Patent Publication Number: US-2022236559-A1

Title: Stereoscopic Head-Up Display with Symmetrical Optical Paths

Description:
BACKGROUND 
     Technical Field 
     The present disclosure is directed to a stereoscopic head-up display with symmetrical optical paths maintaining equal optical paths length of a left eye image and a right eye image in a longer virtual image projection distance or in a greater image magnification, the images on a reflective diffuser being clear, projecting a clear stereoscopic image on both eyes of an observer. 
     Related Art 
     The optical path of an automotive head-up display is usually deployed with a concave mirror  61  to magnify an image on a screen as illustrated in  FIG. 1A , especially configured on a Stereoscopic and Augmented-Reality HUD. In order to have the image fit more closely to reality and reduce vergence-accommodation conflict, a longer virtual image distance (VID), at least 7.5 meters to 20 meters, is required, hence a concave mirror  62  with a greater magnification required as illustrated in  FIG. 1B . 
     Please refer to  FIG. 2 , cited references disclose a single projection module  1  provided with lens time-divisionally projecting a parallax image light of a left eye and a parallax image light of a right eye, a polarizing modulator  2  time-divisionally modulating the two image lights to a polarized parallax image light of the left eye and a polarized parallax image light of the right eye, the left eye parallax polarized image light and the right eye parallax polarized image light are orthogonal in polarizing direction, a beam splitter (a reflective polarizer  3 ) separates the two image lights by reflection and transmission. The reflected image light is projected to a reflective diffuser  5 , the transmitted image light reflected by a reflector  40  behind the beam splitter (the reflective polarizer  3 ) and passing through the beam splitter (the reflective polarizer  3 ) once again, then projecting to the reflective diffuser  5 . The reflective diffuser  5  reflecting and diffusing the polarized parallax image lights of both eyes to the concave mirror  6  to magnify the image displayed and lengthen the virtual image distance. 
     Please refer to  FIG. 3 , the polarized parallax image lights of both eyes are reflected individually to the left eye E 1  and the right eye E 2  of an observer by a windshield  7 , each of both eyes seeing different parallax angles images, thereby forming a stereoscopic image in the brain of the observer. 
     Please refer to  FIG. 4A , the beam splitter (the reflective polarizer  3 ) and the reflector  40  are configured for folding optical paths, accordingly its equivalent projection structure can be regarded as two projection modules  100  on both sides (left and right) respectively as shown in  FIG. 4B , to perform a projection toward the reflective diffuser  5  with a included angle A between the center axles  101  of the two projection modules  100 . 
     Please refer to  FIG. 5A , the embodiment is suitable for a small projection angle difference, that is, the two equivalent projection modules on both sides (left and right) project to the reflective diffuser  5  with a smaller included angle between the center axle  101  of the two projection modules  100 , for example, the included angle A1=5°. The projection modules project the image light to the reflective diffuser  5 , the reflective diffuser  5  reflecting and diffusing the image light to the concave mirror  61 , the concave mirror  61  reflecting the image light to the left eye E 1  or the right eye E 2  of the observer as shown in  FIG. 5B . In some embodiments, a longer virtual image distance (VID) is required, the concave mirror  61  with a greater magnification is configured, that is, the radius of curvature is smaller. In this situation, the two equivalent projection modules on both sides (left and right) project to the reflective diffuser  5  at the same included angle, but the concave mirror  62  will not reflect the image light to the left eye E 1  or the right eye E 2  as shown in  FIG. 5C . 
     Please refer to  FIG. 6A , for a longer virtual image distance (VID) embodiments, the included angle of the two equivalent projection modules projecting to the reflective diffuser  5  shall be enlarged to let concave mirror  62  reflect and focus image light to the eyes of the observer, for example, the included angle A2=10°. As illustrated in  FIG. 6B , the two projection modules on both sides (left and right) with larger included angle project the image light to the reflective diffuser  5 , the reflective diffuser  5  reflecting and diffusing the image light to the concave mirror  62  with a greater magnification, the concave mirror  62  reflecting the image light to the left eye E 1  or the right eye E 2  of the observer. 
     Please refer to  FIG. 7 , the projection device, for example, a DLP or LCD projection device, comprises an image lens  10  provided with a focal distance, an image light L 0  passing through the image lens  10 , then projecting to a screen or a reflective diffuser  5  on the focal distance to form a clear image. 
