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Thread: base down prism

  1. #1
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    base down prism

    I have a comment, question, maybe some technically minded can give me some insight. Much appreciated.

    I received a pair of Varilux DS Comfort in 1.6. Rx is about +3.00 OU add 3.00.

    We edge and mount, check the final lenses for correct rx, pd, height.

    The lenses arrived with Essilor's standard "prism thinning", or equithin as they call it here.

    At the fitting cross there is 4.5 BASE DOWN prism in each lens, and on the lensmeter the mires are practically out of range, hard to measure Rx accurately. Automatic lensmeter shows the lens power is bang on perfect.

    I find it strange that Essilor would surface lenses with that much induced vertical (even though it is OU and equal) Base Down prism, which of course would cause a lot of eyestrain, as the wearer would be forced to look up - not to mention the degradation of sharpness.

    To be honest the lenses were thin, beautiful looking and we had her come to try them on. It was a FAIL and we are re-ordering taking into account this prism effect.

    SO , do you specify vertical prism at the fitting cross when ordering lenses (as a matter of routine), do you check for this when the lenses arrive, is there a maximum amount you will accept and has this been a problem for you too in the past?

    thanks

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    Doh! braheem24's Avatar
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    when edged is the top and bottom of the B thickness equal, if so they're correct. If it's thinner on the top or bottom then it needs more or less prism.

    If your decentration required more then 5mm up in the B, you're going to get more prism to counteract the poor fitting job. Often times it's not the labs fault.

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    Thanks Braheem.

    So if the b on the frame is say 30 and the seg is 21, that is 6 above. That is a poorly fit job?
    And yes, for sure the lens thickness is similar top and bottom. So that make the lens surfacing good? Didnt know that. But couldnt the lab have adjusted the surfacing to reduce the amount of Base down prism at the cross? A little more thickness on one edge isnt that bad, I think.

    Its sort of like if a lens doesn't decenter for PD, well you don't pass it, you call up and say no go. Why is vertical prism at the cross , 4.5 BD a pass for the lab? Surely it should have been caught by someone or some software and reduced to some norm.
    That is my point, what is that norm?

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    Doh! braheem24's Avatar
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    The lab should always error to the top being thicker to reduce BD.

    5BD on a poly is horrible, 5BD on Cr39 is tolerable there is no norm or program, just educated dispensers that know the patient's situation.

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    Master OptiBoarder Darryl Meister's Avatar
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    Why is vertical prism at the cross , 4.5 BD a pass for the lab?
    Prism in a progressive lens should be verified at the prism reference point, not at the fitting cross. This is also the location at which prism is toleranced in optical standards. There are no limits on prism-thinning provided in optical standards, although popular lab management software will generally allow the lab to set reasonable limits based upon clinical judgment.

    For a high plus progressive lens with a high add power like this, up to 3.0 prism diopters of prism-thinning at the prism reference point is not uncommon, particularly with higher fitting heights. Given that the PRP of Varilux lenses is 4 mm below the fitting cross, there would be an additional 0.4 cm * +3.00 D = 1.2 diopters of prism due to the location of the fitting cross.

    If you are uncomfortable with the amount of prism-thinning involved, or the wearer has presented with symptoms of discomfort while wearing the lenses, you can always discuss the use of less prism-thinning with your laboratory. Of course, this will most likely result in a significant increase in lens thickness and weight with this particular job.

    Also keep in mind that, had this been a single-vision job, the optical center would have typically been left along the datum (180) line of the lens. Since the pupil center is 6 mm above the datum with this job, the wearer would have experienced a significant amount (0.6 cm * 3.00 D = 1.8 prism diopters) of vertical yoked prism, either way.

    At least one study has shown that excessive vertical yoked prism may result in initial wearer discomfort and excess postural adjustments (see, for instance, Sheedy and Parsons (1987) "Vertical yoked prism - Patient acceptance and postural adjustment" in Ophthal. Physiol. Opt.). However, over time, many spectacle wearers exhibit significant prism adaptation.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by optimensch View Post
    I have a comment, question, maybe some technically minded can give me some insight. Much appreciated.

