Abstract:
An exemplary embodiment is a system and method for displaying three-dimensional imagery. The system includes an image source, a first projector having light emission, a second projector having a light emission and a twisted nematic liquid crystal rotator disposed in the light emission of the first projector. The first projector and second projector are connected to the image source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional patent application No. 60/179,909 filed Feb. 3, 2000, the entire contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    The invention relates generally to a system and method for displaying three-dimensional imagery with a dual projector three-dimensional stereoscopic projection system, and more specifically, to a dual projector three-dimensional stereoscopic projection system using a twisted nematic liquid crystal rotator or a ½ wave retarder.  
           [0003]    Stereoscopic projection systems enable multiple people to view three-dimensional (3D) images at the same time. Typically stereoscopic projection systems employ either one or two single projection units in the display of 3D stereoscopic images. There are advantages and disadvantages with both single and dual projection unit methods of stereoscopic projection. The invention described herein addresses problems associated with dual unit stereoscopic projection systems. A major problem associated with dual unit systems involves the 3D “cross talk” or “ghosting” effect seen in projected images. This problem typically stems from the method used to encode the left and right perspective images required for stereoscopic imaging.  
           [0004]    There are several types of dual projection systems that use either internal or external methods to alter the polarization of light exiting one or both of the two projectors to produce a projected image with stereoscopic depth information. An internal method is one in which modifications are made inside the projection unit to alter the polarization characteristics of its light output. Likewise an external method is on in which modifications are made outside the projection unit. In either case, a dual unit stereoscopic projection system displays a 3D image by encoding the left perspective image with polarization state P 1  and the right perspective image with an orthogonal polarization state P 2 . Both left and right perspective images are displayed in the same location on a view screen that preserves polarized light. The observer perceives depth from such images by wearing polarized glasses that decode the stereoscopic image on the screen. The left lens of the glasses consists of a polarizing filter that passes P 1  polarized light. Similarly, the right lens of the glasses consists of a polarizing filter that passes P 2  polarized light. Using this configuration the left eye should see only the left perspective image and the right eye should see only the right perspective image.  
           [0005]    In order to realize a dual unit stereoscopic projection system, typically one or both projection units must be modified in such a way that the light output is polarized and that the two polarization states are orthogonal. In addition, the polarization states must match those of the polarizing filters used in the decoder glasses. The most common method employs linearly polarized 3D glasses in a “V” configuration where the left lens consists of a linear polarizing filter with polarization axis oriented −45° from vertical. Similarly the right lens of the decoder glasses consists of a second linear polarizing filter with polarization axis oriented +45° from vertical (or vice versa).  
           [0006]    For a large panel amorphous silicon (a-Si) thin film transistor liquid crystal display (TFT LCD) projector the light output is already polarized with a 45° angle. Typically, one projector is left unmodified and the second projector light output is modified in one of two ways. The first method involves placing a ½-wave retarder (optical phase shift material) in the output path of one of the projectors. The ½-wave retarder is oriented in such a way that the polarization angle of the output light is rotated by 90°. The retarder material may be located either internal or external to the projector casing. The second method involves modifying the LCD panel inside of one projector so that the polarization angle of the output is orthogonal to the unmodified projector. The modification consists of physically reorienting the polarizer and analyzer of the LCD by 90°.  
           [0007]    For projectors in which the light output is not polarized, such as CRT (cathode ray tube) based projectors and DLP (digital light process) projectors, both projectors must be modified. In these cases linear polarizers are placed in the output path of both projectors such that the polarization angles of the two projectors are orthogonal and correspond to the decoding glasses.  
           [0008]    Dual projector three-dimensional (3D) stereoscopic projection systems that utilize a ½-wave retarder to modify the output polarization angle have an inherent “ghosting” or image cross talk problem due to the spectral characteristics of the ½-wave retarder material itself. Many retarder materials have “blue leakage” in which light in the blue wavelengths is not completely rotated by 90°. The result is that one eye will see a faint blue ghost image from the opposite perspective image. Note that different retarder materials may exhibit “leakage” at other wavelengths. If it becomes too prominent, this ghosting effect can degrade or destroy the stereoscopic effect. Further, dual projector 3D stereoscopic projection systems that utilize two cathode ray tube (CRT) or two digital light processor (DLP) projectors (or any other projector with non-polarized light output) suffer a reduction in brightness of at least 50% due to the linear polarizer placed in the light path of each projector.  
