Holography is a technique which enables three-dimensional images to be made. It involves the use of
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
a suitable laser beam part of the laser beam to be directed so that
it illuminates the object and another part so that it illuminates the recording medium
directly, enabling the reference beam and the light which is scattered from the object
onto the recording medium to form an interference pattern
a recording medium which converts this interference pattern into an optical element which modifies
either the amplitude or the phase of an incident light beam according to the intensity of the
interference pattern. an environment which provides sufficient mechanical
and thermal stability that the interference pattern is stable during the time in which
the interference pattern is recorded These requirements are inter-related, and
it is essential to understand the nature of optical interference to see this. Interference
is the variation in intensity which can occur when two light waves are superimposed. The
intensity of the maxima exceeds the sum of the individual intensities of the two beams,
and the intensity at the minima is less than this and may be zero. The interference pattern
maps the relative phase between the two waves, and any change in the relative phases causes
the interference pattern to move across the field of view. If the relative phase of the
two waves changes by one cycle, then the pattern drifts by one whole fringe. One phase cycle
corresponds to a change in the relative distances travelled by the two beams of one wavelength.
Since the wavelength of light is of the order of 0.5μm, it can be seen that very small
changes in the optical paths travelled by either of the beams in the holographic recording
system lead to movement of the interference pattern which is the holographic recording.
Such changes can be caused by relative movements of any of the optical components or the object
itself, and also by local changes in air-temperature. It is essential that any such changes are
significantly less than the wavelength of light if a clear well-defined recording of
the interference is to be created. The exposure time required to record the hologram
depends on the laser power available, on the particular medium used and on the size and
nature of the object(s) to be recorded, just as in conventional photography. This determines
the stability requirements. Exposure times of several minutes are typical when using
quite powerful gas lasers and silver halide emulsions. All the elements within the optical
system have to be stable to fractions of a μm over that period. It is possible to make
holograms of much less stable objects by using a pulsed laser which produces a large amount
of energy in a very short time. These systems have been used to produce holograms of live
people. A holographic portrait of Dennis Gabor was produced in 1971 using a pulsed ruby laser.
Thus, the laser power, recording medium sensitivity, recording time and mechanical and thermal
stability requirements are all interlinked. Generally, the smaller the object, the more
compact the optical layout, so that the stability requirements are significantly less than when
making holograms of large objects. Another very important laser parameter is
its coherence. This can be envisaged by considering a laser producing a sine wave whose frequency
drifts over time; the coherence length can then be considered to be the distance over
which it maintains a single frequency. This is important because two waves of different
frequencies do not produce a stable interference pattern. The coherence length of the laser
determines the depth of field which can be recorded in the scene. A good holography laser
will typically have a coherence length of several meters, ample for a deep hologram.
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 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. A hologram of a shiny object can be
made by locating it very close to the recording plate.
Hologram classifications There are three important properties of a
hologram which are defined in this section. A given hologram will have one or other of
each of these three properties, e.g. an amplitude modulated thin transmission hologram, or a
phase modulated, volume reflection hologram. Amplitude and phase modulation holograms
An amplitude modulation hologram is one where the amplitude of light diffracted by the hologram
is proportional to the intensity of the recorded light. A straightforward example of this is
photographic emulsion on a transparent substrate. The emulsion is exposed to the interference
pattern, and is subsequently developed giving a transmittance which varies with the intensity
of the pattern - the more light that fell on the plate at a given point, the darker
the developed plate at that point. A phase hologram is made by changing either
the thickness or the refractive index of the material in proportion to the intensity of
the holographic interference pattern. This is a phase grating and it can be shown that
when such a plate is illuminated by the original reference beam, it reconstructs the original
object wavefront. The efficiency is greater for phase than for amplitude modulated holograms.
Thin holograms and thick holograms A thin hologram is one where the thickness
of the recording medium is much less than the spacing of the interference fringes which
make up the holographic recording. A thick or volume hologram is one where the
thickness of the recording medium is greater than the spacing of the interference pattern.
