Patent Publication Number: US-2022236572-A1

Title: Free-form prism-lens group and near-eye display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 16/423,005 filed on May 25, 2019, which is a continuation of International Appl. No. PCT/CN2017/113080 filed on Nov. 27, 2017, which claims priority to Chinese Pat. Appl. No. 201611123106.6 filed on Dec. 8, 2016. The disclosures of the above-referenced applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Near-eye displays (NEDs) have seen tremendous development since the birth of virtual reality (VR) and augmented reality (AR). In recent years, commercial NEDs have become prevalent, and companies such as Samsung, Sony, Epson, and Microsoft have launched their NED products. Since these displays need to be worn on head by users in using, a lightweight and compact form is always appreciated to reduce the load of users. 
     SUMMARY 
     The present disclosure relates to an optical system and apparatus for near-eye display. In some examples, there is provided a free-form prism-lens group for use in a near-eye display and a near-eye display apparatus using the free-form prism-lens group. 
     Various embodiments disclosed herein provide a freeform prism-lens group with good stability and wear resistance without additional protection sheets, and a near-eye display device using the same, including a primary prism and an auxiliary lens, and the primary prism is used for achieving main refraction/reflection function, and the auxiliary lens is used for compensating the rays and providing a relatively flat surface, in order to improve the stability and resistance. 
     In an aspect, a free-form prism-lens is provided with smooth front and back surfaces, including: 
     at least a primary prism having a first freeform surface, a second freeform surface, and a third freeform surface and a first freeform auxiliary lens; 
     wherein the first auxiliary lens has a first optical surface proximal to, and with a same shape as, the third freeform surface of the primary prism 
     wherein the third freeform surface of the primary prism is coated with a beam splitter film having predetermined beam splitting ratio, wherein the first auxiliary lens has a second opposing optical surface distal from the primary prism that is planar, aspheric, or spherical with a curvature radius greater than 100 mm. In some embodiments, wherein a second auxiliary lens is included, wherein the second auxiliary lens has a first surface proximal to, and with a same shape as, the first freeform surface of the freeform primary prism. 
     In some embodiments, the first auxiliary lens is coupled to the freeform primary prism by gluing together. 
     In some embodiments, there is a gap with a predetermined thickness of not more than 1 mm is configured between the first surfaces of the second auxiliary lens and the first surface of the freeform primary prism. 
     In some embodiments, the gap is an airgap. 
     In some embodiments, the second auxiliary lens and the freeform primary prism are removably coupled together. 
     In some embodiments, the second auxiliary lens has a second surface distal from the freeform primary prism, wherein the second surface of the second auxiliary lens has an identical shape as the second optical surface of the first auxiliary lens, making environmental light with an unchanged direction after passing through the first auxiliary lens, the primary prism, and the second auxiliary lens in sequence. 
     In some embodiments, the second auxiliary lens has a second surface distal from the freeform primary prism, wherein the second surface of the second auxiliary lens has a different shape with the second optical surface of the first auxiliary lens, thus adding a predetermined diopter compensation to the environmental light. 
     In some embodiments, the second surface of the second auxiliary lens and the second optical surface of the first auxiliary lens are coated with hardening films and antireflection films. 
     As a way of reducing the thickness of the apparatus according to some embodiments of the present disclosure, the second optical surface of the first auxiliary lens and the second surface of the second auxiliary lens do not extend beyond an outer edge of the primary prism in an optical axis direction, such that a maximum thickness of the freeform prism-lens group at the optical axis direction is determined by a maximum outer edge thickness of the primary prism at the optical axis direction. As such, the maximum thickness of the freeform prism-lens group is no more than 15 mm according to some embodiments. 
     In some embodiments, the primary prism has an extended edge portion that extends beyond the freeform surface portion having the beamsplitter film; the extended edge portion has a non-freeform surface; and the freeform portion and the non-freeform surface has a smooth transition. 
     In another aspect, a near-eye display apparatus is provided having the freeform prism-lens group, further including a micro-display (MD), wherein the MD is positioned over the first freeform surface of the freeform primary prism, wherein light emitted from the MD enters the body of the primary prism via the first freeform surface. 
     In some embodiments, a controller and a controllable optical filter are included, wherein the controllable optical filter is attached to the second optical surface of the first auxiliary lens, wherein the controllable optical filter is coupled to the controller and is driven with the MD synchronously. 
     According to various embodiments of the present disclosure, the primary prism made of resinous material is injection-molded with optical resin material with high precision, and the compensating auxiliary lens does not need to be designed individually to reduce the complexity of the surface and the design difficulty. Since both surfaces of the freeform prism-lens group facing to the outside are planar or approximate planar shape. As an optical element in a NED apparatus, it has good extensibility when protecting and attaching external devices, and an attached controllable optical filter may achieve pixel-level control of transmittance, which may effectively decrease the influence of the environmental light on the augmented images, and thus a more vivid and accurate synthesized image can be observed by the eye. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary and the following detailed description of exemplary embodiments of the present invention may be further understood when read in conjunction with the appended drawings, in which: 
         FIG. 1  schematically illustrates the vertical cross-section of the freeform prism-lens group according to some embodiments; 
