I. Overview of holographic films

The history of stereoscopic movies is almost as long as that of ordinary movies. The first large-scale stereoscopic cinema was built in the World Expo held in new york from 65438 to 0939, but when watching it, you need to wear a pair of glasses made of a mating film, and the glasses feel very tired. After World War II, widescreen movies developed rapidly, but in any case, widescreen movies themselves are not three-dimensional. The invention of holographic technology brings hope to real three-dimensional movies. Compared with stereoscopic movies, holographic movies have the following main advantages:

1. Three-dimensional holographic images can be obtained. For such an image, even a slight movement of the observer's head will change the perspective of the image, just like looking at a real object. Moreover, the observer's eyes are in a natural observation state, so he will not feel tired after long-term observation.

2. High information recording density. General holographic film software has high resolution. For example, the PE film of the former Soviet Union has an average diameter of 5~ 12nm, and the resolution is above 1000 lines per millimeter, which is several times higher than that of the color film used for movies at present.

3. When recording the Fourier transform hologram, the information of each small unit of the holographic image is distributed on the whole surface of the hologram, which not only reduces the size of the picture on the film by many times, but also does not cause image degradation due to scratches and dirty spots on the picture. In addition, Fourier transform hologram has an important property suitable for movies, that is, when the hologram moves perpendicular to the optical axis, the reconstructed image is still. Using this feature, we can make a simple and reliable film projector to make the film move slowly and continuously.

4. Holographic film adopts phase recording. Compared with ordinary film, the absorption of light by the film is greatly reduced, so the holographic film will not produce obvious heat when projecting the film.

5. The brightness coefficient of multi-focus holographic screen is many times higher than that of ordinary movie screen, so the former needs less light energy for projection.

Holographic film first appeared in the former Soviet Union in 1976, and the film recorded holograms. It shows a full-length portrait of a girl, holding flowers in front of her face and coming out of the screen. Everyone in the audience can swing their heads up and down, left and right in their seats, and can see her face around the bouquet. Therefore, what you see is a real three-dimensional image. The brightness of the image can be completely comparable to that of ordinary two-dimensional movies.

1982, France publicly exhibited the holographic animation of seagulls flapping their wings and flying to the audience. It is a Fresnel-type multi-slit hologram made by four people from the Film Art Laboratory of Paris University and the Optical Laboratory of Besson University.

1983 10 for the first time in Europe, a holographic film with 24 frames per second was produced on 35mm film by pulsed laser, and then a holographic film with 25 frames per second was produced on 26mm film. One of the realities is that a lady throws soap bubbles at the audience.

Two. Holographic film principle

A film produced and displayed by holography. The image is stereoscopic and has a sense of depth, and its brightness range is much larger than that of ordinary photography and movies.

holographic movies

Holography uses the interference of light waves to record and reproduce images. Hologram is a complete record of light wave front, and only a pattern composed of many thin lines can be seen directly, which has no similarity with the object being photographed. However, when the hologram is irradiated with laser, you can see a completely stereoscopic image behind the hologram without wearing any special glasses. Shooting holographic film requires the use of coherent light sources and high-resolution photographic film, as well as beam splitters and mirrors with specified optical paths, but no objective lens is needed. The shooting process is as follows: the coherent light source is divided into two beams, one of which is directly aimed at the photosensitive plate and called the reference beam; A beam of light is directed at an object, reflected by the object and reaches the photosensitive plate, which is called an object beam. Two beams of light meet in the plane of the photosensitive plate. Because of the different optical paths and phases, interference occurs, resulting in interference patterns, which are recorded by the photosensitive plate and become holographic plates.

The wavelength and direction of the light emitted by the coherent light source are the same, and it is monochromatic light with flat wavefront and continuous progress without interference. This kind of light is an important condition for producing interference patterns. The principle of generating holograms can be traced back to 300 years ago, and some people have done experiments with poor coherent light sources. But it was not until 1960 that laser was invented, and laser was the best coherent light source, so holography developed rapidly.