           [0009]    Thus there is a great need for an improved method and system for generating stereoscopic images of 3D objects, while avoiding the shortcomings of prior art apparatus and methodologies.  
         SUMMARY OF THE INVENTION  
         [0010]    An exemplary embodiment is a system and method for displaying three-dimensional imagery. The system includes an image source, a first projector having light emission, a second projector having a light emission and a twisted nematic liquid crystal rotator disposed in the light emission of the first projector. The first projector and second projector are connected to the image source. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Referring now to the drawings wherein like elements are numbered alike in several FIGURES:  
         [0012]    [0012]FIG. 1 illustrates a prior art dual projector 3D stereoscopic system using two amorphous silicon thin film transistor liquid crystal display projectors and a ½-wave retarder;  
         [0013]    [0013]FIG. 2 illustrates another prior art dual projector 3D stereoscopic system using two amorphous silicon thin film transistor liquid crystal display projectors;  
         [0014]    [0014]FIG. 3 illustrates a prior art dual projector 3D stereoscopic system that utilizes two projectors with non-polarized light output and with the left and right perspective images altered by placing linearly polarizing filters in each projector&#39;s output light path;  
         [0015]    [0015]FIG. 4 illustrates the spectral characteristics of a typical sample of ½-wave retarder;  
         [0016]    [0016]FIG. 5 illustrates a percent transmittance plot for a TN rotator sandwiched between two linearly polarizing filters aligned in parallel;  
         [0017]    [0017]FIG. 6 illustrates a functional diagram of a TN rotator;  
         [0018]    [0018]FIG. 7 illustrates a representation of the cross-section of a typical TN rotator;  
         [0019]    [0019]FIG. 8 illustrates a dual projector 3D stereoscopic system using two amorphous silicon thin film transistor liquid crystal display projectors and an external TN rotator;  
         [0020]    [0020]FIG. 9 illustrates a dual projector 3D stereoscopic system using two amorphous silicon thin film transistor liquid crystal display projectors, an external TN rotator and two external polarizers;  
         [0021]    [0021]FIG. 10 illustrates a dual projector 3D stereoscopic system using dual polysilicon projectors and two ½-wave retarders;  
         [0022]    [0022]FIG. 11 illustrates a signal input diagram depicting green channel interchanging used for stereoscopic image encoding in the dual projector 3D stereoscopic systems of FIGS. 10 and 12;  
         [0023]    [0023]FIG. 12 illustrates a dual projector 3D stereoscopic system using dual polysilicon projectors and two external TN rotators; and  
         [0024]    [0024]FIG. 13 illustrates another dual projector 3D stereoscopic system using dual polysilicon projectors and two external TN rotators.  
     
    
     DETAILED DESCRIPTION  
       [0025]    As discussed, dual projector three-dimensional (3D) stereoscopic projection systems that utilize a ½-wave retarder to modify the output polarization angle have an inherent “ghosting” or image cross talk problem due to the spectral characteristics of the ½-wave retarder material itself. Further, as previously mentioned, dual projector 3D stereoscopic projection systems that utilize two cathode ray tube (CRT) or two digital light processor (DLP) projectors (or any other projector with non-polarized light output) suffer a reduction in brightness of at least 50% due to the linear polarizer placed in the light path of each projector.  
         [0026]    However, using a twisted nematic liquid crystal rotator (TN rotator) plate with LCD based projectors, instead of ½-wave optical retarder material, greatly reduces the amount of cross talk and ghosting due to its superior spectral characteristics. The TN rotator configuration is also superior to using dual CRT or DLP projectors with external polarizers since the transmission loss in the TN rotator is very small compared to the losses in linear polarizer sheets. In addition, the TN rotator can be used with 3-chip polysilicon (p-Si) projection systems in which the polarization of the red and blue light channels is orthogonal to the polarization of the green channel.  
