Download Holografie – lasery

Holografie – lasery
Pavla Štefková
1. HISTORIE HOLOGRAFIE
(Dennis Gabor, Emett Leith a Juris Upatnieks, Uri N. Denisyuk, T. H. Maimam – laser,
Stankoff a Pennington – dichromová želatina, Stehen A. Benton – transmisní holografie,
Lloyd Gross – integrální holografie)
2. POJEM HOLOGRAFIE A HOLOGRAM
(hologarmy – skladové, zakázkové, obtazové, lisované)
3. LASERY
(lasery – základní vlastnosti a druhy)
4. PRINCIP ZÁZNAMU A ZOBRAZENÍ
5. ROZDĚLENÍ A TYPY HOLOGRAFIE A HOLOGRAMŮ
(reflexní, transmisní, plzní, integrální)
6. ZÁZNAMOVÉ MATERIÁLY
(halogenidy stříbra, dichromová želatina (DCG), fotopolymery, atd)
7. POUŽITÍ
8. POUŽITÉ ZDROJE
HISTORIE HOLOGRAFIE
In 1971 the British / Hungarian scientist Dennis Gabor was awarded the Nobel Prize in
Physics for his discovery of holography. Gabor invented holography twenty-four years
beforehand when he was investigating ways to improve the resolving power of electron
microscopes. The resolution was limited by the spherical aberration of electron lenses which
were provided by magnetic fields. Gabor's paper in Nature in 1948 entitled `a new
microscopic principle described how micrographs could be obtained without electron lenses.
Figure: Gabor in-line hologram
Gabors method had two stages. In the first stage, the object was illuminated by an
electron beam. This beam was partially scattered from the object. The scattered wave then
interfered with the primary beam and the interference pattern was recorded on a photographic
plate. In the second stage, coherent light was shone through the developed photograph. A real
and a virtual image of the object were obtained. This method is illustrated in figure. As it was
not possible at the time to produce suitable beams of electrons, Gabor demonstrated the
feasibility of his technique by using visible light instead of electrons. He used a mercury-arc
lamp as his source. The low coherence length of the light produced by the lamp meant that he
could only use very small objects for his holograms.
Holography dates from 1947, when British/Hungarian scientist Dennis Gabor
developed the theory of holography while working to improve the resolution of an electron
microscope. Gabor, who characterized his work as "an experiment in serendipity" that was
"begun too soon," coined the term hologram from the Greek words holos, meaning "whole,"
and gramma, meaning "message." Further development in the field was stymied during the
next decade because light sources available at the time were not truly "coherent"
(monochromatic or one-color, from a single point, and of a single wavelength).
This barrier was overcome in 1960 with the invention of the laser, whose pure, intense
light was ideal for making holograms.
In 1962 Emmett Leith and Juris Upatnieks of the University of Michigan recognized
from their work in side-reading radar that holography could be used as a 3-D visual medium.
In 1962 they read Gabor's paper and "simply out of curiosity" decided to duplicate Gabor's
technique using the laser and an "off-axis" technique borrowed from their work in the
development of side-reading radar. The result was the first laser transmission hologram of 3D objects (a toy train and bird). These transmission holograms produced images with clarity
and realistic depth but required laser light to view the holographic image.
"Train and Bird" is the first hologram ever made with a laser. This pioneer image was
produced in 1964 by Emmett Leith and Juris Upatnieks at the University of Michigan only
four years after the invention of the laser
Their pioneering work led to standardization of the equipment used to make
holograms. Today, thousands of laboratories and studios possess the necessary equipment: a
continuous wave laser, optical devices (lens, mirrors and beam splitters) for directing laser
light, a film holder and an isolation table on which exposures are made. Stability is absolutely
essential because movement as small as a quarter wave- length of light during exposures of a
few minutes or even seconds can completely spoil a hologram. The basic that Leith and
Upatnieks developed is still the staple of holographic methodology.
Leith and Upatnieks preparing to shoot a laser transmission hologram using the "off-axis"
technique borrowed from their work in the development of side-reading radar. (Photo by
Fritz Goro for Life Magazine, 1967)
Also in 1962 Dr. Uri N. Denisyuk of the U.S.S.R. combined holography with 1908
Nobel Laureate Gabriel Lippmann's work in natural color photography. Denisyuk's approach
produced a white-light reflection hologram which, for the first time, could be viewed in light
from an ordinary incandescent light bulb.
Russian scientist Yurii N. Denisyuk, State Optical Institute in Leningrad, USSR, signing a
copy of his book, Fundamentals of Holography. (Photo by Dr. Stephen Benton, 1979)
In 1960 the pulsed-ruby laser was developed by Dr. T.H. Maimam of the Hughes
Aircraft Corporation. This laser system (unlike the continuous wave laser normally used in
holography) emits a very powerful burst of light that lasts only a few nanoseconds (a billionth
of a second). It effectively freezes movement and makes it possible to produce holograms of
high-speed events, such as a bullet in flight, and of living subjects. The first hologram of a
person was made in 1967, paving the way for a specialized application of holography: pulsed
holographic portraiture.
This historic 18"x24" laser transmission, pulsed portrait of Dr. Dennis Gabor, inventor of
holography, was recorded in 1971 by R. Rinehart, McDonnell Douglas Electronics Company,
St. Charles, MO. The portrait commemorated Gabor's winning of the Nobel Prize that year.
(Photo by Daniel E. Quat, 1976)
Shankoff and Pennington developed the use of a dichromated gelatin as a holographic
recording medium in 1967. This made it possible to record a hologram on any clear, nonporous surface. From 1975 - 1984, Rich Rallison (International Dichomate Corp., Draper,
UT) pioneered the use of dichromate holograms that were used as jewelry pendants and other
premium items. This type of holography has been best used for high performance diffractive
optics.
