Patent Publication Number: US-2022229299-A1

Title: Display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to and benefits of Korean Patent Application No. 10-2021-0007461 under 35 U.S.C. § 119, filed on Jan. 19, 2021 in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     This disclosure relates to a display device. 
     2. Description of the Related Art 
     As a technique capable of implementing virtual reality is developed, an interest in a display device related thereto is increasing. The virtual reality may be realistically provided through a three-dimensional display device that can be viewed very closely by a user, such as a head mounted display (HMD) or a face mounted display (FMD). Since the display device is provided in front of a user&#39;s eyes, in case that the display device has low resolution, a screen door effect in which lines are visible between pixels may occur. This is a phenomenon in which a black matrix between pixels is enlarged and viewed through a lens of the display device, and may decrease as the resolution of the display device increases. 
     In a display device having a size that can be worn by a user on his/her head or face, it may be difficult or limited in a process to implement high resolution. 
     The above information disclosed in this background section is only for enhancement of understanding of the background, and therefore, it may contain information that may not form the prior art that may already be known to a person of ordinary skill in the art. 
     SUMMARY 
     Embodiments provide a high-resolution display device. 
     A display device according to an embodiment may include an optical combiner including a first surface; a second surface disposed at a first side of the first surface; and a third surface disposed at a second side of the first surface; a diffractive optical element disposed on the first surface of the optical combiner; a first display panel disposed on the second surface of the optical combiner and displaying a first color image; and a second display panel disposed on the third surface of the optical combiner and displaying a second color image. 
     The diffractive optical element may cause light providing the first color image and light providing the second color image to travel in a direction perpendicular to the first surface of the optical combiner. 
     A display screen of the first display panel may be parallel to the second surface of the optical combiner, and a display screen of the second display panel may be parallel to the third surface of the optical combiner. 
     The display device may further include a lens facing the first surface of the optical combiner. 
     A first distance between a display screen of the first display panel and the diffractive optical element may be equal to a second distance between a display screen of the second display panel and the diffractive optical element. 
     A focal length of the lens may be equal to a sum of the first distance between the display screen of the first display panel and the diffractive optical element and a distance between the diffractive optical element and the lens. 
     The optical combiner may include a fourth surface disposed between the second surface of the optical combiner and the third surface of the optical combiner. The display device may include a third display panel disposed on the fourth surface and displaying a third color image. 
     The fourth surface of the optical combiner may be parallel to the first surface of the optical combiner, and a display screen of the third display panel may be parallel to the fourth surface of the optical combiner. 
     The diffractive optical element may cause light providing the third color image to travel in a direction perpendicular to the first surface of the optical combiner. 
     The optical combiner may include a fourth surface disposed between the second surface of the optical combiner and the third surface of the optical combiner. The display device may include a third display panel disposed on the fourth surface of the optical combiner and displaying a third color image. 
     The optical combiner may have a substantially triangular or substantially trapezoidal cross-sectional shape. 
     The diffractive optical element may be disposed on the first surface of the optical combiner. 
     The diffractive optical element may be patterned on the first surface of the optical combiner. 
     The display device may further include an optical film disposed between the first display panel and the second surface of the optical combiner and between the second display panel and the third surface of the optical combiner. 
       α= x  cos θ 1   , β=x  cos θ 2 .
 
     are satisfied when an angle formed by the first surface and the second surface is θ 1 , an angle formed by the first surface and the third surface is θ 2 , a width of the first color image is α, a width of the second color image is β, and a width of the diffractive optical element is x. 
     A display device according to an embodiment may include an optical combiner including a first surface; a second surface disposed at a first side of the first surface; a third surface disposed at a second side of the first surface; and a fourth surface facing the first surface; a diffractive optical element disposed on the first surface of the optical combiner; a first display panel disposed on the second surface of the optical combiner and displaying a first color image; a second display panel disposed on the third surface of the optical combiner and displaying a second color image; and a third display panel disposed on the first surface of the optical combiner or the fourth surface of the optical combiner to display a third color image. 
     The display device may further include a lens adjacent to the first surface of the optical combiner, and the third display panel may be disposed on the fourth surface of the optical combiner. 
     The display device may further include a lens adjacent to the fourth surface of the optical combiner, the display panel may be disposed on the first surface of the optical combiner, and the diffractive optical element may be disposed between the display panel and the optical combiner. 
     The optical combiner may have a substantially trapezoidal cross-sectional shape, and the first surface of the optical combiner and the fourth surface of the optical combiner may be parallel to each other. 
       α= x  cos θ 1   , β=x  cos θ 2   , γ=x.  
 
