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Thread: Inducing prism?

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    Inducing prism?

    I have a patient with +0.25 OU and 1.50 BI prism 60/57 PD. I thought I would just pull some stock lens, but I could not move the lens enough to get the 1.5 BI prism. I ended up ordering from the lab. Is there a formula where I could change her PD to induce enough prism in her glasses or is it just best to let the lab make the lens?

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    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    Quote Originally Posted by bta89 View Post
    I have a patient with +0.25 OU and 1.50 BI prism 60/57 PD. I thought I would just pull some stock lens, but I could not move the lens enough to get the 1.5 BI prism. I ended up ordering from the lab. Is there a formula where I could change her PD to induce enough prism in her glasses or is it just best to let the lab make the lens?
    Prentice's rule works wonders.

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    Master OptiBoarder OptiBoard Gold Supporter
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    No offense but you should not be finishing lenses if you don't know the prentice rule like the back of your hand...

    or at least have this handy dandy calculator link: http://64.50.176.246/tools/induced.php

    http://www.opticaltraining.com/html/...e_fifteen.html


    Prentice's Rule
    Once it is found that either wanted or unwanted prism is present in any given lens and the direction of the base is determined, the power of the prism can then be calculated utilizing a simple formula called Prentice's Rule. It is important to become familiar with Prentice's Rule since it is used many times each day as a routine matter in the optical dispensary.
    Prentice's Rule states: The power of the prism is equal to the power of the lens in diopters times the amount of decentration in millimeters divided by 10.
    Stated Algebraically:
    =
    F x dec (mm)

    10


    where:
    = prism diopters
    F = power of the lens in diopters
    dec = decentration or distance in mm away from the optical center of the lens.

    Example 1: How much prism is induced 4 mm away from the optical center of a +3.00 D spherical lens?
    Applying Prentice's Rule, F = +3.00 and dec. = 4.0 mm
    so:

    = F x dec/10
    = 3.00 x 4/10
    = 1.2
    Last edited by Tallboy; 07-12-2017 at 03:40 PM.

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    Master OptiBoarder optical24/7's Avatar
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    Prentice Rule is determining the amount of prism induced when moving an OC of a particular power. Cm (moved) X power = prism. In you case 6 X .25 = 1.5 Prism diopters. 6 Cm's = 60mm's. That means if you had a frame w/a ED of 50, with no decentration needed for PD, you'd need 110mm blank to decenter enough to induce 1.5D prism (60 + 50 = 110)
    I hope this helps.







    Edit because my friggin" key pad Is missing letters!
    Last edited by optical24/7; 07-12-2017 at 04:55 PM.

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    Thanks for the help everyone, I will write this down and study it so I can get better.

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    Master OptiBoarder OptiBoard Gold Supporter
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    Quote Originally Posted by bta89 View Post
    Thanks for the help everyone, I will write this down and study it so I can get better.
    You can learn a lot by using the search function on this site. I know I have. Its a tome of optical dispensing knowledge. Pretty soon it will just be in your head when you need it and it will help you be a great dispenser!

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    bta 89--- Scroll down on the OB Home Page to Opticampus and go to "Optical Calculators".

    Pretty cool huh?

    Now support Optiboard and help Steve Machol by scrolling to subscriptions on the home page and commit a few dollars to help him defray the cost of keeping this site open and not beholden to nefarious (internet eye wear) companies!!!

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    sub specie aeternitatis Pete Hanlin's Avatar
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    Just a recommendation, Optical Formulas Tutorial, by Ellen Stoner, et.al. is an invaluable resource for learning, understanding, and the practical understanding of optical formulas related to ophthalmic lenses. My copy (purchase years ago) is dog-eared and worn from many trips through its pages!

    Here's a link: https://www.amazon.com/Ellen-D-Stone...ormulas+Stoner

    The other book I'd recommend is System for Ophthalmic Dispensing, by Brooks and Borish. If you can discipline yourself to read through this entire text (which is admittedly pretty thick), you will find yourself with more ophthalmic knowledge than 90% of opticians in the field. Without being sacrilegious, this is the bible of opticianry. With the two books mentioned above on your shelf (and in your mind), you are completely set for a career in ophthalmic optics.
    Pete Hanlin, ABOM
    Vice President Professional Services
    Essilor of America

    http://linkedin.com/in/pete-hanlin-72a3a74

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    Master OptiBoarder mshimp's Avatar
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    You could decenter a finished .25 all day long and never get to a 1.5 prism! Even if you had a 500mm finished blank.

  10. #10
    sub specie aeternitatis Pete Hanlin's Avatar
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    Well, technically... you would need 60mm of decentration. So, given a round frame with a 48mm A measurement and let's say 2mm decentration for pupil distance, you'd need a blank with a diameter of about 150mm.

    Of course, the ANSI tolerance for prism would also give you a little wiggle room (not than anyone would ever use "wiggle room" :^), so a lens with a diameter of about 130mm or so might just work. Of course, good luck getting the uncut blank into the edger!

    PS- Whoops, should have paid more attention... optical24/7 already did the calculations!
    Pete Hanlin, ABOM
    Vice President Professional Services
    Essilor of America

    http://linkedin.com/in/pete-hanlin-72a3a74

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    Master OptiBoarder mshimp's Avatar
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    OK I exaggerated. However you would never use a finished lens with this prism amount. Also weaker powers don't adhere to Prentice Rule perfectly. Hence it's a "rule"and not a "law".

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