Another major advance in display holography occurred in 1968 when Dr. Stephen A.
Benton invented white-light transmission holography while researching holographic
television at Polaroid Research Laboratories. This type of hologram can be viewed in ordinary
white light creating a "rainbow" image from the seven colors which make up white light. The
depth and brilliance of the image and its rainbow spectrum soon attracted artists who adapted
this technique to their work and brought holography further into public awareness.
Dr. Stephen A. Benton, Massachusetts Institute of Technology, seen through "Crystal
Beginning," a white light transmission hologram produced at the Polaroid Corporation in
1977.
During the same year, Lloyd Cross and Gerry Pethick developed a sand-table system
for making holograms that did not require expensive laboratory optics and an isolation table
for stability during exposures. Optical components were stabilized by using PVC plumbing
pipes inserted into the sand. This revolutionized the medium by making it accessible by
artists. Cross and his associates started the San Francisco School of Holography in 1971, the
first such place for artists and scientists to learn the new medium.
In 1971 Dr. Dennis Gabor was awarded the Nobel Prize in Physics for his discovery of
holography in 1947.
In 1972 Lloyd Cross developed the integral hologram by combining white-light
transmission holography with conventional cinematography to produce moving 3-dimensional
images. Sequential frames of 2-D motion-picture footage of a rotating subject are recorded on
holographic film. When viewed, the composite images are synthesized by the human brain as
a 3-D image.
Lloyd Cross combined holography with cinematography to produce moving images
(holographic stereogram). (Photo by Rosemary H. Jackson, 1978)
POJEM HOLOGRAFIE A HOLOGRAM
Hologram je prostorový obraz. Pomocí dvourozměrného materiálu (emulze na skle,
filmu, plastové fólii) je přesně zobrazen trojrozměrný model. Hologram je věrnou prostorovou
kopií do posledního detailu. O tom svědčí i pojmenování. Význam slova ´HOLOGRAM´ je
´úplný nebo dokonalý záznam´. Vyrábí se v optické laboratoři. Jako zdroj světla pro
zhotovení hologramu se používá laser.
Rekonstrukce je zobrazení holografického obrazu pro pozorovatele. Bez dobrého
světla vidíme na hologramu málo nebo nic. Důležitou podmínkou dobré rekonstrukce je
bodový zdroj světla a správný úhel dopadu světelného paprsku. Na nich (kromě kvality
provedení vlastního hologramu) závisí ostrost, intenzita a barevnost zobrazení. Nejvhodnější
je halogenová žárovka nebo slunce. Úhel dopadu světla pro dokonalou rekonstrukci je
stanoven při výrobě a je neměnnou vlastností daného hologramu. Standardně se vyrábějí
hologramy pro osvětlení ve svislé ose hologramu pod úhlem cca 45° shora.
Skladové hologramy jsou hotové vzory lisovaných nebo obrazových hologramů,
určené k volnému prodeji. Prodávají se jednotlivé hologramy i série podle skladové nabídky.
Větší množství je třeba objednat předem. Na velké odběry je poskytována sleva.
Zakázkové hologramy jsou vyráběny na objednávku pro konkrétního majitele.
Výrobce garantuje zákazníkovi veškerá práva na zhotovování dalších kopií stejného vzoru.
Výroba ražených i obrazových zakázkových hologramů má několik etap: konzultace, grafický
návrh hologramu, technické zpracování designu a modelu až po výrobu hologramů a
případnou aplikaci. Vzhledem k velké rozmanitosti je cena kalkulována individuálně pro
každý obchodní případ jako celek.>
Obrazové (pravé 3D) hologramy jsou obrazem v měřítku 1:1. Hologramy se
zhotovují jednotlivě optickou cestou. To vede k jejich vyšší ceně a menším výrobním sériím.
Jednobarevné tónování je od červené přes zlatou až po zelené a modré provedení. Nosičem
záznamu je sklo nebo film. Tento druh hologramů se vyznačuje velkou hloubkou zobrazované
scény, přesvědčivostí rekonstrukce, širokým úhlem pozorování (jako při pozorování
skutečného objektu). Využívá se k unikátní výzdobě interiérů veřejných i soukromých /hodí
se pro místa s nutností umělého osvětlení/.
Lisované hologramy (duhové) byly vyvinuty pro potřebu zhotovování velkého
množství kopií a jejich cenové dostupnosti. Hologramy jsou lisované do plastové pokovené
nebo průhledné fólie. Finální výrobek je samolepka pro ruční aplikaci nebo fólie pro
strojovou aplikaci (horkou ražbu). Hologram může být barevný; barvy jsou reálné pouze v
jednom úhlu pozorování, v ostatních směrech dochází k duhovému efektu (odtud
pojmenování duhové). Předností, kromě efektivní výroby velkých sérií, je dobrá viditelnost i
při horší kvalitě světla, plná barevnost, větší různorodost vzorů, tvarovaný výsek. Nevýhodou
je menší hloubka obrazu a plná prostorovost pouze v jedné rovině. V souvislosti s vysokým
výskytem padělaného zboží na našem území se stává důležitou otázka zabezpečení originální
produkce a dokladů. Výrobky opatřené zabezpečujícím holografickým symbolem chrání jak
výrobce tak zákazníka. Výrobce má jistotu, že se mu nesnižuje profit, a zákazník dostane
opravdu pravé kvalitní zboží. Lisované hologramy jsou vyhledávaným ochranným prvkem,
jsou účinné pro zvýšení a podporu prodeje, pro reklamní účely, pro drobný dárkový prodej,
pro radost.
