An example of incoherent light would be if you were watching cars coming out of a tunnel, you would likely see many different models and types of cars, traveling at different speeds and at different lengths apart. Now, suppose you started seeing the same model and type of cars, all heading down the highway at the same speed, and the same distance apart. This would be an example of coherent light. Holograms need coherent light to record or playback the image clearly. Incoherent light travels in different frequencies and in different phases.
Coherent light travels in the same frequency and in the same phase.
It is important to use light which is coherent because the information is carried on the crest of each wave. The more points of intersection, the more information. Storing Information Unlike a camera, which has only one point of light reference, a hologram has two or more points of light references. The intersection points of the two light waves contain the wholeinformation of both reference points. This mirror splits the original beam into two beams.