A lens is not finished until it is coated.
Which means it does not reach final test until it is coated either.
Anti-reflective coating can change how a lens behaves under impact. That is
why the impact test has to come after the coating goes on, not before.
The standards are explicit about it. ISO 8980-4 requires at least twenty four
hours to elapse after coating before any type test is carried out - an
acknowledgement, written into the text, that a freshly coated lens is not yet
a finished one. EN 168 and ISO 12312-1 then set out what that coated lens has
to survive. Increased robustness is a 22 mm steel ball in free fall onto the
finished article. Coat it, wait, then drop the ball. Most labs have worked
that way for years.
Smart lenses make it considerably harder.
The stack is no longer one coating. Scratch resistant, hydrophobic,
oleophobic, anti-reflective, and a waveguide bonded in with optically clear
resin. Every layer added is another interface, and every interface is
somewhere a failure can start and then travel.
Test the substrate and you have a number for the substrate. You do not have a
number for the product.
The twenty four hour rule is where this gets interesting. It was written for
anti-reflective coating on an ophthalmic lens, and for that job it is sound.
It was not written for a bonded waveguide assembly. Nobody has yet said how
long the resin in one of those needs before it behaves like the product a
customer will own - or whether the interval is the same one, or ten times it,
or something that depends entirely on the resin system.
So the methods still tell you how to run the test. What has gone missing is
the answer to when.
Engineered for versatility, our proprietary GET.Impact drop ball tester
features an adaptable design that complies with multiple international
standards on demand. One machine, comprehensive testing.
For anyone building in this category, one question: at what point in your
process does the lens actually get hit? Before the stack goes on, or after?
We have been testing lenses since 1989 and we are always glad to compare
notes.