An autostereoscopic optical apparatus (10) for viewing a stereoscopic virtual image comprises a left image to be viewed by an observer (12) at a left viewing pupil (14l) and a right image to be viewed by the observer at a right viewing pupil (14r). The apparatus comprises a left pupil imaging system for forming the left image. A right pupil imaging system forms the right image.

FIELD OF THE INVENTION

This invention generally relates to display apparatus and more particularly relates to an autostereoscopic display apparatus providing a wide field of view, large viewing pupils, and high brightness.

BACKGROUND OF THE INVENTION

The potential value of autostereoscopic display systems is well appreciated for a broad range of data visualization uses and for a wide range of applications that include entertainment, engineering, medical, government, security, and simulation fields. Autostereoscopic display systems include “immersion” systems, intended to provide a realistic viewing experience for an observer by visually surrounding the observer with a three-dimensional (3-D) image having a very wide field of view. As differentiated from the larger group of stereoscopic displays that include it, the autostereoscopic display is characterized by the absence of any requirement for a wearable item of any type, such as goggles, headgear, or special polarized or filter glasses, for example. That is, an autostereoscopic display attempts to provide “natural” viewing conditions for an observer.

An article entitled “3-D displays: A review of current technologies” by Siegmund Pastoor and Mathias Wopking in Displays 17 (1997) surveys various approaches that have been applied for obtaining autostereoscopic display images for one or more viewers. Among the many techniques described in the Pastoor et al. article are electro-holography, volumetric display, direction-multiplexed, diffraction-based, refraction-based, and reflection-based methods for autostereoscopic presentation. While each of these approaches may have merit in one or more specific applications, these approaches have a number of characteristic shortcomings that constrain usability and overall performance. As a group, these conventional approaches have been adapted for autostereoscopic displays, but allow only a narrow field of view and provide limited brightness and poor contrast. Imaging systems employing time-based or spatial multiplexing require complex image processing algorithms in order to provide left- and right-eye images in the proper sequence or with the necessary spatial separation. Time-based multiplexing introduces the inherent problem of image flicker. Spatial multiplexing generally produces an image having reduced resolution. Combining these multiplexing techniques, as is disclosed in European Patent Application EP 0 764 869 A2 to Ezra et al., may provide an increased number of views, but does not compensate for these inherent drawbacks. A number of multiplexing technologies also require tracking of view eye position and compensation for changes in head position. As a further disadvantage, each of the imaging technologies described in the Pastoor et al. article present the viewer with a real image, rather than with a virtual image.

In an article entitled “An Autostereoscopic Display Providing Comfortable Viewing Conditions and a High Degree of Telepresence” by Klaus Hopf inIEEE Transactions on Circuits and Systems for Video Technology, Vol. 10, No. 3, April, 2000, a teleconferencing system employing a spherical mirror is disclosed, recommended particularly for its value in reducing chromatic aberration. However, the optical system disclosed in this article is subject to field curvature constraints, limiting its field of view. Notably, the system described in the Hopf article provides a virtual image; however, due to substantial field curvature, the total field of view of such a system is limited to less than about 15 degrees. While such a narrow field of view may be acceptable for videoconferencing applications, this level of performance would not be useful for a desktop display system.

Virtual imaging provides an advantageous alternative to real image projection, as is used in the apparatus described in the Pastoor article and in EP 0 764 869 A2. In contrast to conventional projection methods for forming a real image, a virtual image is not formed on a display surface. That is, if a display surface were positioned at the perceived location of a virtual image, no image would be formed on that surface. Virtual image display has a number of inherent advantages, as is outlined in U.S. Pat. No. 5,625,372 (Hildebrand et al.) As one significant advantage for stereoscopic viewing, the size of a virtual image is not limited by the size or location of a display surface. Additionally, the source object for a virtual image may be small; a magnifying glass, as a simple example, provides a virtual image of its object. Thus, it can be seen that, in comparison with prior art systems that project a real image, a more realistic viewing experience can be provided by forming a virtual image that is disposed to appear some distance away. Providing a virtual image also obviates any need to compensate for screen artifacts, as may be necessary when projecting a real image.

