Abstract:
Systems including projection mechanisms, screens, and eyeglasses are detailed. The systems significantly reduce perceptible ghosting even when high contrast images (such as dark figures against a white background) are projected.

Description:
FIELD OF THE INVENTION 
   This invention relates to stereoscopic displays in general and more particularly to stereoscopic motion picture projection. 
   BACKGROUND OF THE INVENTION 
   Stereoscopic 3-D imaging requires the presentation of two slightly different sets of images to a viewer; one set corresponds to a left eye viewpoint and the other corresponds to a right eye viewpoint. When the sets of images are presented so that only the left eye of a viewer can see the left eye set of images and the right eye can only see the right eye set of images, the viewer will be able to perceive a 3-D image. 
   Several different methods of separating left and right eye images are known. In the anaglyph method, different colour filters are used. Typically, the left eye and right eye images are projected simultaneously but in different colours, say red and blue respectively, and the viewer wears a pair of glasses fitted with red and blue filters arranged to appropriately separate the images. A major disadvantage of this method is that the resulting 3-D images are deficient in colour information. 
   Another method of image separation involves the use of mutually extinguishing polarizing filters. The filters are placed in front of left and right eye projectors with their polarizing axes at 90 degrees to each other. Viewers wear eyeglasses with polarizing filters arranged in the same orientation as the filters on the projectors. The left and right eye images appear on the screen at the same time, but only the left eye polarized light is transmitted through the left eye lens of the eyeglasses and only the right eye polarized light is transmitted through the right eye lens. This method is inexpensive and allows full colour 3-D images. However, it has limitations in that a substantial amount of unwanted transmission can occur and can result in the formation of objectionable ghost images. For instance, the polarization characteristics of the light can be significantly altered by reflection from a screen, though metallic screen coatings will mitigate this effect. If linear polarizers (which are most effective) are used, ghost images will also increase as the viewer tilts his or her head to the left or right. 
   A third known method involves time multiplexing of left and right eye images. Left and right eye images are presented alternately so that there is only one eye image on the screen at any one moment in time. Viewers wear glasses which alternately block the view of one eye so that only the correct image will be seen by each eye. In other words when a left eye image is projected onto a screen the left eye lens of the glasses will be transparent and the right eye lens will be opaque. When the image on the screen changes to a right eye image, the left lens of the glasses becomes opaque and the right eye lens becomes transparent. The glasses typically have electro-optic liquid crystal shutters and are powered by batteries. This method largely overcomes the problems of unwanted transmission due to head tilt and does not require a special screen to maintain polarization. 
   The liquid crystal shutters that are used in time-multiplexing stereoscopic imaging are usually extinguishing shutters made of at least two linear polarizers on either side of a liquid crystal cell which contains a thin layer of liquid crystal material between two sheets of glass. The two polarizers are oriented with their axes generally orthogonal and the liquid crystal material acts as a variable polarizer influenced by an electric field. Such shutters block a significant proportion of the light when in an opaque state but they have limited transmission when they are in the transparent state, typically about 25–30% of incident light. Liquid crystal shutters have also been found to exhibit poor extinction when used to view high contrast scenes such as dark figures against a white background. Also, poor extinction is noticeable in the corner areas of “wide” screens such as those used by Imax Corporation. 
   When assessing the quality of 3-D motion picture images two figures of merit are used, namely maximum transmission and extinction ratio. Maximum transmission is the percentage of light generated by the projectors which actually reaches the eyes of a viewer. The extinction ratio is defined as a ratio of the brightness of a correct or wanted image to the brightness of an incorrect or unwanted image that leaks through the system. In a 3-D motion picture projection system, the extinction ratio gives an indication of how much ghosting a viewer will perceive. 
   It is an object of the invention to provide an improved method of stereoscopic image separation in which ghosting is reduced or eliminated. 
   SUMMARY OF THE INVENTION 
   According to the invention there is provided a method of presenting stereoscopic images comprising the steps of:
         alternately displaying corresponding left-eye and right-eye images in succession;   alternately and in synchronism with said alternate display of images, blocking the viewer&#39;s right eye when said left-eye images are displayed, and blocking the viewer&#39;s left eye when said right-eye images are displayed, using respective electro-optic liquid crystal shutters, each including a front linear polarizing filter having a first axis of polarization and a rear linear polarizing filter having a second axis of polarization at an angle with respect to said first axis;   wherein the respective liquid crystal shutters are oriented so that the said first axes of polarization of the respective front linear polarizing filters are at an angle with respect to one another;   and wherein said images are displayed by projecting the images onto a screen, and linearly polarizing the projected light so that the left-eye images are polarized along an axis that is parallel to said first axis of the electro-optic shutter for the viewer&#39;s left eye and the right-eye images are polarized along an axis parallel to the first axis of the electro-optic shutter for the viewer&#39;s right eye.       

