Patent Publication Number: US-11662578-B2

Title: Image display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application in a continuation of U.S. application Ser. No. 16/148,026, filed Oct. 1, 2018, which claims priority from Japanese Patent Application No. 2017-194273, filed on Oct. 4, 2017, in the Japanese Patent Office, and Korean Patent Application No. 10-2018-0088669, filed on Jul. 30, 2018, in the Korean Intellectual Property Office, the disclosures of which are herein incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     1. Field 
     Example embodiments relate to image display devices. 
     2. Description of the Related Art 
     Recently, technologies for displaying images with high resolution on image display devices have been actively developed. For example, a head-up display is an image display device that displays a clearer image by correcting chromatic aberration of light emitted from a light source through a prism. 
     The head-up display may include optical information display means such as a cathode ray tube (CRT), an optical system projecting light emitted from the optical information display means in a predetermined direction, and a screen that the light reaches. The optical system may include a plurality of lenses and mirrors to adjust the traveling direction of light emitted from the CRT to reach the screen. 
     SUMMARY 
     One or more example embodiments provide image display devices having a display surface with uniform resolution. 
     According to an aspect of an example embodiment, there is provided an image display device including an optical scanner configured to scan light emitted from a light source, a parallel light generator configured to generate the scanned light as parallel light, a prism configured to refract the parallel light, and a light direction changer including a plurality of points whereon the parallel light refracted by the prism is incident, and configured to reflect or diffract the parallel light and change a traveling direction of the parallel light, wherein the prism is provided on a path of each light traveling from the optical scanner to the light direction changer, and is configured to adjust an optical path difference of parallel light incident on each of the plurality of points of the light direction changer. 
     The prism may include a wedge prism. 
     The optical path difference may be adjusted by changing an apex angle of the wedge prism. 
     The prism may have an apex angle such that the optical path difference is 0. 
     The prism may include glass or resin. 
     The optical scanner may include a micro electro mechanical systems (MEMS) mirror. 
     The parallel light generator may include a parabolic mirror. 
     The parallel light generator may include a condenser lens. 
     The image display device may further include a plane mirror provided between the prism and the light direction changer and configured to reflect light refracted from the prism to be incident on the light direction changer. 
     The image display device may further include a screen on which light having a traveling direction changed by the light direction changer is incident, and an optical system provided between the screen and the light direction changer and configured to guide the light to be incident on the screen. 
     The image display device may further include a diffuser between the light direction changer and the optical system, wherein the diffuser is configured to diffuse the light incident from the light direction changer. 
     A surface of the light direction changer may include a reflective type pattern element. 
     A surface of the light direction changer may include a reflective type element having a cross-sectional pattern of a plurality of rectangular prism shapes. 
     A surface of the light direction changer may include a reflective type element having a cross-sectional pattern of a plurality of triangular prism shapes. 
     A surface of the light direction changer may include a reflective type element having a cross-sectional pattern of a plurality of semi-cylindrical shapes. 
     A surface of the light direction changer may include a reflective type element having a cross-sectional pattern of a plurality of parabolic shapes. 
     A surface of the light direction changer may include a reflective type pattern element having a pattern of a plurality of spherical concave lens array. 
     A surface of the light direction changer may include a reflective type pattern element having a pattern of a plurality of parabolic concave lens array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects will become apparent and more readily appreciated from the following description of the example embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG.  1    is a perspective view showing an image display device according to an example embodiment; 
         FIG.  2    is a side sectional view showing a side surface of the image display device of  FIG.  1    according to an example embodiment; 
         FIG.  3    is a cross-sectional view showing a configuration example of an image display device according to a related example; 
         FIG.  4    is a diagram for explaining a change in a beam diameter of a laser beam propagated by the image display device of  FIG.  3    according to an example embodiment; 
         FIG.  5    is a table showing an example of a relationship between an incidence angle of a laser beam with respect to a pattern element, a maximum optical path difference of the laser beam, and a difference in beam diameters of the image display device of  FIG.  3    according to an example embodiment; 
         FIG.  6    is a table showing an example of a relationship between an incidence angle of a laser beam with respect to a light direction changer of  FIGS.  1  and  2   , a maximum optical path difference of the laser beam, and an apex angle according to an example embodiment; 
         FIG.  7    is a perspective view showing an image display device according to an example embodiment; 
         FIG.  8    is a cross-sectional view showing the image display device of  FIG.  7    according to an example embodiment; 
         FIG.  9    is a cross-sectional view showing an image display device according to an example embodiment; 
         FIG.  10    is a cross-sectional view showing an image display device according to an example embodiment; 
         FIG.  11    shows a modification of a light direction changer included in image display devices according to example embodiments; 
         FIG.  12    shows a modification of a light direction changer included in image display devices according to example embodiments; 
         FIG.  13    shows a modification of a light direction changer included in image display devices according to example embodiments; 
         FIG.  14    shows a modification of a light direction changer included in image display devices according to example embodiments; 
         FIG.  15    shows a modification of a light direction changer included in image display devices according to example embodiments; 
         FIG.  16    shows a modification of a light direction changer included in image display devices according to example embodiments; and 
         FIG.  17    shows a modification of a light direction changer included in image display devices according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to example embodiments, examples of 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. 
