Patent Publication Number: US-10768449-B2

Title: Stereoscopic glasses using tilted filters

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned, U.S. patent application Ser. No. 13/251,456, now U.S. Pat. No. 8,746,888, entitled: “Stereoscopic projector using spectrally-adjacent color bands”, by Silverstein et al.; to commonly assigned, U.S. patent application Ser. No. 13/251,472, now U.S. Pat. No. 8,651,663, entitled: “Stereoscopic projector using scrolling color bands”, by Silverstein et al.; to commonly assigned, co-pending U.S. patent application Ser. No. 13/351,432, entitled: “Stereoscopic glasses using dichroic and absorptive layers”, by Silverstein et al.; to commonly assigned, U.S. patent application Ser. No. 13/351,449, now U.S. Pat. No. 8,947,424, entitled: “Spectral stereoscopic projection system”, by Silverstein et al.; to commonly assigned, U.S. patent application Ser. No. 13/351,495, now abandoned, entitled: “Filter glasses for spectral stereoscopic projection system”, by Silverstein et al.; and to commonly assigned, U.S. patent application Ser. No. 13/351.470, now U.S. Pat. No. 8,864,314, entitled: “Stereoscopic projection system using tunable light emitters”, by Silverstein et al., each of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates to a stereoscopic digital projection system that uses spectrally-adjacent light sources to form left-eye and right-eye images, and more particularly to filter glasses having tilted filters to reduce unwanted flare. 
     BACKGROUND OF THE INVENTION 
     In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. This is particularly true for multicolor cinematic projection systems. Competitive digital projection alternatives to conventional cinematic-quality projectors must meet high standards of performance, providing high resolution, wide color gamut, high brightness, and frame-sequential contrast ratios exceeding 2,000:1. 
     Stereoscopic projection is a growing area of special interest for the motion picture industry. Three-dimensional (3-D) images or perceived stereoscopic content offer consumers an enhanced visual experience, particularly in large venues. Conventional stereoscopic systems have been implemented using film, in which two sets of films and projectors simultaneously project orthogonal polarizations, one for each eye, termed a “left-eye image” and a “right-eye image” in the context of the present disclosure. Audience members wear corresponding orthogonally polarized glasses that block one polarized light image for each eye while transmitting the orthogonal polarized light image. 
     In the ongoing transition of the motion picture industry to digital imaging, some vendors, such as IMAX, have continued to utilize a two-projection system to provide a high quality stereo image. More recently, however, conventional digital projectors have been modified to enable 3D projection. 
     Conventional methods for forming stereoscopic images from these digital projectors have used one of two primary techniques for distinguishing left- and right-eye images. One technique, utilized by Dolby Laboratories, for example, uses spectral or color space separation. The method used is similar to that described in U.S. Pat. No. 7,832,869, entitled “Method and device for performing stereoscopic image display based on color selective filters” to Maximus et al., wherein color space separation is used to distinguish between the left-eye and right-eye image content. The image for each eye is projected using primary Red, Green, and Blue component colors, but the precise Red, Green, and Blue wavelengths that are used differ between left- and right-eye images. To achieve this separation, filters are utilized in the white light illumination system to momentarily block out portions of each of the primary colors for a portion of the frame time. For example, for the left eye, the lower wavelength spectrum of Red, Blue, and Green (RGB) would be blocked for a period of time. This would be followed by blocking the higher wavelength spectrum of Red, Blue, and Green (RGB) for the other eye. The appropriate color adjusted stereo content that is associated with each eye is presented to each spatial light modulator for the eye. The viewer wears viewing glasses with a corresponding filter set that similarly transmits only one of the two 3-color (RGB) spectral sets to each eye. 
     A second approach utilizes polarized light. One method disclosed in U.S. Pat. No. 6,793,341 to Svardal et al., utilizes each of two orthogonal polarization states delivered to two separate spatial light modulators. Polarized light from both modulators is then projected simultaneously. The viewer wears polarized glasses with polarization transmission axes for left and right eyes orthogonally oriented with respect to each other. 
     There are advantages and drawbacks with each approach. Spectral separation solutions, for example, are advantaged by being more readily usable with less expensive display screens. With spectral separation, polarization properties of the modulator or associated optics do not significantly affect performance. However, the needed filter glasses have been expensive and image quality is reduced by factors such as angular shift, head motion, and tilt. Expensive filter glasses are also subject to scratch damage and theft. Promising developments in filter glass design, including the use of layered optical films produced by non-evaporative means by 3M Corp, can help to address the cost problem and make spectral separation techniques more cost-effective. 
     Another drawback of the spectral separation approach relates to difficulties in adjustment of the color space and significant light loss due to filtering, leading to either a higher required lamp output or reduced image brightness. Filter losses have been addressed in U.S. Patent Application Publication 2009/0153752 to Silverstein, entitled “Projector using independent multiple wavelength light sources” wherein independent spectrally-adjacent sources are combined by a beamsplitter to be efficiently directed to a spatial light modulator. One disadvantage of this approach is that these light sources are only utilized approximately half of the time, as the modulator can only provide one eye image in time. While the light sources will likely have a longer life, the initial cost of the display is increase by the cost requirement of two sets of independent sources. 
     With polarization for separating the left- and right-eye images, light can be used more efficiently. U.S. Pat. No. 7,891,816 to Silverstein et al., entitled “Stereo projection using polarized solid state light sources,” and U.S. Pat. No. 8,016,422 to Silverstein et al., entitled “Etendue maintaining polarization switching system and related methods,” describe projection system configurations that fully utilize the light source for both polarization states. However, polarization techniques are disadvantaged by the additional cost and sensitivity of polarization maintaining screens, which typically utilize a structured metallic coating. These coatings are high gain, which improves on axis viewing, but are poor for off axis viewing. Furthermore, the specular reflections with this method can be troubling for some viewers. This effect is further exacerbated when using coherent light, as it leads to higher levels of viewer perceived speckle. Projectors using polarized light are typically more costly due to the difficulty of maintaining high polarization control through high angle optics as well as being more sensitive to dirt and defects. Therefore any gains in efficiency can be somewhat offset by other problems. 
     A continuing problem with illumination efficiency relates to etendue or, similarly, to the Lagrange invariant. As is well known in the optical arts, etendue relates to the amount of light that can be handled by an optical system. Potentially, the larger the etendue, the brighter the image. Numerically, etendue is proportional to the product of two factors, namely the image area and the numerical aperture. In terms of the simplified optical system represented in  FIG. 1  having light emitter  12 , optics  18 , and a spatial light modulator  20 , the etendue of the light source is a product of the light source area A 1  and its output angle θ 1 . Likewise, the etendue of the spatial light modulator  20  equal to the product of the modulator area A 2  and its acceptance angle θ 2 . For increased brightness, it is desirable to provide as much light as possible from the area of light source  12 . As a general principle, the optical design is advantaged when the etendue at the light emitter  12  is most closely matched to the etendue at the spatial light modulator  20 . 
     Increasing the numerical aperture, for example, increases the etendue so that the optical system captures more light. Similarly, increasing the light source size, so that light originates over a larger area, increases etendue. In order to utilize an increased etendue on the illumination side, the etendue of the spatial light modulator  20  must be greater than or equal to that of the light source  12 . Typically, however, the larger the spatial light modulator  20 , the more costly it will be. This is especially true when using devices such as LCOS and DLP components, where the silicon substrate and defect potential increase with size. As a general rule, increased etendue results in a more complex and costly optical design. 
     Efficiency improves when the etendue of the light source is well-matched to the etendue of the spatial light modulator. Poorly matched etendue means that the optical system is either light-starved, unable to provide sufficient light to the spatial light modulators, or inefficient, effectively discarding a substantial portion of the light that is generated for modulation. 
     Solid-state lasers promise improvements in etendue, longevity, and overall spectral and brightness stability. Recently, devices such as VCSEL (Vertical Cavity Surface-Emitting Laser) laser arrays have been commercialized and show some promise, when combined in various ways, as potential light sources for digital cinema projection. However, brightness itself is not yet high enough; the combined light from as many as 9 individual arrays is needed in order to provide the necessary brightness for each color. 
     Laser arrays of particular interest for projection applications are various types of VCSEL arrays, including VECSEL (Vertical Extended Cavity Surface-Emitting Laser) and NECSEL (Novalux Extended Cavity Surface-Emitting Laser) devices from Novalux, Sunnyvale, Calif. 
     However, even with improvements in laser technology and in filter preparation and cost, there is considerable room for improvement in methods of stereoscopic imaging projection. Conventional solutions that use spectral separation of left- and right-eye images are typically light-starved, since at most only half of the light that is generated is available for each eye. Thus, there is a need for a stereoscopic imaging solution that offers increased optical efficiencies with decreased operational and equipment costs. 
     SUMMARY OF THE INVENTION 
     The present invention represents filter glasses for use by an observer of a stereoscopic digital display system that displays stereoscopic images including first-eye images and second-eye images on a display surface, comprising: 
     a first-eye filter having a front surface facing the display surface and an opposite back surface and having transmission characteristics that substantially transmit light from the first-eye images and block light from the second-eye images; 
     a second-eye filter having a front surface facing the display surface and an opposite back surface and having transmission characteristics that substantially transmit light from the second-eye images and block light from the second-eye images; 
     a frame into which the first-eye filter and the second-eye filter are mounted, the frame being adapted to position the first-eye filter in front of the observer&#39;s first eye and to position the second-eye filter in front of the observer&#39;s second eye, the front surfaces of the first-eye filter and the second-eye filter being oriented at a tilt angle of at least 5 degrees relative to vertical so that light from the display surface that is reflected from the first-eye filter and the second-eye filter is directed over the heads of other observers that are seated in front of the observer. 
     This invention has the advantage that the amount of flare light reflected from the front surface of the filter glasses which is directed toward other audience members seated in from of the observer is reduced, thereby reducing the amount of visual noise and improving image contrast. 
     This invention has the advantage that that the amount of flare light reflected from the front surface of the filter glasses which is directed onto the display surface is also reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representative diagram showing factors in etendue calculation for an optical system; 
         FIG. 2  is a schematic block diagram that shows a stereoscopic projection apparatus that uses spectral separation for left- and right-eye images; 
         FIG. 3A  is a schematic diagram showing a prior art color scrolling sequence; 
         FIG. 3B  is a schematic diagram showing a single-channel color scrolling sequence using spectrally-adjacent bands of color according to an embodiment of the present invention; 
         FIG. 4A  is a schematic diagram that shows parts of a single color channel in a stereoscopic digital projection system that uses a single beam scanner to provide two spectrally-adjacent bands of color; 
         FIG. 4B  is a schematic diagram that shows parts of a single color channel in a stereoscopic digital projection system that uses a separate beam scanner to provide each spectrally-adjacent band of color; 
