Art and security applications share a common technique known as the hologram. For artists, holograms offer a double or triple side to one piece of work, allowing for multidimensional art. In the security world, holograms play an important role to decipher between real and fake properties. For instance, the U.S. $20 bill possesses a hologram, which separates it from counterfeits.
Reflection
A reflection hologram shows a true three-dimensional image near its surface. Most galleries use reflection holograms. You also see reflection holograms in movie posters. The hologram is illuminated on a certain spot by incandescent lighting that, when held at a specific angle and distance, changes the look of the picture. Mass-produced holograms such as movie posters and credit cards usually reflect light; they also have a layer of aluminum on the back that causes a mirror effect.
Transmission
A transmission hologram is created when light from a point meets the reference beam, then enters the recording material from the same side as the starting point. Transmission holograms use laser light. The light comes from behind the hologram, and then the image transfers to the observer's side. The transmitted image often projects through a small hologram so that a group of people looking through a window can see the entire hologram.
Embossed
Multicolored embossed holograms have shallow depths of a few millimeters. Security applications often use embossed holograms to protect against manipulation, since they are extremely difficult to copy. Credit cards and passwords use an embossed hologram. The eagle on a Visa card is an embossed hologram. Advertisements, security labels, plastic cards and promotion materials often use embossed holograms to identify and mark the work.
Other Hybrid Holograms
There are many holograms that go between reflection and transmission holograms. Each are a little different. An integral hologram is either a transmission or reflection hologram that is made from photographs of an object or person, outdoor scene or computer graphic. A camera first scans the object, then each view is shown through a LCD screen. Other types of holograms are rainbow, computer generated, multiplex, stereogram, volume and plane.
