Abstract:
A tiled head-mounted display device comprises an optical component including a plurality of prisms with free-form surfaces, and a display component including a plurality of micro-displays ( 6 ), wherein the number of the micro-displays ( 6 ) and the number of the prisms with free-form surfaces is identical, and each prism with free-form surfaces and the corresponding micro-display ( 6 ) constitute a display channel. Each prism is a wedge prism including a first surface ( 2 ), a second surface ( 3 ) and a third surface ( 4 ). The exit pupil planes of each display channel are coincident, thus avoiding pupil aberration and keeping exit pupil diameter and eye clearance same as a single ocular. There is no resolution variance throughout the entire field of view, thus preventing extra trapezoid distortion. The tiled head-mounted display device is compact and lightweight, and provides wide field of view and high resolution. The tiled head-mounted display device can be readily applicable to augmented environments applications by simply adding an auxiliary free-form lens behind the prism with free-form surfaces.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was partially funded by United States (“U.S.”) National Science Foundation Grant No. 0644446. The United States government has certain rights in the invention. 
    
    
     RELATED APPLICATIONS 
     This application is a 371 application of International Application No. PCT/CN2010/072376 filed Apr. 30, 2010. 
     BACKGROUND OF THE INVENTION 
     This invention was partially funded by National Natural Science Foundation of China grant nos. 61205024 and 61178038, and Hi-Tech Research and Development Program of China grant no. 2009AA01Z308. 
     The present invention relates to a head-mounted display device, and in particular, to a tiled head-mounted display device comprising wedge-shaped prisms with free-form surfaces. 
     Head-mounted display devices for virtual environment and augmented environment are popular products in display industry and have been extensively developed in recent years. Head-mounted display device can be used in consumer applications such as 3D movie, video game and sports as well as high end applications such as scientific research, medical/industry training, flight simulation, immersed entertainments. To be a useful and valid display system, the head-mounted display must be capable of generating high fidelity and wide field of view scene. The compactness and lightweight are also preferred to reduce user&#39;s neck fatigue. 
     A head-mounted display device typically consists of three parts: display component, optical system and helmet. In order to reduce the weight of the head-mounted display device, it is crucial to use an optical system with a short focal length and a micro-display. However, there is a trade-off between compactness of optical system and imaging quality of head-mounted display. For head-mounted display device, it is necessary for the optical system to have a large field of view and large exit pupil diameter. The large field of view increases the sense of immersion and allows the users to observe mobile object better. The large exit pupil diameter allows the users to arbitrarily move their eyes during observation without image lost. It would also allow various users with different interpupillary distance to use the system without adjusting the interpupillary distance of the helmet. However, it is difficult to achieve wide field of view, large exit pupil diameter and high resolution at the same time, due to the tradeoff relationships among these parameters. 
     For a conventional head-mounted display system employing a single display channel with a single micro-display for each eye, the relationship between the field of view and the resolution may satisfy R=N/FOV, where R is resolution of the display system, N is resolution of the single micro-display, and FOV is a field of view of the display system. R and FOV are mutually restricted by each other with a given N value, that is, a large field of view will result in a low resolution. Therefore it is difficult to satisfy the requirements of large field of view and high resolution simultaneously in a conventional head-mounted display device employing a single display channel. 
     A tiled head-mounted optical display system based on conventional rotational symmetry oculars is proposed in J. E. Melzer&#39;s paper titled “Overcoming the field-of-view/resolution invariant in head-mounted displays”, Proc. SPIE, Vol. 3362, 284 (1998), L. G. Brown&#39;s paper titled “Applications of the Sensics panoramic HMD ”, SID Symposium Digest 39, 77 (2008), and M. Gutin&#39; paper titled “Automated design and fabrication of ocular optics”, Proc. SPIE 7060, (2008).  FIG. 1   a  shows a schematic view of a tiled optical system and a schematic view showing distortion correction of each display channel of the system.  FIG. 1   b  shows a schematic view of image shown on a screen observed through the system formed by tiling two oculars with rotational symmetry when the micro-displays in the system display an image of regular rectangles. 
     As shown in  FIG. 1   a , the tiled head-mounted optical display system based on the conventional rotational symmetry oculars requires a great number of oculars tiled together in order to obtain a satisfied field of view. The rotational deviation of the respective display channels and the corresponding micro-displays from the user&#39;s viewing axis leads to tilting in the image planes of display channels. In this case, image magnification varies throughout the tiled system, resulting in image distortion on the pupil plane for the displays located at the edges. As shown in  FIG. 1   b , a regular rectangle image displayed by micro-displays is observed as a trapezoid through the rotational tiled oculars. Therefore, the images to be displayed on each display of the tiled optical system needs to be pre-warped, otherwise the user will observe distorted images. For example, the warping for the regular rectangle image is shown in the right portion of  FIG. 1   a . The image displayed on centeral displays is unchanged and is still a regular rectangle, while the image displayed on marginal displays needs to be pre-warped to be as a trapezoid. 
     In addition, in tiling process, the oculars at the edges need to be rotated around the center of the exit pupil of the system, therefore, the eye clearance, that is, the minimum distance from human eyes to the tiled oculars is reduced. As shown in FIG.  1   a , the eye clearance (ec′) of the optical tiled system is less than the eye clearance (ec) of an ocular with a single display channel. Therefore, in order to satisfy the overall requirements of the eye clearance, so that such system can also be used by the users who wear for example glasses or mask, the exit pupil distance or eye clearance of a single ocular must be increased. Moreover, the exit pupil planes of the respective oculars do not coincide with each other but are tilted relative to each other. Therefore, users may see discontinuous images when their eyeballs are moving. The decrease of the effective exit pupil diameter may also lead to pupil aberration. In addition, in the system shown in  FIG. 1   a , the ocular with rotational symmetry is located between the user&#39;s eyes and the micro-displays. If the head-mounted display device is used for augmented environment, a transmissive-reflective optical component needs to be added in order to satisfy the requirements of optical transmission and reflection. In that case, the ocular size will be further increased in order to ensure minimum eye clearance (from half mirror to user&#39;s eyes). For a system tiled by a plurality of oculars, the structure of optical system is greatly complicated, and the weight and size of optical systems are increased significantly. 
     Moreover, the tiling process of the oculars with rotational symmetry is complex, requiring additional processes for the tiling surfaces. For this tiled system, positions and angles of the tiled surfaces of each ocular are different depending on the positions of the oculars in the tiled system. For the ocular at the center, it is necessary to process three or four tilted tiling surface. The processing requirements for oculars at different position are also different. Therefore, it is very difficult to process and assemble this tiled system with relatively high precision. 
     Therefore, there is a demand for a new kind of head-mounted display device having a large field of view and a high-resolution. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect of the invention, there is provided a tiled head-mounted display device comprising: an optical component including a plurality of prisms with free-form surfaces, each prism being a wedge prism comprising a first optical surface, a second optical surface and a third optical surface; and a display component including a plurality of micro-displays. The number of the micro-displays and the number of the prisms with free-form surfaces is identical. Each prism with free-form surfaces and the corresponding micro-display constitutes a display channel. 
     A coordinate system for the tiled head-mounted display device is defined as: global coordinate origin O is the exit pupil center (eye pupil); Z-axis is in a direction along the viewing axis of the user&#39;s eye; Y-axis is perpendicular to Z-axis and extends right above the eye; X-axis is perpendicular to both Y-axis and Z-axis, constituting a Cartesian coordinate. 
     The display channels are tiled in a mosaic pattern, similar to a video wall so that the overall display field of view of the tiled device is equivalent to tiles from individual display channels abutted together. The center of exit pupil of each display channel in the tiled device is located at a common point, i.e. center of the eye pupil. 
     The prism can comprise a first optical surface, a second optical surface and a third optical surface in a counter-clockwise order relative to X-axis. The first optical surface and the second optical surface are free-form surfaces, the third optical surface can be selected from free-form, spherical or aspherical surface. The first optical surface is a transmissive surface, the second optical surface is a concave reflective surface or semi-transmissive and semi-reflective surface, and the third optical surface is a transmissive surface. 
     The free-form surface equation of the first optical surface, the second optical surface and the third optical surface may follow (but are not limited to) any one of conditions (1) to (4), 
                   z   =             c   x     ⁢     x   2       +       c   y     ⁢     y   2           1   +       {     1   -       (     1   +     k   x       )     ⁢     c   x   2     ⁢     x   2       -       (     1   +     k   y       )     ⁢     c   y   2     ⁢     y   2         }       1   /   2           +       ∑     i   =   1     n     ⁢       A   i     ⁢       {         (     1   -     P   i       )     ⁢     x   2       +       (     1   +     P   i       )     ⁢     y   2         }       i   +   1                     (   1   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c x  is radius of curvature in the x direction in the xz-plane, c y  is radius of curvature in the y direction in the yz-plane, k x  is conic coefficient in x direction, k y  is conic coefficient in y direction, A, are aspherical coefficients of 4, 6, 8, 10, . . . 2n orders, P i  are non-rotational symmetry coefficient of 4, 6, 8, 10, . . . 2n orders, and the surface has rotational symmetry about z-axis.
 