     Please refer to  FIG. 8A , cited references disclose the image lens  10  of the projection module projecting the image light separated by the beam splitter (the reflective polarizer  3 ) to form two different optical paths projecting to the reflective diffuser  5 . The two optical paths are different in length, only one of the optical paths length being equal to the focal distance of the image lens  10 . When the two equivalent projection modules of both sides (left and right) project to the reflective diffuser  5  at a small included angle, for example, the included angle is 5°, the optical path length difference is rather small. As illustrated in  FIG. 8B , the optical path length of the polarized image light L 11  is equal to the focal distance of the image lens  10 , thus the polarized image light L 11  forms a clear image on the reflective diffuser  5 . As illustrated in  FIG. 8C , the optical path length of the polarized image light L 12  is slightly shorter than the focal distance of the image lens  10 , thus the polarized image light L 12  being focused not far behind the reflective diffuser  5 , therefore the image being slightly vague. In some embodiments, the optical path length of the polarized image light L 12  is equal to the focal distance of the image lens  10 , thus the polarized image light L 12  forms a clear image on the reflective diffuser  5 , and the polarized image light L 11  being focused not far behind the reflective diffuser  5 , forming a slightly vague image. 
     Please refer to  FIG. 9A , in some embodiments, a lager included angle is required to project to the reflective diffuser  5 , for example, the included angle is larger than 10°, the optical path length passing through the beam splitter (the reflective polarizer  3 ) twice is obviously longer than the optical path length reflected by the beam splitter (the reflective polarizer  3 ), the two optical paths length difference is relatively greater. As illustrated in  FIG. 9B , the optical path length of the polarized image light L 11  is equal to the focal distance of the image lens  10 , thus the polarized image light L 11  forms a clear image on the reflective diffuser  5 . As illustrated in  FIG. 9C , the optical path length of the polarized image light L 12  is shorter than the focal distance of the image lens  10 , the polarized image light L 12  being focused further behind the reflective diffuser  5 , unable to form a clear image on the reflective diffuser  5 , the image being rather blurred and difficult to be identified. In some embodiments, the optical path length of the polarized image light L 12  is equal to the focal distance of the image lens  10 , the polarized image light L 12  forms a clear image on the reflective diffuser  5 , the polarized image light L 11  being focused further behind the reflective diffuser  5 , unable to form a clear image on the reflective diffuser  5 , the image being rather blurred and difficult to be identified. 
     Related technology could be referred to cited references JPH10186522A, TW578011, TW announcement number 396280, CN108919495, TW publication number 200916828, TW publication number 201019031, TW publication number 201214014, TW I349114, TW I359284, TW announcement number 342101, TW M478830, TW I626475, TW M434219 disclose an optical path of display for stereoscopic image. 
     SUMMARY 
     The present disclosure is directed to a stereoscopic head-up display with symmetrical optical paths. The stereoscopic head-up display with symmetrical optical paths comprises the following. 
     A projection module has an image lens time-divisionally projecting a first image light and a second image light alternately. 
     A polarizing modulator modulates the first image light to a first polarized image light and modulates the second image light to a second polarized image light. The first polarized image light and the second polarized image light are orthogonal in polarizing direction. 
     A polarization beam splitter has a beam splitter surface reflecting the first polarized image light and allowing the second polarized image light to pass through. 
     A reflector module has two reflectors configured symmetrically at opposite ends of the beam splitter surface reflecting the first polarized image light and the second polarized image light individually. 
     A reflective diffuser has a plurality of micro-curved mirrors arranged in an array. Optical paths are symmetrical between the polarization beam splitter where the first polarized image light and the second polarized image light are separated and the reflective diffuser where the first polarized image light and the second polarized image light are projected to. The first polarized image light and the second polarized image light inject to the reflective diffuser at different angles. The plurality of the micro-curved mirrors reflect and diffuse the first polarized image light to a receiving area of a first eye. The plurality of the micro-curved mirrors reflect and diffuse the second polarized image light to a receiving area of a second eye. 
     The stereoscopic head-up display with symmetrical optical paths further comprises a windshield and a concave mirror. The concave mirror is configured between the reflective diffuser and the windshield. The reflective diffuser reflects and diffuses the first polarized image light and the second polarized image light to the concave mirror. The concave mirror reflects the first polarized image light and the second polarized image light to the windshield. The windshield reflects the first polarized image light and the second polarized image light individually to the receiving area of the first eye and the receiving area of the second eye. 
     The stereoscopic head-up display with symmetrical optical paths further comprises a shutter module provided with two shutters configured between the reflector module and the polarization beam splitter. Each shutter is individually placed between the symmetrical reflectors and the polarizing beam splitter. The shutters open and close at opposite timing which synchronizes with the projection module time-divisionally projecting the first image light and the second image light. 
     In some embodiments, the polarization beam splitter is a reflective polarizing film. 
     In some embodiments, the polarization beam splitter is a polarizing beam splitter cube. 
     In some embodiments, the stereoscopic head-up display with symmetrical optical paths comprises the following. 
