Patent Publication Number: US-2012026159-A1

Title: Active type display apparatus and driving method for providing stereographic image

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2010-0073201, filed on Jul. 29, 2010, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     1. Field 
     This disclosure relates to an active-type stereographic image display apparatus having a touch function and a driving method thereof. 
     2. Discussion of the Background 
     An autostereographic image technology may be implemented using a parallax phenomenon of two eyes. In order to use the parallax of the two eyes, a display element in which a pixel for the left eye and a pixel for the right eye are separated from each other and an image filter for individually displaying a left eye image and a right eye image may be used. Conventionally, a parallax barrier, a lenticular lens, and the like may be used as the image filter for autostereographic images. 
       FIG. 1  ( a ) is a diagram illustrating a conventional stereographic image apparatus using a lenticular lens, in which a display element  11  and a lenticular lens  12  are adhered to an optically clear adhesive (OCA)  13 , which is interposed between the display element  11  and the lenticular lens  12 . 
       FIG. 1  ( b ) is a diagram schematically illustrating a cross-sectional structure of a conventional capacitive touchscreen. The touch screen may include two transparent films  21  and  22  on which transparent electrode layers  31  and  32  may be deposited, and a protection film  51 . The films may be adhered through optically clear adhesives  41  and  42 . 
     In an example, where an autostereographic imaging and touching function are to be simultaneously implemented, both the stereographic image part of  FIG. 1  ( a ) and the touch part of  FIG. 1  ( b ) may be desired. An autostereographic image part may display a separate image for each of the left and right eyes but the provided image may have a reduced horizontal resolution (i.e., half of that of a 2D display element with the same size, resulting in degradation of image quality). In addition, if the touchscreen is attached to the autostereographic image display, the image quality may be additionally degraded due to the thickness of the apparatus. Further, there may be design and cost problems associated with the increased thickness of the entire display apparatus. 
     In addition, the conventional autostereographic image technology such as the parallax barrier and the lenticular lens may only have a single focal region and accordingly display an image at a fixed focal length. As a result, a user may have to be positioned at optimized distance and viewpoint to appropriately see the provided images. Therefore, if the position of the user is changed, it may be difficult to continuously provide high quality stereographic images. 
     SUMMARY 
     Exemplary embodiments of the present invention provide an active-type stereographic image display apparatus. Exemplary embodiments of the present invention also provide a method for driving an active-type stereographic image. 
     Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
     Exemplary embodiments of the present invention provide, an active-type stereographic image display apparatus including an image display unit which includes pixels in a matrix form, wherein a first eye pixel and a second eye pixel are arranged; a touch sensing unit to sense a touch position through a first transparent electrode layer and a second transparent electrode layer that are arranged to cross with each other, and to separate an image output from the image display unit through a lens array into a first eye image and a second eye image based on a refractive index of light; and a lens driving control unit to control a focal length by controlling the lens array. 
     Exemplary embodiments of the present invention provide, an active-type stereographic image display apparatus including, an image display unit comprising pixels in a matrix form, wherein a first eye pixel and a second eye pixel are arranged in each pixel; a touch sensing unit to sense a touch position through a first transparent electrode layer and a second transparent electrode layer that are arranged in a matrix form to intersect each other, and to separate an image output from the image display unit through a lens array into a first eye image and a second eye image; a lens driving control unit to control a focal length by changing the interval between the image display and the lens array or by changing the radius of curvature of the lens array; and a sensor unit to measure a distance from a user. 
     Exemplary embodiments of the present invention provide, a method for driving an active-type stereographic image including displaying a stereographic image comprising a first eye image and a separate second eye image, sensing a touch position of a user input, separating an image output into a first eye image and a second eye image, and controlling a focal length provided by a lens array. 
     It is to be understood that both forgoing general descriptions and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1(   a ) and  FIG. 1(   b ) are diagrams illustrating structures of a stereographic image display and a touch screen according to a conventional art. 
         FIG. 2  is an exploded perspective view of an active-type stereographic display apparatus according to an exemplary embodiment of the invention. 
         FIG. 3  is a cross-sectional view illustrating an I-I′ section of  FIG. 2  according to an exemplary embodiment of the invention. 
