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Thread: Prism Thinning

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    Prism Thinning

    Optiboarders, an order I recently received from one of my labs has me curious about the miracle of prism thinning.

    I know why prism thinning is implemented in the surfacing of various lenses, but I'm curious about how, say, an anisometropic set of lenses might affect the prism thinning process.

    In order to not manufacture an intolerable amount of induced prism, I know that prism thinning should be yoked and that the amount should not exceed 0.6 times the add (up to 2D for a +3.00 add [citation needed]).

    The job I received yesterday has the following script:

    -1.25 -0.75 x033 +2.75 Add
    +1.25 -0.75 070 0.5 BU +2.75 Add

    With the OS base up prism, the OC of that lens (as measured at the PRP) should be higher than the OD OC, however it's reading about 1D lower. When I called the lab about this, they said that their prism thinning algorithm shows the OCs going in opposite directions and that it factored the patient's BU prism into the equation. We sold the patient a digitally surfaced PAL, but not compensated. The lab owner then proceeded to tell me that if the patient has any issues that I should just give her vodka. Though I appreciate the levity, I just wanted to throw this story to the proverbial wind and have my esteemed Optiboarders-in-arms offer their input.

    Does lensmanmd have his own version of a bat signal?

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    The prism on progressives should be checked at the PRP (prism reference point). Double check where the PRP is for that lens design, looking at that Rx, you may find that if you move even 2mm the prism comes out the way you expect.

    Jim
    Last edited by JH; 02-06-2020 at 07:09 PM.
    Jim Hilker

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    My apologies for wording that kind of oddly (I was in between patients but wanted to get this posted quickly). I am measuring the lenses at the PRP but the OS shows base-down prism instead of base up. The lab owner said that the prism wasn't yoked because their algorithm determined the prism thinning to be ground in opposite directions due to the patient's rx and I hadn't heard of this before.

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    This is real tricky Rx considering that the Rx is opposite in value in each eye. Remember prism thinning has one purpose only and that is make the lens equal thickness on top and bottom so the entire lens can be made thinner. Some require bu thinning to accomplish the equal thickness requirement but the prisms are always yolked . The amount of prism depends on three factors the Rx the shape of the frame and the seg height. If the lab does not use all these factors in their calculations they are not achieving the best possible results. The prescribed prism must be checked at the prp and should have 1/2 diopter difference between the two lenses don’t look for the prism in the lens with the prescribed prism to be as prescribed but look for the difference between both lenses. Even if this is correct your patient might not be happy. His normal gaze is not at the prp but at the fitting cross. You can check the prism difference at the fitting cross you will see it is different than at the prp. You might have to remake the lenses to Achieve the correct prism at normal gaze-position.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    This tour de force article by Darryl Meister from the Opticampus forum should be read by all. To any viewer unfamiliar with just what prism thinning is open the forum click on the course- "Optics of a Progressive".

    Scroll to the very bottom for a simple explanation and diagram.

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    Quote Originally Posted by grudyfan13 View Post
    I am measuring the lenses at the PRP but the OS shows base-down prism instead of base up.
    That is fine. If there is 1D equithin the OD would read 1BD and the OS would read .5BD. As long as there is .5D difference in the correct direction.

    Quote Originally Posted by grudyfan13 View Post
    The lab owner said that the prism wasn't yoked because their algorithm determined the prism thinning to be ground in opposite directions due to the patient's Rx and I hadn't heard of this before.
    They may have misspoke or misunderstood the question. Prism thinning isn't going to be added in opposite directions. You could end up with final prism in opposite directions. Not really in this Rx though. The prism that should be present at the PRP is normally listed on the invoice. All the labs I have dealt with are always in degrees. Probably something like 0.89 @ 269 and .47 @ 271 for your job.

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    Quote Originally Posted by Lensman11 View Post
    This is real tricky Rx considering that the Rx is opposite in value in each eye. Remember prism thinning has one purpose only and that is make the lens equal thickness on top and bottom so the entire lens can be made thinner. Some require bu thinning to accomplish the equal thickness requirement but the prisms are always yolked . The amount of prism depends on three factors the Rx the shape of the frame and the seg height. If the lab does not use all these factors in their calculations they are not achieving the best possible results. The prescribed prism must be checked at the prp and should have 1/2 diopter difference between the two lenses don’t look for the prism in the lens with the prescribed prism to be as prescribed but look for the difference between both lenses. Even if this is correct your patient might not be happy. His normal gaze is not at the prp but at the fitting cross. You can check the prism difference at the fitting cross you will see it is different than at the prp. You might have to remake the lenses to Achieve the correct prism at normal gaze-position.
    Presumably the patient has always has this antimetropia and adding prism thinning isn't going to change that. It will simply move the image slightly up.

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    Any lens form besides a progressive would have the optical center close to the position of normal gaze. When making a bifocal or single vision lens the optical center is usually in the vertical mechanical center. This is not the case with a progressive. When dealing with an Rx with opposing signs with a small amount of prism that will create a visual experience other than what was ordered. This has nothing to do with prism thinning.