It is generally recognized that, in order to minimize vergence/accommodation effects, a 3-D viewing system should display its pair of stereoscopic images, whether real or virtual, at a relatively large distance from the observer. For real images, this means that a large display screen must be employed, preferably placed a good distance from the observer. For virtual images, however, a relatively small curved mirror can be used as is disclosed in U.S. Pat. No. 5,908,300 (Walker et al.). The curved mirror acts as a collimator, forming a virtual image at a relatively large distance from the observer.

From an optical perspective, it can be seen that there would be advantages to autostereoscopic design using pupil imaging. A system designed for pupil imaging must meet a fairly demanding set of requirements, including the following:(a) form separate images at left and right pupils correspondingly;(b) provide the most natural viewing conditions, eliminating any need for goggles or special headgear;(c) present the largest possible pupils to the observer, while limiting crosstalk between left and right views;(d) allow reasonable freedom of movement;(e) provide an ultra-wide field of view; and(f) provide sufficient resolution for realistic imaging, with high brightness and contrast.

It is recognized in the optical arts that each of these requirements, by itself, can be difficult to achieve. An ideal autostereoscopic imaging system must meet the challenge of each of these requirements to provide a more fully satisfactory and realistic viewing experience. Moreover, additional physical constraints presented by the need for a system to have small footprint, and dimensional constraints for interocular separation must be considered, so that separate images directed to each eye can be advantageously spaced and correctly separated for viewing. It is instructive to note that interocular distance constraints limit the ability to achieve larger pupil diameter at a given ultrawide field by simply scaling the projection lens.

Clearly, the value and realistic quality of the viewing experience provided by an autostereoscopic display system using pupil imaging is enhanced by presenting the stereo 3-D image with a wide field of view and large exit pupil. For fully satisfactory 3-D viewing, such a system should provide separate, high-resolution images to right and left eyes. To create a realistic illusion of depth and width of field, the observer should be presented with a virtual image that requires the viewer to focus at some distance.

It is well known that conflict between depth cues associated with vergence and accommodation can adversely impact the viewing experience. Vergence refers to the degree at which the observer's eyes must be crossed in order to fuse the separate images of an object within the field of view. Vergence decreases, then vanishes as viewed objects become more distant. Accommodation refers to the requirement that the eye lens of the observer change shape to maintain retinal focus for the object of interest. It is known that there can be a temporary degradation of a viewer's depth perception when the viewer is exposed for a period of time to mismatched depth cues for vergence and accommodation. It is also known that this negative effect on depth perception can be mitigated when the accommodation cues correspond to distant image position.

There are also other basic optical limitations for immersion systems that must be addressed with any type of optical projection that provides a wide field of view. An important limitation is imposed by the Lagrange invariant. A product of the size of the emissive device and the numerical aperture, the Lagrange invariant determines output brightness and is an important consideration for matching the output of one optical system with the input of another. Any imaging system conforms to the Lagrange invariant, whereby the product of pupil size and semi-field angle is equal to the product of the image size and the numerical aperture. An invariant that applies throughout the optical system, the Lagrange invariant can be a limitation when using, as an image generator, a relatively small spatial light modulator or similar pixel array which operate over a relatively small numerical aperture, since the Lagrange value associated with the device is small. In practical terms, the larger the size of the image source, the easier it is to form an image having a wide field of view and large pupil.

In response to the need for more realistic autostereoscopic display solutions offering a wide field of view, commonly assigned U.S. Pat. No. 6,416,181 (Kessler et al.), incorporated herein by reference and referred to as the '181 patent, discloses an autostereoscopic imaging system using pupil imaging to display collimated left and right virtual images to a viewer. In the '181 disclosure, a curved mirror is employed in combination with an imaging source, a curved diffusive surface, a ball lens assembly, and a beamsplitter, for providing the virtual image for left and right viewing pupils. Overall, the monocentric optical apparatus of the '181 disclosure provides autostereoscopic imaging with large viewing pupils, a very wide field of view, and minimal aberration.

While the autostereoscopic system of the '181 disclosure provides a high-performance immersive display, there is still room for improved embodiments. For example, while the '181 system provides a large viewing pupil, there would be advantages in even further increases in pupil size. At the same time, however, some amount of correction may be needed, since eye movement within a larger viewing pupil can cause some amount of “swim” effect, in which pixels appear to shift position as the eye moves within the viewing pupil. In addition, as is well known in the imaging arts, some amount of spherical aberration is generally inherent in any optical system that employs a curved mirror for image collimation.