     Throughout the specification, it will be understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 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. 
       FIG.  1    is a perspective view showing an image display device  100  according to an example embodiment. 
       FIG.  2    is a side sectional view showing a side surface of the image display device  100  of  FIG.  1    according to an example embodiment. 
     The image display device  100  may include, for example, a head-up display, a head-mounted display, a projector, or the like, but is not limited thereto and may include other image display devices. 
     Referring to  FIGS.  1  and  2   , the image display device  100  may include an optical scanner  120  (for example, a micro electro mechanical system (MEMS) mirror) scanning light emitted from a light source, a parallel light generator  130  generating the scanned light as parallel light, a prism  110  refracting the parallel light, a plane mirror  140  reflecting the refracted parallel light that passed through the prism  110 , and a light direction changer  150  (e.g., a pattern element) changing the traveling direction of the light reflected by the plane mirror  140 . 
     The light emitted from the optical scanner  120  may be the parallel light traveling through a plurality of paths by the parallel light generator  130 . The parallel light may be incident on the plane mirror  140  after passing through the prism  110 . The parallel light incident on the plane mirror  140  may be reflected and become incident on the light direction changer  150 . The direction of the light incident on the light direction changer  150  may be changed and projected on a screen. An optical path difference that may cause a non-uniform resolution of the screen may occur between the lights incident on the light direction changer  150  through the plurality of paths. The prism  110  may adjust the optical path difference between the lights. That is, the prism  110  may reduce or eliminate the optical path difference between the lights. The operating principle of the prism  110  for adjusting the optical path difference will be described later with reference to  FIG.  6   . 
     The optical scanner  120  may include a MEMS mirror. The MEMS mirror functions as the optical scanner  120  scanning a laser beam (for example, red light, green light, blue light, or a combination thereof) emitted from a light source by driving of two axes in the horizontal and vertical directions. The MEMS mirror may project an image onto the light direction changer  150 . The optical scanner  120  may include the MEMS mirror, but is not limited thereto, and may include a member capable of emitting a laser beam other than the MEMS mirror. The optical scanner  120  may also include a member capable of emitting light other than the laser beam. Further, the light emitted from the optical scanner  120  may be incident on the parallel light generator  130 . 
     The parallel light generator  130  may cause the light emitted from the optical scanner  120  to travel in rays parallel to each other. The parallel light generator  130  may include a parabolic mirror. When the light emitted from a focal position of the parallel light generator  130  is reflected at each point of the parallel light generator  130 , the traveling direction of each reflected light may be parallel to the optical axis of the parallel light generator  130 . Therefore, by arranging the optical scanner  120  at the focal position of the parallel light generator  130 , the light emitted from the optical scanner  120  may be reflected by the parallel light generator  130  and be parallel light traveling in a parallel direction. Also, a member other than the parabolic mirror may be used as long as the member may make the light emitted from the optical scanner  120  parallel to the optical axis. For example,  FIGS.  7  and  8    show an image display device  200  that uses a condenser lens  131  instead of the parabolic mirror. Further, for example, a collimator lens may be used instead of the parabolic mirror. However, example embodiments are not limited thereto. The parallel light reflected from the parallel light generator  130  may be incident on the prism  110 . 
