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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.
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