Generating its source image on a small spatial light modulator device, the '181 system overcomes inherent Lagrange invariant conditions by forming an intermediate curved image for projection on a curved diffusive surface. Use of the diffuser with the '181 apparatus is necessary because the image-forming device, typically a reflective LCD or other spatial light modulator, is a relatively small emissive device, measuring typically no more than about 1 inch square. At the same time, however, the use of a diffusive surface effectively reduces overall brightness, introduces some level of graininess to the image, and limits the achievable contrast.

There are other minor drawbacks to autostereoscopic displays that use the design approach of the '181 disclosure. For example, slight “keystoning” aberrations are detectable in a system using the '181 design approach, due to the use of a single curved mirror; moreover, this effect can be compounded by right and left images exhibiting keystoning in opposite orientations with respect to the final image. While spherical lenses such as the ball lenses of the '181 disclosure have overall advantages for maximizing field of view and for minimizing some types of imaging aberration, there are some inherent disadvantages to the use of highly spherical optics, requiring compensation for chromatic effects for example. Curved images can be produced in a number of ways using more conventional optics, which, while not providing some of the advantages of ball lenses, might provide less expensive options for forming intermediate images in an autostereoscopic system.

Thus, it can be seen that there is a need for an improved autostereoscopic imaging apparatus that provides improved brightness, enhanced viewing pupil dimensions, reduced image aberrations, and higher resolution.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an autostereoscopic display device having improved viewing pupil size, brightness, and resolution, with reduced optical aberrations. With this object in mind, the present invention provides an autostereoscopic optical apparatus for viewing a stereoscopic virtual image comprising a left image to be viewed by an observer at a left viewing pupil and a right image to be viewed by the observer at a right viewing pupil, the apparatus comprising:

It is a feature of the present invention that it provides a completely specular autostereoscopic imaging display apparatus, without the need for curved diffusive surfaces. This allows image brightness to be optimized and allows improved contrast over earlier design solutions.

It is an advantage of the present invention that it uses a larger imaging display than previous solutions, relaxing Lagrange invariant constraints on available luminance.

It is a further advantage of the present invention that it provides an improved viewing pupil size when compared with earlier solutions.

It is a further advantage of the present invention that it provides a compact autostereoscopic display system providing a virtual image.

DETAILED DESCRIPTION OF THE INVENTION

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.

For the purposes of the present application, a curved image is an image for which best focus lies in a shape that is substantially spherical. The optical path is simplest when curved images are themselves spherically curved. By forming and using curved intermediate images, for example, rather than flat, planar images, the optics of the present invention take advantage of various symmetrical arrangements and relationships that are favorable for pupil imaging using virtual images, as is described in this section. Curved intermediate images can be formed using fully spherical lenses or using highly spherical lens segments as well as using more conventional image projection optics.

Similarly, for reasons that become apparent upon reading this detailed description, a curved mirror, as described in this application, is preferably spherical, having a single center of curvature.

In the prior art autostereoscopic projection apparatus10described in U.S. Pat. No. 6,416,181 and as shown inFIG. 1, a curved mirror24is employed, in combination with a beamsplitter16to provide an autostereoscopic virtual image to a viewer12at left and right viewing pupils14land14r. For both left and right viewing pupils14land14r, a corresponding image generation system70land70rprovides an initial intermediate curved image that is then projected through a corresponding left or right ball lens assembly30l,30rin order to form an intermediate curved image80at a focal surface of curved mirror24. AsFIG. 1shows, the left and right optical paths cross between beamsplitter16and curved mirror24, due to the nature of imaging using curved mirror24.

Referring toFIG. 2, there is shown, extracted from the more detailed prior art description of the '181 disclosure noted in the background section above, a portion of an image generation system70for providing intermediate curved image80for projection in a stereoscopic projection system82for one viewing pupil14. Here, an image generator74provides a source image from a flat surface, such as from a reflective LCD. A relay lens54directs light from image generator74onto a diffusing element32, so that a curved intermediate image76is formed on a diffusive surface40. Ball lens assembly30, cooperating with beamsplitter16, projects curved intermediate image76toward a front focal surface22of curved mirror24to form intermediate curved image80. Curved mirror24then provides a virtual image of intermediate curved image80for a viewing pupil14.