     The prism  110  may be refract the light reflected by the parallel light generator  130 . The prism  110  may be, for example, a wedge prism having an apex angle α. The prism  110  may be provided on a path of each light traveling from the optical scanner  120  to the light direction changer  150  and may adjust the optical path difference of each light from the optical scanner  120  to the light direction changer  150  through refraction to be 0 or less than a predetermined value. Therefore, the image display devices  100  and  200  may adjust a difference in a beam diameter of the light direction changer  150  to be 0 or less than a predetermined value. 
     In  FIGS.  1  and  2   , the prism  110  is illustrated as a triangular prism having a triangular shape in which the apex angle α is not a right angle, but is not limited thereto. More specifically, as long as the optical path difference of each laser beam is adjusted to be 0 or less than the predetermined value, a rectangular prism in which the apex angle α is a right angle may be used, or a prism having any polyhedral shape may be used. Further, the laser beam transmitted through the prism  110  may be incident on the plane mirror  140 . The apex angle α of the prism  110  capable of adjusting the optical path difference to be 0 or less than the predetermined value will be described later. 
     The plane mirror  140  may be provided between the prism  110  and the light direction changer  150  and may reflect the light that passed through the prism  110  and allow the light to be incident on the light direction changer  150 . The plane mirror  140  may reflect the parallel laser beam transmitted through the prism  110  and allow the parallel laser beam to be incident on the light direction changer  150  while maintaining a parallel state. 
     The light direction changer  150  may include a pattern element. The light direction changer  150  may include a plurality of points where the parallel light refracted by the prism  110  and reflected by the plane mirror  140  is incident thereon. The pattern element may include a predetermined pattern on a surface. Examples of the pattern will be described later with reference to  FIGS.  11  to  17   . The traveling direction of the laser beam reflected by the plane mirror  140  may be changed by reflection or diffraction by the pattern of the pattern element. For example, by adjusting intervals of slits and an aspect ratio of the pattern element, the traveling direction of the light reflected from the plane mirror  140  may be changed to the front direction with respect to the pattern element. 
     The material of the pattern element is not particularly limited. For example, a wire grid may be used to form the predetermined concave-convex pattern on a surface of the pattern element. 
       FIG.  3    is a cross-sectional view showing a configuration example of an image display device  1000  according to a related example. 
       FIG.  4    is a diagram for explaining a change in a beam diameter of a laser beam propagated by the image display device  1000  of  FIG.  3   . 
     Referring to  FIG.  3   , the image display device  1000  may include a MEMS mirror  10 , a parabolic mirror  20 , and a pattern element  30 . In the related example, a prism included in an example embodiment is not provided. Light, for example, a laser beam emitted from the MEMS mirror  10  may be incident on the pattern element  30  as parallel light that is parallel to an optical axis  21  by the parabolic mirror  20 . The pattern element  30  may change the traveling direction of the incident parallel light and project the parallel light onto a screen. 
     An optical path difference of a laser beam may occur in the image display device  1000 , which may cause a difference in resolution of the screen. More specifically, an optical path of a laser beam reflected at a point A of the parabolic mirror  20  and incident on a surface end point a of the pattern element  30  may be greater by an optical path difference ΔOP than an optical path of a laser beam reflected at a point B of the parabolic mirror  20  and incident on a surface end point b of the pattern element  30 . For example, when a length V of the pattern element  30  from the end point a to the end point b is 83.2 mm and an incidence angle θ is 50.0 degree, the optical path difference ΔOP may be about 63.735 mm by Equation 1 below.
 
ΔOP=cos(90−θ)· V   (Equation 1)
 
       FIG.  4    shows a beam diameter along the traveling direction of the laser beam. Referring to  FIG.  4   , light having a strong directivity such as the laser beam and that propagates straight ahead may have the beam diameter gradually narrowing to a certain position and then gradually widening. Here, the beam diameter of the position where the beam diameter is the narrowest is referred to as a beam waist Wo. Resolution may be the highest at the beam waist Wo. Further, beam diameters before and after the beam waist Wo may have a similarity relationship. 
     Referring to  FIG.  3   , to maximize the resolution of a display image generated on a surface of the pattern element  30 , a position of each optical element may be necessarily adjusted such that the beam waist Wo is positioned on the surface of the pattern element  30 . When the beam waist Wo is adjusted to be located at a center point c (the center point between the end points a and b) of the surface of the pattern element  30  (that is, the beam diameter at the center point c is Wo) the resolution of the display image of the center point c may be the highest. For example, in case where an optical path difference between the light incident on the end point a or b and the light incident on the center point c is ΔOP/2, a beam diameter W of the end point a and the end point b may be greater than the beam waist Wo. In this case, the resolution of a display image of the end point a and the end point b may be lower than the resolution of the display image of the center point c. 
