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Thread: Is it possible for PAL design to move the optimal reading zone around the lens?

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    Is it possible for PAL design to move the optimal reading zone around the lens?

    I've read this around the optiboard, using Free Form technology that PAL designs can move the optimal reading zone around the lens. What exactly does this mean?
    Last edited by AustinEyewear; 04-05-2012 at 10:25 PM. Reason: question clarification

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    I'm not sure what you are asking. Some Free Form brands include customizable progressives. You can order a fitting height and also a fixed segment height. I think of the 'fixed segment height' as the distance from the fitting cross to the bottom of the traditionally 5 mm near verification circle. You can tailor the add power progression, to some extent, by ordering both a fitting height and a 'fixed segment height'. This is how opticians can take some control of the power progression and corridor length. (Lens gurus who monitor this site will set me straight if I have not reported accurately.) Is that what you were asking?

    Hope that helps.
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    Thanks Dr. J! I just realized how I should word this question. What I'm actually trying to ascertain, is, will a free form PAL and a molded PAL with same design always be placed on the lens in identical locations?, all other factors being the same, including frame shape. What I mean by same design, is same hour glass shape. I realize the freeform can compensate for factors such as VD, Pano, Frame Wrap, ect. But will the same hour glass shape still exist in identical location of lens?

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    Glad to help

    Quote Originally Posted by AustinEyewear View Post
    Thanks Dr. J! ... will a free form PAL and a molded PAL with same design always be placed on the lens in identical locations?, all other factors being the same, including frame shape. What I mean by same design, is same hour glass shape. I realize the freeform can compensate for factors such as VD, Pano, Frame Wrap, ect. But will the same hour glass shape still exist in identical location of lens?
    I think the general answer to your question is this. If you order a Free Form lens, with a similar minimum fit height as the conventional PAL, yes, the hour glass power profile will be similar - though not exactly the same because of proprietary design - and the Free Form lens should have 20 to 30% less aberrations and a wider field of view. Still general, but a little more specific, aberrations include oblique astigmatism, power error, spherical aberration, coma, and distortion (from Daryl Meister's publications). High sphere powers, high cylinder powers, and high adds will have the most noticeable improvements in optics due to Free Form computer aided lens design.

    The hour glass will not be identical because different brands shape the progressive blend zone harder or softer - even when the fixed segment height is identical.
    Renee Kathleen Jacobs O.D., M.A.
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    Quote Originally Posted by AustinEyewear View Post
    I've read this around the optiboard, using Free Form technology that PAL designs can move the optimal reading zone around the lens. What exactly does this mean?
    Position-wise, the near point might be optimized on the horizontal meridian to accommodate unusually wide or narrow PDs, and for prismatic effects from the distance power (more inset for plus, less for minus). On the vertical meridian, the position might be optimized for frame shape and seg height, and for prismatic effects from the distance power (shorter corridor for minus, longer for plus).
    Robert Martellaro
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    Thanks Robert. This question was spawned by the dual surface PALs that some manufacturers use, where they split the design on both the front and back of the lens. Some use a molded front, and a digitally surfaced back. I'm wondering if the hour glass shape matches up front and back. Or maybe the front doesn't even have the hour glass shape? But if it does, it seems like it would get in the way of making the best possible lens. I'm having trouble with this concept?

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    Quote Originally Posted by AustinEyewear View Post
    Thanks Robert.
    Your welcome.

    This question was spawned by the dual surface PALs that some manufacturers use, where they split the design on both the front and back of the lens. Some use a molded front, and a digitally surfaced back. I'm wondering if the hour glass shape matches up front and back.
    They had better match up!