     are satisfied when an angle formed by the first surface and the second surface is θ 1 , an angle formed by the first surface and the third surface is θ 2 , a width of the first color image is α, a width of the second color image is β, a width of the third color image is γ, and a width of the diffractive optical element is x. 
     According to the embodiments, the resolution of the display device may be increased and luminance may be improved by combining monochrome images. According to the embodiments, there are other advantageous effects that can be recognized throughout the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects and features of the disclosure will become more apparent by describing in detail embodiments thereof with reference to the attached drawings, in which: 
         FIG. 1  illustrates a schematic structure and image superimposition of a display device according to an embodiment. 
         FIG. 2  illustrates a schematic front view showing a structure of a 3D image display device according to an embodiment. 
         FIG. 3  illustrates a schematic side view showing a structure of a display device according to an embodiment. 
         FIG. 4 ,  FIG. 5 , and  FIG. 6  illustrate a design method of a display device according to an embodiment. 
         FIG. 7  and  FIG. 8  each illustrate a schematic side view showing a structure of a display device according to an embodiment. 
         FIG. 9  and  FIG. 10  illustrate a design method of a display device according to an embodiment. 
         FIG. 11  and  FIG. 12  illustrate a 3D image display device according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The disclosure will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments are shown. 
     Further, sizes and thicknesses of constituent elements shown in the accompanying drawings are arbitrarily given for better understanding and ease of description. 
     As used herein, the singular forms, “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In the specification and the claims, the term “and/or” is intended to include any combination of the terms “and” and “or” for the purpose of its meaning and interpretation. For example, “A and/or B” may be understood to mean “A, B, or A and B.” The terms “and” and “or” may be used in the conjunctive or disjunctive sense and may be understood to be equivalent to “and/or.” 
     In the specification and the claims, the phrase “at least one of” is intended to include the meaning of “at least one selected from the group of” for the purpose of its meaning and interpretation. For example, “at least one of A and B” may be understood to mean “A, B, or A and B.” 
     It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. For example, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element without departing from the scope of the disclosure. 
     The spatially relative terms “below”, “beneath”, “lower”, “above”, “upper”, or the like, may be used herein for ease of description to describe the relations between one element or component and another element or component as illustrated in the drawings. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the drawings. For example, in the case where a device illustrated in the drawing is turned over, the device positioned “below” or “beneath” another device may be placed “above” another device. Accordingly, the illustrative term “below” may include both the lower and upper positions. The device may also be oriented in other directions and thus the spatially relative terms may be interpreted differently depending on the orientations. 
     The terms “overlap” or “overlapped” mean that a first object may be above or below or to a side of a second object, and vice versa. Additionally, the term “overlap” may include layer, stack, face or facing, extending over, covering, or partly covering or any other suitable term as would be appreciated and understood by those of ordinary skill in the art. 
     When an element is described as ‘not overlapping’ or ‘to not overlap’ another element, this may include that the elements are spaced apart from each other, offset from each other, or set aside from each other or any other suitable term as would be appreciated and understood by those of ordinary skill in the art. 
     The terms “face” and “facing” mean that a first element may directly or indirectly oppose a second element. In a case in which a third element intervenes between the first and second element, the first and second element may be understood as being indirectly opposed to one another, although still facing each other. 
     It will be understood that when an element such as a layer, film, area, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising”, “includes,” and/or “including,” “has,” “have,” and/or “having,” and variations thereof when used in this specification, will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
     In addition, in the specification, “connected” or “coupled” means that two or more components are not only directly connected or coupled, but two or more components may be connected or coupled indirectly through other components, physically connected or coupled as well as being electrically connected or coupled, or it may be referred to by different names depending on the location or function, but may include connecting or coupling each of parts that may be substantially integral to each other. 
     The phrase “in a plan view” means viewing the object from the top, and the phrase “in a schematic cross-sectional view” means viewing a cross-section of which the object is vertically cut from the side. 
     “About” or “approximately” as used herein is inclusive of the stated value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art, considering the measurement in question and the error associated with measurement of the particular quantity (i.e., the limitations of the measurement system). For example, “about” may mean within one or more standard deviations, or within ±30%, 20%, 10%, 5% of the stated value. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  illustrates a schematic structure and image superimposition of a display device  10  according to an embodiment. 
     The display device  10  may include an optical combiner  100 , a diffractive optical element  200 , display panels  310 ,  320 , and  330 , and a lens  400 . 