         FIG. 2  schematically illustrates the vertical cross-section of the primary prism in  FIG. 1 ; 
         FIG. 3  illustrates the distortion of the displayed image using the freeform prism-lens group in  FIG. 1 ; 
         FIG. 4A  is a schematic diagram illustrating the vertical cross-section of the freeform prism-lens group in  FIG. 1  with a second auxiliary lens with a first amount of diopter compensation provided by the front surface of the second auxiliary lens; 
         FIG. 4B  is a schematic diagram illustrating the vertical cross-section of the freeform prism-lens group in  FIG. 1  with a second auxiliary lens with a second amount of diopter compensation provided by the front surface of the second auxiliary lens; 
         FIG. 4C  is a schematic diagram illustrating a zoomed horizontal cross-section of the freeform prism-lens group in  FIG. 1  through its optical axis; 
         FIG. 5  schematically illustrates the cross-section of the freeform prism-lens group according to some other embodiments; 
         FIG. 6  illustrates the distortion of the displayed image using the freeform prism-lens group in  FIG. 5 ; and 
         FIG. 7  illustrates the NED apparatus with freeform prism-lens according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments disclosed herein will be fully described with reference to the attached drawings. The present disclosure 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 disclosure 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 disclosure. 
     The inventors of the present disclosure have recognized that, employing specifically-designed freeform optical elements can be a promising way to achieve VR or AR display applications. Due to the extra degrees of freedom provided by freeform optics, aberration can be better corrected and improved display quality can be achieved, and the weight and size of the optical element used in the NED system can be reduced. For manufacture considerations, freeform prisms made of resin materials (e.g., polymethyl methacrylate, PMMA) can be massively produced by injection molding technique to lower production costs. However, optical plastics are inferior to optical glasses in the aspect of abrasive resistance. Hence, processes such as hardening treatment, under complex industrial environment and during the long time of normal use, are taken to improve the abrasive resistance and stabilization of the plastic optical elements. 
       FIG. 1  shows an embodiment of the disclosure. The freeform prism-lens group 1 has at least a primary prism  10  for image with freeform surface(s) and a first auxiliary lens  20  for compensation, wherein each freeform optical element has at least two valid optical surfaces and other auxiliary surfaces to form a valid element. The primary prism  10  has at least three valid optical surfaces, which magnifies the image displayed through a MD by refraction and/or reflection. 
     As shown in  FIG. 2 , the primary prism  10  may include three optical surfaces labeled as  101 ,  102 , and  103 , respectively. The first optical surface  101  faces the MD. Light emitted from MD passes through the first optical surface  101  and enters the body of primary prism  10  by refraction, and then propagate to the second optical surface  102 . The light is total internal reflected after the first time to reach the second optical surface  102 . The reflected light from the second optical surface  102  is reflected by the third optical surface  103  and then reach the second optical surface  102  for the second time. After reaching the second optical surface  102  for the second time, the light passes through the second optical surface  102  to the position of exit pupil EPD. The second optical surface  102  of the primary prism  10  is required to satisfy the condition of total internal reflection for light reaches the first time. The second optical surface  102  and the third optical surface  103  are freeform surfaces. The third optical surface  103  coated with a beam splitter film, e.g., a half film or a film with some other splitting ratio reflects the light into the internal of the primary prism  10 . The transmittance of the beamsplitter film can be determined according to the brightness of the MD to ensure the effective use of light, and balance the brightness of the light of the MD and the brightness of the environmental light rays to improve the contrast of the displayed AR image. 
     The primary prism  10  is the major element in the freeform prism-lens group  1  in the aspect of obtaining an enlarged virtual image of the MD. By the curvatures of the second optical surfaces  102  and the third optical surface  103  of the primary prism  10  work in conjunction, the image on the MD could be enlarged. The light passed through the third optical surface  103  that could not reach the user&#39;s eyes, as the loss of the optical energy, has no contribution for the image that the user watches. As a result, the light of the MD is refocused by the primary prism  10 , and an image from MD seen at EP appears to be emitted from some feet away to the eye rather than from about one inch away where the MD is actually placed. 
     Those of ordinary skill in the art can appreciate that, as shown in  FIG. 2 , it is a preferred embodiment to use the way of three freeform surfaces of the primary prism  10  to enhance the ability of enlarging the image and the image aberration adjustment, but it is not limiting. The three effective optical surfaces of the primary prism  10  can be other types of surfaces respectively, e.g. only the third optical surface  103  is the freeform surface, and for the convenience of manufacturing the first optical surface  101  and the second optical surface  102 , they can be other types of surfaces. For example, the first optical surface  101  can adopt a spherical surface or aspheric surface, or the second optical surface  102  can adopt a spherical surface or aspheric surface if the second optical surface  102  can satisfy the condition of total internal reflection for the light first reaching it. Spherical or aspherical surface is conducive to manufacturing and reducing the cost, but the aberration correction ability of spherical or aspherical surface for light of a magnified image is significantly lower than the surface of the freeform surface. As such, the type of surface that holds the third optical surface  103  as a freeform surface is particularly important. 
