VIRTUAL IMAGE DISPLAY DEVICE

Abstract: A head-coupled display device (10) for use in presenting electronically generated visual images to a viewer. The device includes a headpiece (12) worn by a viewer, and an image display screen (20) mounted in the headpiece for presenting electronically generated images. A fresnel lens is positioned adjacent the screen for focusing images from the screen at a selected position for viewing as a virtual image. The lens has a set of concentric converting rings whose surface curvatures act to minimize rectilinear distortion of the virtual images perceived by the viewer over the entire field of view of the image.

Description: VIRTUAL IMAGE DISPLAY DEVICE 1. Field of the Invention The present invention relates to a virtual image display device. 2. Background of the Invention Virtual reality systems are designed to present a user with computer-generated visual, and optionally, auditory cues which allow the user to experience a com¬ puter-simulated reality. At the same time, the user may be equipped with a motion-responsive glove or body suit which permits user interaction with the virtual reality information being presented. For example, the user may be able to touch and "move" an object in the visual field, or experience a changing auditory cue as the image of an object in the visual field is moved with respect to the user's ear. One important application of virtual reality sys¬ tems is in flight simulators, where visual flight information is provided to a trainee-user. Another is for actual in-flight guidance, where visual cues pro¬ cessed from infrared or radar signals are provided to one of the pilot's eyes. Other applications are emer¬ ging, for example in architecture, medicine, engineer- ing, and entertainment, as real-time computer-graphics capabilities are developed. One desired type of virtual reality system employs a head-mounted display (HMD) designed to be worn by a user, and capable of generating visual images which are perceived as three-dimensional virtual images. Head- mounted displays for use in flight simulation systems have been proposed. For example, U.S. Patent No. 4,048,653 describes a head-coupled visual display system which includes a head-mounted unit worn by the observer, and a image projection system for projecting left-eye and right-eye images onto left- and right-eye screens in the head-mounted unit, via an optical train. More recently, the present inventors have described a HMD in which visual images are generated by liquid- crystal display (LCD) screens in a head-mounted unit, and viewed by the observer as three-dimensional virtual images. In general, it is desirable for an observer to experience a wide-angle virtual image, such that even the peripheral vision of the observer is engaged. One aspect in the design of a wide-angle visual system in an HMD is the requirement for a large-diameter lens having a relatively small focal length. Such lenses can be relatively bulky and expensive. Further, sphe¬ rical aberration effects associated with spherical glass lenses can produce pronounced rectilinear dis¬ tortion at the outer regions of the wide-angle image. Heretofore, although a variety of head-coupled image systems have been proposed, none of these has proven capable of achieving distortion-free wide-angle image presentation. 3. Summary of the Invention The invention includes, in one embodiment, a head- mounted display device for use in presenting electroni¬ cally generated visual images to a viewer. The device includes a headpiece adapted to be worn by viewer, and an image display screen mounted in the headpiece for presenting electronically generated images. A fresnel lens is positioned adjacent the screen for focusing images from the screen at a selected position for view- ing as a virtual image which is a selected distance from the viewer's eye. The lens has a set of concen¬ tric converging rings whose surface curvatures act to minimize rectilinear distortion of the virtual images ' perceived by the viewer over the entire field of view of the image. Preferably the device includes such a screen and lens assembly for both left and right eyes, for viewing stereoscopic images. For viewing images over a wide angle, e.g., 60° or more, the lens is preferably a compound lens includes a pair of fresnel lenses positioned front-to-back. The lens diameter is preferably 60 mm or greater in diame¬ ter, and the focal length is preferably 30-50 mm or less. In one preferred embodiment, the lens is about mid-distance between the display screen and the view¬ er's eye, in operative condition, and the virtual image produced by the lens is at least about 10 feet in front of the viewer's eye. Also in a preferred embodiment, the image display screen is a liquid crystal display screen having an array of pixels which make up clusters of RBG pixel groups. The device further includes a partially trans¬ lucent plate positioned adjacent said screen, effective to blur the distinction between the individual pixel elements in such as RBG pixel cluster. In a more general aspect, the invention includes an head- coupled image display device for use in view¬ ing an electronically generated images as a virtual image. The assembly includes an image display screen for presenting electronically generated images, and a fresnel lens for focusing images from the screen at a selected position for viewing by the viewer as a virtu¬ al image which is a selected distance from the viewer's eye. The lens has a set of concentric converging rings whose ring curvatures are designed to produce a desired light-refraction effect, such as reducing outer-field rectilinear distortion over a wide-angle view field. The screen and lens are carried on a head-coupled structure, such as a glasses frame, or a hand-held frame, for positioning the viewer's eye at the