An interpretation of the drop ball test in terms of a statistical model for fracture.

Some well known statistical models for fracture were applied to the problem of impact of glass ophthalmic lenses. Parameters for a flaw distribution function were chosen to fit some previously published results of drop ball testing. The concept of an impact-resistant lens was reformulated in terms of minimizing the most severe flaws. Formulas were derived for the detection parameter, a convenient measure of the ability of any mechanical test to identify those lenses with the worst flaws. It was shown that the fraction of such lenses identified by the drop ball test is very small. As a side note, it was found that it is not significantly more difficult to expose "bad" lenses when they are tested in plastic bags. A method of obtaining useful information from the results of drop ball testing was outlined.