         FIG. 5  is a schematic diagram showing a stereoscopic digital projection system having three color channels, each using the configuration of  FIG. 4A ; 
         FIG. 6A  is a schematic diagram that shows the use of a rotating prism for scanning a single band of color; 
         FIG. 6B  is a schematic diagram that shows the use of a rotating prism for scanning two bands of color; 
         FIG. 6C  is a schematic diagram showing another configuration for using a rotating prism for scanning two bands of color; 
         FIG. 7A  is a schematic diagram that shows uniformizing optics including two lenslet arrays; 
         FIG. 7B  is a schematic diagram that shows uniformizing optics including two integrating bars; 
         FIG. 8  is a schematic diagram showing a beam scanning configuration according to an embodiment of the present invention; 
         FIG. 9  is a schematic diagram of a stereoscopic color scrolling digital projection system having three color channels and using combining optics for arrays of solid-state light emitters; 
         FIG. 10  is a schematic diagram of a stereoscopic color scrolling digital projection system having three color channels according to an alternate embodiment using two spatial light modulators; 
         FIG. 11A  is a graph that shows spectral bands for stereoscopic projection using spectral separation in an interleaved arrangement; 
         FIG. 11B  is a graph that shows spectral bands for stereoscopic projection using spectral separation in an alternate non-interleaved arrangement; 
         FIG. 12A  is a graph that shows spectral transmittances for right-eye and left-eye filters for use with the interleaved spectral band arrangement of  FIG. 3A ; 
         FIG. 12B  is a graph that shows spectral transmittances for right-eye and left-eye filters for use with the non-interleaved spectral band arrangement of  FIG. 3B ; 
         FIG. 13  is a graph illustrating the origin of crosstalk in a wavelength-based stereoscopic imaging system; 
         FIG. 14  is a graph illustrating the angular dependent of the spectral transmission characteristics for left-eye and right-eye eye filters used in commercially available filter glasses; 
         FIGS. 15A and 15B  are cross-section diagrams showing embodiments of right-eye filters having a dichroic filter stack; 
         FIG. 16A  is a graph showing spectral transmittance characteristics for an example right-eye filter using a dichroic filter stack; 
         FIG. 16B  is a graph showing transmitted light provided by the right-eye filter of  FIG. 16A ; 
         FIG. 16C  is a graph showing spectral reflectance characteristics for the right-eye filter of  FIG. 16A ; 
         FIG. 16D  is a graph showing reflected light provided by the right-eye filter of  FIG. 16A ; 
         FIGS. 17A-17D  are cross-section diagrams showing embodiments of right-eye filters having a dichroic filter stack and one or more absorptive filter layers; 
         FIG. 18  is a graph showing spectral transmittance characteristics for an example dichroic filter stack and an example absorptive filter layer appropriate for use in a right-eye filter; 
         FIG. 19A  is a graph showing spectral transmittance characteristics for an example hybrid right-eye filter that combines a dichroic filter stack and an absorptive filter layer; 
         FIG. 19B  is a graph showing transmitted light provided by the hybrid right-eye filter of  FIG. 19A ; 
         FIG. 19C  is a graph showing spectral reflectance characteristics for the hybrid right-eye filter of  FIG. 19A ; 
         FIG. 19D  is a graph showing reflected light provided by the hybrid right-eye filter of  FIG. 19A ; 
         FIG. 20A  is a schematic diagram showing a path of light reflected light from filter glasses for two observers having heads at the same height; 
         FIG. 20B  is a schematic diagram showing a path of light reflected light from filter glasses for two observers having heads at different heights; 
         FIG. 21A  is a side view showing filter glasses with tilted filter elements; 
         FIG. 21B  is a perspective view showing filter glasses with tilted filter elements; 
         FIG. 21C  is a side view showing filter glasses with a hinge for adjusting a tilt angle for tilted filter elements; 
         FIG. 22  is a side view that showing observers wearing filter glasses with tilted filter elements; 
         FIG. 23  is a schematic diagram showing a path of light reflected light from filter glasses with tilted filter elements for two observers having heads at different heights; 
         FIG. 24  is a schematic diagram showing one color channel of a stereoscopic imaging system for forming right-eye and left-eye images using tunable light emitters; and 
         FIG. 25  is a schematic diagram showing a color stereoscopic imaging system for forming right-eye and left-eye images using tunable light emitters. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense. 
     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     Figures shown and described herein are provided to illustrate principles of operation according to the present invention and are not drawn with intent to show actual size or scale. Because of the relative dimensions of the component parts for the laser array of the present invention, some exaggeration is necessary in order to emphasize basic structure, shape, and principles of operation. In addition, various components such as those used to position and mount optical components, for example, are not shown in order to better show and describe components that relate more closely to embodiments of the present invention. 
     Where they are used, the terms “first”, “second”, and so on, do not necessarily denote any ordinal or priority relation, but may be simply used to more clearly distinguish one element from another. 
     The terms “color” and “wavelength band” and “spectral band” are generally synonymous as used in the context of the present disclosure. For example, a laser or other solid-state light source is referred to by its general color spectrum, such as red, rather than by its peak output wavelength (such as 635 nm) or its spectral band (such as 630-640 nm). In the context of the present disclosure, different spectral bands are considered to be essentially non-overlapping. 
     The terms “viewer” and “observer” are used equivalently to refer to a person viewing the stereoscopic display of the present invention. The term “left-eye image” refers to the image formed for viewing by the left eye of the observer. Correspondingly, the term “right-eye image” refers to the image formed for viewing by the right eye of the observer. 
     Embodiments of the present invention address the need for improved brightness in a stereoscopic viewing system using independent adjacent spectral sources. 
     In the context of the present invention, the terms “transmission band” and “pass band” are considered to be equivalent. 
     In the context of the present invention, the term “spectrally-adjacent” relates to nearby spectral bands within the general color spectrum that are used for the component colors that form a color image, typically red, green, blue, and possibly including a fourth color and other additional colors. The corresponding spectrally-adjacent colors for each component color lie in the same color spectrum, but have different wavelength ranges for left- and right-eye images such that the spectral bands are substantially non-overlapping with respect to wavelength. 
       FIG. 2  is a schematic block diagram of an image-forming system that illustrates some of the major components of a stereoscopic digital projection system  110  including a projector apparatus  120  that uses spectral separation for forming left-eye and right-eye images on a viewing screen or other type of display surface  72 . A first set of right-eye light emitters  12 R emit light in a first red spectral band R 1 , a first green spectral band G 1 , and a first blue spectral B 1 . The right-eye light emitters  12 R are used to form a right-eye image for viewing by an observer&#39;s right eye. Similarly, a second set of left-eye light emitters  12 L emit light in a second red spectral band R 2 , a second green spectral band G 2 , and a second blue spectral B 2 . The left-eye light emitters  12 L are used to form a left-eye image for viewing by an observer&#39;s left eye. 
     In a preferred embodiment, the spectral bands associated the left-eye light emitters  12 L and the right-eye light emitters  12 R are all substantially non-overlapping with each other so that filter glasses  74  can be used to effectively separate the light provided by the left-eye light emitters  12 L from the light provided by the right-eye light emitters  12 R. By substantially non-overlapping we mean that the spectral power from one spectral band is negligible for any wavelength where another spectral band is non-negligible. Acceptable results can sometimes be obtained even when there is some small level of overlap between the spectral bands. One criterion that can be used in practice is that less than 5% of the light from one of the spectral bands should overlap with the other spectral band. 
     The filter glasses  74  include a left-eye filter  76 L and a right-eye filter  76 R, together with a frame  62  into which the left-eye filter  76 L and the right-eye filter  76 R are filtered are mounted. The frame  62  is adapted to position the right-eye filter  76 R in front of the observer&#39;s right eye and to position the left-eye filter  76 L in front of the observer&#39;s left eye. The right-eye filter  76 R has spectral transmission characteristics that are adapted to transmit the light in the R 1 , G 1  and B 1  spectral bands from the right-eye light emitters  12 R and to block (i.e., absorb or reflect) the light in the R 2 , G 2  and B 2  spectral bands from the left-eye light emitters  12 L. Likewise, the left-eye filter  76 L has spectral transmission characteristics that are adapted to transmit the light in the R 2 , G 2  and B 2  spectral bands from the left-eye light emitters  12 L and to block the light in the R 1 , G 1  and B 1  spectral bands from the right-eye light emitters  12 R. 
     Projector apparatus  120  can have two separate projector devices, one with color channels intended to serve a left-eye imaging path that projects light from the left-eye light emitters  12 L and the other to serve a right-eye imaging path that projects light from the right-eye light emitters  12 R. However, many designs combine the left-eye and right-eye imaging functions into a single projector, such as to take advantage of inherent alignment characteristics and to reduce the cost associated with components such as projection lenses. Subsequent description in this disclosure gives detailed information on one type of projector that combines left-eye and right-eye imaging paths using color scrolling. It can be appreciated by those skilled in the image projection arts that there are also other methods available for combining stereoscopic left-eye and right-eye images. Embodiments of the present invention can be used with any of a number of types of stereoscopic projection systems that utilize spectral separation techniques. 
     The schematic diagram of  FIG. 3A  shows how a color scrolling sequence can be used to provide a color image from component red (R), green (G), and blue (B) light in conventional practice, for a projection apparatus that is not stereoscopic. A series of image frames  28   a ,  28   b ,  28   c ,  28   d , and  28   e  are shown as they are arranged at different times. Each frame has three bands of light  34   r ,  34   g , and  34   b  having red, green and blue color components, respectively, that are scanned across image region  32 , moving in a vertical direction in the example shown. As a band is scrolled off the bottom of the image frame, it is scrolled into the top of the image frame so that ⅓ of the image frame is covered by each of the color components at any given time. 
     A vertical scrolling motion is generally preferred because horizontal scrolling can be impacted by side to side movement of the viewer whereby the color bands may become perceptible. This is often referred to as a “rainbow effect.” The bands of light in this sequence can be from illumination components, scanned onto the spatial light modulator or may be imaged light from the spatial light modulator. The scanning action is cyclic, recurring at an imperceptible rate for the viewer, at a rate of many times per second (e.g., 144 Hz). As can be seen from this sequence, each image frame  28   a ,  28   b ,  28   c ,  28   d  and  28   e  has each of the three component colors scanned over a different image region. In the image that is formed using this sequence, each frame has red, green, and blue image content, in the respective bands of light  34   r ,  34   g , and  34   b.    
     It can be readily appreciated that the color scrolling scheme of  FIG. 3A , while usable for non-stereoscopic color imaging, presents difficulties for stereoscopic color imaging systems. Providing stereoscopic color requires the scrolling of six different spectral bands, two for each of the component colors. Each source has its own etendue associated with it. Illuminating a single chip with six different sources, each also requiring a gap between them to prevent crosstalk and allowing for chip transition time from each of the color data associated with the particular color would quickly utilize the available etendue or require optically fast lenses. While this is feasible, it is undesirable, since projector brightness is severely constrained and cost of the optics quickly rises with such an arrangement. 
