Patent Publication Number: US-2021191129-A1

Title: Display apparatus including volume grating based combiner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. § 119 to U.S. Provisional Application No. 62/952,699, filed on Dec. 23, 2019 in the U.S. Patent Office and Trademark Office, and Korean Patent Application No. 10-2020-0006743, filed on Jan. 17, 2020 in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     1. Field 
     Example embodiments of the present disclosure relate to a display apparatus, and more particularly, to a display apparatus including a volume grating based combiner. 
     2. Description of Related Art 
     Three-dimensional image display technology has been applied to various fields, and also has been recently applied to image devices for virtual reality (VR) displays and augmented reality (AG) displays. 
     Head mounted displays for VR are commercialized at present and tend to be widely applied in the entertainment industry. In addition, head mounted displays have been developed for medical, educational, and industrial fields. 
     As a development step for VR displays, AR displays have been developed to induce interactions between reality and an imagine as image devices for combining the real world and VR. Interactions between reality and VR are based on a function of providing information on a real situation in real-time and may further increase a reality effect by overlaying a virtual object or information on an environment of the real world. Such an AR display includes a combiner for combining a virtual image with an external real foreground and providing the combined image to an observer. 
     SUMMARY 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of example embodiments. 
     According to an aspect of an example embodiment, there is provided a display apparatus including an image providing device configured to provide an image, and a combiner configured to combine light containing the image, which is emitted from the image providing device, and light containing an external landscape, wherein the combiner includes a plurality of volume gratings configured to diffract the light containing the image emitted from the image providing device, wherein each volume grating of the plurality of volume gratings has a first surface and a second surface facing each other, and wherein each volume grating of the plurality of volume gratings is configured to diffract light incident on the first surface and transmit therethrough light incident on the second surface without diffraction. 
     Each volume grating of the plurality of volume gratings may be configured to diffract light incident on the first surface at a pre-defined incident angle toward a pre-defined direction. 
     The light containing the image, which is provided from the image providing device to each volume grating of the plurality of volume gratings, may be collimated parallel light. 
     Each volume grating of the plurality of volume gratings may be configured to have an optical characteristic having no refractive power with respect to the light incident on the first surface. 
     The light containing the image, which is provided from the image providing device to each volume grating of the plurality of volume gratings, may be diverging light. 
     Each volume grating of the plurality of volume gratings may be configured to collimate by diffraction the diverging light incident on the first surface into parallel light. 
     The combiner may further include a transparent substrate, and wherein the plurality of volume gratings may be provided on the transparent substrate in a two-dimensionally. 
     The image providing device may be further configured to provide the light containing the image toward the first surface of each volume grating of the plurality of volume gratings, and the plurality of volume gratings may be configured to diffract the light containing the image at different angles such that lights respectively diffracted by the plurality of volume gratings travel toward a single region. 
     The first surface of each volume grating of the plurality of volume gratings may have a circular shape, an oval shape, a quadrangular shape, or a hexagonal shape. 
     A distance between two adjacent volume gratings of the plurality of volume gratings may be greater than or equal to a width of each volume grating of the plurality of volume gratings. 
     The width of each volume grating of the plurality of volume gratings may range from 0.5 mm to 1.5 mm. 
     The distance between two adjacent volume gratings of the plurality of volume gratings may gradually increase or decrease away from a center of the combiner. 
     The width of each volume grating of the plurality of volume gratings may gradually increase or decrease away from a center of the combiner. 
     Some of the plurality of volume gratings may be provided in a line along a first row, and remaining ones of the plurality of volume gratings may be provided in a line along a second row adjacent to the first row, and volume gratings provided along the first row and volume gratings provided along the second row are provided to be mismatched with each other in a column direction. 
     Some of the plurality of volume gratings may be provided in a line along a first row, and remaining ones of the plurality of volume gratings may be provided in a line along a second row adjacent to the first row, and volume gratings provided along the first row and volume gratings provided along the second row are provided to face each other in a column direction. 
     The combiner may further include a transparent light guide plate through which light travels by total reflection, and an input coupler configured to guide the light containing the image, which is emitted from the image providing device, to an inside of the transparent light guide plate, wherein the transparent light guide plate has a first surface and a second surface facing each other, and wherein the plurality of volume gratings are provided on the second surface of the transparent light guide plate and configured to diffract the light containing the image toward the first surface of the transparent light guide plate. 
     The plurality of volume gratings may be provided such that the first surface of each volume grating is included inside the transparent light guide plate. 
     The plurality of volume gratings may be provided such that the first surface of each volume grating is in contact with the second surface of the transparent light guide plate. 
     The plurality of volume gratings may include a plurality of first volume gratings configured to diffract, at a first angle, light of a first wavelength band incident on the first surface at a pre-defined incident angle, a plurality of second volume gratings configured to diffract, at a second angle that is different from the first angle, light of a second wavelength band that is different from the first wavelength band, incident on the first surface at the pre-defined incident angle, and a plurality of third volume gratings configured to diffract, at a third angle that is different from the first angle and the second angle, light of a third wavelength band that is different from the first wavelength band and the second wavelength band, incident on the first surface at the pre-defined incident angle. 
     The plurality of first volume gratings, the plurality of second volume gratings, and the plurality of third volume gratings may be alternately provided one by one. 
     The display apparatus may further include an eye tracker configured to track a location of a pupil of an observer and measure a size of the pupil of the observer. 
