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182 Ophthalmic Lenses

Wire Gauze
Goggles made from wire gauze have a very good impact resistance, but
are generally not accepted because they degrade the visual function and
give no protection against splashes of molten metal, etc.

TESTING PROCEDURE FOR SAFETY LENSES
The lenses used as safety eyewear must be tested to establish whether they
are suitable for the specific hazard for which they were designed. The
following factors may be assessed:

Impact Resistance
The impact resistance of all types of lenses may be influenced when the
surface has been abraded. The size and the speed of the missile or particle
also influence the impact resistance of the lens. Larger particles hitting a
lens cause it to bend, and so the fracture is initiated on the back surface.
Smaller particles on the other hand do not cause the lens to bend upon
impact, so the fracture is generally initiated on the front surface. The impact
resistance also increases as the lens thickness increases. Increase in base
curvature also increases the strength of the lens in both heat toughened
and CR lenses.
39
The type of material used for safety eyewear gives an indication of the
mean fracture velocity that can be tolerated. Fig. 12.7 shows the fracture
velocity for some of the materials available. The samples were all of the
same thickness and were struck by a 6.5 mm steel ball. Polycarbonate offers
the greatest fracture resistance of all the lens material.

Fig. 12.7: Fracture velocity of a 3 mm thick sample of different lens materials
when struck by a 6.5 mm diameter steel ball