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    Quote Originally Posted by grudyfan13 View Post
    Does lensmanmd have his own version of a bat signal?
    He does.
    The responses here are on point. At the PRP, the residual vertical should be .5D. If this is the case, and within the .33D +/-, I wouldn’t worry about it.

    IMO, the .5D BU is not necessary, and the prescriber did this to help fusion at the near for a PAL. just like they boost the add by .25D for PAL.
    This is an old school trick to keep patients out of the chair for rechecks. We all know that rechecks are revenue negative.
    Same hold true for old school opticians adding 1mm to the PD AND dropping the SH by 1-2mm. So wrong on all accounts.

    Personally, I wouldn’t have use equithin on this. It really isn’t necessary for such a low RX.

    DRK, what are your thoughts on this?
    Last edited by lensmanmd; 02-07-2020 at 01:16 PM. Reason: Updated thoughts
    I bend light. That is what I do.

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    Quote Originally Posted by Lensman11 View Post
    Any lens form besides a progressive would have the optical center close to the position of normal gaze. When making a bifocal or single vision lens the optical center is usually in the vertical mechanical center.
    Well which is it, OC close to the normal gaze or OC close to the mechanical center? I normally don't see those two things as be coincident. It would be nice for them to be coincident in this patients case, preferably in a small B frame, with little to no pantoscopic tilt. But that is probably all moot with regard to my next point.


    Quote Originally Posted by Lensman11 View Post
    This is not the case with a progressive. When dealing with an Rx with opposing signs with a small amount of prism that will create a visual experience other than what was ordered. This has nothing to do with prism thinning.
    I did assume that since the patient has a +2.75 add and there was no mention of this being their first progressive, which would be fairly important information, that the patient has been successfully wearing progressive with opposite signs. I could be wrong but, seemed like a pretty safe bet. There also exists the possibility that the patients is wearing a lens where the PRP and the fitting cross are in the same location on the 180 line, such as with Seiko progressives.

    I did also assume that your response somehow was in reference to prism thinning as that was the subject of the question. Bad assumption on my part there I guess.

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    In the lensometer, assuming 2 D of equithin, we'll read 2∆ BD OD and 1.5∆ BD OS, for a total prescribed vertical prism imbalance of .5∆ BU OS (or .5∆ BD OD).

    However, as mentioned by Lensman11, and assuming an industry typical 4mm drop, the wearer in the primary gaze will experience about 1∆ BD OS (or 1∆ BU OD), 1.50∆ BD OS due to the anisometropia and subsequent dissimilar dioptric values at 90 degrees, combined with the 0.50∆ BU OS prescribed prism.

    Workarounds include using a PAL with a zero drop, surfacing an opposing prism to cancel the induced prism in the distance, segmented multifocals or SVDO and SVNO with the OCs at the pupil height.

    Hope this helps,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Quote Originally Posted by Lensman11 View Post
    When making a bifocal or single vision lens the optical center is usually in the vertical mechanical center. This is not the case with a progressive. When dealing with an Rx with opposing signs with a small amount of prism that will create a visual experience other than what was ordered. This has nothing to do with prism thinning.
    Hmmm. Just what was ordered? Too many false negatives possible in the current Rx format.

    B

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    If this patient is newly anisometropic ( as in post cataract sx), I would expect epic failure with a PAL. The patient has 2.75D of vertical imbalance. 10mm from wherever the OC's do line up will have 2.75D of prismatic displacement. ( possibly 3.25D since the doc added .5D up on the plus lens)

    If they have had this condition for some time, well, it won't matter much as long as the prismatic imbalance placement is similar to their old Rx. ( they most likely have developed a mono use of their glasses.)

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    Hmmm. Just what was ordered? Too many false negatives possible in the current Rx format.
    I ordered a digitally surfaced in-house PAL from our lab. If you're familiar with IOT designs, I ordered their private label version of the IOT H45.

    If this patient is newly anisometropic ( as in post cataract sx), I would expect epic failure with a PAL
    The patient is not newly anisometropic. After doing a bit of research, I discovered that she had CE surgery in 1999/2000 and has been wearing progressives with success since that time.

    My fears of this impending dispense might be for naught (or maybe my fears should be amplified?): it turns out this patient is essentially monocular due to her progression of AMD. Her BCVAs:
    OD: 20/300
    OS: 20/60

    I know there was a recent thread that was espousing the benefits of having ODs and MDs putting BCVAs on scripts, and I think this patient's story shows why that would be greatly appreciated.

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    Those VAs really beg the question, what is with the .5BU prism OS???

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    Quote Originally Posted by grudyfan13 View Post
    I know there was a recent thread that was espousing the benefits of having ODs and MDs putting BCVAs on scripts, and I think this patient's story shows why that would be greatly appreciated.
    Essential, IMO. If you don't have access to the history (outside RXs), acquire a trial lens set and frame and do it yourself. It will save both the wearer and the optician time and money, and no surprises.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    should have 1/2 diopter difference between the two lenses don’t look for the prism in the lens with the prescribed prism to be as prescribed but look for the difference between both lenses
    <--quoting Lensman11.

    Yeah, what's the prism in the other eye at the prism reference point? Absolute value doesn't matter, but difference does. You just need 1/2^ BU OS net.

