Image Processing Reference
In-Depth Information
Chapter 8
Alternative Approaches
8.1 Introduction
The first part of this tutorial text discussed many of the standard approaches to im-
age stabilization, focusing on optical correction for optical systems. Image-stabili-
zation technology continues to grow and expand and new devices are constantly
evolving. Two devices of particular interest are liquid crystal (LC) spatial light
modulators and orthogonal transfer CCDs. These devices are being introduced into
systems and provide novel, compact means for image stabilization. Microelectro-
mechanical systems (MEMS) are now available for motion and vibration detection
and can be used to provide mechanical sensing and compensation in optical
systems.
8.2 Liquid Crystal Spatial Light Modulators
The idea of using LC as corrective elements dates back to the early 1980s. How-
ever, at that time LC technology was not developed enough to produce usable de-
vices. Things started to change dramatically in the early 1990s, thanks especially to
display-technology research and investments. Now, LC devices are available for
use in laboratory setups and first telescope demonstrations.
To understand how LCs can be used as phase correctors, consider a very sim-
ple, single-element device. The LC material is sandwiched between two glass
plates. Spacers maintain the separation of the glass plates. On the glass plates is de-
posited a thin film of material that is a transparent electrode, usually indium-tin ox-
ide (ITO). The last layer is the alignment layer, used to anchor the molecules as
shown in Fig. 8.1. In conventional display technology, the two faceplates, with ITO
and alignment films, are mounted perpendicularly from each other. The net result is
that the spatial arrangement of the molecules forms a spiral going from one extreme
(the first faceplate) to the orthogonal one on the other side. Because of this spiral ar-
rangement, these are called twisted nematic devices. Phase modulation requires un-
twisted arrangements, where the faceplates are parallel. Normal display technology
is inadequate for phase correction applications for other reasons as well. First, the
optical quality of the faceplates is not very high (Love et al. 1995). The single ele-
ments (pixels) are not controllable individually. Finally, the spacers are not located
at the edge of the devices but are usually small spheres randomly spread throughout
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