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Thread: Calculation of edge thickness in wrap frames.

  1. #1
    Master OptiBoarder Jedi's Avatar
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    Calculation of edge thickness in wrap frames.

    Is there such a formula? What kind of effect (mathematically) does an increase in base curve have on edge thickness.
    "It's not impossible. I used to bull's-eye womp rats in my T-16 back home."


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    lens thickness

    It is called sagittal depth and good book to get is optical formulas by butterworth their web page is www.bh.com

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    ATO Member OPTIDONN's Avatar
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    Boy where is Darryl when you need him?!?!


    Have you checked out his opticampus.com thickness calculator? It should be able to figure it out for you but it wont help much if you are not near a computer.

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    Master OptiBoarder Jedi's Avatar
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    Optidonn,
    I use Darryl's calculator almost everyday, but I'm interested in seeing how much a change in base curve will affect edge thickness. Thanks
    "It's not impossible. I used to bull's-eye womp rats in my T-16 back home."


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    Master OptiBoarder Darryl Meister's Avatar
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    The Thickness Calculator will give you a pretty accurate edge thickness, though I don't currently allow you to specify the base curve separately (the code currently "guesstimates" a base curve based upon the power and index of the lens). I could provide the details of how to calculate thickness, but the equations can be tedious. I have an old article on the subject here:

    http://www.optiboard.com/forums/showthread.php?t=12645

    I'll do some analysis though, and give you an idea as to how much thickness variation you should expect by base curve.

    Also keep in mind that the edge thickness of lens in a wrap frame will depend upon 1) How the lens is edged and 2) How the thickness is measured. For instance, edge thickness calculations using "sag" formulas and such calculate thickness assuming that you are measuring it parallel to the optical axis. In lens design and ophthalmic optics this is frequently referred to as z thickness. A pair of lens calipers, on the other hand, measures the edge thickness perpendicular to the two surfaces (or at least an "average" surface somewhere between them). This is frequently referred to as strap thickness (from the straps of a rimless mounting).

    This difference becomes even more important for a wrapped lens, since the z thickness can be considerably greater than the strap thickness with steeper base curves.
    Darryl J. Meister, ABOM

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    Hi, all.

    My Reflections polys came back with a slight base curve to them, but I am having them re-done with a slightly sharper curve. I'm -0.75 in both eyes. Any special instructions I should give to the lab guy regarding making the lenses/edges -- I'm not sure how knowledgeable about this stuff he is.

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    Bad address email on file QDO1's Avatar
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    Quote Originally Posted by eteman
    It is called sagittal depth and good book to get is optical formulas by butterworth their web page is www.bh.com
    (y*y*f)/(2000(N-1)) from memory - that is the rough and ready formula (y being the radius)
    Last edited by QDO1; 09-26-2005 at 02:56 AM.

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