     A projection module having an image lens time-divisionally projecting a first image light and a second image light. 
     A semi-reflective beam splitter is a semi-reflector provided with a semi-reflective surface partially reflecting a first image light and a second image light and allowing the first image light and the second image light to partially pass through. 
     A reflector module has two reflectors configured symmetrically at opposite ends of the semi-reflective surface reflecting the first image light and the second image light individually. 
     A shutter module has two shutters configured between the reflector module and the semi-reflector. Each shutter is individually placed between the symmetrical reflectors and the semi-reflective beam splitter. The shutters open and close at opposite timing which synchronizes with the projection module time-divisionally projecting the first image light and the second image light. When one of the image lights is projected, one of the shutters opens to project the image light to one of the reflectors. The other shutter closes to block and absorb the image light for preventing the image light from reaching the other reflector. 
     A reflective diffuser has a plurality of micro-curved mirrors arranged in an array. Optical paths are symmetrical between the semi-reflective beam splitter where the first image light and the second image light are separated and the reflective diffuser where the first image light and the second image light are projected to. The first image light and the second image light inject to the reflective diffuser at different angles. The plurality of the micro-curved mirrors reflect and diffuse the first image light to a receiving area of a first eye. The plurality of the micro-curved mirrors reflect and diffuse the second image light to a receiving area of a second eye. 
     In some embodiments, the stereoscopic head-up display with symmetrical optical paths further comprises a windshield and a concave mirror. The concave mirror is configured between the reflective diffuser and the windshield. The reflective diffuser reflects and diffuses the first image light and the second image light to the concave mirror. The concave mirror reflects the first image light and the second image light to the windshield. The windshield reflects the first image light and the second image light individually to the receiving area of the first eye and the receiving area of the second eye. 
     In some embodiments, the stereoscopic head-up display with symmetrical optical paths comprises the following. 
     A projection module has an image lens time-divisionally projecting a first image light and a second image light. 
     A reflective rotatable beam splitter is a rotatable shutter provided with a rotatable beam splitter surface defining a reflection area and a transmission area. The reflection area and the transmission area are alternative rotating to the projection path of the projection module, allowing the reflection area to reflect the first image light or allowing the second image light to pass through the transmission area. 
     A reflector module are two reflectors configured symmetrically at opposite ends of the rotatable beam splitter surface reflecting the first image light and the second image light individually. 
     A reflective diffuser has a plurality of micro-curved mirrors arranged in an array. Optical paths are symmetrical between the reflective rotatable beam splitter where the first image light and the second image light are separated and the reflective diffuser where the first image light and the second image light are projected to. The first image light and the second image light inject to the reflective diffuser at different angles. The plurality of the micro-curved mirrors reflect and diffuse the first image light to a receiving area of a first eye. The plurality of the micro-curved mirrors reflect and diffuse the second image light to a receiving area of a second eye. 
     In some embodiments, the stereoscopic head-up display with symmetrical optical paths further comprises a windshield and a concave mirror. The concave mirror is configured between the reflective diffuser and the windshield. The reflective diffuser reflects and diffuses the first image light and the second image light to the concave mirror. The concave mirror reflects the first image light and the second image light to the windshield. The windshield reflects the first image light and the second image light individually to the receiving area of the first eye and the receiving area of the second eye. 
     In some embodiments, the rotatable shutter is a disc shutter rotating around the center of the disc. The rotating speed of the disc synchronizes with the projection module time-divisionally projecting the first image light and the second image light. When the projection module projects the first image light, the disc rotates to the reflection area. The first image light is reflected by the reflection area. When the projection module projects the second image light, the disc rotates to the transmission area. The second image light passes through the transmission area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  and  FIG. 1B  are schematic diagrams of a conventional automotive head-up display; 
         FIG. 2  is a schematic diagram of a conventional projection device projecting a stereoscopic image; 
         FIG. 3  is a schematic diagram of a conventional automotive head-up display projecting a stereoscopic image; 
         FIG. 4A  and  FIG. 4B  are schematic diagrams of a conventional projection device projecting a stereoscopic image in equivalent optical path; 
         FIG. 5A ,  FIG. 5B  and  FIG. 5C  are schematic diagrams of a conventional projection device projecting a stereoscopic image at different angles; 