   It should be noted that the term “parallel” is to be interpreted broadly in the preceding paragraph and in the claims. Thus, while exact parallelism may represent an ideal condition, acceptable results may be achieved with a deviation of a few degrees. 
   The invention seeks to improve the quality of presentation of stereoscopic images and reduce or eliminate “ghosting”. By offsetting the axes of polarization of the front polarizers of the respective liquid crystal shutters of “alternate eye” 3-D glasses, and alternately displaying left and right eye images which are polarized to “match”, so-called “cross talk” interference between the images (and resulting ghosting) is minimized. Practical limitations of currently available electro-optic shutters to mutually extinguish unwanted images inevitably results in some “leakage” of unwanted image information. The present invention seeks to eliminate that unwanted image by the use of matched polarizers as described previously. It has been found possible to dramatically improve the extinction ratio of the system while retaining high levels of maximum light transmission and acceptable background contrast. 
   It should be noted that the corresponding left and right eye images may overlap in time. This improves the level of maximum light transmission but at the expense of some ghosting. Thus, references herein to “alternate” display of images does not indicate that the images must be presented separately (as is the case with prior art time-multiplexing systems). 
   In a practical example of the invention as applied to a motion picture projection system, linear polarizer filters are placed in front of the projection lenses of a stereoscopic motion picture projector with the polarizing axes of the projector polarizers aligned so that they are parallel to the axes of the linear polarizers on the front of each liquid crystal eyeglass lens. For example, the left liquid crystal eyeglass shutter has a first linear polarizer oriented with the polarizing axis at 45° clockwise with respect to the vertical. The linear polarizer placed in front of the left eye lens of the stereoscopic motion picture projector has an identical orientation; at 45° clockwise from the vertical. Similarly, the right liquid crystal eyeglass shutter has a first linear polarizer oriented with the polarizing axis at 45° counterclockwise with respect to the vertical, and the linear polarizer placed in front of the right eye lens of the stereoscopic motion picture projector is oriented 45° counterclockwise from the vertical. 
   The above arrangement significantly reduces perceptible ghosting at the cost of a slight reduction in overall brightness. The loss of brightness is due to the extra linear polarizer in the optical path and is approximately 10%. Usually a loss of brightness of this magnitude is too large to contemplate, especially in a large format wide screen 3-D motion picture theatre where achieving bright pictures is typically difficult. 
   The invention also provides corresponding apparatus for presenting stereoscopic images, and eyeglasses for use in the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood with reference to the drawings which illustrate a particular preferred embodiment of the invention, as compared with the prior art. 
     In the drawings: 
       FIG. 1  is a schematic illustration of a prior art “alternate eye” 3-D motion picture projection system; 
       FIG. 2  is a view similar to  FIG. 1  illustrating the method and apparatus of the invention; and, 
       FIG. 3  is a graph illustrating temporal multiplexing of the left eye and right eye images in accordance with the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring first to  FIG. 1 , a motion picture projection screen is indicated at  20  and a pair of motion picture projectors for projecting respective series of images onto screen  20  are diagrammatically represented at  22  and  24  respectively. Two projectors have been shown although it is of course to be understood that a single stereoscopic motion picture projector can be used. An example of such a projector is disclosed in U.S. Pat. No. 4,966,454 (Toporkiewicz), the disclosure of which is incorporated herein by reference. In any event, as shown in  FIG. 1 , two projectors are used and alternately project respective “left eye” and “right eye” images onto screen  20  through respective projection lenses  22   a  and  24   a.    