     The optical combiner  100  may provide an optical path between the display panels  310 ,  320 , and  330  and the diffractive optical element  200 . The optical combiner  100  is a medium that transfers an image that is displayed on each of the display panels  310 ,  320 , and  330  to the diffractive optical element  200 . The optical combiner  100  may be an optical element such as a prism or a lens. The optical combiner  100  may be formed of a transparent material such as glass or plastic, by way of non-limiting example. The optical combiner  100  may be entirely formed of a same material or similar material. The optical combiner  100  may have a substantially polygonal column shape (for example, a substantially triangular column or a substantially quadrangular column). 
     The diffractive optical element  200  may change the optical path provided by the optical combiner  100 . The diffractive optical element  200  may refract, transmit, or reflect light to direct light in a direction. For example, the diffractive optical element  200  may refract light forming images displayed by the display panels  310  and  320  toward the lens  400 , and may transmit light forming images displayed by the display panel  330  toward the lens  400 . The diffractive optical element  200  may emit light incident at an angle to a front (in a direction toward the lens  400 ). The diffractive optical element  200  may be a diffraction grating. The diffractive optical element  200  may be attached or connected or coupled to the optical combiner  100 , or may be patterned on a surface of the optical combiner  100 . 
     Each of the display panels  310 ,  320 , and  330  may be a single color display panel capable of providing an image displayed in one of primary colors. The primary colors may include red, green, and blue. The display panels  310 ,  320 , and  330  may provide images of different colors. For example, the display panels  310 ,  320 , and  330  include a first display panel  310  for displaying a first color image IM 1 , a second display panel  320  for displaying a second color image IM 2 , and a third display panel  330  for displaying a third color image IM 3 . The first display panel  310  may include only first color pixels, the second display panel  320  may include only second color pixels, and the third display panel  330  may include only third color pixels. The first color may be green, the second color may be red, and the third color may be blue, but the colors are not limited thereto. For example, the first color may be one of red, green, and blue, the second color may be one of red, green, and blue except for the first color, and the third color may be one of red, green, and blue except for the first color and second color. 
     There is no limitation on a type of display panels included in the display device  10 . For example, the display panels  310 ,  320 , and  330  may be emission display panels including organic or inorganic light emitting diodes. The display panels  310 ,  320 , and  330  may be liquid crystal display panels, as an example. 
     The lens  400  may enlarge an image provided through the diffractive optical element  200 , thereby improving a user&#39;s senses of immersion and presence. In case that an image is enlarged by the lens  400 , a black matrix between pixels is also enlarged, so it may be advantageous to increase the resolution of the display device  10  in order to reduce the screen door effect in which lines are visible between pixels may occur. 
     Monochrome images IM 1 , IM 2 , and IM 3  displayed by the respective display panels  310 ,  320 , and  330  are combined in the diffractive optical element  200  by the optical combiner  100  to be displayed as a multicolor image. The first color image IM 1  displayed by the first display panel  310 , the second color image IM 2  displayed by the second display panel  320 , and the third color image IM 3  displayed by the third display panel  330  may be directed to the diffractive optical element  200  by the optical combiner  100  and may be superimposed in the diffractive optical element  200  to provide a multicolor image. As such, in case that an image is displayed by combining the monochrome images IM 1 , IM 2 , and IM 3 , the resolution of the display device may be increased compared to in case that multiple colors are displayed on a single display panel. For example, the resolution of the display device may be increased by about three times compared to displaying an image by arranging all of the first, second, and third color pixels on a single display panel. 
     The optical combiner  100 , the diffractive optical element  200 , the display panels  310 ,  320 , and  330 , and the lens  400  may be fixed to an optical holder (not illustrated) to maintain their respective positions. 
       FIG. 2  illustrates a schematic front view showing a structure of a 3D image display device according to an embodiment. 
     One of the ways to express a 3D image is a method using binocular disparity, which is a biggest factor in recognizing a 3D effect. For example, in case that different 2D images are respectively provided to a left eye and a right eye, and an image provided to the left eye (hereinafter referred to as “left eye image”) and an image provided to the right eye (hereinafter referred to as “right eye image”) are transferred to a brain, the left eye image and the right eye image are fused to be recognized as a 3D image with depth perception by the brain. The 3D image display device may include a left-eye display device  10 L and a right-eye display device  10 R that respectively provide a left-eye image and a right-eye image with parallax in order for a user to feel a 3D effect. 
     The left eye display device  10 L may include an optical combiner  100 L, a diffractive optical element  200 L, display panels  310 L and  320 L, and a lens  400 L. The right eye display device  10 R may include an optical combiner  100 R, a diffractive optical element  200 R, display panels  310 R and  320 R, and a lens  400 R. The left eye display device  10 L and the right eye display device  10 R may be fixed to a single optical holder or to separate optical holders. 