Forming a Curved Intermediate Image

To eliminate the need for diffusing element32as was required using the approach of the '181 disclosure, the present invention provides an alternate method for forming a curved intermediate image using a specular optical system. Referring toFIG. 3, an intermediate image90is formed by a curved mirror92, which is generally spherical according to the present invention. Image-bearing light from image source94is directed toward curved mirror92through an aperture stop location96, which defines the angle of light from image source94available for forming intermediate image90. Aperture stop location96is at the center of curvature Csof curved mirror92. With aperture stop location96centered at mirror center of curvature Cs, the central rays that pass through Csare reflected back toward this same point. As the traced light rays ofFIG. 3show, curved intermediate image90is typically formed between mirror center of curvature Csand the mirror focal point f. Curved intermediate image90has an image center of curvature Cithat is in a different location from mirror center of curvature Cs. As the distance D between image source94and curved mirror92increases, the respective centers of curvature Ciand Csmove toward each other, coinciding as distance D approaches infinity. At this idealized infinite distance D between image source94and curved mirror92, curved intermediate image90would lie on focal point f and the center of curvature of the image Ciwould coincide with the center of curvature Csof curved mirror24(FIG.1).

Using the overall arrangement ofFIG. 3, image source94can be any of a number of image sources that emit light, such as a display LCD, a CRT, or an OLED or PLED device, for example. Two characteristics of image source94are particularly significant with this arrangement:(i) The image formed on image source94is substantially flat.

There may be some slight curvature to this image, such as would be provided by a CRT; however, the arrangement ofFIG. 3works well when image source94is flat and shows how intermediate image90is formed having a curvature using the methods of the present invention. Since most image display devices form a flat image, there is, then, no need for modification to off-the-shelf display components with this arrangement.(ii) Image source94can be several inches in diameter, and may be well over one foot in diameter. In a preferred embodiment, image source94is a large LCD display, such as a 17-inch display, for example. This is unlike other apparatus for forming a curved intermediate image, such as was shown with reference toFIG. 2, for which a microdisplay, such as a liquid crystal on silicon (LCOS) or DMD component, is used. Use of a larger display device for image source94has particular advantages for increasing both image resolution and brightness.

As described with reference to the Lagrange invariant in the background section above, brightness in an optical system is a product of the emissive area and the solid angle. By allowing image source94to have a large emissive area, the method of the present invention allows substantial brightness levels while, at the same time, allowing light angles to be relatively small. Small light angles are advantageous for maximizing image contrast and minimizing color shifting and other related image aberrations.

Separate image sources94are used for left and right eyes, respectively. Ideally, image source94for left image generation system701and image source94for right image generation system70rare well-matched for image size and color. CRT displays, however, may be at a disadvantage if used as image sources94. Color differences between CRTs may degrade stereoscopic imaging performance. Additionally, as a result of display ageing, CRT image areas may vary dimensionally, effectively causing left/right pixel misalignment. In contrast to CRT displays, LCD displays offer dimensional stability with stable pixel locations, ease of alignment, and simpler mounting.

Ideal Ball Lens Operation

Referring toFIG. 4a, there is shown the concentric arrangement and optical characteristics of ball lens assembly30for directing light from a curved image50. A central spherical lens46is disposed between meniscus lenses42and44. Spherical lens46and meniscus lenses42and44have indices of refraction and dispersion characteristics intended to minimize on-axis spherical and chromatic aberration, as is well known in the optical design arts. An aperture stop48defines a pupil106within ball lens assembly30. Aperture stop48need not be a physical stop, but may alternately be implemented using optical effects such as total internal reflection. In terms of the optics path, aperture stop48serves to define an entrance pupil and an exit pupil for ball lens assembly30.

In a preferred embodiment, meniscus lenses42and44are selected to reduce image aberration and to optimize image quality for the projected image projected. It must be noted that ball lens assembly30could comprise any number of arrangements of support lenses surrounding central spherical lens46. Surfaces of these support lenses, however many are employed, would share a common center of curvature with Cball, the center of curvature of central spherical lens46. Moreover, the refractive materials used for lens components of ball lens assembly30could be varied, within the scope of the present invention. For example, in addition to standard glass lenses, central spherical lens46could comprise a plastic, an oil or other liquid substance, or any other refractive material chosen for the requirements of the application. Meniscus lenses42and44, and any other additional support lenses in ball lens assembly30, could be made of glass, plastic, enclosed liquids, or other suitable refractive materials, all within the scope of the present invention. In its simplest embodiment, ball lens assembly30could simply comprise a single spherical lens46, without additional supporting refractive components.

