That is the reference beam.
When it reaches its target, the modified signal beam collides with the reference beam, causing an interference pattern that is recorded to produce the hologram.
Here the reference beam is added coherently to a Fraunhofer diffraction pattern of the object or formed by a lens (as in the first stage of Figure 9).
All that is required is that the reference beam for each arrives at a slightly different angle from all other reference beams.
A hologram is made by splitting a laser beam in two, aiming one half on the object to be depicted and then recombining it with the other half, the reference beam.
Later, when laser light illuminates that pattern from the same angle as the reference beam, it is scattered to reconstruct an identical wave front of light, which appears to the viewer as a three-dimensional image of the object.
Holography has been adapted to the conventional microscope, which is modified by the inclusion of a separate reference beam so that the light diffracted by the object in the microscope is made to interfere with the light from the reference beam.
Although this worked well for continuous specimens, in the case of particulates it was better to have the reference beam pass through a bare area of the specimen preparation, and by 1950 the use of half-silvered surfaces and slightly tapering slides allowed polarized light to be dispensed with.
The reference beam is coupled into an optical fiber via a piezoelectric-transducer-driven mirror.
In this manner, the reference beam is part of the experiment.
The cracking load increased with this method compared with the reference beam.