                     z   =         c   ⁡     (       x   2     +     y   2       )         1   +     sqrt   ⁡     (     1   -       (     1   +   k     )     ⁢       c   2     ⁡     (       x   2     +     y   2       )           )           +       ∑     j   =   2     66     ⁢       C   j     ⁢     x   m     ⁢     y   n             ,     
     ⁢     j   =         [         (     m   +   n     )     2     +   m   +     3   ⁢   n       ]     /   2     +   1               (   2   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is radius of curvature of surface, C j  is polynomial coefficients, k is conic coefficient, m is an even number;
 
                   z   =         c   ⁡     (       x   2     +     y   2       )         1   +     sqrt   ⁡     (     1   -       (     1   +   k     )     ⁢       c   2     ⁡     (       x   2     +     y   2       )           )           +       ∑     j   =   1     66     ⁢       C     j   +   1       ⁢     Z   j                   (   3   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is radius of curvature of surface, k is conic coefficient, Z j  is Zernike polynomial, C j+1  is coefficients for Z j ;
 
                     z   =             c   x     ⁢     x   2       +       c   y     ⁢     y   2           1   +     sqrt   ⁡     (     1   -       (     1   +     k   x       )     ⁢     c   x     ⁢     x   2       -       (     1   +     k   y       )     ⁢     c   y     ⁢     y   2         )           +       ∑     j   =   1     37     ⁢       C   j     ⁢     x     2   ⁢   m       ⁢     y   n             ⁢           ⁢     
     ⁢           2   ⁢   m     +   n     ≤   10     ,     m   =   0     ,   1   ,   2   ,   3   ,   4   ,   5   ,     n   =   0     ,   1   ,   …   ⁢           ,   10             (   4   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is the vertex curvature, k is the conic constant, c x  is radius of curvature of surface in sagittal direction, c y  is radius of curvature of surface in tangential direction, and C j  is the coefficient for x 2m y n .
 
                   z   =         c   ⁢           ⁢     x   2         1   +       [     1   -       (     1   +   k     )     ⁢     c   2     ⁢     x   2         ]       1   /   2           +     A   ⁢           ⁢     x   4       +     B   ⁢           ⁢     x   6       +     C   ⁢           ⁢     x   8       +     D   ⁢           ⁢     x   10                 (   5   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is radius of curvature, k is conic coefficient, A, B, C, D are aspheric coefficients of 4, 6, 8 and 10 orders, respectively.
 
     According to one embodiment of the present invention, the display channels can be tiled by mechanical tiling methods, the first optical surface, the second optical surface and the third optical surface of each prism satisfy following conditions (6)-(8): 
                   {           0.5   ≤       Z     P   ⁢           ⁢     a   ′         -     Z     P   ⁢           ⁢   a         ≤   4                   Y     P   ⁢           ⁢     a   ′         -     Y     P   ⁢           ⁢   a         ≤   0                   Y     P   ⁢           ⁢     a   ″         -     Y     P   ⁢           ⁢   a         ≥   0                   (   6   )               {               Y     P   ⁢           ⁢     b   ′         -     Y     P   ⁢           ⁢   b         ≥   0                 -   3     ≤       Z     P   ⁢           ⁢     b   ′         -     Z     P   ⁢           ⁢   b         ≤   0                   (   7   )               {             -   4     ≤       Y     P   ⁢           ⁢   c       -     Y     P   ⁢           ⁢     c   ′           ≤   0               0   ≤       Z     P   ⁢           ⁢   c       -     Z     P   ⁢           ⁢     c   ′           ≤   2                   (   8   )               
where R u  is the top marginal ray of the maximum field of view in positive Y direction, R b  is the light ray at lower boundary of the maximum field of view in negative Y direction; P a  is an intersection point at which R b  is transmitted through the first optical surface, P a ′ is an intersection point of R b  with the second optical surface and Pa″ is an intersection point of R b  with the first optical surface upon total reflection; P b  is an intersection point of R u  with the second optical surface ( 3 ), P b′  is an intersection point of R b  with the third optical surface; P c  is an intersection point at which R u  is reflected on the first optical surface, P c ′ is an intersection point of R u  with the third optical surface, Y, Z are coordinates of each point in global coordinate system, respectively.
 
