device in which the light emitted from the organic function layer  30  is outputted to the side of the color filter substrate  40 . 
     It is to be noted that, in this specification, the side of the color filter substrate  40  of the organic EL device  1  in  FIG. 4  is called an “upper side”. Further, in this specification, it is called “to view from above” to view the drawing from the direction of a normal line to the surface on the color filter substrate  40  side of the organic EL device  1 . 
     Since the organic EL device  1  is a top-emission type, the substrate  10  may employ any of a transparent material and a nontransparent material for its base material. As the transparent material, glass, quartz, resin (plastic, plastic film) and the like can be cited, for example. As the nontransparent material, ceramics such as alumina, material made by performing insulating processing such as surface oxidation on a metal sheet of such as stainless steel, a thermosetting resin, a thermoplastic resin, films of these resins (plastic films) and the like can be cited. 
     Although omitted in  FIG. 4 , the driving TFT  12  (see  FIG. 2 ) including a semiconductor film, a gate insulating layer, a gate electrode, a drain electrode and a source electrode is provided for each sun-pixel  2  ( 2 R,  2 G or  2 B) on the substrate  10 . The substrate  10  may be covered with an insulating layer, a planarizing layer or the like made of silicon dioxide (SiO 2 ) or the like, for example. 
     The reflection layer  22  is provided on the substrate  10 . The reflection layer  22  is formed with a light reflective material such as aluminum, silver, or alloy whose major elements are aluminum, silver and the like, for example. 
     The protection layer  26  is provided so as to cover the substrate  10  and the reflection layer  22 . The upper surface of the protection layer  26  is planarized. The protection layer  26  is formed with, for example, an inorganic insulating film such as silicon dioxide (SiO 2 ), silicon nitride (SiN), nitric oxide silicon (SiON) or the like. The protection layer  26  may be formed with organic resin such as an acrylic resin, a polyimide resin or the like. 
     The anodes  24  ( 24 R,  24 G,  24 B) are provided on the protection layer  26 . The anodes  24 R,  24 G and  24  are disposed so as to correspond to the sub-pixels  2 R,  2 G and  2 B, respectively. Layer thicknesses of the anodes  24 R,  24 G and  24  are different from each other in order to adjust an optical distance (light path length) of the optical resonance structure to be explained later, and are set thicker in the order from the anode  24 B, anode  24 G and to anode  24 R. The anode  24  is formed with transparent conductive material such as indium tin oxide (ITO), ZnO 2  or the like, for example. 
     The partition walls  28  are provided on the protection layer  26 . The partition wall  28  has an opening  28   a  to define a region of the sub-pixel  2 . The opening  28   a  is formed one size smaller than the anode  24  when viewed from above. The partition wall  28  is formed along the periphery of the opening  28   a  and overlies the peripheral border of the anode  24  by a predetermined width. The partition wall  28  is formed with an acrylic resin or the like. 
     The organic function layers  30  ( 30 R,  30 G,  30 B) are formed on the anodes  24  and arranged within the opening  28   a  of the partition wall  28 . The organic EL device  1  according to this embodiment includes an organic function layer  30 R that emits light of the red (R) wavelength band, an organic function  30 G that emits light of the green (G) wavelength band, and an organic function layer  30 B that emits light of the blue (B) wavelength band as the organic function layers  30 . In other words, the organic function layers  30 R,  30 G, and  30 B are formed by applying materials thereto that respectively emit light of R, G and B colors, corresponding to the sub-pixels  2 R,  2 G and  2 B. 
     The organic function layers  30 R,  30 G and  30 B are each configured of, for example, a hole transport layer, a light emitting layer and an electron transport layer. In the organic function layers  30 R,  30 G and  30 B, light of different wavelength bands of R, G and B can be obtained by recombining holes injected from the hole transport layer and electrons injected from the electron transport layer in the light emitting layer. These layers constituting the organic function layers  30 R,  30 G and  30 B can be formed using known materials. 
     The cathode  32  is provided so as to cover the partition walls  28  and the organic function layers  30 . The cathode  32  is continuously formed across the plural sub-pixels  2  (organic EL elements  8 ). The cathode  32  functions as a semi-transmissive reflection layer having a property that transmits a part of light that has reached the surface thereof and reflects the other part of the light (that is, semi-transmissive reflectivity). The cathode  32  is formed with magnesium (Mg), silver (Ag), or an alloy the major elements of which are these metals, or the like. 