Hologram je opticky proměnlivý prvek, který mění vlastnosti dopadajícího světla tak,
že z něj vytváří obraz. Tento obraz může mít nejrůznější dynamické či prostorové vlastnosti,
které se společně s mnoha dalšími skrytými prvky bohatě využívají při použití hologramu pro
bezpečnostní účely. Pod mikroskopem objevíme na hologramu systém čar, který není tvořen
žádnými nánosy barvy, ale velmi jemnými vrypy do povrchu hologramu. Hustota vrypů je
větší než tisíc čar na délkový milimetr. Díky této hustotě čar se holografická struktura stává
opticky aktivní a je schopna vytvořit holografický obraz. Tiskařská ani kopírovací technika,
která má až několikařádově nižší rozlišení, není schopna hologram díky jeho velmi jemné
struktuře zapsat ani zkopírovat
Laserové technologie...
představují základní způsob vytváření hologramů. Struktura hologramu, tj. systém
vrypů, vzniká jako jednolitá výplň plochy vymezené grafickým návrhem nebo velikostí
vzájemně interferujících laserových svazků. Laserovou technologií je možné velmi dobře
vytvářet trojrozměrné obrazy stejně tak, jako různé obrazové pohybové efekty s různým
stupněm složitosti a s různým stupněm zabezpečení proti napodobení, či padělání.
LASERY
Holographers use two kinds of lasers to produce holograms: the continuous wave laser
and the pulsed laser. The continuous wave laser emits a steady stream of laser light, whereas
the
pulsed
laser
emits
laser
light
in
bursts.
Continuous wave lasers are far more commonly used in standard holography. As
discussed earlier, the recording of an interference pattern on the film forms a hologram. If the
subject moves, even a microscopic amount, from one moment to the next, two different
interference patterns will be recorded and the holographic image will be dim or may even not
appear at all. An exposure with a continuous wave laser can take from less than a second to
several minutes. During this time there can be no motion at all, including vibrations coming
from the ground; therefore, the laser, optics, and subject must be placed on a vibration
isolation table. Because it is absolutely critical that there is no motion at all, subjects that are
holographed with continuous wave lasers must be ‘dead' or immobile objects that can be
bolted
or
glued
to
the
optical
table
surface.
Pulsed lasers, quite the opposite of continuous wave lasers, emit extremely quick
bursts of very powerful laser light. Exposures can be made in ‘nanoseconds' (billionths of a
second). No vibration table is needed and holograms can be made of people, animals, or even
splashing water because no significant movement takes place in a nanosecond. The reason
pulsed lasers are not used more is because they are significantly more expensive than the
typical continuous wave laser.
Introduction to Lasers
Over the four decades since the first ruby laser was first tested in 1960, literally
hundreds of different lasers have been invented. Nevertheless, only a handful have found
general acceptance for use in volume OEM applications, as well as in the laboratory-the
helium neon laser (HeNe), air-cooled ion laser (argon, krypton, and mixed gas), and the diode
laser have been the most ubiquitous in these applications. Recently, diode-pumped solid-state
lasers (DPSS) and helium cadmium lasers (HeCd) have been used successfully in many OEM
systems, and their applications will increase. Melles Griot manufactures all of these laser
systems, and can provide them in single-unit or volume quantities.
The term "laser" is an acronym for light amplification by stimulated emission of
radiation. The theory behind the laser is beyond the scope of this chapter, but it is important
to identify the basic properties of lasers.
General laser properties
All lasers produce intense beams of light that are monochromatic, coherent, and highly
collimated. The wavelength (color) of the laser light is extremely pure (monochromatic) when
compared to other sources of light, and all of the photons (energy) that make up the laser
beam have a fixed phase relationship (coherence) with one another. This causes the light to
form a beam with a very low rate of expansion (highly collimated) that can travel over great
distances, or can be focused to a very small spot with a brightness that can approximate that
of the sun. Because of these properties, lasers are used in a wide variety of applications in all
walks of life.
All lasers include a gain medium (the source of the laser light, e.g., argon gas), an
excitation source (e.g., a power supply), and a resonator structure (mirrors or reflective
surfaces aligned to reflect some or all of the emitted light back through the gain medium).
Beyond these basic similarities, the lasers are all very different in their size, output, beam
quality, power consumption, and operating life.
The Helium Neon (HeNe) Laser
Specifications
The first to be used in volume applications, there are millions of HeNe lasers in the
field. Even today, only the diode laser (discussed below) is produced in greater volume. The
key to the HeNe's success has been its low cost, small size, long operating life (as much as
50,000 operating hours), and superb beam quality. HeNe lasers produce milliwatts of output
power, and are available in a variety of wavelengths ranging from 543 nm (green) to 3.39 m
m (infrared).
Metrology, reprographics, and bar-code scanning are but a few of the many
applications in which the HeNe laser has found wide acceptance. They are used as alignment
and pointing devices in surgical systems, as precision light sources in clinical and analytical
applications such as cell sorting, as wavelength references in industrial measurement and
positioning systems, and as large-depth-of-field holography sources.
Technical Description of a HeNe Laser
A schematic drawing of a HeNe laser is shown in the picture below. HeNe lasers are
high-voltage low-current devices operating in the "abnormal glow" discharge region.
Discharge current is a few milliamps, and tube discharge voltage ranges from a few hundred
volts to over a kilovolt. HeNe lasers are constructed of glass with a large oxidized-aluminum
"cold" cathode as the electron emitter. The limiting feature on operating life is cathode
degradation caused by erosion of the oxide coating leading to sputtering of aluminum. Tube
life is a strong function of discharge tube diameter.
The Diode Laser
Specifications
Today, diode lasers are readily available at wavelengths varying from 630 nm to 1.6 m
m and above, and at output power ranging from a few milliwatts to several watts. Because of
their small size and exceptional efficiency, diode lasers have replaced the HeNe laser in many
OEM applications, particularly in those where beam quality and wavelength stability, and
unit-to-unit repeatability are not critical.
