Computer holographic video and volume displays are fascinating new technologies. Yet the red-green, polarising and shutter glasses have not been really accepted by the user to date - they are prostheses and affect both comfort and image quality.

The most important information for depth perception comes from stereo vision: the eyes transmit two different views of the same scene to the brain. The same goes in turn for reproduction: each eye has to be presented separately with a view, by means of mirrors or, more commonly nowadays, with colour, polarisation or shutter glasses.

But there is another way: "autostereoscopic" displays do without such spectacles. They use other methods to transfer a left and right image to the respective eyes. Computer holograms and volume displays go even further: they draw three dimensional pictures directly into space, without any stereoscopic tricks.

Among the old but largely unknown ideas is the technique introduced by Sanyo as "Image-Splitter" at this year's fair (Funkaustellung). Although Sanyo has registered a new trademark, the technology has been known since the beginning of the century. A single image contains the two views of the stereo pair, as vertical stripes which are as narrow as possible and which alternately show pieces of the left and right view. A stripe mask in front of the image ensures that the left and right eye of the viewer see only the stripes dedicated to them, as long as the viewer keeps his head at a certain distance from the mask. There is no space for a second viewer.

The three models introduced by Sanyo on the IFA (International Communications Fair) work with LCDs (4.3 to 10.4 inches) with left/right stripes that are only one Pixel wide. In front of the screen is an almost microscopically fine grid, the image splitter.

Dimension Technologies (DTI) has offered a 3D-Display since 1991 that utilizes the same principle in a modified version. A colour-TFT-LCD shows the view for the left eye on the pixel columns with even numbers, and the view for the right eye on the odd number columns. A clever background lighting of 320 white parallel vertical lines renders a light absorbing stripe mask superfluous. Special about it is that the display can be run as a normal 2D-Display (640 columns x 480 rows) or in 3D-Mode (320 columns).

A model by Terumo (Split-Image-Display) uses two TFT-LCDs to produce the stereo pair. A half-mirror combines both pictures thus preserving the full column resolution. Two additional monochrome LCDs provide the stripe lighting behind the TFT-LCDs. A tracking system measures the head position and adjusts the viewing zones by moving image display and stripe display against each other. Instead of blocking the view to one half of the image pair with a mask the view can also be enhanced specifically. By means of a dense row of half-cylinder shaped lenses that directs the two part-images exactly into the corresponding eye.

Such lenticular systems, popular in 3D postcards, have been known since 1908. The method can be refined; if more than two views are used as image stripes the picture can be viewed from different angles (parallax-panoramagram). Kodak and the less well known company Art offer productions of such computer calculated 3D-hardcopies.

Displays based on lenticular systems come from Sanyo and the Henrich Hertz Institute (HHI), Berlin. Both developments are back-projection systems. In the Sanyo product two LCD video projectors project the images for the left and right eye onto a screen (40 inch diagonal). Stripe lenses are located in front of and behind the screen, a double- lenticular system. The rear lens system breaks the two projected images into stripes, the second lens system at the front of the screen guides each stripe into the corresponding eye of the viewer. To experience the 3D-effect, the viewer has to keep at a certain distance with this method (1 metre for the Sanyo product). At this distance from the screen the distance between the ray bundles is the same as the distance between the eyes.

But unlike the Image Splitter, the lens system creates several ray bundles for each spot of the screen; therefore there are several "stereo windows" and theoretically several viewers can sit in front of the display (currently actually only two). With the product the manufacturer is targeting applications in museums, 3D-mini cinemas and video games.

Unlike the Sanyo construction, the HHI display is equipped with a head tracking system that tracks the head of the viewer. An electroluminescent flat screen is being developed as well; it also uses the double lens system and it achieves HDTV resolution.

Together with the company Carl Zeiss, the HHI is currently developing a 3D flat screen for use in medical technology. With the new development, the viewer supposedly is able to move about thanks to a movable lens system plate.

Since 1942 Sharp has used a different concept of 3D-displays called twin-LCD. This system projects the images of two LC-displays separately into the eyes of the viewer. Particularly interesting in this set-up is the folded ray course. Using a half-permeable mirror the system can do with a single light source. If the viewer moves it is sufficient to move the light source in the opposite direction, and the rays meet the viewers eyes again. The light source is guided by a tracking system that not only reacts when the viewer approaches the screen or moves away from it, but also when he moves sideways. Image sources for the twin-LCD are either two laser disk players or a graphics workstation connected to the tracking system. This workstation calculates, in real time, lateral views of the scene enabling the viewer to have a dynamic angular view (motion parallax) up to 40 degrees around the object as long as he does not move too fast (up to 30 cm/sec).

