In recent years it has become common for the birefringence pattern of a thermally toughened glass lens to be examined, and for a judgmental opinion of the impact resistance of that lens to be based on the regularity of the observed pattern Laboratory experiments were carried out to determine the validity of this practice.Commercially available and specially-generated lenses were subjected to modified heat treatments in the laboratory. The birefringence patterns were then examined and the impact resistances determined: these were then compared with those of control groups, tested simultaneously.In these experiments there was no correlation between impact resistance and regularity of birefringence pattern. Reliance on such a correlation for assessing strength is therefore thought to be unjustified and, in some cases, even dangerous. Replication of this work by others is suggested.