In ideal operation, curved image50shares the same center of curvature Cballas ball lens assembly30. When arranged in this fashion, light from curved image50is imaged with low levels of aberration, as is represented in the light rays ofFIG. 4a.

The inherent advantages of a ball lens can be exploited using a modified design, such as using a hemisphere combined with a folding mirror, as is shown in the cross-sectional ray diagram ofFIG. 4b. InFIG. 4b, a hemispheric lens assembly60comprises at least a hemispheric central lens66and a reflective surface62along the meridional plane of the hemisphere. Optionally, one or more meniscus lenses42could also be part of hemispheric lens assembly60. Reflective surface62may be formed over the full surface of the meridional plane or may be formed only along a portion of this surface. As shown inFIG. 4b, hemispheric lens assembly60forms, from curved image50as its object, a curved image64, folding the optical path at the same time. This arrangement can have advantages, for example, where space for optical components is at a premium.

For the purposes of this disclosure, the term “ball lens segment” comprises both fully spherical ball lens assembly30, as shown inFIG. 4aand hemispheric lens assembly60as shown inFIG. 4b.

First Embodiment of Image Generation System

Referring toFIG. 5, there is shown a first embodiment of an image generation system100for forming a curved image10for projection according to the present invention, as disclosed in the commonly-assigned copending U.S. patent application Ser. No. 10/393,236. Curved image10serves the function of intermediate curved image80shown inFIGS. 1 and 2. As described with reference toFIG. 3, image source94provides image-bearing light to curved mirror92through aperture stop location96. Referring now toFIG. 5, a beamsplitter102is used to direct an intermediate image90′ so that it is concentric to ball lens assembly30, which could alternately be embodied as hemispheric lens assembly60, as was shown inFIG. 4b. Because the light is being directed by curved mirror92toward its center of curvature Cs, rather than towards the center of curvature Cballof ball lens assembly30, some portion of the light does not enter the pupil106of ball lens assembly30, thus causing vignetting. Overfilling pupil106of ball lens assembly30compensates for vignetting. Ball lens assembly30re-images intermediate curved image90′ to form curved image110. Beamsplitter102is disposed between the vertex V of curved mirror92and its center of curvature Csas shown in FIG.5.

It must be emphasized that curved mirror92serves as an image generation component that serves image generation system100for forming intermediate curved image110, as shown inFIGS. 3 and 5. This is to be distinguished from ball lens imaging curved mirror24(used later in the optical path, as shown in FIG.1and in subsequent figures) which, in conjunction with beamsplitter16, provides pupil imaging and forms the final virtual image observed by viewer12. Similarly, beamsplitter102serves image generation system100for forming curved image10and is to be distinguished from beamsplitter16(shown in FIG.1and in subsequent figures) which cooperates with curved mirror24to provide pupil imaging and form the virtual image.

Second Embodiment of Image Generation System

Referring toFIG. 6, there is shown an improved embodiment of image generation system100in which a field lens112is positioned along the output axis where intermediate image90is formed. By positioning field lens112at this location, intermediate image90is not substantially changed; however, light from intermediate image90is directed toward center of curvature Cballof ball lens assembly30. Once again, it is significant to observe that ball lens assembly30shares the same center of curvature Cballas intermediate image90, but that this is not identical to the center of curvature Csof curved mirror92or to the imaged center of curvature Cs′, towards which light from curved mirror92is directed. The function of field lens112is, then, to image Csonto Cballwithout substantially affecting the image quality of intermediate image90. By doing this, field lens112essentially redirects light in order to fill pupil106of ball lens assembly30without vignetting.

Common to telescopic, microscopic, and similar “tube” optical systems, field lenses are widely employed in the optical arts, placed at the image location of a first lens, where the image formed at that image location becomes the object of a second lens. In this way, field lens112improves the overall brightness and field of view of the optical system. Background information on field lens use and theory can be found, for example, inModern Optical Engineering, the Design of Optical Systems, by Warren J. Smith, McGraw-Hill, N.Y., pp. 212-213 and in other textbooks known in the optics field.