     The anodes  24  ( 24 R,  24 G,  24 B), the organic function layers  30  ( 30 R,  30 G,  30 B) and the cathode  32  configure the organic EL elements  8  ( 8 R,  8 G,  8 B). The organic EL elements  8 R,  8 G and  8 B are disposed corresponding to the sub-pixels  2 R,  2 G and  2 B. 
     Although not illustrated, a passivation layer is provided on the cathode  32 . The passivation layer is a protection film to prevent deterioration of the organic EL elements  8  caused by entering oxygen, moisture, or the like. The passivation layer is formed with, for example, an inorganic material with low gas transmittance such as SiO 2 , SiN, SiON or the like. 
     On the substrate  10  where the plurality of organic EL elements  8  ( 8 R,  8 G,  8 B) are formed, the color filter substrate  40  is disposed opposing to the substrate  10 . The color filter substrate  40  is configured with a transparent material such as glass. The color filters  42  ( 42 R,  42 G,  42 B) and a light blocking layer  43  are formed on a surface of the substrate  10  side of the color filter substrate  40 . 
     The organic EL device  1  includes, as the color filters  42 , a color filter  42 R corresponding to the red (R) wavelength band, a color filter  42 G corresponding to the green (G) wavelength band and a color filter  42 B corresponding to the blue (B) wavelength band. The color filters  42 R,  42 G and  42 B are respectively disposed corresponding to the sub-pixels  2 R,  2 G and  2 B, and arranged so as to overlap with the organic EL elements  8 R,  8 G and  8 B when viewed from above. The color filters  42 R,  42 G and  42 B selectively pass light of R, G and B wavelength bands from among the light outputted from the organic EL elements  8 R,  8 G and  8 B. 
     The light blocking layer  43  includes openings  43   a  corresponding to the organic EL elements  8 R,  8 G and  8 B, and defines the color filters  42 R,  42 G and  42 B by the openings  43   a.    
     The color filter substrate  40  in which the color filters  42 R,  42 G,  42 B and the light blocking layer  43  are formed is bonded to the substrate  10  via the sealing layer  44 . The sealing layer  44  is formed with a transparent resin, for example, a cured resin such as an epoxy resin or the like. 
     Optical Resonance Structure 
     Next, an optical resonance structure included in the organic EL device  1  according to this embodiment will be described. Optical resonators that cause the light emitted in the organic function layers  30  ( 30 R,  30 G,  30 B) to resonate, are formed between the reflection layer  22  and the cathode  32 . 
     At least part of light emitted in the organic function layers  30  ( 30 R,  30 G,  30 B) resonates guided by the optical resonator, and light of a resonant wavelength corresponding to an optical distance (light path length) of the optical resonator is enhanced. The resonance guided by the optical resonator is carried out while the light travelling back and forth between the reflection layer  22  and the cathode  32 . The light that has resonated in the resonator passes through the cathode  32  so as to be outputted to the upper side. Accordingly, it is possible to enhance the luminance of the light of R, G and B wavelength bands outputted from the organic EL device  1  and obtain the light having a narrower half width. 
     The resonant wavelength in the optical resonator can be adjusted by changing the optical distance between the reflection layer  22  and the cathode  32 . When an optical distance between the reflection layer  22  and the cathode  32  is referred to as “L”, and “λ,” is a peak wavelength of the spectrum of the light which is needed to be taken out from among the light emitted in the organic function layer  30 , the following relational expression holds. Note that (I) (radian) is a phase shift which takes place when light emitted in the organic function layer  30  reflects off at both ends of the optical resonator (for example, at the reflection layer  22  and the cathode  32 ). 
       (2 L )/λ+Φ/(2π)= m  ( m  is an integer)
 
     In the organic EL device  1 , in order for the resonant wavelength of each of the optical resonators to become the predetermined value λ corresponding to each of the light of R, G and B wavelength bands outputted by the sub-pixels  2 R,  2 G and  2 B, the optical distance L of the optical resonator is optimized by appropriately setting the layer thicknesses of the anodes  24 R,  24 G and  24 B. 
     Next, utilization efficiency of light given by the reflection-type volume holograms  130  and  132  of the virtual image display apparatus will be described with reference to the drawings.  FIGS. 5A through 5C  are diagrams for explaining utilization efficiencies of light given by the reflection-type volume hologram. 
     Specifically,  FIGS. 5A through 5C  compare and indicate utilization efficiencies of light given by the reflection-type volume hologram with regard to the light of the green (G) wavelength band, in which the configuration of each electro-optical device that outputs an image forming light is changed.  FIG. 5A  indicates a case in which an organic EL device including an optical resonance structure, like in the organic EL device  1  of this embodiment, is used as the electro-optical device.  FIG. 5B  indicates a case in which an organic EL device without an optical resonance structure is used as the electro-optical device.  FIG. 5C  indicates a case in which a liquid crystal device is used as the electro-optical device. 