     The prisms with free-form surface also satisfy following conditions regarding incident angles of R u  on the first optical surface: 
                   {             θ     m   ⁢           ⁢   i   ⁢           ⁢   1       ≥     arcsin   ⁡     (     1   /   n     )                     θ     m   ⁢           ⁢   i   ⁢           ⁢   2       ≤     arcsin   ⁡     (     1   /   n     )                       (   9   )               
where θ mi1  is an incident angle of R u  emitted from the displays first striking the first optical surface ( 2 ), and θ mi2  is an incident angle of R u  striking the first optical surface ( 2 ) at the second time, n is a refractive index of prism material.
 
     The mechanical tiling methods include a first mechanical tiling method and a second mechanical tiling method. In the first mechanical tiling method, bottom surfaces of two prisms to be tiled are subject to mechanical processing and then cemented together, the bottom surface is positioned between the first optical surface and the second optical surface. In the second mechanical tiling method, side surfaces of two prisms to be tiled are subject to mechanical processing and then cemented together, the side surface intersects with all of the first optical surface, the second optical surface and the third optical surface of the prism. 
     According to another embodiment of the present invention, the display channels can be tiled by optical tiling methods, the first optical surface, the second optical surface and the third optical surface of each prism satisfy following conditions (10)-(12):
 
18≦ Z   Pd ≦28  (10)
 
1.5 ≦Z   Pd′   ≦Z   Pd ≦4  (11)
 
 Z   Pc ≧the eye clearance distance, i.e., 15  (12)
 
where, P c  is the intersection point of R u  with the first optical surface upon total reflection, P d  is the intersection point of chief ray R c  in horizontal field of view with the first optical surface, P d  is the intersection point of R c  with the second optical surface.
 