         FIG. 6A  and  FIG. 6B  are schematic diagrams of a conventional projection device projecting a stereoscopic image at different angles; 
         FIG. 7  is a schematic diagram of a conventional projection device showing an image lens focusing; 
         FIG. 8A ,  FIG. 8B  and  FIG. 8C  are schematic diagrams of a conventional projection device projecting a stereoscopic image with small included angles;  FIG. 8B  is a schematic diagram illustrating the light (twice) passing through a beam splitter forming an optical path;  FIG. 8C  is a schematic diagram illustrating the light reflected by a beam splitter forming an optical path; 
         FIG. 9A ,  FIG. 9B  and  FIG. 9C  are schematic diagrams of a conventional projection device projecting a stereoscopic image with large included angles;  FIG. 9B  is a schematic diagram illustrating the light (twice) passing through a beam splitter forming an optical path;  FIG. 9C  is a schematic diagram illustrating the light reflected by a beam splitter forming an optical path; 
         FIG. 10A ,  FIG. 10B  and  FIG. 10C  are schematic diagrams illustrating a symmetrical optical paths of a stereoscopic image projection of the first embodiment of the instant disclosure; 
         FIG. 11A ,  FIG. 11B  and  FIG. 11C  are schematic diagrams illustrating a symmetrical optical paths of a stereoscopic image projection with small projection included angles of the first embodiment of the instant disclosure; 
         FIG. 12A ,  FIG. 12B  and  FIG. 12C  are schematic diagrams illustrating a symmetrical optical paths of a stereoscopic image projection with large projection included angles of the first embodiment of the instant disclosure; 
         FIG. 13A  and  FIG. 13B  are three-dimensional schematic diagrams illustrating a symmetrical optical paths beam separation and projection of a stereoscopic image projection in an automobile of the first embodiment of the instant disclosure; 
         FIG. 14A  and  FIG. 14B  are schematic diagrams illustrating a light leak of a symmetrical optical paths beam separation of a stereoscopic image projection of the first embodiment of the instant disclosure; 
         FIG. 15A ,  FIG. 15B  and  FIG. 15C  are schematic diagrams illustrating a symmetrical optical paths beam separation and projection with shutters of a stereoscopic image projection of the first embodiment of the instant disclosure; 
         FIG. 16A  and  FIG. 16B  are three-dimensional schematic diagrams illustrating a symmetrical optical paths beam separation and projection with shutters of a stereoscopic image projection of the first embodiment of the instant disclosure; 
         FIG. 17A  and  FIG. 17B  are schematic diagrams illustrating a symmetrical optical paths beam separation and projection with shutters of a stereoscopic image projection of the second embodiment of the instant disclosure; 
         FIG. 18  is a three-dimensional schematic diagram illustrating a symmetrical optical paths beam separation and projection with shutters of a stereoscopic image projection of the second embodiment of the instant disclosure; 
         FIG. 19  is a three-dimensional schematic diagram illustrating a symmetrical optical paths beam separation and projection with shutters of a stereoscopic image projection in an automobile of the second embodiment of the instant disclosure; 
         FIG. 20A ,  FIG. 20B  and  FIG. 20C  are schematic diagrams illustrating a symmetrical optical paths beam separation of a stereoscopic image projection of the third embodiment of the instant disclosure; 
         FIG. 21A  and  FIG. 21B  are schematic diagrams illustrating a symmetrical optical paths beam separation and projection of a stereoscopic image projection of the third embodiment of the instant disclosure; 
         FIG. 22A  and  FIG. 22B  are another schematic diagrams illustrating a symmetrical optical paths beam separation and projection of a stereoscopic image projection of the third embodiment of the instant disclosure; 
         FIG. 23  is a three-dimensional schematic diagram illustrating a symmetrical optical paths beam separation and projection of a stereoscopic image projection in an automobile of the third embodiment of the instant disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 10A - FIG. 16B , the instant disclosure provides an embodiment of a stereoscopic head-up display with symmetrical optical paths comprising the following. 
     As illustrated in  FIG. 10A , a projection module  1  has an image lens  10  time-divisionally projecting a first image light D 1  and a second image light D 2 . The first image light D 1  and the second image light D 2  are provided with images at different parallax angles. 
     A polarizing modulator  2  modulates the first image light D 1  to a first polarized image light L 1  and modulates the second image light D 2  to a second polarized image light L 2 . The first polarized image light L 1  and the second polarized image light L 2  are orthogonal in polarizing direction. 
     A polarization beam splitter  3  has a beam splitter surface  31  reflecting the first polarized image light L 1  and allowing the second polarized image light L 2  to pass through. 
     A reflector module  4  has two reflectors  41 ,  42  configured individually at opposite ends of the beam splitter  31  in a symmetrical manner. As illustrated in  FIG. 10A , the reflectors  41 ,  42  are configured individually at an upper portion and a lower portion of the beam splitter  31 . The reflector  41  reflects the first polarized image light L 1  reflected by the polarization beam splitter  3 . The reflector  42  reflects the second polarized image light L 2  passed through the polarization beam splitter  3 . 