     The image providing device may be further configured to adjust a beam diameter of light incident on the first surface of each volume grating of the plurality of volume gratings, based on information with respect to the location of the pupil of the observer or information with respect to the size of the pupil of the observer. 
     The display apparatus may further include a beam diameter adjustment device configured to adjust a beam diameter of light incident on the first surface of each volume grating of the plurality of volume gratings from the image providing device, based on information with respect to the location of the pupil of the observer or information with respect to the size of the pupil of the observer. 
     The beam diameter adjustment device may include one of an aperture having a variable opening, a lens having a variable focal length, a diffusion plate having a variable diffusion angle, a micro-electromechanical systems (MEMS) mirror array configured to electrically adjust a reflective region, and an actuator configured to move the image providing device to adjust a distance between the combiner and the image providing device. 
     The beam diameter adjustment device may be configured to decrease a beam diameter of light incident on the first surface of each volume grating of the plurality of volume gratings based on a distance between the combiner and the observer decreasing or the size of the pupil of the observer increasing, and to increase the beam diameter of the light incident on the first surface of each volume grating of the plurality of volume gratings based on the distance between the combiner and the observer increasing or the size of the pupil of the observer decreasing. 
     The display apparatus may be a virtual reality (VR) display apparatus, an augmented reality (AR) display apparatus, or a mixed reality (MR) display apparatus of a head mounting type, an eyeglasses type, or a goggle type. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects, features, and advantages of example embodiments will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view of a configuration of a display apparatus according to an example embodiment; 
         FIG. 2  is a cross-sectional view of a configuration of a combiner of the display apparatus illustrated in  FIG. 1 ; 
         FIGS. 3A, 3B, 3C, and 3D  are top views of various arrangements of a plurality of volume gratings arranged on the combiner illustrated in  FIG. 2 ; 
         FIGS. 4A, 4B, and 4C  are views of a process of producing the combiner illustrated in  FIG. 2  by using a holographic printer; 
         FIGS. 5A, 5B, and 5C  are views of a process of producing the combiner illustrated in  FIG. 2  by using a general hologram recording method; 
         FIG. 6  is a view of a configuration of a display apparatus according to another example embodiment; 
         FIGS. 7A, 7B, and 7C  are views of a process of producing a combiner illustrated in  FIG. 6  by using a general hologram recording method; 
         FIG. 8  is a view of a configuration of a display apparatus according to another example embodiment; 
         FIG. 9  is a view of a configuration of a display apparatus according to another example embodiment; 
         FIG. 10  is a view of a configuration of a display apparatus according to another example embodiment; 
         FIG. 11  is an exemplary view of an arrangement of a plurality of volume gratings arranged on a combiner according to another example embodiment; 
         FIGS. 12, 13, 14, 15, 16, and 17  are views of configurations of display apparatuses capable of actively adjusting a beam diameter of light containing an image, according to other example embodiments; 
         FIG. 18  illustrates an example in which a valid reaction area of an opening is adjusted based on pupil information of an observer in the example embodiments illustrated in  FIGS. 12 to 17 ; and 
         FIGS. 19, 20, and 21  show various electronic devices to which a display apparatus according to example embodiments are applicable. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to example embodiments which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the example embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the example embodiments are merely described below, by referring to the figures, to explain aspects. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. For example, the expression, “at least one of a, b, and c,” should be understood as including only a, only b, only c, both a and b, both a and c, both b and c, or all of a, b, and c. 
     Hereinafter, a display apparatus including a volume grating-based combiner will be described in detail with reference to the accompanying drawings. In the drawings, the sizes of constituent elements may be exaggerated for clarity. In addition, the embodiments described below are only illustrative, and various changes in form and details may be made therein. 
     Hereinafter, the term “above” or “on” may include not only directly on in a contact manner but also above in a contactless manner. An expression in the singular includes an expression in the plural unless they are clearly different from each other in context. In addition, when a certain part “includes” a certain component, this indicates that the part may further include another component instead of excluding another component unless there is different disclosure. 
     The use of the term “the” or a similar directional term may correspond to both the singular and the plural. For steps forming the methods, if an order is not clearly disclosed or, if there is no disclosure opposed to the clear order, the steps may be performed in any order deemed proper, and the methods are not necessarily limited to the disclosed order of the steps. 
     In addition, terms such as “ . . . unit”, “ . . . module”, and the like refer to units that perform at least one function or operation, and the units may be implemented as hardware or software or as a combination of hardware and software. 
     Connections or connection members of lines between components shown in the drawings illustrate functional connections and/or physical or circuit connections, and the connections or connection members may be represented by replaceable or additional various functional connections, physical connections, or circuit connections in an actual apparatus. 
     The use of all illustrations or illustrative terms is simply to describe the technical idea in detail, and the scope is not limited due to the illustrations or illustrative terms unless they are limited by claims. 
       FIG. 1  is a view of a configuration of a display apparatus according to an example embodiment. Referring to  FIG. 1 , a display apparatus  100  according to an example embodiment may include an image providing device  20  for providing an image and a combiner  10  configured to combine light L 0  containing the image, which is emitted from the image providing device  20 , and light L 1  containing an external landscape. 
     The image providing device  20  may include a spatial light modulator or a display panel for forming an image containing virtual reality or virtual information. The display panel may include, for example, a liquid crystal display panel or an organic light-emitting display panel. In addition, the image providing device  20  may include a projector for projecting the light L 0  containing the image to the combiner  10 . Therefore, the image providing device  20  may project the light L 0  containing the image, which is formed by the spatial light modulator or the display panel, to the combiner  10 . For example, the image providing device  20  may collimate the light L 0  containing the image to make parallel light, and project the parallel light to the combiner  10 . 