     To help improve image quality and deliver higher brightness, cinematic-quality projection systems for non-stereoscopic imaging often employ separate color channels for each color, typically providing each of a red, green, and blue color channel. A spatial light modulator is provided in each color channel. This arrangement enables the optical design to optimize the design and features of components, such as filters and coatings, for example, to improve their performance for light of the respective wavelengths. 
       FIG. 3B  shows a color scanning arrangement for a stereoscopic projection system according to an exemplary embodiment of the present invention. In this configuration, spectrally-adjacent spectral bands within a single component color spectrum are scrolled across the image region  32 , rather than bands corresponding to the different color components as in the arrangement of  FIG. 2 . In this example, spectrally-adjacent red spectral bands R 1  and R 2  are scrolled, as bands of light  36   a  and  36   b , across image frames  38   a ,  38   b ,  38   c ,  38   d , and  38   e  according to an embodiment of the present invention. The R 1  spectral band is used to provide the left-eye image and the R 2  spectral band is used to provide the right-eye image for the projected stereoscopic image. Similar spectral scrolling mechanisms are provided for each color channel of the stereoscopic image, as will subsequently be described in more detail. Further by maintaining the light of the same color within its own color channel, the optical coatings for the optical components associated with a particular color component can continue to be optimized for the respective color component. 
     The schematic diagrams of  FIGS. 4A and 4B  show parts of a red color channel  40   r  for color scrolling spectrally-adjacent colors in a single color channel, compatible with an embodiment of the present invention. A light source  42   a  emits a beam of light in the R 1  spectral band, and another light source  42   b  emits a beam of light in the R 2  spectral band. Illumination optics  90  provide substantially uniform bands of light onto spatial light modulator  60  for modulation in each of the two spectrally-adjacent spectral bands. Beam scanning optics  92  including a beam scanner  50  provide the cyclical scrolling of the bands of light. It will be recognized that the illumination optics  90  can include multiple lens  48 , some of which may be positioned between the uniformizing optics  44  and the beam scanning optics  92 , with others being positioned between the beam scanning optics  92  and the spatial light modulator  60 . In a preferred embodiment, the illumination optics  90  image an output face of the uniformizing optics  44  onto the spatial light modulator  60 , thereby providing the uniform bands of light. An advantage of this approach is that the light sources  42   a  and  42   b  can be continuously on during projection, providing increased light output over other stereoscopic projection methods. 
     In the configuration of  FIG. 4A , a beam combiner  46  combines the light beams from the light sources  42   a  and  42   b  onto parallel optical axes and directs the spatially-adjacent light beams into uniformizing optics  44 , such as one or more lenslet arrays or uniformizing bars, to provide substantially uniform spatially-adjacent light beams. A beam scanner  50  then cyclically scrolls the combined uniformized light and directs the scrolled combined light beam onto the spatial light modulator  60  through the illumination optics  90 , which provide for beam imaging, shaping and conditioning. In  FIG. 4A , the illumination optics  90  are represented as lens  48 ; however in various embodiments the illumination optics  90  can include different (or multiple) optical components. The beam separation required to prevent crosstalk between the bands of light may be provided by use of spatial or angular separation of the incoming beams of light to beam scanner  50 . In the event that differing angles are utilized, it is generally desired that another element, such as a dichroic beam combiner, be provided downstream of the beam scanner  50  to return the scanned beams of light onto parallel optical axes. 
     The spatial light modulator  60  forms an image frame  38  having corresponding bands of light  36   a  and  36   b . The bands of light  36   a  and  36   b  are cyclically scrolled as described previously. The spatial light modulator  60  has an array of pixels that can be individually modulated according to image data to provide imaging light. The spatial light modulator pixels illuminated by the R 1  spectral band are modulated according to image data for the left-eye image and the spatial light modulator pixels illuminated by the R 2  spectral band are modulated according to image data for the right-eye image. 
     In the alternate configuration of  FIG. 4B , separate uniformizing optics  44  and beam scanners  50  are utilized in the light beams from each of the light sources  42   a  and  42   b  to provide two scanned light beams. A beam combiner  46  then combines the scanned light beams to form a combined scanned light beam, which is directed onto the spatial light modulator  60  using illumination optics  90 . In this case the beam scanning optics  92  includes both beam scanners  50 . 
     The schematic diagram of  FIG. 5  shows a stereoscopic digital projection system  100  that has three color channels (i.e., red color channel  40   r , a green color channel  40   g , and a blue color channel  40   b ). The red color channel  40   r  includes spectrally-adjacent red spectral bands R 1  and R 2 ; the green color channel  40   g  includes spectrally-adjacent green spectral bands G 1  and G 2 ; and the blue color channel  40   b  includes spectrally-adjacent blue spectral bands B 1  and B 2 . Projection optics  70  deliver the imaging light from the three spatial light modulators  60  to a display surface  72 . The viewer observes display surface  72  through filter glasses  74  having left-eye filter  76 L for the left eye and right-eye filter  76 R for the right eye. The left-eye filter  76 L selectively transmits the imaging light for the left-eye image (i.e., light in the R 1 , G 1  and B 1  spectral bands), while blocking (by absorbing or reflecting) the imaging light for the right-eye image (i.e., light in the R 2 , G 2  and B 2  spectral bands). Similarly, right-eye filter  76 R selectively transmits the imaging light for the right-eye image (i.e., light in the R 2 , G 2  and B 2  spectral bands), while blocking the imaging light for the left-eye image (i.e., light in the R 1 , G 1  and B 1  spectral bands). 
     A controller system  80  synchronously modulates the pixels of each spatial light modulator  60  according to image data for the stereoscopic image. The controller system  80  is also coupled to the beam scanners  50  so that it knows which spatial light modulator pixels are illuminated by the different spectrally-adjacent bands at any given time. The spatial light modulator pixels that are illuminated by the first spectral band are modulated according to image data for the left-eye image and the spatial light modulator pixels that are illuminated by the second spectral band are modulated according to image data for the right-eye image. Since the first and second spectral bands are continuously scrolling, the subsets of the spatial modulator pixels that are modulated with the image data for left-eye and right-eye images are continuously changing as well. 
     Projection optics  70  may combine the light beams from the three color channels (e.g., using beam combining optics) and project the combined beam through a single projection lens. Alternately, the projection optics  70  may use three separate projection lenses to project each of the color channels separately onto the display surface  72  in an aligned fashion. 
     As noted earlier with reference to  FIGS. 4A and 4B , the beam scanning optics  92  including one or more beam scanners  50  can be configured to provide band of light scrolling using a number of different arrangements, and can be positioned at any suitable point along the illumination path. Consistent with one embodiment of the present invention,  FIG. 6A  shows a schematic diagram of a beam scanner  50  which includes a single scanning element, namely a rotating prism  52 . In this configuration, a rotating prism  52  can be provided for each of the spectrally-adjacent spectral bands in each of the component color bands. Rotation of the prism  52  redirects the light beam, shown here for the R 1  spectral band, by refraction, so that the light beam position is cyclically scrolled across spatial light modulator  60 . The  FIG. 6A  arrangement is used, for example, in the color channel embodiment shown in  FIG. 4B . 
     In the top diagram of  FIG. 6A , the prism  52  is positioned so that the incident beam is normally incident on a face of the prism. In this case the light beam passes through the prism  52  in an undeflected fashion. In the middle diagram, the prism  52  has been rotated around axis O so that the light beam is incident at an oblique angle onto the face of the prism. In this case, the beam is refracted downward so that it intersects the spatial light modulator at a lower position. In the lower diagram, the prism  52  has been rotated so that the incident beam now strikes a different facet of the prism  42 . In this case, the beam is refracted upward so that it intersects the spatial light modulator  60  at a higher position. It should be noted that the incident beam will generally have a substantial spatial (and angular) extent so that at some prism orientations some of the light rays in the incident beam may strike different faces of the prism. In this way, some of the light rays will be deflected upwards, while others may be deflected downwards. This provides for the band of light to be split between the upper and lower portions of the image frame as shown in image frame  38   e  of  FIG. 3B . 
       FIG. 6B  is a schematic diagram that shows an alternate embodiment for beam scanner  50 , in which a rotating prism  52  simultaneously scans the bands of light for both of the spectrally-adjacent spectral bands in a single color channel (in this example spectral bands R 1  and R 2 ). This configuration is appropriate for use in the example embodiment of  FIG. 4A . In this case, light beams for both of the R 1  and R 2  spectral bands are incident on the prism  52 . As the prism  52  rotates, both of the light beams are simultaneously redirected by refraction. 
       FIG. 6C  is a schematic diagram that shows another alternate embodiment for beam scanner  50 , in which a rotating prism  52  simultaneously scans the bands of light for both of the spectrally-adjacent spectral bands in a single color channel (in this example spectral bands R 1  and R 2 ). In this case, the beams of light incident on the rotating prism come from two different angular directions. Uniformizing optics  44  are used to uniformize each of the spectrally-adjacent light beams. In this example, the uniformizing optics  44  include integrating bars  58 . The illumination optics  90  are split into a first stage  94  and a second stage  96 , each including a plurality of lenses  48 . In this configuration, the lenses  48  in the first stage  94  are arranged to provide telecentricity between the output face of the integrating bars  58  and the prism  52 . Similarly, the lenses  48  in the second stage  96  are arranged to provide telecentricity between the prism  52  and the spatial light modulator  60 . A dichroic combiner  82 , including one or more dichroic surfaces  84 , is used to direct the scanned light beams onto parallel optical axes for illuminating the spatial light modulator  60 . 
     The multi-angle geometry of  FIG. 6C  is similar to that taught by Conner in U.S. Pat. No. 7,147,332, entitled “Projection system with scrolling color illumination.” Connor teaches a projection system having a scrolling prism assembly to simultaneously illuminate different portions of a spatial light modulator with different color bands. White light is divided into different color bands that propagate through the scrolling prism in different directions. The scrolled color bands are reflectively combined so that the different color bands pass out of the scrolling prism assembly parallel. However, Conner does not teach scrolling spectrally-adjacent spectral bands from independent light sources to provide for stereoscopic projection. 
     A rotating prism or other refractive element is one type of device that can be used for the beam scanner  50 . The term “prism” or “prism element” is used herein as it is understood in optics, to refer to a transparent optical element that is generally in the form of an n-sided polyhedron with flat surfaces upon which light is incident and that is formed from a transparent, solid material that refracts light. It is understood that, in terms of shape and surface outline, the optical understanding of what constitutes a prism is less restrictive than the formal geometric definition of a prism and encompasses that more formal definition. While  FIGS. 6A-6C  depict a rectangular prism with a square cross-section, in many instances it is desired to have more than four facets in order to provide improved scanning results. For example, a hexagonal prism, or an octagonal prism can be used in various embodiments. 
     Alternate types of components that can be utilized for beam scanner  50  include rotating mirrors or other reflective components, devices that translate across the beam path and provide variable light refraction, reciprocating elements, such as a galvanometer-driven mirror, or pivoting prisms, mirrors, or lenses. 
