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Thread: PAL heights, whats the confusion?

  1. #1
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    Question PAL heights, whats the confusion?

    There is a huge debate at work. The OD is under the assumption that PAL heights should be taken directly below the pupil. I being an optician say it should be measured from the deepest part of the frame. He claims that works only on FT bifocals. That a pilot frame that the height must be directly below the pupil. I told him that pilot frames are a bad choice for PAL. Who is correct

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    Master OptiBoarder mullo's Avatar
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    Your lab??

    Find out how your lab measures this. The standard is deepest part of frame, although I have seen some claim to go with the frame below pupil. Your doctor/employer is lucky you question this and save him some money. Like I said, talk to your lab guy.gal..................Mullo

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Thumbs up LRP Height

    One of the VCA's standards task forces has produced a standard format for the conveyance of lens blank data which contains some new terms (and excellent definitions of some old ones) which I hope gain acceptance by the industry. One of these is the "Layout Reference Point", which is the point that is used to posiition the lens for blocking. For a single vision lens, it's the Prism Reference Point (generally called the "optical center"); for a multifocal that has an identifiable segment, it's the segment; for a PAL, it's the Fitting Cross. Each of these features corresponds to a measurement taken on the patient, and a dimension that can be measured on the finished spectacles.

    Having defined the Layout Reference Point, it's possible to say that the LRP Height (which we formerly would have called the "Seg Height") is invariably measured the same - it's the height of the LRP above the lower horizontal side of the "frame box" - a rectangle circumscribed about the shape (which may indicate why defining such a point in this way is a Good THing).

    Only the British and a few misguided Australians have insisted on measuring heights "at the PD"; in the absence of a digitized tracing of the frame, these are altogether worthless numbers; given a tracing, it's possible to convert such a number into a proper, box-based one.

    I've not heard of any domestic labs recogninzing any but the boxing-system-based measurements.

  4. #4
    Bad address email on file Rich R's Avatar
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    Deepest part of frame B measurement is the normal.
    I can't imagine trying to measure at p.d., especially on aviator leaves room for error.
    Rich R

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    The Lab guy?

    Often times we leave far too much up to "the lab". An Optician needs desperately to know how to measure seg heights for all lens types. The "lab guy" may have been making hamburgers last week(no offense to my lab friends- dispensers can be in the same boat!). I know many excellent, well-trained laboratory folks who can answer the question as well as anyone, but just calling the lab is not the right answer. Brooks and Borish has a wonderful text to address those issues that can be ordered from the NAO at 1-800229-4828 called System for Ophthalmic Dispensing. To address the question directly, the standard is to measure according to the boxing system, which is the lowest pint on the frame. The ODs are not focused on this type of training in their curriculum now, and it is up to us to make sure we are up to date and know how to do our job- not call the lab!

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    Master OptiBoarder mullo's Avatar
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    Lab guy.

    When referring to lab guy, he is the one making the lenses. We have to be in agreement with him on how he will be cutting the lenses and to minimize redos. It is best to clarify and as I said, usually they will do as you wish. Most do from lowest "B" measurement as Rich R had said......

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    Wink

    Shanbaum,
    Hopefully there are very few misguided Australians. Most labs probably have a list of these misinformed (often stubborn) individuals. We certainly teach the correct method.

    Regards
    David

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    OptiBoardaholic sarahr's Avatar
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    In the UK we measure relative to the Horizontal centre line, or Datum as some of us still like to call it! So if the centre of the pupil was 2mm above the HCL we would order 2Above, or for a Bif maybe 3Below using the box system on the frame rule.

    Surely Shanbaum, you can't be insinuating that the whole of Britain is misguided?!
    :cheers:

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Question Misguided?

    sarahr said:
    Surely Shanbaum, you can't be insinuating that the whole of Britain is misguided?!
    :cheers:
    Hopelessly. Well, maybe not everyone.