    ...it seems like it would get in the way of making the best possible lens.
    I suspect that the lens designers can write software that keeps the PAL performance, especially with the more complex Rxs, closer to the intended design when the progressive optics and power curve are on the same surface.
    Robert Martellaro
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    www.roberts-optical.com
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    Quote Originally Posted by RKJ View Post
    The hour glass will not be identical because different brands shape the progressive blend zone harder or softer - even when the fixed segment I think the general answer to your question is this. If you order a Free Form lens, with a similar minimum fit height as the conventional PAL, yes, the hour glass power profile will be similar - though not exactly the same because of proprietary design - and the Free Form lens should have 20 to 30% less aberrations and a wider field of view. Still general, but a little more specific, aberrations include oblique astigmatism, power error, spherical aberration, coma, and distortion (from Daryl Meister's publications). High sphere powers, high cylinder powers, and high adds will have the most noticeable improvements in optics due to Free Form computer aided lens design.

    height is identical.
    Dear RKJ,
    I study PAL 2 years in Rotlex.
    I've studied some PAL with surfaçagem Free Form, found no less than 20% aberrations, or increased areas proportion of their. There is yes is a decrease of induced astigmatism and a decrease in the area of its maximum power, and greater freedom of choice of the base curve, descentrações adjusted for convergence, and change in the project according to the height of the frame.
    I believe that all projects are Free Form in progress, and in the future we can achieve 30% reductions in aberrations and also improves other parameters, but in the moment, no.

    Best Regards

    Celso Cunha

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    Quote Originally Posted by RKJ View Post
    The hour glass will not be identical because different brands shape the progressive blend zone harder or softer - even when the fixed segment height is identical.
    free-form enables us to break free of the hourglass shape to some degree, Free-form lenses come in Plus, V, Barrel and most common, T shapes.
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    Currently Austin, there are only 2 lenses available in the US that I know of that fully use the near PD and offer complete corridor re-design based on distance power, near power, near pd. The issue is that as the eye rotates through a lens in plus vs minus powers the corridor needs to move and change to compensate for the increase and change in prism to keep the virtual lens in front of the eye at all points through the corridor. When this doesn't happen you will see a decrease of intermediate width.

    The Seiko Surmount and Kodak Unique.

    Quote Originally Posted by AustinEyewear View Post
    Thanks Dr. J! I just realized how I should word this question. What I'm actually trying to ascertain, is, will a free form PAL and a molded PAL with same design always be placed on the lens in identical locations?, all other factors being the same, including frame shape. What I mean by same design, is same hour glass shape. I realize the freeform can compensate for factors such as VD, Pano, Frame Wrap, ect. But will the same hour glass shape still exist in identical location of lens?
    Beer is proof that God loves us and wants us to be happy ~Benjamin Franklin

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    Most of these designs end up in a + Plus shaped lens pattern.

    Quote Originally Posted by AustinEyewear View Post
    Thanks Robert. This question was spawned by the dual surface PALs that some manufacturers use, where they split the design on both the front and back of the lens. Some use a molded front, and a digitally surfaced back. I'm wondering if the hour glass shape matches up front and back. Or maybe the front doesn't even have the hour glass shape? But if it does, it seems like it would get in the way of making the best possible lens. I'm having trouble with this concept?
    Beer is proof that God loves us and wants us to be happy ~Benjamin Franklin

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    I've read this around the optiboard, using Free Form technology that PAL designs can move the optimal reading zone around the lens. What exactly does this mean
    Unfortunately, several posts on OptiBoard have implied that "free-form" technology magically makes lenses better or that it is inherently superior to traditional lenses. This is not correct. And I suspect that a lot of patients out there are getting charged a premium for medicore optical performance due to this kind of overgeneralization. It is a mistake to assume that all free-form lenses are the same or that they all offer comparable benefits.

    Free-form surfacing is simply a manufacturing platform. The potential visual benefits to your patients will only be realized when this manufacturing platform is utilized to fabricate lenses that have first been customized for the wearer. You should carefully review the technical materials from your free-form lens supplier in order to determine what, if any, optical customization is being applied.