         FIG. 4  is a diagram illustrating an operation of controlling a focal length by controlling the position of a lens array in the active-type stereographic image display apparatus to according to an exemplary embodiment of the invention. 
         FIG. 5  is a diagram illustrating an operation of controlling the radius of curvature of the lens array in the active-type stereographic image display apparatus according to an exemplary embodiment of the invention. 
         FIG. 6  is a diagram illustrating an operation of controlling the focal length with reference to  FIG. 5  according to an exemplary embodiment of the invention. 
         FIG. 7  is a diagram illustrating an operation of measuring a position of a user by the active-type stereographic display apparatus according to an exemplary embodiment of the invention. 
         FIG. 8  is a flowchart of a method for driving the active-type stereographic image display apparatus according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure is thorough and will fully convey the scope of the invention to those skilled in the art. Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals are understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure. 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. Furthermore, the use of the terms a, an, etc. does not denote a limitation of quantity, but rather denotes the presence of at least one of the referenced item. It will be further understood that the terms “comprises” and/or “comprising”, or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
     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. 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 the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Hereinafter, an active-type stereographic image display apparatus and a driving method thereof according to exemplary embodiments will be described in more detail with reference to the accompanying drawings. 
       FIG. 2  is an exploded perspective view of an active-type stereographic display apparatus according to an exemplary embodiment of the invention.  FIG. 3  is a cross-sectional view illustrating an I-I′ section of  FIG. 2  according to an exemplary embodiment of the invention. 
     As shown in  FIG. 2 , the active-type stereographic image display apparatus includes an image display unit  110  to output an image, a touch sensing unit  120 , a lens drive control unit  130 , and a protection layer  140 . 
     The image display unit  110  is a display unit. In an example, the display unit  110  may be a liquid crystal display (LCD) panel, an organic light emitting diode (OLED) panel, or other similar displays, which may include pixels in a matrix form. 
     In the image display unit  110 , a pixel line for the left eye and a pixel line for the right eye may cross with each other in various directions, and each pixel may include a pixel for the left eye and a pixel for the right eye, which are arranged alternately. Accordingly, the image display unit  110  may output a separate image through the pixel for the left eye and the pixel for the right eye to allow the two eyes to see different images and to perceive the different images that the two eyes of the user see as a stereographic image. 
     Further, images outputted from the image display unit  110  may be separated into a left eye direction and a right eye direction through a lens array  122 , thereby generating a stereographic effect. 
     The touch sensing unit  120  positioned on the image display unit  110  senses a touch position. In an example, the touching sensing unit  120  may sense a touch position through a first transparent electrode layer  121  and a second transparent electrode layer  123  arranged to cross with each other. Further, the first electrode layer  121  and the second transparent electrode layer  123  separates the images outputted from the image display unit  110  into a left eye image and a right eye image based on the refractive index of light through the lens array  122 . 
     In an example, the touch sensing unit  120  may have a structure in which the first transparent electrode layer  121  and the second transparent electrode layer  123  are separated from each other with the lens array  122  interposed between the two layers integrated into one body. Accordingly, the touch function and the image filter function may be implemented in a single structure. 
     In an example, the touch sensing unit  120  may provide a capacitive touchscreen function through the first transparent electrode layer  121  and the second transparent electrode layer  123  deposited on both surfaces of the translucent lens array  122  to cross with each other. That is, the touch sensing unit  120  may sense a touch position by sensing change in static electricity and current between the first transparent electrode layer  121  and the second transparent electrode layer  123 , which may be separated from each other at an interval, if a touch is initiated by a user. 
     Accordingly, since the image filter part for implementing a stereographic image and the part for implementing the touch function are integrated into one body, the number of layers may be reduced. Further, as the entire layer structure becomes simpler in design, the driving lens control may become easier. In addition, through the integrated structure, a transparent film, which may be used for a supporting member of a transparent conductive material, may be excluded. Thus, the entire thickness of the structure may be reduced, and image quality degradation due to the use of the transparent film may be avoided. 
     Patterns of the first transparent electrode layer  121  and the second transparent electrode layer  123  may be formed into a matrix arrangement to determine a touch position if a user input is received. For example, the pattern of the first transparent electrode layer  121  may be formed in a longitudinal direction, and the pattern of the transparent electrode layer  123  may be formed in a transverse direction to cross with. 