     Based on the focusing of the valid optical surfaces of the primary prism and inconsistency in the thickness of the primary prism, the environmental light rays connecting to the human eyes through the primary prism will cause deformation of the environmental light rays imaging. Therefore, the auxiliary lens needs to be set. The first auxiliary lens  20  is placed on one side of the primary prism  10 . In this embodiment, as shown in  FIG. 1 , the first auxiliary lens  20  is placed outside of the primary prism  10 , e.g., on the side close to the environment. The first optical surface  201  of the first auxiliary lens  20  adjacent to the primary prism  10  is an optical surface having an identical shape with the third optical surface  103  of the primary prism  10 , and is close to the third optical surface  103  of the primary prism  10  by means of gluing together. The second optical surface  202  of the first auxiliary lens  20  facing the environment is also an optical surface for transmitting the environmental light rays into the first auxiliary lens  20  and the primary prism  10 . Because of the second optical surface  202  of the first auxiliary lens  20  facing the environment, preferably, the second optical surface  202  is a planar surface to facilitate protective measures, such as coated with protective film layer. In some embodiments, the light of the MD enters the first auxiliary lens  20  is not used for imaging, therefore, the material for the first auxiliary lens  20  can be different with that for the primary prism, e.g., the first auxiliary lens may not use a special material that is advantageous for reducing optical aberration to reduce the difficulty of the fabrication and the cost, but it is also feasible that the first auxiliary lens and the primary prism use the same material. For the convenience of assembling and positioning, the primary prism  10  is provide positioning structure such as a positioning slot, and the positioning structure can match the shape of the first auxiliary lens  20  or the positioning structure on the first optical surface  201  of the first auxiliary lens. Therefore, the relative position of the first auxiliary lens  20  and the primary prism  10  can be easily determined, resulting in high-precision alignment after the surface of the primary prism  10  is cemented to the first auxiliary lens  20 , and a high image quality of the environmental light rays can be obtained. 
     In some embodiments, the embodiment of the disclosure may include a second auxiliary lens  30 , as illustrated in  FIGS. 1 and 2 . The second auxiliary lens  30  may be placed on the other side of the primary prism  10 , i.e., the first auxiliary lens  20  and the second auxiliary lens  30  are on two different sides of the primary prism  10 . The second auxiliary lens  30  may be a compensation for the light in the other side of the freeform prism-lens group  1 . The first surface  301  of the second auxiliary lens  30  is an optical surface which has a same shape with the second optical surface  102  of the primary prism  10 , and is placed proximal to the second optical surface  102  of the primary prism  10  with an air gap with a predetermined thickness that is no more than 1 mm. To facilitate the assembly, the air gap may be ensured by one or more spacers of predetermined thickness, or by mechanical matching clip and column predetermined lengths with clips and slots wherein clips and columns may be disposed on the second auxiliary lens  30  or the primary prism  10 , and the correspondent clips and slots may be disposed on the primary prism  10  or the second auxiliary lens  30 . The above-mentioned slots, columns or the clips may be manufactured all-in-one-shape with the primary prism  10  or the auxiliary lens  30 , or may be mounted on the primary prism  10  or the second auxiliary lens  30  by other means, e.g., adhering. 
     The second surface  302  of the second auxiliary lens  30  which is opposite to the first surface  301 , i.e., proximal to the exit pupil EP position of this freeform prism-lens group. To achieve a freeform prism-lens group that does not introduce diopter to the environmental light rays, the second surface  302  of the second auxiliary lens  30  has an identical shape with the second optical surface  202  of the auxiliary lens  20  that the surface is face to the environmental light. The second surface  302  of the second auxiliary lens  30  is planar in this embodiment. Since the light from MD is further transmitted through the second auxiliary lens  30  after exiting from the primary prism  10 , the auxiliary lens  30  may be further used to correct aberrations, and it is preferable to use the same material as the primary prism  10 , and the height of the second auxiliary lens  30  may not extend whole surface of the second optical surfaces  202  in the direction perpendicular to the optical axis, but only covers the effective aperture of the exiting light. Since the second optical surface  202  tends to have a tilt to the EPD at a position away from the optical axis, the auxiliary lens  30  partially-covered can effectively reduce the overall thickness and volume of the freeform prism-lens group  1 . In some embodiments, the second optical surface  202  of the first auxiliary lens  20  and the second surface  302  of the second auxiliary lens  30  can be coated with hardening film and anti-reflective film. 
       FIG. 1  illustrates a structure according to some embodiments to obtain a minimum thickness of the freeform prism-lens group  1 . The second optical surface  202  of the first auxiliary lens  20  and the second surface  302  of the second auxiliary lens  30  may not extend an outer edge of the primary prism  10  in the optical axis direction. Such that a maximum thickness of the freeform prism-lens group  1  at the optical direction is determined by a maximum outer edge thickness of the primary prism  10 . According to the primary prism  10  adapting the size of MD, the maximum outer edge thickness of the primary prism  10  will change accordingly. For example, the maximum outer edge thickness of the primary prism  10  at the optical axis direction may be effective controlled when the size of the MD is smaller; the maximum thickness of the primary prism  10  in the optical axis direction can be extension properly when the size of MD is larger. That means, the maximum outer edge thickness of the primary prism at the optical axis direction is adapted to extended, i.e. the maximum outer edge thickness of the primary prism is adapted to the MD adapted to the primary prism. For the MD, the effective maximum outer edge thickness is proportional to the size of the MD. 
     According to the above embodiment of the disclosure, the freeform optical surfaces can be represented by an XYP (XY polynomial) equation: 
     
