**10. Continuous parallax in electronic images projected on diffractive screens**

The depth coding-decoding diffractive principle allows projection of a point source image at any position with respect to a diffractive screen. A computer-controlled figure generator was developed having a thin white light beam being focused at some millimeters from a diffraction grating whose lines were vertical. Because the beam was reflected on a mirror which was made to rotate through a vertical axis, the distance from the focus of the beam to the grating changed. This distance made the necessary degree of coding, which constitutes the horizontal size of a white-light spectrum. The movement was complemented with another two computer-controlled rotating mirrors to generate a luminous point located in any three dimensional position floating in front of the screen (23). A software was in charge of generating animations in the format of line figures. Figure 7 shows that a cube could be drawn without the need to correct any distortion. The image volume is about 100 l, no more is possible due to its reduced brightness. The possibility of white-light laser beams now

Fig. 7. Luminous points in a cube shape in front of the screen.

obtained by TiSa lasers whose spectrum is broadened by means of Bragg optical fibers may

The depth coding-decoding diffractive principle also allows us to project an electronic image at any plane in respect to a diffractive screen. By choosing an oblique plane in which to position a video image, an interesting approach to a holographic TV was obtained: The scene has depth as well as transversal characteristics, giving the illusion of a perfect 3D to most spectators (24). Although the images we show (Fig.8) shows limited resolution, this is due to electronic equipment limitations. The diffractive imaging process has in principle no

After some time observing, some people notice the lack of a perfect relief on the image. The scene can generate a volumetric image by projecting many parallel planes in a rapid sequence, each plane having a the image of a corresponding slice in which the whole image was divided. Animated scenes were made by three-dimensional computer drawing from which four slices were made. They were projected through a computer-controlled rotating mirror to make that at each mirror position the corresponding slice was projected (25, 26). Figure 9 shows a vertical sequence of stereo pairs corresponding to how the scene is viewed.

Fig. 9. Four stereo pair frames of a volumetric scene distributed in four assembled slices.

resolution limitations but those due to diffusion of the screen or speckle noise.

give to the application of this technique new possibilities.

Fig. 8. Vídeo scene appearing in an oblique plane in front of the screen. a) detail. b) complete image, 80 cm high, made of two hundred video lines.

Fig. 8. Vídeo scene appearing in an oblique plane in front of the screen. a) detail. b) complete

image, 80 cm high, made of two hundred video lines.

obtained by TiSa lasers whose spectrum is broadened by means of Bragg optical fibers may give to the application of this technique new possibilities.

The depth coding-decoding diffractive principle also allows us to project an electronic image at any plane in respect to a diffractive screen. By choosing an oblique plane in which to position a video image, an interesting approach to a holographic TV was obtained: The scene has depth as well as transversal characteristics, giving the illusion of a perfect 3D to most spectators (24). Although the images we show (Fig.8) shows limited resolution, this is due to electronic equipment limitations. The diffractive imaging process has in principle no resolution limitations but those due to diffusion of the screen or speckle noise.

After some time observing, some people notice the lack of a perfect relief on the image. The scene can generate a volumetric image by projecting many parallel planes in a rapid sequence, each plane having a the image of a corresponding slice in which the whole image was divided. Animated scenes were made by three-dimensional computer drawing from which four slices were made. They were projected through a computer-controlled rotating mirror to make that at each mirror position the corresponding slice was projected (25, 26). Figure 9 shows a vertical sequence of stereo pairs corresponding to how the scene is viewed.

Fig. 9. Four stereo pair frames of a volumetric scene distributed in four assembled slices.

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

in a similar way to the so-called "edge lit" holograms (29).

**12. Not holographic diffractive screens** 

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,

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

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