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Thread: Formula Question

  1. #1
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    Confused Formula Question

    So not being the trusting sort and experiencing a lot of job coming up off power, I've decided to find a way to double check the curves that our computer system is coming up with. I'm using the formula:

    F2=-(n*F1-t*Fv*F1+n*Fv)/(n-t*F1)
    F2=back curve
    n=index of refraction
    F1=front curve
    t=thickness
    Fv=the back vertex power


    which I figured out using a formula that Daryll gave in another discussion and rearranged it to fit the information that I had available. It seems to work quite well (Thank you Daryll!) The only exception is when I use this on any of our aspheric lenses it comes up way off from what my computer is telling me!

    Now my question is: Is there something with aspheric lenses that I am not taking into account, or is my computer system just really that far off?
    Or is this formula off to begin with?
    I am willing to believe any of these things, but I'm just not sure which one to believe.

    Can anyone help me out?

    Thanks in advance!

  2. #2
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    If Daryll's formula works and the computer doesn't. Daryll is right.
    Now if the computer's formula works, then Daryll isn't necessarily wrong, the computer may be doing some computations and corrections that don't show up in one formula or the other.

    Chip

  3. #3
    ATO Member GAgal's Avatar
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    Darryll's right, but that formula is for spherical lenses. Since aspheric lenses do not have the same curve on all meridians, it is impossible to a completely accurate BC reading with a lens clock. Therefore, your curve reading would be wrong for the equation. The computer should be taking the asphericity into account.
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  4. #4
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by GAgal View Post
    Darryll's right, but that formula is for spherical lenses. Since aspheric lenses do not have the same curve on all meridians, it is impossible to a completely accurate BC reading with a lens clock. Therefore, your curve reading would be wrong for the equation. The computer should be taking the asphericity into account.
    Not necessarily, some programs will use two base curves on an aspheric lens, so that one will be used to compute the power and the other would be used to compute the thickness. The formula for back vertex power is:

    Dv=back vertex power
    Db=Back surface of the lens
    Df=front surface of the lens
    t=thickness in meters
    n=index

    Back Vertex Power
    Dv=Db+[Df/1-(t/n)Df^2]

    It applies in aspheric and sherical blanks, the thickness differs and may programs will use a spherical base curve at 50mm diameter for computing the thickness, since the aspheric value on most lens blanks are not know.

    And I agree with Chip I often find Darryl is more right than software, but it doesn't hurt to check for posterities sake. :)
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  5. #5
    Master OptiBoarder Darryl Meister's Avatar
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    The formula for back vertex power really doesn't rely on the design of the lens. If you are using the correct front curve, center thickness, and refractive index, you should get the correct result for the back curve, at least in the absence of prism.

    That said, how did you determine the front curve of the aspheric lens blank? As Harry alluded to, the actual power of an aspheric surface cannot be measured directly with a lens clock or sag gauge. You should consult the technical specifications provided by the manufacturer.

    Also, have you compensated for the tooling index? Your surfacing software is most likely providing values in the index of your lap tools. Your software might also be compensating for fining pad thickness. You can compensate for the refractive index of your tools using:



    Darryl J. Meister, ABOM

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    What kind of computer system are you using - a basic Rx surfacing calculator or the full lab system type?

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