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    Minimum Material Thickness

    Can I get 1.0 specified thickness in any material or do different materials have different minimum thicknesses? We have been specifying 1.0 thickness on our higher Rx jobs because patients have been coming in with glasses much thinner from online stores than we have been getting from our lab. Same Rx and same frame decentration. The only difference is our CT is coming in at 1.5 or 2.0. Online lenses are coming in at 1.0 CT. The reason I ask if there are minimum CT for different materials is because we are having a hard time getting what we ordered, a 1.0 CT. Its hit or miss. I hope that makes sense.

    #2
    is it the same material?

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      #3
      It's usually 1.67 or 1.74

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        #4
        Up to the lab to decide what minimum thickness they will cut. Anything can be made 1.0, but you may get pushback from the lab if you try on low power Rxs or plastic materials. What can impact those higher powers in high index a lot more than minimum thickness is flatter base curves and asphericity. The only thing online cares about is cosmetics, so they make very flat curves and may or may not also use aspheric (I would suspect they don't because of expense, but then again if they are buying hundreds of thousands of FSV lenses they can probably negotiate the cost difference down to nothing). If all you customer cares about is thin lens, avoid digital, and slap it on an aspheric 0.50 base.
        www.DanielLivingston.com

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          #5
          Originally posted by DanLiv View Post
          Up to the lab to decide what minimum thickness they will cut. Anything can be made 1.0, but you may get pushback from the lab if you try on low power Rxs or plastic materials. What can impact those higher powers in high index a lot more than minimum thickness is flatter base curves and asphericity. The only thing online cares about is cosmetics, so they make very flat curves and may or may not also use aspheric (I would suspect they don't because of expense, but then again if they are buying hundreds of thousands of FSV lenses they can probably negotiate the cost difference down to nothing). If all you customer cares about is thin lens, avoid digital, and slap it on an aspheric 0.50 base.
          Thank you DanLiv, can I ask why to avoid digital lenses?

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            #6
            The only legal material that can be made 1.0 is poly. I don’tthink any lab will make 1.0 lenses in other materials for legal reasons and for spoilage reasons.

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              #7
              How much of an edge thickness difference is it making? Can this extra edge thickness be negated by better frame fitting and edging adjustments?

              Also, is the wearer noticing this, or is it only something you're noticing on the dispensing side?

              If the marginal increase in CT doesn't cause any complaints by the wearer, I'd suggest not trying to fix something that isn't broken. Lenses ground too thin = More risk of warpage, breakage, and all other such problems.

              As to the original question: multiple labs (big brand and also independent) have told me that 1.0 CT can be requested, and they will try to comply with such orders within tolerances... but that they then consider any damage to those lenses to be at the dispenser and wearer's own risk, as it was below their recommended minimum thickness. I found this reply to be reasonable enough, and have not requested 1.0 CT at least for the last 5 years since finding out.

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                #8
                Originally posted by Lensman11 View Post
                The only legal material that can be made 1.0 is poly. I don’t think any lab will make 1.0 lenses in other materials for legal reasons and for spoilage reasons.
                Good "rule of thumb" to follow, but is not always the case...
                ANSI has requirements...
                However, depending on your particular lab account size...

                Best to base end wearer results on "2.0mm" and "Wow" them if you can get it thinner.
                Go ahead and request.
                You've nothing to lose.


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                  #9
                  Originally posted by btoe32 View Post
                  Thank you DanLiv, can I ask why to avoid digital lenses?
                  Digital lenses are designed and calculated with optics as primary, cosmetics secondary. They will have better adherence to optimal corrected curves, which means not as flat, which means not as thin. You may be able to specify a flat base curve on a digital from your lab, but deviating from optimal curves distorts the optics so there's not much point of doing digital then.

                  Originally posted by Lensman11 View Post
                  The only legal material that can be made 1.0 is poly. I don’t think any lab will make 1.0 lenses in other materials for legal reasons and for spoilage reasons.
                  Originally posted by MEB View Post
                  Good "rule of thumb" to follow, but is not always the case...
                  ANSI has requirements...
                  Maybe in your fancy-schmancy license states there's "laws", but out here its the wild wild west, ain't no rules! Any laws about optics or ANSI standards are state laws, and my state has none.

                  I do request 1.0 on high power jobs, and my lab (also not in a licensed state) is usually happy to comply. Occasionally they resist saying it will just be too thin and then I acquiesce and just ask thin as possible. The lenses don't come back as true 1.0, but at least by specifying the lab shoots for 0.3mm of the 1.0 target. If you don't ask there is no required thickness, so you could get a lazy 2.0 or 2.5 and it's still considered "acceptable".​
                  www.DanielLivingston.com

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                    #10
                    Originally posted by Lensman11 View Post
                    The only legal material that can be made 1.0 is poly. I don’tthink any lab will make 1.0 lenses in other materials for legal reasons and for spoilage reasons.
                    I assume you can provide the law that says that?

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                      #11
                      Originally posted by Kwill212 View Post

                      I assume you can provide the law that says that?
                      I think the best term is a liability issue.

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                        #12
                        Originally posted by Uncle Fester View Post

                        I think the best term is a liability issue.
                        um, ok? What does that have to do with a law specifically stating poly is the only material that is ok at 1.0mm CT?