     In case that the left-eye display device  10 L and the right-eye display device  10 R each include two display panels ( 310 L and  320 L;  310 R and  320 R), the left-eye display device  10 L and the right-eye display device  10 R may each include a display panel for displaying a same color among the primary colors. For example, the display panels  310 L and  320 L of the left-eye display device  10 L may respectively display a red image and a green image, and the display panels  310 R and  320 R of the right-eye display device  10 R may respectively display a blue image and a green image. Even in case that the left-eye display device  10 L and the right-eye display device  10 R each display an image in which two monochrome images are superimposed with each other, a user may combine the left-eye image and the right-eye image to recognize the image as a multicolor image. Unlike as illustrated, the left-eye display device  10 L and the right-eye display device  10 R may each include three display panels that display monochrome images of different colors. 
     The left-eye display device  10 L and the right-eye display device  10 R may have differences in colors of images provided by the display panels  310 L and  320 L, and  310 R and  320 R, but they may be structurally the same. Hereinafter, the left-eye display device  10 L and the right-eye display device  10 R will be described as the display device  10  without being distinguished. For example, the display device  10  described herein may be a left-eye display device  10 L or a right-eye display device  10 R. 
       FIG. 3  illustrates a schematic side view showing a structure of a display device  10  according to an embodiment.  FIG. 3  illustrates an example in which one display device  10  may include two display panels  310  and  320 . 
     The optical combiner  100  may have a substantially triangular cross-sectional shape. The optical combiner  100  may have a substantially triangular column shape. The optical combiner  100  may include a first surface S 1 , a second surface S 2 , and a third surface S 3 . 
     The diffractive optical element  200  may be provided or disposed on the first surface S 1  of the optical combiner  100 . The diffractive optical element  200  may be attached or disposed or coupled to the first surface S 1 , or may be patterned on the first surface S 1 . A first surface S 1  may be the largest among the three surfaces S 1 , S 2 , and S 3  of the optical combiner  100 , but the disclosure is not limited thereto. 
     The first display panel  310  may be provided or disposed on the second surface S 2 , and the second display panel  320  may be provided or disposed on the third surface S 3 . A display screen of the first display panel  310  may be parallel to the second surface S 2 , and a display screen of the second display panel  320  may be parallel to the third surface S 3 . The display screen of the first display panel  310  may be spaced apart from the second surface S 2  at an interval, or may be in contact with the second surface S 2 . The display screen of the second display panel  320  may be spaced apart from the third surface S 3  at an interval, or may be in contact with the third surface S 3 . 
     Herein, the display screen represents a surface on which the images IM 1  and IM 2  may be displayed on each of the display panels  310  and  320 , and the display screen may have a width in a horizontal direction and/or a vertical direction. A size of the display screen of the first display panel  310  may correspond to a size of the first color image IM 1 , and a size of the display screen of the second display panel  320  may correspond to a size of the second color image IM 2 . The first color image IM 1  and the second color image IM 2  may be respectively displayed on the display screen of the first display panel  310  and the display screen of the second display panel  320 , and for convenience, they are illustrated in the second surface S 2  and the third surface S 3  of the optical combiner  100 . 
     An optical film OF such as a microlens array film may be positioned or disposed between the first display panel  310  and the second surface S 2  and between the second display panel  320  and the third surface S 3 . 
     The lens  400  may be positioned or disposed adjacent to the first surface S 1  of the optical combiner  100 . The lens  400  may be separated from the first surface S 1  in consideration of a focal distance. 
     The first color image IM 1  displayed by the first display panel  310  and the second color image IM 2  displayed by the second display panel  320  may be directed to the diffractive optical element  200  by the optical combiner  100  and may be superimposed on the diffractive optical element  200 . The first color image IM 1  and the second color image IM 2  which are superimposed may be outputted toward the lens  400  by the diffractive optical element  200 , and a user may view an image in which the first color image IM 1  and the second color image IM 2  are superimposed through the lens  400  (for example, an image displayed by combining the first color image and the second color image). 
       FIG. 4 ,  FIG. 5 , and  FIG. 6  illustrate a design method of a display device  10  according to an embodiment. 
     Referring to  FIG. 4  regarding a design of the optical combiner  100 , a cross-section of the optical combiner  100  in the display device  10  including the two display panels  310  and  320  illustrated in  FIG. 3  is illustrated. A first distance d 1  between the first color image IM 1  and the first surface S 1  and a second distance d 2  between the second color image IM 2  and the first surface S 1  may be the same in order to superimpose the first color image IM 1  and the second color image IM 2  on the first surface S 1  on which the diffractive optical element  200  may be provided or disposed. Unless otherwise stated, in this specification, the distance represents the distance between centers of constituent elements. 
     The following equations can be established in case that a vertex where the first surface S 1  and the second S 2  meet is P, a vertex where the first surface S 1  and the third S 3  meet is Q, an intersection point of the first color image IM 1  and the second color image IM 2  on the first surface S 1  is M, an angle formed between the first and second surfaces S 1  and S 2  is θ 1 , and an angle formed between the first and third surfaces S 1  and S 3  is θ 2 . 
     
