Abstract:
An optical system for a head mounted display includes a light pipe having two parallel surfaces along which modes can travel by total internal reflection. An illumination element allows selection of modes so that the desired modes can be transmitted along the light pipe, either axially or by total internal reflection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent application No. 60/664,254, filed on Mar. 22, 2005, the disclosure of which is incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     This invention was made under U.S. Army Aviation and Missile Command Contract No. N31P4Q-04-C-R280. The government has certain rights to this invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     The propagation of an image through an eyeglass lens or face mask optical plate is known. See, for example, U.S. Pat. No. 5,886,822. Referring to  FIG. 1  herein, the light is directed axially, and reflections from the outside surfaces lens are not desirable. Another known embodiment discusses the use of total internal reflection from the surface of the lens or plate in order to reduce the thickness of the lens, or to increase the field of view. See, for example, U.S. Pat. No. 6,384,982.  
       SUMMARY OF THE INVENTION  
       [0004]     An optical system for a head mounted display is provided that allows selection of desired modes for transmission along a light pipe, either axially or by total internal reflection. More particularly, the system includes a light pipe comprising a solid optical element having two parallel surfaces. An image source includes a display element and an illumination element that includes at least one light source. The image source directs light into the light pipe for transmission to the eye of a user.  
         [0005]     A collimating system between the illumination element and the display element includes a condensing lens element and an aperture located at a focal length of the condensing lens element. In one embodiment, the aperture and the light source are located along an axis of the condensing lens element so that modes from the illumination element that would be transmittable by total internal reflection off the two parallel surfaces along the light pipe are prevented from entering the light pipe.  
         [0006]     In another embodiment, the illumination element includes a further light source, and the collimating system includes a further aperture. The further light source and the further aperture are located off axis of the condensing lens and the display element, so that modes from the illumination element enter the light pipe and are transmitted along the light pipe by total internal reflection of the two parallel surfaces.  
         [0007]     In a still further embodiment, the illumination element includes a plurality of light sources, and the collimating system comprises a plurality of apertures, each aperture associated with a light source. In this manner, a plurality of modes can be transmitted to the light pipe, and at least a portion of the modes are transmitted by total internal reflection off the two parallel surfaces of the light pipe. The light sources can be illuminated in sequence to provide different image synchronously. The light sources can be illuminated simultaneously to provide an image with a wide field of view. The optical system can be configured so that the exit pupil of several modes overlap. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0009]      FIG. 1  is a schematic illustration of a prior art optical system;  
         [0010]      FIG. 2  is a schematic illustration of one mode of total internal reflection of an optical system;  
         [0011]      FIG. 3  is a schematic illustration of an opposite mode of total internal reflection of the optical system of  FIG. 2 ;  
         [0012]      FIG. 4  is a schematic illustration of a variety of virtual images combined in a single optical system;  
         [0013]      FIG. 5  is a schematic illustration of an optical system to remove lift and right virtual images;  
         [0014]      FIG. 6  is a schematic illustration of an optical system in which modes can be selected to illuminate selected virtual images;  
         [0015]      FIG. 7  is a schematic illustration of an optical system in which a variety of total internal reflection modes can be selected; and  
         [0016]      FIG. 8  is a schematic illustration of a further optical system in which a variety of total internal reflection modes can be selected. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     It is the object of this invention to control the propagation of the light through the lens and to make possible or eliminate total internal reflection (TIR) from the lens surface. This control is attained through design of the back light system used to illuminate the LCD.  
         [0018]     We can consider the lens or plate to be a waveguide in which a number of modes can propagate, including the axial mode which does not require any reflection from the waveguide surface. (This is in analogy to a multi-mode optical fiber in which rays can propagate axially or by using reflection at the interface between the fiber core and the cladding.) To control the mode of propagation through the waveguide, it is only necessary to control the light rays incident on the LCD. Rays which are well collimated parallel or at small changes to the optical axis will propagate axially, and rays that pass at larger angles through the LCD may be reflected. The present method allows the modes of the waveguide to be selectively illuminated by addressing the back light. In this way, the field of view of the image may be expanded or contracted.  
         [0019]      FIG. 1  shows the prior art method of delivering an image through an eyeglass lens to the eye. The light is generated by one or more light emitting diodes housed in a backlight enclosure  30  adjacent to an LCD  20 .  FIG. 1  shows the axial path of the light  40  through the lens  10  as it propagates to the eye (not shown) to form a virtual image  50 . The backlight forms a broad area diffused source which in fact emits light not only in the axial direction, but also in a range of angles. Some of the propagation angles are capable of striking the surface of the lens as they propagate to the eye.  FIG. 2  shows an example of such propagation, in which light rays exit the backlight  30  at an angle, pass through the LCD  20  and enter the lens  10 , and reflect from the mirror  11 . The angles cause the light path  60  to be incident on the surface  61  of lens  10  so as to reflect by TIR. This reflection causes a virtual image  70  which is displaced from the center of the field of view of the user.  
         [0020]     An opposite mode is possible for rays that exit the back light with an angle in the opposite direction, as shown in  FIG. 3 . In such a case the light path  65  reflects by TIR from the inside front surface  62  of the lens  10  and results in a virtual image  71  that is displaced to the left of the center of the user&#39;s vision.  
         [0021]     The case in which light is radiated by the back light diffusely, both along the central axis and in a range of angles, causes a plurality of virtual images, as shown in  FIG. 4 . If these images contain the same information content, then they are confusing and not useful. In such a case, it is preferred to eliminate the left image  71  and right image  70 . However, if each of the three virtual images contain different information, then the three images can be merged to form one large virtual image having a nominal width that is three times the width of the single image.  
