CR-39 with premium A/R failing std impact test.

KM · 01-10-2012, 09:38 PM

Has anyone else had this issue : 100% std drop ball test failure of cr-39 lenses with premium A/R ?

Std A/R coated cr-39 are passing fine. First issue was with a lab's private label prem. A/R . 100% failure rate. I asked them to increase thickness to 3.0 min. - still 100% failure !
I informed lab , and when they made no changes , I discontinued use of lab.

Current issue is with a major brand stock cr-39 with premium A/R : 100% failure rate (2.0 ct) . I have sent the failed lenses to their tech lab. But no response as of today.
Seems everyone turns a blind eye to this issue. And that is what I am trying to avoid - a blinded eye on any of our patients.

In the 30 years I have had in this field , this is my first red flag issue for patient safety. A large % of our A/R jobs are finished at labs doing the minimum drop ball batch. Supposedly.
I don't know how to proceed to safely use premium A/R on CR-39.

KM · 01-12-2012, 09:40 PM

Today we reviewed the test results. Seems that the A/R process somehow causes the premium coated cr-39 to fail at a lower impact force than the basic A/R lens.
Even if the impact resistance is at the gov. minimum required, we can still dispense the lens. However; we can expect a higher failure rate with real world impacts.

Obviously, our testing apparatus was putting the lens under a slightly greater impact force than required for std testing. Still, the survival rate(with this slightly higher force)
of the basic A/R coated cr-39 lens over that of the premium A/R coated lens was eye opening.

**MakeOptics** · 01-13-2012, 05:54 PM

It's the tension from the impact on the back surface that leads to failure in drop ball tests. Since AR's, especially premium AR's are harder and more brittle than the lens to prevent scratching they are also more prone to crazing and cracking. This crazing allows the tension to concentrate it's force on a smaller area of the lens and acts as a force multiplier. That is an explanation as to why the coated lens fails even though and uncoated lens would not.