Chou B Ralph, Yuen Gloria S-C, Dain Stephen J
Ophthalmic Standards Laboratory, School of Optometry, University of Waterloo, Waterloo, Ontario, Canada.
Clin Exp Optom. 2011 Nov;94(6):568-74. doi: 10.1111/j.1444-0938.2011.00651.x. Epub 2011 Sep 5.
The aim was to assess the impact resistance of coated and uncoated mid-index spectacle lens materials using the ballistic impact test.
Nominally plano lenses of each material in three thicknesses were obtained. The lenses were flat edged to a 50 mm diameter. Each lens was impacted by a 6.35 mm steel ball. Impact velocities were selected using the Zippy Estimation by Sequential Testing protocol to determine the threshold fracture impact velocity.
Threshold fracture impact velocity generally increased with thickness; however, there was a wide variation in performance among the various lens materials at each thickness. In all but two instances, the differences in impact velocity at each thickness of lens material were significant. Comparison of the data for CR39 and Hoya Phoenix with the results of earlier studies showed that the lens mounting is a significant factor. The fracture velocities found in the present study were significantly lower than the fracture velocities found when the lens edge is restrained in the mounting. A scratch resistant coating reduced the impact resistance of CR39. The effect of the antireflection coating on the fracture velocity depended on the nature of the base scratch-resistant coating.
Mid-index lens materials of the same thickness show widely varying levels of impact resistance under the ballistic test. Impact resistance increases non-linearly with centre thickness. The lens mounting might affect the results of the ballistic impact test. The presence of 'cushion coatings' might enhance impact resistance.
