Holography

Holography. It is an advanced photography technique , which consists of creating three-dimensional images. For this, a laser beam is used, which microscopically records a photosensitive film. This, when receiving the light from the appropriate perspective, projects an image in three dimensions.

Summary

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  • 1 History
  • 2 Hologram
  • 3 Scheme of image reconstruction
  • 4 Advantages of the holographic method
  • 5 Source

History

In 1948 Dennis Gabor proposed a new method of obtaining images radically different from those used in ordinary optical instruments. For this discovery he was awarded in 1971 with the Nobel Prize in Physics. The essence of this method consists of the following.

The images obtained with ordinary optical instruments (photographic camera, projection flashlight, cinematographic projector, eye, etc.) record the intensity of the waves, that is, the square of their amplitude

The wave phase is lost in this case. Gábor proposed to use the phenomenon of interference to record the relationships between the frequencies and the wave phases and then use the figure obtained to reconstruct the relationships between the amplitudes. If only one wave parameter, its amplitude, is recorded in ordinary photography, by Gábor’s method complete information is recorded on all the wave parameters: frequency, phase and amplitude. The resulting interference figure in this case is called a hologram, and the method of obtaining the image is holography.

Hologram

To obtain the hologram, a light beam 1 is directed on a semi-transparent mirror M that divides it into two beams. Reference beam 2 falls directly on photographic plate F; beam object or signal 3 illuminates object S and diffuses. Part of the scattered light falls on the photographic plate, where it interferes with the reference beam. The interference pattern that occurs is fixed to the photosensitive emulsion. This is the hologram.

Due to its external appearance, the hologram is not at all like the object. It is a system of interference maxima and minima analogous, for example, to a Newton’s ring system

Attention should be drawn to the fact that between beams 2 and 3 there is a great difference in gear, from a few tens of centimeters to several meters. This creates certain difficulties in the process of obtaining the hologram.

The semi-transparent mirror divides each light beam into two, which, when they meet, must give an interference figure. But this only occurs if oscillations belonging to the same wave train are found at the given point in space. This should last a time interval commensurable with the train’s passing time. The length of the train L must be tens or even hundreds of times greater than the gear difference (fig. 66.20, a). In this case the succession of the trains at the observation point will be co-phase, the waves will be coherent and the interference figure will be observed (but if the gear difference turns out to be approximately equal to the length of the train), the succession of the trains in the reference wave and in the object wave it is carried out independently and the interference figure disappears.

The coherence and chromaticity of ordinary light sources are insufficient to obtain the holograms. Precisely for this reason, as Gábor said, holography suffered a fifteen-year lethargy. It was not until 1962 – 1963 that quantum optical generators or lasers were invented that emit highly coherent light with a train length several thousand times greater than that of trains from ordinary light sources (for example, from mercury lamps). With the coherent light of the lasers good quality holograms are achieved.

Image reconstruction scheme

Reconstruction beam 4 of coherent light strikes the hologram at the same angle as reference beam 2 strikes the photographic plate. By diffusing into the interference maxima and minima recorded in the hologram, the light is transformed into two beams, one diverging 5 and the other converging 6.

Beam 6 gives a real relief image of object 87. This has the defect, as seen in the figure, of being a mirror image, which is not always convenient.

Diverging beam 5 is generally used for observation. The eye in front of it looks through the hologram as through a window and sees the virtual image of object S, which exactly matches the object.

A method of obtaining color holograms was proposed in 1962 by Yu. N. Denisiuk1 based on Lipman’s idea of ​​interference color photography. In the figure the reference wave 1 and the object wave 2 impinge on both sides on a thick layer of E1 photosensitive emulsion in which a standing wave system is produced. For reconstruction the hologram is illuminated at the same angle with the reconstruction wave 3 that is diffused in the bellies of the standing wave. The observer perceives the diffuse beam 4 and sees the virtual image S • The peculiarity of the color hologram is that the bellies formed by the waves of different length are in different. Therefore,

For the discovery of the method of obtaining color holograms, Yu.N. Denisiuk was awarded the Lenin Prize.

Advantages of the holographic method

1-In ordinary photography each part of the emulsion presents a separate part of the object. So the information contained in one part of the photograph has no relation to the information contained in another part. The destruction of any part of a photograph means the loss of the corresponding information.

In the hologram each of its parts contains information about the whole figure, so the image that is obtained, even from a small part of the hologram, is a complete and correct representation of the object, although less bright and sharp. This is analogous to how, using a small piece of lens, the image of an object can be obtained than with the entire lens, although its quality is somewhat worse. As an information store, the hologram is much more secure than ordinary photography.

2- The hologram is characterized by having a much greater capacity for information than photography. Thus, if a page of printed text can fit on a sheet of photographic paper or a 6 x 9 mm plate, depending on the quality of the emulsion, 100 to 300 holograms can be inscribed on the same surface. Nowadays, with the abrupt increase experienced by printing production, the problem of compact information storage has become more acute and in the future it will become even more acute. Holography gives the possibility to solve this problem.

3- Holography can solve the problem of creating stereoscopic cinema and color television.

4- If the length l ‘of the image reconstruction wave is greater than the length? of the wave with which the hologram was obtained, the image will be greater than the object in the proportion? ‘D. This allows the magnification and separating power of the microscope to be greatly increased. But this microscope is for now a future thing.

We note, however, that this also has a limit: the 2 ‘wavelength of the reconstruction beam must be several times less than the distance between the interference fringes. In the opposite case, the emulsion will be a homogeneous medium for this wave (62.8) and the holographic effect will disappear.

5- Acoustic holography is of great interest . Coherent sound waves are very easily obtained and sound (or ultrasound) propagates well in liquids and solids. So it’s easy to get an acoustic three-dimensional hologram of opaque objects. If the image is then reconstructed with visible light, we can see the internal structure of these bodies, for example, the structure of a metal rod, a concrete beam or the entrails of the body. For both technique and medicine this offers colossal interest. The main difficulty encountered in this case is the methods of recording and setting the acoustic holograms.

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