         [0022]      FIG. 5  illustrates how the left and right images can be removed by modification of the back light to form a condensing illuminator. In  FIG. 5  we show that the backlight ( 30  in  FIG. 1 ) has been replaced by a collimated backlight system comprising: an LED  31 , a condensing lens  100 , and an aperture  101 . The aperture would typically be fitted with a diffuser to form a light source of known size and numerical aperture. The aperture  101  is placed at a distance from the condensing lens  100 , typically close to or equal to the focal length of the lens. Newton&#39;s lens equation shows that this creates an image of the aperture at infinity, meaning that the rays from the aperture are collimated. The degree of collimation depends on the width d of the aperture. Owing to the collimation, rays that would have reflected from the inside of the lens by TIR are removed and only the central virtual image is created. The display viewing optics  110  (in the embodiment illustrated, a light pipe  112  with reflecting surface  114 , beam splitter  116 , and reflecting surface  118 ) then re-image the aperture at the exit pupil of the system. Thus this invention provides a method of removing parasitic virtual images in a multi-mode waveguide lens.  
         [0023]      FIG. 6  shows the result of moving the aperture  101  off center. In such a case, the rays are collimated, but they propagate at an angle to the axis of the system. This means by positioning the aperture correctly, rays can be selected to propagate in a mode that uses a selected number of TIR bounces to reach the eye of the user, so as to illuminate one virtual image corresponding to the mode.  
         [0024]     A system can be constructed in which any number of TIR modes can be selected. In  FIG. 7  we illustrate the case in which the center mode is combined with two TIR modes. In this case, three LEDs  201 ,  202 ,  203  are combined with three apertures so that three distinct modes are created when the rays pass through lens  220 . The central mode  205  propagates through the eyeglass lens or face mask to create the central virtual image. The ray bundle  204  reflects by TIR from the inside surface of the lens ( 61  in  FIG. 2 ) to form a virtual image on the right ( 70  in  FIG. 2 ). The ray bundle  206  reflects from the inside surface of the lens ( 62  in  FIG. 3 ) to form the left virtual image ( 71  in  FIG. 3 ). The total internal reflection also folds the optical path so that the exit pupils of the three images overlap. If the three LEDS are simultaneously illuminated, three images with equivalent information content are created in the field of view of the user. The left and right images will be seen to be reversed (left to right). However, if the LEDs are sequentially illuminated, and the LCD  210  is provided with three different images synchronously with the LED illumination, the user will see three distinct images. Such LCDs are know as field sequential LCDs and are sold by a number of companies, such as Kopin Corporation, which sells a field sequential Cyberdisplay.  
         [0025]     In the preferred embodiment of the invention, the optical system is designed so that exit pupil of the several modes overlaps, so that all the images are visible from a single eye position. Other configuration are possible wherein eye motion is required to view different modes. Optical design techniques my also be used to define the position and size of the light source to eliminate mode-mixing whereby the image of one mode receives stray light while another mode is illuminated. This would result in ghost images and/or loss of contrast. The optical design may be performed by any method known in the art, including first order optical design and manual or computerized ray tracing, for example using commercial optical software such as ZEMAX, CODE V, or OSLO.  
         [0026]      FIG. 8  shows a ray trace of a multi-mode design utilizing TIR reflections. Three separate light sources  801 ,  802 , and  803  can be independently pulsed to illuminated the display  810  when the corresponding information is shown. The illustration shows the illumination of the center region of the display, but similar design can be applied to the entire region of the display. The source light is collimated by the condenser lens  815 . After passing through the display, the light passes through the field lens  820  and the air gap  825  to enter the light pipe  830 . The air gap is necessary to allow for TIR at that interface for the modes that require it. The first reflection at the mirror is at an acute angle, so typically the surface  828  must be made reflective by the application of a reflective coating, cementing of an external mirror, or by any other method known in the art. The rays then propagate through the light pipe using different numbers of TIR reflections. The last reflection is again at an acute angle off of surface  835  which must be made reflective similar to surface  828 . Upon exiting the light pipe the light passes a second air gap  850  which allows for TIR reflections at the surface, through the eye lens  855 .  
         [0027]     The system design must allow for the eye&#39;s pupil  860  to be placed at a reasonable eye relief from the last mechanical part. The eye relief must be at least several millimeters and is preferably at least 10 mm and even more preferably between 12 and 35 mm. For a head mounted display it is preferable that all the modes be visible from the same pupil position, so that the user does not have to physically move the display relative to his/her head. As the eye can scan, however, this motion may be used to access different portions of the image of some of the modes.  
         [0028]     The number of reflections characterize the mode. In the system shown, the center mode  840  has 7 reflections, of which 5 are TIR reflections off the parallel surfaces of the pipe. The left mode  841  has 9 reflections, of which 7 are TIR. The right mode  842  has 5 reflections, of which 3 are TIR. Certain source locations may illuminate pixels for more than one mode (for example the left pixel of the right image mode and the right pixel of the center mode). To avoid this, the source size must be constrained so that none of the rays from a source for a given mode form a virtual image in a location associated with any other mode.  
         [0029]     It can be seen that by proper design of a lens or facemask, the positions and spacings of the virtual images corresponding to the various modes can be controlled. By choosing the lens thickness appropriately, the virtual images can be made to overlap, touch without overlap, or may be made to have a space between them. The images on the LCD can be expanded, contracted, or distorted electronically to adjust the spacing between virtual images which are thus superposed, so as to produce one seamless virtual image. Any number of optical propagation modes can be used to increase the width of the final superposed image, the limiting factor being the speed at which the LCD can be refreshed. For superimposing three images, the LCD should be capable of flashing images at 180 frames per second in order that the user does not see flicker in the image.  
         [0030]     Although the above description is of a monocular system, two such systems can be joined to create a binocular or stereo display.  
         [0031]     The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.