Impact resistance of Transitions Plus spectacle lenses

Although it has been shown that photochromic glass industrial lenses do not provide adequate occupational impact protection, they are frequently requested by outdoor workers wanting sunglasses. The recent introduction of Transitions Plus (TP), a photochromic plastic lens, may meet the needs of these workers. The impact resistance of 60 TP lenses of dress thickness was studied using a ballistic test. Lenses were edged and mounted in metal industrial frames. The completed spectacles were placed on a headform for ballistic testing using a 6.5 mm (1/4 in) steel ball propelled from an airgun. The lenses were divided into test groups that received: (1) a single impact at the geometrical center by a ball traveling at 18 m/s (59 ft/s) and (2) 50 consecutive impacts at 18 m/s, or (3) a single impact at 46.5 m/s (152 ft/s). All lenses passed the single impact test at 18 m/s. Four lenses broke under repeated impact. All lenses subjected to the high-speed impact test failed, either breaking or being dislodged from the frame. These findings show that dress thickness TP lenses provide impact protection from small blunt missiles traveling at moderate speed, and that they are durable under repeated low energy impact. However, thicker lenses must be used when the wearer has a high risk of ocular exposure to small high-speed missiles in the workplace. It can be concluded that TP lenses may be used in occupational eyewear for many outdoor workers who want photochromic sunglass lenses, provided that other needs for vision and occupational safety are also satisfied.