     Meanwhile, the beam waist Wo in the case of assuming an ideal Gaussian beam may be calculated by Equation 2 below. The beam diameter W of the end point a and the end point b may be calculated by Equation 3 below. In Equation 2, A denotes a wavelength of the laser beam. 
     
       
         
           
             
               
                 
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                     Equation 
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                   W 
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       FIG.  5    is a table showing a specific example of the relationship between the incidence angle θ of a laser beam with respect to the pattern element  30 , the maximum optical path difference ΔOP of the laser beam, and a difference in beam diameters of the image display device  1000  of  FIG.  3   . 
     The optical path difference ΔOP shown in  FIG.  5    may be obtained by using Equation 1, and the length V from the end point a to the end point b of the pattern element  30  may be 83.2 mm. 
     Referring to  FIG.  5   , the relationship of the incidence angle θ, the optical path difference ΔOP, the beam waist Wo may be calculated by using Equation 2, and the beam diameter W may be calculated by using Equation 3. For example, the laser beam may be incident on the pattern element  30  such that the incident angle θ is 7.7 degrees. In this case, the maximum optical path difference ΔOP may be 11.171 mm, the beam waist Wo may be 0.061 mm, and the beam diameter W may be 0.086 mm at a point away from the beam waist Wo by ΔOP/2. The larger the incident angle θ, the greater the difference in the beam diameter on the surface of the pattern element  30 , and thus lower the resolution of a display image. Also, the beam waist Wo and the beam diameter W of  FIG.  5    may be calculated when the wavelength A of Equation 2 is about 523 nm. 
     As described with reference to  FIGS.  3 ,  4 , and  5   , the image display device  1000  may have a non-uniform resolution according to an optical path difference occurring between laser beams incident on the pattern element  30  and a characteristic of the beam diameter W varying depending on traveling position of the laser beam. Meanwhile, the image display device  100  according to an example embodiment of  FIGS.  1  and  2    may have a uniform resolution by including the prism  110  adjusting the optical path difference between laser beams and reducing or eliminating the optical path difference. 
       FIG.  6    is a table showing a specific example of a relationship between the incidence angle θ of a laser beam with respect to the light direction changer  150  of  FIGS.  1  and  2   , a maximum optical path difference ΔOP of the laser beam, and the apex angle α. 
     As described above, the prism  110  may adjust an optical path difference of each light from the optical scanner  120  to the light direction changer  150 . The prism  110  may adjust the optical path difference to be 0 or less than a predetermined value. Therefore, the image display device  100  may adjust a difference in a beam diameter of the light direction changer  150  to 0 or less than the predetermined value. 
     For example, the image display device  100  may change the beam diameter W in the light direction changer  150  to be the same as the beam waist Wo. An optical path length L of the prism  110  (the traveling path of light in the vacuum) may be calculated from Equation 4 below by a path length D (the actual length through which light travels) of the prism  110  and a refractive index n of the prism  110 .
 
 L=n·D   (Equation 4)
 
     In other words, when the prism  110  has the refractive index n larger than 1, the optical path length L may be greater than the path length D. This principle may be used to adjust the optical path difference of the light traveling in a plurality of paths. The installation position and the installation angle of each member and the apex angle α of the prism  110  may be determined such that the optical path difference is 0 or less than the predetermined value. 
     For example, when the length V from one end of the light direction changer  150  to the other end is 83.2 mm and the incidence angle θ of the laser beam with respect to the light direction changer  150  is 50.0 degree, the maximum optical path difference ΔOP may be 63.735 mm according to Equation 1. The apex angle α of the prism  110  that makes the maximum optical path difference ΔOP almost zero may be determined as 57.15 degree. Also,  FIG.  6    is only an example, and each value may vary depending on the installation position and the installation angle of each member. 
     When the refractive index n is greater than 1, a material of the prism  110  is not particularly limited. For example, the material of the prism  110  may include glass, resin, or the like. Since the Abbe number of glass tends to be greater than that of resin, the difference in the refractive index n caused by the wavelength difference of the laser beam may be reduced by using glass as the material of the prism  110 . Also, it may be easier to process glass than resin in a plane manner. 