     A reflective diffuser  5  has a plurality of micro-curved mirrors arranged in an array. Optical paths are symmetrical between the polarization beam splitter  3  where the first polarized image light L 1  and the second polarized image light L 2  are separated and the reflective diffuser  5  where the first polarized image light L 1  and the second polarized image light L 2  are projected to. The first polarized image light L 1  and the second polarized image light L 2  inject to the reflective diffuser  5  at different angles. As illustrated in  FIG. 10B , the plurality of the micro-curved mirrors of the reflective diffuser  5  reflect and diffuse the first polarized image light L 1  to a first area R 1 , the first area R 1  extending to a receiving area of one eye. The plurality of the micro-curved mirrors of the reflective diffuser  5  reflect and diffuse the second polarized image light L 2  to a second area R 2 , the second area R 2  extending to a receiving area of the other eye. The polarization beam splitter is a reflective polarizing film (as shown in  FIG. 10A ) or a polarizing beam splitter cube (as shown in  FIG. 10C ). 
     Please refer to  FIG. 11A , the separated first polarized image light L 1  and the separated second polarized image light L 2  project individually to two different optical paths LP 1 , LP 2  passing by the symmetrical arranged reflectors  41 ,  42  and reflected to the reflective diffuser  5 . When the two optical paths LP 1  and LP 2  reach the reflective diffuser  5  at a smaller angle difference, for example, an included angle between LP 1  and LP 2  is 5°, the two optical paths LP 1 , LP 2  being equal in length and symmetrical, the optical paths length from the two polarized image lights L 1 , L 2  to the reflective diffuser  5  are equal to a focal distance of the image lens  10 , thereby the two polarized image lights L 1 , L 2  forming a clear image on the reflective diffuser  5  as shown in  FIG. 11B  and  FIG. 11C . 
     Please refer to  FIG. 12A , when the two optical paths LP 1  and LP 2  reach the reflective diffuser  5  at a greater angle difference, for example, an included angle between LP 1  and LP 2  is greater than 10°, due to the symmetrical optical paths, no optical path length difference is occurred. The optical paths length from the two polarized image lights L 1 , L 2  to the reflective diffuser  5  are equal to the focal distance of the image lens  10 , thereby the two polarized image lights L 1 , L 2  forming clear images on the reflective diffuser  5  as shown in  FIG. 12B  and  FIG. 12C . 
     Please refer to  FIG. 13A , the instant disclosure provides an embodiment of a stereoscopic head-up display with symmetrical optical paths comprising a windshield  7  and a concave mirror  6 . The concave mirror  6  is configured between the reflective diffuser  5  and the windshield  7 . The reflective diffuser  5  reflects and diffuses the first polarized image light L 1  and the second polarized image light L 2  to the concave mirror  6 . The concave mirror  6  reflects the first polarized image light L 1  and the second polarized image light L 2  to the windshield  7 . The windshield  7  reflects the first polarized image light L 1  and the second polarized image light L 2  individually to the receiving area of the first eye E 1  and the receiving area of the second eye E 2 . The polarization beam splitter  3  is a reflective polarizing film or a polarizing beam splitter cube. The first polarized image light L 1  and the second polarized image light L 2  are separated by the polarization beam splitter  3 , reflected to the reflective diffuser  5  by the symmetrical two reflectors  41 ,  42  individually. The reflective diffuser  5  reflects and diffuses the first polarized image light L 1  and the second polarized image light L 2  to the concave mirror  6 , magnifying an image displayed and lengthening a virtual image distance. The concave mirror  6  reflects the first polarized image light L 1  and the second polarized image light L 2  to the windshield  7 . The windshield  7  reflects the first polarized image light L 1  and the second polarized image light L 2  individually to the receiving area of the first eye E 1  and the receiving area of the second eye E 2  as shown in  FIG. 13B , the left eye and the right eye individually seeing different parallax angles images, thereby forming a stereoscopic visual image in the brain of an observer. 
     The first polarized image light L 1  and the second polarized image light L 2  are orthogonal in polarizing direction. One of the polarized image lights is reflected on the polarization beam splitter  3  while the other polarized image light passes through the polarization beam splitter  3  to separate the image lights beams. In an ideal embodiment, the first polarization beam splitter  3  reflects the entire first polarized image light L 1  and allow the entire second polarized image light L 2  to pass through. On a practical occasion, when the light passes through the interface of two different media, different proportions of reflections and transmission may occur. As shown in  FIG. 14A , most of the first polarized image light L 1  is reflected, partial of a transmitting light L 10  entering the optical path of the second polarized image light L 2 . As shown in  FIG. 14B , most of the second polarized image light L 2  passes through, partial of a reflecting light L 20  entering the optical path of the first polarized image light L 1 . Neither of the above separate the beam completely. Thus, the left eye sees slightly a right eye&#39;s image, for example 1/40 luminance of the right eye&#39;s image, the right eye seeing slightly a left eye&#39;s image, for example, 1/40 luminance of the left eye&#39;s image. 