       
         
           
             Z 
             = 
             
               
                 
                   c 
                   ⁡ 
                   
                     ( 
                     
                       
                         x 
                         2 
                       
                       + 
                       
                         y 
                         2 
                       
                     
                     ) 
                   
                 
                 
                   1 
                   + 
                   
                     sqrt 
                     ⁡ 
                     
                       ( 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               1 
                               + 
                               k 
                             
                             ) 
                           
                           ⁢ 
                           
                             
                               c 
                               2 
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   x 
                                   2 
                                 
                                 + 
                                 
                                   y 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               + 
               
                 
                   ∑ 
                   
                     n 
                     = 
                     0 
                   
                   
                     6 
                     ⁢ 
                     6 
                   
                 
                 ⁢ 
                 
                   
                     C 
                     j 
                   
                   ⁢ 
                   
                     x 
                     m 
                   
                   ⁢ 
                   
                     y 
                     m 
                   
                 
               
             
           
         
       
       
         
           
             j 
             = 
             
               
                 
                   [ 
                   
                     
                       
                         ( 
                         
                           m 
                           + 
                           n 
                         
                         ) 
                       
                       2 
                     
                     + 
                     m 
                     + 
                     
                       3 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       n 
                     
                   
                   ] 
                 
                 / 
                 2 
               
               + 
               1 
             
           
         
       
     
     wherein c is the vertex curvature (CUY), k is the conic constant, and C j  is the polynomial coefficient. 
     For the first example embodiment of the disclosure, from the environmental light to the exit pupil, there are a first auxiliary lens  20 , a primary prism  10 , and a second auxiliary lens  30  in sequence. All of the three optical elements are freeform optical elements and may be fabricated by high-precision optical resin molded. The first auxiliary lens  20  is glued with the primary prism  10  and there is a 0.5 mm air gap between the second auxiliary lens  30  and the primary prism  10 . The whole freeform prism-lens group  1  has a total thickness of not more than 15 mm, and a field-of-view of the freeform prism-lens group  1  is larger than 50°. The first optical surface  201  and the first surface  301  have an identical shape with the third optical surface  103  and the second optical surface  102  of the primary prism  10  respectively, and the second optical surface  202  and the second surface  302  are planar surfaces. The description of the optical surface  201 , 202 , 203  is omitted in the surfaces of the first embodiment of the disclosure in table  1 . Accordingly, for the light provided by the MD used in the first embodiment of the disclosure, the image distortion of the freeform prism-lens group  1  on the EPD is shown in  FIG. 3 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Surface 
                 Radius of curvature (mm) 
               
               
                   
                   
               
             
            
               
                   
                 302, 202 
                 Planar (∞) 
               
               
                   
                 102, 301 
                 −130.534623433003 
               
               
                   
                 103, 201 
                 −44.9621213993141 
               
               
                   
                 101 
                 −18.4176668625078 
               
               
                   
                 MD 
                 Planar (∞) 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 surface 
                 surface 
               
               
                   
                 103 
                 101 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 X 
                 0 
                 0 
               