       
         
           
             
               
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     In the above equations, a 1  indicates a distance between the vertex P and vertex M, b 1  indicates a distance between the vertex Q and the intersection point M, a indicates a distance between the first color image IM 1  and the vertex P, and b indicates a distance between the second color image IM 2  and the vertex Q. The intersection point M may correspond to a center of the diffractive optical element  200 . 
     θ 1  and θ 2  may be changed depending on a period of a lattice, patterning, for example, in the diffractive optical element  200 . Light has a different refractive index depending on a wavelength thereof, and thus θ 1  and θ 2  may be different. θ 1  and θ 2  may be determined depending on a characteristic of the diffractive optical element  200  and colors of the monochrome images IM 1  and IM 2  provided by the display panels  310  and  320 . In case that θ 1  and θ 2  are determined, optimal positions of the diffractive optical element  200  and the display panels  310  and  320  on the optical combiner  100  may be determined depending on the above equations. 
     For example, in case that θ 1  and θ 2  are determined as about 45° and about 59.67°, a may be about 30.80 mm, b may be about 18.01 mm, a 1  may be about 43.55 mm, and b 1  may be about 35.68 mm. In case that the first color image IM 1  is a green image and the second color image IM 2  is a red image, θ 2  may be greater or smaller than θ 1 . Each value of θ 1  and θ 2  and a large-small relationship therebetween may depend on the characteristic of the diffractive optical element  200 . 
     A method of designing a size of each of the display panels  310  and  320 , more precisely, a size of the display screen of each of the display panels  310  and  320 , will be described with reference to  FIG. 5 . 
     The following equations can be established in case that a width of the first color image IM 1  corresponding to the width of the display screen of the first display panel  310  is α, a width of the second color image IM 2  corresponding to the width of the display screen of the second display panel  320  is ( 3 , and a width of the diffractive optical element  200  is x. 
         a′=a   n  cos θ 1   , b′=b   n  cos θ 2  
 