     In each of  FIGS. 5A through 5C , the horizontal axis represents a wavelength (unit: nm); while the vertical axis represents diffraction efficiency of the reflection-type volume hologram and also represents spectrum intensity of the organic EL device or the liquid crystal device. Note that the same reflection-type volume hologram is used in  FIGS. 5A through 5C . 
     The reflection-type volume hologram, as described earlier, has a diffraction structure including an interference fringe corresponding to a predetermined wavelength band, and selectively diffracts and reflects light of the predetermined wavelength band while passing therethrough light other than the light of the predetermined wavelength band. Note that a diffraction spectrum of the reflection-type volume hologram is narrow in width, and in the examples of  FIGS. 5A through 5C , the half width thereof is around 15 nm, for example. In the virtual image display apparatus, light that is diffracted and reflected by the reflection-type volume hologram reaches the eye of a viewer, whereas light that passes through the reflection-type volume hologram does not reach the eye of the viewer and not used. 
     First, the case of using a liquid crystal device illustrated in  FIG. 5C  is described. In the liquid crystal device, light outputted from a light source is modulated in a liquid crystal layer, then light of a specified wavelength band (green in this case) having passed through a color filter is outputted. As shown in  FIG. 5C , the half width of light outputted from the liquid crystal device is wider, and is around five times the half width of a diffraction spectrum of the reflection-type volume hologram. Of the light outputted from this liquid crystal device, light that falls in a diffraction spectrum range of the reflection-type volume hologram (indicated by diagonal lines in  FIG. 5C ) is diffracted, reflected and used by the reflection-type volume hologram. Meanwhile, of the light outputted from the crystal liquid device, light outside of the diffraction spectrum range of the reflection-type volume hologram (indicated by dots in  FIG. 5C ) passes through the reflection-type volume hologram and is not used. 
     As described above, in the case where a liquid crystal device is used in the virtual image display apparatus, of the light outputted by the liquid crystal device, light that reaches the eye of a viewer is small in quantity, and light that does not reach the eye of the viewer is extremely large in quantity. Accordingly, the luminance of an image (virtual image) viewed by the viewer is lower in comparison with an original image produced in the liquid crystal device, and in turn the visibility thereof is extremely lowered. Therefore, in order to ensure an appropriate luminance of a virtual image which is viewed by the viewer, electric power to drive the light source of the liquid crystal device is needed to be larger. 
     As shown in  FIG. 5B , the half width of light outputted from the organic EL device without an optical resonance structure is narrower than that of the liquid crystal device, and is around three times the half width of the diffraction spectrum of the reflection-type volume hologram. Accordingly, with the organic EL device, because an amount of light that passes through the reflection-type volume hologram and is not used becomes less (indicated by dots in  FIG. 53 ) in comparison with the case of using the liquid crystal device, the utilization efficiency of light can be improved. 
     As shown in  FIG. 5A , the half width of light outputted from the organic EL device having an optical resonance structure is narrower than that of the organic EL device without an optical resonance structure, and has a value close to the half width of the diffraction spectrum of the reflection-type volume hologram. Accordingly, with the organic EL device having an optical resonance structure, an amount of light that passes through the reflection-type volume hologram and is not used becomes further less (indicated by dots in  FIG. 5A ) in comparison with the case of using the organic EL device without an optical resonance structure, so that the utilization efficiency of light is further improved. 
     Moreover, the peak of light outputted from the organic EL device having an optical resonance structure is higher than that of the organic EL device without an optical resonance structure. Accordingly, with the organic EL device having an optical resonance structure, the amount of light that is diffracted, reflected, and guided to reach the eye of the viewer by the reflection-type volume hologram is larger (indicated by diagonal lines in  FIG. 5A ) in comparison with the case of using the organic EL device without an optical resonance structure. 
     As described thus far, with the virtual image display apparatus  100  according to the first embodiment of the invention, by including the organic EL device  1  having an optical resonance structure, the utilization efficiency of the light that the organic EL device  1  outputs is improved and the light that reaches the eye of a viewer is large in quantity, thereby making it possible to enhance the luminance of a virtual image viewed by the viewer and in turn enhance the visibility thereof. Accordingly, the virtual image display apparatus according to this invention can be appropriately used as a visual image display apparatus like a see-through type HMD in which an outside scene is transmitted and viewed as a background. 