     When multiple beam scanners  50  are utilized, it is critical to synchronize the rotation of all of the beam scanners  50 , and subsequently the image data associated with the different spectral bands. One method, not depicted, is to configure the optical arrangement such that a single motor is used to control the moving optical elements for at least two of the beam scanners  50 . For example a single axle can be used to drive multiple prisms  52  using a single motor. In some embodiments, a single rotating prism  52  can be used to scan multiple spectral bands by directing light beams through the prism  52  from multiple directions, or by directing light beam through different portions of the prism  52  (as shown in  FIG. 6B ). 
     As shown in the examples of  FIGS. 4A, 4B, and 5 , beam paths for the spectrally-adjacent spectral bands can be aligned with each other to illuminate spatial light modulator  60  using the beam combiner  46 . The beam combiner  46  can be a dichroic beam combiner, or can use any other type of beam combining optics known in the art. 
     The uniformizing optics  44  condition the light beams from the light sources  42   a  and  42   b  to provide substantially uniform beams of light for scanning. In the context of the present disclosure, the term “substantially uniform” means that the intensity of the beam of light incident on the spatial light modulator  20  appears to be visually uniform to an observer. In practice, the intensity of the uniformized light beams should be constant to within about 30%, with most of the variation occurring being a lower light level toward the edges of the uniformized light beams. Any type of uniformizing optics  44  known in the art can be used, including integrating bars or lenslet arrays. 
       FIG. 7A  shows an example of uniformizing optics  44  that can be used for the embodiment of  FIG. 4A . The uniformizing optics  44  use a pair of lenslet arrays  54  to uniformized the light beams. One of the spatially-adjacent light beams (e.g., for the R 1  spectral band) is passed through the top half of the lenslet arrays  54 , while the other spatially-adjacent light beam (e.g., for the R 2  spectral band) passes through the bottom half of the lenslet arrays  54 . An opaque block  56  is provided between the light beams for the spectrally-adjacent spectral bands, to help prevent crosstalk. In this manner a single lenslet array structure may be utilized per color band thereby reducing costs. 
       FIG. 7B  shows another example of uniformizing optics  44  that can be used for the embodiment of  FIG. 4A . In this case, the uniformizing optics  44  use a pair of integrating bars  58  to uniformized the light beams. One of the spatially-adjacent light beams (e.g., for the R 1  spectral band) is passed through the upper integrating bar  58 , while the other spatially-adjacent light beam (e.g., for the R 2  spectral band) passes through the lower integrating bar  58 . 
     As mentioned earlier, in a preferred embodiment, the output face(s) of the uniformizing optics  44  are imaged onto the spatial light modulator  60  using the illumination optics  90 , where the imaging light passes through the beam scanning optics  92 . It will be obvious to one skilled in the art that many different configurations for the illumination optics  90  can be used to provide this feature.  FIG. 8  shows one embodiment where the illumination optics  90  are divided into first stage  94  and second stage  96 , each including two lenses  48 . The lenses  48  in the first stage  94  form an image of the output faces of integrating bars  58  at an intermediate image plane  98  corresponding to the position of the prism  52 , which is a component of the beam scanner  50 . The second stage  96  forms an image of the intermediate image plane  98  onto the spatial light modulator  60 , thereby providing substantially-uniform bands of light  36   a  and  36   b . The bands of light are scanned across the spatial light modulator as the prism  52  is rotated. The lenses  48  can be used to adjust the magnification of the intermediate image according to the size of the prism  52 , and to adjust the magnification of the scanned bands of light according to the size of the spatial light modulator  60 . 
     The controller system  80  ( FIG. 5 ) synchronously modulates the pixels of each spatial light modulator  60  according to image data for the stereoscopic image. Logic in the controller system  80  coordinates the image data for the left- and right-eye image content with the corresponding positions of each band of light  36   a  and  36   b . The controller system  80  may be a computer or dedicated processor or microprocessor associated with the projector system, for example, or may be implemented in hardware. 
     Embodiments of the present invention are well suited to using solid-state light sources such as lasers, light-emitting diodes (LEDs), and other narrow-band light sources, wherein narrow band light sources are defined as those having a spectral bandwidth of no more than about 15 nm FWHM (full width half maximum), and preferably no more than 10 nm. Other types of light sources that could be used include quantum dot light sources or organic light emitting diode (OLED) light sources. In still other embodiments, one or more white light sources could be used, along with corresponding filters for obtaining the desired spectral content for each color channel. Methods for splitting polychromatic or white light into light of individual color spectra are well known to those skilled in the image projection arts and can employ standard devices such as X-cubes and Phillips prisms, for example, with well-established techniques for light conditioning and delivery. 
     The use of lasers provides a significant advantage in reducing the bandwidth of the spectrally-adjacent spectral bands, thereby allowing more separation between the adjacent bands and increased color gamut. This is desirable in that the filters on each eye are inevitably sensitive to angle whereby the wavelength of the filter edge transitions shift due to non-normal incidence. This angular sensitivity is a commonly known problem in all optical filter designs. Therefore using a reduced bandwidth emission helps to solve this problem enabling this common shift to occur without substantially impacting crosstalk. Many lasers have bandwidths on the order of 1 nm. While this may seem ideal, there are other factors, such as speckle reduction, which benefit from broader spectral bands. (Speckle is produced by the interference of coherent light from defects on optical components.) While speckle can occur using any type of light source, it is most pronounced with narrow band light sources such as LEDs, and even more so with Lasers. A more desirable bandwidth would fall between 5-10 nm as a compromise to provide adequate spectral separation while reducing the sensitivity to speckle. A spectral separation of between 15-20 nm is generally sufficient to mitigate the filter angular sensitivity issues. 
     The schematic diagram of  FIG. 9  shows a stereoscopic digital projection system  100  using a common optical path for projection optics  70 . The stereoscopic digital projection system includes a red color channel  40   r , a green color channel  40   g  and a blue color channel  40   b . Each color channel includes one or more arrays of light sources (e.g., laser array sources) for each of a pair of spectrally-adjacent spectral bands. Light sources  42   a  emit light beams in the left-eye spectral bands (R 2 , G 2  and B 2 ), and light sources  42   b  emit light in the spectrally-adjacent right-eye spectral bands (R 1 , G 1  and B 1 ). Light-redirecting prisms  30  are used in each color channel to redirect the light beams from the light sources  42   a  and  42   b  into a common direction to form a combined light beam including spatially-adjacent light beams for the right-eye and left-eye spectral bands (e.g., the R 1  and R 2  spectral bands). The light beams from the right-eye spectral band (e.g., the R 1  spectral) will be grouped on one side of the combined light beam, and the light beams from the left-eye spectral band (e.g., the R 2  spectral) will be grouped on the other side of the combined light beam. One type of light-redirecting prism  30  that can be used for this purpose is described in the aforementioned, commonly-assigned, co-pending U.S. Patent Application Publication 2009/0153752 entitled “Projector using independent multiple wavelength light sources” by Silverstein, which is incorporated herein by reference. 
     The combined light beam for each component color channel is directed through uniformizing optics  44 , beam scanning optics  92  and illumination optics  90 , and is reflected from dichroic surface  68  to provide scanned first and second bands of light  36   a  and  36   b  onto the corresponding spatial light modulators  60 . A controller system  80  ( FIG. 5 ) synchronously modulates the spatial light modulator pixels according to image data for the stereoscopic image, wherein the spatial light modulator pixels illuminated by the first band of light (e.g., R 1 ) are modulated according to image data for the left-eye image and the spatial light modulator pixels illuminated by the second band of light (e.g., R 2 ) are modulated according to image data for the right-eye image. 
     The modulated imaging light beams provided by the spatial light modulators  60  are transmitted through the dichroic surfaces  68  and are combined onto a common optical axis using a dichroic combiner  82  having multiple dichroic surfaces  84 . The combined light beam is projected onto a display surface (not shown) using the projection optics  70  for viewing by observers wearing filter glasses  74  ( FIG. 5 ). 
     The embodiment illustrated in  FIG. 9  uses three spatial light modulators  60 , one for each component color channel (i.e., red, green and blue). Each spatial light modulator  60  is illuminated with scrolling bands of light having spectrally-adjacent spectral bands within a particular component color channel. The spatial light modulators tend to be one of the more expensive and complex components of the stereoscopic digital projection system  100 . 
       FIG. 10  illustrates a schematic diagram for an alternate embodiment of a stereoscopic digital projection system  110  that utilizes only two spatial light modulators  60 L and  60 R, one associated with a left-eye image forming system  41 L and one associated with a right-eye image forming system  41 R. The left-eye image forming system  41 L includes three left-eye light sources  43 L, one for each component color spectrum (R 1 , G 1  and B 1 ). Similarly, the right-eye image forming system  41 R includes three right-eye light sources  43 R, one for each component color spectrum (R 2 , G 2  and B 2 ). The right-eye light sources  43 R are spectrally-adjacent to the corresponding left-eye light sources  43 L. 
     Each of the image forming systems include uniformizing optics  44 , beam scanning optics  92 , illumination optics  90  and a dichroic surface  68  to direct the scanned beams of light onto spatial light modulators  60 L and  60 R. In this case, the left-eye image forming system  41 L provides three scanned bands of light  34   r ,  34   g  and  34   b , corresponding to the red, green and blue spectral bands (R 1 , G 1  and B 1 ), respectively. Likewise, the right-eye image forming system  41 R provides three scanned bands of light  35   r ,  35   g  and  35   b , corresponding to the red, green and blue spectral bands (R 2 , G 2  and B 2 ), respectively. 
     A controller system (not shown) synchronously modulates the pixels of the spatial light modulator  60 L in the left-eye image forming system  41 L according to image data for the left-eye image, wherein the pixels illuminated by the each band of light (R 1 , G 1  and B 1 ) are modulated according to the image data for the corresponding color channel of the left-eye image. Likewise, the controller system synchronously modulates the pixels of the spatial light modulator  60 R in the right-eye image forming system  41 R according to image data for the right-eye image, wherein the pixels illuminated by the each band of light (R 2 , G 2  and B 2 ) are modulated according to the image data for the corresponding color channel of the left-eye image. 
     A dichroic combiner  82  including a dichroic surface  84  is used to combine the imaging light from the left-eye image forming system  41 L and the right-eye image forming system  41 R onto a common optical axis for projection onto a display surface using projection optics  70 . The dichroic surface  84  is preferably a spectral comb filter having a series of notches that transmits the spectral bands (R 2 , G 2  and B 2 ) corresponding to the imaging light for the right-eye light sources  43 R while reflecting the spectral bands (R 1 , G 1  and B 1 ) corresponding to the imaging light for the left-eye light sources  43 L. Spectral comb filters can be fabricated using any technique known in the art, such as multi-layer thin-film dichroic filter coating methods and co-extruded stretched polymer film structure fabrication methods. Another type of dichroic filter that can be used to provide a spectral comb filter for use as dichroic surface  84  is a rugate filter design. Rugate filters are interference filters that have deep, narrow rejection bands while also providing high, flat transmission for the rest of the spectrum. Rugate filters are fabricated using a manufacturing process that yields a continuously varying index of refraction throughout an optical film layer. Rugate filters feature low ripple and no harmonic reflections compared to standard notch filters, which are made with discrete layers of materials with different indices of refraction. 
     By way of example, and not by way of limitation, Tables 1 and 2 list example spectrally-adjacent spectral bands according to embodiments of the present invention. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Exemplary interleaved spectrally-adjacent spectral bands 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Right-Eye Image 
                 Left- Eye Image 
               
