Other Hologram Types
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, 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 we already discussed. 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 emulsion 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 monochrome 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.
The Bragg effect can also change the way the hologram reflects light, especially in holograms that you can view in white light. At different viewing angles, the Bragg effect can be different for different wavelengths of light. This means that you might see the hologram as one color from one angle and another color from another angle. The Bragg effect is also one of the reasons why most novelty holograms appear green even though they were created with a red laser.