near conjugate of the lens. These and other objects and features of the inven¬ tion will become more fully apparent when the following detailed description of the invention is read in con- junction with the accompanying drawings. Brief Description of the Drawings Figure 1 is a perspective view of a head-mounted display device containing the display optics of the present invention; Figures 2 shows the display optics for one eye in the head-mounted device of Figure 1; Figure 3 shows a portion of the face of a liquid crystal display array in the display optics; Figure 4 shows the face of a fresnel lens in the display optics; Figure 5 is a ray diagram illustrating how the fresnel lens assembly illustrated in Figure 5 acts to reduce rectilinear image distortion ' at outer-field regions of the image screen; and Figure 6 illustrates how a colored image is produced by the display optics. Detailed Description of the Invention Figure 1 shows a head-mounted display (HMD) or display device 10 constructed according to one embodi¬ ment of the invention. The device generally includes a headpiece 12 designed to worn by a viewer, and an optical assembly 14 for producing wide-angle, electron- ically generated virtual images to each eye of the viewer, as described below. Also shown in the device, although not forming part of the present invention, is a pair of earphones, such as earphone 16, which are used in sending auditory cues, preferably linked to the observed visual images, to the user. The headpiece includes a frame 17 designed to fit on the viewer's head and a mask 18 which fits over the viewer's eye region. Although the structure used in coupling the optical assembly to a viewer's eye in the Figure 1 embodiment is a headgear which engages front, back, and sides of the user's head, other types of head-mounted structures, such as a glasses-type frame are contemplated. In a more general embodiment, the optical assembly is carried on a support structure, such as a desk- mounted support, or a hand-held support which functions to hold the optical assembly at a relatively fixed position with respect to the viewer's eyes, for viewing a virtual image produced by the assembly. For example, a desk-mounted support might include a base mounted on the desk, and a movable arm which is positionable to place the optical assembly, which is carried on the arm, in position for viewing. With reference to Figures 1-3, the optical assem- bly in the device includes an image display screen 20 mounted in the headpiece for presenting electronically generated images to the viewer. In the embodiment il¬ lustrated, the image display screen is produced by a pair of liquid crystal display (LCD) units 22, 24. According to standard construction, each LCD unit in¬ cludes a source of unpolarized light, a first polariz¬ er, a polarization rotating array (which creates indi¬ vidual pixels) , and a second polarizer which serves as an analyzer. (These components are represented by a single plate in the Figures l and 2) . The cutaway of device 22 in Figure 1, and Figure 3, shows an array 26 of pixels, of pixels 28, which form the image display screen for the user's right eye in the device. LCD units having a 62 x 48 mm square pixel array, and about 102,000 pixels per array are commercially available, such as from Epson (Japan) . Electronically generated images are supplied to the two image screens from a suitable image generator (not shown) connected to the LCD units through a suit- able link, such as indicated at 29. The image genera¬ tor may be, for example, a pair of graphics worksta¬ tions, such as Silicon Graphics workstations, designed for generating position-dependent images which (a) change to reflect the relative position of the head- piece worn by the viewer, and (b) are reconstructed by the viewer as a three-dimensional image. Such image generators are can be assembled from commercially available work station and graphics software compo¬ nents. Alternatively, the image screen for presenting electronically created images could be provided by one or a pair of CRT's which may be mounted, for example, at the sides of the headpiece and which project their images via a lens or optical fiber bundle to a suitable viewing screen positioned at the front region of mask 18, or other types of display screens for presenting electronically generated images. Other devices for producing electronically generated images, such as video cameras or the like, are also contemplated. The screen at which images are presented to the viewer is preferably located in the mask at a position which is about 5-10 cm from the plane of the viewer's pupils. The distance between the screen and the view¬ er's eyes will be dictated by the optical properties of the lens(es) employed in the optical assembly, and the selected positions of the conjugates in the optical assembly, i.e., the selected focal positions on oppo¬ site sides of the lens, as will be described below. Although the screen in device 10 is positioned directly in front of the viewer, it will be appreciated that any screen configuration which is effective to generate an image in front of the viewer is suitable. For example, the screen may be placed at the top of the mask, with the screen images projected downwardly onto a half-silvered mirror positioned to direct a portion of the image directly toward the viewer's eyes. Such a configuration would allow the viewer to view the electronically generated image as well as to see "through" the image. In this configuration, the fres- nel lens desribed below is preferably positioned between the screen and the half silvered mirror. Positioned adjacent the viewing side of the LCD device is a slightly cloudy or rough-surfaced film 29 which acts to blur light from individual pixels, and reduce viewer distraction due to individual light array