               
                   
                 Component Color 
                 Spectral Bands 
                 Spectral Bands 
               
               
                   
                   
               
               
                   
                 Red 
                 625-640 nm 
                 655-670 nm 
               
               
                   
                 Green 
                 505-520 nm 
                 535-550 nm 
               
               
                   
                 Blue 
                 442-456 nm 
                 470-484 nm 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Exemplary non-interleaved spectrally-adjacent spectral bands 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Right-Eye Image 
                 Left- Eye Image 
               
               
                   
                 Component Color 
                 Spectral Bands 
                 Spectral Bands 
               
               
                   
                   
               
               
                   
                 Red 
                 625-640 nm 
                 655-670 nm 
               
               
                   
                 Green 
                 535-550 nm 
                 505-520 nm 
               
               
                   
                 Blue 
                 442-456 nm 
                 470-484 nm 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 11A  shows spectral bands R 1 , G 1 , and B 1  for the right eye and spectral bands R 2 , G 2 , and B 2  for the left eye for each component color according to the Table 1 arrangement. Each of the spectral bands has a corresponding central wavelength (λ R1 , λ G1 , λ B1 , λ R2 , λ G2 , λ B2 ) and a corresponding bandwidth (W R1 , W G1 , W B1 , W R2 , W G2 , W B2 ). For the  FIG. 11A  arrangement, the spectral bands observe an interleaved ordering according to the central wavelengths for the respective spectral bands: λ B1 &lt;λ B2 &lt;λ G1 &lt;λ G2 &lt;λ R1 &lt;λ R2 . 
     The bandwidths can be characterized using an appropriate measure of width for the spectral bands. Typically, the bandwidths are defined to be the wavelength separation between the lower edge (i.e., the “cut-on edge”) of the spectral band and the upper edge (i.e., the “cut-off edge”) of the spectral band. In a preferred embodiment, the bandwidths are full-width half-maximum bandwidths where the lower and upper edges correspond to the wavelengths where the spectral power in the spectral band falls to half of its peak level. In other embodiments the lower and upper edges can be determined according to other criteria. For example the edges can be defined to be the wavelengths where the spectral power falls to a specified level other than half of the peak level (e.g., the 10% power level or the 25% power level). Alternatively, the bandwidth can be characterized using some other measure of the width of the spectral band (e.g., a multiple of the standard deviation of the spectral power distribution for the spectral band). 
     In the example shown in  FIG. 11A , the bandwidth of each spectral band is about 10-15 nm, while the separation between adjacent spectral bands is 15 nm or more. Various embodiments may use light emitters having different bandwidths, or may have different separations between the adjacent spectral bands. The minimum bandwidth for typical light emitters that would be used for digital projection systems would be about 1 nm, corresponding to the bandwidth of a single laser. 
     The central wavelengths of the spectral bands can be characterized using any appropriate measure of the central tendency for the spectral bands. For example, in various embodiments, the central wavelengths can be peak wavelengths of the spectral bands, centroid wavelengths of the spectral bands, or the average of the lower and upper edge wavelengths. 
       FIG. 11B  shows spectral bands R 1 , G 1 , and B 1  for the right eye and spectral bands R 2 , G 2 , and B 2  for the left eye for each component color according to the Table 2 arrangement. In this case, the spectral bands observe a non-interleaved ordering according to the central wavelengths for the respective spectral bands where: λ B1 &lt;λ B2 &lt;λ G2 &lt;λ G1 &lt;λ R1 &lt;λ R2 . The rearrangement of the G 1  and G 2  spectral bands in the  FIG. 11B  arrangement relative to the ordering in the  FIG. 11A  arrangement is generally advantageous for simplifying filter glass coating design and for other purposes, as will subsequently be described in more detail. 
     It should be noted that there will generally be slight color gamut differences between right- and left-eye imaging paths associated with the use of the different red, green and blue primaries. As a result, different color processing, including white balance and color correction transforms, will generally be needed to account for the spectral bands associated with the primary colors used for left-eye and right-eye imaging paths. White balancing can be performed, for example, by adjusting the brightness of one or more light emitters, by applying transforms to individual color channels, by adjusting illumination timing or by using filtration to adjust color intensity. Color correction transforms are used to determine control signals for each of the color channels to produce a desired color appearance associated with a set of input color values. Color correction transforms will generally also include some form of gamut mapping to determine appropriate output colors for cases where the input color values are outside of the color gamut associated with the color primaries used for the left-eye and right-eye imaging paths. Color correction operations can be performed by applying color correction matrices, or by applying other forms of color transforms such as three-dimensional look-up tables (3-D LUTs). Methods for determining color transforms that are appropriate for a particular set of color primaries are well-known in the art. 
     Because additional spectral bands are available for wavelength-based stereoscopic imaging systems, there may be additional color gamut available that can be utilized when the system is used for non-stereoscopic imaging applications. An example of a technique that can be used for this purpose is described in commonly assigned U.S. Patent Application Publication No. 2011/0285962 entitled “2D/3D Switchable Color Display Apparatus with Narrow Band Emitters” by Ellinger et al. 
     The right-eye filter  76 R and the left-eye filter  76 L in filter glasses  74  ( FIG. 5 ) have spectral transmittance characteristics that are designed to transmit the spectral bands associated with the corresponding left-eye or right-eye image and block the spectral bands associated with the other eye.  FIG. 12A  illustrates an example of a right-eye filter transmittance  78 R for right-eye filter  76 R and a left-eye filter transmittance  78 L for left-eye filter  76 L that can be used in accordance with the interleaved spectral band arrangement shown in  FIG. 11A . The right-eye filter transmittance  78 R transmits most of the light in the right-eye spectral bands (R 1 , G 1 , B 1 ) while blocking most of the light in the left-eye spectral bands (R 2 , G 2 , B 2 ). Likewise, the left-eye filter transmittance  78 L transmits most of the light in the left-eye spectral bands (R 2 , G 2 , B 2 ) while blocking most of the light in the right-eye spectral bands (R 1 , G 1 , B 1 ). In this example, both the right-eye filter transmittance  78 R and the left-eye filter transmittance  78 L is a “comb filter” that includes two contiguous bandpass filter transmission bands  77 B and one contiguous edge filter transmission band  77 E. A transmission band is considered to be contiguous provided that it has at least some minimum specified transmission percentage (e.g., 50%) over all wavelengths within the transmission band. 
     The right-eye filter  76 R and the left-eye filter  76 L should generally be designed to transmit at least 50% of the light from the corresponding eye spectral bands in order to avoid causing a significant loss in image brightness. Preferably, this value should be 80% or higher. To prevent objectionable cross-talk, the right-eye filter  76 R and the left-eye filter  76 L should generally be designed to transmit less than 5% of the light from the opposite eye spectral bands. Preferably, this value should be less than 2% to ensure that the crosstalk is substantially imperceptible. 
       FIG. 12B  illustrates an example of a right-eye filter transmittance  79 R for right-eye filter  76 R and a left-eye filter transmittance  79 L for left-eye filter  76 L that can be used in accordance with the non-interleaved spectral band arrangement shown in  FIG. 11B . In comparison to the arrangement shown in  FIG. 12B , it can be seen that the filters in the arrangement of  FIG. 12A  have the advantage that they require fewer edge transitions. In particular, both the right-eye filter transmittance  78 R and the left-eye filter transmittance  78 L use only a single bandpass filter transmission band  77 B, together with a single edge filter transmission band  77 E. This is made possible by the fact that due to the reordering of the spectral bands there is no intervening left-eye spectral band between the right-eye green spectral band G 1  and the right-eye red spectral band R 1 . Likewise, there is no intervening right-eye spectral band between the left-eye blue spectral band B 2  and the left-eye green spectral band G 2 . Each of the filters in the arrangement of  FIG. 12B  require only three edge transitions (from low transmittance to high transmittance or from high transmittance to low transmittance), whereas the filters in the arrangement of  FIG. 12A  each require five edge transitions. In general, the complexity of a filter design increases with the number of edge transitions, and with the sharpness of the edge transitions that are required. The fabrication of filters with fewer bandpass filter transmission bands (and therefore fewer edge transitions) is therefore significantly less complex, requiring fewer filter layers, and as a result is less expensive. This is an important advantage since the filter glasses  74  must be manufactured in large quantities for use by each viewer in the audience who is viewing the projected stereoscopic image. Another advantage of the arrangement of  FIG. 12B  is that there are fewer opportunities for generating crosstalk since there are fewer edge transitions where an opposing eye spectral band can leak into a transmission band. 
     The left-eye filter  76 L and the right-eye filter  76 R in filter glasses  74  ( FIG. 5 ) can be made using any fabrication technique known in the art. In some embodiments, one or both of the left-eye filter  76 L and the right-eye filter  76 R are dichroic filters that includes an optical surface having a multi-layer thin-film coating. The multi-layer thin-film coating can be designed to provide appropriate filter transmittances, such as the right-eye filter transmittance  78 R and the left-eye filter transmittance  78 L of  FIG. 12A  and the right-eye filter transmittance  79 R and the left-eye filter transmittance  79 L of  FIG. 12B . Techniques for designing and fabricating multi-layer thin-film coatings having specified spectral transmittance characteristics are well known in the art. 
     In other embodiments, one or both of the left-eye filter  76 L and the right-eye filter  76 R are multi-layer dichroic filters that are fabricated using a co-extruded stretched polymer film structure. One method for fabricating such structures is described in U.S. Pat. No. 6,967,778 to Wheatley et al., entitled “Optical film with sharpened bandedge,” which is incorporated herein by reference. According to this method, a coextrusion device receives streams of diverse thermoplastic polymeric materials from a source such as a heat plastifying extruder. The extruder extrudes a multi-layer structure of the polymeric materials. A mechanical manipulating section is used to stretch the multi-layer structure to achieve the desired optical thicknesses. 
     Crosstalk is an undesirable artifact that can occur in stereoscopic imaging systems where the image content intended for one of the observer&#39;s eyes is contaminated with the image content intended for the other eye. This can create the appearance of perceptible “ghost images” where the viewer sees faint images of objects in the scene that are spatially offset from the main images. To avoid objectionable crosstalk it is important that the amount of light from the left-eye light emitters  12 L that is transmitted by the right-eye filter  76 R is a small fraction of the amount of light from the right-eye light emitters  12 R that is transmitted by the right-eye filter  76 R. Likewise, the amount of light from the right-eye light emitters  12 R that is transmitted by the left-eye filter  76 L should be a small fraction of the amount of light from the left-eye light emitters  12 L that is transmitted by the left-eye filter  76 L. 
       FIG. 13  illustrates the origin of crosstalk in a wavelength-based stereoscopic imaging system. The figure shows a close up of the wavelength range that includes the right-eye red spectral band R 1  and the left-eye red spectral band R 2 . A left-eye filter transmittance  79 L is shown that transmits the majority of the light in the left-eye red spectral band R 2  while blocking the majority of the light in the right-eye red spectral band R 1 . However, it can be seen that there is a small overlap region  75  where a small amount of the light from the right-eye red spectral band R 1  is transmitted by the left-eye filter transmittance  79 L. This transmitted right-eye light will reach the observer&#39;s left eye, producing crosstalk and resulting in a faint ghost image. 
     Various metrics can be used to characterize the amount of crosstalk. One such metric is given by the following equation: 
                     C     R   →   L       =         ∫         P   R     ⁡     (   λ   )       ⁢       T   L     ⁡     (   λ   )       ⁢   d   ⁢           ⁢   λ         ∫         P   L     ⁡     (   λ   )       ⁢       T   L     ⁡     (   λ   )       ⁢   d   ⁢           ⁢   λ         ×   100             (     1   ⁢   A     )                 C     L   →   R       =         ∫         P   L     ⁡     (   λ   )       ⁢       T   R     ⁡     (   λ   )       ⁢   d   ⁢           ⁢   λ         ∫         P   R     ⁡     (   λ   )       ⁢       T   R     ⁡     (   λ   )       ⁢   d   ⁢           ⁢   λ         ×   100             (     1   ⁢   B     )               
where C R→L , is the amount of crosstalk from the right-eye image that contaminates the left-eye image, C L→R  is the amount of crosstalk from the left-eye image that contaminates the right-eye image, P L (λ) and P R (λ) are the spectral power distributions for the light from the left-eye light emitters  12 L and the right-eye light emitters  12 R, respectively, T L (λ) and T R (λ) are the spectral transmittances for the left-eye filter  76 L and the right-eye filter  76 R, respectively, and λ is the wavelength. It can be seen that the metrics given by Eqs. (1A) and (1B) compute the percentages of the undesired light that is passed by the filters relative to the amount of desired light that is passed by the filters. Generally, the amount of crosstalk should be less than 5% under all viewing conditions to avoid objectionable artifacts, and preferably it should be less than 2% to ensure that the crosstalk is substantially imperceptible.
 