     The optical tiling methods comprise a first optical tiling method and a second optical tiling method. In the first optical tiling method, the bottom surfaces of both prisms to be tiled are directly cemented together. The bottom surface of the prism is positioned between the first optical surface and the second optical surface. In the second optical tiling method, the side surfaces of the respective prisms are directly cemented. The side surface of each prism intersects with the first optical surface, the second optical surface and the third optical surface of the prism. 
     The prism with free-form surfaces can be made of a material having a refractive index N d1  of 1.4&lt;N d1 &lt;1.8 and an Abbe number V d1  above 20. 
     The prism with free-form surfaces has a first order focal length of 14&lt;f&lt;27 mm. 
     In a first embodiment, the head-mounted display device comprises a first display channel and a second display channel tiled by the first mechanical tiling method, the first display channel is rotated by a first angle in YOZ plane about X-axis of a global coordinate system, the second display channel is rotated by ±180° about Z axis which is in a direction along viewing axis of a human eye, and then rotated by the first angle in the opposite direction about X-axis of the global coordinate system. The tiled head-mounted display according to the first embodiment has a horizontal field of view of at least 50° and a vertical field of view of at least 40°. 
     In a second embodiment, the tiled head-mounted display device of claim  3 , wherein the head-mounted display device comprises a first display channel and a second display channel tiled by the second mechanical tiling method, the first display channel is rotated by a second angle in XOZ plane about Y-axis of a global coordinate system, the second display channel is rotated by the second angle in the opposite direction about Y-axis of the global coordinate system. The tiled head-mounted display according to the second embodiment has a horizontal field of view of at least 70° and a vertical field of view of at least 30°. 
     In a third embodiment, the head-mounted display device comprises a first display channel, a second display channel and a third display channel rotated about Y-axis of a global coordinate system by a predetermined angle, the second display channel is tiled with the first display channel and the third display channel using second mechanical tiling method, respectively. The tiled head-mounted display according to the third embodiment has a horizontal field of view of at least 100° and a vertical field of view of at least 30°. 
     In a fourth embodiment, the head-mounted display device comprises a first display channel, a second display channel, a third display channel and a fourth display channel rotated about Y-axis of the global coordinate system by a predetermined angle, the second display channel is tiled with the first display channel and the third display channel with the second mechanical tiling method, respectively, the third display channel is tiled with the second display channel and the fourth display channel using the second mechanical tiling method, respectively. The tiled head-mounted display according to the fifth embodiment has a horizontal field of view of at least 120° and a vertical field of view of at least 30°. 
     In a fifth embodiment, the head-mounted display device comprises a first display channel, a second display channel, a third display channel and a fourth display channel, the first display channel and the third display channel are rotated by a second angle in XOZ plane about Y-axis of a global coordinate system, the second display channel and the fourth display channel are rotated by the second angle in the opposite direction about Y-axis of the global coordinate system, the first display channel and the second display channel are tiled with the second mechanical tiling method, the third display channel and the fourth display channel are tiled with the second mechanical tiling method, the first display channel and the third display channel are tiled with the first mechanical tiling method, and the second display channel and the fourth display channel are tiled with the first mechanical tiling method. The tiled head-mounted display according to the fifth embodiment has a horizontal field of view of at least 70° and a vertical field of view of at least 50°. 
     In a sixth embodiment, the head-mounted display device comprises a first display channel, a second display channel, a third display channel, a fourth display channel, a fifth display channel and a sixth display channel, the first display channel, the second display channel and the third display channel are rotated about Y-axis of a global coordinate system by a predetermined angle, the fourth display channel, the fifth display channel and the sixth display channel are rotated about Y-axis of a global coordinate system by the predetermined angle, the second display channel is tiled with the first display channel and the third display channel using the second mechanical tiling method respectively, the fifth display channel is tiled with the fourth display channel and the sixth display channel using the second mechanical tiling method respectively, the first display channel and the fourth display channel are tiled with the first mechanical tiling method, the second display channel and the fifth display channel are tiled with the first mechanical tiling method, the third display channel and the sixth display channel are tiled with the first mechanical tiling method. The tiled head-mounted display according to the sixth embodiment has a horizontal field of view of at least 100° and a vertical field of view of at least 50°. 
     In a seventh embodiment, the head-mounted display device comprises a first display channel, a second display channel, a third display channel, a fourth display channel, a fifth display channel, a sixth display channel, a seventh display channel and an eighth display channel, the first display channel, the second display channel, the third display channel and the fourth display channel are rotated about Y-axis of a global coordinate system by a predetermined angle, the fifth display channel, the sixth display channel, the seventh display channel and the eighth display channel are rotated about Y-axis of a global coordinate system by the predetermined angle, the second display channel is tiled with the first display channel and the third display channel using the second mechanical tiling method respectively, the third display channel is tiled with the second display channel and the fourth display channel using the second mechanical tiling method respectively, the sixth display channel is tiled with the fifth display channel and the seventh display channel using the second mechanical tiling method respectively, the seventh display channel is tiled with the six display channel and the eighth display channel using the second mechanical tiling method respectively, the first display channel and the fifth display channel are tiled with the first mechanical tiling method, the second display channel and the sixth display channel are tiled with the first mechanical tiling method, the third display channel and the seventh display channel are tiled with the first mechanical tiling method, the fourth display channel and the eighth display channel are tiled with the first mechanical tiling method. The tiled head-mounted display according to the seventh embodiment has a horizontal field of view of at least 120° and a vertical field of view of at least 50°. 
     In a eighth embodiment, the tiled head-mounted display system comprises a first display channel and a second display channel tiled by the first optical tiling method, the first display channel is rotated by a first angle in YOZ plane about X-axis of a global coordinate system, the second display channel is rotated by ±180° about Z axis and then rotated by the first angle in the opposite direction about X-axis of the global coordinate system. The tiled head-mounted display according to the eighth embodiment has a horizontal field of view of at least 50° and a vertical field of view of at least 40°. 
     In a ninth embodiment, the tiled head-mounted display system comprises a first display channel and a second display channel tiled by the second optical tiling method, the first display channel is rotated by a second angle in XOZ plane about Y-axis of a global coordinate system, the second display channel is rotated by the second angle in the opposite direction about Y-axis of the global coordinate system. The tiled head-mounted display according to the ninth embodiment has a horizontal field of view of at least 70° and a vertical field of view of at least 30°. 
     In a tenth embodiment, the tiled head-mounted display system comprises a first display channel, a second display channel, a third display channel and a fourth display channel, the first display channel and the third display channel are rotated by a second angle in XOZ plane about Y-axis of a global coordinate system, the second display channel and the fourth display channel are rotated by the second angle in the opposite direction about Y-axis of the global coordinate system, the first display channel and the second display channel are tiled with the second optical tiling method, the third display channel and the fourth display channel are tiled with the second optical tiling method, the first display channel and the third display channel are tiled with the first optical tiling method, and the second display channel and the fourth display channel are tiled with the first optical tiling method. The tiled head-mounted display according to the tenth embodiment has a horizontal field of view of at least 70° and a vertical field of view of at least 50°. 
     According to yet another aspect of the present invention, the tiled head-mounted display system can further comprise an auxiliary lens with free-form surfaces. Each lens cooperates with the corresponding prism with free-form surfaces, so that the user is able to see external scenery for augmented reality application. The second optical surface of the prism is a semi-transmissive and semi-reflective mirror surface. 
     The optical tiled head-mounted display device according to the present invention is compact and lightweight, and the exit pupil planes of all display channels are coincident, thus avoiding pupil aberration and keeping pupil diameter and eye clearance constant. Furthermore, there is no resolution variance in the overall field of view, thus preventing additional distortion. The tiled head-mounted display device according to the present invention can be readily applicable to augmented reality. In comparison, for a conventional head-mounted display device to be used in augmented reality, it is necessary to introduce a half mirror into the device to fold optical path in order to achieve optical transmission, thus requiring a complex and bulky structure. 
     Furthermore, the surfaces of the prisms according to the present invention for optical tiling can be formed continuous together as larger optical surfaces, each larger surface can be fabricated in one time and thus it does not require additional processing for the tiling surface. In addition, all optical surfaces of the prism of the display channels in the head-mounted display device can be formed integrally, thus reducing difficulty and complexity of the tiling process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention. 
         FIG. 1   a  and  FIG. 1   b  are schematic views showing a tiled head-mounted display system based on conventional oculars with rotational symmetry; 
         FIG. 2   a  and  FIG. 2   b  are two-dimensional schematic view and three-dimensional schematic view of one display channel in a tiled head-mounted display system according to the present invention; 
         FIG. 3  is a two dimensional schematic view of a single display channel in a tiled head-mounted display system according to the present invention with a mechanical tiling method; 
         FIG. 4  is a two dimensional schematic view of a single display channel in a tiled head-mounted display system according to the present invention with an optical tiling method; 
         FIG. 5   a  is a schematic view of a tiled head-mounted display system according to a first embodiment of the present invention;  FIG. 5   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a first embodiment of the present invention; 
         FIG. 6   a  is a schematic view of a tiled head-mounted display system according to a second embodiment of the present invention;  FIG. 6   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a second embodiment of the present invention; 
         FIG. 7   a  is a schematic view of a tiled head-mounted display system according to a third embodiment of the present invention;  FIG. 7   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a third embodiment of the present invention; 
         FIG. 8   a  is a schematic view of a tiled head-mounted display system according to a fourth embodiment of the present invention;  FIG. 8   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a fourth embodiment of the present invention; 
         FIG. 9   a  is a schematic view of a tiled head-mounted display system according to a fifth embodiment of the present invention;  FIG. 9   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a fifth embodiment of the present invention; 
         FIG. 10   a  is a schematic view of a tiled head-mounted display system according to a sixth embodiment of the present invention;  FIG. 10   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a sixth embodiment of the present invention; 
         FIG. 11   a  is a schematic view of a tiled head-mounted display system according to a seventh embodiment of the present invention;  FIG. 11   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a seventh embodiment of the present invention; 
         FIG. 12  is a two dimensional schematic view of one display channel in a tiled head-mounted display system for augmented environment according to the present invention; 
         FIG. 13   a  and  FIG. 13   b  are two-dimensional schematic view and three-dimensional schematic view of a tiled head-mounted display system for augmented environment according to the present invention; and 
         FIG. 14  is a schematic view showing the tiled head-mounted display system for augmented environment according to the present invention wore by a user. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments according to the present invention will be fully described with reference to the attached drawings. The present invention may, however, be embodied in various forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete and will fully convey the scope of the invention to those skilled in the art. Moreover, the features of the respective embodiments can also be combined in ways other than the specific embodiments described hereafter, and the technical solutions based on such combination will still fall within the scope of the present invention. 
       FIG. 2   a  and  FIG. 2   b  are two-dimensional schematic view and three-dimensional schematic view of one display channel in a tiled head-mounted display system according to the present invention. In the drawings, the coordinate system is defined as: global coordinate origin O is exit pupil center; Z-axis is in a direction along the viewing axis of user&#39;s eye; Y-axis is perpendicular to Z-axis and extends right above the eye; X-axis is perpendicular to both Y-axis and Z-axis, constituting a Cartesian coordinate. One display channel of a tiled head-mounted display device according to the present invention can comprise a prism with three free-form optical surfaces and a micro-display. Because the display channel is designed by reverse optical path design, that is, the rays coming from exit pupil are refracted and reflected by the prism successively with free-form surfaces to the micro-display. For the convenience of description, the elements and surfaces are numbered starting from the exit pupil. Reference number  1  represents the exit pupil. The prism comprises a first optical surface  2 , a second optical surface  3  and a third optical surface  4  in a counter-clockwise order relative to X-axis. The first optical surface  2  and the second optical surface  3  are free-form surfaces, the third optical surface  4  may be selected from free-form, spherical or non-spherical surface. The first optical surface  2  can be a concave transmissive surface on the user side, for example. The second optical surface  3  can be a concave reflective surface or semi-transmissive and semi-reflective surface on the user side for magnifying image, for example. When the tiled head-mounted display device is used for virtual environments, the third optical surface  4  can be a concave transmissive surface on the user&#39;s side. As shown in  FIG. 2   a , in reality, the light path in the head-mounted display device according to the invention starts from the micro-display  6 . The lights emitted by the micro-display  6  such as a LCD enter into the prism through the third optical surface  4 , and then are subject to total reflection on the inner side of the first optical surface  2 , then reflected by the second optical surface  3  and finally enter into user&#39;s eye through the first optical surface  2 . 
     The free-form surface equation of the first optical surface  2 , the second optical surface  3  and the third optical surface  4  may follow (but are not limited to) any one of conditions (1) to (5). 
                   z   =             c   x     ⁢     x   2       +       c   y     ⁢     y   2           1   +       {     1   -       (     1   +     k   x       )     ⁢     c   x   2     ⁢     x   2       -       (     1   +     k   y       )     ⁢     c   y   2     ⁢     y   2         }       1   /   2           +       ∑     i   =   1     n     ⁢       A   i     ⁢       {         (     1   -     P   i       )     ⁢     x   2       +       (     1   +     P   i       )     ⁢     y   2         }       i   +   1                     (   1   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c x  is radius of curvature in the x direction in the xz-plane, c y  is radius of curvature in the y direction in the yz-plane, k x  is conic coefficient in x direction, k y  is conic coefficient in y direction, A, are aspherical coefficients of 4, 6, 8, 10, . . . 2n orders, P i  are non-rotational symmetry coefficient of 4, 6, 8, 10, . . . 2n orders, and the surface has rotational symmetry about z-axis;
 
