Image Processing Reference
In-Depth Information
8
Chapter 1
1.6 Beam Wander vs. Image Jitter
It is common at this point to look at beam wander and image jitter and ask what dif-
ferentiates them. Consider a cooperative optical communication system that has a
separated, unconnected transmitter and receiver. The transmitter used to propagate
the beam can be affected by vibration and the effects of the atmosphere such that
the beam can wander over the surface of the receiver. The receiver can also be af-
fected by local vibrations as well as the effects of beam wander, which will change
the angle of arrival of the wavefront from the transmitted beam.
One goal of the designer of an optical communications system is to reduce the
effects of beam wander and image jitter on the receiver. This is also true for imag-
ing systems that acquire and image an object, and continues to follow or track the
object while obtaining a high-quality image. In order to achieve these goals, scien-
tists need the ability to cover a large range of angles and hold this to a high level of
precision. This naturally breaks into two distinct categories of angular control: one
related to steering, the other to stabilization.
In a transmitter and receiver system, the objective is to get the information from
the transmitter into the receiver as effectively as possible. This goal becomes sig-
nificantly more difficult to meet when the transmitter, receiver, and the propagation
medium are all in motion. This is further complicated when the information trans-
ferred must be kept private. In this case, there are significant advantages for the
transmitter to incorporate techniques for keepping the beam on the receiver and for
the receiver to acquire as much of the signal as possible onto its detector. Ideally,
they will work cooperatively to support the need for a private transmission.
The importance of image stabilization has long been recognized in astron-
omy, particularly with the use of large telescopes. The problem for astronomers is
that the objects in the sky (the transmitters) are not really directed at the receiver.
In this case, the tracking and pointing of the telescope mount serves to provide
coverage over the range of the whole sky and acts to provide a level of beam steer-
ing. However, the starlight passing through the atmosphere jitters under turbu-
lence, causing the image to wander. Astronomers then rely on image stabilization
to control the image position in the focal plane to within a few tens of microns.
One advantage of image-stabilization systems on telescopes is that they also cor-
rect for the effects of a poor tracking mount; i.e., beam wander that is introduced
by the telescope mount.
Nearly all modern, professional telescopes incorporate some form of image
stabilization in the camera system. The same concept has been extended to ad-
vanced telescopes in order to correct for more than image motion, actually correct-
ing for the dynamic diffractive effects of the earth's atmosphere over very small
scales. Such systems are known as adaptive optics systems.
The need for beam steering and image stabilization is, of course, not limited to
astronomical and communications systems. Both the commercial and defense es-
tablishments are incorporating these concepts. Some examples are in the commer-
cial market such as image-stabilized binoculars and video cameras, while laser