     To overcome the light leak, as shown in  FIG. 15A , the instant disclosure provides an embodiment of a stereoscopic head-up display with symmetrical optical paths comprising a shutter module  8  configured between the reflector module  4  and the polarization beam splitter  3 . The symmetrical two reflectors  41 ,  42  are individually provided with shutters  81 ,  82  in the front thereof. The two shutters  81 ,  82  open and close at opposite timing. The timing synchronizes with the projection module  1  time-divisionally projecting the first image light D 1  and the second image light D 2  to overcome the problem of incomplete beam separation. On an occasion of partial light leak, the shutters  81 ,  82  are capable of blocking the leaked light to separate the beam completely. As shown in  FIG. 15B , when the first image light D 1  is projected, the shutter  81  opens, the first polarized image light L 1  supposed to be reflected entirely on the polarization beam splitter  3 , though the partial transmitting light L 10  entering the optical path of the second polarized image light L 2 , the closing shutter  82  preventing the partial transmitting light L 10  from reaching the reflector  42  by blocking and absorbing the partial transmitting light L 10 . As shown in  FIG. 15C , when the second image light D 2  is projected, the shutter  82  opens, the second polarized image light L 2  supposed to be entirely passing through the polarization beam splitter  3 , though the partial reflecting light L 20  entering the optical path of the first polarized image light L 1 , the closing shutter  81  preventing the partial reflecting light L 20  from reaching the reflector  41  by blocking and absorbing the partial reflecting light L 20 . 
     The shutter module  8  is an electronic shutter or a mechanical shutter. As shown in  FIG. 16A , for example, the electronic shutter deploys electronic signals to control an opaque (closed) and a transparent (open) liquid crystal shutters  81 ,  82  of liquid crystal lens. As shown in  FIG. 16B , for example, the mechanical shutter deploys a partial area of a disc for blocking (closing), the other area for passing through (opening), the rotatable shutters  83 ,  84  rotating around the center of the disc. The shutter module  8  working at opposite timing which synchronizing with the projection module are provided between the symmetrical reflectors  41 ,  42  and the polarization beam splitter  3 . On an occasion that the polarization beam splitter  3  is unable to separate the beam completely, the leaked light being blocked, preventing the leaked light from entering the other optical path to achieve about 1/1000 effect, that is, one of the eyes sees only 1/1000 luminance of the other eye image, thereby substantially improving the stereoscopic visual quality. 
     Please refer to  FIG. 17A - FIG. 19 , the instant disclosure provides an embodiment of a stereoscopic head-up display with symmetrical optical paths comprising the following. 
     A projection module  1  has an image lens  10  time-divisionally projecting a first image light D 1  and a second image light D 2 . 
     A semi-reflective beam splitter is a semi-reflector  9  provided with a semi-reflector surface  91  partially reflecting the first image light D 1  and the second image light D 2 , and allowing the first image light D 1  and the second image light D 2  to partially pass through. 
     A reflector module  4  has two reflectors  41 ,  42  configured symmetrically at opposite ends of the semi-reflective surface  91  reflecting the first image light D 1  and the second image light D 2  individually. 
     A shutter module  8  has two shutters  81 ,  82  configured between the reflector module  4  and the semi-reflector  9 . The symmetrical reflectors  41 ,  42  are individually provided with shutters  81 ,  82  in the front thereof. When the first image light D 1  is projected, the shutter  81  opens, enabling the partial reflected first image light D 1  to inject to the reflector  41 , then the partial reflected first image light D 1  is projected by the reflector  41 . The other shutter  82  closes, making the partial transmitted first image light D 1  to be blocked and absorbed. When the second image light D 2  is projected, the shutter  82  opens, enabling the partial transmitted second image light D 2  to inject to the reflector  42 , then the partial transmitted second image light D 2  is projected by the reflector  42 . The other shutter  81  closes, making the partial reflected second image light D 2  to be blocked and absorbed. 
     A reflective diffuser  5  has a plurality of micro-curved mirrors arranged in an array. The optical paths are symmetrical between the semi-reflector  9  where the first image light D 1  and the second image light D 2  are separated and the reflective diffuser  5  where the first image light D 1  and the second image light D 2  are projected to. The first image light D 1  and the second image light D 2  inject to the reflective diffuser  5  at different angles. The plurality of the micro-curved mirrors reflect and diffuse the first image light D 1  to a receiving area of one of the eyes. The plurality of the micro-curved mirrors reflect and diffuse the second image light D 2  to a receiving area of the other eye. 