     The combiner  10  is configured to combine the light L 0  containing the image, which is projected from the image providing device  20 , and the light L 1  containing the external landscape and provide the combined image to an observer. For example, the combiner  10  may be configured to change a traveling direction of the light L 0  containing the image, which is projected from the image providing device  20 , to a particular direction by diffracting the light L 0  containing the image and transmit the light L 1  containing the external landscape therethrough without diffraction. The external light L 1  contains a real landscape existing in front of the observer and not an artificial image modulated and generated by a separate spatial light modulator or displayed by a separate display panel. Therefore, the observer may observe both an artificially generated virtual image and a real landscape. Accordingly, the display apparatus  100  according to the example embodiment may be applied to implement augmented reality (AR) or mixed reality (MR). For example, the display apparatus  100  according to the example embodiment may be a near-eye AR display apparatus. For example, when the display apparatus  100  is used, the combiner  10  may be arranged in front of an eye of the observer. 
     To diffract the light L 0  containing the image, which is projected from the image providing device  20 , such that the light L 0  containing the image travels toward the eye of the observer, the combiner  10  may include a plurality of volume gratings VG 1 , VG 2 , and VG 3 . A volume grating may include an interference pattern formed by interference between reference light and signal light. The interference pattern may vary according to a wavelength of the reference light, an incident angle of the reference light, a traveling direction of the signal light, and a form of the signal light. When light having the same wavelength as the reference light is incident on this volume grating at the same incident angle as that of the reference light, the reference light is diffracted by the interference pattern of the volume grating such that the same light as the signal light is reproduced. Therefore, according to interference patterns of volume gratings, various forms of signal light may be generated. 
     According to the example embodiment, each of the plurality of volume gratings VG 1 , VG 2 , and VG 3  is configured to diffract light incident at a pre-defined particular incident angle toward a pre-defined particular direction. For example, the light L 0  containing the image, which is projected from the image providing device  20 , acts as the reference light, and the plurality of volume gratings VG 1 , VG 2 , and VG 3  may be configured to diffract the light L 0  containing the image, which is incident at the pre-defined particular incident angle, and reproduce signal light containing an image, which travels toward the eye of the observer. 
     The plurality of volume gratings VG 1 , VG 2 , and VG 3  are locally formed on the surface of the combiner  10  and separated from each other. The plurality of volume gratings VG 1 , VG 2 , and VG 3  arranged at different locations may diffract the light L 0  containing the image at different angles such that lights respectively diffracted from the plurality of volume gratings VG 1 , VG 2 , and VG 3  travel toward a single same region and are incident on the eye of the observer, particularly, the pupil of the eye of the observer. For example, the volume grating VG 1  located to be higher than the eye of the observer may be configured to diffract downward the light L 0  projected from the image providing device  20 , the volume grating VG 2  located at the same height as the eye of the observer may be configured to diffract, in a normal direction with respect to the grating VG 2 , the light L 0  projected from the image providing device  20 , and the volume grating VG 3  located to be lower than the eye of the observer may be configured to diffract upward the light L 0  projected from the image providing device  20 . 
     The image providing device  20  may be configured to individually project the light L 0  containing the image to the plurality of volume gratings VG 1 , VG 2 , and VG 3 . For example, the image providing device  20  may generate a plurality of parallel light beams respectively corresponding to the plurality of volume gratings VG 1 , VG 2 , and VG 3  and individually project the plurality of parallel light beams to the plurality of volume gratings VG 1 , VG 2 , and VG 3 , respectively. The image providing device  20  may be configured to project the light L 0  containing the image toward the entire region of the combiner  10 . In this case, the light L 0  is diffracted only in regions in which the plurality of volume gratings VG 1 , VG 2 , and VG 3  are formed among the entire region of the combiner  10 . In any case, the light L 0  containing the image, which is projected from the image providing device  20 , may be incident on the plurality of volume gratings VG 1 , VG 2 , and VG 3  at the same incident angle. 
     In addition, each volume grating VG 1 , VG 2 , or VG 3  may be configured such that light diffracted by each volume grating VG 1 , VG 2 , or VG 3  and traveling toward the eye of the observer is parallel light. Therefore, the light L 0  which contains the image and is incident on each volume grating VG 1 , VG 2 , or VG 3  may be parallel light, and light traveling from each volume grating VG 1 , VG 2 , or VG 3  toward the eye of the observer may also be parallel light. In this case, each volume grating VG 1 , VG 2 , or VG 3  may have an optical characteristic having no refractive power like a plane mirror. 
       FIG. 2  is a cross-sectional view of a configuration of the combiner  10  of the display apparatus  100  illustrated in  FIG. 1 . Referring to  FIG. 2 , the combiner  10  may include a transparent substrate  11  made of a material such as glass or plastic transparent to visible light and a photosensitive layer  12  arranged on the surface of the transparent substrate  11 . The plurality of volume gratings VG 1 , VG 2 , and VG 3  are formed inside the photosensitive layer  12 . Each volume grating VG 1 , VG 2 , or VG 3  may have a thin thickness t of, for example, about 100 μm. Therefore, the combiner  10  and the display apparatus  100  may be produced with a sufficiently thin thickness. 
