**11. Lensless projection on diffractive screens**

The projection by means of lenses or mirrors put limitations in focalization because it is usual to deal with oblique projections. A proposal to solve this problem by elliminating focusing was made by Lunazzi (27, 28) based on employing a linear luminous source, a long filament of a halogen lamp horizontally located behind the screen. The image is a consequence of the shadow projected by each part of the filament. The shadow on the screen can only be seen from a position which is precisely opposite to the filament part. A continuous sequence of shadows is generated rendering horizontal parallax. The angular extension of the filament in respect to the screen center defines the angular field for observation. The image is very peculiar (Fig. 10) because although it does not have inverted depth it shows an object whose closer parts are smaller than those which are farther away from the observer, as in a common shadow. The objects can only be transmission and not diffusing objects, which is not a problem because we can envision its application for elements like liquid crystal displays (Fig.11).

Fig. 10. Continuous parallax image projected with a linear source. Left: original view. Right: anaglyphic stereo representation with no color channel.

In this proposal discontinuity exists on the slicing, the more slices dividing the scene, the more perfect it appears. Slicing on video scenes should be made by a vertical white-light

The projection by means of lenses or mirrors put limitations in focalization because it is usual to deal with oblique projections. A proposal to solve this problem by elliminating focusing was made by Lunazzi (27, 28) based on employing a linear luminous source, a long filament of a halogen lamp horizontally located behind the screen. The image is a consequence of the shadow projected by each part of the filament. The shadow on the screen can only be seen from a position which is precisely opposite to the filament part. A continuous sequence of shadows is generated rendering horizontal parallax. The angular extension of the filament in respect to the screen center defines the angular field for observation. The image is very peculiar (Fig. 10) because although it does not have inverted depth it shows an object whose closer parts are smaller than those which are farther away from the observer, as in a common shadow. The objects can only be transmission and not diffusing objects, which is not a problem because we can envision its application for

Fig. 10. Continuous parallax image projected with a linear source. Left: original view. Right:

anaglyphic stereo representation with no color channel.

strip shaped beam sweeping on the scene, but has not been implemented yet.

**11. Lensless projection on diffractive screens** 

elements like liquid crystal displays (Fig.11).

Fig. 11. A LCD transmission watch as seen in the linear source projection

An improvement of this technique that can be expected would be a way to make the image appear in front of the screen and a way to illuminate from inside of its transparent support, in a similar way to the so-called "edge lit" holograms (29).

### **12. Not holographic diffractive screens**

If the term "holographic" corresponds to Gabor's idea of wave reconstruction, it should be applied to cases in which the interest is precisely the reproduction of waves, like in the case of imaging or holographic interferometry techniques. In that sense, the construction of a diffractive element by interferential means does not give to it the holographic characteristics. If the name "holographic" is given because a three-dimensional continuous parallax image results on the element, it is because the popular sense of the term is being employed. That is why the term "diffractive screen" was widely employed in this text. To reinforce the idea

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

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

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

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

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

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

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

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

It is common to say that holography is the technique allowing to reconstruct the wave amplitude and phase, but if we remain with this sole idea we could not properly analyze the holographic image. Considering that the phase of light cannot be seen, not even a detector with enough capability exists. What our eyes see are rays, directions of propagation. My personal vision of holography comes from my initiation as a 14-year-old photographer

This allows me to concentrate on the images and not on a mental visualization of Maxwell equations and their application, in which even complex-conjugated light fields are present. The third dimension can only be clearly perceived through binocular vision, which most popular imaging systems do not permit at present time. To demonstrate the existence of

can be provided, the adding of views reduces the brightness of the image.

not known yet.

consumes much computer processing power.

not employing visible light for that (32).

making stereo pictures.

unnecessary; a diffractive screen can accomplish the task equally.

sizes, filling large volumes to be seen from a large distance.

achieved at a considerably energy cost, low speed and short duration.

**14. How to evaluate parallax with ordinary images of a system** 

public.

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 diffractive screens.

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).