       FIG.  7    is a perspective view showing an image display device  200  according to an example embodiment. 
       FIG.  8    is a cross-sectional view showing the image display device  200  of  FIG.  7   . 
     Referring to  FIGS.  7  and  8   , the image display device  200  is the same as the image display device  100  shown in  FIGS.  1  and  2   , except that the parallel light generator  130  of  FIGS.  1  and  2    is replaced by a condenser lens  131 . 
       FIG.  9    is a cross-sectional view showing an image display device  300  according to an example embodiment. 
     Referring to  FIG.  9   , the image display device  300  may include a translucent screen  160  at the front end of the light direction changer  150  and an optical system  161  between the light direction changer  150  and a screen  160 . The optical system  161  may allow a laser beam to be incident on the screen  160 . Accordingly, the image display device  300  may function as a head-up display. The screen  160  may display a virtual image by using the laser beam incident through the optical system  161 . A user may recognize the virtual image. 
       FIG.  10    is a cross-sectional view showing an image display device  400  according to an example embodiment. 
     Referring to  FIG.  10   , the image display device  400  may further include a diffuser  162  between the light direction changer  150  and the optical system  161 . Accordingly, a laser beam incident from the light direction changer  150  may be focused on the diffuser  162 . The diffuser  162  may serve as a secondary image plane. The diffuser  162  may diffuse the incident laser beam, thereby expanding a visual region which is a region where a display image may be recognized. Also, the configurations of  FIGS.  9  and  10    are merely examples, and the arrangement of the screen  160 , the optical system  161 , and the diffuser  162  may be changed. 
     Also, the configurations discussed above are merely examples, and the configurations of the image display devices  100 ,  200 ,  300 , and  400  are not limited thereto. For example, the image display devices  100 ,  200 ,  300 , and  400  may not include the plane mirror  140 . More specifically, when each laser beam is incident on a surface of the light direction changer  150  at the predetermined incident angle θ or the optical path difference of each laser beam is close to 0, the plane mirror  140  may be omitted. 
     Also, the above-described example embodiments may be applied to any image display device other than a head-up display. For example, the image display devices  100 ,  200 ,  300 , and  400  may include a projection optical system at the front end of the light direction changer  150  to serve as a projector. As described above, the image display devices  100 ,  200 ,  300 , and  400  may include an arbitrary optical system at the front end of the light direction changer  150  to operate as different types of image display devices. 
       FIG.  11    shows a top view and a cross-sectional view of a modification of the light direction changer  150  included in the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments. 
     Referring to  FIG.  11   , the light direction changer  150  may include a reflective type pattern element. The light direction changer  150  may have a wire grid or the like on its surface to form a pattern. For example, the light direction changer  150  may have a shape in which a plurality of rectangular shape prisms are disposed in parallel such that the prisms face a front direction with respect to the light direction changer  151 . Intervals of slits and an aspect ratio of the pattern may be determined such that the traveling direction of a first order diffracted ray is a front direction with respect to the light direction changer  150 . Also, the traveling direction of a laser beam changed by the light direction changer  150  is not limited to the front direction with respect to the light direction changer  150  and may be changed according to the configurations of the image display devices  100 ,  200 ,  300 , and  400 . 
       FIG.  12    shows a top view and a cross-sectional view of a modification of a light direction changer  151  included in the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments. 
     Referring to  FIG.  12   , the light direction changer  151  may include a reflective type pattern element having a pattern of a rectangular prism shape on its surface. For example, the light direction changer  151  may have a shape in which a plurality of sawtooth shape prisms are disposed in parallel such that the prisms face a front direction with respect to the light direction changer  151 . The light direction changer  151  may change the traveling direction of a laser beam to a predetermined first direction by reflecting the laser beam on an inclined surface formed on the surface. 
       FIG.  13    shows a top view and a cross-sectional view of a modification of a light direction changer  152  included in the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments. 
     Referring to  FIG.  13   , the light direction changer  152  may include a reflective type pattern element having a pattern of a prism shape on its surface. More specifically, the light direction changer  152  may have a shape in which a plurality of triangular prisms are disposed in parallel such that the apex angle α is directed forward with respect to the light direction changer  152 . The light direction changer  152  may change the traveling direction of a laser beam to a predetermined first direction by reflecting the laser beam on an inclined surface formed on the surface. 
