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16 Ophthalmic Lenses

a lens material with low specific gravity will produce a lighter lens.
Reduction of weight improves wearing comfort, and eliminates the
indentations from nose pads that are produced by wearing heavy glasses.
However, low density does not always mean a lighter lens – the amount of
material depends on the refractive index and the minimum thickness. But
low specific gravity is very important for prescriptions that require high
powered plus or minus lenses. Patients with high power prescriptions are
usually advised to use higher index glasses. Although they offer thinner
lenses, they are not necessarily more comfortable to wear because of their
high specific gravity. The following table shows the specific gravity of
various lens materials:

Material Refractive Index Specific Gravity
Polycarbonate 1.590 1.20 Gram/cc
CR 39 1.498 1.32 Gram/cc
Crown glass 1.523 2.54 Gram/cc
High index glass 1.60 2.63 Gram/cc

Impact Resistance

Fig. 2.4: Drop ball test for impact resistance
In 1971, the Food and Drug Administration (FDA) adopted a procedure to
ensure the level of protection provided to the consumer for “street wear”.
More stringent standards are applied for industrial protection. The standard
involves dropping a 5/8” steel ball from a height of 50” onto the lens and
the lenses that survive this test are deemed impact resistant. The
implementation of this standard in the USA led to increased use of hard
resin lenses. Only a sample of hard resin lenses is needed to be tested for
impact resistance, whereas every glass lens is needed to be tested. This
accelerated the shift towards the plastic lenses (Fig. 2.4).