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180 Ophthalmic Lenses

2. It is difficult to determine whether the lens has been toughened, as there
is neither a stress pattern nor a conventional method that will provide
this information.
3. It is not exactly suited for crown glasses. A special type of glass is needed
for the best results which are more expensive.
When chemically or heat toughened glass lenses fracture, they usually
show a radial fracture pattern, although concentric cracks can also occur.
Therefore, only a few splinters of glass are produced and the fragments
tend to stay in the spectacle frame.

Special Heat Toughened Safety Lenses
Special heat toughened lenses became widely available around 1970. The
technology involved was developed by Chance Pilkington and is currently
the property of Norville Optical Co. The lenses are heated in the same way
as air-quenched lenses, but then cooled by immersion in oil. These lenses
are somewhat thinner than the usual heat-toughened variety, and can be
identified with a polarizing strain viewer. The strain pattern is obviously
very different to that found in the conventional heat-toughened lens.

Plastics

There are different materials available in plastic material family:

CR 39
CR was introduced by the Columbia Chemical Division of the Pittsburg
39
Plate Glass Company in 1941 and first became available as a spectacle lens
material in 1950s. The word “CR” stands for Columbia Resin and it was
the 39th batch or formula made by the laboratory. It is a thermosetting
material and offers greater impact resistance. However, when the lens
breaks, it produces sharper fragments. The scratches on the lens surface do
not obviously affect the impact resistance.

Polycarbonate
Polycarbonate material was first produced in the late 1950, since then there
has been a steady increase in its quality and use. Initially, the ophthalmic
use of this material was restricted to industrial safety lenses as it was quite
grey in appearance. But now polycarbonate lenses are readily available,
not only as safety lenses, but also as high quality spectacle lenses for dress
wear. Polycarbonate lenses are thermoplastic polymer that melts at a fairly
low temperature and can be molded and remolded easily by a process of
heating and cooling. When the strength of polycarbonate is compared to
that of metals, the mechanical strength is fairly low, but it is very light and