ABSTRACT
Coherence [1] may be defined as the property of wave-like states
that enables it to exhibit interference (i.e., addition of wave
functions). In the case of laser light, this interference gives rise
to a speckle field consisting of areas/volumes of higher and lower
intensity, which gives the field a distinct “grainy” pattern (Fig. 4.1).
A speckle pattern can be subjective or objective: If the visible laser
light illuminates a wallpaper or some other rough surface, the
viewer will see laser speckles in the image plane [2]. If either the
viewer or the illuminated target moves, the speckles will shift, and
the direction of the perceived movement depends on the viewer’s
eyes. This is called a subjective speckle pattern since the details
of the pattern depend on the parameters of the viewing system
(for example the shape of the eye). An objective speckle pattern is
independent of the parameters of the viewing system and can be
imaged by using, for example, a photographic plate or optical sensor
without an objective.
