The second and third beams are modified versions of the reference beam.
To retrieve the data, the reference beam alone is shined through the crystal.
The stored data is read through the reproduction of the same reference beam used to create the hologram.
These two sources are called the reference beam and the signal beam.
When the information is to be retrieved or read out from the hologram, only the reference beam is necessary.
Readout is achieved by illumination with the reference beam alone.
Light reflecting from the object interferes with the reference beam.
As can be seen in Figure 2, the laser light is split into an object beam and a reference beam.
The other beam necessary to write a hologram is the reference beam.
(In holography, these would be the signal and reference beams).