     The lens array  122  is an image filter in an optical structure, which uses a light refraction phenomenon to display a left eye image and a right eye image by separating the images outputted through the image display unit  110  into a left side image and a right side image. Since the lens array  122  uses light refraction, it may be designed in consideration of complex factors including the pixels for the left eye and the pixels for the right eye of the image display unit  110 , the size and shape of the lens, distances, and the like. The image display unit  110  may employ a method of separately operating the pixel for the left eye and the pixel for the right eye, and image contents may also be separately produced to provide a stereographic image. In a case of displaying a 2D image such as text which does not require a stereographic image, the same image may be displayed on the pixel for the left eye and the pixel for the right eye. 
     In an example, the hemispheres of the lenses of the lens array  122  may be arranged to be aligned with the pixels for the left eye and the pixels for the right eye of the image display unit  110  positioned thereunder. The pixels for the left eye and the right eye may be aligned such that the left eye image and the right eye image are accurately separated through each lens into a stereographic image. 
     Further in an example, a convex lens type or a concave lens type of lenticular lens may be used in the lens array  122  to provide an autostereographic image. In addition, a structure capable of controlling the focus of the stereographic image through a shape change or a position change of the lens array  122  may be applied to provide an autostereographic image. 
     In an example, the lens driving control unit  130  may control the focal length of the stereographic image provided through the shape change or the position change of the lens array  122  by controlling the lens array  122 . Generally, a singe focal region in which a stereographic image is seen at a limited viewpoint may be arranged by changing the transmittance of light using a fixed hemispheric lens. In comparison, by controlling a distance between the image display unit  110  and the lens array  122  through the lens driving control unit  130 , multiple focusing regions may be implemented and the focal length of the stereographic image may be adjusted depending on the distance from the user. Alternatively, similar effects may be achieved by changing the radius of curvature of the lenses constituting the lens array  122 . 
     The protection layer  140  is arranged to cover the upper part of the touch sensing unit  120 . In an example, the protection layer  140  may include a protection film, protection glass, or similar protective parts. In addition, the protection layer  140  may be arranged to maintain a reference height so as not to deform or break the outer shapes of the lens hemispheres on the lens array  122  by pressure exerted during a touch input. Accordingly, the protection film  140  may be removed or replaced with another material. 
       FIG. 4  is a diagram illustrating an operation of controlling a focal length by controlling the position of the lens array in the active-type stereographic image display apparatus to according to an exemplary embodiment of the invention. 
     A stereographic image is displayed at a distance, provided by the pixels for the left eye  112  and the pixels for the right eye  111  of the image display unit  110 , which passes through the lens array  122 . Accordingly, a user may be able to see the image at a fixed viewpoint between the lens array  122  and the user. 
     In an example, the lens driving control unit  130  (not shown in  FIG. 4 ) may adjust the focus of the stereographic image by controlling the interval (distance) between the image display unit  110  and the lens array  122 . The image display unit  110  may include the pixels for the left eye  112  and the pixels for the right eye  111 . 
       FIG. 4  illustrates a case where the user&#39;s viewpoint changes from X to X′. As shown in  FIG. 4 , in the case where the user&#39;s viewpoint changes from X to X′, the distance between the image display unit  110  and the lens array  122  is controlled to increase to a length of A+a from the original length A. Similarly, the distance from the lens array  122  to the user&#39;s viewpoint is reduced to the length of B−b from the original length of B. The total focal length L from the image display unit  110  to the viewpoint at which the stereographic image is displayed is reduced to the length of L−b from the original length of L, thereby reducing the focal length of the stereographic image. 
     On the other hand, if the interval between the image display unit  110  and the lens array  122  is reduced to a length of A−a from the original length of A, the total focal length L may be increased to a length of L+b from the original length of L, thereby increasing the focal length of the stereographic image. In the case where the distance between the lens array  122  and the user is increased to a length of B+b from the original length of B, the distance between the pixel of the image display unit  110  and the lens array  122  may be reduced to length of A−a from the original length of A, thereby increasing the focal length of the stereographic image. 
     The relationships described above may be calculated using Equation 1 and Equation 2. Equation 2 may be calculated from Equation 1. 
     