    As to prescribing, yeah, I can see a "savvy doc" trying to "split the imbalance" (or at least a partial split) by putting some of the vertical imbalance in the distance portion. Just approximately, the vertical imbalance on downgaze when reading in the near zone is 2.5 BU OS. So, if the doc puts some of that prism in front of the eye in straight-ahead viewing, there is less downgaze doubling. It's spreading the misery. I don't like to "split the vertical imbalance" but, hey, a 1/2^ isn't a big whoop one way or another.

    I would've used a shorty-corridor. If they couldn't handle it, it's the slab-off treatment. But this patient has a very mature add power, so probably has already adapted long ago. Who knows how their poor brain and eye muscles are doing it?

    [Yes, it's entirely possible that the patient had a vertical phoria WITH anisometropia. I've seen that a few times, believe it or not. Sometimes the vertical phoria is antagonistic to the prism imbalance on downgaze, but sometimes it's simpatico.]

    But the above is theoretical. After reading further, this case is different.

    This macular degeneration case makes that low amount of prism meaningless. It makes the imbalance on downgaze meaningless, too. The patient is monocular for central viewing, no doubt, what with 20/300 vision. And sadly the "good eye" is only 20/60. So, why the prism? I can only think of some attempt at prismatic aid to eccentric viewing? https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3223708/
    Last edited by drk; 02-11-2020 at 08:54 AM.

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    Quote Originally Posted by drk View Post
    As to prescribing, yeah, I can see a "savvy doc" trying to "split the imbalance" (or at least a partial split) by putting some of the vertical imbalance in the distance portion. Just approximately, the vertical imbalance on downgaze when reading in the near zone is 2.5 BU OS. So, if the doc puts some of that prism in front of the eye in straight-ahead viewing, there is less downgaze doubling. It's spreading the misery. I don't like to "split the vertical imbalance" but, hey, a 1/2^ isn't a big whoop one way or another.
    We used to do that on segmented multifocals, positioning the distance OC at the "top of the seg", usually for low values of imbalances, avoiding the cost of a slab but still minimizing asthenopia.

    Regardless, how many prescribers give this much thought beyond what happens at the refractor head? Probably less than a couple or so hundred, and maybe half that many ophthalmic opticians.

    This macular degeneration case makes that low amount of prism meaningless. It makes the imbalance on downgaze meaningless, too. The patient is monocular for central viewing, no doubt, what with 20/300 vision. And sadly the "good eye" is only 20/60. So, why the prism? I can only think of some attempt at prismatic aid to eccentric viewing? https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3223708/
    Maybe because it was habitual, and no one wanted to rock the boat. I've seen a few folks, with similar VAs, unable to accept the removal of their decades long prism Rx, usually with higher values though. Case-by-case I guess.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
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    Quote Originally Posted by drk View Post
    <--quoting Lensman11.

    As to prescribing, yeah, I can see a "savvy doc" trying to "split the imbalance" (or at least a partial split) by putting some of the vertical imbalance in the distance portion. Just approximately, the vertical imbalance on downgaze when reading in the near zone is 2.5 BU OS. So, if the doc puts some of that prism in front of the eye in straight-ahead viewing, there is less downgaze doubling. It's spreading the misery. I don't like to "split the vertical imbalance" but, hey, a 1/2^ isn't a big whoop one way or another.
    Uh, am I lost here? Adding BU prism to the OS is going to increase the near vertical imbalance.

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    What's up? drk's Avatar
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    I get confused some times.

    OK, say we have 2.5BU induced vertical on downgaze, without any monkey business.

    If we get a BU lens in downgaze, we want to offset that a little by putting some BD in downgaze. So we surface a little BD throughout the whole lens.

    So, yeah, you're right.

    So, yeah, that's not "splitting the prism".

    Thank you.
    Last edited by drk; 02-11-2020 at 06:08 PM.

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    Quote Originally Posted by optical24/7 View Post
    ....... ( possibly 3.25D since the doc added .5D up on the plus lens)....

    Sometimes I feel like I'm farting into the wind....

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    Quote Originally Posted by drk View Post
    I get confused some times.

    OK, say we have 2.5BU induced vertical on downgaze, without any monkey business.

    If we get a BU lens in downgaze, we want to offset that a little by putting some BD in downgaze. So we surface a little BD throughout the whole lens.

    So, yeah, you're right.

    So, yeah, that's not "splitting the prism".

    Thank you.
    Introducing prism to neutralize induced prism on the primary gaze is an accepted practice for dealing with PALs and anisometropia. It's SOP if you order a slabbed PAL from Zeiss. And you're right, it's not quite splitting prism, it's more of a rob Peter to pay for Paul type scenario, that is, as we decrease the imbalance at far, it increases at near.

    Best regards,

    Robert Martellaro
    Last edited by Robert Martellaro; 02-13-2020 at 11:22 AM.
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    Well, after much deliberation and wait, the patient came in late last week and picked up her newest set of high-fashion spectacles. The verdict? The patient seemed pretty happy with everything, all things considered. Thanks to everyone for their input.

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