Monday, December 28, 2009


Holography (from the Greek, ὅλος-hólos whole + γραφή-grafē writing, drawing) is a technique that allows the light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the same position relative to the recording medium as it was when recorded. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the recorded image (hologram) appear three dimensional.
The technique of holography can also be used to optically store, retrieve, and process information. While holography is commonly used to display static 3-D pictures, it is not yet possible to generate arbitrary scenes by a holographic volumetric display.

Viewing the hologram
The holographic recording is the random variation in intensity which is an objective speckle pattern, and not the regular lines which are likely to be due to interference arising from multiple reflections in the glass plate on which the photographic emulsion is mounted. It is no more possible to discern the subject of the hologram from this than it is to identify the music on an audio CD by looking at the structure of the CD surface. When this hologram is illuminated by a divergent laser beam, the viewer will see the object used to make it (in this case, a toy van) because the light is diffracted by the hologram to reconstruct the light which was scattered from the object.
When one looks at a scene, each eye captures a portion of the light scattered from the scene, and the lens of the eye forms an image of the scene on the retina, in which light from each angular position is focused to a specific angular position in the image plane. Since the hologram reconstructs the whole of the scattered light field that was incident on the hologram, the viewer sees the same image whether it is derived from the light field scattered from the object, or the reconstructed light field produced by the hologram, and is unable to tell whether he or she is looking at the real or the virtual object. If the viewer moves about, the object will appear to move in exactly the same way whether he or she is looking at the original light field or the reconstructed light field. If there are several objects in the scene, they will exhibit parallax. If the viewer is using both eyes (stereoscopic vision), he or she will get depth information when viewing the hologram in exactly the same way as when he or she is viewing the real scene.
It should be clear from this why a hologram is not a 3D photograph. A photograph records an image of the recorded scene from a single viewpoint, which is defined by the position of the camera lens. The hologram is not an image, but an encoding system which enables the scattered light field to be reconstructed. Images can then be formed from any point in the reconstructed beam either with a camera or by eye. It was very common in the early days of holography to use a chess board as the object, and then take photographs at several different angles using the reconstructed light to show how the relative positions of the chess-pieces appeared to change.
Since each point in the hologram contains light from the whole of the original scene, the whole scene can, in principle, be reconstructed from an arbitrarily small part of the hologram. To demonstrate this concept, the hologram can be broken into small pieces and the entire object can still be seen from each small piece. If one envisions the hologram as a "window" on the object, then each small piece of hologram is just a part of the window from which it can still be viewed, even if the rest of the window is blocked off.

Viewing and authoring
The object and the reference beams must be able to produce an interference pattern that is stable during the time in which the holographic recording is made. To do this, they must have the same frequency and the same relative phase during this time, that is, they must be mutually coherent. Many laser beams satisfy this condition, and lasers have been used to make holograms since their invention, though it should be noted that the first holograms by Gabor used 'quasi-chromatic' light sources. In principle, two separate light sources could be used if the coherence condition could be satisfied, but in practice a single laser is always used.
In addition, the medium used to record the fringe pattern must be able to resolve the fringe patterns and some of the more common media used are listed below. The spacing of the fringes depends on the angle between object and reference beam. For example, if this angle is 45°, and the wavelength of the light is 0.5μm, the fringe spacing is about 0.7μm or 1300 lines/mm. A working hologram can be obtained even if all the fringes are not resolved, but the resolution of the image is reduced as the resolution of the recording medium reduces.
Mechanical stability is also very important when making a hologram. Any relative phase change between the object and reference beams due to vibration or air movement will cause the fringes on the recording medium to move, and if the phase changes is greater than π, the fringe pattern is averaged out, and no holographic recording is obtained. Recording time can be several seconds or more, and given that a phase change of π is equivalent to a movement of λ/2 this is quite a stringent stability requirement.
A good holography laser will typically have a coherence length of several meters, ample for a deep hologram. Certain pen laser pointers have been used to make small holograms. The size of these holograms is not restricted by the coherence length of the laser pointers (which can exceed several meters), but by their low power of below 5 mW.
The objects that form the scene must, in general, have optically rough surfaces so that they scatter light over a wide range of angles. A specularly reflecting (or shiny) surface reflects the light in only one direction at each point on its surface, so in general, most of the light will not be incident on the recording medium. It should be noted that the light scattered from objects with a rough surface forms an objective speckle pattern that has random amplitude and phase.
The reference beam is not normally a plane wavefront; it is usually a divergent wavefront that is formed by placing a convex lens in the path of the laser beam.
To reconstruct the object exactly from a transmission hologram, the reference beam must have the same wavelength and curvature, and must illuminate the hologram at the same angle as the original reference beam (i.e. only the phase can be changed). Departure from any of these conditions will give a distorted reconstruction. While nearly all holograms are recorded using lasers, a narrow-band lamp or even sunlight is enough to recognize the reconstructed image.
The reconstructed hologram is enlarged if the light used to reconstruct the hologram has a higher wavelength. This initially generated some interest since it seemed to be possible to use X-rays to make holograms of molecules and view them using visible light. However X-ray holograms have not been created to date. This effect can be demonstrated using a light source which emits several different frequencies.
Exact reconstruction is achieved in holographic interferometry where the holographically reconstructed wavefront interferes with the live wavefront, to map out any displacement of the live object, and gives a null fringe if the object has not moved.
Google SEO sponsored by Red Dragon Electric Cigarette Products