Holography

Holography (from the Greek, ???? – holos whole + ????? – graphe writing) is the science of producing holograms; it is 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, retrieve, and process information. It is common to confuse volumetric displays with holograms, particularly in science-fiction works such as Star Trek, Star Wars, Red Dwarf, and Quantum Leap.

• Read about the holography inventor – Dennis Gabor.

 

• Overview
• Technical description
• Holographic recording process
• Holographic reconstruction process
• Materials
• Mass replication
• Real-time holography
• Digital holography
• Holography in art

Overview

Holography was discovered in 1948 by Hungarian physicist Dennis Gabor (1900–1979), for which he received the Nobel Prize in physics in 1971. He received patent GB685286 on the invention. The discovery was an unexpected result (or serendipity as Dennis would say) of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England, but the field did not really advance until the discovery of the laser in 1960.

The first holograms which recorded 3D objects were made by Emmett Leith and Juris Upatnieks in Michigan, USA in 1963 and by Yuri Denisyuk in the Soviet Union.

There are several types of holograms which can be made. The very first holograms were “transmission holograms”, which were viewed by shining laser light through them. 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 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 (a Denisyuk 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.

One of the most promising recent advances in the short history of holography has been the mass production of low-cost solid-state lasers — typically used by the millions in DVD recorders and other applications, but sometimes also useful for holography. These cheap, compact, solid-state lasers can compete well with the large, expensive gas lasers previously required to make holograms, and are already helping to make holography much more accessible to low-budget researchers, artists, and dedicated hobbyists.

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 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.

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).

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 or the interference pattern.

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 her viewpoint and see the image rotate exactly as the original object would.

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.

In simple holograms 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. Also certain pen laser pointers have been used to make small holograms (see External links). The size of these holograms is not restricted by the coherence length of the laser pointers (which can exceed 1 m), but by their low power of below 5 mW.

Materials

It is possible to store the diffraction gratings that make up a hologram as phase gratings or amplitude gratings. In the former type the optical distance (i.e. the refractive index or in some cases the thickness) in the material is modulated. In amplitude gratings the modulation is in the absorption. Amplitude holograms have a lower efficiency than phase holograms and are therefore used casino games online more rarely. Most materials used for phase holograms reach the theoretical diffraction efficiency for holograms, which is 100% for thick holograms (Bragg diffraction regime) and 33.9% for thin holograms (Raman-Nath diffraction regime, holographic films of typically some ?m thickness).

The table below shows the principal materials for holographic recording. Note that these do not include the materials used in the mass replication of an existing hologram, which are described in the following section. The resolution limit given in the table indicates the maximal number of interference lines per mm of the gratings. The required exposure is for a long exposure. Short exposure times (less than 1/1000th of second, such as with a pulsed laser) require a higher exposure due to reciprocity failure.

General properties of recording materials for holography:

Material	Reusable	Processing	Type of hologram	Max. eff.	Required exposure [mJ/cm2]	Resolution limit [mm-1]
Photographic emulsions	No	Wet	Amplitude	6%	0.001–0.1	1,000–10,000
			Phase (bleached)	60%
Dichromated gelatin	No	Wet	Phase	100%	10	10,000
Photoresists	No	Wet	Phase	33%	10	3,000
Photothermo- plastics	Yes	Charge and heat	Phase	33%	0.001	500 – 1,200
Photopolymers	No	Post exposure	Phase	100%	1 – 1,000	2,000 – 5,000
Photochromics	Yes	None	Amplitude	2%	10 – 100	> 5,000
Photorefrac- tives	Yes	None	Phase	100%	0.1 – 50,000	2,000 – 10,000

Mass replication

An existing hologram can be replicated, either in an optical way similar to holographic recording, or in the case of surface relief holograms, by embossing. Surface relief holograms are recorded in photoresists or photothermoplastics, and allow cheap mass reproduction. Such embossed holograms are now widely used, for instance as security features on credit cards or quality merchandise.