elements seen close to the eye. The ability of the film to blur individual elements is also useful in a color LCD device, for diffusing different color pixels into larger multi-color elements, as will be described below with reference to Figure 6. A variety of semi-translucent film material, such as velum, polyethylene sheet material with a slightly roughened surface, or material with an internal light- scattering polymer. The film is preferably 1 to 2 mm from the image screen (pixel array) of the LCD device. According to an important feature of the inven¬ tion, the optical assembly includes a fresnel lens, such as the compound fresnel lens shown at 30 in Figures 1 and 2. In the embodiment shown, compound lens 30 includes two identical fresnel lens elements, or lenses, 32, 34 which are disposed in a front-to-back arrangement as seen in Figure 2. That is, the front (upper side) of lens 34 in the figure faces the back (lower side) of lens 32. The purpose of the two-lens configuration is to decrease the focal length of the compound lens by about 1/2, at a selected lens diame¬ ter, to produce a view angle from the viewer's eye of at least about 60°. The two lenses are separated by a distance of about 2-3 mm, with their optical centers aligned. The spacing between the two lens is effective to minimize moire pattern effects from the surface patterns of the two lens. Fresnel lens 32, which is representative, will be described with reference to Figure 2, which shows the lens through a diameter cross section, and Figure 4, which shows the square lens in planar view. The lens is formed of a series of concentric rings, such as rings 36, 38, whose surface curvatures form a parabolic lens surface, as indicated by dash-dot line 40 in Figure 2. In this embodiment, the surface curvatures of the lens rings act to reduce rectilinear distortion effects—either pincushion or barrel type distortions— seen at outer regions of optical images formed by spherical lenses, at an infinite conjugate. The effect of lens curvature on rectilinear distortion will be seen below with respect to Figure 5. More generally, the lens curvatures are selected to achieve a desired image-ray refraction effect, for producing desired corrections or image effects in the virtual image, at selected regions of the image. In the embodiment illustrated, the lens ring curvatures are corrected for spherical aberrations at an infinite conjugate, i.e., where substantially parallel rays are focused at the lens focal point, and where the near conjugate corresponds to the focal point of the lens. For finite conjugates, different ring curvatures would be required to reduce rectilinear distortion, as will be appreciated from Figure 5 below. Thus, in a more general case, to correct for rectilinear distortion, the lens ring curvatures are selected to correct for spherical aberration at a given conjugate pair, and the eye is placed at one of conjugates, usually the near conjugate. Alternatively, the fresnel lens ring curvatures can be selected to correct or compensate refraction effects produced by other lens element in a compound lens. For example, in a compound fresnel lens containing a conventional glass optical element, the fresnel lens ring curvatures may be selected to correct or compensate for spherical aberration effects due to the glass lens element. As defined herein, "fresnel lens" may mean a single fresnel lens, a compound fres¬ nel lens, such as shown in Figure 2, and a compound lens containing both fresnel lens elements(s) and lens element(s) with continuous surface curvatures, such as standard glass lens elements. Fresnel lenses having a selected surface curvature (ring curvatures) can be formed conventionally from a master lens formed by optical lathe. Subsequent dupli- cations are prepared readily by e bossure or casting processes. Suitable lenses for use in the lens confi¬ guration shown in Figure 2 are available from Fresnel Technologies (Ft. Worth, TX) , catalog lens no 4. Each of the fresnel lenses forming the compound lens in the device has a preferred focal length of about 60 mm or less, and a preferred diameter of about 65 mm or greater. As indicated in Figure 2, the square lens has roughly the same dimensions as the image screen, for receiving and focusing pixel information over the entire array. When the identical two lens are placed in tandem, as illustrated in Figure 2 , the focal length of the compound lens is approximately 1/2 that of each individual lens, and preferably less than about 50 mm, more preferably about 30 mm or less. At the exemplified lens parameters, i.e., a 65 mm lens diame¬ ter and a 30 mm focal length, the field of view is about 90°. The compound lens is placed in mask 18 with its near conjugate at or near the expected position of the viewer's pupil. The distance between the lens and the image screen is determined by the desired viewing conjugates, i.e., the desired position of the virtual image from the viewer. In the embodiment shown, the image conjugate is the infinite conjugate (at least about 10 feet from the viewer) which is achieved by placing the lens one focal length from the image screen, e.g., about 30 mm. By moving the screen fur¬ ther from the lens, the image conjugate is reduced, having the effect of moving the virtual image closer to the viewer. Although not shown, the mask may be designed to hold interchangeable fresnel lens or lens combinations, to achieve different selected imaging effects with the different lens surface curvatures. Figure 5 illustrates how the optical assembly of the invention focuses an image from the image screen onto the viewer's eye. In the assembly shown, the compound fresnel lens 30 of Figure 2 has been replaced by a single fresnel lens 32 for simplicity, it being understood that the compound lens functions identically but with one-half