Base Line Fracture Resistance Studies of Tempered and Non‐tempered Glass Ophthalmic Lenses

Groups of plano ophthalmic lenses 48 mm round and 2.2 mm thick were tested in a drop‐ball tower conforming to both F.D.A. regulations and ANSI standards. Lenses were impacted with steel balls of different sizes from different heights to provide a scale of increasing equal energy increments. One of the energy levels was provided by the standard 5/8‐inch ball dropped from a height or 50 inches. The fracture resistance (drop ball energy) of anncaled (untempered), heat tempered and chemically tempered lenses are compared graphically and statistically. The annealed group had the lowest mean fracture energy over the narrowest range. The chemically tempered group had the highest mean fracture energy over the widest range. Experiments attempting to predict which untempered lenscs would be most fracture resistant when tempered are described and interpreted in terms of modern glass strength theory.