The first step in the embossing process is to make a stamper by electrodeposition of nickel on the relief image recorded on the photoresist or photothermoplastic. When the nickel layer is thick enough, it is separated from the master hologram and mounted on a metal backing plate. The material used to make embossed copies consists of a polyester base film, a resin separation layer and a thermoplastic film constituting the holographic layer.

The embossing process can be carried out with a simple heated press. The bottom layer of the duplicating film (the thermoplastic layer) is heated above its softening point and pressed against the stamper so that it takes up its shape. This shape is retained when the film is cooled and removed from the press. In order to permit the viewing of embossed holograms in reflection, an additional reflecting layer of aluminum is usually added onto the hologram recording layer.

Real-time holography

Beyond conventional holography, there exists a technique whereby the steps used to form a hologram are performed simultaneously in a material that can be refreshed. That is, recording, developing and reconstructing occur simultaneously, and the medium allows continuous updating of the (dynamic) image — a “real-time” hologram. In this process, the film is replaced by either a passive material or an active electro-optical device (such as a spatial light modulator).

When a passive material is employed in place of the film, the real-time holographic interaction is often referred to as an all-optical process. That is, the only input to the process is optical energy (not electric currents or acoustic energy). The specific laser parameters (wavelength, polarization, intensity, etc.) and the material are selected so that the optical properties of the material are modified by the presence of the laser beam, such as the refractive index or the absorptive properties of the medium.

In a real-time hologram, therefore, the material that replaces film must be capable of changing in response to a varying set of recording beams and input image information. Examples of such materials are referred to as nonlinear optical materials, and can be realized using a variety of media such as photorefractive crystals, atomic vapors and gases, semiconductors or semiconductor heterostructures (such as quantum wells), plasmas and, even liquids. In this case, the local absorption and/or phase in the nonlinear material will be exposed, and will track changes in the interference pattern formed by the recording beams. As the interference pattern changes, the local absorption and/or phase pattern in the material will also change and replace the original pattern.

The simultaneous recording and reconstruction of a hologram is referred to as degenerate four-wave mixing (DFWM), as there are four optical beams that interact to form the real-time hologram: a pair of recording beams, a readout beam, and the resultant output, or reconstructed beam. The search for novel nonlinear optical materials for real-time holography is an active area of research.

The field of real-time holography and its potential applications is presently being pursued by researchers in aerospace, communications, image processing and machine vision. Examples of applications include high-energy lasers with enhanced performance for welding and materials processing, high-bandwidth free-space and optical fiber communication links, real-time pattern recognition systems and robust virtual reality systems. As an example of the latter application, MIT’s Spatial Imaging Group is developing systems that employ real-time holography to create machines which allow interactivity between a user and a three-dimensional mid-air projected image.

Digital holography

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. In this case, the reconstruction process can be carried out by digital processing of the recorded hologram by a standard computer. A 3D image of the object can later be visualized on the computer screen or TV set (i.e. a CLARO Holoscreen).

Holography in art

Salvador Dalí claimed to have been the first to employ holography artistically. He was certainly the first and most notorious surrealist to do so, but the 1972 New York exhibit of Dalí holograms had been preceded by the holographic art exhibition which was held at the Cranbrook Academy of Art in Michigan in 1968 and by the one at the Finch College gallery in New York in 1970, which attracted national media attention.

The Dalí Holograms were mastered in St. Louis, at the McDonnell Douglas Company who had just invested in a Ruby Pulse Laser and decided to, aside from meteorological purposes, make industrially oriented projection Holograms for presentations and trade shows. In London Dalí assembled his models by hanging objects with wires inside of wooden frames. This technique allowed for overlapping and differences in depth.

Since then the quality of the holograms has increased dramatically, mainly due to better holographic emulsions. As of 2005 there are many artists who use holograms in their creations.