     Please refer to  FIG. 17A ,  FIG. 17B  and  FIG. 18 , the shutter module  8  is capable of dealing with the light leak, in the first embodiment of the instant disclosure, a combination of the polarizing modulator  2  and polarization beam splitter  3  is replaced with the semi-reflective and semi-transmissive (e.g., 50% reflection/50% transmission) semi-reflector  9 . The two shutters  81 ,  82  open and close at opposite timing. The timing synchronizes with the projection module  1  time-divisionally projecting the first image light D 1  and the second image light D 2  to achieve a similar effect of the first embodiment of the instant disclosure. Thus, the projection module  1  doesn&#39;t deploy with the polarizing modulator  2  in the front thereof, instead directly projecting the image light to the semi-reflector  9 . When the projection module  1  projects the first image light D 1 , the first image light D 1  reaches the two shutters  81 ,  82  simultaneously. The shutter  81  opens, enabling the first image light D 1  reflected by the semi-reflector  9  to inject to the reflector  41 , then the reflected first image light D 1  is reflected to the reflective diffuser  5 . The reflective diffuser  5  reflects and diffuses the first image light D 1  to the first area R 1 , the first area R 1  extending to a receiving area of one of the eyes. The other shutter  82  closes, the first image light D 1  passing through the semi-reflector  9  being blocked and absorbed. When the projection module  1  projects the second image light D 2 , the second image light D 2  reaches the two shutters  81 ,  82  simultaneously. The shutter  82  opens, the second image light D 2  passing through the semi-reflector  9  to inject to the reflector  42 , then the transmitting second image light D 2  is reflected to the reflective diffuser  5 . The reflective diffuser  5  reflects and diffuses the second image light D 2  to the second area R 2 , the second area R 2  extending to a receiving area of the other eye. The other shutter  81  closes, the second image light D 2  reflected by the semi-reflector  9  being blocked and absorbed. 
     Although nearly half of the light is blocked and absorbed by the closing shutter module  8  making the light utilization rate lower to less than 50%, the polarizing modulator in the first embodiment of the instant disclosure has a similar phenomenon. The polarizing modulator  2  has a light transmittance lower than 50%. A combination of the semi-reflector  9  and the shutter module  8  with a symmetrical optical paths design, removing the polarizing modulator  2  and the polarization beam splitter  3  from the combination, thereby reducing the cost and achieving a light-leak-proof effect. 
     Please refer to  FIG. 19 , the instant disclosure provides an embodiment of a stereoscopic head-up display with symmetrical optical paths further comprising a windshield  7  and a concave mirror  6 . The concave mirror  6  is configured between the reflective diffuser  5  and the windshield  7 . The reflective diffuser  5  reflects and diffuses the first image light D 1  and the second image light D 2  to the concave mirror  6 . The concave mirror  6  reflects the first image light D 1  and the second image light D 2  to the windshield  7 . The windshield  7  reflects the first image light D 1  and the second image light D 2  individually to the receiving area of the first eye E 1  and the receiving area of the second eye E 2 . 
     Please refer to  FIG. 20 - FIG. 23 , the instant disclosure provides an embodiment of a stereoscopic head-up display with symmetrical optical paths comprising the following. 
     A projection module  1  has an image lens  10  time-divisionally projecting a first image light D 1  and a second image light D 2 . 
     A reflective rotatable beam splitter is a rotatable shutter  85  provided with a rotatable beam splitter surface  850  centered on a shaft defining a reflection area  851  and a transmission area  852 . The reflection area  851  and the transmission area  852  are alternative rotating to the projection path of the projection module  1 , enabling the reflection area  851  to reflect the first image light D 1  and allowing the second image light D 2  to pass through the transmission area  852 . 
     A reflector module  4  has two reflectors  41 ,  42  configured symmetrically at opposite ends of the rotatable beam splitter surface  850  reflecting the first image light D 1  and the second image light D 2  individually. 
     A reflective diffuser  5  has a plurality of micro-curved mirrors arranged in an array. The optical paths are symmetrical between the rotatable shutter  85  where the first image light D 1  and the second image light D 2  are separated and the reflective diffuser  5  where the first image light D 1  and the second image light D 1  are projected to. The first image light D 1  and the second image light D 2  inject to the reflective diffuser  5  at different angles. The plurality of the micro-curved mirrors reflects and diffuses the first image light D 1  to a receiving area of a first eye E 1 . The plurality of the micro-curved mirrors reflect and diffuse the second image light D 2  to a receiving area of a second eye E 2 . 
