**5. Diffractive screens for stereo imaging**

Once holography was discovered and its first practical applications performed, the possibility of employing a hologram not for creating images but for generating vision zones of projected images appeared. The roughness properties that Gabor wanted for the screen to

In that way a Lippman-Bragg-Denisyuk hologram was made for the screen and its evolution made it capable of rendering color images by means of the three-chromatic procedure. It was discontinued, but constituted the only cinema system that can receive the academic term of "holographic", because recording was made by interference of the object light (a sequence of laser pulses) with a reference beam, recording and diffraction followed, and the concept of a holographic screen was introduced, giving continuous parallax and allowing the observer to move his head. The only visual shortcoming maybe came from the strict monochromaticity of the laser light beams. Living subjects were reproduced. The system cost was, of course, very high. While in need of more precise information, it was certain that at least eight people could watch a twenty minutes movie on the 1 m x 1 m square screen at

The very common use of photography has always rendered plane images and that is why it is normal to think that a lens image is plane. But we know that the images are threedimensional and that it is the detecting system which makes the result to be in a plane. By closing or opening the aperture of a lens, one always captures the same plane scene but the sharpness of those elements not precisely focused diminishes at large apertures. When opening the diaphragm from his smaller diameter, the out-of-focus situation of the additional rays, which do not fit in the image generated by the center of the lens, happens because of the new perspectives added by the lens area being unobstructed. It is important to notice that they correspond to the viewpoint of each area part being unobstructed on the

One of the first applications of this property was a three-dimensional photography system employing only one camera, opening to light only one area on each side of the lens, one transmitting through one color filter and the other through another, photographing in only

The horizontal parallax transmitted by a slit and placed on a lens may be employed for

Lunazzi (12) projected a scene directly from objects on a diffractive screen of 15 cm x 30 cm employing an ordinary slide projector objective. The 6x enlarged image gives the precise impression of a holographic one, but has more focusing depth limitations. "Direct holoprojection of objects" was a name given to this technique in which the horizontal

Son (13) employed this parallax property to project sequentially a set of views from a multimedia projector. Each slit position on the projector corresponded to a vision zone for the screen and the observer could have a different view within a discrete sequence of lateral positions. The image persistence on the retina gives the illusion of simultaneous viewing, but it is necessary for the system to put all views in the time of one ordinary movie frame (1/24s), so that a set of views may be projected. At 24 views per second, for example, the frame capability of the electronic multimedia system needed is about the square value of that of an ordinary projector. In the present state of the art it seems not possible to achieve this at a high definition resolution. Employing many projectors at close lateral positions is a possibility to reduce the frame rate needs and to obtain a brighter image, but it is only possible if the screen has a low scattering level in order to avoid the simultaneous view of all

a few meters distance.

**7. On the parallax of a lens image** 

lens. Continuous parallax is then allowed.

generating multiple views and even continuous parallax.

extension of a lens is the fundamental property (Figure 4).

one shot a direct anaglyph.

projections.

diffuse light vertically for the observer to see the whole projected image can then be obtained by making the hologram of a rectangular shape diffuser. The converging image which results from illuminating in a direction which is opposite to that corresponding to the reference beam occupies the zone where one observer's eye must be located. And the focusing property of the screen generates another observation zone for the other eye, provided that the angular separation of the two projectors to one screen point matches the binocular separation for the observer, obtaining by diffraction the effect of Ivanov's screen by refraction and reflection.

The idea was patented (8), but I do not know if any prototype was made.

It was conceived as a thick (Lippmann-Bragg-Denisyuk) hologram to avoid the simultaneous viewing of more than one image due to the spectral dispersion, so that the proposed solution was purely monochromatic. It still presents two of Ivanov's screen problems: different angular separation for binocular vision at different distances from the screen and the observer having to keep his head at an almost fixed position. While adding the lack of poly-chromaticity, it dispenses with a three-chromatic procedure to be applied for color reproduction.