     In addition, each volume grating VG 1 , VG 2 , or VG 3  may have a first surface S 1  and a second surface S 2  facing each other. For example, when the display apparatus  100  is used, the first surface S 1  of each volume grating VG 1 , VG 2 , or VG 3  may be arranged toward the eye of the observer, and the second surface S 2  of each volume grating VG 1 , VG 2 , or VG 3  may be arranged toward an external landscape. In this case, each volume grating VG 1 , VG 2 , or VG 3  may be configured to diffract light incident on the first surface S 1  and transmit therethrough light incident on the second surface S 2  without refraction. The image providing device  20  may be arranged to project the light L 0  containing the image to the first surface S 1  of each volume grating VG 1 , VG 2 , or VG 3 . Then, the plurality of volume gratings VG 1 , VG 2 , and VG 3  may diffract the light L 0  containing the image toward the eye of the observer and transmit the external light L 1  therethrough without refraction. 
       FIG. 2  shows, in the cross-sectional view, that three volume gratings VG 1 , VG 2 , and VG 3  are arranged in a vertical direction, but embodiments are not necessarily limited thereto. For example, a greater number of volume gratings may be arranged on the transparent substrate  11  in a two-dimensional array form. For example,  FIGS. 3A to 3D  are top views of various arrangements of a plurality of volume gratings arranged on the combiner  10  illustrated in  FIG. 2 . 
     First, referring to  FIG. 3A , the combiner  10  may include a plurality of two-dimensionally arranged volume gratings VG 1   a , VG 1   b , VG 1   c , VG 2   a , VG 2   b , VG 2   c , VG 3   a , VG 3   b , and VG 3   c . As shown in  FIG. 3A , the plurality of volume gratings VG 1   a , VG 1   b , VG 1   c , VG 2   a , VG 2   b , VG 2   c , VG 3   a , VG 3   b , and VG 3   c  may be arranged in a line in a row direction and a column direction. For example, a plurality of volume gratings VG 1   a , VG 1   b , and VG 1   c  may be arranged in a line along a first row, a plurality of volume gratings VG 2   a , VG 2   b , and VG 2   c , may be arranged in a line along a second row, and a plurality of volume gratings VG 3   a , VG 3   b , and VG 3   c  may be arranged in a line along a third row. In addition, the plurality of volume gratings arranged along different rows may be arranged to face each other in the column direction. For example, a plurality of volume gratings VG 1   a , VG 2   a , and VG 3   a  may be arranged in a line along a first column, a plurality of volume gratings VG 1   b , VG 2   b , and VG 3   b  may be arranged in a line along a second column, and a plurality of volume gratings VG 1   c , VG 2   c , and VG 3   c  may be arranged in a line along a third column. Although  FIG. 3A  shows that a plurality of volume gratings are arranged in three rows and in three columns, this is only an example, and embodiments are not necessarily limited thereto. For example, volume gratings may be arranged in two rows or two columns, or volume gratings may be arranged in four or more rows or four or more columns. 
     Each volume grating may have, for example, a quadrangular shape. A distance d between two adjacent volume gratings may be greater than or equal to a width w of each volume grating. For example, the width w of each volume grating may be about 0.5 mm to about 1.5 mm. Because a size of each volume grating is very small, each volume grating may act as a pinhole. Therefore, because image information is delivered to the observer through a small opening such as a pinhole, a depth of focus may be long. In addition, an angle of view may increase by using a plurality of volume gratings. 
     The widths w of the plurality of volume gratings may not all be the same. In addition, the distances d between the plurality of volume gratings may not be all the same. For example, sizes of the widths w of the plurality of volume gratings and the distances d between the plurality of volume gratings may vary according to relative locations from the eye of the observer. For example, the width w of each volume grating may gradually increase or decrease away from the center of the combiner  10 , and the distance d between two adjacent volume gratings may gradually increase or decrease away from the center of the combiner  10 . 
     In addition, referring to  FIG. 3B , a plurality of volume gratings VG 1   a , VG 1   b , VG 2   a , VG 2   b , VG 2   c , VG 3   a , and VG 3   b  may be two-dimensionally arranged to be mismatched with each other. For example, a plurality of volume gratings VG 1   a  and VG 1   b  may be arranged in a line along the first row, a plurality of volume gratings VG 2   a , VG 2   b , and VG 2   c  may be arranged in a line along the second row, a plurality of volume gratings VG 3   a  and VG 3   b  may be arranged in a line along the third row, and the volume gratings VG 1   a  and VG 1   b  of the first row and the volume gratings VG 3   a  and VG 3   b  of the third row may be arranged to be mismatched with the volume gratings VG 2   a , VG 2   b , and VG 2   c  of the second row. For example, the volume gratings VG 1   a  and VG 1   b  of the first row and the volume gratings VG 3   a  and VG 3   b  of the third row may be located between the volume gratings VG 2   a , VG 2   b , and VG 2   c  of the second row in a horizontal direction. 
     Although  FIGS. 3A and 3B  show that each volume grating has a quadrangular shape, this is only an example, and a shape of each volume grating is not necessarily limited to the quadrangular shape. For example, as shown in  FIG. 3C , each volume grating VG 1   a , VG 1   b , VG 2   a , VG 2   b , VG 2   c , VG 3   a , or VG 3   b  may have a circular or oval shape. As shown in  FIG. 3D , each volume grating VG 1   a , VG 1   b , VG 2   a , VG 2   b , VG 2   c , VG 3   a , or VG 3   b  may have a hexagonal shape. In addition, although  FIGS. 3A to 3D  show that a plurality of volume gratings in one combiner  10  have the same shape, embodiments are not necessarily limited thereto. For example, a plurality of volume gratings may have different shapes according to beam cross-sectional shapes of the light L 0  containing the image, which is projected from the image providing device  20 , and locations of the plurality of volume gratings. 