       α= x  cos θ 1   , β=x  cos θ 2  
 
       0&lt;β&lt;2 b  
 
     In the above equations, a′ indicates a distance between the first color image IM 1  and the vertex P, b′ indicates a distance between the second color image IM 2  and the vertex Q, a n  indicates a distance between the diffractive optical element  200  and the vertex P, and b n  indicates a distance between the diffractive optical element  200  and the vertex Q. 
     For example, in case that x is about 20 mm, θ 1  is about 45°, and θ 2  is about 59.67°, α is about 14.14 mm, and β is about 10.09 mm. As such, the width of the display screen of each of the display panels  310  and  320  may depend on the two angles θ 1  and θ 2  of the optical combiner  100  and a width x of the diffractive optical element  200 . The width x of the diffractive optical element  200  may correspond to or substantially correspond to the width of an image area provided by the diffractive optical element  200 . 
     Referring to  FIG. 6 , in case that b n  is 0 in case that θ 2  is greater than θ 1 , it is possible to minimize the size of the optical combiner  100 . A reason that a n  is greater than 0 is because the first distance d 1  between the first color image IM 1  and the diffractive optical element  200  and the second distance d 2  between the second color image IM 2  and the diffractive optical element  200  are the same. In contrast, in case that θ 1  is greater than θ 2 , it is possible to minimize the size of the combiner  100  by designing a n  as 0. 
     Respective distances from the display panels  310  and  320  to the lens  400  and the focal length of the lens  400  may be designed to be the same in order to increase image sharpness. As a result, the focal length of the lens  400 =the first distance d 1  between the first color image IM 1  and the diffractive optical element  200 +the distance between the diffractive optical element  200  and the lens  400 =the second distance d 2  between the second color image IM 2  and the diffractive optical element  200 +the distance between the diffractive optical element  200  and the lens  400 . Herein, the first distance d 1  may correspond to a distance between the display screen of the first display panel  310  and the diffractive optical element  200 , and the second distance d 2  may correspond to a distance between the display screen of the second display panel  320  and the diffractive optical element  200 . 
       FIG. 7  and  FIG. 8  each illustrate a schematic side view showing a structure of a display device according to an embodiment.  FIG. 7  and  FIG. 8  illustrate an example in which one display device  10  may include three display panels  310 ,  320 , and  330 . 
     Referring to  FIG. 7 , the optical combiner  100  may have a substantially trapezoidal cross-sectional shape. The optical combiner  100  may have a substantially quadrangular column shape. The optical combiner  100  may include a first surface S 1 , a second S 2 , a third surface S 3 , and a fourth surface S 4 . The first and fourth surfaces S 1  and S 4  may face each other, and may be parallel. The first surface S 1  may be largest among the four surfaces S 1 , S 2 , S 3 , and S 4  of the optical combiner  100 , but the disclosure is not limited thereto. 
     The diffractive optical element  200  may be provided or disposed on the first surface S 1  of the optical combiner  100 . The diffractive optical element  200  may be attached or disposed or coupled to the first surface S 1 , or may be patterned on the first surface S 1 . 
     The first display panel  310  may be provided or disposed on the second surface S 2 , the second display panel  320  may be provided or disposed on the third surface S 3 , and the third display panel  330  may be provided or disposed on the fourth surface S 4 . The display screen of the first display panel  310  may be parallel to the second surface S 2 , the display screen of the second display panel  320  may be parallel to the third surface S 3 , and the display screen of the third display panel  330  may be parallel to the fourth surface S 4 . The display screen of the first display panel  310  may be spaced apart from the second surface S 2  at an interval, or may be in contact with the second surface S 2 . The display screen of the second display panel  320  may be spaced apart from the third surface S 3  at an interval, or may be in contact with the second surface S 2 . The display screen of the third display panel  330  may be spaced apart from the fourth surface S 4  at an interval, or may be in contact with the second surface S 2 . An optical film OF such as a microlens array film may be positioned or disposed between the first display panel  310  and the second surface S 2 , between the second display panel  320  and the third surface S 3 , and between the third display panel  330  and the fourth surface S 4 . 
     The lens  400  may be positioned or disposed adjacent to the first surface of the optical combiner  100  and the diffractive optical element  200 . The lens  400  may be separated from the first surface S 1  in consideration of a focal distance. 
     The first color image IM 1  displayed by the first display panel  310 , the second color image IM 2  displayed by the second display panel  320 , and the third color image IM 3  displayed by the third display panel  330  may be directed to the diffractive optical element  200  by the optical combiner  100 , and may be superimposed in the diffractive optical element  200 . The diffractive optical element  200  may refract the first color image IM 1  and the second color image IM 2  and may transmit the third color image IM 3 . The first color image IM 1 , the second color image IM 2 , and the third color image IM 3 , which are superimposed, may be outputted toward the lens  400  by the diffractive optical element  200 , and a user may view an image in which the first color image IM 1 , the second color image IM 2 , and the third color image IM 3  are superimposed through the lens  400 . 
     The display device  10  illustrated in  FIG. 8  may have a structure in which the optical combiner  100  is inverted in the display device  10  illustrated in  FIG. 7 . In case that a difference from the display device  10  illustrated in  FIG. 7  is described, the diffractive optical element  200  may be provided or disposed on the first surface S 1  of the optical combiner  100 , and the third display panel  330  may also be provided or disposed on the first surface S 1 . The diffractive optical element  200  may be positioned or disposed between the third display panel  330  and the optical combiner  100 . The lens  400  may be positioned or disposed adjacent to the fourth surface S 4 . 
     The first color image IM 1  and the second color image IM 2  may be directed to the diffractive optical element  200  by the optical combiner  100 , and the first color image IM 1 , the second color image IM 2 , and the third color image IM 3  may be superimposed in the diffractive optical element  200 . The diffractive optical element  200  may reflect the first color image IM 1  and the second color image IM 2  at an angle (for example, in a direction that is perpendicular or substantially perpendicular to the first surface S 1 ), and may transmit the third color image IM 3 . The first color image IM 1 , the second color image IM 2 , and the third color image IM 3 , which are superimposed, may be directed toward the lens  400  by the diffractive optical element  200 , and a user may view an image in which the first color image IM 1 , the second color image IM 2 , and the third color image IM 3  are superimposed through the lens  400 . 
       FIG. 9  and  FIG. 10  illustrate a design method of a display device  10  according to an embodiment. 
     Referring to  FIG. 9  regarding a design of the optical combiner  100 , a cross-section of the optical combiner  100  in the display device  10  including the three display panels  310 ,  320 , and  330  illustrated in  FIG. 7  is illustrated. A first distance d 1  between the first color image IM 1  and the first surface S 1 , a second distance d 2  between the second color image IM 2  and the first surface S 1 , and a third distance d 3  between the third color image IM 3  and the first surface S 1  may be the same in order to superimpose the first color image IM 1 , the second color image IM 2 , and the third color image IM 3  on the first surface S 1  on which the diffractive optical element  200  is provided. 
     Similar to the design of the optical combiner  100  in the display device  10  including the two display panels  310  and  320 , the following equations can be satisfied in case that a vertex where the first surface S 1  and the second S 2  meet is P, a vertex where the first surface S 1  and the third S 3  meet is Q, an intersection point of the first color image IM 1  and the second color image IM 2  on the first surface S 1  is M, an angle formed between the first and second surfaces S 1  and S 2  is θ 1 , and an angle formed between the first and third surfaces S 1  and S 3  is θ 2 . 
     