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    Master OptiBoarder MVEYES's Avatar
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    Thumbs up Sucessful fitting

    I measure similar to sarahr but check to see the position of the patients head as they walk. If they walk head back I have them stare at an object that is straight out in front of them and lift their chin a bit and mark lid margin. Then measure from the mean of the b measurement down to the mark. That is where I align the laser marks.
    Any of you who do VSP insurance and order online know that they force you to measure from the bottom of the frame up to the cross (4 mm above the laser marks).

    :D Jerry
    The mighty oak tree was once a little nut that held its ground

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Thumbs down Re: Sucessful fitting

    MVEYES said:
    Any of you who do VSP insurance and order online know that they force you to measure from the bottom of the frame up to the cross (4 mm above the laser marks).
    :D Jerry
    As I mentioned, the Layout Reference Point for a PAL is the fitting cross, NOT the line on which the laser engravings appear - which are usually colinear with the Prism Reference Point (I know of no exception, though there is no standard that mandates this configuration).

  12. #12
    Master OptiBoarder Joann Raytar's Avatar
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    I agree. Jerry, if the goal is to end up with the fitting cross on center over the pupil, wouldn't fitting the 180 line at the lower lid be risking error? What if the patient had a lower lid the drooped a bit?

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    Master OptiBoarder MVEYES's Avatar
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    Question Jo

    You're right about droopy lids. I do change my technique with those patients. But by fitting on the 180, I have many more happy patients then when I started out with the Grolman fitting device that placed the fitting cross on the pupil. We are seeing a lot of repeat progressive wearers and it is easy using the laser engravings to mark relative positions on the new eyewear. It works well.

    I feel that each person is an individual and I analyze position after interviewing. I also have a demonstrator for new PAL wearers that allows them some feeling about the height I propose setting the PAL in their new frame.




    :cheers: Jerry
    The mighty oak tree was once a little nut that held its ground

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    OptiBoard Professional Mike Fretto's Avatar
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    Maybe I missed something somewhere but to assume the fitting cross is 4mm above the laser markings on all progressives is a mistake some lenses are 2mm above.
    Mike

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    Bad address email on file John R's Avatar
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    Re: LRP Height

    shanbaum said:
    Only the British and a few misguided Australians have insisted on measuring heights "at the PD"; in the absence of a digitized tracing of the frame, these are altogether worthless numbers; given a tracing, it's possible to convert such a number into a proper, box-based one.
    Mmm, We see a lot of shapes during a day and by far of the varifocals the most just have a dot where the pupil is..No numbers just a dot relating to the fitting cross.
    Remember dispensing is not my field, but why is it wrong to measure heights at the PD ?

    I like the idea of the VCA'S of standard measurements for everbody. Have you got any data on what they have come up with.

  16. #16
    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Re: Re: LRP Height

    John R said:
    Remember dispensing is not my field, but why is it wrong to measure heights at the PD ?
    Everything. It creates a pointless dependency between the PD and LRP Height - unless the bottom of the frame is a horizontal line, the LRP Height would vary with the PD. If you measure the PD a little wide or narrow, you'll get the LRP Height wrong; the magnitude of the error is proportional to the steepness of the shape.

    In the absence of a tracing, the point on the eyewire directly below the patient's pupil is not known - it depends on the shape. Providing the vertical dimension at that point doesn't help, because, again depending on the shape, the endpoints of that line could be anywhere (well, lots of places, anyway). That's why, in that case, the number just has no meaning at all.

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    Master OptiBoarder MVEYES's Avatar
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    Thumbs up Mike

    You're right. That's why I reference the lens with the laser markings.
    Most of the progressives I use in my practice have fitting crosses 4 mm above the laser markings.


    :cheers: Jerry
    The mighty oak tree was once a little nut that held its ground

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Re: Mike

    MVEYES said:
    You're right. That's why I reference the lens with the laser markings.



    :cheers: Jerry
    I promise, the guy who designed the lens wants the pupil at the Fitting Cross, regardless of its height above the PRP or the engravings.

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