     As described above, a volume grating includes an interference pattern formed by interference between reference light an signal light. A method of forming an interference pattern of a volume grating having the optical characteristics described above includes a hologram recording method and a method using a holographic printer. 
     For example,  FIGS. 4A to 4C  are views of a process of producing the combiner  10  illustrated in  FIG. 2  by using a holographic printer. First, referring to  FIG. 4A , reference light R and signal light S are simultaneously emitted on the same location of the photosensitive layer  12  on the transparent substrate  11 , such that the reference light R interferes with the signal light S. A beam diameter of the reference light R and a beam diameter of the signal light S are smaller than a size of one volume grating VG 1 . An interference pattern formed by the reference light R and the signal light S is a hogel that is a unit constituting the volume grating VG 1 . The one volume grating VG 1  may be constructed by a plurality of two-dimensionally arranged hogels. An interference pattern of the hogels is determined by a beam profile of the signal light S. The beam profile of the signal light S may be modulated to a desired shape by a spatial light modulator SLM. For example, the beam profile of the signal light S may vary according to a computer generated hologram (CGH) signal applied to the spatial light modulator SLM of the holographic printer. 
     Therefore, an optical characteristic of the volume grating VG 1  formed on the combiner  10  may be more easily determined according to the CGH signal applied to the spatial light modulator SLM of the holographic printer. In addition, a location and a shape of the volume grating VG 1  formed on the combiner  10  may be more easily determined according to a forming location and an arrangement shape of the plurality of hogels. For example, an interference pattern and a location of the plurality of hogels may be determined by considering a pre-defined optical characteristic of the volume grating VG 1 . In addition, the one volume grating VG 1  may be formed by calculating a CGH signal for forming a determined interference pattern of a hogel and then by sequentially forming a plurality of hogels in the photosensitive layer  12  while applying the calculated CGH signal to the spatial light modulator SLM. 
     After completing the one volume grating VG 1  in this way, as shown in  FIG. 4B , a subsequent volume grating VG 2  may be formed by moving a location at which hogels are formed. The volume grating VG 2  may be formed by previously determining an interference pattern and a location of a plurality of hogels in consideration of an optical characteristic of the volume grating VG 2  to be formed and sequentially forming the plurality of hogels inside the photosensitive layer  12  while applying, to the spatial light modulator SLM, a CGH signal calculated based on the interference pattern and the location. In this way, the combiner  10  may be produced by forming a plurality of two-dimensionally arranged volume gratings with a holographic printer. 
     After completing all of a plurality of volume gratings, a residual part of the photosensitive layer  12  in which no volume gratings are formed may remain as it is or be removed. For example, referring to  FIG. 4C , after removing a residual part of the photosensitive layer  12  in which the plurality of volume gratings VG 1 , VG 2 , and VG 3  are not formed, a transparent protective layer  13  covering the plurality of volume gratings VG 1 , VG 2 , and VG 3  may be formed on the transparent substrate  11 . 
     In addition,  FIGS. 5A to 5C  are views of a process of producing the combiner  10  illustrated in  FIG. 2  by using a hologram recording method. First referring to  FIG. 5A , the reference light R and the signal light S are simultaneously emitted on the same location of the photosensitive layer  12  on the transparent substrate  11 , such that the reference light R interferes with the signal light S. The location on which the reference light R and the signal light S are emitted is the same as a location of the volume grating VG 1  to be formed. The reference light R is incident on the photosensitive layer  12  at the same incident angle as an incident angle at which the light L 0  containing the image, which is emitted from the image providing device  20 , is incident on the volume grating VG 1 . In addition, the reference light R is the same parallel light as the light L 0  containing the image, which is emitted from the image providing device  20 . The signal light S is incident on the photosensitive layer  12  in the same direction as a direction in which the light L 0  containing the image is diffracted by the volume grating VG 1  and then travels. In addition, a beam cross-sectional shape of the reference light R and a beam cross-sectional shape of the signal light S may be the same as a shape of the volume grating VG 1  to be formed. In this case, the volume grating VG 1  may be formed by a single occurrence of interference between the reference light R and the signal light S. 
     As shown in  FIG. 5B , the volume grating VG 2  may be formed by emitting the reference light R and the signal light S on the same location on the photosensitive layer  12  as a location of the volume grating VG 2 . The signal light S is incident on the photosensitive layer  12  in the same direction as a direction in which the light L 0  containing the image is diffracted by the volume grating VG 2  and then travels. Likewise, as shown in  FIG. 5C , the volume grating VG 3  may be formed by emitting the reference light R and the signal light S on the same location on the photosensitive layer  12  as a location of the volume grating VG 3 . The signal light S is incident on the photosensitive layer  12  in the same direction as a direction in which the light L 0  containing the image is diffracted by the volume grating VG 3  and then travels. After forming all of the plurality of volume gratings VG 1 , VG 2 , and VG 3 , as shown in  FIG. 4C , a part of the photosensitive layer  12  in which the plurality of volume gratings VG 1 , VG 2 , and VG 3  are not formed may be removed, and the transparent protective layer  13  covering the plurality of volume gratings VG 1 , VG 2 , and VG 3  may be formed on the transparent substrate  11 . 
