Lenses at 1.0 C.T.

[Jim G]

I'm starting this tread as a continuation to the "Velocity and 1.0 centers" as part of that is about edging thin lenses and the issue of 1.0 c.t.'s deserves it's own forum.

First off, the prevailing interpretation of the FDA impact standard is that it really doesn't matter what the manufacturer says (and, they no longer really say anything), the person/company performing the final act of manufacturing is responsible for drop-ball testing. Typically, that would be edging!

We do routine impact testing; I'll share some thoughts and results. First, a surfaced lens will perform differently than a cast lens (typically, worse impact resistance). Second, surfaced lenses vary signiifcantly in their impact resistance as a result of the manufacturing variables. Third, adding a hard coat will lessen the impact resistance of a lens. Forth, adding an AR coat will do likewise. Lastly, using the two in combination (recommended) will render some lens materials unable to pass the test.

From our experience, the only surfaced lenses that will pass impact testing at a 1.0 c.t. are poly, Trivex, 1.60 and 1.66. These will probably also pass with a hard coat and an AR coating. The flip side is that anything less than 1.3 is very difficult to surface. So, even tho possible, I'd enfourage you to allow your labs to surface to 1.3 to 1.5 even tho they'll pass impact at 1.0.

Steve, to your point about a cushion coat. I am only aware of one lab currently using one as part of their process and it has a direct connection to a manufacturer. In other words, while the technology is available, it is not currently being used by the vast majority of labs (we're working on a UV curable one that looks promising but not yet available).

Last word. EDGING RETAILERS: YOU'RE RESPONSIBLE FOR IMPACT TESTING.

How's that grab ya?

[Steve Machol]

Good post Jim!

I realized after posting my answer about the 'buffer' coats that it might be confusing. I really only did that because Sola makes finished Spectralite available with 1mm centers, and some people may be misled into thinking that this means it can be safely ground to 1.0mm.

The fact is Spectralite 1mm requires a buffer coating to meet FDA impact compliance. If someone grinds it to less than the recommended 1.5mm, they almost certainly will fail impact testing unless they apply their own buffer coat. And as you point out, this technology is not generally available yet.

Nonetheless, you're absolutely right to point out that the laboratory is ultimately responsible for ensuring impact compliance. If they strictly adhere to the lens manufacturers processing recommendations, then the are probably 'safe'. However any deviation from this - adding hard coats, A/R coating, grinding under the specified thickness limits, etc. - places the responsibility clearly on the processing lab.

[willsaake]

Thanks for the information guys. I am definitely beginning to understand all this about 1.0's now. I never knew a finishing lab was responsible for impact testing except in the case of glass. We have an independent lab that does all of our surfacing. If grinding lenses down to 1.0 only saves .5mm on the edge then why does anyone bother? I routinely see 1.1 and 1.2 and quite often 1.0's all the time. It seems rather absurd to go to all the trouble if all you save is .5. Another things though, if a finishing lab does not do impact testing at all are they breaking any rule that says they have to impact test? We did impact testing at Visionworks but that was only when we were manufacturing crown glass. We never did it on anything else.

[Jim G]

Couple things going on here. The OLA (Optical Laboratory Association) has accepted a proposal to develop a statistical sampling to determine a combination of factors (substrate, surfacing, hardcoat, ARC, etc.) that may render a lens unable to pass the impact standard. This should have significan implications.

Lens manufacturers have started to change the way they indemnify subsequent users of their products. This is probably best explained by the following two quotes taken from semi-finished lens packaging:

"The enclosed . . . lens bank has been manufactured in compliance with FDA CRF 801.410. When laboratory ground to (the companies) surfacing chart specifications, the lens will be in compliance with FDA impact regulation 21 CFR 801.410. Any modification of this lens other than normal surfacing, edging or tinting, will require impact testing to assure compliance with the above regulation." This is representative of the "old" wording for a CR-39 lens at a minimum center thickness of 2.0mm.

The second quote:

"(This lens is) capable of being processed to meet applicable impact resistance requirements FDA regulation 21 CFR 801.410 and ANSI Z8031, Z87.1.

"Conformance to this standard is the responsibility of all subsequent processors...Improper operations may induce stress which will reduce the impact resistance significantly. Therefore, the processor cutting, milling, drilling or grinding the lenses to its finished shape, size, thickness and curvature and applying a coating or tinting/dying, is responsible for the performance . . . of the finished product and should verify the performance through appropriate verification testing." Believe it or not, this in on a poly package.

My guess is that the first statement will evolve into the second. Either way, the last operation is considered to be the one responsible for testing.

[LensLdy]

The message on the back of the envelope of the Hoya stock 1.71 with a 1.0CT is similar to the 1st statement mentioned.