The recorded hologram is now a three dimensional structure, and it can be shown that incident
light is diffracted by the grating only at a particular angle, known as the Bragg angle.
If the hologram is illuminated with a light source incident at the original reference
beam angle but a broad spectrum of wavelengths; reconstruction occurs only at the wavelength
of the original laser used. If the angle of illumination is changed, reconstruction will
occur at a different wavelength and the colour of the re-constructed scene changes. A volume
hologram effectively acts as a colour filter. Transmission and reflection holograms
A transmission hologram is one where the object and reference beams are incident on the recording
medium from the same side. In practice, several more mirrors may be used to direct the beams
in the required directions. Normally, transmission holograms can only
be reconstructed using a laser or a quasi-monochromatic source, but a particular type of transmission
hologram, known as a rainbow hologram, can be viewed with white light.
In a reflection hologram, the object and reference beams are incident on the plate from opposite
sides of the plate. The reconstructed object is then viewed from the same side of the plate
as that at which the re-constructing beam is incident.
Only volume holograms can be used to make reflection holograms, as only a very low intensity
diffracted beam would be reflected by a thin hologram.
Holographic recording media The recording medium has to convert the original
interference pattern into an optical element that modifies either the amplitude or the
phase of an incident light beam in proportion to the intensity of the original light field.
The recording medium should be able to resolve fully all the fringes arising from interference
between object and reference beam. These fringe spacings can range from tens of micrometers
to less than one micrometer, i.e. spatial frequencies ranging from a few hundred to
several thousand cycles/mm, and ideally, the recording medium should have a response which
is flat over this range. If the response of the medium to these spatial frequencies is
low, the diffraction efficiency of the hologram will be poor, and a dim image will be obtained.
Standard photographic film has a very low or even zero response at the frequencies involved
and cannot be used to make a hologram - see, for example, Kodak's professional black and
white film whose resolution starts falling off at 20 lines/mm — it is unlikely that
any reconstructed beam could be obtained using this film.
If the response is not flat over the range of spatial frequencies in the interference
pattern, then the resolution of the reconstructed image may also be degraded.
The table below shows the principal materials used for holographic recording. Note that
these do not include the materials used in the mass replication of an existing hologram,
which are discussed in the next section. The resolution limit given in the table indicates
the maximal number of interference lines/mm of the gratings. The required exposure, expressed
as millijoules of photon energy impacting the surface area, is for a long exposure time.
Short exposure times require much higher exposure energies, due to reciprocity failure.
Copying and mass production An existing hologram can be copied by embossing
or optically. Most holographic recordings have surface relief
patterns which conform with the original illumination intensity. Embossing, which is similar to
the method used to stamp out plastic discs from a master in audio recording, involves
copying this surface relief pattern by impressing it onto another material.
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 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
on the hologram recording layer. This method is particularly suited to mass production.
The first book to feature a hologram on the front cover was The Skook by JP Miller, featuring
an illustration by Miller. That same year, "Telstar" by Ad Infinitum became the first
record with a hologram cover and National Geographic published the first magazine with
a hologram cover. Embossed holograms are used widely on credit cards, banknotes, and high
value products for authentication purposes. It is possible to print holograms directly
into steel using a sheet explosive charge to create the required surface relief. The
Royal Canadian Mint produces holographic gold and silver coinage through a complex stamping
process. A hologram can be copied optically by illuminating
it with a laser beam, and locating a second hologram plate so that it is illuminated both
by the reconstructed object beam, and the illuminating beam. Stability and coherence
requirements are significantly reduced if the two plates are located very close together.
