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Thread: How to calculate axial edge thickness & lift

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    How to calculate axial edge thickness & lift

    Hi everyone! I'm a student, and we just started learning about RGP's. I'm having trouble figuring out how to calculate axial edge lift and axial edge thickness. Here are the parameters I have:

    Tricurve -1.50D tricurve RGP
    CT = 0.14mm
    OAD/OZD: 9.0/7.8
    BCR = 7.80mm/43.25D
    n = 1.47

    I do know that s1 + ET = s2 + CT, but I don't know how to calculate s1 and s2 (looked all over the internet & I couldn't find it except in blocked google books).

    Thank you so much in advance!
    Last edited by ocean4787; 10-29-2011 at 08:03 AM.

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    I'm a bit rusty.....getting old ya know, ......but I think s1 refers to the saggital depth of optical zone, and s2 is the saggital depth of the peripheral curve.

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    Quote Originally Posted by ocean4787 View Post
    Hi everyone! I'm a student, and we just started learning about RGP's. I'm having trouble figuring out how to calculate axial edge lift and axial edge thickness. Here are the parameters I have:

    Tricurve -1.50D tricurve RGP
    CT = 0.14mm
    OAD/OZD: 9.0/7.8
    BCR = 7.80mm/43.25D
    n = 1.47

    I do know that s1 + ET = s2 + CT, but I don't know how to calculate s1 and s2 (looked all over the internet & I couldn't find it except in blocked google books).

    Thank you so much in advance!

    The base curve radius is 43.25 and the power needed is -1.50, this can be used to find the front curve power:

    front curve + back curve = power
    front curve + (-43.25) = (-1.50)
    front curve = (-1.50) + 43.25
    front curve = 41.75

    The front curve radius can be determined next:

    front curve radius = (index - 1) / front curve
    front curve radius = 1.47 - 1 / 41.75
    front curve radius = 0.47 / 41.75
    front curve radius = 11.3mm

    The lift is the difference between the back curve and the front curve plus the center thickness at the overall diameter, the axial thickness is the same formula except you use the back curves diameter instead of the overall diameter:

    Axial Lift = sag(41.75)@9.0mm + CT - sag(7.8)@9.0mm
    Axial Thickness = sag(41.75)@9.0mm + CT - sag(7.8)@7.8mm

    The sag of the curves are calculated using the following formula:

    sag(r)@d = r - sqrt(r^2 - (d/2)^2]

    Good question, on a side note the new terminology used for RGP's is just GP's I don't know if that will get you extra credit or not.

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by uncut View Post
    I'm a bit rusty.....getting old ya know, ......but I think s1 refers to the saggital depth of optical zone, and s2 is the saggital depth of the peripheral curve.
    Close I can see how you came up with that as a hypothesis.

    S1 refers to the sag of the front curve which must be calculated in this instance and s2 refers to the base curve or back curve radius' sag.

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    Master OptiBoarder OptiBoard Silver Supporter rdcoach5's Avatar
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    In general, it's easy to think of it this way. The 2nd curve is 1.5 diop flatter than the radius and the peripheral curve is 1.5 diop flatter tha the 2nd curve. a lot of us used to do the quad curve. Int , secondary and peripheral with all the curves narrower especially the peripheral curve. Often we specified .1 at 12.00 or 13.00 for a lot of uplift at the edge but then rolled the edge to prevent sharpness.

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    Quote Originally Posted by rdcoach5 View Post
    In general, it's easy to think of it this way. The 2nd curve is 1.5 diop flatter than the radius and the peripheral curve is 1.5 diop flatter tha the 2nd curve. a lot of us used to do the quad curve. Int , secondary and peripheral with all the curves narrower especially the peripheral curve. Often we specified .1 at 12.00 or 13.00 for a lot of uplift at the edge but then rolled the edge to prevent sharpness.
    Fantastic information, I too would often have the lab tip the edge with a 12.50 or 18.00 depending on a flat or steep lens. I keep my outside curves to 1.5mm of the total overall diameter of the lens, figure 0.75mm for a secondary curve, I usually make the secondary curve 1.00mm flatter than the base curve as my rule of thumb + a few tenths depending on if I vault the cornea or have any astigmatism, I use a nomogram to determine that.

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    ocean4787
    How to calculate axial edge thickness & lift
    Hi everyone! I'm a student, and we just started learning about RGP's. I'm having trouble figuring out how to calculate axial edge lift and axial edge thickness. Here are the parameters I have:

    Tricurve -1.50D tricurve RGP
    CT = 0.14mm
    OAD/OZD: 9.0/7.8
    BCR = 7.80mm/43.25D
    n = 1.47

    I do know that s1 + ET = s2 + CT, but I don't know how to calculate s1 and s2 (looked all over the internet & I couldn't find it except in blocked google books).

    Thank you so much in advance!

    Hi, I've just joined the board and realise that what I am about to say is to criticise existing members early in a relationship.

    You have been given insufficient information to calculate both axial edge lift and axial thickness.


    If you refer to : BS EN ISO 18369-1:2006+A1:2009 Ophthalmic Optics - Contact lens (Please note that it is an International Standard)

    axial edge lift is defined as: distance between a point on the back of a contact lens at the edge and the vertex sphere, measured parallel to the contact lens axis.

    So in your case Ocean you need to know the vertex sphere (BOZR which you have at 7.80mm) and the back peripheral zone radius at the Total Diameter (which apparently you have not been given).

    I'm sorry if this has added to your confusion, if you order a rigid lens with a specified axial edge lift then when you view the fluorescein fit on the eye the edge lift is shown by the bright green ring around the edge of the lens, and tells by how much the lens is (hopefully) proud of the cornea at that point. Some GP lens types will specify the radial edge lift which the difference between the BOZR and BPZR. These two measuremnts can be confused but are in fact different especially in value.

    If in doubt refer to the above ie ISO 18639-1:2006+A1:2009

    Regards
    Ian

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by IaninIOM View Post
    Hi, I've just joined the board and realise that what I am about to say is to criticise existing members early in a relationship.

    You have been given insufficient information to calculate both axial edge lift and axial thickness.


    If you refer to : BS EN ISO 18369-1:2006+A1:2009 Ophthalmic Optics - Contact lens (Please note that it is an International Standard)

    axial edge lift is defined as: distance between a point on the back of a contact lens at the edge and the vertex sphere, measured parallel to the contact lens axis.

    So in your case Ocean you need to know the vertex sphere (BOZR which you have at 7.80mm) and the back peripheral zone radius at the Total Diameter (which apparently you have not been given).

    I'm sorry if this has added to your confusion, if you order a rigid lens with a specified axial edge lift then when you view the fluorescein fit on the eye the edge lift is shown by the bright green ring around the edge of the lens, and tells by how much the lens is (hopefully) proud of the cornea at that point. Some GP lens types will specify the radial edge lift which the difference between the BOZR and BPZR. These two measuremnts can be confused but are in fact different especially in value.

    If in doubt refer to the above ie ISO 18639-1:2006+A1:2009

    Regards
    Ian
    Thank you Ian, I did make the assumption that the lens had a peripheral curve that allowed the lens to come to a edge thickness of equal value to the lenses center thickness.

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