   A pair of “alternate eye” 3-D glasses such as would be worn by a viewer of the images projected onto screen  20  is represented at  26  and has respective left and right lenses  28  and  30  in the form of liquid crystal shutters. The shutters are triggered alternately in synchronism with the projection of images onto screen  20  so that the right lens  30  is opaque (and the viewer&#39;s right eye blocked) when left eye images appear on the screen and, conversely, the left eye lens is opaque and the viewer&#39;s left eye is blocked when right eye images appear on the screen. Shutters of the type are well-known in the art and are disclosed for example in U.S. Pat. No. 4,424,529 (Roese, et al.), the disclosure of which is incorporated herein by reference. The lenses  28  and  30  will be described in more detail later in connection with  FIG. 2 . For present purposes, it is sufficient to note that, while shutters of this type are reasonably efficient at blocking light, some leakage of light can occur and can result in unacceptable ghosting, particularly when the glasses are used to view high contrast scenes such as dark figures against a white background. Also, poor extinction is noticeable in the corner areas of “wide” screens such as those used by Imax Corporation. 
   As seen in  FIG. 1 , a left eye image is being projected onto screen  20  from projector  22 . The left lens  28  of the eyeglasses  26  is in its transmissive state while the right lens  30  is opaque. The image  32  on screen  20  is clearly visible through the left lens  28  of the eyeglasses. However, a ghost image  32   a  leaks through the opaque right lens  30  of the eyeglasses, providing an objectionable perception to the viewer. The converse situation of course arises when right eye images are projected and the left lens of the eyeglasses is opaque; i.e. objectionable “ghosts” of the right eye image leak through the opaque left lens  28 . 
     FIG. 2  shows the same components as in  FIG. 1 , except that linear polarizing filters  34  and  36  have been placed in front of the respective projection lenses of projectors  22  and  24 . Also in  FIG. 2 , the two lenses  28  and  30  of the eyeglasses  26  have been shown in more detail. 
   Referring to lens  28  by way of example, the lens includes a front polarizing filter  38  having an axis of polarization indicated at  40 , and a rear polarizing filter  42  having an axis of polarization  44  at an angle (e.g. 90°) with respect to the axis  40  of the front polarizing filter. Similarly, lens  30  has a front polarizing filter  46  with an axis of polarization  48  and a rear polarizing filter  50  with an axis of polarization  52  at an angle to axis  48 . Located between the two polarizers in each lens is a cell comprising a thin layer of liquid crystal material between two sheets of glass. The two cells are indicated at  54  and  56  respectively. As is well known in the art, the liquid crystal material acts as a variable polarizer influenced by an electric field. Thus, in the transmissive state, the liquid crystal material in effect “twists” the light as it travels between the front and rear polarizers, so that the light is transmitted through the lens. In the “off” state, this twisting effect does not occur and light is not transmitted since the axes of polarization of the two polarizers are not in line. 
   In accordance with the invention, the front linear polarizing filters  38  and  40  of the respective eyeglass lenses are deliberately arranged with their axes of polarization ( 40  and  48  respectively) at an angle with respect to one another, preferably 90° (orthogonal). 
   The two polarizing lenses  34  and  36  that are placed in front of the lenses of the respective projectors  22  and  24  are “matched” to the front polarizing filters  38  and  40  of the respective left and right lenses of the eyeglasses. In other words, the filter  34  that is front of the projector  22  (the left eye image projector) is arranged with its axis of polarization (denoted  58 ) parallel to the axis of polarization  40  of the front polarizer  38  of the left eyeglass lens  28 . Similarly, the filter  36  that is placed in front of the right eye image projector  24  is arranged with its axis of polarization ( 60 ) parallel to the axis of polarization  48  of the front polarizer  46  of the right eye lens  30 . At the instant shown in  FIG. 2 , a left eye image is being projected onto screen  20  and is polarized, say, 45° clockwise from the vertical as indicated by axis  58  of filter  34 . In contrast with the situation in  FIG. 1  in which this image light is not polarized, there can be no leakage through the right eye lens  30  of the eyeglasses  26 . In the embodiment of  FIG. 2 , any of this left eye image light that strikes the right lens  30  will first encounter the front polarizer  46  which is orthogonally polarized (at 45° counterclockwise from the vertical) so that there will be no leakage of left eye image light into the right eye lens. The converse situation will of course obtain when right eye images are projected and the left eyeglass lens  28  is in the opaque state. 