An index matching fluid is often used between the plates to minimize spurious interference
between the plates. Uniform illumination can be obtained by scanning point-by-point or
with a beam shaped into a thin line. Reconstructing and viewing the holographic
image When the hologram plate is illuminated by
a laser beam identical to the reference beam which was used to record the hologram, an
exact reconstruction of the original object wavefront is obtained. An imaging system located
in the reconstructed beam 'sees' exactly the same scene as it would have done when viewing
the original. When the lens is moved, the image changes in the same way as it would
have done when the object was in place. If several objects were present when the hologram
was recorded, the reconstructed objects move relative to one another, i.e. exhibit parallax,
in the same way as the original objects would have done. 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. A holographic image can also be obtained using
a different laser beam configuration to the original recording object beam, but the reconstructed
image will not match the original exactly. When a laser is used to reconstruct the hologram,
the image is speckled just as the original image will have been. This can be a major
drawback in viewing a hologram. White light consists of light of a wide range
of wavelengths. Normally, if a hologram is illuminated by a white light source, each
wavelength can be considered to generate its own holographic reconstruction, and these
will vary in size, angle, and distance. These will be superimposed, and the summed image
will wipe out any information about the original scene, as if superimposing a set of photographs
of the same object of different sizes and orientations. However, a holographic image
can be obtained using white light in specific circumstances, e.g. with volume holograms
and rainbow holograms. The white light source used to view these holograms should always
approximate to a point source, i.e. a spot light or the sun. An extended source will
not reconstruct a hologram since its light is incident at each point at a wide range
of angles, giving multiple reconstructions which will "wipe" one another out.
White light reconstructions do not contain speckles.
Volume holograms
A volume hologram can give a reconstructed beam using white light, as the hologram structure
effectively filters out colours other than those equal to or very close to the colour
of the laser used to make the hologram so that the reconstructed image will appear to
be approximately the same colour as the laser light used to create the holographic recording.
Rainbow holograms
In this method, parallax in the vertical plane is sacrificed to allow a bright well-defined
single colour re-constructed image to be obtained using white light. The rainbow holography
recording process uses a horizontal slit to eliminate vertical parallax in the output
image. The viewer is then effectively viewing the holographic image through a narrow horizontal
slit. Horizontal parallax information is preserved but movement in the vertical direction produces
colour rather than different vertical perspectives. Stereopsis and horizontal motion parallax,
two relatively powerful cues to depth, are preserved.
The holograms found on credit cards are examples of rainbow holograms. These are technically
transmission holograms mounted onto a reflective surface like a metalized polyethylene terephthalate
substrate commonly known as PET. Fidelity of the reconstructed beam
To replicate the original object beam exactly, the reconstructing reference beam must be
identical to the original reference beam and the recording medium must be able to fully
resolve the interference pattern formed between the object and reference beams. Exact reconstruction
is required in holographic interferometry, where the holographically reconstructed wavefront
interferes with the wavefront coming from the actual object, giving a null fringe if
there has been no movement of the object and mapping out the displacement if the object
has moved. This requires very precise relocation of the developed holographic plate.
Any change in the shape, orientation or wavelength of the reference beam gives rise to aberrations
in the reconstructed image. For instance, the reconstructed image is magnified if the
laser used to reconstruct the hologram has a shorter wavelength than the original laser.
Nonetheless, good reconstruction is obtained using a laser of a different wavelength, quasi-monochromatic
light or white light, in the right circumstances. Since each point in the object illuminates
all of the hologram, the whole object can be reconstructed from a small part of the
hologram. Thus, a hologram can be broken up into small pieces and each one will enable
the whole of the original object to be imaged. One does, however, lose information and the
spatial resolution gets worse as the size of the hologram is decreased — the image
becomes "fuzzier". The field of view is also reduced, and the viewer will have to change
position to see different parts of the scene. Applications
Art Early on, artists saw the potential of holography
as a medium and gained access to science laboratories to create their work. Holographic art is often
the result of collaborations between scientists and artists, although some holographers would
regard themselves as both an artist and a scientist.
Salvador Dalí claimed to have been the first to employ holography artistically. He was
certainly the first and best-known surrealist to do so, but the 1972 New York exhibit of
Dalí holograms had been preceded by the holographic art exhibition that 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.