The Air-Cooled Ion
Specifications
Air-cooled ion lasers produce up to 100 mW of output power, with excellent beam
quality and stability, at wavelengths ranging from the violet (458 nm) to the near infrared
(752 nm). The most prominent of these gas lasers is argon, with its strongest output at 514 nm
(green) and 488 nm (blue). The krypton laser produces is known for its red (647 nm) and
yellow (568 nm). The two gases can be combined to produce a laser with quasi-white all-lines
output.
Air-cooled ion lasers can be operated either multiline (with several wavelengths
operating simultaneously), or at a single wavelength. The choice of wavelength can be fixed
by adjusting the mirror coatings to select the appropriate wavelength, or varied, by using a
tunable Littrow prism as a wavelength-selecting element.
Because of their high power, excellent beam quality, and blue-green wavelength,
argon lasers have been used extensively in high-speed printing applications and in critical
cell-sorting and classifying applications.
Technical Description of Air-Cooled Ion Lasers
Argon-Ion Lasers
Specifications
Argon-ion lasers operate at a wide variety of wavelengths ranging from the violet to
the green. The strongest lines are the blue 488-nm line and the green 514-nm line. All Melles
Griot lasers are cooled by forced air. Systems are available in multiline, single-line, and linetunable configurations.
Argon/Krypton (Mixed Gas) Lasers
Specifications
These air-cooled lasers combine the characteristics of both argon-ion and krypton-ion
lasers, but with reduced output power. They operate at a variety of wavelengths ranging from
the blue to the red. The strongest lines are blue (488 nm), green (520 nm), yellow (568 nm),
and red (647 nm). Systems are available in multiline, single-line, and line-tunable
configurations. If a single red line is required, a pure krypton-ion laser will give more output.
If blue-green output is most important, an argon-ion laser may be a better choice.
Krypton-Ion Lasers
Specifications
Air-cooled krypton-ion lasers produce output in a wide variety of wavelengths
including red (647 nm and 676 nm), and infrared (752 nm). (Krypton lasers also can produce
yellow output at 568 nm as well as a variety of lines in the blue and green, but for stability
reasons.
The Helium Cadmium (HeCd) Laser
Specifications
The HeCd laser is a member of the same laser family as the HeNe, and is the only
relatively economical, continuous-wave source for violet (440 nm) and ultraviolet (325 nm)
output. Because of its excellent beam quality, it has been used extensively for 3-D
stereolithography applications, as well as for exposing holographs.
Technical Description of HeCd Lasers
A schematic diagram of a HeCd laser is shown below. HeCd lasers are low-current
high-voltage devices that operate in the "abnormal glow" discharge region. A heated cadmium
reservoir near the anode of the discharge tube vaporizes sufficient material to maintain the
proper partial pressure. The cadmium is transported down the bore by electrophoresis, and
condenses in cool trapping areas at the cathode end of the tube. A helium pump maintains
constant helium pressure over the life of the tube. Discharge current, helium pressure, and
cadmium pressure must all be very tightly controlled.
Diode-Pumped Solid State (DPSS) Lasers
Specifications
Diode-pumped solid-state (DPSS) microchip lasers are an exciting new tool for OEM
applications that combines the beam quality of a gas laser and the small size and efficiency of
a diode laser with single-line output in the blue (457 nm), green (532 nm), or infrared (1064
nm).
L stands for Light.
The amazing thing about lasers is that they harness the power of light and control it.
Laser light has several special qualities:
•
•
•
•
it comes in one color
it comes in a thin beam
it can be made very intense or not intense at all
and it can be focused to a tiny spot.
These characteristics are very different from light from a lightbulb. Lightbulb light:
•
•
•
•
has many colors mixed together
doesn't come in a narrow beam
cannot be focused to as small a spot
and cannot be as intense as a laser without expending tremendous amounts of
energy.
The A in laser stands for amplification
Amplification means that a very bright intense beam of light can be created. The laser
may be activated by a few photons, but then many, many more are generated. The initial light
is amplified to make a very bright compact beam.
The S in laser stands for stimulated
Stimulated means that the photons are amplified by stimulating an atom to release
more photons. An atom can exist in an excited state, similar to a bow when it is stretched.
When the atom relaxes it emits a photon.
This is similar to releasing the string of the bow and letting the arrow fly. Normally, an
atom will relax from its excited state anytime it feels like it. However, if another photon
comes by that has the same energy as the atom in the excited state, the atom will decide to
give off its photon and have it join the other. The atom is stimulated by another photon to
release its photon.
The E in laser stands for emission
Emission refers to the giving off of photons. The excited atom emits a photon when another
photon comes by. In 1917, Einstein described this process as Stimulated Emission.
More about Emission...
The photons bounce between the two mirrors until enough photons have been emitted
that some pass through the semi-silvered mirror on one end. These are the photons which are
seen as the laser beam.
The R in laser stands for radiation
Radiation is a word that has a bad reputation. It is a general term for anything that is
radiated, or given off by an object. For lasers, radiation refers to the photons which are being
emitted.
Light Amplification by the Stimulated Emission of Radiation
PRINCIP ZÁZNAMU A ZOBRAZENÍ
Recording Stage of a hologram
Laser beams can be used to create a hologram. A laser beam entering from the left is
split into two parts by a partially reflecting mirror acting as a beam splitter. The lower beam
then illuminates an object and scatters toward the plate on which the hologram is to be
formed. Simultaneously, a reference beam of coherent laser light, the upper beam, is sent
directly to the plate, where it interferes with the light from the objecd. The interference pattern
produced on the photographic plate stores both intensity and phase information about the light
arriving from the object, giving a more complete record of the object's appearance.