In one embodiment, surface S1of field lens112is concentric with mirror center of curvature Csand therefore does not deviate chief rays towards Cball. In such an embodiment, surface S2, not concentric with mirror center of curvature Cs, operates to bend chief rays toward Cball. Alternately, surface S2could be concentric with mirror center of curvature Cs, surface S1performing the operation of bending chief rays toward Cball. Embodiments with either surface S1or S2concentric with Csor Cballrepresent the most straightforward approaches to the design of field lens112; other designs could have neither surface S1nor S2concentric with mirror center of curvature Csor Cball, however, these designs could be more complex.

As was noted above with reference toFIG. 4b, the use of hemispheric ball lens assembly60may have advantages for simplifying the optical path. Referring toFIG. 7a, there is shown an alternative arrangement to that ofFIG. 6, using hemispheric ball lens assembly60with field lens112. In the arrangement ofFIG. 7a, field lens112again operates to image Csonto Cball., where Cballis optically the center of curvature of hemispheric ball lens assembly60.

Providing Advantages of Telecentric Light

Still referring toFIG. 7a, an optional focusing optical element98is employed for providing improved, uniform brightness across the field. Disposed against the surface of image source94, or very near this surface, focusing optical element98acts as a type of field lens for directing light emitted from image source94. Referring toFIG. 8, there is shown, in schematic form, the function of focusing optical element98, focusing the emitted light from image source94to its focal point foe, coincident with mirror center of curvature Cs, which is the entrance pupil of the image generation system. By doing this, focusing optical element98forces telecentricity for light emitted from image source94, thereby optimizing the brightness and contrast of the image provided to the optical system through aperture stop location96. As a result, curved image110has optimum brightness across the field. In one embodiment, focusing optical element98is a Fresnel lens. Among other devices that could be employed as focusing optical element98are holographic optical elements, diffraction optical elements, two-cylinder Fresnel lenses, or even a more conventional curved surface lens, for example.

Considerations for Beamsplitter102

As is shown inFIGS. 5 and 6, beamsplitter102must accept incident light over a range of angles, so that where beamsplitter102is made of glass, rays at extreme sides of the field effectively encounter different thicknesses of glass. For this reason, it can be seen that there are advantages to providing beamsplitter102having minimal thickness of glass or plastic. Thus, beamsplitter102may be a thin glass or thin plastic type or a pellicle type beam splitting device.

Referring toFIG. 7b, a wedge beamsplitter104can be used as an alternative. Wedge beamsplitter104has substrate thickness varied so that the cross-sectional profile of wedge beamsplitter104is wedge-shaped. The difference in substrate thickness of wedge beamsplitter104provides an alternative solution that helps to compensate for optical path length and angle differences across its surface. The use of wedge beamsplitter104within image generation system100has advantages, since its thicker substrate is mechanically less fragile than a pellicle or thin beamsplitter102.

Embodiment for Stereoscopic Viewing

Referring toFIG. 9, there is shown a perspective view of a left and a right eye image generation system100iand100rwithin autostereoscopic display apparatus10. The task of generating a curved image to be displayed to each eye uses the basic components ofFIGS. 6,7aand7b. Left and right image sources941and94r provide, through respective left and right aperture stop locations961and96r and reflected from left and right beamsplitter1021and102r, light from images to their respective left and right curved mirrors921and92r. Respective left and right curved intermediate images, through respective left and right beamsplitter102land102r, are formed near their respective left and right field lenses112land112r, which redirect light to their respective left and right ball lens segments130land130r. Left and right ball lens segments130land130rare basically hemispheric in the configuration ofFIG. 9, similar to hemispheric lens assembly60as shown inFIG. 4b. Left and right ball lens segments130land130rare provided with left and right reflective surfaces132land132rand perform the dual function of folding the light path and projecting the intermediate images towards curved mirror24. With the arrangement ofFIG. 9, each image generation system100land1100rcan produce the appropriate image intended for left and right viewing pupils14land14r, the viewing pupils typically range in size between 5 mm and 60 mm. The left and right viewing pupils14land14rare separated by the interocular distance, which typically ranges between 55 mm and 75 mm. Curved images generated by left and a right eye image generation systems100land100rform left and right intermediate curved images110land110rfor collimation using large curved mirror24to provide, by cooperation with beamsplitter16, virtual images at left and right viewing pupils14land14r, in the same manner as is disclosed in U.S. Pat. No. 6,416,181. Referring back toFIG. 1, curved image110ofFIG. 7aor7bcorresponds to intermediate curved image80inFIG. 1, for example.

