andrewNZ: Not an answer to the question, but if you buy a pair of glasses with glass lenses, you won't have a problem with scratched lenses. The cost often isn't any more than plastic, sometimes less.
I am not careful with sunglasses. I drop them, I clean them with my shirt, and I leave them kicking around all sorts of places.
I had a pair that was @15 years old and bined them when the frame finally broke. I have another pair that is 5-10 years and they have some minor scratches from being dropped on the road and me head butting things. My new pair is >1 year old and doesn't have a scratch on it.
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