    I received a pair of Varilux DS Comfort in 1.6. Rx is about +3.00 OU add 3.00.

    We edge and mount, check the final lenses for correct rx, pd, height.

    The lenses arrived with Essilor's standard "prism thinning", or equithin as they call it here.

    At the fitting cross there is 4.5 BASE DOWN prism in each lens, and on the lensmeter the mires are practically out of range, hard to measure Rx accurately. Automatic lensmeter shows the lens power is bang on perfect.

    I find it strange that Essilor would surface lenses with that much induced vertical (even though it is OU and equal) Base Down prism, which of course would cause a lot of eyestrain, as the wearer would be forced to look up - not to mention the degradation of sharpness.

    To be honest the lenses were thin, beautiful looking and we had her come to try them on. It was a FAIL and we are re-ordering taking into account this prism effect.

    SO , do you specify vertical prism at the fitting cross when ordering lenses (as a matter of routine), do you check for this when the lenses arrive, is there a maximum amount you will accept and has this been a problem for you too in the past?

    thanks


    My suggestion to you is to begin specifying the DOC position in all eyewear if you have not done so in the past. Laboratories seem to love keeping the OC at the middle of the blank, rather than where the patient needs it. They seem to like to "compensate your lens powers", too.

    They also tried, in my instance, to even fudge the position of the DOC in relationship to standard segs. I always specify all DOC verticals, in order to avoid unwanted prism vertically.
    Eyes wide open

  7. #7
    Master OptiBoarder LENNY's Avatar
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    Quote Originally Posted by Darryl Meister View Post
    Prism in a progressive lens should be verified at the prism reference point, not at the fitting cross. This is also the location at which prism is toleranced in optical standards. There are no limits on prism-thinning provided in optical standards, although popular lab management software will generally allow the lab to set reasonable limits based upon clinical judgment.

    For a high plus progressive lens with a high add power like this, up to 3.0 prism diopters of prism-thinning at the prism reference point is not uncommon, particularly with higher fitting heights. Given that the PRP of Varilux lenses is 4 mm below the fitting cross, there would be an additional 0.4 cm * +3.00 D = 1.2 diopters of prism due to the location of the fitting cross.

    If you are uncomfortable with the amount of prism-thinning involved, or the wearer has presented with symptoms of discomfort while wearing the lenses, you can always discuss the use of less prism-thinning with your laboratory. Of course, this will most likely result in a significant increase in lens thickness and weight with this particular job.

    Also keep in mind that, had this been a single-vision job, the optical center would have typically been left along the datum (180) line of the lens. Since the pupil center is 6 mm above the datum with this job, the wearer would have experienced a significant amount (0.6 cm * 3.00 D = 1.8 prism diopters) of vertical yoked prism, either way.

    At least one study has shown that excessive vertical yoked prism may result in initial wearer discomfort and excess postural adjustments (see, for instance, Sheedy and Parsons (1987) "Vertical yoked prism - Patient acceptance and postural adjustment" in Ophthal. Physiol. Opt.). However, over time, many spectacle wearers exhibit significant prism adaptation.

    Best regards,
    Darryl
    Do you think this could be the reason high hyperopes raise their heads when walking in SV?

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    Darryl, thanks for the very detailed and informative reply.
    Perhaps it is the technical way you measure the prism reference point (this is, for example, 4mm below the fitting cross in a varilux comfort?) but because my client was really uncomfortable, I was curious about the amount of prism she exerienced looking straight ahead, which of course is at the fitting cross. I put the glasses on myself and the image jumps up in a really big way (4.5 bd will do that). So you are looking upwards (which is really not comfortable) and the image quality is degraded.

    What would happen if I ordered the lenses with no prism thinning and even with 1 or 2 base UP yolked prism instead? Would it be awfully thick on top? Is there an app out there to calculate this ?
    thanks again.