               
                 Y 
                 0 
                 0 
               
               
                 X 2   
                 −0.00536 
                 0.058254 
               
               
                 XY 
                 0 
                 0 
               
               
                 Y 2   
                   6.24E−05 
                   8.23E−04 
               
               
                 X 3   
                 0 
                 0 
               
               
                 X 2 Y 
                 4.49E−05 
                   9.97E−04 
               
               
                 XY 2   
                 0 
                 0 
               
               
                 Y 3   
                 −0.00025 
                 0.000159 
               
               
                 X 4   
                 −5.02E−06 
                 −3.03E−04 
               
               
                 X 3 Y 
                 0 
                 0 
               
               
                 X 2 Y 2   
                 −1.20E−06 
                   6.18E−04 
               
               
                 XY 3   
                 0 
                 0 
               
               
                 Y 4   
                   2.42E−05 
                 −1.57E−04 
               
               
                 X 5   
                 0 
                 0 
               
               
                 X 4 Y 
                 −1.03E−06 
                 −8.79E−05 
               
               
                 X 3 Y 2   
                 0 
                 0 
               
               
                 X 2 Y 3   
                   3.24E−07 
                   4.40E−05 
               
               
                 XY 4   
                 0 
                 0 
               
               
                 Y 5   
                 −1.10E−06 
                   1.61E−04 
               
               
                 X 6   
                 −2.21E−08 
                   1.78E−05 
               
               
                 X 5 Y 
                 0 
                 0 
               
               
                 X 4 Y 2   
                 −1.66E−07 
                   3.66E−05 
               
               
                 X 3 Y 3   
                 0 
                 0 
               
               
                 X 2 Y 4   
                 −8.13E−08 
                   6.15E−05 
               
               
                 XY 5   
                 0 
                 0 
               
               
                 Y 6   
                   7.61E−08 
                 −7.74E−06 
               
               
                 X 7   
                 0 
                 0 
               
               
                 X 6 Y 
                 −7.00E−09 
                   1.21E−06 
               
               
                 X 5 Y 2   
                 0 
                 0 
               
               
                 X 4 Y 3   
                   3.73E−08 
                   1.08E−06 
               
               
                 X 3 Y 4   
                 0 
                 0 
               
               
                 X 2 Y 5   
                 −4.08E−08 
                 −4.94E−06 
               
               
                 XY 6   
                 0 
                 0 
               
               
                 Y 7   
                   5.84E−09 
                 −8.97E−06 
               
               
                 X 8   
                   4.19E−10 
                 −9.55E−08 
               
               
                 X 7 Y 
                 0 
                 0 
               
               
                 X 6 Y 2   
                   2.83E−09 
                 −1.63E−06 
               
               
                 X 5 Y 3   
                 0 
                 0 
               
               
                 X 4 Y 4   
                 −2.53E−09 
                 −1.86E−06 
               
               
                 X 3 Y 5   
                 0 
                 0 
               
               
                 X 2 Y 6   
                   6.28E−09 
                 −1.09E−06 
               
               
                 XY 7   
                 0 
                 0 
               
               
                 Y 8   
                 −1.88E−09 
                   1.03E−06 
               
               
                 X 9   
                 0 
                 0 
               
               
                 X 8 Y 
                   7.30E−11 
                 −8.44E−08 
               
               
                 X 7 Y 2   
                 0 
                 0 
               
               
                 X 6 Y 3   
                 −2.85E−10 
                   1.09E−07 
               
               
                 X 5 Y 4   
                 0 
                 0 
               
               
                 X 4 Y 5   
                   2.19E−10 
                 −4.56E−09 
               
               
                 X 3 Y 6   
                 0 
                 0 
               
               
                 X 2 Y 7   
                 −3.38E−10 
                   3.83E−07 
               
               
                 XY 8   
                 0 
                 0 
               
               
                 Y 9   
                   5.04E−11 
                   1.50E−07 
               
               
                 X 10   
                 −2.57E−12 
                 −1.82E−10 
               
               
                 X 9 Y 
                 0 
                 0 
               
               
                 X 8 Y 2   
                 −9.01E−12 
                   4.05E−08 
               
               
                 X 7 Y 3   
                 0 
                 0 
               
               
                 X 6 Y 4   
                   1.95E−12 
                   2.25E−10 
               
               
                 X 5 Y 5   
                 0 
                 0 
               
               
                 X 4 Y 6   
                   1.03E−11 
                   6.35E−08 
               
               
                 X 3 Y 7   
                 0 
                 0 
               
               
                 X 2 Y 8   
                 −1.04E−11 
                 −4.96E−08 
               
               
                 XY 9   
                 0 
                 0 
               
               
                 Y 10   
                   6.81E−12 
                 −2.42E−08 
               
               
                   
               
            
           
         
       
     
     In the first embodiment, the freeform prism-lens group  1  does not provide diopter correction of the transmitted environmental light, since the second surface  302  and the second optical surface  202  are both planar surfaces. Users with myopia or hyperopia may need to wear vision correction lens  40  for vision correction when using the freeform prism-lens group  1 , as shown in  FIG. 4A . The above-mentioned vision correction lens  40  that is nearer to the exit pupil may be placed on one side of the second auxiliary lens  30 , and the side is the opposite side of the second auxiliary lens  30  to the primary prism  10 . The vision correction lens  40  may be firmly fixed in front of the eye by means of extra mechanism, e.g., frames and arms of the frames (not shown in  FIG. 4A ). Or, the vision correction lens  40  (usually two pieces are needed for two eyes) may be made in the form of polarized clips and mounted on the assembled freeform prism-lens group  1  by snap closure of both sides of the clips. 
