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184 Ophthalmic Lenses
Table 12.1: A Comparison of the major properties
of glazing materials for eye – protectors
Material Impact Hardness Chemical Thermost- Fracture Resista Weight
Resistance Resistance ability Pattern -nce to Hot
Particles
Large Small
Missile Missile
Glass
Heat Good Good Good Very Good Very Good Fair Poor Heaviest
toughened
Chemically Good Poor Good Very Good Very Good Fair Poor Heavy
toughened
Laminated Fair Fair Good Good Fair Poor Poor Heavy
Plastics
PMMA Fair Fair Poor Good Fair Good Very Good Light
Good Good Fair Good Good Fair Good Light
CR 39
Polycarbo- Very Very Poor Fair Good Very Good Good Light
nate Good Good
(coated)
TYPES OF LENS HOUSING
The lens housing may be made of metal or plastics. Nickel alloy is usually
used to make metal housing and polycarbonate, polyamide, cellulose
acetate, etc can be used for plastic lens housing. These materials may be
used to manufacture:
1. Spectacle frames
2. Goggles
3. Face shields
4. Helmets
Spectacle Frames
Cellulose acetate, polyamide, polycarbonate and many other kinds of
plastics can be used to make plastic spectacle frames. They can also be
made from metal wire also, but these frames have been shown to cause
more damage upon impact than a plastic frame. Side shield spectacle
frames are very good as eye-protectors. But these side shields must not
restrict the wearers field of vision and hence they should be made of
transparent material that does not discolour with age. Injection molded
side shields with polycarbonate materials are best, as their shape does
not alter.