Page 243 - Modern Optical Engineering The Design of Optical Systems

Optical Materials 223

subject to cold flow under pressure. The thermal expansion coefficient
5
is almost 10 times that of glass, being 7 or 8 10 /°C.
The change of index with temperature for plastics is very large
(about twenty times that of glass) and negative. Thus, maintaining
focus over a range of temperature is a significant problem for plastic
optics. Often they must be athermalized as well as achromatized. The
density of plastics is low, usually to the order of 1.0 to 1.2. The charac-
teristics of some of the most widely used optical plastics are summarized
in Fig. 10.9.
Another optical application for plastics is in replication. In this
process a precisely made master mold is vacuum-coated with a release,
or parting layer, plus any required high- or low-reflection coatings.
(The nature of the release layer is usually considered proprietary, but
very thin layers of silver, salt, silicon, or plastic have been publicly
mentioned.) Next, a few drops of low-shrinkage epoxy are pressed out
into a thin (ideally about 0.001- or 0.002-in-thick) layer between the
master and a closely matching substrate. The substrate may be Pyrex,
ceramic, or very stable aluminum (for reflector optics), or glass (for
refracting optics). When the epoxy has cured, the master is removed

Figure 10.9 Properties of several optical plastics. (From Lytle and Altman.) Note that
index values may vary significantly from one manufacturer to another.