       
         
           
             
               
                 
                   
                     
                       1 
                       A 
                     
                     + 
                     
                       1 
                       B 
                     
                   
                   = 
                   
                     
                       1 
                       L 
                     
                     = 
                     
                       
                         2 
                         r 
                       
                       . 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
             
               
                 
                   B 
                   = 
                   
                     
                       
                         AL 
                         
                           ( 
                           
                             A 
                             - 
                             L 
                           
                           ) 
                         
                       
                        
                       
                           
                       
                        
                       where 
                        
                       
                           
                       
                        
                       A 
                     
                     &lt; 
                     
                       L 
                       . 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equations 1 and 2, A may be a distance from the image display unit  110  to the lens array  122 , B may be a focal length from the lens array  122  to a point where an image is displayed, and L may be a focal length from the image display unit  110  and the point where the image may be displayed. In addition, r may be the radius of curvature of the lens in the lens array  122  and may be a fixed value regardless of the position of the user or the interval between the image display unit  110  and the lens array  122 . The lens driving control unit  130  (not shown) may control the value of A automatically or according to a user input depending on the user&#39;s requirement so as to display an optimized stereographic image. 
     Referring to Equations 1 and 2, it can be seen that if the distance between the image display unit  110  and the lens array  122  is reduced to a length of A−a from the original length of A using the lens driving control unit  130  (not shown), the viewing distance may be increased to a length of B+b from the original length of B. Therefore, if the lens driving control unit  130  using such a principle is added to the lower end of the lens array  122  to control the length of A, the user may see the stereographic image at a desired distance. 
     As described above, the lens driving control unit  130  may control the focal length of the lens array  122  by changing the interval between the image display unit  110  and the lens array  122 . For example, the lens driving control unit  130  may reduce the interval between the image display unit  110  and the lens array  122  to increase the focal length if the distance from the user is or becomes greater than a reference value. Similarly, the interval between the image display unit  110  and the lens array  122  may increase to increase the focal length if the distance from the user is or becomes smaller than the reference value. 
     The lens driving control unit  130  may change the position of the lens array  122  using a thin film motor. For example, the lenticular lens may be controlled using the thin film motor. The thin film motor may be implemented by configuring a rotation shaft and a magnetic field coil of the motor as thin films through the microelectromechanical systems (MEMS) technique, which uses vapor deposition and etching methods used in a semiconductor manufacturing process. Rotation of the thin film motor may drive the lenticular lens by changing a direction of force using an upward or downward movement of the gears. 
     In addition, the lens driving control unit  130  may change the position of the lens array  122  using a piezoelectric film of which the amount of contraction or expansion may be changed according to an applied electric signal. For example, if two sheets of piezoelectric films having different contraction or expansion directions are attached to the lenticular lens, and a voltage is applied thereto, physical deformation occurs in a direction in which the film expands. Accordingly, as the contraction or expansion amounts can be controlled by a change in the voltage applied, control over the lenticular lens is possible. 
     Alternatively, although not shown in the provided figure, the embodiments as described may be used to move the position of the image display unit  110 , or a combination of both the image display unit  110  and lens array  122  to control the interval between the image display unit  110  and the lens array  122 . 