the focal length. Also in this assembly, the lens is positioned one focal length from image screen 20, as indicated, for producing a virtual image at least about 10 feet from the viewer. The figure shows the position of the viewer's eye, where the pupil aperture is indicated by the opening 42. The pupil aperture is typically about 3 mm in diameter, with the focal point of the lens, indicated at 43, being position at the center of the aperture. According to an important feature of the lens design in the Figure-1 embodiment, the effective lens ring surface curvatures act to minimize rectilinear distortions which can occur at outer regions of the visual field. This feature is achieved in a fresnel lens whose ring surface curvatures collectively define a parabolic lens surface facing the image screen, and which act to focus onto the viewer's pupil, light rays which are directed substantially normal to the lens surface, as illustrated in Figure 5. In the figure, two equal-length arrows 54, 56 are formed by image screen 20 at center and edge regions of the screen, respectively. The light rays making up the center arrow, and indicated at 57, are refracted at the center portion of lens 32, where lens curvature is substantially the same for both spherical and parabolic surfaces. The two converging rays 58, 60 shown focused into the pupil in Figure 6, represent the chief rays from left and right pixels in the screen which form the rays making up the arrow. To construct the virtual image perceived by the viewer, the rays 58, 60 are projected, in the present embodiment, to an infinite conjugate, i.e., at least about 10 feet in front of the user, to give a virtual image 62 of the arrow. Similarly, the virtual image constructed from the light rays making up arrow 56, is constructed from refracted rays 64, 66 formed by chief rays striking the lens in a substantially normal direc¬ tion and refracted into the viewer's pupil, as in Figure 5. Rays 64, 66, when projected to the infinite conjugate, give the virtual image 64 of the arrow having substantially the same dimensions as image 62 of the center arrow. That is, the perceived rectilinear size of images formed on the screen is substantially independent of the position on the screen where the images are formed. Also illustrated in the figure is the rectilinear distortion in image size which would occur if outer- field images from screen 20 were focused by a conven- tional spherical mirror, such as indicated by lens sur¬ face 68 in dash-dot lines in the figure. It is assumed here that the spherical aberration produced by a sphe¬ rical lens would act to focus light rays from outer- field images, and directed normal to the lens surface, at a point closer to the lens than at the true focal length. Accordingly, rays which are focused at the focal point (through the viewer's pupil) would origi¬ nate from rays which strike the lens at a slightly oblique angle, as shown in dash-dot lines at 70. When the refracted rays, shown by dash-dot lines 72, 74, are projected to the infinite conjugate, the image 68 of arrow 56 which is formed is substantially longer than image 62 of the center arrow. This type of rectilinear distortion in which outer-field images show distorted enlargement, is known as pincushion distor- tion. A reverse type of distortion, known as barrel distortion, would occur if aberrations in the lens surface caused outer-field images to be focused behind the actual focal point of the lens. In the configuration shown, rectilinear distor¬ tions are minimized by forming a fresnel lens (or pair of lenses) whose ring curvatures (a) are facing the image screen, and (b) define a substantially parabolic lens surface. Other fresnel-lens surface curvatures would be required to correct for rectilinear distortion in other lens configurations, such as where the lens is used in combination with a glass lens which exhibits spherical aberrations. In the latter case, the fresnel lens ring surface curvatures would be selected to com- pensate for spherical aberration produced the glass lens, e.g., by a lens ring curvatures which have an exaggerated parabolic curvature in the outer lens rings. Figure 6 illustrates an optical assembly 80 like that described above, for use in projecting color images to the viewer. The figure shows a portion of an LCD array 82 containing groups of RBG pixels forming three-pixel color groups, such as group 82 formed of pixels 84, 85, 86. A partially translucent or cloudy plate 82 in the assembly, functions, as described above, to blur the distinction between individual pixels in the image screen, and particularly in the present embodiment, to blur the distinction between adjacent different-color pixels, to produce a diffuse color "element", such as indicated at 88 on the plate, which is formed of the three contributing color pixels. The color elements are focused, as above, by a fresnel lens 90 onto a viewer's pupil, for producing a colored virtual image with minimized rectilinear distortion. From the foregoing, it will be appreciated how various objects and features of the invention are met. The optical assembly provides a large-diameter, small focal length lens for wide angle viewing. At the same time, the fresnel lens can be designed in lens ring curvatures to eliminate or minimize rectilinear distor¬ tion effects, or create other selected image-refraction effects. The invention thus overcomes a limitation in head-coupled display in achievingwide-angle, relative¬ ly distortion free image presentation to a viewer. Another important advantage, in a head-mounted device, " is the relatively light weight of the lens and lens assembly. Although the invention has been described with respect to particular embodiments, it will be apparent that various changes and modifications can be made without departing from the invention.

