Strength of glass lenses processed in an ultrasonically stimulated chemtempering bath.

Comparative experiments were performed on conventionally chemtempered lenses and ones chemtempered in an ultrasonically stimulated, slowly oscillated salt bath. With ultrasonic stimulation, a substantially shorter treatment time was found to result in greater strength and greater surface enrichment of K+ ions. The K+-rich surface layer was as deep or deeper than that found in conventionally chem-tempered lenses. The results suggest that the ultrasonic stimulation continually enriches the salt bath with K+ ions in the region of the lens surface.

Strength of thin chemtempered lenses: static load testing.

Static load tests were conducted on heat-tempered and chemtempered plano white crown glass lenses from five different optical laboratories. With both ball-on-ring and ring-on-ring loading, chemtempered lenses considerably thinner than 2.0 mm were found to be as failure resistant as 2.0-mm-thick heat-tempered lenses. A similar result was obtained previously using the drop-ball test. It is shown that the theory of brittle fracture can be used to relate the results of different tests and provides a rational basis for comparing the relative performance of chemtempered and heat-tempered lenses.

Strength of thin chemtempered lenses: drop-ball testing.

Failure heights were measured in drop-ball tests for both chemtempered and heat-tempered plano, white crown glass lenses from five different optical laboratories. It was found that (1) failure height was proportional to the square of the lens thickness, (2) chemtempered lenses substantially thinner than 2.0 mm are as resistant to breakage as 2.0-mm-thick heat-tempered lenses, and (3) a close correlation existed between results of single-drop and multiple-drop tests and between results of tests using rigid and compliant mounts.

Basic principles of lens fracture testing.

An analysis of different tests used or proposed for evaluation of the resistance of ophthalmic lenses to breakage is presented. The tests considered subject the lenses to different strain rates, areas under stress, and energy losses. The analysis examines effects of these variables as well as effects of differences in lens-failure stress and lens thickness on test results.