     If one surface of the vision correction lens  40  is planar, the vision correction lens  40  may be adhered to the second surface  302  of the second auxiliary lens  30 . The adhering method may increase the fixation, but decrease the degree of flexible and also introduces complexity in assembly. A person skilled in this art would know that, in the embodiment of the disclosure, the vision correction lens  40  may be fabricated all-in-one-shape with the second auxiliary lens  30 , as shown in  FIG. 4B . In this way, the second auxiliary lens  30  with diopter compensation ability is removable couple to the primary prism  10  with a predetermined air gap, and the second surface  302  of the second auxiliary lens  30  has a different shape with the second optical surface  202  of the first auxiliary lens  20 , so that the second auxiliary lens  30  with additional vision correction function may fit users with different vision ability. 
     Because the second optical surface  202  and the second surface  302  of the freeform prism-lens group  1  in the embodiment have smooth surfaces, the freeform prism-lens group  1  has a good size extensibility. For example, the primary prism  10  needs to effectively magnify the image on the MD without obvious distortion, which calls for curved optical surfaces  102  and  103 , and the curved optical surfaces  102  and  103  preventing the freeform prism-lens group  1  from being extended to desired range (For example, limited by the size of MD, the freeform prism-lens group  1  may not be easily extended to the size of a normal eyeglass with diopter compensation ability, e.g., 70 mm in diameter), because the extension for the edge area according to curvature will result in a distorted image of the MD and a negative effect on both the surface shapes of first and second auxiliary lenses  20  and  30  and the space in front of and behind the primary prism  10  that can be utilized to arrange for the auxiliary lenses  20  or  30 . If the freeform prism-lens group  1  has a smooth front surface  302  and a smooth back surface  202  as in the first embodiment, as shown in  FIG. 4C  that horizontal image along the optical axis, the primary prism  10  may have a portion of above-mentioned freeform surface (i.e. freeform shape area) on the central area of the second optical surface  102  and the third optical surface  103  having the beam splitter film. For edge portion outside the freeform surface portion, the shape of the surface can be extended easily. Symmetrically, for example, the primary prism  10  may have an extended edge portion that is non-freeform surface, e.g., spherical, aspheric or planar surface. The connection area between the freeform portion and the non-freeform surface is optimally a smooth transition area (i.e., smooth transition area). Accordingly, surfaces  201  and  301  of auxiliary lenses  20  and  30  may have identical shapes with surfaces  102  and  103  of the primary prism  10  respectively in order to obtain smooth surfaces  202  and  302  of the freeform prism-lens group  1 . Rays from environmental light will not change direction after passing through the freeform prism-lens group  1 , so the user may observe an undistorted environment (equivalent to wearing a plain glass with certain thickness). The freeform prism-lens group  1  may have a good size extensibility, e.g., freeform prism-lens group  1  can be extended to a desired size but not limited by the size of the micro-display MD. The freeform prism-lens group  1  can be conveniently fabricated to the shape users wanted after the size extended, e.g., similar to conventional vision correction lens. 
       FIG. 5  illustrates a structure according to some other embodiments, wherein the freeform prism-lens group  1 A includes at least a freeform primary prism  10 A having at least one freeform surface, and a first freeform auxiliary lens  20 A having at least one freeform surface. Either the freeform primary prism  10 A or the first freeform auxiliary lens  20 A has at least two optical surfaces and other auxiliary surfaces to form a valid prism or lens shape. The freeform primary prism has at least three valid optical surfaces, namely  101 A,  102 A, and  103 A. And by refraction and/or reflection effect of surfaces  101 A to  103 A, the displayed image is enlarged. The first freeform auxiliary lens  20 A has a first optical surface  201 A that is deposed proximal to the third optical surface  103 A of the primary prism  10 A and has a same shape with the third optical surface  103 A. Different from the first embodiment, the first freeform auxiliary lens  20 A has a second optical surface  202 A distal from the primary prism, which is aspheric, or smooth spherical with a radius of curvature greater than 100 mm. 
     Similar to the first embodiment, the freeform prism-lens group  1 A also includes a second auxiliary lens  30 A which is placed on the other side of the freeform primary prism  10 A, i.e., the first freeform auxiliary lens  20 A and the second auxiliary lens  30 A are on different sides of the freeform primary prism  10 A. The freeform prism-lens group  1 A may be used to further correct aberrations and improve the imaging quality of the environmental light rays. The first optical surface  201 A of the first freeform auxiliary lens  20 A may have an identical shape with the third optical surface  103 A of the primary prism  10 A and may be cemented to the third optical surface  103 A. The first surface  301 A of the second auxiliary lens  30 A may have an identical freeform shape with the second freeform surface  102 A of the primary prism  10 A and there may be a predetermined thin air gap between the first surface  301 A and the second optical surface  102 A. The second auxiliary lens  30 A can be easily removable coupled to the freeform primary prism  10 A. 
     Since the second optical surface  202 A is spherical or aspheric, for the modified embodiment, the shape of the second surface  302 A of the second auxiliary lens  30 A is optimally not limited. The second surface  302 A of the second auxiliary lens  30 A which can match different vision users may be manufactured further. For users with normal visual ability, the second surface  302 A may have an identical shape with the second optical surface  202 A. And for user with certain amount of myopia or hyperopia and/or astigmatism, the above-mentioned amount of diopter compensation is realized by the difference in the shapes of the second surface  302 A and the second optical surface  202 A. In addition, the second auxiliary lens  30 A can be replaced if severe abrasion occurs or the visual ability of the user changes, for the second auxiliary lens  30 A is removable. Hence, the cost of the freeform prism-lens group  1 A for different visual ability users is reduced and the service life of the freeform prism-lens group  1 A is prolonged. 
     Similar to the first embodiment, the freeform surfaces in the modified embodiment may be described by SPS XYP. As shown in table 3 (the surface parameters of the modified embodiment for users with normal visual ability), for the modified embodiment, the air gap between the second auxiliary lens  30 A and the freeform primary prism  10 A is 0.4 mm. The maximum thickness of the freeform prism-lens group  1 A is not greater than 15 mm when measured from the second optical surface  202 A of the first freeform auxiliary lens  20 A to the second surface  302 A of the second auxiliary lens  30 A. The field-of-view of the modified embodiment is larger than 50°. For the light from MD, the distortion image of the freeform prism-lens group  1  on EPD modified is illustrated in  FIG. 6 . 
     Similar to the first embodiment, the dimension of the freeform prism-lens group  1 A can be scaled, if only the front surface  302 A and the back surface  202 A are spherical surfaces with a curvature of greater than 100 mm or aspheric surfaces. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Surface 
                 Radius of curvature (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 302A, 202A 
                 −500, −300, −250, −150, or −80 
               