A plan for the future: with further light sources in the optics several viewers could sit in front of the twin-LCD.

All these stereoscopic displays do not create a real three dimensional image but a spatial illusion, since the human spatial vision can be deceived with two flat pictures. Serious problems can arise from the conflict between accommodation and convergence. The partial images have full depth of field with objects appearing focused at any distance. Gradual loss of the spatial impression, head and eye aches and even nausea and vertigo could be the results. Also, by moving the head one does not gain new views, the movement parallax is missing. As in the Sharp development it can be simulated with a computerized tracking system. The three particularly important spatial informations for three dimensional viewing are, - apart from movement parallax- accommodation, convergence and binocular parallax; they are created only through actual objects or real 3D-Images.

The fault list of stereoscopic displays is long: fixed viewer position, only one or two viewers, no 360 degrees viewing. To overcome these problems 3D-images have to be drawn into the real space. To depict three dimensional scenes in space, holography is the most popular technique. The usual method of creating holograms is to split a laser light bundle in two parts, of which one reaches the film directly (reference ray) and one is reflected from the object onto the film. The film itself does not carry a directly recognizable image as information but only a phase difference, an interference pattern of bright and dark lines holding the coded spatial information. The 3D image appears when the hologram is lit appropriately, suspended in space and seemingly real like an object. Depending on the recording method a hologram can also store the colours of the photographed scene. Today, high resolution reproductions in natural colours are only possible from actually existing objects. To be able to animate computer models in space like on a screen would be video-holography.

Wouldn't it be possible to simulate the holographic recording process on computer, track the course of the light wave front and thus create a synthetic computer-hologram, without laser, film and a real object?

The Spatial Imaging Group of the Massachusetts Institute of Technology began working on the problem four years ago and their results are impressive.

Starting points of the calculations are 3D-objects created with CAD software. A computer generates a hologram from this data which is stored in an image memory. With a special real time-holography-display ("holovideo") the three dimensional image can be produced in real time. The computer-holographic pattern is transferred as acoustic oscillations into a crystal acousto-optic-modulator (AOM). The oscillations change the refraction index of the crystal both geometrically and timewise. A laser traveling through the AOM is modified according to the holographic interference pattern similar to a normal hologram on a photographic plate, and a 3D-image is formed in space. An additional optical deflection system reflects the synthetic hologram row by row to the viewer, with a rate of 40 pictures per second.

The biggest problem of this technique is the huge amount of data. An object of 10x10x10 cm with a 30 degree viewing angle makes a hologram of 25 Gigabyte; the required data transmission rate at 60 pictures per second in 8-bit-resolution would be 12 Terabytes. So far workstations have been occupied for minutes to hours creating the computer generated holograms (CGH), even with the necessary massive cut backs in resolution.

Real time video holography was achieved with a connection machine and data reduction (such as neglect of the vertical parallax, restriction to 11 degrees viewing angle, 3 cm object size). New coding techniques reduce calculating time to a hundredth and enable workstations to calculate 6-Mbyte-holograms in less than two seconds. Fully coloured interactive video holography in real time has come within reach.

The ideal three dimensional depiction is an image in space that one can walk around and view the object from all sides. Image generation in space (as opposed to one or two planes) could be achieved by stacking sections of the object image in rapid sequence into space. But for this the odrawing boardoe has to fill a volume which is made possible through projection planes that oscillate through space (multiplanar displays). The idea has been realized in several ways.

Since the mid-sixties oscillating or rotating projection planes have been used. One of these concepts utilizes a flexible concave mirror. Made to vibrate by a speaker membrane, the focal length of the mirror (varifocal mirror) changes. This way the image of a cathode ray tube is focused on different planes. The apparent image depth is far greater than the amplitude of the mirror. In 1988, BBN produced such a device commercially. A disadvantage of the principle are the optical distortions of the image; a 360 degree view is impossible.

In 1991, researchers from Canterbury University, New Zealand, took up a patent from 1964 and improve it: a phosphorus coated disk rotates inside a Cathode Ray Sphere (instead of the known cathode ray tube). Two electron guns and a conventional deflection unit make the phosphored screen glow. When the screen rotates, a translucent three dimensional image is visible. The electron guns are shifted by 120 degrees and positioned below the equator to generate better 360 degree viewing. The problem is the limited size and the high demands on the quality of the vacuum.

