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Thread: Vertical Imbalance questions.

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    Vertical Imbalance questions.

    I am able to figure the vertical imbalance of an oblique axis in the 90-degree meridian for slab off lenses. However, I'm not able to find anything on how to figure the prism amount and prism angle to compensate for it. I am really in the dark on the angle. The prism amount should be what the imbalance is, but for even a normal job this isn't always the case with our software. Also, how do you work prescribed prism into the equation?

    P.S. When entering the vertical imbalance equation into an Excel spreadsheet, why do you have to divide 57.296 into the axis, when I don't in my graphing calculator? example: (COS(70/57.296))

    Thanks for any help.

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    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by Bob Price
    The prism amount should be what the imbalance is, but for even a normal job this isn't always the case with our software.
    If you are moving the OC in two different directions then the prism amount will be different. For example if you lay a ST out for surface 6mm in and 6mm down then you will need to move the OC horizontally and vertically, how much will depend on power in those meridians.

    Example -1.00 -2.00 X 45

    Power @ 90 = -2.00
    Power @ 180 = -2.00
    PD 60/57
    OC 3mm above seg
    We need to move the OC 4.5mm Horizontally and 3mm vertically
    Ph=.45 X 2
    Ph=0.9 D

    Pv=.3 X 2
    Pv=0.6 D

    P=square root(V + H)
    P=square root(1.2)
    P=1.09

    This gives your combined horzontal and verical component. Now you need to find what axis to grind the prism in.

    tan(x)=V/H
    tan(x)=0.6/0.9
    x=33.6 degrees

    So you would grind 1 D of prism @ 33.6 degrees. Remember the axis will depend on what quadrant you need to grind in. For example if you want to move the OC up and in for the OD then our axis will remain 33.6 degrees, but if we want to move the OC up and out then it will be 180-33.6=146.4 degrees.

    Quote Originally Posted by Bob Price
    P.S. When entering the vertical imbalance equation into an Excel spreadsheet, why do you have to divide 57.296 into the axis, when I don't in my graphing calculator? example: (COS(70/57.296))
    1 Radian = 57.296 degrees
    To convert type DEGREES into the excel equation.

    Hope this answers your question

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    Send an e-mail to administrator@orionoptical.com for Dr. Mancusi, who has a Ph.D. in mathematics and has been in the optical business all his life.

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    [QUOTE=lensgrinder]If you are moving the OC in two different directions then the prism amount will be different. For example if you lay a ST out for surface 6mm in and 6mm down then you will need to move the OC horizontally and vertically, how much will depend on power in those meridians.


    Actually, I was hoping to figure for the slab-off that will be ground for near imbalance. After you figure for the imbalance at the reading depth, how do you figure the prism amount, depending on prescribed prism in the distance or equithin, and how do you determine the angle that this slab-off will be ground at? I also was curious why our system sometimes ground a different amount of prism then what the imbalance was at near, when there wasn't any equithin or prescribed prism in the distance.

    I'm figuring the reading depth of a bifocal at 5mm. Is a trifocal 9mm? What is it for a progressive?

    I appreciated your response, as I was able to learn from it. Thanks.
    Last edited by Bob Price; 01-15-2006 at 12:49 PM.

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    RETIRED JRS's Avatar
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    Bob, how have you been?

    The default in Excel is Radian - that is why you see the conversion error. You hand held has a "DRG" key and you have it set for degrees so the conversion is automatic.

    In Excel use =RADIANS(), or =DEGREES(). If you need to see examples type "functions" in the little help box in the upper right corner. View the math and trig areas.

    Perhpas some day I'll get a chance to come back up your way. Tell you mom hello, and congrats on the promotion.
    J. R. Smith


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    Quote Originally Posted by Bob Price


    Actually, I was hoping to figure for the slab-off that will be ground for near imbalance. After you figure for the imbalance at the reading depth, how do you figure the prism amount, depending on prescribed prism in the distance or equithin and how do you determine the angle that this slab-off will be ground at? I also was curious why our system sometimes ground a different amount of prism then what the imbalance was at near, when there wasn't any equithin or prescribed prism in the distance.
    A slab-off will create two optical centers on one lens. It is sometimes referred to as bicentric grinding. You will put the slab on lens with the most minus least plus in the 90th meridian. For example:
    OD +3.00
    OS -3.00
    Reading depth 10mm
    OD 3D BU
    OS 3D BD
    Total imbalance is 6D
    We will grind a 6D Slab on the left eye @ the reading level. This is an approximation because when we read we look down as well as in. Reading level, if not measured, is generally 10mm, but if you want to be exact, measure. If you have cylinder do not forget to find the power in the 90th meridian. Do remember these calculations are approximations. You would need to use more exact formulas to get closer.

    Keep in mind you still need to grind the distance OC also which is what I posted earlier.

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    Master OptiBoarder Darryl Meister's Avatar
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    Quote Originally Posted by Bob
    The prism amount should be what the imbalance is, but for even a normal job this isn't always the case with our software. Also, how do you work prescribed prism into the equation?
    Keep in mind that you are using an approximation (e.g., the sin^2 formula) to calculate prism in an oblique cylinder, so you are not actually calculating the precise amount of prism vertical prism. However, your software may be calculating the exact amount of prism. Also, you need to ensure that both you and your surfacing software are using the same vertical distance from the optical center to the near point, or the prism amount will also be different. It sounds as though you may not know what reading level your software is actually using.

    Otherwise, once you have calculated the correct vertical prism imbalance amount at the reading level, this would also be the same prism to use for your slab-off (unless the doctor has specified a different amount).
    Darryl J. Meister, ABOM

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