     A number of factors influence the level of crosstalk that occurs in the stereoscopic digital projection system  110  ( FIG. 2 ). These factors include the amount of wavelength separation between the left-eye spectral bands and the right-eye spectral bands, the sharpness of the edge transitions for the light-emitter spectral bands, the sharpness of the edge transitions for the filter transmission bands, and the alignment between the light-emitter spectral bands and the filter transmission bands. Since the locations of the edge transitions for the filter transmission bands is sometimes a function of the incidence angle (e.g., for dichroic filters), the amount of crosstalk may be a function of viewing angle. 
     The wavelength separation between the left-eye spectral bands and the right-eye spectral bands is a particularly important factor that must be considered during the design of a digital projection system in order to avoid crosstalk. The wavelength separation can be defined to be the wavelength interval between the upper edge (i.e., the “cut-off edge”) of the lower spectral band to the lower edge (i.e., the “cut-on edge”) of the higher spectral band. This distance is characteristically measured from the half-maximum point on each band edge. For example,  FIG. 13  shows the wavelength separation S between the right-eye red spectral band R 1  and the left-eye red spectral band R 2 . The amount of wavelength separation that is necessary to eliminate objectionable crosstalk will depend on the sharpness of the edge transitions in the filter transmittance, as well as other effects such as variability of the edge transition location with incidence angle. 
     The variation of the locations of the edge transitions with angle of incidence for a set of commercially available filters intended for use with wavelength-based stereoscopic imaging systems is illustrated in  FIG. 14 . Graph  130  shows a pair of measured spectral transmittance curves for a right-eye filter for normally incident light as well as light incident at a 20° angle of incidence. It can be seen that the edge transitions shift about 5-10 nm toward the short wavelength direction. These wavelength shifts occur as a result of the longer path length that the light takes through the dichroic filter stack. Since the shifts occur towards the short wavelength direction, they are sometimes called “blue shifts.” Graph  135  shows an analogous pair of spectral transmittance curves for a left-eye filter, which exhibit similar shifts in the edge transitions. 
     Because of the variability in the locations of the edge transitions, it is generally desirable that the wavelength separation between the left-eye spectral bands and the right-eye spectral bands be large enough to accommodate the range of edge transition positions associated with the range of expected viewing angles without inducing objectionable crosstalk artifacts. U.S. Patent Application Publication No. 2010/0060857, entitled “System for 3D Image Projection Systems and Viewing,” to Richards et al. notes this problem and recommends sizing “guard bands” or notches between the respective spectral bands for each eye, such as between the green color channel spectral bands G 1  and G 2 , for example. 
     In a preferred embodiment, the light emitters are narrow-band light sources, such as solid-state lasers, having bandwidths that are no more than about 15 nm. Accordingly, if the central wavelengths of each spectral band for a particular color are chosen to be at least 25 nm apart, this will provide wavelength separations between the bands of at least 10-15 nm, which is sufficient to provide substantial protection against crosstalk given properly designed filters. For this and other reasons, the use of narrow-band solid state light sources is advantaged over conventional approaches that use filtered white light sources, wherein the bandwidths of the spectral bands typically exceed 40 nm for individual primary colors. (The larger bandwidth is necessary in conventional filtered white light sources as further narrowing of the spectrum reduces the system optical efficiency.) 
     The left-eye filter  76 L and the right-eye filter  76 R can be made using any spectral filter technology known in the art. One type of spectral filters of particular interest for wavelength-based stereoscopic imaging systems are dichroic filters made using thin-film dichroic filter stacks. Dichroic filters are fabricated by coating a plurality of transparent thin film layers having markedly different refractive indices on a substrate. The thin film layers can be deposited in various forms and using various methods, including vacuum coating and ion-deposition, for example. The material is deposited in alternating layers having thicknesses on the order of one-quarter wavelength of the incident light in the range for which the coating is designed. Materials used for the coating layers can include dielectrics, metals, metallic and non-metallic oxides, transparent polymeric materials, or combinations thereof. In an alternate embodiment, one or more of the dichroic filter stack layers is deposited as a solution of nanoparticles. Where polymer materials are used, one or more of the filter stack layers can be formed from extruded materials. 
     The thicknesses and indices of refraction of the thin film layers in the dichroic filter stack can be adjusted to control the spectral transmittance characteristics. One important advantage of using filters made using dichroic filter stacks is that given enough layers, the shape of the spectral transmittance curves can be accurately controlled, and very sharp edge transitions can be achieved. This enables filters to be provided that selectively transmit one set of spectral bands while blocking the other set. 
     However, one characteristic of dichroic filters that can be disadvantageous for stereoscopic imaging application is that the light that is not transmitted through the filter is reflected back off the filter. The undesirable effects of this effect is illustrated in  FIG. 15A . Imaging light from display surface  72  is directed toward an observer wearing filter glasses  74  (not shown in  FIG. 15A ) that include right-eye filter  76 R disposed in front of the right eye  194  of the observer. The right-eye filter  76 R in this case includes a dichroic filter stack  86  on a front surface  66 F of a transparent substrate  88 , such as a glass or plastic substrate. (The front surface  66 F faces the display surface  72 , while the opposite rear surface  66 R faces the observer.) 
     The incident light includes right-eye incident light  196 R comprising right-eye image data and left-eye incident light  196 L comprising left-eye image data. The right-eye incident light  196 R is substantially transmitted through the right-eye filter  76 R as right-eye transmitted light  198 R and will be incident on the observer&#39;s right eye  194  to enable the observer to view the right-eye image. The left-eye incident light  196 L is substantially reflected back into the viewing environment as left-eye reflected light  197 L. This reflected light can be scattered around in the viewing environment and can contaminate the viewed image as “flare” light that would be transmitted through the left-eye filter  76 L ( FIG. 5 ) into the observer&#39;s left eye. The problem of flare light is exacerbated as the audience size increases. The reflected light from each pair of filter glasses  74  can be inadvertently directed back toward the display screen or to other objects or structures in the viewing area, increasing the amount of visual noise and reducing image contrast. 
     Some of the left-eye flare light from a direction behind the observer may be incident on rear surface  66 R of the right-eye filter  76 R. This light is shown as left-eye incident light  186 L, which will be substantially reflected from the dichroic filter stack and will be directed back into the right eye  194  as left-eye reflected light  187 L. The origin of this light may be direct reflections off the filter glasses  74  worn by other viewers that are seated behind the observer, or may be light that may have been reflected off of other surfaces. 
     The left-eye filter  76 L, which is not shown in  FIG. 15A , has a similar structure and complementary behavior, substantially transmitting the intended image-bearing light emitted for the left-eye image while substantially blocking the unwanted light for the right-eye image. 
       FIG. 15B  shows an arrangement similar to that shown in  FIG. 15A  where the dichroic filter stack is on the rear surface  66 R of the substrate  88 . The overall behavior of the right-eye filter  76 R is identical to that of  FIG. 15A , although this configuration has the advantage that the dichroic filter stack  86  may be less likely to be damaged by scratching it since it is less exposed. 
     To further illustrate the problem of unwanted reflected light,  FIG. 16A  shows a typical right-eye dichroic filter transmittance  170 R that can be used with a wavelength-based stereoscopic projection system that uses right-eye light emitters having right-eye spectral bands R 1 , G 1  and B 1  and left-eye light emitters having left-eye spectral bands R 2 , G 2  and B 2 . It can be seen that dichroic filter transmittance  170 R is arranged to transmit most of the light in the right-eye spectral bands R 1 , G 1 , B 1 , while blocking most of the light in the left-eye spectral bands R 2 , G 2 , B 2 . 
       FIG. 16B  is a graph showing the light that is transmitted through the right-eye filter  76 R according to the right-eye dichroic filter transmittance  170 R as a function of wavelength. In this example, the transmitted right-eye light  175 R includes more than 90% of the incident light in the right-eye bands, and the transmitted left-eye light  175 L includes about 3% of the light in the left-eye spectral bands. As discussed earlier, the transmitted left-eye light  175 L will be a source of crosstalk in the viewed stereoscopic image. 
     For dichroic filters, the dichroic filter reflectance R D (λ) will be approximately equal to:
 
 R   D (λ)≈(1− T   D (λ))  (2)
 
where T D (λ) is the dichroic filter transmittance.  FIG. 16C  shows a right-eye dichroic filter reflectance  171 R corresponding to the right-eye dichroic filter transmittance of  FIG. 16A . It can be seen that the right-eye dichroic filter reflectance  171 R reflects the majority of the light in the left-eye spectral bands R 2 , G 2 , B 2 .
 
