Center Thickness

[hipoptical]

For example one of the labs I use will charge extra for a poly with a 1mm center thickness and that is due to the fact that when they surface beyond their set limits they will test every pair and some times they break. Higher breakage means higher cost.

No lab I have ever worked in or run tested every lens surfaced to 1.0 CT. I challenge labs to even be able to prove that ANY testing is done regularly. Smoke- being blown in places unmetionable....
(Just my lowly opinion, which I will hold ontil proof is shown otherwise. I've been in meetings with managers and owners throughout the nation, and testing of any kind is a joke. Everyone still talks about it- but all tongue-in-cheek, unless talking to a customer. Then, of course we all test: impact, scratch, weather, durability, you name it! I will attest that our testing is documented here for our own tests, and at COLTS for the tests they perform on our products.)
Your lab charges for 1.0 CT because they can, not because they have to. If they have good processes and controls, breakage is not an issue. They likely use stock lenses in many cases, anyway.

[HarryChiling]

No lab I have ever worked in or run tested every lens surfaced to 1.0 CT. I challenge labs to even be able to prove that ANY testing is done regularly. Smoke- being blown in places unmetionable....

The testing is done once on a number of lenses not just one.

Cumulative Binomial Sampling Table at 100% Confidence with Zero Consecutive Failures

# of lenses - 100% confidence that the % fracture is no worst than

120 - 6.5%
130 - 6.0%
140 - 5.5%
150 - 5.0%
170 - 4.5%
190 - 4.0%
220 - 3.5%
250 - 3.0%
310 - 2.5%
380 - 2.0%
510 - 1.5%
760 - 1.0%
850 - 0.9%
950 - 0.8%
1090 - 0.7%
1270 - 0.6%
1520 - 0.5%
1900 - 0.4%
2530 - 0.3%
3800 - 0.2%
7600 - 0.1%

In order to certify a manufacturer, probability analysis during Product and
Process Validation, as apart of the COLTS FDA Certification Program, must
show (with a confidence level of 99%) that the Reference Lens has an expected
fracture rate of 6.5% or less at 50” of drop height.
8.1.2 Confirmation of this probability prediction is accomplished though cumulative
product testing using Cumulative Binomial distribution. After drop ball testing 120
consecutive Reference Lenses (and achieving zero fractures), it can be
concluded with 100% certainty that the Reference Lens is less than 6.5%
defective. Since the Reference Lens represents that weakest product
combination, any other lens type produced by the manufacture performs at less
percent fracture than the Reference Lens results.

(By-the-way: I make 1.56 with a 1.0 CT and it passes drop ball testing at over 100 inches even after A/R.)

I will attest that our testing is documented here for our own tests, and at COLTS for the tests they perform on our products.

I am just saying that the lens you said you made to pass a 100 inch drop ball test could be a fluke. If you did not perform the test on a number of lenses that would allow you to say with confidence that 99% of the lenses would fracture less than 6.5% of the time in testing. Then they do not meet 21 CFR 801.410 and are not impact resistant lenses. The numbers provided above come from COLTS Laboratorys and is their process for testing lenses.

Your lab charges for 1.0 CT because they can, not because they have to. If they have good processes and controls, breakage is not an issue. They likely use stock lenses in many cases, anyway.

It is true they charge me for it because they can, that is because it is more difficult to make thinner lenses without running into problems. I have done surfacing for a number of years and the thinner they get the more headaches I would have to deal with. This leads to me determining what my machines and staff are capable of without increasing the number of breakages, and if we get a request for thinner you better believe I would charge more for that lens just in case I broke it. Even the best trained people are going to run into higher breakages when they are trying to surface lenses very thin.

I believe that you may have made a 1.56 index lens with a 1.0mm CT with AR (assuming both sides wich actually makes the lens more likely to break) and dropped the steel ball from 100 inches on it without it breaking, but if you think I am gullable enough to believe that a lens to this spec can withstand that abuse without a high number of them shattering you are dead wrong.

[hipoptical]

Harry,
If you would read my posts, I have said clearly that we have done extensive testing on a regular basis, not one lens and hoped for the best.
This thread was not supposed to be about what I do, but about how thin one CAN surface different products. The point is, that in the good 'ole USA we have lots of regulations, and other places do not. Those countries that do not have the regulations we have are free to do what they are capable of. I've not heard of an excessive number of injuries to the eye, or of people going blind because they wear thin lenses. The advice the person was looking for was how thin CAN you go... not how thin Harry and the Regulators think you SHOULD go;).
I push the envelope... sounds like you stay well within the boundaries set. I am comfortable with my position, and you with yours. The difference I think, is that I am fine with where you are, but you seem to be distraught with what I say I can do. I would just advise anyone to only work within the boundaries that one is comfortable with. Some people are pioneers, some are settlers. I don't settle.
Unless there's anything more, I think I'm done with this thread, since it seems like the one who started it is gone. I don't want to appear as though you or I are hijacking. (If there is more that needs to be discussed I will be happy to.)

[hipoptical]

I believe that you may have made a 1.56 index lens with a 1.0mm CT with AR (assuming both sides wich actually makes the lens more likely to break) and dropped the steel ball from 100 inches on it without it breaking, but if you think I am gullable enough to believe that a lens to this spec can withstand that abuse without a high number of them shattering you are dead wrong.