         [0027]    In general, the embodiments described herein use a TN rotator or a ½ wave retarder to optically alter the images for a variety of dual projector applications. Note that the embodiments may also include any projector having one color channel polarized orthogonal to the other two color channels, and are not limited to the specific projectors described herein. However, for ease of illustration, the embodiments shown in the figures illustrate the TN rotator(s), ½ wave retarders and touch up polarizers external to the projectors. However, other embodiments may include the TN rotator(s), ½ wave retarders and touch up polarizers (and a combination thereof) positioned within the projectors or even a combination of internal and external positions. FIG. 1 illustrates a prior art dual projector 3D stereoscopic system  10  using two amorphous silicon (a-Si) thin film transistor liquid crystal display (TFT LCD) projectors  12  and  14  and a ½-wave retarder  16 . A characteristic of this type of projector is that the exiting light is polarized at a 45° angle from the vertical due to the LCD&#39;s output analyzer (linearly polarizing filter). The “reference” polarization state of the first projector  12  is labeled P 1 . A ½-wave retarder plate  16  is typically positioned either externally (as shown in FIG. 1) or internally to produce an orthogonal polarization state, P 2 , for the second projector  14 . The disadvantage of this prior art method lies in the fact that the ½-wave polarizing material is wavelength dependent and allows 3D cross talk in either the blue or red colors of the spectrum.  
         [0028]    [0028]FIG. 2 illustrates another prior art dual projector 3D stereoscopic system  20  using two a-Si TFT LCD projectors. In FIG. 2, however, the LCD panel  26  of the first projector  22  is modified to output linearly polarized light with a polarization axis orthogonal to the normal, unmodified projector. The LCD polarizer  24  of the first projector  22  has state P 2  and the LCD analyzer  28  has state P 1 . However, in the second projector  30 , the LCD polarizer  36  has state P 1  and the LCD analyzer  40  has state P 2 . The result is that the light output of the first projector  22  has a linear polarization axis of +45° and the light output of the second projector  30  has a polarization axis of −45°. A disadvantage of this prior art method is that the LCD display works best when the LCD polarizer and analyzer are unchanged. The modified LCD display will exhibit 3D cross talk and must be corrected with “touch-up” polarizers that reduce the light output of the projectors.  
         [0029]    [0029]FIG. 3 illustrates a prior art dual projector 3D stereoscopic system  60  using two projectors  62  and  64  having non-polarized light output. In this system, the left and right perspective images are altered (or encoded) by placing linearly polarizing filters  66  and  68  in each projector&#39;s output light path. As FIG. 3 shows, the first projector  62  has a linear polarizer positioned such that the polarization axis is +45° with respect to the vertical, and the second projector  64  has a linear polarizer positioned such that the polarization axis is 45° with respect to the vertical. As previously discussed, over half the light output of each projector is absorbed in the polarizing filters  66  and  68 , thus, reducing brightness by at least 50 percent.  
         [0030]    [0030]FIG. 4 illustrates the spectral characteristics  70  of a typical sample of ½-wave retarder, such as is produced by 3M Corporation. The graph shows the extinction of the ½-wave retarder. Extinction is defined as the percent transmittance of ½-wave retarder sandwiched between two linearly polarizing filters, whose polarization axes are aligned in parallel, and the optical axis of the ½-wave retarder is oriented at +/−45° from the polarization axis of the polarizing filters. The vertical axis represents the percent transmittance and the horizontal axis represents the light wavelength in units of nanometers. The horizontal axis spans the visible spectrum with blue on the left (around 500 mn) green in the middle (around 600 nm) and red on the right around (700 nm). Ultraviolet is on the far left and infrared is on the far right. FIG. 4 demonstrates the extinction between the P 1  and P 2  polarization states used to encode 3D images. As shown, that ½-wave retarder has good extinction in the “green” region of the spectrum but has much worse extinction in the blue and red regions. In other words, if this ½-wave retarder were used in a dual projector 3D stereoscopic system, persons viewing 3D images would most likely see blue or purple ghosting of the images. Note that a high level of percent transmittance results in heavy 3D cross talk or light “leakage” between each eye, resulting in possible ghosting of the image. Therefore, the best 3D quality results when the percent transmittance is as low as possible.  