[Bradmain]

May I be commercial here? It is my understanding that Hoya Lens of America (my employer) is the only manufacturer / lab that can grind down to 1.0 to 1.3 CT on their materials because they have a cushion coat that is applied to their surfaced lenses (and still pass drop ball).

They do this on their surfaced 1.60, 1.71 and Phoenix (the Trivex 1.53) materials and, of course, finished stock.

Sorry for the shameful plug, but someone had to do it.

Brad Main:drop:

[LensLdy]

Well, Bradmain, there you go--somebody had to do it and you did it. I guess that sets the record straight!:cheers:

[Jim G]

We've done extensive drop ball testing and find that ALL Trivex, poly, 1.6 and 1.66 will pass drop-ball at 1.0 C.T. using an Ultra Optics backside coating and AR. This is without the "cushion" coat you mention.

This doesn't mean we like to surface the stuff that thin. Because of warpage, we like to keep 'em to at least 1.3 or so.

Many of the manufacturers routinely use a "cushion-coat" on their finished single vision lenses; this isn't limited to just Hoya.

[LensLdy]

Jim G,

Being a newcomer to this forum, I'm curious to know your optical background. Sounds to me like you work for a lab or a manufacturer of lenses--is that common knowledge and I'm just behind the eight ball again? I came to that conclusion because you spoke of testing several lenses with a 1.0 CT. Knowing a person's perspective gives a different interpretation to their posts, know what I mean?

[Jim G]

I'm with Walman Optical, based in the home office in Mpls. If you don't know of us, we're involved in all area of the ophthalmic business from instruments to frames to contact lenses to a few wholesale labs to Ultra Optics, the primary supplier of backside scratch coating equipment and chemistry to wholesale labs around the world.

We feel that one of our responsibilities as a wholesale lab is to ensure that products perform as advertised. We therefore established an in-house testing program 18 years ago. I ran it for awhile until we had the good fortune to hire a character by the name of Dan Torgersen who, along with his full-time position with us, is the Technical Director of the Optical Laboratory Association. Dan and I work closely to test a variety of products in a variety of areas but, we're primarily concerned with impact resistance, scratch resistance, coating adhesion and UV attenuation.

As a result of our testing, we have the following statement in our price catalog: "...some combinations of lens materials, center thicknesses, scratch resistant coatings and AR coatings DO NOT PROVIDE AN ADEQUATE RESERVE OF IMPACT STRENGTH for dress eyewear...for that reason, we will not supply certain combinations of lens materials, center thicknesses and/or treatments." This is simply to protect our customers...sometimes from themselves!

Additionally, because of Ultra Optics, we work with the lens
manufacturers to ensure compatibility of our coatings with their lenses. Testing here is similar to that done for Walman.

Thanks for asking.

[LensLdy]

Jim G,

Those are some impressive credentials! Nice to make your acquaintance!

LensLdy

[harry a saake]

:hammer: While i have wrote this on another thread, i think you all are placing too much emphasis on how thin you can get a lens. The reality of fact is whatever you save in the center on a lens is all that you will save on the edge. Why would you want to bother doing that for a .5 mm. Nobody is going to see this, plus you leave yourself open for a host of problems, as warpage, flexing and potentially a lawsuit.there is not much point in going below 1.8

[JRS]

Yes Harry, it does sound rather foolish.

The thickness of a piece of 24lb paper (quality printer paper) is 0.1mm thick.

So going from a [more] solid center foundation of say 1.3mm to 1.0mm is not hardly noticiable - except it probably cost the lab more time and money to achieve it.

[LensLdy]

1.8CT????? By today's standards, that's almost SAP thickness! You ask why bother to go to a 1.0 CT when the difference in thinness isn't significant from a 1.3 or a 1.5 CT. They do it because they can, it's that simple. And from a marketing point of view, it makes them just a smidge better and more technologically advanced than the other guys. Just like having an 8 cylinder car is faster than a 6 or 4 cylinder one.:cheers:

[harry a saake]

:finger: lens lady, apparently you have some facts mixed up. First just because you can do anything does not neccessarily make it a good idea. You could jump off the brooklyn bridge, but would it be smart? As far as your anology on the cylinders of cars, an 8 being faster then a six or a four, its apparent your not well versed in cars, as i can show you many a four cylinder that an 8 will not touch. As far as your belief about the lens companies showing they can do it better, NO, all they have shown is that they made it thinner. The unfortunate part of all this is that some of you will never believe this until you get hit with a lawsuit,and ruin the rest of your life. I dont know if any of you remember the optician in Chicago who sold a Rayban to an undercover agent. While making an arrest, the agent was punched, the lens shattered, ruined his eyesight, and they sued. Guess what. They won. The optician was suppose to know better. Tell me something lens lady, are you going to put a pair of these lenses in a rec spec, with their 1.0 ct.:hammer:

