How many holograms have you got in your pocket? If you're carrying
any money, the answer is probably "quite a few." Holograms are
those shiny, metallic patterns with ghostly images floating inside
them that help to defeat counterfeiters: they're very hard to
reproduce so they help to stop people printing illicit copies of
banknotes. Credit cards usually have holograms on them too and
software packages also frequently have hologrammatic seals to prove
their authenticity. What else can you use holograms for? Let's take a
closer look at what they are and how they're made!
Photo: Criss-crossing laser beams are the secret scientific power behind
holograms. Although you need a laser to make a hologram, you can view most holograms in ordinary light. Laser experiment photo courtesy of NASA.
Light is an amazing form of energy that zaps through our world at
blistering speeds: 300,000 km (186,000 miles) per second—enough to
whip from the Sun to Earth in just over 8 minutes. We see things
because our eyes are sophisticated light detectors: they constantly
capture the light rays bouncing off nearby objects so our brain can
construct an ever-changing impression of the world around us. The
only trouble is that our brain can't keep a permanent record of what
our eyes see. We can recall what we thought
we saw, and we can recognize images we've seen in the past, but we can't easily recreate
images intact once they've disappeared from view.
Back in the 19th century, ingenious inventors helped to solve this
problem by discovering how to capture and store images on chemically
treated paper. Photography,
as this became known, has revolutionized
the way people see and engage with the world—and it gave us
fantastic forms of entertainment in the 20th century in the form of
movies and TV. But no matter how
realistic or artistic a photograph
appears, there's no question of it being real. We look at a photo and
instantly see that the image is dead history:
the light that captured the objects in a photograph vanished long ago and can never
be recaptured.
Photo: The security hologram on a banknote helps to stop
people making fake copies. Holograms are harder to reproduce than other security devices. Tilt this banknote hologram and you can see the picture change.
Tilt it one way and you can see the number 10 (it's a British £10 note); tilt it the other
way and you can see the image of a mermaid.
What is a hologram?
"The photographic record obtained with optical apparatus of [this] kind... has been termed a
'hologram' and this name will be used hereinafter."
Holograms are a bit like photographs that never die. They're sort
of "photographic ghosts": they look like three-dimensional photos
that have somehow got trapped inside glass,
plastic, or metal.
When you tilt a credit-card hologram, you see an image of something like a
bird moving "inside" the card. How does it get there and what
makes it seem to move? What makes it different from an ordinary
photograph?
Suppose you want to take a photograph of an apple. You hold a
camera in front of it and, when you press the shutter button to take
your picture, the camera lens opens briefly and lets light through to
hit the film (in an old-fashioned camera) or the light-sensitive image sensor chip
(the CCD or CMOS chip in a digital camera). All the
light traveling from the apple comes from a single direction and enters a single lens, so the camera
can record only a two-dimensional pattern of light, dark, and color.
If you look at an apple, something different happens. Light
reflects off the surface of the apple into your two eyes and your
brain merges their two pictures into a single stereoscopic
(three-dimensional) image. If you move your head slightly, the rays
of light reflected off the apple have to travel along slightly
different paths to meet your eyes, and parts of the apple may now
look lighter or darker or a different color. Your brain instantly
recalculates everything and you see a slightly different picture.
This is why your eyes see a three-dimensional image.
A hologram is a cross between what happens when you take a
photograph and what happens when you look at something for real. Like
a photograph, a hologram is a permanent record of the light reflected
off an object. But a hologram also looks real and three-dimensional
and moves as you look around it, just like a real object. That
happens because of the unique way in which holograms are made.
How do you make a hologram?
You make a hologram by reflecting a laser
beam off the object you
want to capture.
Photo: Laser beams—the bright, perfect power behind holograms. Photo
courtesy of US Air Force and
Wikimedia Commons.
In fact, you split the laser beam into two separate
halves by shining it through a half-mirror (a piece of glass coated with a thin
layer of silver so half the laser light is reflected and half passes through—sometimes called a semi-silvered mirror). One half of the beam bounces off a mirror, hits the object, and reflects onto the photographic
plate inside which the hologram will be created. This is called the object
beam. The other half of the beam bounces off another mirror and hits
the same photographic plate.
This is called the reference beam.
A hologram forms where the two beams meet up in the plate.
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How holograms work
Photo: The dove on this credit-card hologram seems to rotate as you tilt the card in the light.
Laser light is much purer than the ordinary light in a flashlight (torch) beam.
In a flashlight beam, all the light waves are random and jumbled up.
Light in a flashlight beam runs along any old how, like schoolchildren racing
down a corridor when the bell goes for home time.
But in a laser, the light waves are coherent:
they all travel precisely in step, like soldiers marching on parade.
When a laser beam is split up to make a hologram, the light waves
in the two parts of the beam are traveling in identical ways. When
they recombine in the photographic plate, the object beam has
traveled via a slightly different path and its light rays have been
disturbed by reflecting off the outer surface of the object. Since the
beams were originally joined together and perfectly in step,
recombining the beams shows how the light rays in the object beam have been changed
compared to the reference beam. In other words, by joining the two beams back
together and comparing them, you can see how the object changes
light rays falling onto it—and
that's simply another way of saying "what the object looks like."