       FIG. 16D  is a graph showing the light that is reflected from the right-eye filter  76 R according to the right-eye dichroic filter transmittance  170 R as a function of wavelength. In this example, the reflected right-eye light  176 R includes less than 10% of the incident light in the right-eye bands, and the reflected left-eye light  176 L includes about 97% of the light in the left-eye spectral bands. As discussed earlier, this reflected light can be a source of objectionable flare in the viewing environment. 
     In some embodiments, the problem of unwanted reflected light is mitigated using a hybrid filter design as shown in  FIG. 17A . With this approach, the right-eye filter  76 R includes both a dichroic filter stack  86 , as well at least one wavelength-variable absorptive filter layer  87 . Absorptive filters absorb a fraction of the light at a particular wavelength, while transmitting the remainder of the light. (Some small fraction of the light may also be reflected.) It is generally not possible to produce absorptive filters having spectral transmittance characteristics with sharp edge transitions at arbitrary wavelengths as can be done with dichroic filter designs. Therefore, absorptive filters are typically not suitable to provide the high degree of color separation required for wavelength-based stereoscopic imaging system. However, the combination of absorptive filter layers with dichroic filter layers has been found to provide significant performance advantages relative to the use of pure dichroic filters. 
     In accordance with embodiments of the present invention, the dichroic filter stack is designed to transmit 60% or more of the light from the right-eye light emitters and reflect 60% or more of the light from the left-eye light emitters. Preferrably, the dichroic filter stack should transmit at least 90% or more of the light from the right-eye light emitters and reflect at least 90% of the light from the left-eye light emitters. 
     Likewise, the absorptive filter layers  87  are designed to transmit a larger percentage of the light in the right-eye spectral bands that the light in the left-eye spectral bands. Preferably, the absorptive filter layers  87  should transmit a large majority of the light in the right-eye spectral bands, while absorbing a large majority of the light in the right-eye spectral bands. 
     Taken together, the hybrid right-eye filter is adapted to transmit 50% or more of the light from the right-eye light emitters, while blocking most of the light in the left-eye light emitters so that the amount of transmitted light from the left-eye light emitters is less than 5% of the transmitted light from the right-eye light emitters. The absorption characteristics of the absorptive filter layers  87  are such that the amount of left-eye incident light  196 L reflected from the right-eye filter  76 R is substantially reduced relative to configurations that use only a dichroic filter stack  86  (e.g, the configurations shown in  FIGS. 15A-15B ). In a preferred embodiment, the right-eye filter  76 R should absorb a majority of the left-eye incident light  196 L such that less than 50% of the left-eye incident light  196 L is reflected. Ideally, the right-eye filter  76 R should absorb a large majority (e.g., more than 90%) of the left-eye incident light  196 L. 
     In some embodiments, the absorptive filter layers  87  can be coated on top of the dichroic filter stack  86 . In other embodiments, the absorptive filter layers  87  can be provided by doping the thin film layers or substrate. 
     Wavelength-variable absorptive materials that are useful for providing absorptive filter layers  87  include relatively narrow-band absorbing dyes and pigments, such as ABS  647  and ABS  658  available from Exciton of Dayton Ohio; Filtron A Series dye absorbers and Contrast Enhancement notch absorbers available from Gentex Corp. of Simpson, Pa., or other molecular chemistries. 
     Other classes of wavelength-variable absorptive materials that can be used in accordance with the present invention include metamaterials or resonant plasmonic structures. Metamaterials are structurally shaped nano-structures that can be tuned to absorb light, An example of such a material is described by Padilla in the article entitled “New metamaterial proves to be a ‘perfect’ absorber of light” (Science Daily, May 29, 2008). Similarly, plasmonic absorbers have been created by use of typically reflective metals structured at sub-wavelength scales such those described by Aydin et al. in the article “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers” (Nature Communications, pp. 1-7, Nov. 1, 2011). 
     Still other absorber structures can be utilized such as photonic crystals where photonic crystals are utilized to guide light through multiple passes through absorption materials. For example, Zhou et al. describe absorption enhancements using photonic crystals in the article “Photonic crystal enhanced light-trapping in thin film solar cells” (Journal of Applied Physics, Vol. 103, paper 093102, 2008). 
     Still another approach to spectral filtration uses naturally derived nanoparticle absorbers such as colored films created by dipping a substrate in a solution of viruses or protein molecules. In some embodiments, the virus or protein molecules can be self-assembling. One example of absorbers using nonparticle virus molecules has been developed by Seung-Wak Lee at University of California, Berkeley and is described in an article entitled “No paint needed! Virus patterns produce dazzling colour” (New Scientist, p. 18, Oct. 29, 2011). 
     In some embodiments, a plurality of absorptive filter layers  87  can be used. For example, individual absorptive filter layers  87  can be provided for to selectively absorb light in each of the spectral bands R 2 , G 2 , B 2  that comprise the left-eye incident light  196 L. Alternately, a single absorptive filter layer  87  can be used to selectively absorb light in portions of a plurality of the spectral bands R 2 , G 2 , B 2 . 
     In the configuration of  FIG. 17A , the dichroic filter stack  86  is positioned over the front surface  66 F of the substrate  88 , and the absorptive filter layer  87  is positioned over the dichroic filter stack  86 . In order to achieve the stated advantages the absorptive filter layer  87  must be positioned between the light source (e.g., the display surface  72 ) and the dichroic filter stack  86  so that the unwanted light is absorbed before it can be reflected by the dichroic filter stack  86 . As illustrated in  FIG. 17B , the absorptive filter layer  87  and the dichroic filter stack  86  can alternatively be positioned in other arrangements as long as they maintain the proper relative positions. In this example, the dichroic filter stack  86  is positioned over the rear surface  66 R while the absorptive filter layer  87  is positioned over the front surface  66 F. 
     The arrangements of  FIGS. 17A and 17B  will be ineffective to prevent the reflection of left-eye incident light  186 L that is incident on the rear surface  66 R of the right-eye filter  76 R (e.g., after reflecting off of filter glasses worn by other viewers). This light will interact with the dichroic filter stack  86  before it reaches the absorptive filter layer  87 , and will therefore still be reflected as left-eye reflected light  187 L. 
       FIGS. 17C and 17D  show arrangements that are analogous to  FIGS. 17A and 17B , respectively, where a second absorptive filter layer  86  is positioned over the rear surface  66 R. In this way, both the left-eye incident light  196 L and the left-eye incident light  186 L will be substantially absorbed, although at the cost of a slightly lower transmittance for the right-eye incident light  196 R. In such embodiments, the right-eye filter  76 R should preferably absorb a majority of the left-eye incident light  186 L such that less than 50% of the left-eye incident light  186 L is reflected. Ideally, the right-eye filter  76 R should absorb a large majority (e.g., more than 90%) of the left-eye incident light  186 L. 
     In other embodiments, the layers can be distributed in other arrangements, or can be combined with additional layers. For example, additional protective layers can be positioned over one or both of the dichroic filter stack  86  or the absorptive filter layer  87  to provide scratch resistance or fade resistance. An anti-reflection coating can also be used to reduce first-surface reflections. In some embodiments, the anti-reflection coating can be formed with a plurality of thin film layers, which can optionally be included as part of the dichroic filter stack  86 . 
       FIG. 18  shows an example of a right-eye absorptive filter transmittance  172 R that can be used for the absorptive filter layer  87  ( FIG. 17A ), in combination with the right-eye dichroic filter transmittance  170 R of  FIG. 16A . If a dichroic filter stack  86  and an absorptive filter layer  87  with these spectral properties are used in the hybrid filter arrangement of  FIG. 17A or 17B , the combined transmittance of the hybrid filter T H (λ) can be calculated as follows:
 
 T   H (λ)≈ T   D (λ) T   A (λ)  (3)
 
where T D (λ) is the dichroic filter transmittance and T A (λ) is the absorptive filter transmittance. (This assumes that the substrate transmittance is approximately equal to 1.0.) The combined reflectance of the hybrid filter R H (λ) can be calculated as follows:
 
 R   H (λ)≈ R   D (λ)( T   A (λ)) 2 =(1− T   D (λ))( T   A (λ)) 2   (4)
 
where R D (λ) is the dichroic filter reflectance, which is equal to 1−T D (λ) by Eq. (2). This equation is based on the assumption that the reflected light is transmitted through the absorptive filter layer  87 , reflected by the dichroic filter stack  86 , and then transmitted through the absorptive filter layer  87  a second time. It makes the assumption that first surface reflectances can be neglected.
 
