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Thread: blended slab lines

  1. #1
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    Confused blended slab lines

    There seems to be a new trend developing in the area of slab-offs. With the advent of new lathes and other generators, it seems as if the old way of doing a bi-centric grind is going away. My question is this....Has anybody noticed a drop in the quality of slab lenses when they are cut by one of these new generators? It seems to me to be a sub-standard way of doing it. The "blend" in the slab line creates a field of chaotic curves that would seem to me to be in a difficult place for a patient to adapt to. This seems similar to the unwanted "swim" associated with the chaotic curves of a progressive lens. Any thoughts on the subject???

  2. #2
    Master OptiBoarder Darryl Meister's Avatar
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    The "blend" in the slab line creates a field of chaotic curves that would seem to me to be in a difficult place for a patient to adapt to.
    Can you describe in a bit more detail what you mean by a "blended" slab-line and the curves it produces?

    While I imagine that it is possible to produce a surface with a progressive increase in prism (that is, a "blended slab-off," similar to a "blended bifocal"), I don't think you could fine and polish such a surface using conventional cylinder machines or toric surfacers. Also, it would almost certainly induce unwanted cylinder power (blur) across the transition.

    A relatively easy way to reduce the visibility of the slab-off line is simply to apply an AR coating to the lens, which makes it more difficult to see the boundary between the two surface curves.

    Surfacing slab-off lenses takes a great deal of skill, particularly if the prescription has cylinder power... Is it possible that you just got a lens with a less-than-spectacular looking slab-off line? Remember that you could always use those pre-cast, reverse slab-off lenses, instead.

    Best regards,
    Darryl

  3. #3
    One eye sees, the other feels OptiBoard Silver Supporter
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    JP,

    I'am not sure if this pertinant but "Epic Labs" can slab up to 1.5^ with the line nearly invisible. My *guess* is that they won't go over 1.5^ because it would look like what you are describing...horrible.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    The process that I'm talking about is the new lathes on the market that essentially cut a slab-off in one step. They're the same machines that can produce the front surface on a progressive lens. As I understand it thses machines are not able to make an abrupt cut (anything with an instant surface change such as a flat top bifocal or a sharp slab line.) The lenses they produce are not polished in conventional ways with a tool. They are instead polished with a spongy ball similar to a soapy nerf ball. So when they produce a slab-off the transition from one prism amount to another can not be made without a blend of some kind.

    PS-The reverse slabs are fine but are limiting in the materials and seg styles available. So this is a problem for any lens which requires a back side slab-off.

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    Master OptiBoarder Darryl Meister's Avatar
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    The process that I'm talking about is the new lathes on the market that essentially cut a slab-off in one step.
    If you're referring to a free-form generator with a soft-pad polishing process, then I would just ask the lab to surface them conventionally -- or at least in two steps. You're right; you're probably not going to get a well-defined slab-off line in that situation if they are doing it in one step. If they cannot surface them regularly, and you are not satisfied with the quality of the slab-offs coming out of their free-form process, perhaps you should consider ordering them from a specialty lab -- or at least have your lab farm out the work.

    My *guess* is that they won't go over 1.5^ because it would look like what you are describing...horrible
    It is possible to get nice looking slab-offs above 1.5 PD though; you just need to baby the lens through the process (particularly the first fine). But, of course, this is why slab-offs cost more money. Frankly, many ECPs may not even bother prescribing a vertical imbalance correction below 2.0 PD.

    Best regards,
    Darryl

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    Darryl,

    Thanks for the info so far. This does bring me full circle back to my original question however. In your opinion would this be an adequate substitue for a traditional bi-centric slab or even a molded reverse slab? My guess is that on low slab amounts(maybe 2 or less)the difference would be minimal because the crispness of the line on a traditional slab below 2 isn't easy to achieve. But, like you pointed out, slabs below 2 aren't that common. People walk around with 1-2 diopters of uncorrected prism imbalance all the time. But when slab amounts start getting into the 3-4 diopter range people generally have a more difficult time with double vision because of imbalance. With a free form generator, labs will be trying to produce slabs with these higher amounts of slab-off prism which would make for a larger blended area to deal with. I know of a few labs who have these lathes and undoubetdly will try to use them to their full potential.

    PS-For the sake of argument the lens in question required a 4.5 diopter slab in plastic transitions. I've seen plenty of 4.5 diopter slabs with crisp lines. This was definitely produced on a free form generator. Again, do you think this is the way the industry will turn when it comes to slabs?

    Thanks a bunch!
    JP

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    Master OptiBoarder Darryl Meister's Avatar
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    Frankly, you already understand the issues involved surprisingly well. Honestly, I can't say that I've ever given slab-off production on a free-form generator any thought. And, unfortunately, commenting with any insight into the matter would at the very least require evaluating some actual lens samples and discussing the issues involved with one of our lens designers familiar with free-form manufacturing.

    As you pointed out, a slab-off line does not represent a smooth change in surface curvature, but rather an abrupt change. Speaking mathematically, the surface has a discontinuous first derivative across the line, while a smooth progressive lens surface has continuous first and second derivatives. Moreover, even if you could produce a perfect bi-centric surface off of the free-form generator, soft-pad polishing would still be a problem. Even if the polishing was also numerically controlled, the "ramp" in the surface that results in the slab-line would still be rounded to some degree as the pad moved over it.

    The best approach for such lenses might be to deliberately design a smooth, well-controlled "blending" region. The amount of unwanted cylinder power will depend upon both the amount of prism and the length of the transition, and an optimal balance between the length of the transition and the amount of blur/distortion would have to be determined. I doubt you'd want the transition to extend beyond 2 or 3 mm (which is also the transition length for many blended bifocals), but the lower limit of the length of the transition would be set by the mechanical limitations of the surfacing and polishing process, as we've discussed earlier.

    Interesting stuff, though.

    Best regards,
    Darryl

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    Thank you for posting this ! It is a real pleasure to read about such an interesting subject.

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    We do slabs on a regular basis. If the amount of slab is below 1.5^ we advise not to use a slab. If they insist on a slab we up prism to 2^. It is difficult to grind slabs less than 2^, and may be unecessary. The greater the slab prism, the easier the slab line is to control and produces a more defined slab line. I was able to see first hand the slab produced on the DAC free form generator. The slab line was not well defined, but I was impressed with the process and finished product.
    Joseph Felker
    AllentownOptical.com

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