a laser, interference, diffraction, light intensity recording and suitable illumination

of the recording. 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

image appear three-dimensional. The holographic recording itself is not an

image; it consists of an apparently random structure of either varying intensity, density

or profile.

Overview and history The Hungarian-British physicist Dennis Gabor,

was awarded the Nobel Prize in Physics in 1971 "for his invention and development of

the holographic method". His work, done in the late 1940s, built on pioneering work in

the field of X-ray microscopy by other scientists including Mieczysław Wolfke in 1920 and WL

Bragg in 1939. The discovery was an unexpected result of research into improving electron

microscopes at the British Thomson-Houston Company in Rugby, England, and the company

filed a patent in December 1947. The technique as originally invented is still used in electron

microscopy, where it is known as electron holography, but optical holography did not

really advance until the development of the laser in 1960. The word holography comes from

the Greek words ὅλος and γραφή.

The development of the laser enabled the first practical optical holograms that recorded

3D objects to be made in 1962 by Yuri Denisyuk in the Soviet Union and by Emmett Leith and

Juris Upatnieks at the University of Michigan, USA. Early holograms used silver halide photographic

emulsions as the recording medium. They were not very efficient as the produced grating

absorbed much of the incident light. Various methods of converting the variation in transmission

to a variation in refractive index were developed which enabled much more efficient holograms

to be produced. Several types of holograms can be made. Transmission

holograms, such as those produced by Leith and Upatnieks, are viewed by shining laser

light through them and looking at the reconstructed image from the side of the hologram opposite

the source. A later refinement, the "rainbow transmission" hologram, allows more convenient

illumination by white light rather than by lasers. Rainbow holograms are commonly used

for security and authentication, for example, on credit cards and product packaging.

Another kind of common hologram, the reflection or Denisyuk hologram, can also be viewed using

a white-light illumination source on the same side of the hologram as the viewer and is

the type of hologram normally seen in holographic displays. They are also capable of multicolour-image

reproduction. Specular holography is a related technique

for making three-dimensional images by controlling the motion of specularities on a two-dimensional

surface. It works by reflectively or refractively manipulating bundles of light rays, whereas

Gabor-style holography works by diffractively reconstructing wavefronts.

Most holograms produced are of static objects but systems for displaying changing scenes

on a holographic volumetric display are now being developed.

Holograms can also be used to store, retrieve, and process information optically.

In its early days, holography required high-power expensive lasers, but nowadays, mass-produced

low-cost semi-conductor or diode lasers, such as those found in millions of DVD recorders

and used in other common applications, can be used to make holograms and have made holography

much more accessible to low-budget researchers, artists and dedicated hobbyists.

It was thought that it would 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.

How holography works

Holography is a technique that enables a light field, which is generally the product of a

light source scattered off objects, to be recorded and later reconstructed when the

original light field is no longer present, due to the absence of the original objects.

Holography can be thought of as somewhat similar to sound recording, whereby a sound field

created by vibrating matter like musical instruments or vocal cords, is encoded in such a way that

it can be reproduced later, without the presence of the original vibrating matter.

Laser Holograms are recorded using a flash of light

that illuminates a scene and then imprints on a recording medium, much in the way a photograph

is recorded. In addition, however, part of the light beam must be shone directly onto

the recording medium - this second light beam is known as the reference beam. A hologram

requires a laser as the sole light source. Lasers can be precisely controlled and have

a fixed wavelength, unlike sunlight or light from conventional sources, which contain many

different wavelengths. To prevent external light from interfering, holograms are usually

taken in darkness, or in low level light of a different color from the laser light used

in making the hologram. Holography requires a specific exposure time, which can be controlled

using a shutter, or by electronically timing the laser.

Apparatus A hologram can be made by shining part of

the light beam directly onto the recording medium, and the other part onto the object

in such a way that some of the scattered light falls onto the recording medium.

A more flexible arrangement for recording a hologram requires the laser beam to be aimed

through a series of elements that change it in different ways. The first element is a

beam splitter that divides the beam into two identical beams, each aimed in different directions:

One beam is spread using lenses and directed onto the scene using mirrors. Some of the

light scattered from the scene then falls onto the recording medium.

The second beam is also spread through the use of lenses, but is directed so that it

doesn't come in contact with the scene, and instead travels directly onto the recording

medium. Several different materials can be used as

the recording medium. One of the most common is a film very similar to photographic film,

but with a much higher concentration of light-reactive grains, making it capable of the much higher

resolution that holograms require. A layer of this recording medium is attached to a

transparent substrate, which is commonly glass, but may also be plastic.

Process When the two laser beams reach the recording

medium, their light waves, intersect and interfere with each other. It is this interference pattern

that is imprinted on the recording medium. The pattern itself is seemingly random, as

it represents the way in which the scene's light interfered with the original light source

— but not the original light source itself. The interference pattern can be considered

an encoded version of the scene, requiring a particular key — the original light source

— in order to view its contents. This missing key is provided later by shining

a laser, identical to the one used to record the hologram, onto the developed film. When

this beam illuminates the hologram, it is diffracted by the hologram's surface pattern.

This produces a light field identical to the one originally produced by the scene and scattered

onto the hologram. The image this effect produces in a person's retina is known as a virtual

image. Holography vs. photography

Holography may be better understood via an examination of its differences from ordinary

photography: A hologram represents a recording of information

regarding the light that came from the original scene as scattered in a range of directions

rather than from only one direction, as in a photograph. This allows the scene to be

viewed from a range of different angles, as if it were still present.