               
                   
                 102A, 301A 
                 −110.0294863 
               
               
                   
                 103A, 201A 
                 −27.5533493 
               
               
                   
                 101A 
                 22.17088364 
               
               
                   
                 MD 
                 Planar (∞) 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 surface 
               
               
                   
                   
                 103A 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 X 
                 0 
                 0 
               
               
                 Y 
                 0 
                 0 
               
               
                 X 2   
                 0.007597 
                 −0.01624 
               
               
                 XY 
                 0 
                 0 
               
               
                 Y 2   
                 0.008138 
                 0.218469 
               
               
                 X 3   
                 0 
                 0 
               
               
                 X 2 Y 
                   5.67E−05 
                   0.00457 
               
               
                 XY 2   
                 0 
                 0 
               
               
                 Y 3   
                   1.77E−05 
                   0.030221 
               
               
                 X 4   
                   5.74E−06 
                 −0.00063 
               
               
                 X 3 Y 
                 0 
                 0 
               
               
                 X 2 Y 2   
                   1.74E−05 
                 −0.00142 
               
               
                 XY 3   
                 0 
                 0 
               
               
                 Y 4   
                 −1.69E−06 
                 0.001379 
               
               
                 X 5   
                 0 
                 0 
               
               
                 X 4 Y 
                   3.92E−08 
                 −0.00033 
               
               
                 X 3 Y 2   
                 0 
                 0 
               
               
                 X 2 Y 3   
                   9.83E−07 
                 −0.00043 
               
               
                 XY 4   
                 0 
                 0 
               
               
                 Y 5   
                   1.76E−07 
                 −1.09E−05 
               
               
                 X 6   
                 −1.66E−08 
                   4.19E−06 
               
               
                 X 5 Y 
                 0 
                 0 
               
               
                 X 4 Y 2   
                 −3.60E−07 
                 −5.99E−05 
               
               
                 X 3 Y 3   
                 0 
                 0 
               
               
                 X 2 Y 4   
                 −4.00E−07 
                 −3.21E−05 
               
               
                 XY 5   
                 0 
                 0 
               
               
                 Y 6   
                   2.81E−07 
                 −5.79E−07 
               
               
                 X 7   
                 0 
                 0 
               
               
                 X 6 Y 
                 −4.87E−09 
                   2.09E−06 
               
               
                 X 5 Y 2   
                 0 
                 0 
               
               
                 X 4 Y 3   
                 −1.60E−08 
                 −5.22E−06 
               
               
                 X 3 Y 4   
                 0 
                 0 
               
               
                 X 2 Y 5   
                 −1.46E−08 
                   7.02E−07 
               
               
                 XY 6   
                 0 
                 0 
               
               
                 Y 7   
                 −4.47E−09 
                   3.18E−07 
               
               
                 X 8   
                   5.60E−10 
                 −1.09E−08 
               
               
                 X 7 Y 
                 0 
                 0 
               
               
                 X 6 Y 2   
                   3.50E−09 
                 2.77E−07 
               
               
                 X 5 Y 3   
                 0 
                 0 
               
               
                 X 4 Y 4   
                   1.25E−08 
                 −2.40E−07 
               
               
                 X 3 Y 5   
                 0 
                 0 
               
               
                 X 2 Y 6   
                   2.26E−09 
                   1.90E−07 
               
               
                 XY 7   
                 0 
                 0 
               
               
                 Y 8   
                 −6.03E−09 
                   7.84E−09 
               
               
                 X 9   
                 0 
                 0 
               
               
                 X 8 Y 
                   3.09E−11 
                 −5.33E−09 
               
               
                 X 7 Y 2   
                 0 
                 0 
               
               
                 X 6 Y 3   
                   8.49E−11 
                   1.77E−08 
               
               
                 X 5 Y 4   
                 0 
                 0 
               
               
                 X 4 Y 5   
                   1.53E−10 
                 −6.81E−09 
               
               
                 X 3 Y 6   
                 0 
                 0 
               
               
                 X 2 Y 7   
                   7.69E−11 
                   7.04E−09 
               
               
                 XY 8   
                 0 
                 0 
               
               
                 Y 9   
                   2.85E−11 
                 −1.10E−09 
               
               
                 X 10   
                 −3.36E−12 
                 −1.56E−11 
               
               
                 X 9 Y 
                 0 
                 0 
               
               
                 X 8 Y 2   
                 −9.91E−12 
                 −2.88E−10 
               
               
                 X 7 Y 3   
                 0 
                 0 
               
               
                 X 6 Y 4   
                 −7.39E−11 
                   5.08E−10 
               
               
                 X 5 Y 5   
                 0 
                 0 
               
               
                 X 4 Y 6   
                 −7.65E−11 
                 −1.49E−10 
               
               
                 X 3 Y 7   
                 0 
                 0 
               
               
                 X 2 Y 8   
                   1.03E−11 
                   4.54E−11 
               
               
                 XY 9   
                 0 
                 0 
               
               
                 Y 10   
                   3.95E−11 
                 −4.27E−11 
               
               
                   
               
            
           
         
       
     