Three. Recording and Reproduction of Holographic Films

1. Fresnel type

Holographic film is shot in two steps. The first photo of the master hologram H 1 is shown in Figure 2. The horizontal slit S is placed in front of the holographic plate. When the slit S is moved in the vertical direction, the continuously moving object Oi is photographed. The master hologram H 1 thus obtained consists of a series of narrow strips of holograms hi, and their reconstructed images have parallax in the horizontal direction and time difference in the vertical direction. H 1 is reconstructed by using the conjugate light wave R 1 * of the reference light R1during recording. The holographic plate is placed in the middle of the reconstructed image, and the reference light R2 is introduced, and then the Fresnel hologram H2 is recorded, as shown in Figure 3. If H2 is irradiated with the conjugate light wave R2* of R2, a series of slit real images of the main hologram H 1 can be reproduced, and the observer can see the images reproduced from the slit hologram corresponding to this slit image. Since the slit S moves vertically during recording, a continuously moving image can be observed by moving the observation point up and down on the phase plane of the slit, or the angle of illumination light of the hologram H2 can be changed by fixing the position of the observer, or the position of the reproduced image of the slit moves up and down in the vertical direction, so that an animation with vertical parallax can be observed.

Fig. 4 is an optical path diagram of a hologram H2 shot by a two-color laser. First, the main hologram H 1r shot by red laser is placed, and the red reference light forms a 45-degree angle with the normal line of the dry plate. Then, the 8E56 dry plate with H2r of 50cm*60cm is shot on it, and then the Fresnel hologram of the real image is reproduced by the main hologram H 1g shot by green laser instead of H 1r, and the green reference light comes from the other side of the normal line of the dry plate. It should be noted that H2r should be placed so that its latex surface faces the reference light, while the latex surface of H2g faces away from the reference light. The exposed dry plate was developed by D- 19, bleached and dried, and the latex of H2r and H2g adhered to each other, thus obtaining the final holographic H2.

As shown in Figure 5, H2 reproduces the animation device, and the illuminating red light and green light enter the projection device along the 45-degree angle direction on both sides of H2 normal. When the distance from illumination light to H2 is 2.75 meters, the distance from slit real image to H2 is 2.24 meters ... The reconstructed slit real image of hologram H2 is 2.24 meters long, which is enough for six people to watch at the same time.

2. Rainbow type

Applying the principle of rainbow hologram to multi-slit hologram can realize white light reproduction. First, the master hologram H 1 is recorded and synthesized according to the optical path shown in Figure 2, and the light source is He-Ne laser. When H 1 is irradiated and reproduced with conjugated reference light R 1*, two vertical slits Sl and Sr are placed in front of H 1, as shown in Figure 6. Then, the hologram H2 is shot at the moderate position of each reconstructed image, and the reference light is in the horizontal plane, so as to ensure that H2 has a high spatial frequency in the horizontal direction. When reproducing H2, if the illumination light is conjugate reference light R2*, the slit real images of H 1 and the vertical slit real images SL' and Sr' are reproduced in space, as shown in fig. 7. If white light is used as illumination light, as shown in fig. 8, rainbow images of slits Sl and Sr will be reproduced in space. The observer can see the reconstructed image through the slit real image. When the observer moves horizontally, he will see reconstructed images of different colors. If you move the observation point vertically on the slit plane, you can see the holographic animation image.

3. Reflective type

The synthetic master hologram H 1 is still shot by the optical path shown in Figure 2, and the recording light wave is He-Ne laser. Then, the real image of H 1 is reproduced with the conjugate reference light R 1* as the object of the second recording. At this time, the reflection hologram H2 can be reproduced with white light and has a large field of view. By illuminating the hologram H2 with illumination light from a white light source at different angles, a moving image can be simply reproduced.

4. Difficulties in holographic film application

1. High power pulse laser. It is necessary to develop a pulse laser with three primary colors of red, green and blue. The pulse energy is 1 ~ 10J, the repetition frequency is 25Hz, and the required drying length is more than 10m. The sensitivity of the general holographic film in the sub-bureau is three pulse energies 1J laser energy 100m? M scene, 10J shooting 1000m? The scene of m.

2. Holographic movies. Is it required that the sensitivity of the film for one exposure should be about 5~ 10uJ/cm? Cm, the diffraction efficiency is about 50%~70%, and the signal-to-noise ratio is about 50~ 100.

3. Holographic screen. The screen should be able to shoot realistic three-dimensional color images of three primary colors, with a screen width of 6~8m, which can accommodate 200~400 seats of audience.

4. The quality of holographic film images. Mainly to eliminate the image caused by laser speckle.

5. The sound quality of holographic films. In the slit holographic film system, the film transmission speed is slow, which will bring some difficulties to the recording and playback of sound.