                     z   =         c   ⁡     (       x   2     +     y   2       )         1   +     sqrt   ⁡     (     1   -       (     1   +   k     )     ⁢       c   2     ⁡     (       x   2     +     y   2       )           )           +       ∑     j   =   2     66     ⁢       C   j     ⁢     x   m     ⁢     y   n             ,     
     ⁢     j   =         [         (     m   +   n     )     2     +   m   +     3   ⁢   n       ]     /   2     +   1               (   2   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is radius of curvature of surface, C j  is polynomial coefficients, m is an even number in the present invention;
 
                   z   =         c   ⁡     (       x   2     +     y   2       )         1   +     sqrt   ⁡     (     1   -       (     1   +   k     )     ⁢       c   2     ⁡     (       x   2     +     y   2       )           )           +       ∑     j   =   1     66     ⁢       C     j   +   1       ⁢     Z   j                   (   3   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is radius of curvature of surface, k is conic coefficient, Z j  is Zernike polynomial, C j+1  is coefficients for Z j ;
 
                     z   =             c   x     ⁢     x   2       +       c   y     ⁢     y   2           1   +     sqrt   ⁡     (     1   -       (     1   +     k   x       )     ⁢     c   x     ⁢     x   2       -       (     1   +     k   y       )     ⁢     c   y     ⁢     y   2         )           +       ∑     j   =   1     37     ⁢       C   j     ⁢     x     2   ⁢   m       ⁢     y   n             ⁢     
     ⁢           2   ⁢   m     +   n     ≤   10     ,     m   =   0     ,   1   ,   2   ,   3   ,   4   ,   5   ,     n   =   0     ,   1   ,   …   ⁢           ,   10             (   4   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is the vertex curvature, k is the conic constant, c x  is radius of curvature of surface in sagittal direction, c y  is radius of curvature of surface in tangential direction, and C j  is the coefficient for x 2m y n .
 
                   z   =         c   ⁢           ⁢     x   2         1   +       [     1   -       (     1   +   k     )     ⁢     c   2     ⁢     x   2         ]         1   /   2     ⁢                   +     A   ⁢           ⁢     x   4       +     B   ⁢           ⁢     x   6       +     C   ⁢           ⁢     x   8       +     D   ⁢           ⁢     x   10                 (   5   )               
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, c is the radius of curvature, k is conic coefficient, A, B, C, D are aspheric coefficients of 4, 6, 8 and 10 orders, respectively.
 
