optical rules

[mauroventura]

Hello Boys.
Some our customers (..opticians) , sometimes ,send me some edged lenses to check their defective surface coating (A.R., A.S.) .
Not Always, but often , i found big A.R. cracks , due to a deformation of the lens (i mean a gain of 2 , 3 front diopters).
When i try to explain that, obviously , the origin of the problem can be a trouble of compatibility from the frame curve and the lens front curve , their answer is that a good tolerance between this two curve rays must be 'normal'. So the lenses are defective! .
:drop:
I would like to know your opinions , because i'm confused:hammer:
The lenses are negative cr 39 stock lenses and the frame material in most cases is tri-acetate(in very few cases the material is metal).


Mauro

[Websta]

I have seen issues like this, but more so with Poly lenses. The problems may lie in the lens, which in some cases, is cut too long. Then the fitter will heat the frame and squeeze the lens into the frame causing the cracks (lens will bow slightly on minus lenses)
The other problem may be that the stock lenses are too thin. We have found this in some cases. The edger pressure could cause the lens to be "squashed" flat causing the crazing.

Another long shot is that the store staff try to adjust the lenses/frames and put the whole job in the heater too long. This will cause the AR to craze in some instances (depending on heater temp and length of time)
CR will usually take temps up to around 80°C, then they will start cracking.

OR it could just be a dodgy AR coating!