I'm not talking here about the 'holograms' on special release DVDs, which give you two or three aspects of the same object -- sometimes, two or three entirely different pictures -- which you can view simply by moving your head from side to side. Real holograms show a continuously varying view as you move your head. If it's a large one, move slowly to the side and gradually the objects in the background that are obscured by a foreground detail are revealed. They also tend to be spooky, displayed in darkened rooms by oddly positioned lights.
But they are not magic at all. They are simply a variation on the photograph. Indeed, the glass plates or plastic film upon which they are made are essentially the same as black and white photographic film, merely tuned for their particular task.
The first holograms were created -- and a Nobel Prize duly awarded to their inventor some years later -- in 1947. But they remained impracticable until 1960, when the Laser was invented. Laser stands for 'Light Amplification by the Stimulated Emission of Radiation', but it's more conveniently thought of as coherent light.
Step outside on a sunny day, and the white light that illuminates everything is actually a mess. It consists of an incoherent stew of radiation. The light waves come in a near-infinite range of wavelengths. Each wavelength is perceived by the eye as a different colour (which we fake on TV screens using just the wavelengths for red, green and blue). All mixed together, they appear white. They are scattered during their entry to and passage through the atmosphere, and a significant proportion of the light has already bounced of terrestrial objects, so it's going in all directions.
Coherent light of the kind produced by a laser goes in one direction -- which is why your laser pointer makes a nice tight dot on the presentation screen. And it's of a single wavelength -- which is why that dot is red (other colours are available, but rarer).
Holograms are made by shooting a laser beam through a lens, which widens the beam so it's big enough to illuminate the object you're trying to take a picture of, then through the special photographic film (which is canted at an angle, typically 45 degrees) onto the object. Some of this light bounces back. Some goes further (into the pits of the coin that's your object), and therefore take a tiny bit longer to bounce back.
The light that bounces back encounters the laser beam which is still coming its way and causes an interference pattern. The film captures that interference pattern.
Process the film and you have a hologram ready for viewing. To see it, shine a light from the back at the same angle as the original laser and you have the full three dimension representation of the original object.
Is all this of any use? Aside, that is, from peculiar 3D pictures that work best with odd lighting in a dark room?
Heard of HVD? That stands for Holographic Versatile Disc. If those last two words look familiar, that's because they are the last two words of DVD as well.
A single layer DVD holds 4.7 gigabytes of data. The heirs apparent, Blu Ray and HD-DVD, hold between 15 and 27 gigabytes on a disc the same size. Using holographic techniques, a bunch of companies including Fuji and Konica are busily specifying a 12cm disc, the same size as the DVD and the CD, capable of holding nearly four terabytes (one terabyte equals one thousand gigabytes) of data. Practically, the companies are aiming for just one terabyte.
But that's still enough space for over 400 hours of DVD quality video!