       FIG. 5  is a diagram illustrating an operation of controlling the radius of curvature of the lens array in the active-type stereographic image display apparatus according to an exemplary embodiment of the invention. 
     The lens array  122  may be implemented using a shape memory polymer or a smart material having the same function so as to change the radius of curvature of each lens. Such special materials may be driven by control values externally applied, such as, electricity, temperature, pressure, humidity, and magnetic field. 
       FIG. 5  illustrates changes in shape of the lens according to varying control values applied externally. More specifically,  FIG. 5  ( a ) illustrates an initial state in which a control value is not applied externally,  FIG. 5  ( b ) illustrates a case where the hemispheric shape of the lens is gently curved by applying a control value, and  FIG. 5  ( c ) illustrates a case where the control value is applied at a maximum level. 
     As the radius of curvature of the hemispheric shape becomes reduced, the focal length may be increased. Contrastingly, as the radius of curvature becomes increased, the focal length of the stereographic image may be reduced. In an example, if the distance the image display unit  110  from the user becomes greater than a reference value, the lens driving control unit  130  may reduce the radius of curvature of the lens array  122  to deform the lens to be gentle thereby increasing the focal length. Similarly, if the distance the image display unit  110  from the user becomes smaller than the reference value, the length driving control unit  130  may increase the radius of curvature of the lens array  122  to deform the lens to be sharp thereby reducing the focal length. 
     As described above, the lens driving control unit  130  may control the focal length of the lens array  122  by changing the radius of curvature of the lens array  122 . That is, multiple focus regions may be provided by controlling the hemispheric shapes of the lenses, and the focal length of the stereographic image may be controlled to implement the stereographic image display in which the image focus may be actively controlled depending on the user&#39;s requirement. Further, the user may change single focus region to multiple focus regions through an input unit of the active-type stereographic image display apparatus, i.e., through buttons or a touch input. 
       FIG. 6  is a diagram illustrating an operation of controlling the focal length with reference to  FIG. 5  according to an exemplary embodiment of the invention.  FIG. 6  illustrates an operation of the lens array  122  for changing the shape of the lens according to the control value applied externally. More specifically, a case where the user&#39;s viewpoint is changed from X in  FIG. 6  ( a ) to X′ in  FIG. 6  ( b ) is illustrated. 
     As described above, the lens array  122  may be made of a shape memory polymer or a smart material of which the shape may be changed according to the control value applied externally. In the lens array  122  using such a material, the hemispheric shape of the lens may be automatically changed by an operation of the user or by measuring the distance from the user. The lens driving control unit  130  may increase or reduce the value of the radius of curvature of the hemispheric shape of the lens so as to display the optimized stereographic image in consideration of the user&#39;s viewpoint. 
     As shown in  FIG. 6  ( a ), the distance between the image display unit  110  and the lens array  122  is A, the radius of curvature of the lens in the lens array  122  is R, the distance from the lens array  122  to the user&#39;s viewpoint is B, and the total focal length from the image display unit  110  to the viewpoint at which the stereographic image is displaced is L. 
     As shown in  FIG. 6  ( b ), if the user moves from X to X′, in order to increase the focal length, the lens driving control unit  130  may reduce the radius of curvature of the lens array  122  from R to r, while length of A is fixed. Accordingly, the distance from the lens array  122  to the user&#39;s viewpoint is increased to a length of B+b, and the total focal length from the image display unit  110  to the viewpoint at which the stereographic image is displayed is increased from an original length of L to a length of L+b. Contrastingly, if the radius of curvature of the lens in the lens array  122  is increased, the total focal length is reduced to a length of L−b from the original length of L. 
       FIG. 7  is a diagram illustrating an operation of measuring a position of a user by the active-type stereographic display apparatus according to an exemplary embodiment of the invention. 