     The micro-display in each display channel can be any types of flat panel displays such as LCD displays, OLED displays. The prism can be formed of plastic or glass optical material by injection molding, micromachining, which will not be discussed in detail herein. 
     The tiled head-mounted display device according to the present invention includes a number of display channels each comprising the prism with free-form surface described above and a corresponding micro-display. The display channels are tiled so that the overall field of view of the tiled device can be substantially a summation of individual field of view of each display channel and the center of exit pupil of each display channel in the tiled device is coincident with each other. 
     Tiling methods of the tiled head-mounted displays according to the present invention can be categorized as mechanical tiling methods and optical tiling methods.  FIG. 3  is a two dimensional schematic view of a single display channel in a tiled head-mounted display system according to the present invention with a mechanical tiling method.  FIG. 4  is a two dimensional schematic view of a single display channel in a tiled head-mounted display system according to the present invention with an optical tiling method. 
     The tiling methods comprise cementing the prisms with free-form surfaces of display channels by methods such as adhering, gluing, bonding and welding or integrating the prisms by injection molding. 
     When display channels are tiled by so-called mechanical tiling method according to the present invention as shown in  FIG. 3 , the first optical surface  2 , the second optical surface  3  and the third optical surface  4  of each prism  101  may satisfy following conditions (6)-(8): 
                   {           0.5   ≤       Z     P   ⁢           ⁢     a   ′         -     Z     P   ⁢           ⁢   a         ≤   4                   Y     P   ⁢           ⁢     a   ′         -     Y     P   ⁢           ⁢   a         ≤   0                   Y     P   ⁢           ⁢     a   ′         -     Y     P   ⁢           ⁢   a         ≤   0                   (   6   )               {               Y     P   ⁢           ⁢     b   ′         -     Y     P   ⁢           ⁢   b         ≥   0                 -   3     ≤       Z     P   ⁢           ⁢     b   ′         -     Z     P   ⁢           ⁢   b         ≤   0                   (   7   )               {             -   4     ≤       Y     P   ⁢           ⁢   c       -     Y     P   ⁢           ⁢     c   ′           ≤   0               0   ≤       Z     P   ⁢           ⁢   c       -     Z     P   ⁢           ⁢     c   ′           ≤   2                   (   8   )               
where R u  is the top marginal ray of the maximum field of view in positive Y direction, R b  is the light ray at lower boundary of the maximum field of view in negative Y direction; P a  is an intersection point at which R b  is transmitted through the first optical surface  2 , P a ′ is an intersection point of R b  with the second optical surface  3  and P a ″ is an intersection point of R b  with the first optical surface  2  upon total reflection; P b  is an intersection point between R u  and the second optical surface  3 , P b ′ is an intersection point between R b  and the third optical surface  4 ; P c  is an intersection point at which R u  is reflected on the first optical surface  2 , P c ′ is an intersection point between R u  and the third optical surface  4 , Y, Z are coordinates of each point in global coordinate system, respectively.
 
     The prism  101  must also satisfy following conditions regarding incident angles of R u  on the first optical surface  2 : 
                   {             θ     m   ⁢           ⁢   i   ⁢           ⁢   1       ≥     arcsin   ⁡     (     1   /   n     )                     θ     m   ⁢           ⁢   i   ⁢           ⁢   2       ≤     arcsin   ⁡     (     1   /   n     )                       (   9   )               
where θ mi1  is an incident angle of R u  emitted from the displays first striking the first optical surface  2 , and θ mi2  is an incident angle of R u  striking the first optical surface  2  at the second time, n is a refractive index of prism material.
 
     Mechanical tiling methods can include a first mechanical tiling method and a second mechanical tiling method. In the first mechanical tiling method, bottom surfaces  12  of two prisms are subject to mechanical processing such as cutting and polishing and then cemented together. In  FIG. 3 , the bottom surface  12  indicated by dotted lines is positioned between the first optical surface  2  and the second optical surface  3 . In the second mechanical tiling method, side surfaces of two prisms, for example, side surfaces  22  and  23  in  FIG. 2   a  are subject to mechanical processing such as cutting and polishing and then cemented together. The side surface may intersect with all of the first optical surface  2 , the second optical surface  3  and the third optical surface  4  of prism. 
     When display channels are tiled by so-called optical tiling method according to the present invention as shown in  FIG. 4 , the first optical surface  2 , the second optical surface  3  and the third optical surface  4  of each prism  102  may satisfy following conditions (10)-(12):
 
18≦ Z   Pd ≦28  (10)
 
1.5 ≦Z   Pd′   −Z   Pd ≦3  (11)
 
 Z   Pd ≧the eye clearance distance, i.e., 15  (12)
 
where, P c  is an intersection point of R u  with the first optical surface  2  upon total reflection, P d  is an intersection point of chief ray R c  in horizontal field of view and the first optical surface  2 , P d  is an intersection point of R u  with the second optical surface  3 .
 