Alternate Embodiment for Stereoscopic Viewing, Using Left and Right Mirrors

The embodiment ofFIG. 9showed the use of a single curved mirror24with beamsplitter16for forming both left and right virtual images. In the alternate embodiment ofFIG. 16, there is shown how autostereoscopic display apparatus10, using the same left and a right eye image generation systems100land100rdescribed with respect toFIG. 9, can employ separate left and a right curved mirrors,24land24r, one for each corresponding viewing pupil14land14r. The alternate arrangement ofFIG. 16is advantaged over the arrangement ofFIG. 9that uses a single curved mirror24because each left and right ball lens segment130land130rcan be positioned on-axis with respect to its corresponding curved mirror24l,24r. In the arrangement ofFIG. 9, on the other hand, left and night ball lens segments130land130rare offset slightly to either side of the optical axis of single curved mirror24. This causes a slight keystoning aberration in each optical path. Disadvantageously, keystoning effects for left and right images are in opposite directions, degrading the quality of the image at extreme edges of the field. To some extent, this effect can be corrected or mitigated electronically, by pre-distorting the image data.

Designs using left and right curved mirrors24land24r, such as shown inFIG. 16allow on-axis imaging, minimizing or eliminating keystoning effects. However, designs using multiple mirrors can be disadvantaged due to mechanical placement constraints; it is difficult to arrange both left and right curved mirrors24land24rand beamsplitter16without some obstruction and consequent reduction of field width. Another design consideration relates to the relative positioning of left and right ball lens segments130land130r. Using dual curved mirrors24land24r, positioning constraints-for both left and right ball lens segments130land130rcan be relaxed somewhat, easing space requirements for imaging support components and allowing the size of ball lens segment130components to be relatively larger, providing a larger viewing pupil14. By contrast, the embodiment ofFIG. 9requires that ball lens segments130land130rbe positioned closely together, but allows a more compact design at the same time.

The same general principles used for forming a virtual image with the prior art configuration of FIG.1and the configuration ofFIG. 9also apply to the dual mirror configuration ofFIG. 16, with necessary modifications for separate left and right image paths, as would be appreciated by those skilled in the imaging arts. For example, left and right intermediate curved images110land110rare formed at focal surfaces of left and right curved mirrors respectively in order to provide virtual imaging.

Alternate Embodiment Using Broad Range of Image Generation Systems100

While ball lens segments130land130rshown inFIGS. 9 and 16provide corresponding intermediate left and right curved images110land110rhaving optimal curvature, a broader range of image-forming optical systems could be used for this purpose. For example, lower cost conventional projection optics could be deployed to form intermediate left and right curved images110land110rhaving reasonably good curvature, with some compromises in imaging performance. Referring toFIG. 17, for example, there is shown an arrangement of components using more conventional projection optics for left and right image generation systems100land100r.

The ideal spatial relationships for pupil placement provided by left and right image generation systems100land100r, left and right curved mirrors24land24r, and beamsplitter16, as represented inFIG. 17, include the following:(i) the center, of curvature C24lof left curved mirror24lis optically coincident with the exit pupil of left image generation system100l;(ii) the center of curvature C24rof right curved mirror24ris optically coincident with the exit pupil of right image generation system100r;(iii) the center of curvature C24lof left curved mirror24land center of curvature C24rof right curved mirror24rare separated by the ocular distance between left and right viewer pupils14land14r;(iv) left viewer pupil14lis optically at the center of curvature C24lof left curved mirror24l;(v) right viewer pupil14ris optically at the center of curvature C24rof right curved mirror24r;(vi) left curved image110lis optically centered at left viewing pupil14land at the focal surface of left curved mirror24l; and(vii) right curved image110ris optically centered at right viewing pupil14rand at the focal surface of right curved mirror24r.