Sorry- but I did mean to respond to this...
Facts:
1.56 index material
A/R both sides (vacuum technology, not nano- or sol-gel)
And it was actually 120 inches, on a number of lenses, quite repeatable, and documented


I don't think you're gullible. I think you accept things that are known. I think you believe that what's normal and widely done is all there is. I think you're a sheep. I think you're a settler.
(And I don't say that harshly or to be rude or demeaning. People are just different. I have to find better and different ways to do things or I get bored. Now, just to make you feel better, you may like to know that we normally still produce this product with CT of 1.8-2.0; just because I can do something doesn't mean I always do. I will do it if asked, though.)
As Red Green says, have fun out there- and keep your stick on the ice.

[HarryChiling]

Sorry- but I did mean to respond to this...
Facts:
1.56 index material
A/R both sides (vacuum technology, not nano- or sol-gel)
And it was actually 120 inches, on a number of lenses, quite repeatable, and documented


I don't think you're gullible. I think you accept things that are known. I think you believe that what's normal and widely done is all there is. I think you're a sheep. I think you're a settler.
(And I don't say that harshly or to be rude or demeaning. People are just different. I have to find better and different ways to do things or I get bored. Now, just to make you feel better, you may like to know that we normally still produce this product with CT of 1.8-2.0; just because I can do something doesn't mean I always do. I will do it if asked, though.)
As Red Green says, have fun out there- and keep your stick on the ice.

A sheep , you must be trying to aggrevate me. Thats OK I am not saying it can't be done, but to have this pass test repeatably, yes I question that. The fact that you say it passes 100 inches and now its 120 inches. I have a fishing story that goes the same way every time I tell it :D . Yes that was a little nip to aggrevate you. Any way I am sure it ca be done, but you are right I don't think it should be done (opinion). I have surfaced thinner myself, I once had a lens surfaced so thin that the edges were almost like paper. I gave the guy the lens didn't charge him, didn't write it up and told him I don't know him. It's not that I am a settler, I play by the rules. The fact of the matter is that the US IS obbsesed with impact resistance, and you are absolutely right that it is due to people being sue crazy. I would rather live within the rules than outside.

Some people are pioneers, some are settlers.

I would differ in opinion if you are trying to say that surfacing beyond a point is pioneering, it is dangerous to the patient if taken too far. It is not difficult to surface a lens down that thin, but you are correct in that it would require some knowledge. Like pressure settings on your polishers, and wich blocks to use to get thin especially on the high pluses. The lab I worked in before my current position we had the glass coburn blocks and alloy for the pluses so that the diameter of the block was smaller than the wax, but the people that were used to working with the wax would always get waves in their lenses, because they would not let the alloy cool long enough. These are things that contribute to the fact that if I had to do a high plus it would cost more, the blanks are still the same price, but the process is different and sometimes the settings have to be changed, and everyone in the lab was not familiar with the finess involved with the different processes. I would have to charge more, because the breakage was higher. Now I work in a practice that has a finishing lab only and I understand that any lab I order from might charge me a fee or premium for surfacing beyond their limits.

just because I can do something doesn't mean I always do. I will do it if asked, though

I would hope that if asked, you would charge a fee for this expertise or pioneering that you are doing.

Like you said in your previous post I think it boils down to opinion. I guess we learned where each other stands and the guy that started the thread actually didn't get his question answered :cheers: .

[hipoptical]

This last thing....
100 inches, 120 inches? Not a fish story or a change in the truth. Just a flaw in me. If you ask me what time it is, I will say that it is now 1:30, when it is actually 1:21 as I write. The reason I said 120 the last time was to give you the actual facts, not just a rough figure. Our drop ball unit actually goes to 150 inches, but the lenses would not CONSISTENLY pass at that level. That's why I didn't bring that up. I only say this to say that no matter how little you believe, the truth doesn't change. (Many people think Jesus was nothing special, that doesn't affect the truth, either.) Your perception is your opinion, the truth may be different.

[HarryChiling]

100 inches, 120 inches? Not a fish story or a change in the truth. Just a flaw in me. If you ask me what time it is, I will say that it is now 1:30, when it is actually 1:21 as I write.

Again that on was a pot shot for the sheep comment :p, no offence meant :D. I can believe it, but I question the consistency of it passing was all I was saying.

[hipoptical]

I take no offense to any of this discussion; I sense that you are sparring with me in a friendly way, a good discussion with no anger on either side.
The only problem with a forum like this is that I can't convince you or anyone else that what I'm saying is ALL true. If you were in my facility, I could prove it, but the reality is that it doesn't matter. I am somewhat encouraged knowing that there are some out there who do not believe that I can consistently produce these types of lenses, with consistent results. It just makes it that much better when we do, and other labs are questioned as to why they cannot, but we can...:D

[harry a saake]

Samuel, what you are saying about your patients makes no sense, what do they do walk in to your dispensary with a pair of calipers?, Samuel, how would they know if it was 1.0 or 1.5. The only thing that makes sense that you said was they want the thinnest lenses they can get, but Samuel that also means within reason. The only way i will ever believe what you said is, i want to see that person who can tell .5 mm difference, most opticians cant no less the public.

[Samuel Jong]

Samuel, what you are saying about your patients makes no sense, what do they do walk in to your dispensary with a pair of calipers?, Samuel, how would they know if it was 1.0 or 1.5. The only thing that makes sense that you said was they want the thinnest lenses they can get, but Samuel that also means within reason. The only way i will ever believe what you said is, i want to see that person who can tell .5 mm difference, most opticians cant no less the public.

You are right that they didn't know if it was 1.0 or 1.5, I just fulfil their request thinner than what they are wearing now. They want it and I can make it, and go ahead by ignoring the safety center thickness, although it doesn't affect much the edge thickness. Once again, I just to satisfy the patients with their new lenses - thinner.

As you know, sometimes the patients bring their new lenses to the other optical shops in order to be confirmed whether they get thinner lens or not.