was wondering what would be the best crown glass, cr-39 with yellow tint, poly and off topic but how many nanometers or below a certain measurement is damaging to the cornea? i have an electromagnetic spectrum sheet in front of me but all i see wavelengths below 400 nm are considered harmful to the eye and below 100nm is absorbed by the cornea which causes conjunctivitis and keratitis. lost my system for dispensing book so trying to gather as much info i can. and would 380nanometers - 780nanometers be considered visible light?