During the 1970s, a number of art studios and schools were established, each with their
particular approach to holography. Notably, there was the San Francisco School of Holography
established by Lloyd Cross, The Museum of Holography in New York founded by Rosemary
H. Jackson, the Royal College of Art in London and the Lake Forest College Symposiums organised
by Tung Jeong. None of these studios still exist; however, there is the Center for the
Holographic Arts in New York and the HOLOcenter in Seoul, which offers artists a place to
create and exhibit work. During the 1980s, many artists who worked
with holography helped the diffusion of this so-called "new medium" in the art world, such
as Harriet Casdin-Silver of the USA, Dieter Jung of Germany, and Moysés Baumstein of
Brazil, each one searching for a proper "language" to use with the three-dimensional work, avoiding
the simple holographic reproduction of a sculpture or object. For instance, in Brazil, many concrete
poets found in holography a way to express themselves and to renew Concrete Poetry.
A small but active group of artists still use holography as their main medium, and many
more artists integrate holographic elements into their work. Some are associated with
novel holographic techniques; for example, artist Matt Brand employed computational mirror
design to eliminate image distortion from specular holography.
The MIT Museum and Jonathan Ross both have extensive collections of holography and on-line
catalogues of art holograms. Data storage
Holography can be put to a variety of uses other than recording images. Holographic data
storage is a technique that can store information at high density inside crystals or photopolymers.
The ability to store large amounts of information in some kind of media is of great importance,
as many electronic products incorporate storage devices. As current storage techniques such
as Blu-ray Disc reach the limit of possible data density, holographic storage has the
potential to become the next generation of popular storage media. The advantage of this
type of data storage is that the volume of the recording media is used instead of just
the surface. Currently available SLMs can produce about 1000 different images a second
at 1024×1024-bit resolution. With the right type of media, this would result in about
one-gigabit-per-second writing speed. Read speeds can surpass this, and experts believe
one-terabit-per-second readout is possible. In 2005, companies such as Optware and Maxell
produced a 120 mm disc that uses a holographic layer to store data to a potential 3.9 TB,
which they plan to market under the name Holographic Versatile Disc. Another company, InPhase Technologies,
is developing a competing format. While many holographic data storage models have used
"page-based" storage, where each recorded hologram holds a large amount of data, more
recent research into using submicrometre-sized "microholograms" has resulted in several potential
3D optical data storage solutions. While this approach to data storage can not attain the
high data rates of page-based storage, the tolerances, technological hurdles, and cost
of producing a commercial product are significantly lower.
Dynamic holography In static holography, recording, developing
and reconstructing occur sequentially, and a permanent hologram is produced.
There also exist holographic materials that do not need the developing process and can
record a hologram in a very short time. This allows one to use holography to perform some
simple operations in an all-optical way. Examples of applications of such real-time holograms
include phase-conjugate mirrors, optical cache memories, image processing, and optical computing.
The amount of processed information can be very high, since the operation is performed
in parallel on a whole image. This compensates for the fact that the recording time, which
is in the order of a microsecond, is still very long compared to the processing time
of an electronic computer. The optical processing performed by a dynamic hologram is also 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. In optics, addition and
Fourier transform are already easily performed in linear materials, the latter simply by
a lens. This enables some applications, such as a device that compares images in an optical
way. The search for novel nonlinear optical materials
for dynamic holography is an active area of research. The most common materials are photorefractive
crystals, but in semiconductors or semiconductor heterostructures, atomic vapors and gases,
plasmas and even liquids, it was possible to generate holograms.
A particularly promising application is optical phase conjugation. It allows the removal of
the wavefront distortions a light beam receives when passing through an aberrating medium,
by sending it back through the same aberrating medium with a conjugated phase. This is useful,
for example, in free-space optical communications to compensate for atmospheric turbulence.