Reconstruction Stage of a hologram
An observer can view the object recorded in the hologram by illuminating the film
with a laser beam. The light scattered off the hologram toward the observer's eyes has the
same intensity and phase information as the light that would have come through the window
of the film from the real object, producing a realistic three-dimensional view of a virtual
object which no longer actually exists in space.
The fundamental idea behind a hologram is to illuminate the object and the plate
directly with a plane of coherent light, as illustrated. Each point on the plated received light
reflected from every point on the object, just as an imaginary plane placed there in space
would. Since the lighting is coherent, the rays all combine at each point in a unique way
indicative of the phase of the light wave at that imaginary plane. The trick to capturing this
combined phase information on the film is the use of a second direct coherent beam, extracted
from the same laser source, celled the reference beam. This second beam is projected directly
onto the plate, where it interferes with the waves arriving from the object in a way that can be
captured photographically: dark spots appear where the two beams interfere destructively and
bright spots where they interfere constructively. It in effect converts the phase information at
the plate, unrecordable by the film, into intensity information that is recordable. Later, when
the object has been completely removed, anyone can reconstruct the entire plane of light
coming from it, complete with the necessary phase information, by simply illumination the
plate with a coherent laser beam, as shown above.
To the observer a virtual object appears, looking identical to the original. The
observer's eyes each see a slightly different view, since the impinging beams are identical to
those that would have reached his or her eyes through the imaginary plane, and the object can
even be viewed from different angles as long as they are small enough to pass through the part
of the imaginary plane covered by the original photographic plate. This is the origin of the
three-dimensional effect we commonly associate with holograms.
Production of Transmission hologram
There are two main types: the transmission type, viewed by light passing through it
towards the observer, and the reflection type, viewed by light reflected back from the plate to
the observer. They are made by slightly different techniques.
Viewing a reflection hologram
When two light waves pass through each other each wave acts like a bump to the other.
And the result is like rapids of light. The standing wave patterns are stationary even though
the light waves energy continues to move. When waves meet they perform addition and
subtraction. When two waves of equal size meet at their high points (called crests), they add
together to make a wave twice as high at that point. Conversely, where two waves of equal
size meet at their low points (call troughs) they add together to become twice as low. And
when one wave at its high point meets another wave at its low point they subtract and cancel
out. But it isn't really cancelled out in the sense of being destroyed. Its more a case of there
being no light at that spot. If you follow the wave down its path just a drop further it will be
meeting the other wave at a different relationship and once again be visible. Its a situation of
infinite possibilities. Just like the patterns possible as the waves of two pebbles meet in a
pond. At any point you may notice that the standing wave pattern has produced a place where
the waves have added together to get higher or subtracted to become lower or even just gone
flat. There are few terms that are used to describe the possible encounters. If the waves add
and get higher its called constructive interference. If the waves subtract or cancel altogether
its called destructive interference
Imagine the interference pattern as a fingerprint of the encounter of two individual waves.
Each object you make a hologram of creates its own interference pattern that identifies it.
In holography, there are two basic waves that come together to create the interference
pattern. First and foremost is the wave that bounces off the object we are making a hologram
of. Since it bounces off the object, thereby taking its shape, it is called the object wave. You
can't have interference without something to interfere with. So a second wave of light that has
not bounced off an object is used to perform this function. It is called the reference wave.
When an object wave meets a reference wave creating a standing wave pattern of interference,
it is photographed and called a hologram. Semi-transparent mirror divides laser beam into two
beams. The first beam which is called a signal beam, is directed by mirror, expanded by lens
and it illuminates object. The second beam, called a reference beam, is also directed by
mirror, expanded by lens and it falls directly onto photoplate. The photoplate registers an
interference pattern between the bearing beam and the light beam, reflected from the object. A
transmission hologram appears after an ordinary photo-chemical treatment (hologram of
Leith-Upatnieks). If such a hologram is exposed to a laser light beam, you may see a 3-d
image of the object. The transmission hologram does not reconstruct the image in ordinary
white light, and it is necessary to copy it to the reflection hologram
How a Hologram Is made
The diagram below illustrates a typical holographic layout on a vibration-free table top
in a completely darkened room. The exposure must be made in an environment void of any
movement or vibration. Even movement to the degree of half a wave length of light will
prevent the recording of the image on the film.
A beam of laser light is optically separated into two beams. One, the reference beam,
is directed toward a piece of holographic film and expanded (its diameter increased) so that
the light covers the film evenly and completely. The second (object) beam is directed at the
subject of the composition and similarly expanded to illuminate it.
When the object beam reflects off the subject, it carries with it information about the
location, size, shape and texture of the subject. Some of this reflected object beam then meets
the reference beam at the holographic film, producing an interference pattern which is
recorded in the light sensitive emulsion.
After the film is developed, the hologram is illuminated at the same angle as the
"reference" beam during the original exposure to reveal the 3-D image.
A hologram must be illuminated to produce the image. Although laser light is used to
make holograms, holograms are usually illuminated with normal incandescent spotlights. To
see the image, the viewer must look at the film. "Projected" images appear in the space
between the film and the viewer. Images cannot be projected from the film to a distant point
as in cinematography. The projected image of Princess Leia from the chrome-dome of R2D2
in the 1977 movie Star Wars was a film special effect – not holography. Perhaps holography
will make this a reality in the future.
ROZDĚLENÍ A TYPY HOLOGRAFIE A HOLOGRAMŮ
Holography is a three-dimensional imaging technique. It uses laser light to record the
patterns of light waves reflected from an object onto the emulsion of light sensitive film (or
glass plates). When that film is developed, and re-exposed to laser light (or normal
incandescent light like most holograms today), it re-creates -- in space -- all the points of light
that originally came from the object. The resulting image, either behind or in front of the
holographic film, has all the dimensions of the original object and looks so real that you are
tempted to reach out and touch it -- only to find nothing there but focused light.