     Second Embodiment 
     Hereinafter, the configuration of a virtual image display system according to a second embodiment of the invention will be described. The virtual image display apparatus of the second embodiment differs from the first embodiment in that the configuration of an organic EL device is different from that of the first embodiment; however, other constituent elements than this one are approximately the same as those of the first embodiment. Therefore, the configuration of the organic EL device according to the second embodiment will be mainly discussed below with reference to the drawings. 
     Organic EL Device 
       FIG. 6  is a schematic cross-sectional view illustrating the structure of the organic EL device according to the second embodiment. Although an organic EL device  1 A according to the second embodiment differs from the organic EL device  1  of the first embodiment in that the organic EL device  1 A does not include a color filter, other constituent elements than this one are approximately the same. Note that in the second embodiment, same reference numerals are given to the same constituent elements as those of the first embodiment, and the description thereof will be omitted. 
     As shown in  FIG. 6 , the organic EL device  1 A includes the reflection layer  22 , the protection layer  26 , the anodes  24 , the partition walls  28 , the organic function layers  30 , the cathode  32 , the sealing layer  44 , and a sealing substrate  45  on the substrate  10 . In other words, the organic EL device  1 A includes, in place of the color filter substrate  40  in the organic EL device  1 , the sealing substrate  45  without the color filters  42 . Accordingly, the organic EL device  1 A outputs the light that has not passed through the color filters  42 . 
     The sealing substrate  45 , like the color filter substrate  40 , is configured with a transparent material such as glass. The sealing substrate  45  has a function to protect the organic EL elements  8  against an impact shock or the like from outside. Note that it is possible to remove the sealing substrate  45  if the sealing layer  44  can satisfactorily protect the organic EL elements  8 . 
     The organic EL device  1 A according to the second embodiment does not have a color filter. However, in the virtual image display apparatus according to the second embodiment, like in the virtual image display apparatus  100  according to the first embodiment, of the image forming light outputted by the organic EL device  1 A, light outside of the diffraction spectrum wavelength range of the reflection-type volume holograms  130 ,  132  (see  FIG. 1 ) is not diffracted and reflected, and is not used in displaying a virtual image. To rephrase, even if the organic EL device  1 A does not have a color filter, the reflection-type volume holograms  130 ,  132  substantially cut off other light than the light within the range of the wavelength band necessary for displaying a virtual image. 
     Accordingly, with the virtual image display apparatus according to the second embodiment, the following effects can be obtained in addition to the effects obtained in the first embodiment; that is, it is possible to further enhance the luminance of a virtual image viewed by the viewer and further improve the visibility of the virtual image because the image forming light outputted by the organic EL device  1 A can be used without the light passing through the color filter. Moreover, since the color filter is not needed, it is possible to make the organic EL device  1 A thinner and to lessen the manufacturing man-hour of the organic EL device  1 A in comparison with the first embodiment. 
     It is to be noted that the above embodiments are intended only to explain some aspects of the invention, and any variations and applications can be made arbitrarily within the range and spirit of the invention. As the variations, the following can be cited, for example. 
     Variation 1 
     The virtual image display apparatus  100  according to the above embodiments includes the reflection-type volume hologram  132  at a position opposed to the light incidence port of the light guide member  120 , and the reflection-type volume hologram  130  at a position opposed to the light output port; however, the invention is not limited thereto. The virtual image display apparatus may include a light path changing unit such as a reflection mirror in place of the reflection-type volume hologram at either a position opposed to the light incidence port of the light guide member  120  or a position opposed to the light output port. If the virtual image display apparatus includes a reflection-type volume hologram at at least one of a position opposed to the light incidence port of the light guide member  120  and a position opposed to the light output port, it is possible to selectively use the light of a necessary wavelength band for displaying a virtual image. 
     Variation 2 
     In the organic EL devices  1  and  1 A of the above embodiments, the organic function layers  30  ( 30 R,  30 G,  30 B) are each formed by being applied different materials each of which emits light of R, G or B color; however, the invention is not limited thereto. The organic function layers  30  may be formed with a material that emits white light, that is, a material that emits light of equal to or more than four wavelength bands including the R, G and B wavelength bands. To rephrase, the number of wavelength bands in the light emitted by the organic function layers  30  may exceeds the number of resonant wavelengths in the optical resonance structure. 