         [0031]    [0031]FIG. 5 illustrates a percent transmittance plot  72  for a TN rotator sandwiched between two linearly polarizing filters aligned in parallel. In this example, the TN rotator is made with liquid crystal (such as is manufactured by BASF) using 1 mm optically flat glass for the substrate. Note that a TN rotator is typically comprised of a sandwich of liquid crystal material between two glass plates. In the example of FIG. 5, the spacing between the glass plates is 10 micrometers. The overall percent transmittance has a much more uniformly flat spectrum than does the ½-wave retarder material. Thus, dual projector 3D stereoscopic projections systems using a TN rotator have better results than systems using a ½-wave retarder.  
         [0032]    [0032]FIG. 6 illustrates a functional diagram  80  of a TN rotator  86 . The TN rotator is constructed to rotate the input light&#39;s  82  electric field vector orientation a total of 90°. As shown, light entering the TN rotator  86  has an E-field orientation  84  of +45° with respect to the vertical. As light passes through the TN rotator, the E-field vector is rotated so that its orientation becomes 45° with respect to the vertical at the output  88 . Note that TN rotators may be constructed such that the E-field vector may be rotated at any angle. In fact, for dual projector 3D stereoscopic projection systems in which certain p-Si or DILA (digital image light amplifier) projectors are used, a 0° to 45° TN rotator may be preferable. Such an embodiment is discussed below.  
         [0033]    [0033]FIG. 7 illustrates a representation of the cross-section of a typical TN rotator  90 . No matter what method is used to actually make a TN rotator, the basic result is to achieve a TN rotator with a cell in which the liquid crystal molecules are arranged in a helical chain. In the example of FIG. 7, several chains of liquid crystal molecules  92  having a helical structure (in which ¼ of a turn is realized in the helix) are shown. Starting from the top of the cell  94 , liquid crystal molecules are represented as being oriented parallel to the page. Moving down, the molecules gradually rotate in orientation until they are pointing into the page at the bottom of the cell  96 . Such a helical structure acts as a microscopic wave-guide for the light and causes the rotation of the light&#39;s E-field vector. In one embodiment, liquid crystal molecules may be aligned at the surface of the glass substrate using methods well known in the art of making liquid crystal cells and displays. These methods typically involve the application of a polyamide coating to the glass substrate. After the polyamide is applied, it is mechanically rubbed with felt or other soft material in the direction in which the liquid crystal molecules should be aligned. Thus, for a 0° to 90° TN rotator application, the polyamide is rubbed at 0° relative to the desired input E-field vector on the input glass substrate. For the output glass substrate, the polyamide is rubbed 90° with respect to the first rubbing direction.  
         [0034]    In the embodiment of FIG. 8, a dual projector 3D stereoscopic system  100  uses two a-Si TFT LCD projectors  12  and  14 , and an external TN rotator  102  to obtain the P 2  polarization state for the second projector  14 . In the embodiment of FIG. 9, a dual projector 3D stereoscopic system  110  uses two a-Si TFT LCD projectors  12  and  14 , an external TN rotator  102  and two external “touch-up” polarizers  112  and  114 . Polarizers  112  and  114  are used to “clean up” the final light output. Note that some of the light output from a projector may not be polarized in the same direction, therefore, polarizers  112  and  114  may be used to remove the “stray” polarized light. In other words, polarizers  112  and  114  may be used to filter light that may be polarized in undesired directions. Also, note that for one embodiment, polarizers  112  and  114  are applied directly to the outputs of the two projectors  12  and  14 . Polarizers  112  and  114  are aligned such that the axis of polarization is parallel to that of the projector itself. Thus, the polarizers further decrease the cross talk for projectors that do not have optimum native polarization characteristics for 3D images.  
