The Impact Resistance of Eye Protector Lens Materials

The general experimental conditions for drop‐ball testing are reviewed and relevant parameters identified and discussed. In particular, the effect of variations in commencing height and increment size are demonstrated. It is recommended that the formula h c = x ‐ 2s be used to estimate a suitable commencing height for experimental work, where h c = commencing height: x = estimated mean fracture height: and s = estimated standard deviation. Examining the strength of thermally toughened glass and allyl resin (CR39) with drop‐ball tests shows that: (a) Strength increases with lens thickness: for both materials. 3 mm lenses are indicated but 2 mm lenses contra‐indicated: (b) strength increases with increasing base curvature but in the range examined (6.00 D base curve—10.00 D base curve), the effect is small: and (c) mean fracture height increases with decreasing ball size and the relative superiority of 3 mm glass over 3 mm allyl resin lenses diminishes with decreasing ball size.