Holography is a form of photography that allows an image to be recorded in three dimensions. There are two basic categories of holograms: transmission and reflection. Transmission holograms create a 3-D image when light that is all one wavelength, travels through them. Reflection holograms create a 3-D image when laser light or white light reflects off of their surface. There are holograms on most driver's licenses, ID cards and credit cards. 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.
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. 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. If you tear a hologram in half, you can still see the whole image in each piece. The same is true with smaller and smaller pieces.
The holograms you can buy as novelties or see on your driver's license are reflection holograms. These are usually mass-produced using a stamping method. When you develop a holographic emulsion, the surface of the emulsion collapses as the silver halide grains are reduced to pure silver. This changes the texture of the emulsion's surface. One method of mass-producing holograms is coating this surface in metal to strengthen it, and then using it to stamp the interference pattern into metallic foil. A lot of the time, you can view these holograms in normal white light. You can also mass-produce holograms by printing them from a master hologram, similar to the way you can create lots of photographic prints from the same negative.
But reflection holograms can also be as elaborate as the transmission holograms. There are lots of object and laser setups that can produce these types of holograms. A common one is an inline setup, with the laser, the mixture and the object all in one line. The beam from the laser starts out as the reference beam. It passes through the emulsion, bounces off the object on the other side, and returns to the emulsion as the object beam, creating an interference pattern. You view this hologram when white or colorless light reflects off of its surface. You're still seeing a virtual image -- your brain's interpretation of light waves that seem to be coming from a real object on the other side of the hologram.
Reflection holograms are often thicker than transmission holograms. There is more physical space for recording interference fringes. This also means that there are more layers of reflective surfaces for the light to hit. You can think of holograms that are made this way as having multiple layers that are only about half a wavelength deep. When light enters the first layer, some of it reflects back toward the light source, and some continues to the next layer, where the process repeats. The light from each layer interferes with the light in the layers above it. This is known as the Bragg effect, and it's a necessary part of the reconstruction of the object beam in reflection holograms. In addition, holograms with a strong Bragg effect are known as thick holograms, while those with little Bragg effect are thin.
In movies, holograms can appear to move and recreate entire animated scenes in midair, but today's holograms can only mimic movement. You can get the illusion of movement by exposing one holographic emulsion multiple times at different angles using objects in different positions. The hologram only creates each image when light strikes it from the right angle. When you view this hologram from different angles, your brain interprets the differences in the images as movement. It's like you're viewing a holographic flip book. You can also use a pulsed laser that fires for a minute fraction of a second to make still holograms of objects in motion.
Dennis Gabor invented holograms in 1947. He was attempting to find a method for improving the resolution of electron microscopes. However, lasers, which are necessary for creating and displaying good holograms, were not invented until 1960. Gabor used a mercury vapor lamp, which produced monochrome blue light, and filters make his light more coherent. Gabor won the Nobel Prize in Physics for his invention in 1971. However, I believe that holograms will be in the future to thwart counterfeiters, for surgery purposes, to recreate images of the human brain and for animated billboards.
Recently hailed by Life Magazine as one of the medical breakthroughs for the 21st century, Voxel's medical holograms give doctors a 3-D view of the human body. A hologram of a CT or MRI scan would allow brain surgeons, for example, to measure the exact size, depth, and location of a tumor. Voxel, a company in Laguna Hills, California, creates holograms out of a patient's CT (computer tomography) or MRI (magnetic resonance imaging) scans. CT scans are high-resolution X rays that "photograph" cross-sectional slices of bones, as well as blood vessels and soft tissue, like the brain. MRI scans are similar but use magnetic fields to peer at soft tissue in the body. Both scans provide very detailed pictures of a person's anatomy, but only as flat images. That's where holograms come in.
