Image Processing Reference
In-Depth Information
Chapter 1
Figure 1.2 Effect of a lens on an unperturbed and tilted wavefront. The focal plane
shows a displacement in the position of the focused spot due to the wavefront tilt.
of the optical system, the focused spot will have a different appearance from that of
the plane wavefront. The simplest effect to be considered is a plane wave tipped
with respect to the optical axis. On passing through the optical system, this wave-
front is displacement from the original focused spot position. This is illustrated in
Fig. 1.2.
Since the atmosphere is dynamic, the tilt it adds to the wavefront varies with
time, so the focused spot moves about over time. The atmosphere can introduce
much more complex shapes on the wavefront and the result will be to change the
appearance of the image spot until it loses its diffraction-limited form and becomes
little more than a large blur.
The motion of the wavefront can be considered from two perspectives: move-
ment at the transmitter and movement of the image itself.
1.4 Beam Wander
Many sources, including instabilities in the beam projector and the effects of the at-
mosphere on the transmitted beam, can introduce beam wander. One can see beam
motion by looking at a series of short-exposure images and comparing them to the
long-exposure image resulting from their integration, as illustrated in Fig. 1.3,
showing a spot that is several beamwidths in diameter. The short-exposure image
shows that the characteristic beamwidth is still present, but that the beam actually
moves or wanders with time as it builds up the dimensions of the long-exposure im-
Beam wander due to the atmosphere is caused by large cells of air with differ-
ing indices of refraction moving across the path of the beam (Hardy 1998). Since
there can be many refractive cells along the light's path through the atmosphere and
the air at different heights can move in different directions, the overall pattern of the
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