     In another aspect of the disclosure, the freeform prism-lens group  1 / 1 A in the first embodiment and the modified embodiment can be utilized as the optical system of an NED apparatus.  FIG. 7  illustrates a typical application of optical see-through NED apparatus, e.g., head-mounted display (HMD) wherein the freeform prism-lens group  1 / 1 A and the micro-display MD in the first embodiment or the other embodiments may be included. The freeform prism-lens group  1 / 1 A is placed in front of one eye, and usually a pair of freeform prism-lens groups  1 / 1 A are utilized for two eyes. Due to the effect of beamsplitter in the freeform prism-lens group  1 / 1 A, light rays from the real-world scene (i.e., the environmental light rays), e.g., ray  114 , and the rays from the micro-display MD, e.g., ray  116 , will be synthesized at the position of the eye, so the image  132  is observed by the eye. In image  132 , part of the real scene  120 , e.g. the grove, and the image  104  for augmented reality from the micro-display MD, e.g. birds that are not in real existence, can be observed. In the field of entertainment, images that are not in close relation to the real scene may be augmented. For example, birds may be replaced by dolphins, so that a fantasy scene where dolphins are leaping among the grove can be observed. In the field of advertisement, the augmented images of particular objects or goods may be displayed to the user, e.g., a bottle of soft drink on table. The NED apparatus in the disclosure may have applications in many other fields. 
     In general, a user may wear HMD anywhere, like indoor or outdoor. 
     Various information can be collected to determine the type of the augmented image to be displayed and the position of the augmented image to be displayed. For example, the position of the user, the visual direction of the user, and the position of floor, wall or maybe furniture (in the indoor situation) may be used to determine where the augmented image is to be placed in a proper place of the environmental light. The location of the user&#39;s head may be obtained by pose tracking and inertial units which may be mounted on the head of the user (e.g. on the NED in this disclosure), and then the visual direction of the user is determined. One or more depth cameras may be used by pose tracking to get the three-dimensional model of the user. Similarly, one or more depth cameras may be used to get the position of floor, wall, or other objects in the environment. Various sensors and controllers within the range of existent technology may be utilized to get all the data needed in the NED in this disclosure. The above-mentioned controller(s) may be in the range of general data processors and controllers, e.g., central processing units (CPUs) or other microprocessors. The above-mentioned controllers(s) may be placed near the sensors to facilitate signal transmission or data processing, or the above-mentioned controllers may be placed somewhere not so close to the freeform prism-lens group and the sensors and are connected to the sensors by data cables. 
     The micro-display MD can only add but not decrease the rays coming from the environmental light. Therefore, limited by the display principal and the brightness of the micro-display MD, dark colors may not be well displayed. Particularly, black pixel of the images cannot be augmented in the micro-display viewing path. When dark images are to be augmented in the environment, the user may still feel the translucent environment or feel the ghosted augmented images. In the circumstance of strong augmented reality or mixed reality, rays from the real scene are expected to be partially removed in order to display augmented images of any colors or brightness with a more vivid effect. To achieve this target, as simple way in the existent technology, the HMD may further include an optical filter to decrease the amount of the environmental light rays that comes into the HMD to improve the contrast of the augmented images. For example, the widely-used films with photochromic property may be added to reduce rays when the environmental light rays are too strong and to protect the eyesight. However, removal of rays from the whole environment is not accurate enough for more general applications, and it is expected to selectively remove the environmental light rays from particular field angles. 
     Because the surface  202 / 202 A of the auxiliary lens  20 / 20 A that is close to the environment has a planar or smooth shape, it is possible to add a controllable optical filter  50  which is attached on the surface  202 / 202 A. The controllable optical filter  50  may be in the form of a transmitting LCD panel, electrochromic film, or other controllable optical filter devices. The above-mentioned transmitting LCD panel may be obtained by removing substrates, backlights, and scattering layers from traditional LCDs. The above-mentioned LCD panel may include one or more transmitting LCD chips, wherein LCD chips control the amount of rays passing through. For example, this kind of LCD chips are used in projectors. The controllable optical filter  50  may include grids of pixels, wherein the transmittance of each pixel can be controlled individually between the minimum transmittance and the maximum transmittance. Ideally, the minimum transmittance is zero and the maximum transmittance is 100%, but a limited range between these two values is acceptable. For example, a monochromatic LCD panel with not more than two polarized filters may have a transmittance range of 50%-20% or 10% for each pixel, and the maximum resolution is same as the LCD panel. At the minimum of 50%, the controllable optical filter  50  may seem slightly colored, and this is acceptable. It&#39;s ideal that the highest transmittance for the above-mentioned LCD panel is set to be 100%. In practice, a parameter, alpha, which is between 0-100% may be determined, where 0 is the lowest transmittance achievable (i.e., the opaquest) and 100% corresponds to the highest transmittance achievable (i.e. the most transparent). Each pixel may be assigned an alpha value by the optical filter transmittance control circuit. 
     In the scene rendering for augmented objects, it should be noted which part of the real scene is in front of the augmented objects. If the augmented object is front of the real scene, the pixels on the corresponding area on the controllable optical filter  50  will have zero or low alpha values, wherein the corresponding real scene is obscured to avoid undesired rays that disturbs the augmented objects. If the augmented object is to be behind the real scene, the alpha values of the pixels on the controllable optical filter  50  are set to be 100% or high to make rays of the real scene enter the NED system, and pixels on the corresponding area of the micro-display MD are shut off. Since the corresponding area may be pixel-based, the situation that part of the augmented object is front of the real scene, part of the augmented object is to be behind the real scene and part of the augmented object is overlap with real scene can be handled. To improve the stability of the augmented image, the alignment of the extra pixels of the transmittance of controllable optical filter and the augmented image is maintained. The correspondence of the pixels may refer to the condition where pixels on the controllable optical filter  50  match the pixels on the micro-display MD. In more detail, each pixel on the controllable optical filter  50  attached on surface  202 / 202 A may have an identical-sized with the pixel on the micro-display MD, and the controllable optical filter  50  and the micro-display MD are driven simultaneously. 
     The controllable optical filter  50  may be controlled by environmental light ray sensors placed on the enclosure of the NED. When the environmental light rays are too strong, the transmittance of the controllable optical filter  50  may decrease independently from the augmented images displayed by the micro-display MD. The optical filter transmittance control circuit may be the CPU or other microprocessors of the NED, or may be an extra control circuit that can communicate with the CPU of the NED. 
     Moreover, the transmittance of the controllable optical filter  50  may react to the feedback of other sensors, e.g., eye-tracker which can be fulfilled by a camera placed inside of the enclosure of the NED to track the position of the eye relative to the enclosure of the NED. In general, eye-tracking involves the capture of eye images and computer vision technology that determines the position of the pupil relative to the eye socket. Another eye tracking method may use one or more photo detectors and a LED array. By a known position of the tracing camera, the position of the eye relative to any other stable object (e.g., the controllable optical filter  50  or the freeform prism-lens group  1 / 1 A) on the enclosure may be obtained. In most cases, it is enough to track one of the two eyes, since two eyes move consistently. But it is possible to track each eye separately and determine the position where the augmented image is to be displayed according to the position of each eye. 
     The AR image controller may also react to the feedback of the environmental light ray sensor and/or the eye tracker, and enhance the control to the augment image. For example, the brightness of image displayed on the micro-display MD may be adjusted according to the above-mentioned feedback. 
     The controllable optical filter  50  is in its maximum transmittance condition for the environmental light ray see-through path when no augmented image is displayed. When the controllable optical filter  50  is not driven simultaneously with the micro-display MD, e.g., the controllable optical filter  50  is kept to block all rays from the environmental light, all the rays that enters the eye comes from the micro-display MD viewing path. Thus, the NED in this disclosure works as a HMD for virtual reality. That means an optical see-through HMD. 
     According to the freeform prism-lens group and the NED apparatus thereof in this disclosure, the auxiliary lenses need not to be specially designed when the design of the primary prism is finished, and the design difficulties in the design of surfaces are thus decreased. The front and back optical surfaces of the prism-lens group are planar or close to planar, which facilitates the protective measures aimed to improve the durability and attachment of additional elements without the increase of maximum thickness of the freeform prism-lens group. In the NED apparatus, controllable optical filter may achieve pixel-level control of transmittance, which may effectively decrease the influence of the environmental light rays on the augmented images, and thus a more vivid and accurate synthesized image can be observed by the eye. 
     These and other advantages of embodiments of the present disclosure will be apparent to those skilled in the art from the foregoing specification. Accordingly, it will be recognized by skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the disclosure.