A three-dimensional image of an object made by holography.
If you look at these holograms from different angles, you see objects from different perspectives, just like you would if you were looking at a real object. Some holograms even appear to move as you walk past them and look at them from different angles. Others change colors or include views of completely different objects, depending on how you look at them.
Holograms have other surprising traits as well. If you cut one in half, each half contains whole views of the entire holographic image. The same is true if you cut out a small piece -- even a tiny fragment will still contain the whole picture. On top of that, if you make a hologram of a magnifying glass, the holographic version will magnify the other objects in the hologram, just like a real one.
Once you know the principles behind holograms, understanding how they can do all this is easy. This article will explain how a hologram, light and your brain work together make clear, 3-D images. All of a hologram's properties come directly from the process used to create it, so we'll start with an overview of what it takes to make one.
A Closer Look To produce a simple hologram, a beam of coherent, monochromatic light, such as that produced by a laser, is split into two beams. One part, the object or illumination beam, is directed onto the object and reflected onto a high-resolution photographic plate. The other part, the reference beam, is beamed directly onto the photographic plate.
The interference pattern of the two light beams is recorded on the plate. When the developed hologram is illuminated from behind (in the same direction as the original reference beam) by a beam of coherent light, it projects a three-dimensional image of the original object in space, shifting in perspective when viewed from different angles. Appropriately enough, the word hologram comes from the Greek words holos, "whole," and gramma, "message." If a hologram is cut into pieces, each piece projects the entire image, but as if viewed from a smaller subset of angles. The large amount of information contained in holograms makes them harder to forge than two-dimensional images. Many credit cards, CDs, sports memorabilia, and other items include holographic stickers as indicators of authenticity. Holography is used in many fields, including medicine, data storage, architecture, engineering, and the arts.
If you want to see a hologram, you don't have to look much farther than your wallet. There are holograms on most driver's licenses, ID cards and credit cards. If you're not old enough to drive or use credit, you can still find holograms around your home. They're part of CD, DVD and software packaging, as well as just about everything sold as "official merchandise."
Unfortunately, these holograms -- which exist to make forgery more difficult -- aren't very impressive. You can see changes in colors and shapes when you move them back and forth, but they usually just look like sparkly pictures or smears of color. Even the mass-produced holograms that feature movie and comic book heroes can look more like green photographs than amazing 3-D images.
On the other hand, large-scale holograms, illuminated with lasers or displayed in a darkened room with carefully directed lighting, are incredible. They're two-dimensional surfaces that show absolutely precise, three-dimensional images of real objects. You don't even have to wear special glasses or look through a View-Master to see the images in 3-D. |