Holography

Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions. The technique of holography can also be used to optically store and retrieve information. Holograms are common in science-fiction, most notably Star Trek, Star Wars, and Red Dwarf.

Technical description

The difference between holography and photography is best understood by considering what a Black & White (B&W) 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 colour photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different colour filters), which allows a limited reconstruction of the wavelength of the light, and thus its colour.

Related Topics:
Intensity - Light - Amplitude - Electric field - Colour - Filter - Wavelength

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However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost. In a hologram, both the amplitude and the phase of the light (usually at one particular wavelength) are recorded. When reconstructed, the resulting light field is identical to that which emanated from the original scene, giving a perfect three-dimensional image (albeit, in most cases, a monochromatic one, though colour holograms are possible).

Related Topics:
Phase - Monochromatic

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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 which is combined with the light from the scene or object (the object beam). Optical interference 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.

Related Topics:
Interference - Superposition - Diffraction grating - Hologram

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Holographic reconstruction process

Once the film is processed, if 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 where the depth information is encoded entirely in the zoneplate angle). Because both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move their viewpoint and see the image rotate exactly as the original object would.

Related Topics:
Diffraction - Zoneplate

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Because of the need for interference between the reference and object beams, holography typically uses a laser in production. The light from the laser is split into two beams, one forming the reference beam, and one illuminating the object to form the object beam. A laser is used because the coherence of the beams allows interference to take place, although early holograms were made before the invention of the laser, and used other (much less convenient) coherent light sources such as mercury-arc lamps.

Related Topics:
Laser - Coherence - Mercury

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The coherence length of the beam determines the maximum depth the image can have. A laser will typically have a coherence length of several meters, ample for a deep hologram. Small pen laser pointers tend to have a smaller coherence length and were considered too small to do holography. That has been shown to be incorrect, and people have successfully made small holograms with laser pens. Large analogue holograms cannot be made with laser pens due to their lower power (typically 1mW to 5mW). Digital holography does not suffer from this problem.

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