High strength ophthalmic lens

Abstract

Abstract An ophthalmic glass lens has an internal zone in which the tensile stress is less than three kg./mm.2 and a compressively stressed, ion-exchanged surface zone that is at least four mils in depth and in which the compressive stresses generate a modulus of rupture greater than 35,000 psi. The lens is molded from a preferably lime-free, alkali silicate glass containing, by weight on the oxide basis, 12-20% Na2O plus K2O, 10-20% ZnO plus MgO plus TiO2, and 1-5% Al2O3, and is chemically strengthened by an exchange of potassium ions for sodium ions in the surface layer of the lens at a temperature below the strain point of the glass. A surface abraded lens will withstand fracture from the impact of a 5/8 inch diameter steel ball dropped from a height of 50 inches, but if broken by a point impact, has a breakage pattern of a few large pieces.

Inventors

• 👤 D Boyd
• 👤 J Williams

Bibliographic Details