     Please refer to  FIG. 20A , the rotatable shutter  85  is a disc shutter rotating around the center of the disc in clockwise or counterclockwise, the two radii from the center point of the disc defining one area as a reflection area  851  and the other area as a transmission area  852 . In the embodiment of the instant disclosure, the diameter from the center point of the disc defines two areas as the reflection area  851  and the transmission area  852 . The reflection area  851  is a reflective surface plated with silver or aluminum, or plated with coating to increase the light reflection. The transmission area  852  is made of a transparent material, such as glass, resin or crystal, or plated with coating to increase the light transmission. The rotatable shutter  85  is deployed to replace the combination of the polarizing modulator  2 , the polarization beam splitter  3  and the two shutters  81 ,  82 . The rotatable shutter  85  is configured at the position of the replaced polarization beam splitter  3 . The rotating speed of the rotatable shutter  85  synchronizes with the projection module  1  time-divisionally projecting the first image light D 1  and the second image light D 2 . As illustrated in  FIG. 20B , when the projection module  1  projects the first image light D 1 , the rotatable shutter  85  rotates the reflection area  851  to the projection optical path of the projection module  1 , the first image light D 1  being reflected by the reflection area  851 . As illustrated in  FIG. 20C , when the projection module  1  projects the second image light D 2 , the rotatable shutter  85  rotates the transmission area  852  to the projection optical path of the projection module  1 , the second image light D 2  passing through the transmission area  852 . 
     Please refer to  FIG. 21A  and  FIG. 21B , when the projection module  1  projects the first image light D 1 , the rotatable shutter  85  rotates the reflection area  851  to the projection optical path of the projection module  1 . The reflection area  851  reflects the first image light D 1  to the reflector  41 . The reflector  41  reflects the first image light D 1  to the reflective diffuser  5 . The reflective diffuser  5  reflects and diffuses the first image light D 1  to the first area R 1 , the first area R 1  extending to the receiving area of one of the eyes. 
     Please refer to  FIG. 22A  and  FIG. 22B , when the projection module  1  projects the second image light D 2 , the rotatable shutter  85  rotates the transmission area  852  to the projection optical path of the projection module  1 . The second image light D 2  passes through the transmission area  852  to the reflector  42 . The reflector  42  reflects the second image light D 2  to the reflective diffuser  5 . The reflective diffuser  5  reflects and diffuses the second image light D 2  to the second area R 2 , the second area R 2  extending to the receiving area of the other eye. 
     Please refer to  FIG. 23 , the instant disclosure provides an embodiment of a stereoscopic head-up display with symmetrical optical paths further comprising a windshield  7  and a concave mirror  6 . The concave mirror  6  is configured between the reflective diffuser  5  and the windshield  7 . The reflective diffuser  5  reflects and diffuses the first image light D 1  and the second image light D 2  to the concave mirror  6 . The concave mirror  6  reflects the first image light D 1  and the second image light D 2  to the windshield  7 . The windshield  7  reflects the first image light D 1  and the second image light D 2  individually to the receiving area of the first eye E 1  and the receiving area of the second eye E 2 . 
     The above three embodiments of the instant disclosure deploy a single projection module along with a beam splitter and a symmetrical optical paths to achieve a clear stereoscopic visual effect. In the first embodiment, the image light is emitted by the image lens of the projection module, passing by the polarizing modulator time-divisionally modulate two image lights to two polarized image lights, the two polarized image lights being orthogonal in polarizing direction, the polarization beam splitter separate the two polarized image lights by reflecting and transmitting and become the polarized image light of the left eye and the polarized image light of the right eye, the symmetrical optical paths structure projecting the polarized image light of the left eye and the polarized image light of the right eye to the reflective diffuser at different angles, reflecting and diffusing individually to the receiving area of the left eye and the receiving area of the right eye to achieve a clear stereoscopic visual effect. The first embodiment is further deployed with two shutters to solve the light leak caused by the polarization beam splitter. 
     Since the two shutters can solve light leak problem, the second embodiment deploys a semi-reflective beam splitter to replace the combination of the polarizing modulator and the polarization beam splitter along with the symmetrical optical paths structure to achieve a clear stereoscopic visual effect. 
     The third embodiment deploys a reflective rotatable beam splitter to replace the combination of the polarization beam splitter and two shutters along with the symmetrical optical paths structure to achieve a clear stereoscopic visual effect. 
     It is worth mentioning in the above three embodiments the optical paths are symmetrical between the beam splitter (the polarization beam splitter  3 , the semi-reflector  9 , the rotatable shutter  85 ) where the image lights (the polarized image lights L 1  and L 2 , the image light D 1  and D 2 ) are separated and the reflective diffuser  5  where the image lights reach after reflected by the reflectors  41 ,  42 , thereby maintaining the image optical paths length of both eyes equal, the image on the reflective diffuser  5  being clear, projecting a clear stereoscopic visual effect in a longer virtual image projection distance or in a greater image magnification.