       
         
           
             
               
                 a 
                 1 
               
               = 
               
                 a 
                 
                   cos 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     θ 
                     1 
                   
                 
               
             
             , 
             
               
                 b 
                 1 
               
               = 
               
                 b 
                 
                   cos 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     θ 
                     2 
                   
                 
               
             
           
         
       
       
         
           
             
               
                 a 
                 1 
               
               + 
               
                 b 
                 1 
               
             
             = 
             
               
                 a 
                 
                   cos 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     θ 
                     1 
                   
                 
               
               + 
               
                 b 
                 
                   cos 
                   ⁢ 
                   
                       
                   
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                     θ 
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               d 
               1 
             
             = 
             
               
                 d 
                 2 
               
               = 
               
                 
                   d 
                   3 
                 
                 = 
                 
                   
                     a 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     tan 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       1 
                     
                   
                   = 
                   
                     b 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     tan 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       2 
                     
                   
                 
               
             
           
         
       
       
         
           
             a 
             = 
             
               
                 ( 
                 
                   
                     a 
                     1 
                   
                   + 
                   
                     b 
                     1 
                   
                 
                 ) 
               
               ÷ 
               
                 ( 
                 
                   
                     1 
                     
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
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                       tan 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
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                       tan 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
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                       ⁢ 
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         θ 
                         2 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             b 
             = 
             
