Impact Resistance of Variously Mounted Ophthalmic Lenses

An experimental investigation was conducted to determine the frangibility of ophthalmic glass lenses loaded by the impact of steel spheres with diameters ranging from 6.35 to 28.6 mm when dropped from heights up to 1.905 m. These lenses were supported either simply, by a ring similar to that used in U.S. Food and Drug Administration (FDA) drop tests, or while inserted in three different commercially used frame configurations placed on a head form. Strains up to 4000 mU strain were recorded by gauges cemented to the lenses, and impact forces on the glass up to 10.2 kN were determined, with many lenses surviving. Plano lenses tested according to FDA standards experienced peak values of 2300 mU strain. The maximum force transmitted to the head form at the nose piece was found to be 91 N as determined by embedded crystal transducers. A series of special tests on circular piano lenses inserted in specially constructed rings and a circular eyewire indicated that eyewire tension is not a significant factor in impact resistance of lenses under both static and dynamic loading.