     Volume gratings formed by the holographic printing method or the hologram recording method described above may react only to light incident in a particular direction. For example, the volume gratings diffract only the light L 0  containing the image, which is obliquely incident on the first surface S 1  of  FIG. 2  at a predetermined angle. The volume gratings may transmit therethrough, without diffraction, light incident in a different direction or at a different angle from that of the light L 0  containing the image. Therefore, the combiner  10  according to the example embodiment may efficiently combine the light L 0  containing the image and the light L 1  containing the external landscape, and provide the combined light to the observer. 
     In addition, the combiner  10  used in the display apparatus  100  according to the example embodiment may be more easily produced using a holographic printer. When a holographic printer is used, an optical characteristic of volume gratings formed in the combiner  10  may be more easily determined according to a CGH signal applied to the spatial light modulator SLM of the holographic printer, and a location and a shape of the volume gratings formed in the combiner  10  may be more easily determined according to a location and an arrangement of hogels. Therefore, optical efficiency of the combiner  10  with respect to the light L 0  containing the image may be optimized, and the combiner  10  having high transmittance and low noise with respect to the light L 1  containing the external landscape may be implemented. 
       FIG. 6  is a view of a configuration of a display apparatus  110  according to another example embodiment. Referring to  FIG. 6 , the light L 0  containing the image, which is projected from the image providing device  20  to the plurality of volume gratings VG 1 , VG 2 , and VG 3  in the combiner  10 , may be diverging light having a beam diameter gradually increasing in a traveling direction. In this case, each volume grating VG 1 , VG 2 , or VG 3  may be configured to diffract diverging light incident on the first surface S 1  (see  FIG. 2 ) thereof and collimate the diffracted light into parallel light. 
     For example, the volume grating VG 1  located to be higher than the eye of the observer may be configured to diffract downward the light L 0  projected from the image providing device  20  and collimate the diffracted light into parallel light. The volume grating VG 2  located at the same height as the eye of the observer may be configured to diffract, in a normal direction, the light L 0  projected from the image providing device  20  and collimate the diffracted light into parallel light. The volume grating VG 3  located to be lower than the eye of the observer may be configured to diffract upward the light L 0  projected from the image providing device  20  and collimate the diffracted light into parallel light. Accordingly, the plurality of volume gratings VG 1 , VG 2 , and VG 3  shown in  FIG. 6  may have an optical characteristic having positive (+) refractive power similar to a concave mirror. 
     The plurality of volume gratings VG 1 , VG 2 , and VG 3  shown in  FIG. 6  may also be formed by a holographic printing method or a hologram recording method. For the holographic printing method, a CGH signal may be calculated by considering that the light L 0  containing the image, which is projected from the image providing device  20  to the plurality of volume gratings VG 1 , VG 2 , and VG 3 , is diverging light. In addition, the plurality of volume gratings VG 1 , VG 2 , and VG 3  may have the optical characteristic illustrated in  FIG. 6  by forming a plurality of hogels according to the CGH signal applied to the spatial light modulator SLM. 
       FIGS. 7A to 7C  are views of a process of producing the combiner  10  illustrated in  FIG. 6  by using a general hologram recording method. Referring to  FIGS. 7A to 7C , the reference light R is the same diverging light as the light L 0  containing the image, which is emitted from the image providing device  20 . The other process illustrated in  FIGS. 7A to 7C  is the same as described with reference to  FIGS. 5A to 5C . 
       FIG. 8  is a view of a configuration of a display apparatus  120  according to another example embodiment. Referring to  FIG. 8 , a combiner  30  of the display apparatus  120  may include a transparent light guide plate  31  through which light travels by total reflection and an input coupler  35  configured to guide light containing an image, which is emitted from the image providing device  20 , to the inside of the transparent light guide plate  31 . For example, the input coupler  35  may have a prism shape and be arranged between a light incident surface of the transparent light guide plate  31  and the image providing device  20 . 
     The transparent light guide plate  31  may have a first surface  31   a  and a second surface  31   b  facing each other. When the display apparatus  120  is used, the combiner  30  may be arranged such that the first surface  31   a  of the transparent light guide plate  31  faces the eye of the observer. The plurality of volume gratings VG 1 , VG 2 , and VG 3  may be configured to diffract the light containing the image toward the first surface  31   a  of the transparent light guide plate  31  by being arranged on the second surface  31   b  of the transparent light guide plate  31 . The light containing the image, which is diffracted by the plurality of volume gratings VG 1 , VG 2 , and VG 3 , may exit from the first surface  31   a  of the transparent light guide plate  31  and be incident on the eye of the observer. 
     In the example embodiment illustrated in  FIG. 8 , the combiner  30  may not include a transparent substrate and a photosensitive layer. The plurality of volume gratings VG 1 , VG 2 , and VG 3  may be included and buried inside the transparent light guide plate  31 . For example, the first surfaces S 1  of the plurality of volume gratings VG 1 , VG 2 , and VG 3  may face the first surface  31   a  of the transparent light guide plate  31  in the inside of the transparent light guide plate  31 . In addition, the second surfaces S 2  of the plurality of volume gratings VG 1 , VG 2 , and VG 3  may coincide with the second surface  31   b  of the transparent light guide plate  31 . 
       FIG. 9  is a view of a configuration of a display apparatus  120   a  according to another example embodiment. Referring to  FIG. 9 , unlike the configuration of the combiner  30  illustrated in  FIG. 8 , a combiner  30 ′ of the display apparatus  120   a  may further include the transparent substrate  11  and the photosensitive layer  12 . In this example, the plurality of volume gratings VG 1 , VG 2 , and VG 3  may be arranged such that the first surfaces S 1  thereof are in contact with the second surface  31   b  of the transparent light guide plate  31 . 