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    Lenny,
    I never noticed the hyperopes raising their heads when walking in SV, but I think having 5 prism diopters base down OU would do that. Interesting observation.

  10. #10
    Master OptiBoarder Darryl Meister's Avatar
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    Quote Originally Posted by Lenny
    Do you think this could be the reason high hyperopes raise their heads when walking in SV
    Possibly. A base down prismatic effect can make things look like you're walking up hill until you adapt to it, but raising your chin will reduce the prismatic effect by bringing the line of sight closer to the optical center of the lens.

    Quote Originally Posted by optimensch
    but because my client was really uncomfortable, I was curious about the amount of prism she exerienced looking straight ahead, which of course is at the fitting cross
    If you traced the source of her discomfort to the base down prism effect, then I would certainly consider discussing this with the laboratory. Just keep in mind that you will be inducing even more vertical prism during near vision.

    Quote Originally Posted by optimesch
    I put the glasses on myself and the image jumps up in a really big way (4.5 bd will do that). So you are looking upwards (which is really not comfortable) and the image quality is degraded.
    You may experience "jump" when you first put these or any prescription glasses on but, of course, the effect will be constant afterward. Also, if you're using a customized free-form lens that is supposed to be optically optimized, the prism shouldn't degrade image quality. You would need to confirm with the lens manufacturer what, if any, optical optimization is being applied for this product.

    Quote Originally Posted by optimensch
    What would happen if I ordered the lenses with no prism thinning and even with 1 or 2 base UP yolked prism instead? Would it be awfully thick on top? Is there an app out there to calculate this ?
    You would need pretty complex software calculations to accurately determine the difference in lens thickness. However, the approximate difference in edge thickness due to prism can probably be estimated using the following formula:



    where t is the approximate difference in edge thickness and H is the height of the lens. For your job, reducing the prism-thinning by 1 prism diopter will probably increase the edge thickness difference between the upper and lower edges of a lens over a 30 mm B dimension by:



    Or about one-half millimeter for every prism diopter. The center thickness will also increase by roughly half this amount. These are just very rough estimates though. But cutting back on some of the prism-thinning is probably the simplest solution, if you have isolated the problem to vertical yoked prism and feel that the patient will not adapt to the prism.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by uncut View Post

    My suggestion to you is to begin specifying the DOC position in all eyewear if you have not done so in the past. Laboratories seem to love keeping the OC at the middle of the blank, rather than where the patient needs it. They seem to like to "compensate your lens powers", too.
    That's going to cause the same issues discussed here, but in the near. Prism thinning is used to provide cosmesis but in some cases it will screw with optics learn to take it out when necessary.

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    well the image degradation due to dispersion, chromatic abberation, seeing through the 4.5 base down, even a pretty good material, like tl16 with an abbe of over 40, when your seeing through 4.5 prism there is a loss of sharpness.

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by optimensch View Post
    well the image degradation due to dispersion, chromatic abberation, seeing through the 4.5 base down, even a pretty good material, like tl16 with an abbe of over 40, when your seeing through 4.5 prism there is a loss of sharpness.
    Very true, yolk prism is used in PAL lenses to even out thickness and provide optimum cosmetic's but it could also be had to place the optical center such that chromatic aberrations are equal at both the DRP and NRP, this is sometimes the best compromise. I would also choose the corridor length to reduce this disparity.

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    What was the patient previously wearing ? Was the previuos RX comparable and what amount of prism was present in the old Rx if it was a progressive. Assuming a similar Rx for the old glasses, try matching that Rx's base curve and prism.

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    the patient had been wearing comforts (i believe 1.49), and there was no prism thinning, at the cross there was around 2.5 BD (as it turns out). The new comforts (DS, TL16) were 1 diopter flatter curve (5 instead of 6) and they were prism thinned and now 4.5 BD at the cross. Of course I did not verify the prism in her old glasses before ordering the new lenses, and didn't specify if I wanted prism thinning when the lenses were ordered. usually we go for esthetics (the lab went knife edge as much as possible), but this turns out to be a very instructive case, which is why I brought it up on OB. Instructive for me anyway, and if anyone learns from it, that's great.