Claims: IT IS CLAIMED: 1. A head-coupled display device for use in presenting electronically generated wide angle visual images to a viewer, comprising a headpiece adapted to be worn by viewer, an image display screen mounted in the headpiece for presenting electronically generated images, and a fresnel lens for focusing images from the screen at a selected position for viewing as a virtual image which is a selected distance from the viewer's eye, said lens having a set of concentric converging rings whose surface curvatures act to minimize rectilinear distortion of the virtual images perceived by the viewer over the entire field of view of the image. 2. The display device of claim 1, wherein the lens ring curvatures are selected to correct for spherical aberration at selected conjugates, one of which is a near conjugate, and the lens is positioned so that the viewer's eye is at the near conjugate. 3. The display device of claim 2, wherein the lens is corrected for spherical aberration at an infinite conjugate, the lens is positioned with its focal point adapted to correspond to viewer's pupil and midway between the display screen and the viewer's pupil. 4. The display device of claim 3, wherein the fresnel ring curvatures define a parabolic surface. 5. The display device of claim 1, wherein said fresnel lens is a compound lens which includes a pair of fresnel lenses positioned front-to-back. 6. The display device of claim 5, wherein the lens diameter is at least about 60 mm, and the focal length is no greater than about 50 mm. 7. The display device of claim 6, wherein the field of view is at least about 60°. 8. The display device of claim 1, wherein the image display screen is a liquid crystal display screen having an array of pixels which make up clusters of RBG pixel groups, and said device further includes a partially translucent plate positioned adjacent said screen, effective to blur the distinction between the individual pixel elements in such as RBG pixel cluster. 9. The display device of claim 1, which includes two such display screens and fresnel lenses, for left and right viewer eyes. 10. The display device of claim 1, wherein the fresnel lens is interchangeable with fresnel lenses having different effective lens curvatures. 11. An optical device for viewing electronically generated images, comprising an image display screen for presenting electroni¬ cally generated images, a fresnel lens for focusing images from the screen at a selected position for viewing by the viewer as a virtual image which is a selected distance from the viewer's .eye, said lens having a set of concentric converging rings whose surf ce curvatures are selected to achieve a desired image-ray refraction effect, and head-coupling means for supporting the screen and lens at a position for viewing by the viewer. 12. The display device of claim 11, for use in producing a wide-angle image, wherein the lens surface curvatures are selected to correct for spherical aberration at selected conjugates, one of which is a near conjugate, and the lens is positioned so that the viewer's eye is at the near conjugate. 13. The display device of claim 12, wherein the lens is corrected for spherical aberration at an infinite conjugate, the lens is positioned with its focal point adapted to correspond to viewer's pupil and- midway between the display screen and the viewer's pupil. 14. The display device of claim 13, wherein the fresnel ring curvatures define a parabolic surface. 15. The display device of claim 11, wherein said fresnel lens is a compound lens includes a pair of fresnel lenses positioned front-to-back. 16. The display device of claim 15, wherein the lens diameter is at least about 60 mm, and the focal length is no greater than about 50 mm. 17. The display device of claim 17, wherein the field of view is at least about 60°. 18. The display device of claim 11, wherein the image display screen is a liquid crystal display screen having an array of pixels which make up clusters of RBG pixel groups, and said device further includes a partially translucent plate positioned adjacent said screen, effective to blur the distinction between the individual pixel elements in such as RBG pixel cluster.

Inventors: TEITEL MICHAEL A, TEITEL, MICHAEL, A.
Applicants: VPL RESEARCH INC, VPL RESEARCH INC.
Application year: 1992
Main classification: G09G1
IPC codes: G02B27