A BALLISTIC EVALUATION OF THE IMPACT RESISTANCE OF SPECTACLE LENS MATERIALS

The impact resistance of chemically and thermally hardened glass lense s of 2.2- and 3.0-mm thicknesses was evaluated using a ballistic test. Each lens was edged and mounted into a spectacle frame, which was pla ced on a standard headform. A 6.5-mm steel ball was fired from an air gun at the center of the lens at increasing speeds until the lens brok e. The multiple impact data were used to plot cumulative breakage curv es. We found that heat-treated photochromic glass and heat-treated cro wn glass fail at similar missile speeds and that chemical treating con siderably improves the impact resistance of crown glass but not of pho tochromic glass. The poorer performance of photochromic lenses indicat es that they should not be prescribed when optimal impact protection i s required. Plastic lenses show superior performance.