A photograph can be recorded using normal light sources whereas a laser is required

to record a hologram. A lens is required in photography to record

the image, whereas in holography, the light from the object is scattered directly onto

the recording medium. A holographic recording requires a second

light beam to be directed onto the recording medium.

A photograph can be viewed in a wide range of lighting conditions, whereas holograms

can only be viewed with very specific forms of illumination.

When a photograph is cut in half, each piece shows half of the scene. When a hologram is

cut in half, the whole scene can still be seen in each piece. This is because, whereas

each point in a photograph only represents light scattered from a single point in the

scene, each point on a holographic recording includes information about light scattered

from every point in the scene. It can be thought of as viewing a street outside a house through

a 4 ft x 4 ft window, then through a 2 ft x 2 ft window. One can see all of the same

things through the smaller window, but the viewer can see more at once through the 4 ft

window. A photograph is a two-dimensional representation

that can only reproduce a rudimentary three-dimensional effect, whereas the reproduced viewing range

of a hologram adds many more depth perception cues that were present in the original scene.

These cues are recognized by the human brain and translated into the same perception of

a three-dimensional image as when the original scene might have been viewed.

A photograph clearly maps out the light field of the original scene. The developed hologram's

surface consists of a very fine, seemingly random pattern, which appears to bear no relationship

to the scene it recorded. Physics of holography

For a better understanding of the process, it is necessary to understand interference

and diffraction. Interference occurs when one or more wavefronts are superimposed. Diffraction

occurs whenever a wavefront encounters an object. The process of producing a holographic

reconstruction is explained below purely in terms of interference and diffraction. It

is somewhat simplified but is accurate enough to provide an understanding of how the holographic

process works. For those unfamiliar with these concepts,

it is worthwhile to read the respective articles before reading further in this article.

Plane wavefronts A diffraction grating is a structure with

a repeating pattern. A simple example is a metal plate with slits cut at regular intervals.

A light wave incident on a grating is split into several waves; the direction of these

diffracted waves is determined by the grating spacing and the wavelength of the light.

A simple hologram can be made by superimposing two plane waves from the same light source

on a holographic recording medium. The two waves interfere giving a straight line fringe

pattern whose intensity varies sinusoidally across the medium. The spacing of the fringe

pattern is determined by the angle between the two waves, and on the wavelength of the

light. The recorded light pattern is a diffraction

grating. When it is illuminated by only one of the waves used to create it, it can be

shown that one of the diffracted waves emerges at the same angle as that at which the second

wave was originally incident so that the second wave has been 'reconstructed'. Thus, the recorded

light pattern is a holographic recording as defined above.

Point sources

If the recording medium is illuminated with a point source and a normally incident plane

wave, the resulting pattern is a sinusoidal zone plate which acts as a negative Fresnel

lens whose focal length is equal to the separation of the point source and the recording plane.

When a plane wavefront illuminates a negative lens, it is expanded into a wave which appears

to diverge from the focal point of the lens. Thus, when the recorded pattern is illuminated

with the original plane wave, some of the light is diffracted into a diverging beam

equivalent to the original plane wave; a holographic recording of the point source has been created.

When the plane wave is incident at a non-normal angle, the pattern formed is more complex

but still acts as a negative lens provided it is illuminated at the original angle.

Complex objects To record a hologram of a complex object,

a laser beam is first split into two separate beams of light. One beam illuminates the object,

which then scatters light onto the recording medium. According to diffraction theory, each

point in the object acts as a point source of light so the recording medium can be considered

to be illuminated by a set of point sources located at varying distances from the medium.

The second beam illuminates the recording medium directly. Each point source wave interferes

with the reference beam, giving rise to its own sinusoidal zone plate in the recording

medium. The resulting pattern is the sum of all these 'zone plates' which combine to produce

a random pattern as in the photograph above. When the hologram is illuminated by the original

reference beam, each of the individual zone plates reconstructs the object wave which

produced it, and these individual wavefronts add together to reconstruct the whole of the

object beam. The viewer perceives a wavefront that is identical to the wavefront scattered

from the object onto the recording medium, so that it appears to him or her that the

object is still in place even if it has been removed. This image is known as a "virtual"

image, as it is generated even though the object is no longer there.

Mathematical model A single-frequency light wave can be modelled

by a complex number U, which represents the electric or magnetic field of the light wave.

The amplitude and phase of the light are represented by the absolute value and angle of the complex

number. The object and reference waves at any point in the holographic system are given

by UO and UR. The combined beam is given by UO + UR. The energy of the combined beams

is proportional to the square of magnitude of the combined waves as:

If a photographic plate is exposed to the two beams and then developed, its transmittance,

T, is proportional to the light energy that was incident on the plate and is given by

where k is a constant. When the developed plate is illuminated by

the reference beam, the light transmitted through the plate, UH is equal to the transmittance

T multiplied by the reference beam amplitude UR, giving

It can be seen that UH has four terms, each representing a light beam emerging from the

hologram. The first of these is proportional to UO. This is the reconstructed object beam

which enables a viewer to 'see' the original object even when it is no longer present in

the field of view. The second and third beams are modified versions

of the reference beam. The fourth term is known as the "conjugate object beam". It has

the reverse curvature to the object beam itself and forms a real image of the object in the

space beyond the holographic plate. When the reference and object beams are incident

on the holographic recording medium at significantly different angles, the virtual, real and reference

wavefronts all emerge at different angles, enabling the reconstructed object to be seen

clearly. Recording a hologram

Items required

To make a hologram, the following are required: a suitable object or set of objects