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Safety Lenses 181

Fig. 12.5: Polycarbonate lenses

as a spectacle lens material they are very strong. Lenses can be produced
by both molding and surfacing techniques with good quality impact
resistant lenses. When hit polycarbonate crazes and the crazed areas that
surrounds the point of impact can be seen easily by the way in which light
is scattered through the lens. The amount of crazing produced is dependent
on the time period of the load and the temperature of the material. When
the temperature is low, the material crazes more readily and fractures by
small high velocity particles occur at a low velocity.
The surface of polycarbonate is fairly soft and scratches very easily.
To overcome this problem the usual practice is to hard coat the lens to
protect its surface. Polycarbonate lenses are more difficult to glaze than
CR lenses. Machines dedicated to this material are fitted with special
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edging wheels, as it is very difficult to cut using standard diamond-edging
wheels.

Cellulose Acetate
This material has not been used to manufacture ophthalmic lenses as it is
too soft to hold its shape accurately. However, it is used to make both frames
and side-shields and to make covers to protect the surface of lenses.

Fig. 12.6: Eye protectors and face shields