Hobbyist use
Since the beginning of holography, experimenters have explored its uses. Starting in 1971,
Lloyd Cross started the San Francisco School of Holography and started to teach amateurs
the methods of making holograms with inexpensive equipment. This method relied on the use of
a large table of deep sand to hold the optics rigid and damp vibrations that would destroy
the image. Many of these holographers would go on to
produce art holograms. In 1983, Fred Unterseher published the Holography Handbook, a remarkably
easy-to-read description of making holograms at home. This brought in a new wave of holographers
and gave simple methods to use the then-available AGFA silver halide recording materials.
In 2000, Frank DeFreitas published the Shoebox Holography Book and introduced the use of
inexpensive laser pointers to countless hobbyists. This was a very important development for
amateurs, as the cost for a 5 mW laser dropped from $1200 to $5 as semiconductor laser diodes
reached mass market. Now, there are hundreds to thousands of amateur holographers worldwide.
By late 2000, holography kits with the inexpensive laser pointer diodes entered the mainstream
consumer market. These kits enabled students, teachers, and hobbyists to make many kinds
of holograms without specialized equipment, and became popular gift items by 2005. The
introduction of holography kits with self-developing film plates in 2003 made it even possible
for hobbyists to make holograms without using chemical developers.
In 2006, a large number of surplus Holography Quality Green Lasers became available and
put Dichromated Gelatin within the reach of the amateur holographer. The holography community
was surprised at the amazing sensitivity of DCG to green light. It had been assumed that
the sensitivity would be non-existent. Jeff Blyth responded with the G307 formulation
of DCG to increase the speed and sensitivity to these new lasers.
Many film suppliers have come and gone from the silver-halide market. While more film
manufactures have filled in the voids, many amateurs are now making their own film. The
favorite formulations are Dichromated Gelatin, Methylene Blue Sensitised Dichromated Gelatin
and Diffusion Method Silver Halide preparations. Jeff Blyth has published very accurate methods
for making film in a small lab or garage. A small group of amateurs are even constructing
their own pulsed lasers to make holograms of moving objects.
Holographic interferometry
Holographic interferometry is a technique that enables static and dynamic displacements
of objects with optically rough surfaces to be measured to optical interferometric precision.
It can also be used to detect optical-path-length variations in transparent media, which enables,
for example, fluid flow to be visualized and analyzed. It can also be used to generate
contours representing the form of the surface. It has been widely used to measure stress,
strain, and vibration in engineering structures. Interferometric microscopy
The hologram keeps the information on the amplitude and phase of the field. Several
holograms may keep information about the same distribution of light, emitted to various
directions. The numerical analysis of such holograms allows one to emulate large numerical
aperture, which, in turn, enables enhancement of the resolution of optical microscopy. The
corresponding technique is called interferometric microscopy. Recent achievements of interferometric
microscopy allow one to approach the quarter-wavelength limit of resolution.
Sensors or biosensors
The hologram is made with a modified material that interacts with certain molecules generating
a change in the fringe periodicity or refractive index, therefore, the color of the holographic
reflection. Security
Security holograms are very difficult to forge, because they are replicated from a master
hologram that requires expensive, specialized and technologically advanced equipment. They
are used widely in many currencies, such as the Brazilian 20, 50, and 100-reais notes;
British 5, 10, and 20-pound notes; South Korean 5000, 10,000, and 50,000-won notes; Japanese
5000 and 10,000 yen notes; and all the currently-circulating banknotes of the Canadian dollar, Danish krone,
and Euro. They can also be found in credit and bank cards as well as passports, ID cards,
books, DVDs, and sports equipment. Other applications
Holographic scanners are in use in post offices, larger shipping firms, and automated conveyor
systems to determine the three-dimensional size of a package. They are often used in
tandem with checkweighers to allow automated pre-packing of given volumes, such as a truck
or pallet for bulk shipment of goods. Holograms produced in elastomers can be used as stress-strain
reporters due to its elasticity and compressibility, the pressure and force applied are correlated
to the reflected wavelength, therefore its color.