    That said, certain free-form lens suppliers offer you the ability to customize features such as the height of the near zone, inset of the near zone, or even the relative width of the near zone:
    • Some lens designs vary the height of the near zone using a variable corridor length in order to offer consistent reading utility at shorter fitting heights.
    • Some lens designs vary the inset of the near zone based upon the distance PD, distance prescription (to account for prism), and/or reading distance.
    • Some lens designs vary the relative width of the near zone in order to offer a lifestyle-oriented design with greater emphasis on either reading vision or far vision.
    • Some lens designs vary the relative "softness" of the distance and near zones in order to account for head movement propensity.


    This question was spawned by the dual surface PALs that some manufacturers use, where they split the design on both the front and back of the lens. Some use a molded front, and a digitally surfaced back.
    Yes, you can place some or all of the progressive optics on either the front surface, back surface, or split between both. There may be optical, cosmetic, or mechanical reasons to choose one approach over the other. Or the free-form lens supplier may simply be utilizing an approach that safely circumvents existing patents or IP. Because progressive lenses represent relatively "thin" optical systems, however, the distribution of the optics between the front and back surfaces will generally produce only minor differences in optical performance.

    Currently Austin, there are only 2 lenses available in the US that I know of that fully use the near PD and offer complete corridor re-design based on distance power, near power, near pd. The Seiko Surmount and Kodak Unique.
    ZEISS free-form progressive lenses have offered this technology for 10 years.

    Best regards,
    Darryl
    Last edited by Darryl Meister; 04-22-2012 at 10:52 AM.
    Darryl J. Meister, ABOM

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    Thanks for responding Darryl. I think I've been under somewhat of a false pretense that has recently been cleared up. I was assuming the PAL design in a free-form progressive was completely modified on an individual basis. I have since been enlightened on this.

    To boil it all down to the lowest common denominator....... my new understanding is that the design remains in place, all free form does, is correct for the aberrations which are induced by the mis-match of the base curve. The PAL design shape remains in place, now it can be further optimized using free-form. Please be sure to correct me if I am wrong and I would love to get additional clarity on the subject.

    Understanding that the design is not changed, helps me understand how some lens manufacturers can split the PAL design on both sides, the front using a molded PAL and on back using a free-form PAL. I could not comprehend how this could be done until I realized that the PAL design shape does not change. I was worried that the front side could take on a shape completely different than the backside, and create unwanted interference with one another.

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    It sounds like to me, that you are asking about a specific lens and whether the design changes when done in freeform???? If that is correct, your understanding is correct. As to all lenses prepared in freeform processing, some "designs" change to optimize the optics for the patient, based off fit and frame shape and rx. That's my 2 cents.

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    Quote Originally Posted by EyeCare Rich View Post
    It sounds like to me, that you are asking about a specific lens and whether the design changes when done in freeform???? If that is correct, your understanding is correct. As to all lenses prepared in freeform processing, some "designs" change to optimize the optics for the patient, based off fit and frame shape and rx. That's my 2 cents.
    Yes Rich, In general, I was referring to a specific design. As an example, previously I was under the impression that if a certain design was selected, the corridor width might actually change because of all the parameters that were accounted for. For example, some Rx's may lend themselves to a wider corridor than others, and I thought free form algorithms would take advantage of this.

    But now I am under the impression the width will be the same for any patient, regardless of Rx, just more of it usable because of the free-form algorithms account for the base curve and just make more of it clearer, not wider. Man its hard to talk about this stuff without a whiteboard..... Hope that makes sense?
    Last edited by AustinEyewear; 04-23-2012 at 05:58 PM.

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    free form does, is correct for the aberrations which are induced by the mis-match of the base curve. The PAL design shape remains in place, now it can be further optimized using free-form. Please be sure to correct me if I am wrong and I would love to get additional clarity on the subject
    Really, all free-form does is cut a surface onto a lens blank. It does not improve or optimize anything. Free-form surfacing can be, and often is, utilized to cut a conventional progressive surface onto a lens blank, which results in a lens comparable in performance to traditional, semi-finished lenses. The advantages of free-form manufacturing for the wearer is as an enabling technology.

