**13. Comparation of electronic image systems employing diffractive screens with other volumetric systems**

Non-glasses autostereo systems are not practical for its wide-spread use for commercial purposes yet (30)(31), so that the many systems which has been proposed for it are to be kept under discussion.

Let us consider the following volumetric systems: lenticular, a parallax barrier, a curvature controlled mirror, a rotating screen under projection, a rotating LED display, an electrooptic hologram generator, a fast recording of integral hologram, a laser focalization on crystal, a laser focalization on air, and flying luminous sources.

Among the lenticular systems let us consider only those in which small semi-cylindrical vertical lenses are in contact with a conventional screen. To reach the number of views necessary to produce an apparent continuous view sequence many views must be displayed simultaneously, making a large bandwidth necessary or much processing by very special digital techniques. This is the same as happens for the diffractive screen systems to add several slices or views. Maybe an advantage can be proved in the case of the slicing oblique plane system, because the continuous parallax does not demand electronic processing power.

the fact can be mentioned that similar elements can be constructed by direct recording techniques like lithography, for example. A 5 m diameter diffractive lens was made for an astronomic space project (27), a technique that could be similar to one for the construction of

It was made in the form of a mosaic, already explored with holographic techniques (10).

Fig. 12. Picture of a 5 m diameter diffractive lens made by litography (by R. Hyde- S. Dixit).

Non-glasses autostereo systems are not practical for its wide-spread use for commercial purposes yet (30)(31), so that the many systems which has been proposed for it are to be

Let us consider the following volumetric systems: lenticular, a parallax barrier, a curvature controlled mirror, a rotating screen under projection, a rotating LED display, an electrooptic hologram generator, a fast recording of integral hologram, a laser focalization on

Among the lenticular systems let us consider only those in which small semi-cylindrical vertical lenses are in contact with a conventional screen. To reach the number of views necessary to produce an apparent continuous view sequence many views must be displayed simultaneously, making a large bandwidth necessary or much processing by very special digital techniques. This is the same as happens for the diffractive screen systems to add several slices or views. Maybe an advantage can be proved in the case of the slicing oblique plane system, because the continuous parallax does not demand electronic processing power.

**13. Comparation of electronic image systems employing diffractive screens with** 

crystal, a laser focalization on air, and flying luminous sources.

diffractive screens.

**other volumetric systems** 

kept under discussion.

The parallax barrier system can be disregarded for its long duration presentation applications because of the necessity to maintain the head position during the observation. Relief is inverted when deviating from the right position. Although more than two views can be provided, the adding of views reduces the brightness of the image.

To make large diffractive screens seems to be as easy as making large holograms, while examples of lenticular or parallax barrier screens which are larger than one square meter are not known yet.

The curvature controlled mirror employs fast moving parts, being limited in size to a few decimeters by air resistance and noise generation. The rotating screen systems presents the same problem; its advantage is the ability to show 360 degrees images. The images of the rotating screen systems can not fill more space than the screen does, appearing in fact within a transparent cylinder, never in front or in the back of it, the effect that more impresses the public.

The electro-optic system, already named as "holographic video", developed by Benton, never recorded live video scenes but computer made ones. It has size limitations and consumes much computer processing power.

The fast recording of holograms has been possible in telecommunications with new photosensitive materials. A scanning frame was recorded every two seconds, which is still ten times more than required for video, and not at large size yet. To reach the video velocity much bandwidth and processing capability will also be needed and, if reaching the 1/20s frame speed, the persistence of the images on the retina makes the photosensitive material unnecessary; a diffractive screen can accomplish the task equally.

There are systems creating luminous points within crystals and images can be seen from almost any position, having more than 360 degrees viewing capability, but the images remains within the supporting crystal and it seems neither practical nor possible to have large size images. A new interesting possibility appears to make the same in a liquid, while not employing visible light for that (32).

Other systems focussing intense laser beams on air may generate 360 degrees views, but presents a low resolution and high cost, as well as noise or dangerous luminous or temperature levels. One can think that this systems could be useful for working at large sizes, filling large volumes to be seen from a large distance.

Finally, the same can be said of the recent system made of small flying sources, minihelicopters whose position can be remotely computer controlled: large sizes could be achieved at a considerably energy cost, low speed and short duration.
