Letter: Material world
Author: Frank Norville
In an otherwise well-presented article Ocular Protection in Practice (Optician, 21.9.18) its Zeiss authors have seemingly neglected some significant data effectual to their quote ‘even higher (>n=1.50) index materials do not provide maximum UV protection up to 400nm.’ I think they will find 1.76 index n=1.60 TXA and newer MR8 1.60 clearly do reach a UV400 absorption category, as would the whole new range of 1.59 polycarbonate, 1.60, 1.67 and 1.74 indices when specified in their latest 3G material mix enabling a 410 UV cut off.
Many other regular index materials (excepting CR39) nearly reach UV400 missing target by just 5nm (395nm) ie Trivex Tribrid 1.67, 1.74. The exception being CR39 here the authors quote CR39 as providing UV protection up to 355. Perhaps in theory but collecting and testing lenses ‘from the field’ will reveal that the race to the bottom for the cheapest CR39 has driven their UV cut off to as low as 310nm for some.
Readers are likely to agree the author’s statement ‘children’s eyes are particularly susceptible to UV damage’ so more than anything this article should alert those practitioners and optical businesses currently dispensing inferior UV block CR39 to children that UV400 CR39 makes for a huge amount of sense or even more so Trivex break resistant lenses virtually UV400 (395nm).
Perhaps prescribers need to ask themselves why they can justify still prescribing sub UV400 CR39 lenses anyway.