Image Processing Reference
In-Depth Information
the beam and forming an image. In practice, most optical systems are sufficiently
complex that the separation between these two concepts is quickly blurred.
This chapter provides an overview of a wavefront, the effect of an optical sys-
tem on the wavefront, the effects of image motion, and the value of using image sta-
bilization to improve image quality. This will be used to motivate the use of active
image stabilization in advanced optical systems, including telescopes and optical
communication systems.
1.2 Wavefronts
A wavefront is a surface of uniform phase moving in space and time. This wave-
front can be easily pictured by considering the light from a star as a point source re-
leasing light uniformly in all directions. In a given instant, the light from the star
leaves the surface and propagates outward into space. The light rays travel from the
star in the form of a sinusoidally varying, traveling wave. The crests and troughs of
the wave identify points of constant phase of the rays and the wavefront of light.
Physically close to the star, the curvature of the wavefront is very large. However,
as the distance from the star increases, the curvature approaches zero. Over large
distances, say from the star to Earth, the wavefront's radius of curvature increases
until it is indistinguishable from a plane wave as shown in Fig. 1.1. Certainly, on
the scale of most optical imaging telescopes, all information about the physical
shape of the star is lost from the plane wavefront. When a plane wave is focused in a
telescope, the resulting image structure is defined by the shape of the telescope ap-
erture rather than the light source (Hecht 2002).
If the wavefront from a star is intercepted by a telescope located outside Earth's
atmosphere, the wavefront has minimal distortion and is essentially flat. If, on the
other hand, the telescope is located on earth, then the wavefront must pass through
Figure 1.1 Formation of a propagating plane wave from a distant star. (Earth image
credit NASA, Astronomy Picture of the Day.)
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