     Similar to mechanical tiling methods, optical tiling methods can also comprise a first optical tiling method and a second optical tiling method. In the first optical tiling method, the bottom surfaces  11  of both prisms in two display channels to be tiled can be directly cemented together or the two prisms can be formed integrally by injection molding, the bottom surface  11  of the prism is positioned between the first optical surface  2  and the second optical surface  3 ; In the second optical tiling method, the side surface of the respective prisms  11  such as side surfaces  23  can be directly cemented or the two prisms can be formed integrally by injection molding. The side surface intersects with the first optical surface, the second optical surface and the third optical surface of the prism. 
     Because display channels are tilted with respect to their origin position in the mechanical tiling device, the tiled head-mounted display formed by the mechanical tiling methods has a smaller exit pupil diameter and eye clearance compared with the tiled head-mounted displays formed by the optical tiling methods. For the tiled head-mounted display formed by the mechanical tiling methods, it is required to correct trapezoidal distortion. For the tiled head-mounted displays formed by the optical tiling methods, it is not necessary to correct trapezoidal distortions due to positional change and the user will not see discontinuous images even if his/her eyeballs are moving. In some cases, user might see stitches between different display channels in the field of view in the tiled head-mounted display formed by the mechanical tiling methods while the tiled head-mounted display formed by the optical tiling methods does not suffer from this problem. 
     The embodiments of both the mechanical tiled head-mounted displays and the optical tiled head-mounted displays mentioned above will be described in detail hereafter. The invention, however, is not limited to the specific embodiment described in follows. 
     First Embodiment 
       FIG. 5   a  is a schematic view of a tiled head-mounted display system according to a first embodiment of the present invention;  FIG. 5   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a first embodiment of the present invention. As shown in  FIG. 5   a , a head-mounted display device according to the first embodiment of the present invention can comprise a first display channel  501  and a second display channel  502  tiled by the first mechanical tiling method as described above. The first display channel  501  is rotated by a first angle in YOZ plane about X-axis of a global coordinate system. The second display channel  502  is rotated by ±180° about Z axis which is in a direction along viewing axis of a human eye, and then rotated by the first angle in the opposite direction about X-axis of the global coordinate system. As shown in  FIG. 5   b , the tiled head-mounted display according to the first embodiment of the present invention has a horizontal field of view of at least 50° and a vertical field of view of at least 40°. 
     Second Embodiment 
       FIG. 6   a  is a schematic view of a tiled head-mounted display system according to a second embodiment of the present invention;  FIG. 6   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a second embodiment of the present invention. As shown in  FIG. 6   a , a head-mounted display device according to the second embodiment of the present invention can comprise a first display channel  601  and a second display channel  602  tiled by the second mechanical tiling method as described above. The first display channel  601  is rotated by a second angle in XOZ plane about Y-axis of a global coordinate system. The second display channel  602  is rotated by the second angle in the opposite direction about Y-axis of the global coordinate system. As shown in  FIG. 6   b , the tiled head-mounted display according to the second embodiment of the present invention has a horizontal field of view of at least 70° and a vertical field of view of at least 30°. 
     Third Embodiment 
       FIG. 7   a  is a schematic view of a tiled head-mounted display system according to a third embodiment of the present invention;  FIG. 7   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a third embodiment of the present invention. As shown in  FIG. 7   a , a head-mounted display device according to the third embodiment of the present invention can comprise a first display channel  701 , a second display channel  702  and a third display channel  703  rotated by a predetermined angle about Y-axis of a global coordinate system. The second display channel  702  is tiled with the first display channel  701  and the third display channel  703  using the second mechanical tiling method, respectively. As shown in  FIG. 7   b , the tiled head-mounted display according to the third embodiment of the present invention has a horizontal field of view of at least 100° and a vertical field of view of at least 30°. 
     Fourth Embodiment 
       FIG. 8   a  is a schematic view of a tiled head-mounted display system according to a fourth embodiment of the present invention;  FIG. 8   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a fourth embodiment of the present invention. As shown in  FIG. 8   a , a head-mounted display device according to the fourth embodiment of the present invention can comprise a first display channel  801 , a second display channel  802 , a third display channel  803  and a fourth display channel  804  rotated by a predetermined angle about Y-axis of a global coordinate system. The second display channel  802  is tiled with the first display channel  801  and the third display channel  803  using the second mechanical tiling method, respectively. The third display channel  803  is tiled with the second display channel  802  and the fourth display channel  804  using the second mechanical tiling method, respectively. As shown in  FIG. 8   b , the tiled head-mounted display according to the fourth embodiment of the present invention has a horizontal field of view of at least 120° and a vertical field of view of at least 30°. 
     Fifth Embodiment 
       FIG. 9   a  is a schematic view of a tiled head-mounted display system according to a fifth embodiment of the present invention;  FIG. 9   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a fifth embodiment of the present invention. As shown in  FIG. 9   a , a head-mounted display device according to the fifth embodiment of the present invention can comprise a first display channel  901 , a second display channel  902 , a third display channel  903  and a fourth display channel  904 . The first display channel  901  and the third display channel  903  are rotated by a second angle in XOZ plane about Y-axis of a global coordinate system. The second display channel  902  and the fourth display channel  904  are rotated by the second angle in the opposite direction about Y-axis of the global coordinate system. The first display channel  901  and the second display channel  902  are tiled with the second mechanical tiling method. The third display channel  903  and the fourth display channel  904  are tiled with the second mechanical tiling method. The first display channel  901  and the third display channel  903  are tiled with the first mechanical tiling method. The second display channel  902  and the fourth display channel  904  are tiled with the first mechanical tiling method. As shown in  FIG. 9   b , the tiled head-mounted display according to the fifth embodiment of the present invention has a horizontal field of view of at least 70° and a vertical field of view of at least 50°. 
     Sixth Embodiment 
       FIG. 10   a  is a schematic view of a tiled head-mounted display system according to a sixth embodiment of the present invention;  FIG. 10   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a sixth embodiment of the present invention. As shown in  FIG. 10   a , a head-mounted display device according to the sixth embodiment of the present invention can comprise a first display channel  1001 , a second display channel  1002 , a third display channel  1003 , a fourth display channel  1004 , a fifth display channel  1005  and a sixth display channel  1006 . The first display channel  1001 , the second display channel  1002  and the third display channel  1003  are rotated by a predetermined angle about Y-axis of a global coordinate system. The fourth display channel  1004 , the fifth display channel  1005  and the sixth display channel  1006  are rotated by a predetermined angle about Y-axis of a global coordinate system. The second display channel  1002  is tiled with the first display channel  1001  and the third display channel  1003  using the second mechanical tiling method respectively. The fifth display channel  1005  is tiled with the fourth display channel  1004  and the sixth display channel  1006  using the second mechanical tiling method respectively. The first display channel  1001  and the fourth display channel  1004  are tiled with the first mechanical tiling method. The second display channel  1002  and the fifth display channel  1005  are tiled with the first mechanical tiling method. The third display channel  1003  and the sixth display channel  1006  are tiled with the first mechanical tiling method. As shown in  FIG. 10   b , the tiled head-mounted display according to the sixth embodiment of the present invention has a horizontal field of view of at least 100° and a vertical field of view of at least 50°. 