Non-optical holography In principle, it is possible to make a hologram
for any wave. Electron holography is the application of
holography techniques to electron waves rather than light waves. Electron holography was
invented by Dennis Gabor to improve the resolution and avoid the aberrations of the transmission
electron microscope. Today it is commonly used to study electric and magnetic fields
in thin films, as magnetic and electric fields can shift the phase of the interfering wave
passing through the sample. The principle of electron holography can also be applied
to interference lithography. Acoustic holography is a method used to estimate
the sound field near a source by measuring acoustic parameters away from the source via
an array of pressure and/or particle velocity transducers. Measuring techniques included
within acoustic holography are becoming increasingly popular in various fields, most notably those
of transportation, vehicle and aircraft design, and NVH. The general idea of acoustic holography
has led to different versions such as near-field acoustic holography and statistically optimal
near-field acoustic holography. For audio rendition, the wave field synthesis is the
most related procedure. Atomic holography has evolved out of the development
of the basic elements of atom optics. With the Fresnel diffraction lens and atomic mirrors
atomic holography follows a natural step in the development of the physics of atomic beams.
Recent developments including atomic mirrors and especially ridged mirrors have provided
the tools necessary for the creation of atomic holograms, although such holograms have not
yet been commercialized. Things often confused with holograms
Effects produced by lenticular printing, the Pepper's Ghost illusion, tomography and volumetric
displays are often confused with holograms. The Pepper's ghost technique, being the easiest
to implement of these methods, is most prevalent in 3D displays that claim to be "holographic".
While the original illusion, used in theater, involved actual physical objects and persons,
located offstage, modern variants replace the source object with a digital screen, which
displays imagery generated with 3D computer graphics to provide the necessary depth cues.
The reflection, which seems to float mid-air, is still flat, however, thus less realistic
than if an actual 3D object was being reflected. Examples of this digital version of Pepper's
ghost illusion include the Gorillaz performances in the 2005 MTV Europe Music Awards and the
48th Grammy Awards; and Tupac Shakur's virtual performance at Coachella Valley Music and
Arts Festival in 2012, rapping alongside Snoop Dogg during his set with Dr. Dre.
During the 2008 American presidential election, CNN debuted its tomograms to "beam in" correspondents
including musician will.i.am as "holograms". An even simpler illusion can be created by
rear-projecting realistic images into semi-transparent screens. The rear projection is necessary
because otherwise the semi-transparency of the screen would allow the background to be
illuminated by the projection, which would break the illusion.
Crypton Future Media, a music software company that produced Hatsune Miku, one of many Vocaloid
singing synthesizer applications, has produced concerts that have Miku, along with other
Crypton Vocaloids, performing on stage as "holographic" characters. These concerts use
rear projection onto a semi-transparent DILAD screen to achieve its "holographic" effect.
In 2011, in Beijing, apparel company Burberry produced the "Burberry Prorsum Autumn/Winter
2011 Hologram Runway Show", which included life size 2-D projections of models. The company's
own video shows several centered and off-center shots of the main 2-dimensional projection
screen, the latter revealing the flatness of the virtual models. The claim that holography
was used was reported as fact in the trade media.
Holography in fiction
Holography has been widely referred to in novels, TV and movies.
See also
References
Reference sources
Further reading
External links Commercial Holographic Printing
Holographic Maker leading art & commercial hologram collection
Leading Holographic Company in India Vendor of holographic screens
International Hologram Manufacturers Association The Nobel prize lecture of Dennis Gabor
MIT's Spatial Imaging Group with papers about holographic theory and Holographic video
Medical Applications of Holograms How Stuff Works – holograms
Walker, John. "Holographic Art". Glossary of Art, Architecture & Design since 1945,
3rd. ed. Center for the Holographic Arts, New York
– a non-profit organisation promoting holography Specular holography art site
Faster way to produce holographic tiles Abrasion, hand-drawn holograms
Holoforum – A place to discuss holography Animations demonstrating holography by QED
Smart Future Solutions Holographic display How To Create A Hologram
U.S. Patent 3,506,327 — "Wavefront reconstruction using a coherent reference beam" — E. N.
Leith et al.