     The active-type stereographic image display apparatus may be provided with a sensor unit  150  to sense the position of the user or may automatically adjust the focus using a method of calculating distance. In an example where the sensor unit  150  is provided, the lens driving control unit  130  may change the focal length of the lens array  122  based on the distance of the user from the stereographic image display apparatus as measured by the sensor unit  150 . In an example, the sensor unit  150  may be provided with a distance sensor, an optical sensor, and the like to measure the distance from the user&#39;s viewpoint to the active-type stereographic image display apparatus in real time. 
     Further, the active-type stereographic display apparatus may adjust the focal length at which the stereographic image is projected according to an operation by the user. In addition, the sensor unit  150  and the lens driving control unit  130  may cooperate with each other to automatically measure the distance between the user and the image display unit  110  and actively adjust the focal length. 
       FIG. 8  is a flowchart of a driving method of the active-type stereographic image display apparatus according to an exemplary embodiment of the invention. For convenience,  FIG. 8  will be described as if the method were performed by active-type stereographic image display apparatus described above. However, the method is not limited as such. 
     In an example, the active-type stereographic image display apparatus displays a stereographic image from which a left eye image and a right eye image may be separated through the image display unit  110  and the touch sensing unit  120  thereon (S 110 ). 
     As described above, the image display unit  110  may include the pixels in a matrix form and has a structure in which the pixel for the left eye and the pixel for the right eye which may be arranged alternately in each pixel. 
     The touch sensing unit  120  may be configured to sense a touch position through a first transparent electrode layer  121  and a second transparent electrode layer  123 . In addition, the first transparent electrode layer  121  and a second transparent electrode layer  123  may be formed to intersect each other, and separate the images output from the image display unit  110  through the lens array  122  into the left eye image and the right eye image based on the refractive index of light. Furthermore, the touch sensing unit  120  may be configured to interpose the lens array  122  between the first transparent electrode layer  121  and the second transparent electrode layer  123 . Lastly, the first transparent electrode layer  121  and a second transparent electrode layer  123  may be opposed to each other, thereby simultaneously implementing the image filter function for a stereographic image and the capacitive touch screen function for a touch input. 
     In addition, the active-type stereographic image display apparatus may control the focal length through the lens driving control unit  130  by controlling the lens array  122  mounted in the touch sensing unit  120  (S 120 ). In the focal length controlling process (S 120 ), the active-type stereographic image display apparatus first measures the distance from the user through the sensor unit  150  mounted inside or based on distance calculation (S 121 ). If the measured distance from the user is equal to or greater than the reference value (S 122 ), the lens driving control unit  130  increase the focal length by controlling the lens array  122  (S 123 ). Contrastingly, if the distance from the user measured in S 121  is equal to or smaller than the reference value (S 122 ), the lens driving control unit  130  reduces the focal length by controlling the lens array  122  (S 124 ). 
     During the focal length controlling process (S 120 ), the lens driving control unit  130  may control the focal length of the lens array  122  by changing the interval between the image display unit  110  and the lens array  122 . Alternatively, the lens driving control unit  130  may control the focal length of the lens array  122  by changing the radius of curvature of the lens array  122 . For example, if the distance from the user is increased, the lens driving control unit  130  may increase the focal length by reducing the radius of curvature of the lens array  122 , or by reducing the distance between the image display unit  110  and the lens array  122 , or by combination of the two methods. On the other hand, if the distance from the user is reduced, the lens driving control unit  130  may decrease the focal length by increasing the radius of curvature of the lens array  122 , or by increasing the interval between the image display unit  110  and the lens array  122 , or by combination of the two methods. 
     Further, in an example, in the method of controlling the focal length by changing the distance between the image display unit  110  and the lens array  122 , the distance between the image display unit  110  and the lens array  122  may be controlled by changing the position of the image display unit  110 , or by changing the position of the lens array  122 , or by combination of the two methods. In addition, the two methods of controlling the distance and of controlling the radius of curvature may be used independently or in combination. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.