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                          #13
                          Originally posted by DanLiv View Post
                          Maybe in your fancy-schmancy license states there's "laws", but out here its the wild wild west, ain't no rules! Any laws about optics or ANSI standards are state laws, and my state has none.
                          ​​​​​​Likewise here in the Third World! 🤣

                          But, on a practical note: some lab managers I've had the chance to speak with were kind enough to indulge my curiosity on the practical side of requesting the thinnest possible lens, and shared the following with me.

                          Requesting the thinnest possible lens means center thickness (CT) of 1.0 mm, which most contemporary lab machines could probably accommodate. But this means increased risk of lens warpage at one of more stages of processing, breakage during edging and/or transport, or even the lens flattening slightly during processing due to the weight of the alloy block (then returning to a more curved shape after deblocking, which may affect the end result). So such labs would prefer to work with CT slightly thicker than 1.0 mm (and as DanLiv has pointed out, it doesn't have to be exactly that, sometimes just requesting the thinnest is perfectly fine for all parties involved).

                          Also, in some cases (read: anisometropia), the system may grind one side or the other thicker as part of the balancing calculations, so whatever thickness you get is likely in your optical best interests.

                          Going back to my earlier suggestion: if the wearer is happy, don't try to fix what isn't broken. And if despite your specific requests the final lenses are still a hair too thick, I'd recommend trying to adjust bevels etc before asking the labs to try for 1.0 CT, all things considered.

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                            #14
                            Since 1964 there is a federal law not any state law that requires all eyeware to pass a drop ball test. To pass the test glass lenses must be treated either chemically or by heat hardening. Plastic lenses do not have to be checked individually but you must batch check your process and record the results. No plastic lens other than poly or Trivex can pass a drop ball test at 1.0 center. This is basic requirements to manufacture eyeware it should be written on every lens box you receive from every manufacturer. Adding any coating to plastic lens greatly reduces the impact resistance and will require you to make lenses thicker to pass the drop ball test. This law is what made plastic replace glass as the thicker glass lenses were heavy and frames were getting larger in the 60’s making glass almost unusable.

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                              #15
                              There is a lot of misinformation here, I’ll try to clear it up.

                              1st off, almost any lens (minus) can have a 1.0mm ct. but there are fabrication issues when doing this, mostly waves/inconstant powers. But also liability issues, (as Fester stated). ANSI is not a law. It is a standardization institution. It gives “recommendations “ as to safety of products primarily. Most industries adhere to their recommendations. Recommendation for impact resistance is for lens manufacturers ( labs, lens molders, ect) to do “batch” impact resistance. ( they also don’t state what constitutes a “batch”.) If you or a manufacturer produced a lens that would fail an impact resistance test, the optical police aren’t coming after you…but the injured plaintiff will because it would be a slam dunk case. Everyone ( including ANSI recommendations) knows that a .5mm ct glass lens can shatter easily as example). So, because of fabrication AND liability issues, some labs will not go below a certain minimum ct.

                              Now then, stock lenses (particularly minus) can be consistently injection molded with thin centers. Check your own stock lenses. Most will have 1.0-1.2 centers. It’s much easier to control variables ( thickness/power constancy) with molding. Easier to do documented batch testing.

                              As far as digital lenses…Lens designers do not only go for optics but also cosmetics. To achieve thinner perceived ​​​​​​ lenses, you need to decrease its plate height, ( plate hgt is how tall a lens sits on a flat surface, concave down, measured to the highest apex of the convex surface). Imagine a -1.00 on a 7 base lens compared to a -1.00 put on a 4 base. With the same ct and diameter, they will not be any different on thickness, but the 4 base will appear ​​​​​​ thinner. So, to close this part, the 1st priority of digital lens designers is cosmetics. If optics were primary, they would stick with best curve theory.

                              Now, when you reduce the plate height, you are going against best curve theory, ( Tscherning ellipse). Best curve theory is choosing a particular base curve for a given power, ( to minimize off axis astigmatism). Going back to our 7 bc and 4bc in the example above, Tschernings says use a 7 base. But hey! I want them to look thinner! I want them on a 4 base! Enter aspheric/atoric lenses. Using aspheric/atoric curves we can minimize the off axis astigmatism using flatter curves than what best curve would call for.

                              For folks newer to this field, to explain aspheric/atoric lenses, think of a bulls eye, with rings radiating out from the center. On a minus lens, as the curve goes out from the bulls eye center, it’s increasing. Think of a lens that goes from a 4 base center to eventually a 6 base curve on the outer edges. There are specific amounts of asphericity needed for a given power to help minimize the off axis astigmatism created by going flatter than best curve. Atoric is the same except you compensate the added cylinder (compound power) by a different amount of asphicity along it’s axis. Plus lenses achieve asphericity just the opposite, they are steeper in the center, then reduce curvature as they go out from the center. Old timers will remember the old “4 drop” cataract lens. That’s exactly what we did at Universal Opthalmics when we made molds and cast them in cr…

                              As for ordering a flatter than recommended from digital labs, it depends. There are overlaps in the software programming for power ranges. Your requested Rx might be in that area. But it really all comes down to the software programing. Programmers are not gonna write the elaborate software for every possible base curve/power scenario, “ Hey! What do ya mean you can’t put my -1.00 on a 17 base curve! I thought you were a digital lab!! “

                              I hope this cleared up some of the stuff this thread brought up.

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