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Abstract
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PURPOSE: To evaluate the penetration resistance of
common spectacle and safety lenses to high-velocity projectiles and to examine
the current recommendations regarding the use of such lenses. METHODS: The
penetration threshold of glass, high-index, and polycarbonate spectacle and
safety lenses was determined by firing BBs, pellets, and 0.22-caliber
projectiles. The mass, diameter, velocity, and energy of each projectile were
measured, and the center thickness, mass, and refractive power of each lens were
recorded. The penetration threshold for each lens type was calculated, and a
comparison of impact resistance was made. RESULTS: The maximum velocity of the
BB was measured at 221 m/s; pellets at 210 m/s; CB.22 rimfire projectiles at 204
m/s; and 0.22 rimfire projectiles at 290 m/s. The BB penetration thresholds were
as follows: glass lenses = 84.0 m/s (2.4 J) and high-index lenses = 107.7 m/s
(4.09 J). Polycarbonate spectacle and safety lenses were not penetrated by BBs
regardless of their velocity; however, these lenses were penetrated by CB.22
rimfire projectiles (89.2 J). Review of the penetrated lenses showed that center
thickness and refractive power was comparable between the various groups of
projectiles. CONCLUSIONS: Polycarbonate offers a vastly superior degree of
penetration resistance compared with other commonly used lens materials. The
current recommendations regarding the use of polycarbonate in prescription and
protective lenses, as endorsed by the American Academy of Ophthalmology and the
American Academy of Pediatrics, must be reevaluated. |