The Basics of Holography

Coherent Light

The basis of holography is coherent light. So what is this? A coherent light source produces light waves that all have the same frequency and phase. The best known source of coherent light is the laser. One of the benefits of coherent light is it's very easy to describe mathematically. Another benefit of coherent light is it produces predicatable interference patterns when two coherent light sources interact with each other. These interference patterns can be described mathematically, which is the basis for computational holography.

Basic Film Holography

The recording of holograms on film started in the mid 1960s and this is a good starting point for describing how holography works. The film used for holography is a special high resolution film that is more sensitive to light than the film that was used in cameras. The process starts with a laser as the source of coherent light. The beam of light is then split into two separate beams by a beam spliter. One beam of light illuminates the object to be recorded and this light is reflected onto the film plate. The other beam, called the reference beam directly illuminates the film plate. When the two beams reach the film plate they interfer with each other and form the interference pattern which is the recorded hologram.

This process is shown below.

After the film has been developed the hologram can be reconstructed. This is accomplished by illuminating the developed film with a laser. This need not be the same laser as was used to produce the hologram, but if it has a different frequency the hologram will be scaled. The interaction between the illuminating laser and the film recreates the original wavefront from the object, which is what we normally call a hologram. This process is shown below.

So what does an interference pattern look like? The image below shows an interference pattern that was computed by our software. While there is definitely a correlation between the interference pattern and the object recorded in it, this relationship is not evident to most viewers.

The process described here is the simplest form of hologram. There are many other kinds of holograms and this has been an active research area since the mid 1960s. Books on holography describe the different kinds of holograms. Since the physics and mathematics of holography is well understood we can algorithmically produce the interference patterns required for holography in a computer. Our goal is to be able to compute these holograms at interactive speed so they can be used as an alternative to existing 3D display technologies.