     The organic EL device has an optical resonance structure, and the optical distance of the optical resonance structure is optimized so that the resonant wavelength in each of the sub-pixels  2 R,  2 G and  2 B corresponds to each of the light of R, G and B wavelength bands even in a case where the organic function layers  30  have a configuration in which the light of white is emitted. Moreover, in the virtual image display apparatus, because, of the light outputted from the organic EL device, light outside of the ranges of diffraction spectrum wavelengths of the reflection-type volume holograms  130  and  132 , is not diffracted and reflected, it is possible to selectively use the light of a necessary wavelength band for displaying a virtual image. In the case where the organic function layers  30  are formed with a material that emits white light, it is possible to form the organic function layers  30  in the same layer across the sub-pixels  2 R,  2 G and  2 B. Further, by forming the organic function layers  30  in the same layer across the sub-pixels  2 R,  2 G and  2 B, because it is not necessary to carry out patterning individually for each of the sub-pixels  2 R,  2 G and  2 B, this technique may be preferably applied in a case such that the sub-pixels  2 R,  2 G and  2 B are less than 20 μm in size. 
     Variation 3 
     In the organic EL devices  1  and  1 A of the above-described embodiments, although light of three-kind wavelength bands, i.e., R, G and B wavelength bands, is guided to resonate, light of 1, 2, 4 or more-kind wavelength bands may be guided to resonate. Of the light of plural wavelength bands emitted by the organic function layers  30 , it is preferable that light of at least part of the plural wavelength bands be guided to resonate. For example, if there exist three-kind wavelength bands in the light that is emitted by the organic function layers  30 , the number of the wavelength bands that are guided to resonate in the resonator may be equal to or less than three. In the case where the organic function layers  30  have a configuration in which white light is emitted, the number of the wavelength bands that are guided to resonate in the resonator may be four, three, two, or just one. On the other hand, the number of the wavelength band guided to resonate in the resonator may be equal to or greater than five. 
     It is preferable that wavelength bands and peak wavelengths diffracted and reflected by the reflection-type volume hologram  130  or  132  be set so as to correspond to wavelength bands and peak wavelengths guided to resonate in the resonator. For example, if there are three kinds of wavelength bands that resonate in the resonator, it is preferable that three kinds of wavelength bands and peak wavelengths be provided which are diffracted and reflected by the reflection-type volume hologram  130  or  132 , corresponding to the wavelength bands and peak wavelengths that resonate in the resonator. Through this, light enhanced by the resonator can be efficiently diffracted and reflected by the reflection-type volume hologram  130  or  132 . Note that the wavelength bands and peak wavelengths diffracted and reflected by the reflection-type volume hologram  130  or  132 , are not needed to be completely the same as the wavelength bands and peak wavelengths guided to resonate in the resonator; that is, the wavelength bands diffracted and reflected by the reflection-type volume hologram  130  or  132  may be narrower in width, and/or the peak wavelengths may be deviated due to some manufacturing conditions or the like. The wavelength bands and the peak wavelengths diffracted and reflected by the reflection-type volume hologram  130  or  132  and the wavelength bands and the peak wavelengths guided to resonate in the resonator may be set so as to enhance the utilization efficiency of light. 
     Variation 4 
     The organic EL devices  1  and  1 A of the above-described embodiments have a configuration in which light of three-kind wavelength bands, i.e., light of R, G and B wavelength bands is emitted; however, the invention is not limited thereto. The organic EL devices  1  and  1 A may have a configuration in which, as the light of N-kind wavelength bands, light of one, two, four or more kinds of wavelength bands is emitted. 
     Variation 5 
     In the organic EL devices  1  and  1 A of the above-described embodiments, the optical distance of the optical resonator is optimized by changing each of the layer thicknesses of the anodes  24  corresponding to the sub-pixels  2 R,  2 G and  2 B; however, the invention is not limited thereto. The optical distance of the optical resonator may be optimized by changing the layer thickness of the insulating layer interposed between the reflection layer  22  and the cathode  32 , corresponding to the sub-pixels  2 R,  2 G and  2 B, or by laminating a plurality of insulating layers or conductive layers. 
     Variation 6 
     In the organic EL devices  1  and  1 A of the above-described embodiments, although glass, quartz, resin (plastic, plastic film), ceramics and the like are cited as a material of the substrate  10 , a semiconductor substrate such as silicon may also be cited. In this case, transistors that configure the switching TFT  11 , the driving TFT  12 , the data line driving circuit  14 , the scanning line driving circuit  15  and the like are not needed to be a thin film transistor including a semiconductor thin-film layer, and may be a transistor with a channel being formed in the semiconductor substrate itself. In addition, the substrate  10  may be configured with an SOI substrate.