Apart from other concepts with rotating projection planes (i.e. spirals), techniques with self glowing planes have been invented. In 1977 a system by MIT that packed 4096 red LEDs on a rotating disk was patented. The glow pattern, however, could only be changed during stops. Future developments aim to integrate the electronics for image generation and storage on the rotting disk. The mechanical strains due to the fast rotation limit the technique. Therefore, rotating planes onto which the image points are projected are of interest again.

In the late seventies Prof. R. Hartwig and a group of Heidelberg researchers suggested a system with a rotating spiral plane (helix). Such a 3D-Display with a rotating helix was produced in a collaborative project of a secondary school in Stade and the "Institut fuer Flugfuehrung" of Braunschweig University. The projection plane rotates with 1200 rpm in a glass cylinder. The semi-transparent plane is a 360 degree helix plane. Red, blue and green laser rays parallel to the rotation axis hit the helix and generate a light point on it that is visible from all sides. A computer generates the image in real time. The image resolution however is restricted by the limited laser deflection frequency and the computing speed.

Kodak also uses the Hartwig principle but substituted the laser system with a video projector, which required the installation of a complicated, self-rotating lens to solve the focusing problems.

Texas Instruments' "Omni View" development is working on the same concept since 1988; a prototype of 36 inch diameter was made available to the Navy in 1992. The projection plane of this display consists of two helices wound like mirror images. The plane rotates with 600 rpm and thus fills a cylinder volume. Five laser systems - one for each space quadrant and an additional one to produce higher resolution (up to 15000 voxels, volume picture elements) - with three lasers (red, yellow, green) each, create coloured three dimensional images visible from all sides. Light rays are synchronized with the rotation of the disk and are split into small packets of up to 10000 per second. A scanner lays the laser rays in a grid pattern on the plane in the x and y directions; the z coordinate results from the synchronisation with the rotation.

Disadvantages of all such multi-plane displays are the mechanical strain and the complicated "targeting". Most have dead zones, dark space areas that can not be targeted, such as the axes of rotating projection planes. It would be better if the space points could be targeted directly, without having to use the rotating plane as a bridge from the Pixel-flatland to the Voxel-space.

Real volume imaging is still a dream, but the dream has a solid basis - stepwise excitation of fluorescence. Already in 1920 it was observed that atoms of mercury vapour can be activated to after-glow (fluorescence) when exposed to two different subsequent light pulses. For fluorescence atoms have to have three suitable energy levels; they are activated at the crossing point of two invisible infrared light rays from the first to the second level and from there to the third. Returning to their basic level they emit visible light - the crossing point of the infrared rays glows. Experiments of that kind were successful with an erbium enriched calcium fluoride crystal, creating glowing space points that can be targeted. The glow, however, is only visible in total darkness. So far researchers have not found an efficient medium for larger displays. There is speculation about suitable physical processes other than fluorescence; voxels of photochemically activated molecules could scatter light in colours.

Spatial Vision

The ability to see three dimensionally depends on a multitude of visual informations that are gathered by our eyes and their motorics. Some of the information is gained by one eye alone, other information requires both eyes.

The laws of geometrical perspective were already known to Euclid. Renaissance painters were the first to utilize linear perspective and the depiction of light and shadow at the same time to give the viewer a perception of depth. The aerial perspective (the obluing of the horizonoe) followed and, with Leonardo da Vinci, even depth of field: "Sfumato".

The (apparent) size of known objects and the mutual position of several objects also aid in estimating distances: if an object covers another object it lies in front of that, not behind it. When we move, the relative position of objects changes. All these depth informations can be obtained by one eye.

With two eyes stereoscopic vision is possible, enabling a human to perceive distance differences with an accuracy of 1:1000. This is based on the two images received by the retinas. They differ due to the distance between the eyes. From the differences the brain models a spatial impression. How this exactly works is not fully understood yet. The magic pictures and optical paradoxes of an M.C. Escher both challenge and confuse our visual intelligence.

When adjusting to a point at a certain distance both eyes turn until their viewing lines cross at that point. The muscle tension expended for accommodation and convergence also provides very important information. At distances beyond 6 metres these oculo- motoric effects disappear; the eyes are focused to infinity and are nearly parallel. If convergence and accommodation contradict each other then nausea, headaches and the loss of depth perception are the results.

Translation (1996) of original German article
from: Adam Schwarz