How to break a lens

Abstract

Over the years, the author has familiarized himself with far too many of the classic methods for turning a single optical element into any number of smaller pieces. In the vast majority of these cases the application of gravity was the destructive element. Resting a large lens on a yet unbeveled edge to produce the classic "clamshell" is a well known example. Another ever popular technique using gravity assist is trying to carefully pull one's fingers out from under an optical element when placing it on a surface plate. Fig 1. A classic "clamshell" fractureIt is the purpose of this paper to introduce more subtle methodologies that can catastrophically fracture a lens. These involve material choice, lens geometry, temperature environments and mounting techniques. When these parameters are carefully chosen, a traditionally benign test can produce some truly spectacular fractures.Accurately predicting glass failure demands fairly detailed analysis. 1 It is essential to have thorough knowledge of material properties for all the glass components of the system. It is often the case that these parameters are not easily obtained or in obscure units. The geometry of the surrounding structure must also be well known. An example of such an analysis is presented. Of course the analysis was performed after the test catastrophe. As expected, the analysis confirmed that, yes indeed, the lens was perfectly designed to fracture under the test conditions.