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Thread: Using different refractive indices for each eye. Any issues?

  1. #1
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    Question Using different refractive indices for each eye. Any issues?

    I've always had this rule I follow and I can't remember who passed it on or where I read up on it: when dispensing, to always match the refractive index for both eyes, irrespective of the Rx.

    Today I was asked by a colleague, "but why?" And I couldn't find an optical reason why not (blonde moment..?) given this Rx:

    R +5.00 /-1.50 x 38
    L +0.75

    The situation arised when the colleague was finding the best pricing solution, and was asking which lens to use. i.e., grind aspheric high index for the R eye, and stock aspheric CR39 in the L eye.

    Aside from matching AR coats or extra edge thickness for nylon/rimless frames - are there any genuine optical reasons for matching indices? Perhaps matching base curves? Or was this one of those rules that created itself and became habitual with no merit?

    -Memoir.

    ps: A second 'rule' came into mind which I wouldn't mind collaborating with you all, "for single vision plus powers, if grind, grind both eyes"..?

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    Aniseikonic Lenses

    I have used different refractive index lenses for each eye with no problems. When ordering the eyewear specify aniseikonic lenses from your lab. From my experience, if your lab questions the order - use a different lab. There are plenty of qualified opticians and software programs available to calculate the best lenses for the patient. The completed eyewear's top priority is that the images of each eye match each other. For this reason, instead of magnifying the image of one eye, it may be easier to minify the image of another. This may allow for a more cosmetically acceptable spectacles, or at least lenses that are easier to manufacture, and therefore, less costly. I hope this helps.

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    I mix indices all the time to try to "ballance" thickness. As long as the same design (comfort to comfort, etc.) is utilzed I have never had a patient have a problem with this.


    Chip

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    Underemployed Genius Jacqui's Avatar
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    I've always stayed away from mixing, but there are times......

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    Dear Memoir, long time no hear! I can think of no reason optically speaking not to mix refractive indices. I can only think of reasons why one should, particularly in order to create effective aniseikonic lenses.
    With regard to grinding both lenses if only one needs to be grind; can't think of a good reason for that either besides matching base (front) curves. I don't see why this would particularly apply only to plus powers however. Your theory of "Or was this one of those rules that created itself and became habitual with no merit?" is probably true.

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    lens-o-matic bhess25's Avatar
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    Quote Originally Posted by Memoir View Post
    ps: A second 'rule' came into mind which I wouldn't mind collaborating with you all, "for single vision plus powers, if grind, grind both eyes"..?


    ct. thats pretty much it!
    equal opportunity offender!!

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    ABOM Wes's Avatar
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    Lenses with differing refractive indices may also have differing abbe values. If you're doing a job with prescribed prism and you wish to use lenses of differing index, make sure to use materials wiith similar abbe values, and the highest abbe possible.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

    “As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein

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    I have been wearing SV lenses 1,6 on L (about -2,50) and CR39 on R (-0,75) with same base curve in a rimless frame, no problems at all, balanced
    edge thickness, one of my best glasses for night driving. However as previoulsy mentioned I would stay away from mixing different base curves.
    Besides, most RX LABs should be able to manufacture the lower script in the matching edge thickness of the higher script using the same material.

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    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by Memoir View Post
    The situation arised when the colleague was finding the best pricing solution, and was asking which lens to use. i.e., grind aspheric high index for the R eye, and stock aspheric CR39 in the L eye.
    Before Trivex, I used to dispense SV aspheric Spectralite. One thing I learned is that the semi-finished and finished versions of this lens used two different p values, or degrees of asphericity. This loss of symmetry off-axis might cause problems for sensitive individuals when the Rx is similar for both eyes.

    I've always had this rule I follow and I can't remember who passed it on or where I read up on it: when dispensing, to always match the refractive index for both eyes, irrespective of the Rx.
    I've not heard this before. Maybe it should be base curve instead of refractive index- most PAL manufacturers recommend the same BC for semi-finished PALs, even with dissimilar RXs, keeping the corridor length, inset, and general design the same for eyes.

    Other concerns are cosmetic in nature- polished edges and AR coatings looking identical with different materials.

    I suspect that the front surface design should be as similar as possible for both eyes, especially for the above mentioned PALs, and that refractive index differences shouldn't cause any trouble for the wearer.
    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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    Master OptiBoarder OptiBoard Silver Supporter rdcoach5's Avatar
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    Quote Originally Posted by Wes View Post
    Lenses with differing refractive indices may also have differing abbe values. If you're doing a job with prescribed prism and you wish to use lenses of differing index, make sure to use materials wiith similar abbe values, and the highest abbe possible.
    Wes, have you had any problem with poly on one eye and any other index on the oother?

  11. #11
    ABOM Wes's Avatar
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    I really try to avoid mixing indices, and I avoid low abbe materials for prescribed prism orders. I've mixed spherical poly with aspheric 1.67 before with cosmetically and optically acceptable results, but its not something I care to do often. Circumstances vary.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

    “As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein

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    Optimentor Diane's Avatar
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    For the Rx you mentioned, using different indices would be beneficial to the patient. Chip is correct in that you should use the same design and Wes is correct to use similar abbe values if at all possible.

    Chip, I'm surprised you didn't mention to just use contact lenses, and forget the specs..:) No problem for aniseikonia or vertical imbalance.

    Diane
    Anything worth doing is worth doing well.

  13. #13
    OptiWizard Yeap's Avatar
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    I think if you able to match the BC, Abbe value and AR coating then shouldn't be an issue.
    Yeap


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