Holographic Movie Device

The subject of the present invention is a device for providing holographic movies or motion pictures and so obtaining for spectators the effect of relief.

It is known that if it is desired to record by means of holography objects having dimensions greater than 1 m, the amount of light diffused by this object and striking the sensitive surface is small. To form a hologram, which receives both this diffused light and a coherent reference wave, exposure times greater than a second must be used with lasers having a power of the order of a watt. This is due to the low sensitivity of the photographic support media of high resolution required for holographic recording. It is then not possible to record the 20 images per second required for the projection of movies.

It is further known, particularly from the article by J. P. Huignard and A. Marrakchi published in "Optics Communications", volume 38 (1981), page 249, to amplify a wavefront in photorefractive crystals such as Bi.sub.12 SiO.sub.20 (B.S.O.), by mixing two waves.

The present invention, using optical amplification, allows a hologram to be obtained requiring an exposure time compatible with the cinema.

It is an object of this invention to provide a holographic motion picture device for recording successive holographic images of a scene on a photosensitive film and means for projecting holographic images, wherein the recording comprises at least one laser which illuminates the scene and which further supplies the spherical reference wave incident on the film. The wave diffused by the film is incident to the induced refraction index variation medium, which medium is further illuminated by a flat wave also obtained from the beam supplied by the laser. The wave front obtained from illuminating the scene is amplified in this medium, and the amplified wave front strikes the film.

Other features and advantages of the invention will be clear from the following description, illustrated by the figures which show:

FIG. 1, the diagram of an amplification of a wave front;

FIG. 2, the simplified diagram of recording successive holograms, in accordance with the invention;

FIG. 3, the projection diagram of the holograms; and

FIG. 4, the diagram for mounting the three color source for holographic movies in color.

FIG. 1 shows the known principle of amplifying a wave front .SIGMA..sub.1 by means of an induced refraction index variation medium C which is preferably a B.S.O. crystal.

To obtain the optical amplification, it is possible to use the following physical effects:

optical Kerr effect for example in CS.sub.2,

saturated absorption in gases (Na vapor; SF.sub.6. . . ),

photorefractive effect: (BSO, LiNBO.sub.3, BaTiO.sub.3 etc . . . )

thermal effects.

In certain media, particularly in the B.S.O. crystal, it is known that, in order to have high amplification, a phase shift must be produced between the reference wave and the index variation network, by moving for example the crystal or the interference strata.

Amplification is obtained by means of a reference wave .SIGMA..sub.R and it results from the self-diffraction of this reference wave in the dynamic phase grating, induced in medium C.

The amplified wave front .SIGMA..sub.2 is identical to the incident wave front .SIGMA..sub.1, except for the amplification coefficient.

The incident wave .SIGMA..sub.1 and the reference wave .SIGMA..sub.R are at the same optical frequency and the same goes for the amplified wave .SIGMA..sub.2.

It is demonstrated in the document already cited by J. P. Huignard and A. Marrakchi that under these conditions the intensity of the wave transmitted by the interaction medium can be written in the form: ##EQU1## where .beta.=I.sub.R /I.sub.1

and I.sub.1, I.sub.2 and I.sub.R are respectively the intensities of the waves .SIGMA..sub.1, .SIGMA..sub.2 and the reference wave .SIGMA..sub.R, l is the length of the crystal, .alpha. its absorption coefficient and .GAMMA. the gain.

.GAMMA. is given by the expression: ##EQU2## where .DELTA.n.sub.S is the variation at saturation and .psi. the angle between the waves .SIGMA..sub.1 and .SIGMA..sub.R.

If .beta. is very large, expression (1) can be simplified and becomes:

From the expression (1) we find the value l.sub.o of l, which gives the greatest amplification which is such that:

By way of example, we will take the following numerical values:

refraction index at saturation, .DELTA.n.sub.S =10.sup.-4

absorption coefficient, .alpha.=2 cm.sup.-1

angle between the wave fronts in the medium, cos .psi..perspectiveto.1

reference intensity ratio on signal .uparw.=10.sup.9

We find from (2) and (4)

from which according to (4)

FIG. 2 shows the formation of a hologram of a scene P on a film F.

The light source is a laser L.sub.1, which illuminates the scene to be recorded P through two semi-transparent mirrors m.sub.1 and m.sub.2.

The optical power taken by the semi-transparent mirror m.sub.1 provides, by means of a lens l.sub.1 the flat reference wave .SIGMA..sub.R which illuminates the photorefractive medium C. Semitransparent mirror m.sub.2 takes off the optical power required for obtaining, by means of lens l.sub.2, the oblique reference spherical wave for the hologram on film F.

The light diffused by scene P falls on a lens l.sub.3 providing the signal wave front .SIGMA..sub.1 incident on the photorefractive medium, which supplies by amplification the wave front .SIGMA..sub.2 which falls on the film.

The hologram on film F is then obtained by interference of the amplified wave .SIGMA..sub.2 and the holographic reference wave R, supplied by lens l.sub.2.

Taking into account the amplification, the power of wave .SIGMA..sub.2 is such that the exposure time, for a laser L.sub.1 supplying a power of the order of 1 watt, is sufficiently short to allow a film to be produced for the cinema.

After development of the film F, the holograms may be projected, as shown in FIG. 3. A laser L.sub.2 provides the spherical wave for lighting the recorded hologram, through mirror m.sub.3 and lens l.sub.3. A lens l.sub.4 reflects the image practically to infinity. The image may thus be seen by a number of spectators.

In an example of application, we have:

Linear dimensions of the scene P to be filmed.omega.3 m

Shooting distance =10 m

Power of the recording laser L.sub.1 =2 W

Area of the amplifying medium=1.times.1 cm.sup.2

Length l.sub.o of the amplifying medium=1 cm

Effective gain of the amplifying medium=.GAMMA.-.alpha.=10 cm.sup.-1

Intensity of the incident signal .SIGMA..sub.1 on the amplifying medium-I.sub.1 =5 nW/cm.sup.2

Intensity of the amplified wave .SIGMA..sub.2, I.sub.2 =1000 .mu.W/cm.sup.2

Area of the hologram=1 cm.sup.2

Exposure time of the hologram=50 ms

Writing energy for the hologram=5 .mu.J.

Intensity of the reference wave R of the holograms=1 mW.

The exposure time of 50 ms is sufficient for a film of high resolution, required in holography, and allows the necessary rate to be obtained for the cinema.

On the other hand, without amplification the exposure time under the same conditions would have been 200 sec, which does not allow this rate to be obtained.

According to a variation of the invention, color holographic movies are provided.

For this, laser L.sub.1 of FIG. 2 is replaced by an assembly 1, shown in FIG. 4, comprising a blue laser L.sub.B, a green laser L.sub.V and a red laser L.sub.R, as well as a mirror M and two dichroic mirrors N.sub.1 and N.sub.2. These mirrors allow the beam from the three lasers L.sub.B, L.sub.V and L.sub.R to be superimposed. The same assembly 1 serves for projection.

Amplifying media such as B.S.O. may be used for the simultaneous amplification of the three wave fronts. In fact, the B.S.O. crystal is sensitive in the 450-650 nm range.

To conclude, a new device has been described for providing monochrome or trichrome movies in relief.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.