This information is burned permanently into the photographic plate by
the laser beams. So a hologram is effectively a permanent record of
what something looks like seen from any angle.
Now this is the clever part. Every point in a hologram
catches light waves that travel from every point in the object. That
means wherever you look at a hologram you see exactly how light would
have arrived at that point if you'd been looking at the real object.
So, as you move your head around, the holographic image appears to
change just as the image of a real object changes. And that's why
holograms appear to be three-dimensional. Also, and this is really neat, if you break a hologram into tiny pieces, you can still
see the entire object in any of the pieces: smash a glass hologram of a cup into bits and you can still see the entire cup in any of the bits!
(Hyperphysics has a more detailed explanation of exactly what we mean when we say "a piece of a hologram contains the whole object".)
What can we use holograms for?
Photo: The hologram on this DVD case is designed to deter copyright piracy.
Until the 1980s, holograms were a slightly wacky scientific idea.
Then someone found a way of printing them onto metallic film and they
became an incredibly important form of security. Proper glass
holograms look much more impressive than the tiny metallic ones you
see on banknotes and credit cards and you often see them used in
jewelry or other decorative items: you can even have holographic
pictures hanging on your wall with eyes that really do follow you
around the room! In the 1980s, a British theater even projected a
hologram of Laurence Olivier on stage to save the actor (who was, by
then, quite elderly) the hassle of appearing in person each night.
Lots of artists have experimented with making holographic pictures,
including the Spanish surrealist
Salvador Dali. Holograms also have important medical and scientific uses. In a technique called
holographic interferometry, scientists can make a hologram of
something like an engine part and store it as a "three-dimensional
photograph" for later reference. If they make another hologram of
the engine part at some later date, comparing the two holograms
quickly shows up any changes in the engine that may indicate signs of
wear or impending failure.
No-one's yet found a good way of making moving pictures with
holograms, but it's probably only a matter of time. Once that
happens, we can look forward to three-dimensional holographic TV and
a whole new era of super-realistic entertainment!
Who invented holograms?
Holograms were invented by a brilliant Hungarian-born physicist named
Dennis Gabor (1900–1979) while he was
working in the UK. He'd been researching optical physics in the 1940s, and carried out his breakthrough work in
holography in the early 1950s. The remarkable thing about his invention is that it was many years ahead of its time:
lasers, which made holography practical, did not appear until the 1960s. As Gabor's many
patents show, he was a prolific inventor with wide-ranging interests across many different areas of physics.
In the 1930s, he invented new kinds of electron multipliers
and cathode-ray tubes; in the 1940s, he was experimenting with
photography and projection, which
set him on the road toward holography; later inventions included composite fabrics
for use in television equipment, and various innovations in recording and transmitting sound. Towards the
end of his life, Gabor's brilliant contribution was recognized by the award of the world's top science prize, the
Nobel Prize in Physics 1971,
"for his invention and development of the holographic method."
Artwork: Dennis Gabor's original sketch of his 1950s holographic apparatus. Monochromatic light (yellow) enters at the bottom (1), passes through various prisms (blue) and lenses (gray) and is split into two beams. The low-intensity
object beam on the left passes through the specimen on a slide (red, 10); the high intensity reference beam on the right continues in parallel without touching the specimen. The beams are recombined in a photographic plate (21/22) at the top after passing through more lenses (gray) and prisms (blue). Artwork from US Patent #2,770,166: Improvements in and relating to optical apparatus for producing multiple interference patterns by Dennis Gabor, courtesy of US Patent and Trademark Office.
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Don't want to read our articles? Try listening instead
The Hologram: Principles and Techniques by Martin J. Richardson and John D. Wiltshire. Wiley/IEEE Press, 2018. An interesting and engaging mixture of history, science, and practical, hands-on technology.
Holograms:
A Cultural History by Sean Johnston. Oxford University Press, 2016. How have holograms changed the way we see the world?
Optics by Eugene Hecht. Pearson, 2016. This classic undergraduate textbook is a good place to start for a general grounding in light and optics.
If you're trying to understand the broad significance of holographic technology and how
it's changed over the years, these three articles by Professor Sean F. Johnston are well worth a look:
A Cultural History of the Hologram by Sean F. Johnston, Leonardo, Vol. 41, No. 3 (2008), pp. 223–229. An interesting look at the development of holography over the last few decades, with a timeline that shows how technical advances enabled a growing range of everyday applications.
Holographic Memories by Demetri Psaltis and Fai Mok, Scientific American, Vol. 273, No. 5 (November 1995), pp. 70–76. Back in the 1990s, scientists believed optical technologies such as holography would revolutionize computer memory.
Holography: Out of the Lab at Last: Popular Science, March 1984: How people were getting excited about holograms over a quarter century ago!
US Patent #3,561,838: Holographic imaging by Dennis Gabor, CBS. Filed March 24, 1967, issued Feb 9, 1971. One of Gabor's later patents, outlining a simpler way to produce higher-quality holographic images.
Hologram from LCD Projector by Baybren, Instructables.com. How to project a low-resolution "hologram" with an ordinary TV projector.
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