True 3-D Displays
The True 3-D display is a volumetric 3-D display projected with a beam of light that is scanned in X, Y, and Z axes—creating a three-dimensional sculpture of light that appears to float in space. Viewers of the True 3-D display can point and focus their eyes at different objects at different depths in the display, bringing them in and out of focus just like real objects at different distances. For a simple illustration of this effect, move your mouse cursor over the image to the right as if it were your gaze. As your cursor points at the statue in the foreground, it comes into sharp focus, and the brick wall in the background is slightly blurred. As you move the mouse cursor to the brick wall, the statue is slightly blurred—just as would be the case if you were viewing real objects at different distances.
This ability to render objects with correct focus cues is not present in conventional 3-D stereoscopic displays, which do not create a 3-D volume of light for viewing, but rather display one flat 2-D image to the right eye and a different 2-D image to the left, creating a partial illusion in the visual system of seeing a 3-D object. The incomplete illusion of 3-D created by stereoscopic displays creates cue conflicts in the visual system, and can contribute to eye fatigue and headaches. In particular, because all of the light is coming from a flat 2-D surface, conventional stereoscopic displays require viewers to focus the lenses of their eyes at one fixed distance (the focus distance of 2-D image surface) while pointing their eyes at a different distance (the apparent depth created by the stereoscopic effect). This is unnatural, because the neural processes that control the focus of the eyes (called “accommodation”) and where the two eyes are pointing or converging (i.e., “vergence”) are linked together, so the eyes automatically try to focus at the same distance to which they are pointing. This forced decoupling of reflexively-linked processes when viewing conventional stereo displays can fatigue eyes, cause discomfort, compromise image quality, and may lead to pathologies in developing visual systems.
Our multi-planar True 3-D displays generate accommodation cues that match vergence and stereoscopic retinal disparity demands, and can display images and objects at viewing distances throughout the full range of human accommodation (from 6.25 cm to infinity), better mimicking natural vision and minimizing eye fatigue.
Scanned light displays biaxially scan a color- and luminance-modulated beam of light, serially moving a single pixel in 2-D across the retina to form an image (for an illustration, see the video clips to the right). We have integrated a variable-focusing element into a scanned light display to enable a voxel to be triaxially-scanned throughout a 3-D volume. The light is not projected onto a screen but rather creates a 3-D volume of light that is viewed directly by the eye. By positioning the 3-D volume between the surface of a lens and its focal length, the 3-D volume can be magnified to occupy a virtual space stretching from the lens to distant horizon. As when viewing real 3-D objects, the eyes can focus upon different points within the 3-D volume.
We have designed and constructed a number of scanned voxel display prototypes using this approach, but we will briefly describe a recent prototype that presents full-color, stereoscopic, multi-planar video directly to each eye, using a scanning beam of light. Before the beam is raster-scanned in the X and Y axes, it is first “scanned” in the Z-axis with a deformable membrane mirror (DMM) MOEMS device. The DMM contains a thin silicon nitride membrane, coated with a reflective layer of aluminum, that is stretched in front of an electrode. The shape of the reflective membrane is controlled by applying bias and control voltages to the membrane and electrode. With no applied voltage (left side of Figure) the membrane forms a flat mirror and a collimated beam reflected from its surface remains collimated. With an applied voltage, the reflective membrane is electrostatically-deflected toward the electrode, forming a concave parabolic surface that will focus a beam of light to a near point (right side of Figure). Intermediate voltage levels shift the focal point anywhere between the near point and optical infinity (i.e., a collimated beam).
|The deformable membrane mirror (DMM) is used to dynamically change the focus of the beam before it is XY-scanned. The beam is shown entering from the bottom of the figure, and being reflected to the right. If no voltage is applied across the membrane and electrode (left side of figure) the membrane remains flat and doesn’t change the focus of a beam reflected from its surface. If a voltage is applied (right side of figure) the membrane electrostatically deflects toward the electrode, creating a concave parabolic mirror that shifts beam focus closer.|
After being scanned in the Z-axis with the deformable membrane mirror, the beam is scanned in the X-axis with a spinning polygon mirror and scanned in the Y-axis with a galvanometric mirror scanner, completing the triaxial scan. This 3-D scanned voxel volume is optically divided with fold mirrors and relayed to left and right eyes. See the figure below for a graphical overview of the complete optical system.
|The viewer brings different depth planes into focus by naturally shifting the accommodation of the eyes’ lenses. By changing the voltage to the DMM rapidly, a frame-sequential multiplanar image is generated. (A) The viewer accommodates his/her eye to the distance and thus the house in the background plane is in focus while the tree in the foreground plane is somewhat blurred. (B) The viewer accommodates near, bringing the tree into focus on the retina while the house is shifted out of focus.|
Commercial realizations of our prototype scanned voxel displays can include a lightweight head mounted display (HMD), ideal for wearable computing and augmented reality applications, or a stand-alone desktop display, designed to be viewed from a distance. Using batch microfabrication techniques, the MOEMS scanners can be produced at low cost. Red laser diodes are inexpensive, allowing portable monochrome red scanned voxel displays to be manufactured affordably. A portable full color system would currently require higher manufacturing costs. Blue laser diodes are expensive and have shorter lifetimes, but it is anticipated that both cost and lifetime will improve in the next few years. Small prototype green semiconductor lasers capable of MHz rate luminance modulation have been demonstrated by Corning, Novalux, and OSRAM, and will soon reach large-scale commercial production.
Non-fatiguing 3D displays can be used for all 3D viewing applications for which conventional stereoscopic systems are typically used. There are, however, some applications for which they are critical. Surgeons are increasingly using minimally-invasive methods (e.g., endoscopy and laproscopy) which require looking at displays for many continuous hours. 3D displays enable surgeons to better guide endoscopes around obstructions within the narrow spaces of the body, but doctors must remain in top mental form throughout long surgeries, so it is crucial that these displays be non-fatiguing and comfortable. The guidance of minimally-invasive surgery tools is a form of teleoperation, and other forms of teleoperation—such as the piloting of remote UAVs (Unmanned Autonomous Vehicles)—also can greatly benefit from 3D displays that can be comfortably viewed for extended durations. Finally, as 3D displays are used for video games, we should not present young children with sensory conflicts that could lead to pathologies in their developing visual systems. While surgeons must spend hours concentrating on displays during surgery, children often voluntarily spend even longer periods concentrating on video game displays.
This research is supported by a grant from the National Science Foundation Major Research Instrumentation program, NSF/BES grant number 0421579.
For more information about True 3D displays, please contact Dr. Brian Schowengerdt.