Unlike photography or painting, holography can render a subject with complete
dimensional fidelity. A hologram creates everything your eyes see -- depth, size, shape,
texture, and relative position – from many points of view. In fact, the term "hologram" was
coined from the Greek words holos, meaning "whole," and gramma, meaning "message."
More than one 3-dimensional image can be recorded on the same piece of film. For
example, the two-channel hologram, "Brain/Skull" (produced by The Polaroid Corporation in
the 1990s), displays two different images as it is viewed from left to right.
There are two basic types of holograms -- reflection and transmission. They can be
distinguished by the way in which they are illuminated.
Reflection holograms are lit from the front, reflecting the light to you as you view it,
like a painting or photograph hung on a wall. Different film emulsions produce images with
different characteristics.
1. Silver Halide is the emulsion of choice for most artists and holographers who use silver
halide glass plates to achieve the highest quality images. Holographers also use silver halide
film, which is cheaper, less fragile and easier to handle but does not produce the depth,
resolution and projection possible with glass plates. However, film is used successfully for
longer production runs.
2. Dichromated Gelatin (DCG) is a chemical-gelatin mix that is coated onto a piece of glass.
Dichromates produce very bright images in a golden-yellow color. The images have the least
range of depth, but they are viewable in normal room light without special spotlights. This
makes Dichromates suitable as small gift products for the consumer gift market or the
premium & incentive industry. Dichromates have largely been replaced with cheaper mass
produced photo polymer holograms. The medium still works best for high performance
diffractive optics and probably will for a long time to come. Bright, clean DCG masters are
still made for copying into photo polymers.
From 1975 - 1984, Rich Rallison (International Dichomate Corp., Draper, UT) pioneered the
production of glass sandwich dichromate holograms that were used as jewelry pendants, key
chains, paper weights, and other premium items. He produced these various sized dicromates
(up to 3") with Jason Sapan, Holograhic Studios, NYC in 1978-79.
3. Photo polymer is the newest of the recording materials. Developed by Polaroid and Dupont,
photo polymers have a plastic backing and are suitable for long production runs. The image
depth of photo polymers is slightly less than that of silver halide; however, the images are
brighter, with a wider angle of view.
Transmission holograms are lit from the rear (like a photographic transparency) and
bend light as it passes through the hologram to your eyes to form the image.
1. Laser transmission holograms are made with lasers, like all holograms, but also must be lit
with lasers to be viewed. Therefore, the images appear in the color of the laser used in
illuminating them for viewing, usually red (helium neon laser). Other types of holograms use
a laser transmission hologram as the master, from which copies are made. This is the earliest
type of hologram developed by Leith and Upatniks in 1962.
2. White light transmission holograms are illuminated with incandescent light (white light)
and produce images that contain the rainbow spectrum of colors. The colors change as the
viewer moves up and down and are often called "rainbow" holograms. Holographers have
developed considerable control over the colors displayed in this type hologram to produce
images in a specific color or in near full, natural color.
This is the first full color hologram, a 4 x 5" full color one step white light transmission
hologram by Dr. Stephen A. Benton, Herbert Mingace, Jr. and William R. Walter, The
Polaroid Corporation, in 1979.(Photo by S.A. Benton, Collection of MIT Museum,
Cambridge, MA)
Other techniques have been developed to record living subjects, to show movement
and to extend the mass-production capabilities of the medium.
Pulsed holography uses a quick, intense burst of laser light to record the subject in few
nanoseconds – too quick for movement to be a factor. This is similar to strobe or flash
photography and has been used extensively for portraitures.
Integral holography, developed by Lloyd Cross, combines holography with cinematography
to record a stereogram as a white-light transmission hologram. Several frames of 2-D motion
picture footage are converted frame-by-frame to narrow, slit holograms stacked side-by-side
on a piece of holographic film. Early integrals were mounted in a circular or semi-circular
format and produced an image that appeared in the center with several seconds of movement.
Any subject that can be recorded or reproduced as movie footage, video or computer graphics
can be made into a holographic image.
This is a series of photographs taken of "Kiss II," an integral hologram produced by Lloyd
Cross, inventor of the process. The hologram -- which was made from approximately 360
frames of motion picture footage -- was typically mounted in a semi-circular, wall-mounted
display and illuminated by a single light bulb below. The floating, 3-dimensional image of
Pam Brazier blows a kiss and winks as the viewer walks by. (Photo by Daniel Quat, 1977)
Embossed holograms are transmission holograms with a mirror backing that are applied to
magazines promotional items and credit cards. The holographic information is transferred
from light sensitive glass plates to nickel embossing shims. The holographic images are
"printed" by stamping the interference pattern onto plastic and then backing the images with a
light reflecting foil. The resulting hologram can be duplicated millions of times for a few
cents apiece.
ZÁZNAMOVÉ MATERIÁLY
Requirements
A hologram may be recorded in a medium as a variation of absorption or phase or
both. The recording material must respond to incident light pattern causing a change in its
optical properties. In the absorption or amplitude modulating materials, the absorption
constant changes as a result of exposure, while the thickness or the refractive index changes
due to the exposure in phase modulating materials. In the phase modulating materials there is
no absorption of light and all the incident light is available for image formation, while the
incident light is significantly absorbed in an amplitude modulating medium. Thus a phase
material can produce a higher efficiency than an amplitude material. Also in phase
modulating media the amount of phase modulation can be made as large as desired.
The resolution capability of a recording material depends on its modulation transfer
function. The nonlinear effects of the recording material are minimized for obtaining high
quality holographic images.