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    Always start from the patients previous file if you can and even with that you must next check the present glasses they are wearing, then listen to any complaints they might have about the present glasses before making changes. That means checking the present pair for prism, base curves etc.

    A general rule of thumb is that prism thinning will be in the amount of 0.6 X the add power or 2/3 of the add power. In this case with a 3.00 add that would be 2.00 diopters of prism. So if the rx's were similar but the old pair was not prism thinned then that accounts for the 2.00 difference you have noted. Using the 2/3 rule of thumb makes for simple mental math .

    Check this out as images are better explanantions : http://www.opticampus.com/cecourse.p...rism_thinning/

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    that's a great piece of info Idispense, thanks.
    Honestly, other than un-yolked (obviously) vertical/horizontal prism for phoria/tropia etc., i tend to rarely consider the unyolked vertical prism, this is obviously something that needs more consideration.

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    This may not be the right place to post this and I dont intend to hijack this thread , but on this topic I might say that many encounter difficulties, both Drs and opticians. One way to build your practice and referrals is to encourage opticians in your area to consult with you should they experience any "unsolvable" issues I would also encourage you as a Dr to never write "Drs change of Rx" on a Rx card with a note on the back that the patient can read. Most attempts to do this are for the sole purpose of orchestrating a free redo . Most that I have seen always involve a completly new Rx with an attempt to shift blame to the optician.

    Gain the respect of opticians by not doing this and call the optician in person to discuss what needs to be done to satisfy the patient. No matter who might be at fault, these matters should be between yourself and the optician first.

    It will build your practise through approachableness and respect.

    Providing VA info would also be very helpful. How can I know if I have done my job correctly if I don't know what to aim for ?

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    Quote Originally Posted by PhiTrace View Post
    That's going to cause the same issues discussed here, but in the near. Prism thinning is used to provide cosmesis but in some cases it will screw with optics learn to take it out when necessary.
    I disagree with the premise that "near" issues will be caused.....that would only happen with a deficient brand of multifocal.....which(IMO) is a good way to weed out the brands that are of suspect quality and functionality.
    Eyes wide open

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    When Executive style lenses were made, there was never any prismatic thinning done to reduce the upper lens thickness, why on earth would anyone do it now? It's just silly.

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    Quote Originally Posted by optimensch View Post
    I received a pair of Varilux DS Comfort in 1.6. Rx is about +3.00 OU add 3.00.
    Was there a bump in the add over the old eyeglasses? High add PAL adaptation and subsequent wear is usually not a pleasant experience for the wearer. The overdone prism thinning adds to the misery.

    If you need to minimize induced vertical prism and/or vertical imbalance, one might select a PAL that has the fitting cross on the 180 line.
    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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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Quote Originally Posted by MikeAurelius View Post
    When Executive style lenses were made, there was never any prismatic thinning done to reduce the upper lens thickness, why on earth would anyone do it now? It's just silly.
    Yes there was, maybe just not in Minnesota!

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    Quote Originally Posted by shanbaum View Post
    Yes there was, maybe just not in Minnesota!
    LOL!! Back in the day, we made hundreds of these every month, and I don't recall EVER seeing a request for prismatic thinning. I even remember a dispenser telling me not to use prismatic thinning on a job.

    My point is that this: by using prismatic thinning, you are modifying the prescription, something only an OD or MD can do. You are giving the patient something that was not prescribed, that affects his/her vision, and frankly, adds misery to a lens that doesn't have the best overall optics in the world.

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    Quote Originally Posted by uncut View Post
    I disagree with the premise that "near" issues will be caused.....that would only happen with a deficient brand of multifocal.....which(IMO) is a good way to weed out the brands that are of suspect quality and functionality.
    You can disagree all you want, the math won't change.

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    Keep in mind prism thinning brings the OC lower in a plus lens, so if the patient is complaining of reading you're barking up the wrong yolk.

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