       FIG.  14    shows a top view and a cross-sectional view of a modification of a light direction changer  153  included in the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments. 
     Referring to  FIG.  14   , the light direction changer  153  may include a reflective type pattern element having a pattern of a plurality of cylindrical shapes disposed in parallel on its surface. The cylindrical shape includes a semi-cylindrical shape in which a part of a cylinder is cut off. The light direction changer  153  may change the traveling direction of a laser beam by reflecting the laser beam on the surface of the cylindrical shape. Also, since the light direction changer  153  has the semi-cylindrical shape on the surface, the laser beam may be reflected and then travel, be diffused, in various directions according to a position where the laser beam is reflected from. Therefore, the light direction changer  153  may expand a visual region. 
       FIG.  15    shows a top view and a cross-sectional view of a modification of a light direction changer  154  included in the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments. 
     Referring to  FIG.  15   , the light direction changer  154  may include a reflective type pattern element having a pattern of a plurality of parabolic shapes disposed in parallel on its surface. More specifically, the light direction changer  154  may have any parabolic shape on the surface. The light direction changer  154  may change the traveling direction of a laser beam by reflecting the laser beam on the parabolic surface. Also, since the light direction changer  154  has the parabolic shape on its surface, the laser beam may be reflected and then travel, be diffused, in various directions according to a position where the laser beam is reflected from. Therefore, the light direction changer  154  may expand a visual region, like the light direction changer  153  of  FIG.  14   . 
       FIG.  16    is a top view and a cross-sectional view showing a modification of a light direction changer  155  included in the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments. 
     Referring to  FIG.  16   , the light direction changer  155  may include a reflective type lattice element having a pattern of a spherical concave lens array shape on its surface. The light direction changer  155  may change the traveling direction of a laser beam by reflecting the laser beam on the surface. Further, since the light direction changer  155  has a surface shape where spherical concave lenses are arranged in a lattice array shape, the laser beam may be reflected and then travel, be diffused, in various directions according to a position where the laser beam is reflected. Therefore, the light direction changer  155  may expand a visual region, like the light direction changer  153  of  FIG.  14   . 
       FIG.  17    is a top view and a cross-sectional view showing a modification of a light direction changer  155  included in the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments. 
     Referring to  FIG.  17   , the light direction changer  156  may include a reflective type lattice element having a pattern of a parabolic concave lens array shape on its surface. The light direction changer  156  may change the traveling direction of a laser beam by reflecting the laser beam on the surface. Further, since the light direction changer  156  has a surface shape like that parabolic concave lenses are arranged in a lattice array shape, the laser beam may be reflected and then travel, be diffused, in various directions according to a position where the laser beam is reflected. Therefore, the light direction changer  156  may expand a visual region, like the light direction changer  153  of  FIG.  14   . 
     Also, the surface of the parabolic concave shape may have a characteristic of reflecting parallel light and converging light at a focus position. That is, since the laser beam reflected from the surface of the parabolic concave shape converges at a focus position of each parabolic surface, a second focal plane may be generated by the convergence of focus positions of parabolic surfaces. Thereby, the image display device  100  may allow a user to recognize as if a display image is being projected on a plurality of focal planes. In other words, the image display device  100  may realize the 3D display by controlling the display content time-serially according to a direction in which the laser beam is reflected by the light direction changer  156 . 
     Also, a material and a manufacturing method of each member used as the light direction changers  150  to  156  described with reference to  FIGS.  11  to  17    are not particularly limited. 
     As described above, each of the image display devices  100 ,  200 ,  300 , and  400  according to example embodiments may include the optical scanner  120  scanning light emitted from a light source, the parallel light generator  130  generating the scanned light as parallel light, the prism  110  refracting the parallel light, and the light direction changer  150  changing the traveling direction of the light. The prism  110  may adjust the optical path difference of each light to be substantially 0 or less than a predetermined value. Therefore, the image display devices  100 ,  200 ,  300 , and  400  may adjust the difference in the beam diameter on the light direction changer  150  to be substantially 0 or less than the predetermined value. For example, the image display device  100  may change the beam diameter on the light direction changer  150  to be the same as a diameter of the beam waist Wo. 
     According to example embodiments of the present disclosure, uniformity of resolution of a display by an image display device may be improved. 
     It should be understood that embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each 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.