     The plurality of volume gratings VG 1 , VG 2 , and VG 3  described above may be configured to react to light of all wavelengths. For example, each of the plurality of volume gratings VG 1 , VG 2 , and VG 3  described above may be configured to diffract all of incident light of a red band, incident light of a green band, and incident light of a blue band in the same direction. However, in this case, diffraction efficiency of the plurality of volume gratings VG 1 , VG 2 , and VG 3  may be relatively low. Therefore, when volume gratings are configured to react only to light of a particular wavelength band, diffraction efficiency of the volume gratings may be improved. 
     For example,  FIG. 10  is a view of a configuration of a display apparatus  130  according to another example embodiment. Referring to  FIG. 10 , a combiner  40  of the display apparatus  130  may include a plurality of first volume gratings VG 1 R, VG 2 R, and VG 3 R configured to diffract only light of the red band, which is incident on the first surface S 1  at a pre-defined particular incident angle, a plurality of second volume gratings VG 1 G, VG 2 G, and VG 3 G configured to diffract only light of the green band, which is incident on the first surface S 1  at a pre-defined particular incident angle, and a plurality of third volume gratings VG 1 B, VG 2 B, and VG 3 B configured to diffract only light of the blue band, which is incident on the first surface S 1  at a pre-defined particular incident angle. The plurality of first volume gratings VG 1 R, VG 2 R, and VG 3 R may diffract light of the red band at a pre-defined particular angle, for example, toward the eye of the observer. In addition, the plurality of second volume gratings VG 1 G, VG 2 G, and VG 3 G may diffract light of the green band at a pre-defined particular angle, for example, toward the eye of the observer, and the plurality of third volume gratings VG 1 B, VG 2 B, and VG 3 B may diffract light of the blue band at a pre-defined particular angle, for example, toward the eye of the observer. Then, the observer may view a color image including the red light, the green light, and the blue light. 
       FIG. 10  shows that the plurality of first volume gratings VG 1 R, VG 2 R, and VG 3 R, the plurality of second volume gratings VG 1 G, VG 2 G, and VG 3 G, and the plurality of third volume gratings VG 1 B, VG 2 B, and VG 3 B are alternately arranged in the vertical direction one by one. However, this is only an example, and an arrangement of volume gratings is not necessarily limited thereto. For example,  FIG. 11  is an exemplary view of an arrangement of a plurality of volume gratings arranged on the combiner  40 , according to another example embodiment. As shown in  FIG. 11 , a plurality of first volume gratings R configured to diffract light of the red band, a plurality of second volume gratings G configured to diffract light of the green band, and a plurality of third volume gratings B configured to diffract light of the blue band may be alternately arranged in the horizontal direction one by one. In addition, for example, the plurality of first volume gratings R, the plurality of second volume gratings G, and the plurality of third volume gratings B may be alternately arranged in a diagonal direction one by one. 
     In the display apparatuses according to the example embodiments described above, because a size of volume gratings is small, a long depth of focus may be achieved according to the principle of pinhole. However, because a size of volume gratings cannot be zero, an image may not be focused according to a location change of the pupil of the observer or a size change of the pupil. In this case, to focus an image, an effective reaction area of an opening may be actively adjusted in response to a location change of the pupil of the observer or a size change of the pupil. However, because it is difficult to actively change a size of volume gratings, a beam diameter of light containing an image, which is projected from the image providing device  20  and incident to the volume gratings, may be actively adjusted. 
     For example,  FIGS. 12 to 17  are views of configurations of display apparatuses capable of actively adjusting a beam diameter of light containing an image according to other example embodiments; 
     Referring to  FIG. 12 , a display apparatus  140  may further include an aperture  50 , an eye tracker  141 , and a processor  145 . The aperture  50  is a beam diameter adjustment device arranged between the image providing device  20  and the combiner  10  and configured to adjust a beam diameter of light containing an image, which is to be incident on the plurality of volume gratings VG 1 , VG 2 , and VG 3 . To this end, the aperture  50  may have an opening which variably changes. The eye tracker  141  may measure a location and a size of the pupil of the observer in real-time and provide a measurement result to the processor  145 . The processor  145  may control an operation of the aperture  50  based on information about the location or the size of the pupil of the observer, which is provided from the eye tracker  141 . For example, the processor  145  may increase or decrease a diameter of the opening of the aperture  50  in response to a location change or a size change of the pupil of the observer. 
     In addition, referring to  FIG. 13 , a display apparatus  140   a  may further include an adjustable lens  60 , the eye tracker  141 , and the processor  145 . The adjustable lens  60  may be, for example, a variable focus lens between the image providing device  20  and the combiner  10 . Because a beam diameter of light containing an image, which is incident on the combiner  10 , changes according to a change in a focal length of the adjustable lens  60 , the adjustable lens  60  may function as a beam diameter adjustment device. The processor  145  may increase or decrease the focal length of the adjustable lens  60  in response to information about a location or a size of the pupil of the observer, which is provided from the eye tracker  141 . 