[LensLdy]

No, Harry, I wouldn't put a lens with a 1.0 CT in a Rec Spec because it doesn't belong there. Generally a 1.0 CT is found on hi index lenses and nobody in their right mind would put a hi index lens in a Rec Spec. A Rec Spec would require a poly lens or the new Phoenix by Hoya which is made of the new Trivex material, which has a 1.3 CT

As for the optician in Chicago who got sued and lost, it would seem that he didn't ask a lot of questions about his patient's life style or else he would have known that the possiblity of danger existed in the patient's occupation.

I'm sorry you felt the need to nit pick on the issue of 8, 6, and 4 cylinder cars. I'm sure if you took a poll of the general population, the overwhelming majority of people would think that an 8 cylinder car goes faster than a 4 cylinder one. Yes, I'm sure there are exceptions, but ...I was speaking in general terms. And any question I have about cars I could ask my hubby--he's a genius when it comes to cars!

And I always thought that thinner was BETTER when it came to high minus lenses
:idea:

[harry a saake]

Lens Lady, no i wasn,t really trying to nit pick, just make a point. As far as thinner always being better however, is just simply not true. As far as i anm concerned and this is my own personal view, you have to be nuts to put any 1.3 lens in a rec spec. i dont care who makes it or what they claim, because something that thin is going to break.
.........Over the years all kind of manufactures have come out with all kinds of claims as to what there lenses or products could do. Is that a surprise, after all they are selling it.
..............Next i would ask you do you have enough substantiated documentation, that a 1.3 lens made of trivex, is really going do withstand a racquetball traveling in well over excess of 100 mph, or a tennis ball, or a baseball?
............ Somewhere you have to ask yourself ,WHY DO I WANT TO TAKE THE CHANCE.

[chip anderson]

Lenslady: Thinner edges is better on high minus lenses, the point is this should not be gained by making the center too thin to be strong. Thinner edges can be obtained by lens index, aspheric design, frame size/PD, and other tricks. But if you make a lens .05 mm thinner at center, you make the edge only
.05 mm thinner at the edge. This is negligable and will make the impact resistance much lower.

Chip

[Rich R]

I have to agree that thinner is better looking cosmetically, I would not grind them that thin for myself, I have seen how much lenses that thin bend in the edging process.
Rich R.

[LensLdy]

So you don't feel secure about using a lens with a 1.3 CT in a Rec Spec, but you feel safer using a poly, which if it's a stock lens, has a 1.5 CT. That measly .2mm makes you feel more confident in the strength of the lens? I've seen the tests done on poly and Trivex and I would have no problem using a Trivex lens in a situation where I would have ordinarily used a poly lens. Not only would I be giving my patient a lens with better optics because of the higher Abbe value, but it would be lighter in weight. And I definitely would use this lens for kids for the very same reasons! I've always felt guilty about giving poly to a child. I mean, here it is, usually their 1st pr. of glasses, and I'm putting lenses on them that have the biggest problem with chromatic aberration in the industry. Not a very nice way to say welcome to the world of eyeglasses, in my opinion.

[harry a saake]

:bbg: Lens Lady, in my shop no rec specs go out unles they are 3.0 ct. No, i would not feel comfortable with poly at 2.0. Its poly at 3.0

[karen]

Jim, I have what may be a stupid question but this wouldn't be the first time I asked one so here goes... Can you help me to understand why applying a hard coat or AR would make a lens less impact resistant????

[Steve Machol]

Originally posted by karen
Jim, I have what may be a stupid question but this wouldn't be the first time I asked one so here goes... Can you help me to understand why applying a hard coat or AR would make a lens less impact resistant????

Simply put, one of the primary factors in the impact resistance of a curved object is the relative 'brittleness' of its concave surface - providing, of course, the lens is struck from the convex side. Hard coatings can add some degree of brittleness to a surface, but in practice this is generally not enough to change the impact resistance significantly However when combined with an AR Coating which is promarily a very thin and brittle 'glass-like' layer, the reduction in impact resistance can be significant.

[LensLdy]

Steve,

Would that still hold true if 1 of the layers of the AR coating was a shock absorption or cushion coat layer? I know Seiko and Hoya both use an AR that has a layer like this, which enables their 1.0 CT lenses to pass drop ball testing.

[Steve Machol]

Actually the 'shock' absorber layers are not part of the AR. They go on underneath the hard coating and are sometimes the only way that a given lens material and thickness can meet FDA impact requirements when AR coated. These layers definitely do help.