     Seventh Embodiment 
       FIG. 11   a  is a schematic view of a tiled head-mounted display system according to a seventh embodiment of the present invention;  FIG. 11   b  is a schematic view illustrating field of view of the tiled head-mounted display system according to a seventh embodiment of the present invention. As shown in  FIG. 11   a , a head-mounted display device according to the seventh embodiment of the present invention can comprise a first display channel  1101 , a second display channel  1102 , a third display channel  1103 , a fourth display channel  1104 , a fifth display channel  1105 , a sixth display channel  1106 , a seventh display channel  1107  and an eighth display channel  1108 . The first display channel  1101 , the second display channel  1102 , the third display channel  1103  and the fourth display channel  1104  are rotated by a predetermined angle about Y-axis of a global coordinate system. The fifth display channel  1105 , the sixth display channel  1106 , the seventh display channel  1107  and the eighth display channel  1108  are rotated by a predetermined angle about Y-axis of a global coordinate system. The second display channel  1102  is tiled with the first display channel  1101  and the third display channel  1103  using the second mechanical tiling method respectively. The third display channel  1103  is tiled with the second display channel  1102  and the fourth display channel  1104  using the second mechanical tiling method respectively. The sixth display channel  1106  is tiled with the fifth display channel  1105  and the seventh display channel  1107  using the second mechanical tiling method respectively. The seventh display channel  1107  is tiled with the six display channel  1106  and the eighth display channel  1108  using the second mechanical tiling method respectively. The first display channel  1101  and the fifth display channel  1005  are tiled with the first mechanical tiling method. The second display channel  1102  and the sixth display channel  1106  are tiled with the first mechanical tiling method. The third display channel  1103  and the seventh display channel  1107  are tiled with the first mechanical tiling method. The fourth display channel  1104  and the eighth display channel  1108  are tiled with the first mechanical tiling method. As shown in  FIG. 11   b , the tiled head-mounted display according to the seventh embodiment of the present invention has a horizontal field of view of at least 120° and a vertical field of view of at least 50°. 
     Eighth Embodiment 
     A tiled head-mounted display system according to a eighth embodiment of the present invention can comprise a first display channel and a second display channel tiled by the first optical tiling method as described above, which is similar to the illustration of  FIG. 5   a  in structure. The first display channel is rotated by a first angle in YOZ plane about X-axis of a global coordinate system. The second display channel is rotated by ±180° about Z axis and then rotated by the first angle in the opposite direction about X-axis of the global coordinate system. The tiled head-mounted display according to the eighth embodiment of the present invention has a horizontal field of view of at least 50° and a vertical field of view of at least 40°. 
     Ninth Embodiment 
     A tiled head-mounted display system according to a ninth embodiment of the present invention can comprise a first display channel and a second display channel tiled by the second optical tiling method as described above, which is similar to the illustration of  FIG. 6   a  in structure. The first display channel is rotated by a second angle in XOZ plane about Y-axis of a global coordinate system. The second display channel is rotated by the second angle in the opposite direction about Y-axis of the global coordinate system. The tiled head-mounted display according to the ninth embodiment of the present invention has a horizontal field of view of at least 70° and a vertical field of view of at least 30°. 
     Tenth Embodiment 
     A tiled head-mounted display system according to a tenth embodiment of the present invention can comprise a first display channel, a second display channel, a third display channel and a fourth display channel, which is similar to the illustration of  FIG. 9   a  in structure. The first display channel and the third display channel are rotated by a second angle in XOZ plane about Y-axis of a global coordinate system. The second display channel and the fourth display channel are rotated by the second angle in the opposite direction about Y-axis of the global coordinate system. The first display channel and the second display channel  902  are tiled with the second optical tiling method. The third display channel  903  and the fourth display channel  904  are tiled with the second optical tiling method. The first display channel  901  and the third display channel  903  are tiled with the first optical tiling method. The second display channel  902  and the fourth display channel  904  are tiled with the first optical tiling method. The tiled head-mounted display according to the tenth embodiment of the present invention has a horizontal field of view of at least 70° and a vertical field of view of at least 50°. 
     Tiled Head-Mounted Display Device for Augmented Environment Applications 
     In tiled head-mounted display devices according to the ten embodiments discussed above, if the second optical surface  3  of the prism in each display channel is coated with a reflective film to be formed as a reflective surface, the tiled head-mounted display devices can be mainly used for virtual environment application. If the second optical surface  3  of the prism in each display channel can be coated with a transflective film to be formed as a semi-transmissive and semi-reflective surface, an head-mounted display device for augmented environment application can be formed by adding an auxiliary lens with free-form surfaces in the display device, so that the lens and the prisms can constitute a focus-free system and allow user to see through the display device to observe outside real world.  FIG. 12  is a two dimensional schematic view of one display channel in a tiled head-mounted display system for augmented environment according to the present invention. Therefore the embodiments as discussed above can be used for augmented environment by adding an auxiliary lens with free-form surfaces. 
       FIG. 13   a  and  FIG. 13   b  are two-dimensional schematic view and three-dimensional schematic view of a tiled head-mounted display system for augmented environment according to the present invention. As shown in  FIG. 13   a , the tiled head-mounted display device can comprise display channels each comprising a prism with free-form surfaces  1302 , a micro-display device  1302  and a lens with free-form surfaces  1304 . The display channels can be tiled by the first mechanical tiling method, thus a part of prism  1302  and a part of lens  1304  are removed during tiling. 
       FIG. 14  is a schematic view showing the tiled head-mounted display system for augmented environment according to the present invention wore by a user. As shown in  FIG. 14 , the tiled head-mounted display system for augmented environment according to the present invention can be used as a single ocular system for just one eye. Alternatively, it could also be used as a binocular head-mounted display system for both eyes. 
     The optical tiled head-mounted display device according to the present invention is compact and lightweight, and the exit pupil planes of all display channels are coincident, thus avoiding pupil aberration and keeping pupil diameter and eye clearance constant. Furthermore, there is no resolution variation in the field of view, thus preventing additional distortion. The tiled head-mounted display device according to the present invention can be readily applicable to augmented reality. In comparison, for a conventional head-mounted display device to be used in augmented reality, it is necessary to introduce a semi-reflective semi-transmissive mirror into the device to fold optical path in order to achieve optical transmission, thus requiring a complex and bulky structure. 
     Furthermore, the surfaces of the prisms according to the present invention for optical tiling can be formed continuous together as larger optical surfaces, they can be fabricated in one time and thus it does not require additional processing for the tiling surface. In addition, all optical surfaces of the prisms of the display channels in the head-mounted display device can be formed integrally, thus reducing difficulty and complexity of the tiling process. 
     The invention being thus described, it will be obvious that the same may be varied and modified in many ways. Such variations and modification are not to be regarded as a departure from the spirit and scope of the invention, and all such variation and modifications as would be obvious to those skilled in the art are intended to be included within the scope of the following claims.