       FIG. 19A  shows a right-eye hybrid filter transmittance  173 R calculated from the spectral transmittances in  FIG. 18  using Eq. (3). The right-eye hybrid filter transmittance  173 R is superimposed on a set of right-eye spectral bands R 1 , G 1  and B 1  and a set of left-eye spectral bands R 2 , G 2  and B 2 . Comparing  FIG. 19A  to  FIG. 16A , it can be seen that the right-eye hybrid filter transmittance  173 R is quite similar to the right-eye dichroic filter transmittance  170 R. 
       FIG. 19B  is a graph showing the light that is transmitted through the right-eye filter  76 R according to the right-eye hybrid filter transmittance  173 R as a function of wavelength. In this example, the transmitted right-eye light  175 R includes about 81% of the incident light in the right-eye bands, which is a slight degradation relative to the dichroic-only configuration that was plotted in  FIG. 16B . However, the transmitted left-eye light  175 L includes only about 1% of the light in the left-eye spectral bands. This represents about a 3× reduction in the amount of cross-talk relative to the dichroic-only configuration. This reduction in cross-talk is an added benefit of the hybrid filter approach. 
       FIG. 19C  shows a right-eye hybrid filter reflectance  174 R calculated using Eq. (4). In comparison to  FIG. 16C , it can be seen that the reflectivity in the wavelength regions corresponding to the left-eye spectral bands R 2 , G 2 , B 2  is significantly reduced. 
       FIG. 19D  is a graph showing the light that is reflected from the right-eye filter  76 R according to the right-eye hybrid filter reflectance  174 R as a function of wavelength. In this example, the reflected right-eye light  176 R includes less than 7% of the incident light in the right-eye bands, and the reflected left-eye light  176 L includes about 8% of the light in the left-eye spectral bands. This represents more than a 12× reduction in the amount of reflected left-eye light relative to the dichroic-only solution. This will provide a significant reduction in the amount of flare light that results from reflections off the filter glasses  74 . 
     It should be noted that absorptive filter layers  87  can be used to supplement the spectral separation provided by dichroic filter stacks  86  to form hybrid filters whether the stereoscopic imaging system uses interleaved spectral bands (as in the examples discussed relative to  FIG. 12A  and  FIGS. 19A-19D ) or non-interleaved spectral bands (such as the configuration shown in  FIG. 12B ). A general design principle is that the absorptive filter layers  87  used with the filter for a particular eye should absorb more of the spectral bands associated with the opposite eye image and less of the spectral bands associated with the image-forming light for the particular eye. 
     As has been noted, reflection of “flare light” that is reflected from filter glasses  74  worn by other viewers can reduce the contrast of the projected image seen by an observer and can add visual noise that detracts from the stereoscopic viewing experience. To illustrate this,  FIGS. 20A and 20B  illustrate a scenario where some incoming light  230  from display surface  72  is reflected from filter glasses  74  worn by a rear observer  160  and is directed as reflected light  235  onto the rear side of filter glasses  74  worn by a front observer  162 . As was discussed relative to  FIGS. 15A and 15B , some of this light can be reflected back into the eyes of front observer  162 . This effect can be more or less pronounced, depending on whether or not the heads of rear observer  160  and front observer  162  are at the same height as shown in  FIG. 20A , or at different heights as shown in  FIG. 20B . With typical seating arrangements, the head of front observer  162  is at a lower elevation than that of the rear observer  160  as shown in the  FIG. 20B  configuration. In the worst case scenario, the filter glasses  74  use dichroic filters that reflect most or all of the light from the spectral bands that are not transmitted to the eyes of the rear observer  160 . When front observer  162  is directly in front and relatively level with rear observer  160 , those functionally identical filter glasses  74  on front observer  162  will now highly reflect the wrong spectral content from any light that happens to strike the back surface of the filters, substantially degrading stereoscopic image quality and contrast. Even when the reflected light of filter glasses  74  does not directly land on the back side of the filter glasses  74  for the front observer  162 , some of that light will return to the projection screen further decreasing image quality and contrast for all viewers. While curved filters spreads this light out more than flat filters, much of the light will still land on the screen. 
       FIGS. 21A and 21B  illustrate filter glasses  200  having a modified design to mitigate the degradation of image quality due to light reflected from the right-eye filter  76 R and the left-eye filter  76 L according to an embodiment of the present invention. The side view of  FIG. 21A  and perspective view of  FIG. 21B  show filter glasses  200  that are configured to reduce image degradation due to back reflection by redirecting reflected light at a skewed angle, upwards with respect to the viewer position, so that it is directed away from the display surface  72  ( FIG. 20A ) other viewers sitting in front of the wearer of the filter glasses  200 . A frame  210  including rims  215  dispose the right-eye filter  76 R and the left-eye filter  76 L at a tilt angle θ relative to vertical, so that reflected light is directed upwards and away from other viewers seated ahead of the wearer of the filter glasses  200 . 
     For typical viewing environments, the tilt angle θ is preferably between about 5 to 20 degrees. A larger tilt angle may be preferred for embodiments where there is a very short distance between the wearer of the filter glasses  200  and the display surface  72 . An extreme example would be an observer sitting approximately one screen height away from the display surface at a vertical position approximately ¼ of a screen height from the bottom. In this case, light from the bottom of the display surface  72  reaches the filter glasses  200  from a direction about 14 degrees below the horizontal and light from the top of the display surface  72  reaches the filter glasses from a direction about 37 degrees above the horizontal. Thus the filters would need to be tipped up to a tilt angle of approximately 37 degrees in order for all of reflected light to be directed over the top of the display surface  72 . This level of angular tilt may not be practical from an aesthetics point of view. Most audience viewers prefer to be at center level or higher with the screen suggesting a maximum tip of 26 degree would be more practical. Significant benefits can be realized even when the tilt angle θ is less than this level since the light from all viewers returning to the screen is additive, therefore any reduction in the stray light provides a corresponding image quality improvement. 
     For cases where the left-eye filter  76 L and the right-eye filter  76 R include dichroic filter stacks, the tilting of the filters will generally cause the edge transitions in the spectral transmittance curves to shift as has been discussed earlier. In this case, it may be desirable to adjust the dichroic filter designs to provide the desired spectral transmittance characteristics. 
     In some embodiments, the frame  210  include optional opaque side shields  220  that block at least some of the stray light from reaching the rear surface of the left-eye filter  76 L and the right-eye filter  76 R. In a preferred embodiment, the rims  215  are made using a moldable material and the tilt angle θ is provided by appropriately molding the shape of the rims  215 . In an alternate embodiment illustrated in  FIG. 21C , the frame  210  include a hinge mechanism  225  that enables the rims  215  to be pivoted to provide a variable tilt angle θ. In this way, the tilt angle can be adjusted as appropriate for the viewing environment. 
     In the illustrated embodiments, the front and back surfaces of the left-eye filter  76 L and the right-eye filter  76 R are shown to be substantially planar and behave as flat plates. In other embodiments, the left-eye filter  76 L and the right-eye filter  76 R may be provided as curved plates with spherical or aspherical curved surfaces. In this case, the tilt angle is defined relative to a best fit plane through the curved surfaces. 
       FIGS. 22 and 23  shows filter glasses  200  worn by rear observer  160  and front observer  162 , according to an embodiment of the present invention. The rims  215  in the filter glasses  200  are arranged to orient the left-eye filter  76 L and the right-eye filter  76 R at an appropriate tilt angle so that reflected light  235  produced when incoming light  230  from the display surface  72  (not shown in  FIG. 22 ) is reflected from the left-eye filter  76 L and the right-eye filter  76 R of the filter glasses  200  worn by the rear observer  160  is directed over the heads of other observers (e.g., front observer  162 ). As a result, the reflected light  235  from the filter glasses  200  for the rear observer  160  is less likely to negatively impact the image quality seen by the front observer  162 . Preferably, the reflected light  235  is directed over the top of the display surface  72  so that it does not add flare light to the displayed image. 
     In an alternate embodiment of the present invention, there is provided a stereoscopic imaging apparatus that uses one or more tunable light sources to provide the different spectral bands in at least one of the color channels. Referring to  FIG. 24 , there is shown a schematic diagram of a red imaging channel  140   r  that has a red tunable light emitter  152   r , such as a tunable narrow-band, solid-state laser, for example. The red tunable light emitter  152   r  can selectively provide light in at least two different states. In the first state, the red tunable light emitter  152   r  provides light in the R 1  spectral band that is used to form the right-eye image, and in the second state the red tunable light emitter  152   r  provides light in the R 2  spectral bands that is used to form the left-eye image. As shown in timing chart  154 , the controller system  80  is adapted to control the red tunable light emitter  152   r  so that it alternately emits light in the R 1  and R 2  spectral bands according to a defined temporal sequence. In order to switch without being detectable to the viewer, the red tunable light emitter  152   r  must be capable of switching between the color states at a high rate, such as at about 60 Hz, for example. 
     The emitted light is conditioned by optical components (e.g, uniformizing optics  44  and one or more lenses  48 ) to illuminate spatial light modulator  60 . The pixels of spatial light modulator  60  are synchronously controlled by the controller system  80  according to image data for the corresponding right-eye or left-eye image. The resulting image is then projected to display surface  72  using projection optics  70  as described previously. 
     As illustrated in  FIG. 25 , the red tunable light emitter  152   r  of  FIG. 24  can be combined with a green tunable light emitter  152   g  and a blue tunable light emitter  152   b  that provide right-eye and left-eye image content in the blue and green color channels, respectively, to form color stereoscopic imaging system  150 , having red imaging channel  140   r , green imaging channel  140   g  and blue imaging channel  140   b . Each tunable light emitter emits light in at least two different spectral bands, typically of the same primary color (red, green, or blue). In this configuration, the projection optics  70  can include a beam combining system, such as the dichroic combiner  82  described with reference to  FIG. 9 , for example. 
     It can be appreciated that the stereoscopic imaging system  150  which uses tunable light emitters has advantages over other types of wavelength-based stereoscopic imaging systems that require multiple light sources or require multiple banks of filters for filtering light from a single polychromatic (white) light source. For example, the configuration described relative to  FIG. 5 , requires six different light emitters rather than the three light emitters of  FIG. 25 . Furthermore, the configuration of  FIG. 5  also requires three beam scanners  50  to switch between the two color states. 
     Another useful feature of some types of tunable light emitters is that they can be used to provide some amount of wavelength “jitter” about a central wavelength either through creation of multiple simultaneous modes, high frequency mode hopping or higher frequency tuning around the central spectral band, so that the emitted light varies at each moment with respect to wavelength. In this case, when the controller system  80  controls the tunable light emitters to operate in their first state the tunable light emitters can be configured to sequentially emit light having two or more different peak wavelengths within a first spectral band, and when the tunable light emitters to operate in their second state the tunable light emitters can be configured to sequentially emit light having two or more different peak wavelengths within a second spectral band that is spectrally adjacent to the first spectral band. Randomness of the spectral output within the wavelength range of the spectral band reduces undesirable effects of highly coherent light, such as speckle, common to many types of laser projection systems. 
     The red, green and blue tunable light emitters  152   r ,  152   g  and  152   b  of  FIG. 25  can be any type of tunable light source known in the art. In some embodiments, the tunable light emitters are solid-state light sources, such as tunable light-emitting diodes (LEDs) or tunable lasers. Tunable lasers change emitted output wavelength using one of a number of different possible mechanisms. One such approach involves the control of an optical cavity using micro-electromechanical systems (MEMS) devices capable of rapidly switching between mechanical states as described in the article “760 kHz OCT scanning possible with MEMS-tunable VCSEL” by Overton (Laser Focus World, p. 15, July 2011). In the described device, an electrostatically actuated dielectric mirror is suspended over the top of a laser structure in order to adjust the wavelength. 
     An alternate approach to providing a suitable tunable laser is to use a bistable laser. Feng et al., in an article entitled “Wavelength bistability and switching in two-section quantum-dot diode lasers” (IEEE Journal of Quantum Electronics, Vol. 46, pp. 951-958, 2010), disclose the use of two-section mode-locked quantum dot lasers that switch in discrete integer multiples in 50 picoseconds. The operation of this device is based on the interplay of the cross-saturation and self saturation properties in gain and absorber and the quantum-confined Stark effect in absorber. This type of laser can be easily tuned by varying a current injection level or a voltage level. 
     A type of tunable LED that can be used in accordance with the present invention is described by Hong, et al. in an article entitled “Visible-Color-Tunable Light-Emitting Diodes,” Advanced Materials, Vol. 23, pp. 3284-3288 (2011). These devices are based on gallium nitride nanorods coated with layers of indium gallium nitride to form quantum wells. The thicknesses of the layers vary naturally when they are produced and, by changing the applied voltage, current can be pushed through different layers, thereby providing different colors of emitted light. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, the light emitters used in the various embodiments can be of any type known in the art, and can include arrays of lasers or other emissive devices combined onto the same optical axis using prisms or other combining optics. 
     Optical systems, typically represented by a lens or a block in the schematic drawings provided, could include any number of optical components needed to guide and condition the illumination or imaged light. 
     The spatial light modulator  60  in each color channel can be any of a number of different types of spatial light modulator, such as a liquid crystal array or a Digital Light Processor available from Texas Instruments, Dallas, Tex. (a type of digital micro-mirror array) for example. 
     In some embodiments, a color channel can have two spatial light modulators, one corresponding to each eye of the observer, so that there are six spatial light modulators in a stereoscopic digital projection system. Alternately, each color channel can have a single spatial light modulator as in  FIG. 5 , shared between left-eye and right-eye image content using color scrolling or some other resource-sharing method, such as alternately activating the different spectral bands according to a timing pattern. 
     In some embodiments, additional filtering can be provided in the illumination path to attenuate the spectral content from one or more of the light emitters so that the adjacent spectral bands are substantially non-overlapping. 
     While the invention has been described with reference to a stereoscopic digital projection system which projects images onto a display screen, it will be obvious to one skilled in the art that the invention can also be applied to other types of stereoscopic digital display systems that do not involve projection. For example, stereoscopic digital soft-copy displays can be used to directly form the left-eye and right-eye stereoscopic images on a display surface. The soft-copy display can use any type of display technology known in the art such as LED displays and LCD displays. 
     PARTS LIST 
     
         
           12  light emitter 
           12 L left-eye light emitters 
           12 R right-eye light emitters 
           18  optics 
           20  spatial light modulator 
           28   a  image frame 
           28   b  image frame 
           28   c  image frame 
           28   d  image frame 
           28   e  image frame 
           30  light redirecting prism 
           32  image region 
           34   b  band of light 
           34   g  band of light 
           34   r  band of light 
           35   b  band of light 
           35   g  band of light 
           35   r  band of light 
           36   a  band of light 
           36   b  band of light 
           38  image frame 
           38   a  image frame 
           38   b  image frame 
           38   c  image frame 
           38   d  image frame 
           38   e  image frame 
           40   r  red color channel 
           40   g  green color channel 
           40   b  blue color channel 
           41 L left-eye image forming system 
           41 R right-eye image forming system 
           42   a  light source 
           42   b  light source 
           43 L light source 
           43 R light source 
           44  uniformizing optics 
           46  beam combiner 
           48  lens 
           50  beam scanner 
           52  prism 
           54  lenslet array 
           56  block 
           58  integrating bar 
           60  spatial light modulator 
           60 L spatial light modulator 
           60 R spatial light modulator 
           62  frame 
           66 F front surface 
           66 R rear surface 
           68  dichroic surface 
           70  projection optics 
           72  display surface 
           74  filter glasses 
           75  overlap region 
           76 L left-eye filter 
           76 R right-eye filter 
           77 B bandpass filter transmission band 
           77 E edge filter transmission band 
           78 L left-eye filter transmittance 
           78 R right-eye filter transmittance 
           79 L left-eye filter transmittance 
           79 R right-eye filter transmittance 
           80  controller system 
           82  dichroic combiner 
           84  dichroic surface 
           86  dichroic filter stack 
           87  absorptive filter layer 
           88  substrate 
           90  illumination optics 
           92  beam scanning optics 
           94  first stage 
           96  second stage 
           100  stereoscopic digital projection system 
           110  stereoscopic digital projection system 
           120  projector apparatus 
           130  graph 
           135  graph 
           140   b  blue imaging channel 
           140   g  green imaging channel 
           140   r  red imaging channel 
           150  stereoscopic imaging system 
           152   b  blue tunable light emitter 
           152   g  green tunable light emitter 
           152   r  red tunable light emitter 
           154  timing chart 
           160  rear observer 
           162  front observer 
           170 R right-eye dichroic filter transmittance 
           171 R right-eye dichroic filter reflectance 
           172 R right-eye absorptive filter transmittance 
           173 R right-eye hybrid filter transmittance 
           174 R right-eye hybrid filter reflectance 
           175 R transmitted right-eye light 
           175 L transmitted left-eye light 
           176 R reflected right-eye light 
           176 L reflected left-eye light 
           186 L left-eye incident light 
           187 L left-eye reflected light 
           194  right eye 
           196 R right-eye incident light 
           196 L left-eye incident light 
           197 L left-eye reflected light 
           198 R right-eye transmitted light 
           200  filter glasses 
           210  frame 
           215  rims 
           220  side shield 
           225  hinge mechanism 
           230  incoming light 
           235  reflected light 
         A 1  area 
         A 2  area 
         B spectral band 
         B 1  spectral band 
         B 2  spectral band 
         G spectral band 
         G 1  spectral band 
         G 2  spectral band 
         O axis 
         R spectral band 
         R 1  spectral band 
         R 2  spectral band 
         S wavelength separation 
         θ tilt angle 
         θ 1  angle 
         θ 2  angle