Plane hologram is another type of widely used holograms. This is a transmission hologram in which the angle difference between the object beam and reference beam is less than 90 degrees. Plane hologram means that the holographic information is primarily inside the two-dimensional plane of the emulsion. These holograms are also known as thin holograms, requiring coherent or highly filtered playback source. They are holograms of focused images and widely used in packaging. This type of holograms are thin holograms because the emulsion has a thickness of around only 7/millionths of a meter.

Recording Process of Plane Holograms

The image plane is set in such a way that the object is optically suspending in front of the hologram. Or it can be that just a part of the object is in front of the hologram and the other part is behind it. Or it can also be that everything is suspending in the back. The basic idea behind a plane hologram is that the reference beam is hitting the film from the same side as the object beam. This type of hologram has a relatively uniform fringe pattern. Thus if your film is in a fixed plane and your object is in a stationary position, when you rotate the incidence angle of the reference beam, you can make out whether you are making a plane or volume holograms. If your angle is below 90 degrees it's plane hologram and between 90 degrees and 180 degrees it's volume hologram.

Difference Between Plane and Volume Hologram

As the angle difference between the object beam and the reference beam changes, so also the spacing of the patterns in the emulsion. So long the angle difference is less than 90 degrees, the hologram is a plane hologram. It is possible to use the same emulsion and make both plane and volume holograms depending on the angle difference between reference and object beam. A very important point for differentiation between the two occurs when the reference beam swings around its arc of possible positions. In a plane hologram the reference beam hits the film from the same side as the object beam. In a volume hologram, the reference beam takes a 180 degree turn, it hits the film from the opposite side as the modulated object beam.