Prescription glasses made from any plastic material greatly reduce this risk, much more so than for children who do not require eyewear. Although glass lenses offer protection against some risks, they can shatter if the impact force is great enough, such as when falling against a hard object. Glass is also heavier than plastic, which makes the eyewear uncomfortable and increases the likelihood of its falling off during rough play. …show more content…
It will protect the eyes from many impacts, but not all. Depending on the speed and other circumstances, a fall while riding a bicycle might generate sufficient impact to shatter CR-39. Informal play of certain sports, such as baseball, may also shatter CR-39. On the other hand, polycarbonate has ten times the impact resistance of CR-39. For eyewear use, polycarbonate is practically shatterproof.
This material has many applications requiring impact resistance. For example, the canopies of some fighter planes are made from this material to protect the pilot from bird strike. It is also the standard lens material for safety glasses. Of course, glasses with polycarbonate lenses do not automatically make them safety glasses. Safety glasses have additional frame requirements and must meet ANSI standards.
Polycarbonate has a few drawbacks. Its optical quality is not as good as CR-39 (standard plastic) and it costs more. While standard plastic costs $5, polycarbonate is $15. However, both materials will cost the same if you factor in ultraviolet protection. CR-39 allows some transmission of UVA and will require a UV blocking coating for full protection. This coating costs $10. Polycarbonate does not require a UV coating because it naturally blocks all