No single material possesses all the requirements of a holographic material. A material
is yet to be discovered which will have the high sensitivity of silver halides, high diffraction
efficiency and index modulation capability of DCG and photopolymers, recyclability of
photorefractive crystals, and useful at all laser wavelengths.
The silver halide materials have been the most popular choice of the holographers for
obvious reasons of high exposure sensitivity over a wide range of spectral regions and high
resolving power. These materials are suitable for transmission as well as reflection holograms,
both of amplitude and phase type.
The recording sensitivity of DCG has been extended to red wavelength making it
possible to record multicolour reflection holograms. This material is very difficult to handle
and the recorded holograms are sensitive to environmental conditions, yet this is a very good
material suitable for very high efficiency and low noise holograms. Photopolymers are
expected to replace DCG as these are also capable of producing large index modulation and
high diffraction efficiencies and are free from the disadvantages of DCG. Photopolymers do
not require lengthy controlled processing techniques and can be processed in situ. The
photopolymer holograms are insensitive to environmental changes.
Photoresists are suitable for producing surface relief holograms for making masters
needed for replication by embossing techniques. This material is most sensitive to
ultravioletlue light only. Efforts have recently been made to make photoresist material
sensitive at red wavelength.
Photorefractive crystals are very promising materials for real-time holography. They
can be recycled. Photothermoplastics can also be recycled several hundred times and are most
suitable for holographic interferometry.
Silver Halide Emulsion
The photographic emulsions are the most convenient and commonly used materials for
recording holograms. These are available in a wide range of spectral sensitivities. The
emulsion contains silver halide microcrystals, dispersed in gelatin deposited on a glass
substrate or on a plastic film. The average size of the grains in the holographic emulsion is
about 0.08-0.03 micrometer. To decrease the scattering the crystals must be smaller than the
wavelength of light so that Rayleigh theory becomes applicable. The emulsions with smaller
grains are `slower' than those with larger grains. The emulsion thickness range from 5 -15
micrometer. They can be used for recording thin or volume holograms of amplitude or phase
type. These materials have excellent shelf life.
The unexposed silver halide crystals remain in emulsion after development. These are
still photosensitive and limit the life of the developed emulsion. They can be removed by
fixing with sodium thiosulphate (hypo) which forms a number of water-soluble silvery
complexes along with a few water- insoluble complexes.
It is better to use two developing agents rather than one as their activity together is
greater than individual one. Most developing agents work efficiently only in alkaline solution
with pH>7. The developing agent together with an alkali may be sufficient to develop an
emulsion but the solution will lose its reducing power on reaction with atmospheric oxygen.
Therefore a preservative like sodium sulphite is added in large quantity. A restrainer is also
added to prevent the developer from attacking unexposed crystals. KBr or an organic
antifoggent may be used as a restrainer. The restrainer hinders the production of development
fog. A chemical developer contains five principal ingredients viz.
Hardened Dichromated Gelatin (DCG)
Dichromated gelatin (DCG) is the best for volume phase holograms capable of
producing very high diffraction efficiency and low scattering noise. Unfortunately because of
poor self life of DCG plate, it cannot be commercialized. The dichromated gelatin has the
following unique characteristics which make it an ideal material for hologram recording.
•
•
•
•
•
DCG has resolution capability extending beyond 5000 lines/mm. Its response
is uniform over a broad range of spatial frequencies from 100 lines/mm to
5000 lines/mm.
The refractive index modulation capacity of DCG is very high.
DCG has low absorption over a wide range of wavelengths.
DCG can give reconstructions even without development.
DCG hologram can be redeveloped to get any desired refractive index
modulation and peak diffraction wavelength.
Photoresists
Photoresists are organic materials for producing thin relief phase holograms.
Holograms in photoresist are ideal for making nickel masters which are used for
embossing replicas.
Photoresists are of negative or positive working type. In negative working photoresist
the unexposed areas are dissolved in the development, while in the positive working
photoresist, the exposed areas are dissolved away in the developer. The negative working
photoresist requires a larger exposure usually through the plate so that the expo sed
photoresist adheres to the substrate during development.
The most widely used photoresist is Shipley Microposit 1350 which is a positive
working resist. The sensitivity of this resist is maximum in the ultraviolet. It has adequate
sensitivity at 458 nm of argon ion laser and 442 nm of He-Cd laser. However, the sensitivity
at 488 nm is poor.
The material is coated on a glass substrate by spin method to a layer 1 to 2 micrometer
thick. It is then baked at 75 deg C for 15 min. On exposure to light, the layer thickness
changes.
Photothermoplastics
Thermoplastic is a material for producing surface relief thin phase holograms. The
thermoplastic material repeatedly softens and hardens when heated and cooled. The material
has a multilayer structure on a substrate of glass or film. It consists of three thin layers: doped
tin or indium oxide (a transparent conductor), polyvinyl carbazole sensitized with trinitro-9fluorenone (photoconductive organic polymer) and staybelite Ester 10 (thermoplastic
substance, a resin). The material has high sensitivity over the whole visible spectrum.
Photochromic Materials
Photochromics are real-time recyclable materials. They require no processing for
development and can be erased and reused. The holograms can be readout during or
immediately after the recording. This property is useful for holographic interferometry. There
is no inherent resolution limit since they are essentially grain free and operate on atomic or
molecular scale. Their storage capacity is high since the storage process occurs throughout the
volume of the material.
Photochromic materials change their transmission spectrum in response to exposure of
light of appropriate wavelength. They, in general, become dark under the action of shortwave
visible or ultraviolet radiation. They are bleached by exposure to longwave visible or infrared
radiation. This reversibility of the colour change distinguishes these materials from other
photosensitive materials. Both organic and inorganic materials have been studied for
photochromism.
