Holography and mass image storage

Principle

Holography is an advanced form of photography that allows an image to be recorded in 3-D. This technique can also be used to optically store, retrieve, and process information.

Fig. 1 Identigram as a security element in a German Identity card (Personalausweis)

Several types of holograms can be made. The first holograms were "transmission holograms", which were viewed by shining laser light through them and looking at the reconstructed image of the other side. A later refinement, the "rainbow transmission" hologram allowed viewing by white light and is commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission holograms are now commonly formed as surface relief patterns in a plastic film, and they incorporate a reflective aluminum coating which provides the light from "behind" to reconstruct their imagery. Another kind of common hologram is the true "white-light reflection hologram" which is made in such a way that the image is reconstructed naturally using light on the same side of the hologram as the viewer.

Technical description

The difference between holography and photography is best understood by considering what a black and white photograph actually is: it is a point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a color photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different color filters), which allows a limited reconstruction of the wavelength of the light, and thus its color. Recent low-cost solid-state lasers are performed to make holograms

Fig. 1 Principle of making a hologram. (See Light: beam splitter for its working principle.)

Light, being a wave phenomenon, is characterized also by its phase. In a photograph, the phase of the light from the original scene is lost, and with it the three-dimensional effect. In a hologram, information from both the intensity and the phase is recorded. When illuminating the hologram with the appropriate light, it diffracts part of it into exactly the same wave (up to a constant phase shift invisible to our eyes) which emanated from the original scene, thus retaining the three-dimensional appearance. Also color holograms are possible.

Holographic recording process

To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam (see Fig. 1) which is combined with the light from the object (the object beam). If these two beams are coherent, optical interference (see Huygens’principle and Light: diffraction) between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram. The central miracle of holography is that when the recorded grating is later illuminated by a substitute reference beam, the original object beam is reconstructed, producing a 3D image.

These recorded fringes do not directly represent their respective corresponding points in the space of a scene (the way each point on a photograph represents a single point in the scene being photographed). Rather, a small portion of a hologram's surface contains enough information to reconstruct the entire original scene, but only what can be seen from that small portion as viewed from that point's perspective. This is possible because during holographic recording, each point on the hologram's surface is affected by light waves reflected from all points in the scene, rather than from just one point. It is as if, during recording, each point on the hologram's surface were an eye that could record everything it sees in any direction. After the hologram has been recorded, looking at a point in that hologram is like looking "through" one of those eyes.

To demonstrate this concept, you could cut out and look at a small section of a recorded hologram; from the same distance you see less than before, but you can still see the entire scene by shifting your viewpoint laterally or by going very near to the hologram, the same way you could look outside in any direction from a small window. What you lose is the ability to see the objects from many directions, as you are forced to stay behind the small window.

Holographic reconstruction process

When the processed holographic film is illuminated once again with the reference beam, diffraction from the fringe pattern on the film reconstructs the original object beam in both intensity and phase (except for rainbow holograms). Because both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move his or her viewpoint and see the image rotate exactly as the original object would.

Fig. 2 Principle of reconstruction of the image.

It is possible to store the diffraction gratings that make up a hologram as phase gratings or amplitude gratings of various specific materials.

Application

High tech applications are numerous in (astro)physics, but large scale application in mass storing medical images etc. is coming soon. On bank notes, credit cards etc. it are normal safely features. Because of the need for coherent interference between the reference and object beams, laser light is used to record holograms. But formerly other coherent light sources such as Hg-arc lamps, see Light: sources). In simple holograms, the coherence length of the beam determines the maximum depth the image can have. The coherence length L is:

L = λ²/(n Δλ).

where λ is the central wavelength of the source, n is the refractive index of the medium, and Δλ is the spectral width of the source. In sunlight and incandescent holograms on credit cards have depths of a few mm. A good holography laser will typically have a coherence length of several meters.

Holography can be applied to a variety of uses other than recording images.

Holographic data storage stores information at high density inside crystals or photopolymers and has the potential to become the next generation of popular storage media with possibly 1 gigabit/s writing speed and 1 terabit/s readout speed. Four terabyte disks are nearly commercially available.

An alternate method to record holograms is to use a digital device like a CCD camera instead of a conventional photographic film. This approach is often called digital holography.

More Info

Dynamic holography

The discussion above describes static holography, with sequentially recording, developing and reconstructing. A permanent hologram is produced.

There exist also holographic materials which don't need the developing process and can record a hologram in a very short time (optical parallel processing of the whole image). Examples of applications of such real-time holograms include phase-conjugate mirrors ("time-reversal" of light), optical cache memories, image processing (pattern recognition of time-varying images) and optical computing.

The fast processing compensates the fact that the recording time. The optical processing performed by a dynamic hologram is much less flexible than electronic processing. On one side one has to perform the operation always on the whole image, and on the other side the operation a hologram can perform is basically either a multiplication or a phase conjugation. But remember that in optics, addition and Fourier transform (see Fourier analysis) are already easily performed in linear materials, the second simply by a lens. This enables some applications like a device that compares images in an optical way.

Holonomic brain theory

The fact that information about an image point is distributed throughout the hologram, such that each piece of the hologram contains some information about the entire image, seemed suggestive about how the brain could encode memories. The fact that spatial frequency encoding displayed by cells of the visual cortex was best described as a Fourier transform of the input pattern. Also the cochlea makes a Fourier transform. This holographic idea leaded to the term "holonomic".