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Thread: Prism on near reference point

  1. #1
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    Prism on near reference point

    hi all.

    I'm PAL lens wholesaler and I have some questions about the prism on near reference point.

    One optician often reject pal lens due to above problem. so, I ask of him.

    Why are you rejecting lenses?

    He talk to me that when he receive pal lens, he check lens.

    distance power, add power and zero point of prism from DRP to NRP.

    If the zero point of prism line towards temporal direction, He regards the pal lens is inferior goods.

    Unfortunately, I can't accept his claim.

    If you have any idea about this matter, pls explain me.

    Regards,

  2. #2
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    The PRP or Prism Reference Point on a PAL is halfway between the marking found on the lens, which are 34mm apart making the markings about 17mm between the two markings. This is the point of the lens that prism should be measured, keep in mind you may see vertical prism which is used to thin progressives, but you should see no horizontal prism unless it was prescribed. In the US the acceptable tolerance is 0.33D horizontal prism for powers less than 3.37 and equal to or less than +/- 1mm from specified monocular PD (actually it is stated 0.67 imbalance which could be split between both lenses or all in one lens, but I won't accept anything above 0.33D and my lab has never given me a hard time for not accepting it).

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    Click image for larger version. 

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Quote Originally Posted by kc0724 View Post
    Click image for larger version. 

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    Not because of unwanted horizontal prism, given that it looks like there's zero horizontal prism at the PRP - assuming that no horizontal prism was prescribed. Your graphic looks normal for a toric lens with low sphere power and axis somewhere in the second quadrant (e.g. 160, in minus cyl form).

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    @kc0724: What is the power of the lens in your diagram?

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    It happened a long time ago. so, I can't remember the prescription.

    is it related to axis of cylinder?

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    Quote Originally Posted by kc0724 View Post
    It happened a long time ago. so, I can't remember the prescription.

    is it related to axis of cylinder?
    Not necessarily, If that was say a +2.00 with a +2.00 Add, I too would reject it for Horizontal prism. If I follow your tangent(lensometer dots) for the distance o.c upwards to the check point. The patient then would be dealing with at least a diopter of base in prism for distance and about 1.5 diopters base out for near. This issue crops up in surfacing, unwanted horizontal prism while surfacing really creates havoc for patients. Your optician may be rejecting because of this reason. Prism created during blocking, layout, surfacing all adds to any prism created within lens during blank production.


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    By the way, How is the exact amount of regular prism on pals, Is it depends on the power far lens?

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    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by Lado View Post
    By the way, How is the exact amount of regular prism on pals, Is it depends on the power far lens?
    If you are talking about prism thinning (to equate lens thickness on the top and bottom) the usual method is to grind .6 X Add power or 2/3rd of the add power base down OU.

    Ideally, you would factor in the distant power at 90, add power, fitting hight, decentration and frame shape. But the above is a close approximation.

    If you are talking about prescribed prism, you grind it conventionally, the same way as on SV or segmented lenses. It's not power dependent, only dependent on what the prescriber finds the patient needs.

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    Thanks, it really helped me , but dont you have any literature about it, I ll appreciate it.

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    Master OptiBoarder optical24/7's Avatar
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    "Introduction to Ophthalmic Optics" by Meister/Sheedy or "System for Ophthalmic Dispensing" by Brooks/Borish are both excellent reference books.

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    Lets supose we have this: R: +2.00 L: +3.00 with ADD 3.00 OU . So we have a Progresive lens, WITH NO PRISM REQUIRED When you check it up :
    how much of vertical prism you will find in: THE FAR ZONE, THE NEAR ZONE AND IN THE MIDDLE OF LASER DOTS, AND ALSO IF THERE WILL BE SOME HORIZONTAL PRISM in any zone.

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Prism, IMHO, is such a tricky issue mainly because "typical" insurance-driven eye exams and refractions make little time allowance for proper vetting of orthoscopy.
    Also, current rx paradigms don't provide a vocabularlu that would communicate important info about *degree*, meaning phoric-tendencies.
    Only armed with this info could we more properly apply todays fabrication tolerances in a truly efficient and smart manner.

    In the absence of such a vocabulary, we tend to apply costly adherence to strict ANSI interpretation.

    My 2 cents

    b

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