         [0035]    [0035]FIG. 10 illustrates a dual projector 3D stereoscopic system  120  using dual p-Si projectors  122  and  124  and two ½-wave retarders  142  and  144 . The system  120  employs three separate p-Si LCD displays: one display for each color: red, green, or blue. Since each p-Si LCD display necessarily requires its own light path, the three separate light paths are recombined into a single, full-color, light source prior to the entering projection lens. A three-input, one output optical combiner cube may be used to recombine these light paths. Currently, there are two types of combiner cubes. The first type of combiner cube causes all of the light exiting the combiner cube (and therefore the projector) to have the same polarization state. However, the second type of combiner cube (as may be used in the system  120  of FIG. 10) causes the red and blue light  126  to have one linear polarization state  146  and the green light  128  to have an orthogonal polarization state  150 . Note that an embodiment for a dual projector 3D stereoscopic system  190  using the first type of combiner cube is shown in FIG. 13 and discussed below. In the system of FIG. 10, ½-wave retarders  142  and  144  are applied to the output of the projectors  122  and  124  to alter or “encode” the light. The first projector  122  vertically polarizes the red and blue light  126 , and horizontally polarizes the green light  128 . Similarly, the second projector  124  vertically polarizes the red and blue light  130 , and horizontally polarizes the green light  140 . The first ½-wave retarder  142  is rotated −22.5° in order to rotate the polarization angle of both the red-blue light  146  and the green light  150  by a total of  45 °. Similarly the second ½-wave retarder  144  is rotated +22.5° to achieve a rotation angle of +45° for both the red-blue  148  and green light  152 . Thus, the polarization axis of the red-blue light  146  (from the first projector  122 ) is aligned with the polarization axis of the green light  152  (from the second projector  124 ). Similarly, the polarization axis of the green light  150  (from the first projector  122 ) is aligned with the polarization axis of the red-blue light  148  (from the second projector  124 ). In other words, a 3D stereoscopic image can be viewed if the first projector  122  is configured to display the red-blue light of the right perspective image, the green light of the left perspective image, and the second projector  124  is configured to display the red-blue light of the left perspective image and the green channel of the right perspective image.  
         [0036]    [0036]FIG. 11 illustrates a signal input diagram  160  depicting green light channel interchanging used for stereoscopic image encoding in the dual projector 3D stereoscopic systems of FIGS. 10 and 12. Two component video or VGA computer sources  162  and  164  are shown, the first source  162  for the left perspective image and the second source  164  for the right perspective image. Two p-Si projectors  166  and  168  are also depicted. To encode the 3D stereoscopic image being displayed, the green channel of the left perspective image source  170  is interchanged with the green channel of the right perspective image source  172 . As is known in the art, both the left and right perspective image sources  162  and  164  should be synchronized for proper operation.  
         [0037]    However, as previously discussed, undesirable image ghosting is likely in systems using a ½-wave retarder. Thus, the embodiment of FIG. 12 illustrates a dual projector 3D stereoscopic system  180  using dual polysilicon projectors  122  and  124  and two external TN rotators  182  and  184 . The system  180  of FIG. 12 is configured similarly to the system  120  of FIG. 10, except that a TN rotator is used in FIG. 12 instead of the  2 -wave retarder used in FIG. 10. In this embodiment, a 0° to 45° TN rotator  182  is applied external to the first projector  122 , and a 0° to −45° TN rotator  184  is applied external to the second projector  124 . As discussed, the input configuration of FIG. 11 may be used for this system  180 .  
         [0038]    The embodiment of FIG. 13 illustrates another dual projector 3D stereoscopic system  190  using dual p-Si projectors  122  and  124  and two external TN rotators  196  and  198 . In this embodiment, the polarization angle  192  out of the projectors  122  and  124  is vertical for all colors. Thus, a 0° to 45° TN rotator  196  is applied external to the first projector  122 , and a 0° to −45° TN rotator  198  is applied external to the second projector  124 . However, the input configuration of FIG. 11 is not used in this embodiment since each color channel has the same polarization axis orientation.  
         [0039]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.