     In addition, referring to  FIG. 14 , an image providing device  20   a  of a display apparatus  140   b  may be a holographic image forming device capable of actively adjusting a beam diameter. The display apparatus  140   b  may also further include the eye tracker  141  and the processor  145 . The processor  145  may control the image providing device  20   a  in response to information about a location or a size of the pupil of the observer, which is provided from the eye tracker  141 , and the image providing device  20   a  may increase or decrease a beam diameter of light containing an image according to the control of the processor  145 . In this case, because the image providing device  20   a  may directly adjust the beam diameter of the light containing the image, a separate beam diameter adjustment device may not be used. 
     Referring to  FIG. 15 , a display apparatus  140   c  may further include a diffusion plate  70 , the eye tracker  141 , and the processor  145 . The diffusion plate  70 , used as a beam diameter adjustment device, may be arranged between the image providing device  20  and the combiner  10 , and may have a variable diffusion angle. For example, the diffusion plate  70  may change a diffusion angle thereof while a haze characteristic is changed according to an electrical control. The processor  145  may increase or decrease the diffusion angle of the diffusion plate  70  in response to information about a location or a size of the pupil of the observer, which is provided from the eye tracker  141 . 
     Referring to  FIG. 16 , a display apparatus  140   d  may further include a micro-electromechanical systems (MEMS) mirror array  80 , the eye tracker  141 , and the processor  145 . The MEMS mirror array  80  used as a beam diameter adjustment device may include a plurality of micro-mirrors which are between the image providing device  20  and the combiner  10  and electromechanically operate. The MEMS mirror array  80  may freely adjust a direction of light reflected from the plurality of micro-mirrors by independently controlling slopes of the plurality of micro-mirrors. For example, the MEMS mirror array  80  may electrically adjust a reflective region. The processor  145  may adjust a beam diameter of light to be incident on the combiner  10  by controlling the MEMS mirror array  80  in response to information about a location or a size of the pupil of the observer, which is provided from the eye tracker  141 . 
     Referring to  FIG. 17 , a display apparatus  140   e  may further include an actuator  90 , the eye tracker  141 , and the processor  145 . The actuator  90  may move the image providing device  20  to adjust a distance between the combiner  10  and the image providing device  20 . For example, the actuator  90  may include a linear motor. Because a beam diameter of light containing an image, which is incident on the combiner  10 , varies according to the distance between the combiner  10  and the image providing device  20 , the actuator  90  may be used as a beam diameter adjustment device. The processor  145  may adjust the distance between the combiner  10  and the image providing device  20  by controlling the actuator  90  in response to information about a location or a size of the pupil of the observer, which is provided from the eye tracker  141 . 
       FIG. 18  illustrates an example in which an effective reaction area of an opening is adjusted based on pupil information of the observer in the example embodiments illustrated in  FIGS. 12 to 17 . To this end, a size (e.g., a width or a diameter) of the plurality of volume gratings VG 1 , VG 2 , and VG 3  may be a little bit larger than a size necessary in a general case. For example, when the effective reaction area of the opening at a normal location of the pupil of the observer is about 1 mm, the size of the plurality of volume gratings VG 1 , VG 2 , and VG 3  may be about 1.5 mm. In addition, a beam diameter of light containing an image, which is to be incident on the plurality of volume gratings VG 1 , VG 2 , and VG 3 , may be adjusted according to a location change of the observer. For example, the beam diameter of the light containing the image at the normal location of the pupil of the observer may be adjusted to about 1 mm. In addition, as a distance between the observer and the combiner  10  is closer, or when a size of the pupil of the observer is larger, the beam diameter of the light containing the image, which is to be incident on each volume grating VG 1 , VG 2 , or VG 3  decreases. As the distance between the observer and the combiner  10  increases, or when the size of the pupil of the observer decreases, the beam diameter of the light containing the image, which is to be incident on each volume grating VG 1 , VG 2 , or VG 3  increases. 
     According to the example embodiments described above, a change in a location or a size of the pupil of the observer may be actively responded. Particularly, because the plurality of volume gratings VG 1 , VG 2 , and VG 3  of the combiner  10  react to only light of a particular wavelength, which is incident at a particular angle, even when a size of the plurality of volume gratings VG 1 , VG 2 , and VG 3  increases, a transmittance of external light does not significantly decrease. Therefore, even without sacrificing the transmittance of external light, a change in a location or a size of the pupil of the observer may be responded. 
       FIGS. 19 to 21  show various electronic devices to which the display apparatus according to the above-described example embodiments is applicable. As shown in  FIGS. 19 to 21 , the display apparatus may constitute a wearable device. In other words, the display apparatus may be applied to a wearable device. For example, the display apparatus may be applied to a head mounted display (HMD). In addition, the display apparatus may be applied to glasses-type displays, goggle-type displays, and the like. Wearable electronic devices shown in  FIGS. 19 to 21  may be operated in conjunction with a smartphone. Such a display apparatus may be a virtual reality (VR) display apparatus, an AR display apparatus, or an MR display apparatus manufactured in the form of head mounted type, glasses type, or goggles type capable of providing VR or a virtual image and a real external landscape together. 
     In addition, the display apparatus may be provided in a smartphone, and the smartphone itself may be used as a VR display apparatus, an AR display apparatus, or an MR display apparatus. In other words, the display apparatus may be applied to a small electronic device (mobile electronic device) that is not the wearable device as shown in  FIGS. 19 to 21 . In addition, application fields of the display apparatus may vary. For example, the display apparatus may be applied not only to implementing VR, AR, or MR, but also to other fields. For example, the display apparatus may also be applied to a small television or a small monitor that a user may wear 
     It should be understood that example embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each example embodiment should typically be considered as available for other similar features or aspects in other embodiments. While one or more example embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.