Holograms are recorded in photochromic materials generally by selective optical
erasing or bleaching of material which has been darkened by uniform exposure of light. When
the material is exposed to an interference pattern of erase light, the transmission increases at
the bright portion of the pattern due to bleaching effect. This creates an absorption hologram.
To erase the hologram, it is illuminated by the switching light which darkens the crystal
uniformly.
Photodichroics
Photodichroism can also be used to create switched and unswitched states in suitable
crystals. This happens through selective alignment of anisotropic absorption centres indu ced
by exposure to linearly polarized light . The material selectively absorbs light of a certain
polarization only. Na-doped KCl crystal is a good example of photodichroism.
Photorefractive Crystals (Electro-Optic Materials)
Photorefractive crystals are excellent for recording volume phase holograms in realtime. These materials have excellent resolution, efficiency, storage capacity, sensitivity and
reversibility. The application of these materials for hologram recording was first considered
by Chen et al., who suggested that "optical damage" effect in these crystals could be exploited
to record a thick hologram.
POUŽITÍ HOLOGRAFIE (APLIKACE)
A hologram can be made not only with the light waves of a laser, but also with sound
waves and other waves in the electro-magnetic spectrum. Holograms made with X-rays or
ultraviolet light can record images of particles smaller than visible light, such as atoms or
molecules. Microwave holography detects images deep in space by recording the radio waves
they emit. Acoustical holography uses sound waves to "see through" solid objects.
Holography's unique ability to record and reconstruct both light and sound waves
makes it a valuable tool for industry, science, business, and education. The following are
some applications:
Double-exposed holograms (holographic interferometry) provide researchers with crucial
heat-transfer data for the safe design of containers used to transport or store nuclear materials.
A telephone credit card used in Europe has embossed surface holograms which carry a
monetary value. When the card is inserted into the telephone, a card reader discerns the
amount due and deducts (erases) the appropriate amount to cover the cost of the call.
Supermarket scanners read the bar codes on merchandise for the store's computer by using a
holographic lens system to direct laser light onto the product labels during checkout.
Holography is used to depict the shock wave made by air foils to locate the areas of highest
stress. These holograms are used to improve the design of aircraft wings and turbine blades.
A holographic lens is used in an aircraft "heads-up display" to allow a fighter pilot to see
critical cockpit instruments while looking straight ahead through the windscreen. Similar
systems are being researched by several automobile manufactures.
Magical, totally unique and lots of fun --candy holograms are the ultimate snack technology.
Chocolates and lollipops have been transformed into holographic works of art by molding the
candy's surface into tiny, prism-like ridges. When light strikes the ridges, it is broken into a
rainbow of brilliant iridescent colors that display 3-D images.
Researchers at the University of Alabama in Huntsville are developing the sub- systems of a
computerized holographic display. While the work focuses on providing control panels for
remote driving, training simulators and command and control presentations, researchers
believe that TV sets with 3-D images might be available for as little as $5,000 within the next
ten years.
Holography is ideal for archival recording of valuables or fragile museum artifacts.
Scientists at Polaroid Corp. have developed a holographic reflector that promises to make
color LCDs whiter and brighter. The secret lies in a transmission hologram that sits behind an
LCD and reflects ambient light to produce a white background.
The arrival of the first prototypical optical computers, which use holograms as storage
material for data, could have a dramatic impact on the overall holography market. The yet-tobe-unveiled optical computers will be able to deliver trillions of bits of information faster than
the current generation of computers.
Independent projects at IBM and at NASA's Jet Propulsion Laboratory have demonstrated the
use of holograms to locate and retrieve information without knowing its address in a storage
medium, but by knowing some of its content.
To better understand marine phytoplankton, researchers have developed an undersea
holographic camera that generates in-line and off-axis holograms of the organisms. A
computer controlled stage moves either a video camera or a microscope through the images,
and the organisms can be measured as they were in their undersea environment
Application areas
1. holographic optical elements (HOEs)
2. optical computing
3. optical metrology & microscopy
4. non-destructive testing (NDT)
5. three-dimensional imaging (display holography)
POUŽITÉ ZDROJE
• http://web.bham.ac.uk/ceu716/mid-term/node4.html
(historie holografie)
• http://www.holophile.com/history.htm
(historie holografie, aplikace holografie)
• http://www.hologram.cz/
(definice holografie a základní rozdělení hologramů)
• http://www.optaglio.cz/technologie.htm
(definice hologramu)
• http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/stereosc/chap3/chapter3_6_e.html
(zjednodušené vysvětlení pojmů holografie a hologramu)
• http://www.mellesgriot.com/products/lasers/technology.asp
(lasery, základní rozdělení)
• http://www.litiholographics.com/technology/tech_lasers.htm
(lasery používané v holografii)
• http://physics.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fwww.thetech.
org%2Fhyper%2Flasers%2Foverview.html
(popis laserů)
• http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/stereosc/chap3/chapter3_6_e.html
(princip zobrazení)
• http://ihome.cuhk.edu.hk/~s016969/physproj/Spectrum/Visible/hologram1.htm
(definice holografie a princip zobrazení a záznamu)
• http://www.holography.ru/phys2eng.htm
(podrobně popsané principy zobrazení hologramů)
• http://www.holophile.com/about.htm
(princip zobrazení, záznamové materiály, druhy holografie a hologramů)
• http://www.shadow.net/~holodi/feature1.htm
(silver halide záznamový materiál)
• http://hololight.virtualave.net/materials.html
(druhy a poisy jednotlivých záznamových materiálů pro holografii)
• http://www.mit.edu/afs/athena/course/other/mas450/www/handouts/Ch-01.html
(aplikace hologarfie)
• Miroslav Miler: Holografie (teoretické a experimentální základy a její použití)