Holograms
Last updated: July 7, 2008.
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: The security hologram on a banknote helps to stop
people making fake copies. Holograms are harder to reproduce than other security devices.
How can you store a beam of light?
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.
Photo: Light is amazing. This is what sunlight hitting Earth looks like
from the Space Shuttle.
Photo by courtesy of NASA Johnson Space Center and
Great Images in NASA.
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.
What is a hologram?
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?
Photo: 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.
A hi-res version of this image is available for commercial use from our
Flickr page.
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 CCD
chip (in a digital camera). All the
light travelling 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 stereosopic
(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 holograms work
Laser light is much purer than the ordinary light in a torch beam.
In a torch beam, all the light waves are random and jumbled up.
Light in a torch 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 travelling in identical ways. When
they recombine in the photographic plate, the object beam has
travelled 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, all the pieces still contain enough information to recreate the complete hologram:
smash a glass hologram of a cup into bits and you can still see the entire cup in any of the bits!
What can we use holograms for?
Photo: The dove on this credit-card hologram seems to rotate as
you tilt the card in the light.
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!
Photo: The hologram on this DVD case is designed to deter copyright piracy.
Further reading