    However, some manufacturers utilize additional software that redesigns the lens based upon data specific to the individual, such as his or her prescription and frame fitting requirements, immediately prior to free-form fabrication. If such software has not been used to redesign the lens in "real time," the surface cut onto the lens blank will offer no meaningful visual benefits to the wearer. Consequently, free-form technology is valuable to the wearer to the extent that it enables real-time optical design for the individual.

    Understanding that the design is not changed, helps me understand how some lens manufacturers can split the PAL design on both sides, the front using a molded PAL and on back using a free-form PAL
    The design can be altered or changed, prior to fabrication, if sufficiently advanced optical design software it utilized. For instance, in addition to customizing the lens design for the prescription or fitting requirements of the wearer, the corridor length or dimension of the viewing zones can be modified for the wearer. However, the actual placement of the progressive optics, on the front, back, or split between both surfaces, will only have a minor influence on optical performance.P

    lacing the optics on the back surface, for instance, minimizes skew distortion slightly and brings the viewing zones slightly closer to the eye, which will increase the field of view slightly. Optically customing the lens design for the wearer's specific prescription and fitting requirements, on the other hand, may significantly increase the wearer's field of view compared to traditional lenses.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Thanks again Darryl. I have a good understanding of the entire freeform process from a very high level - from the advanced optical design software algorithms to the machines that surface the lenses, sorry about that, I was being lazy when I used free form to describe the whole process. What I don't understand are the designs themselves. Part of my handicap, is that I really don't even know how traditional PAL's were molded. If I may continue with the corridor width question, would a traditional molded PAL (same design) have one corridor width for all PALs regardless of Rx? Or would they vary the corridor width based on Rx/material to achieve best possible width across a range of Rx's, and in turn, create a multitude of molds for different ranges of Rx's?

    The reason I ask this specific question, is because it seems the free-form based PAL designed would allow you to create an unlimited number of corridor widths that would only be limited by the pts Rx and material used even when using the same PAL design. The width may not vary much, but why not squeeze as much performance out of the lens as possible? However, I was recently told that this is not done, and the design would be the same regardless.

    Bottom line, is I'm trying to understand how much a particular design would be tweaked? Of course the advanced optical design software will correct for the aberrations caused by the base curve mismatch which will inherently increase viewing zones. But how much of the design can be tweaked to further increase the viewing zones and then correct for aberrations caused by base curve mismatch?

    Its seems to me, that the designer would attempt to spread all viewing zone out as much as possible, which they would be limited by the Rx and material used, and once they did this, apply the algorithms to correct base curve mismatch.

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    Quote Originally Posted by AustinEyewear View Post
    would a traditional molded PAL (same design) have one corridor width for all PALs regardless of Rx? Or would they vary the corridor width based on Rx/material to achieve best possible width across a range of Rx's, and in turn, create a multitude of molds for different ranges of Rx's?
    The latter, based on base curve and add power. See "multi-design progressive lenses". Not all semi-finished PALs are this 'smart', so research your product line carefully.

    The reason I ask this specific question, is because it seems the free-form based PAL designed would allow you to create an unlimited number of corridor widths that would only be limited by the pts Rx and material used even when using the same PAL design. The width may not vary much, but why not squeeze as much performance out of the lens as possible? However, I was recently told that this is not done, and the design would be the same regardless.
    Rodenstock and Hoya have PALs that can be ordered with a design bias towards better distance, near, or intermediate.

    Of course the advanced optical design software will correct for the aberrations caused by the base curve mismatch which will inherently increase viewing zones.
    I'm not so sure that there will be a increase in zone width(s)- that's primarily a function of the fundamental PAL design philosophy...unless, the Rx is very strong, and/or with significant prescribed astigmatism, especially with oblique axes. Marginal astigmatism and off-axis power error can be reduced however.

    Its seems to me, that the designer would attempt to spread all viewing zone out as much as possible, which they would be limited by the Rx <snip>
    Specifically, limited by the Add power and corridor length. See Minkwitz's Theorem. http://www.ncbi.nlm.nih.gov/pubmed/16276325
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