               
                 ( 
                 
                   
                     a 
                     1 
                   
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                     1 
                   
                 
                 ) 
               
               ÷ 
               
                 ( 
                 
                   
                     1 
                     
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                       ⁢ 
                       
                           
                       
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                       ⁢ 
                       
                           
                       
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                       ⁢ 
                       
                           
                       
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                         θ 
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     θ 1  and θ 2  may be changed depending on a period of a lattice, patterning, by way of example in the diffractive optical element  200 . Light has a different refractive index depending on a wavelength thereof, and thus θ 1  and θ 2  may be different. θ 1  and θ 2  may be determined depending on a characteristic of the diffractive optical element  200  and colors of the monochrome images IM 1  and IM 2  provided by the first and second display panels  310  and  320 . In case that θ 1  and θ 2  are determined, optimal positions of the diffractive optical element  200  and the first and second display panels  310  and  320  on the optical combiner  100  may be determined depending on the above equations. A position of the third display panel  330  may be optimized by enabling the third distance d 3  to be equal to the first distance d 1  and the second distance d 2 . 
     A method of designing a size of each of the display panels  310 ,  320 , and  330 , more precisely, a size of the display screen of each of the display panels  310 ,  320 , and  330 , will be described with reference to  FIG. 10 . 
     The following equations can be established in case that a width of the first color image IM 1  corresponding to the width of the first display screen of the first display panel  310  is α, a width of the second color image IM 2  corresponding to the width of the second display screen of the second display panel  320  is β, and a width of the diffractive optical element  200  is x. 
         a′=a   n  cos θ 1   , b′=b   n  cos θ 2  
 
       α= x  cos θ 1   , β=x  cos θ 2  
 
       0&lt;β&lt;2 b  
 
     The widths of the display screens of the first and second display panels  310  and  320  may depend on the two angles θ 1  and θ 2  of the optical combiner  100  and the width x of the diffractive optical element  200 . Since the third display panel  330  is parallel to the diffractive optical element  200 , a width γ of the display screen of the third display panel  330  may be the same as the width x of the diffractive optical element  200 . The width x of the diffractive optical element  200  corresponds to the width of an image area provided by the diffractive optical element  200 . 
     Respective distances from the display panels  310 ,  320 , and  330  to the lens  400  and the focal length of the lens  400  may be designed to be the same in order to increase image sharpness. As a result, the focal length of the lens  400 =the first distance d 1  between the first color image IM 1  and the diffractive optical element  200 +the distance between the diffractive optical element  200  and the lens  400 =the second distance d 2  between the second color image IM 2  and the diffractive optical element  200 +the distance between the diffractive optical element  200  and the lens  400 =the third distance d 3  between the third color image IM 3  and the diffractive optical element  200 +the distance between the diffractive optical element  200  and the lens  400 . Herein, the first distance d 1  may correspond to a distance between the display screen of the first display panel  310  and the diffractive optical element  200 , the second distance d 2  may correspond to a distance between the display screen of the second display panel  320  and the diffractive optical element  200 , and the third distance d 3  may correspond to a distance between the display screen of the third display panel  330  and the diffractive optical element  200 . 
       FIG. 11  and  FIG. 12  illustrate a 3D image display device according to an embodiment. 
       FIG. 11  generally illustrates the 3D image display device, and  FIG. 12  illustrates an internal structure of the display device  10  by removing a portion of a holder  500 . 
     Referring to  FIG. 11 , the 3D image display device may include a left-eye display device  10 L and a right-eye display device  10 R, which may form a pair. The left-eye display device  10 L and the right-eye display device  10 R may be separate or integral with each other. 
     Referring to  FIG. 12 , the display device  10  may include a holder  500 , an optical combiner  100  accommodated therein, a diffractive optical element  200 , display panels  310  and  320 , and a lens  400 L. The diffractive optical element  200  may be provided or disposed on a long side of the optical combiner  100 . The display panels  310  and  320  may be vertically positioned as illustrated, but may be horizontally or diagonally positioned. Monochrome images displayed by the display panels  310  and  320  may be superimposed through the optical combiner  100  and the diffractive optical element  200  to be displayed as a multicolor image, and may be finally provided to a user through the lens  400 . 
     While the disclosure has been described in connection with what is considered to be practical embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the disclosure and the appended claims.