   This arrangement significantly reduces perceptible ghosting at the cost of a slight reduction in overall brightness. The loss of brightness is due to the extra linear polarizer in the optical path as compared with the embodiment of  FIG. 1  and will typically amount to about 10%. Usually, a loss of brightness of this magnitude is too large to contemplate, especially in a large format wide screen 3-D motion picture theatre where achieving bright pictures typically is difficult. However, it has been found in practice that this loss of brightness is acceptable and does not represent a practical obstacle. 
   For the sake of clarification,  FIG. 3  illustrates the alternate projection of left and right eye images of the inventive method. Left and right eye images are alternately displayed and the glasses are oppositely triggered with the same temporal frequency. The left and right eye images are alternately displayed in a repeating on/off cycle in which the “on” and “off” portions of the cycle are of equal length (a “50/50” duty cycle), so that there are never left and right eye images on the screen at the same time (although this is not essential). When a left image is projected, the left lens of a pair of 3-D eyeglasses is transparent (time period T), whereas the right eye lens is opaque (time period O). Likewise, when a right eye image is projected the left lens is opaque. 
   Alternate projection of left and right eye images can be achieved, for example, by projecting the images from two separate filmstrips using two projectors that are synchronized with one another. Alternatively, a single rolling loop projector capable of so-called “alternate image” projection from two separate filmstrips can be used. In either case, provision must be made for the images to be differently polarized. 
   The electro-optic shutters incorporated in the eyeglasses worn by the viewer must be activated in synchronism with projection of the images. This can be accomplished in a variety of ways, for example by suitable electrical circuitry for triggering the shutters in synchronization with the projector or projectors. U.S. Pat. No. 5,002,387 (Baljet et al.) discloses a projection synchronization system in which infrared signals are used to synchronize prior art blocking shutters in a time multiplexing stereoscopic system. The disclosure of this patent is incorporated herein by reference. 
   The following discussion will further illustrate the advantages of the invention, as compared with the prior art: 
   Figures of merit for the inventive method can be calculated for comparison by including the effects of adding aligned polarizers to the projection lenses. The table below illustrates the advantages of the invention. The first column contains the three image quality figures of merit for the prior art method of 3-D motion picture projection using linear polarizers in front of the projection lenses and in eyeglasses worn by members of the audience. The second column contains the two figures of merit for the inventive 3-D method. The extinction ratio of the inventive shutters is increased dramatically (over 10,000%). The maximum transmission when using the inventive method is only marginally decreased. Overall the quality of a 3-D presentation is greatly improved when using the inventive method. 
   
     
       
             
           
             
             
             
           
             
             
             
             
           
         
             
                 
             
             
               Figure of Merit Table 
             
           
        
         
             
                 
               LC Shutter 
               Invention 
             
             
                 
                 
             
           
        
         
             
                 
               Transmission 
               30% 
               30 × .9 = 27% 
             
             
                 
               Extinction Ratio 
               150:1 
               15,000:1 
             
             
                 
               (on axis) 
             
             
                 
               Extinction Ratio 
                10:1 
                1,000:1 
             
             
                 
               (off axis) 
             
             
                 
                 
             
           
        
       
     
   
   The invention addresses several limitations and disadvantages of prior art systems. It provides a 3-D image separation method that has a high extinction ratio especially in scenes of high contrast and is not susceptible to ghosting caused by head tilting. 
   The above description should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the preferred embodiments of this invention. For example although polarizing filters are described, other optically extinguishing filters such as colour or wavelength band pass filters could be used.