With this spatial arrangement of optical components, a real image of the exit pupil of left image generation system100land a virtual image of left curved image110lare formed at left viewing pupil14l. Correspondingly, a real image of the exit pupil of right image generation system100rand a virtual image of right curved image110rare formed at right viewing pupil14r.

It must be emphasized that the relationships listed in (i)-(vii) above are ideal spatial relationships; optimal pupil imaging is obtained when the requirements of (i)-(vii) are satisfied. In practice, some amount of tolerance error is acceptable, provided that viewing pupils14land14rare formed at suitable positions for the observer.

Correcting for Spherical Aberration

As was described with reference toFIG. 4a, ball lens assembly30unavoidably exhibits some amount of aberration, which is largely corrected using central lens46in combination with meniscus lenses42and44. Referring toFIG. 10, there is shown, for ball lens assembly30of a preferred embodiment, tangential and sagittal aberration curves150for a 20 mm pupil, showing a significant amount of correction.

However, as a result of residual spherical aberration due to higher order aberrations, the size of viewing pupils14l,14ris still somewhat limited. Due to this residual aberration, movement of the eyes of observer12within viewing pupils14l,14rcan cause some amount of image “swim”.

Spherical aberration is a recognized problem in optical systems that employ a concave mirror, such as astronomical telescopes for example. To compensate for this type of aberration, the Schmidt optical system, as described inModern Optical Engineering, the Design of Optical Systems, by Warren J. Smith (cited above), pp. 393-394, employs an aspheric corrector plate. In the Schmidt system, a thin, aspheric corrector plate is positioned at the center of curvature of the curved mirror.

Comparing aberration curves150in FIG.11andFIG. 12, the improvement using the Schmidt solution can be readily seen, for similar imaging conditions and a 32 mm pupil.FIG. 11corresponds to the optical arrangement ofFIG. 6, where there is no correction for spherical aberration due to residual higher order aberrations.FIG. 12corresponds to the optical arrangement of FIG.13.

FIG. 13shows a'side view of a simplified optical path for image generation system100using an aspheric corrector element140disposed near the center of curvature of curved mirror92. Light from image source94is directed through aspheric corrector element140, which is optically conjugate to the center of curvature of curved mirror92. The curved intermediate image formed by curved mirror92lies near field lens112, which directs the light to a ball lens segment130. Ball lens segment130creates curved image110for curved mirror24(not shown in FIG.13).

Placing aspheric corrector element140near the center of curvature of curved mirror92effectively images aspheric corrector element140into the pupil of ball lens segment130; that is, corrector element104and the pupil of ball lens segment130are optically conjugate. This allows aspheric corrector element140to provide effective correction across the full field of view. As a result, pupil size can be increased to 50 mm, with minimal aberration, as shown in aberration curve150of FIG.14. By way of example,FIG. 15shows the sag that is required of the aspheric surface for a corrector plate as aspheric corrector element140in a preferred embodiment.

In an alternate embodiment, aspheric corrector element140could be a compound lens that corrects chromatic as well as spherical aberration. Such an arrangement would require more complexity than the design of a single-component aspheric corrector element140, but would simplify the design requirements of ball lens segment130. For example, where a compound lens is used for aspheric corrector element140, it may be possible to use only a single element as ball lens segment130.

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 scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, field lens112can be more complex than is shown here, having different curvature, composition, or coatings. Image source94, a transmissive LCD device in one embodiment, can be any of a number of types of image source, including film, CRT, LCD, and digital imaging devices: Image source94could be an emissive array, such as an organic light emitting diode (OLED) array, for example. In order to take advantage of the benefits of monocentric imaging, curved mirror92will be substantially spherical in most embodiments; however, some slight shape modifications might be used, with the corresponding changes to supporting optics and to optional aspheric corrector element140. Either ball lens assembly30or hemispheric lens assembly60could serve as the ball lens segment for either or both left and right image generation systems. Separate left and right curved mirrors24could be used to improve the image quality of each viewing pupil14l,14r, reducing undesirable “keystoning” effects that can result from off off-axis positioning of left and right ball lens segments130l,130r. Curved mirror24could be fabricated as a highly reflective surface using a number of different materials.

Thus, what is provided is an apparatus and method for autostereoscopic image display having improved brightness, pupil size, and resolution.

Parts List