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Thread: Need Help For Ophthalmic Question!

  1. #1
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    Need Help For Ophthalmic Question!

    ANY ONE CAN HELP ME ON THIS QUESTION....TQ!


    The prescription R +6.00/+2.00x90 at 15mm is to be dispensed at 10mm.
    The base curve of the spectacle lens is to be -6.00D and the vertex power allowance -0.80D. The refrative index is 1.50.


    Find (i) The presciption at the correct vertex distance.
    (ii) The cross curve of the spectacle lens.
    (iii) The compensated front surface power.
    (iv) The axial thickness


    PLS SHOW THE ANSWER AND THE HOLE FORMULA.....TQ

  2. #2
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by cyinteo
    ANY ONE CAN HELP ME ON THIS QUESTION....TQ!


    The prescription R -6.00/+2.00x90 at 15mm is to be dispensed at 10mm.
    The base curve of the spectacle lens is to be +6.00D and the vertex power allowance -0.80D. The refrative index is 1.50.


    Find (i) The presciption at the correct vertex distance.
    (ii) The cross curve of the spectacle lens.
    (iii) The compensated front surface power.
    (iv) The axial thickness


    PLS SHOW THE ANSWER AND THE HOLE FORMULA.....TQ
    (i) the prescription at the vertex distance you specified is -5.83 +1.90
    (ii) the cross curve is -9.59
    (iii) the front surface power is -5.66
    (iv) the axial thickness you need a diameter and a minimum thickness.

    I assumed that you meant +6.00 base lens and a -6.00 prescription. The way you had it written the lens would be a double convex. To find out the equations visit www.technicalopticians.org
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    Master OptiBoarder lensgrinder's Avatar
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    The cross curve, if using the compensated power of: +6.18 +2.14 X 90 will be +14.32. For example the base curve will be the flattest curve on a toric surface and the cross curve will be the steepest. You would put the compensated Rx on the optical cross so that you have +6.18 @ 90 and +8.32 @ 180. Now you subtract +8.32 -(-6.00) and you get +14.32 since we cannot figure thickness.

    Hope this helps.

  4. #4
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by lensgrinder
    The cross curve, if using the compensated power of: +6.18 +2.14 X 90 will be +14.32. For example the base curve will be the flattest curve on a toric surface and the cross curve will be the steepest. You would put the compensated Rx on the optical cross so that you have +6.18 @ 90 and +8.32 @ 180. Now you subtract +8.32 -(-6.00) and you get +14.32 since we cannot figure thickness.

    Hope this helps.
    I think he may have written the question wrong. A +6.00 on a -6.00 base is a little suspicious to me, but other than that I got what you got on the cross except I used the true curve since this makes it equal to the tools.

    The equations that you would need are the true power formula for converting the base curve from marked to true curve. The vertex compensation formula for converting the Rx to the vertex compensated values. The lensmakers formula for coming up with the power. For the thickness you would need the sagittal depth formula, however it would require additional information. A tip when compensating the powers break them down into their respective meridians then compensate, or you will be off. All these formulas are available on the www.technicalopticians.org site.

    PS - If you need a knowledgable individual on these boards, lensgrinder is a assistant profesor of opticianry, so if he is willing to help you know you are going to get it right :)
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  5. #5
    Master OptiBoarder Darryl Meister's Avatar
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    He is referring to the back curve as the Base curve in this situation.

    I'm going to assume that he is working in minus cylinder form, since a plus cylinder lens (with the toric surface on the front) would require a different vertex power allowance for each meridian. Besides, you would not refer to the back surface as the "Base curve" for a plus cylinder lens. I'm also going to assume that the vertex power allowance applies to the final, compensated prescription, since there is a vertex distance change involved.

    The first thing to do is to compensate the prescription.

    Determine the new power of each meridian using:

    Fnew = Fold / (1 + delta * Fold)

    where (delta) is the change in vertex distance in meters (for instance, -0.005 in this case, since you are moving the lens backward).

    For your particular prescription, this gives you:

    Fsph = 6.00 / [1 + (-0.005) * 6.00] = +6.19 D

    For the sphere meridian, and

    Fcyl = 8.00 / [1 + (-0.005) * 8.00] = +8.33 D

    for the cylinder meridian (the sum of the sphere power and cylinder power). Since the new cylinder power is equal to the difference between the sphere meridian and the cylinder meridian, this makes the new prescription:

    +6.19 +2.14 x 090, as confirmed by lensgrinder's optical cross.

    Or, since we will be working from now on in minus cylinder form,

    +8.33 -2.14 x 180

    The cross curve for this prescription using a -6.00 D back Base curve would simply be equal to the sum of the minus cylinder power and the back Base curve:

    Cross curve = -6.00 + (-2.14) = -8.14 D

    This is your cross curve.

    The nominal, non-compensated front curve would be equal to the sphere power less the back Base curve, giving you:

    8.33 - (-6.00) = +14.33 D, as lensgrinder indicated.

    Now, simply add the vertex power allowance to your nominal front curve to arrive at your compensated front curve:

    14.33 + (-0.80) = +13.53 D

    This is your compensated front curve.

    Vertex power allowance is simply the difference between the compensated front surface power and the nominal front surface power.

    The expression relating the nominal and compensated front surface powers is given by:

    1 / Fcomp = 1 / Fnom + CT / n

    where Fcomp is the compensated front surface power, Fnom is the nominal front surface power, CT is the center thickness in meters, and n is the refractive index (1.500 in this case). You already know the index and you can solve for two of these terms:

    1 / Fcomp = 0.0739 and 1 / Fnom = 0.0698

    Leaving you with:

    0.0739 = 0.0698 + CT / 1.500

    Rearranging to solve for CT gives you:

    CT = 1.500 * (0.0739 - 0.0698) = 0.0062 m

    This represents a center thickness of roughly 6.2 mm.

    However, note that if the toric surface is on the front (plus cylinder form) instead, you would actually need two vertex power allowances -- one for each meridian. But the question would have to be phrased differently in order to allow for a plus cylinder lens.
    Darryl J. Meister, ABOM

  6. #6
    Master OptiBoarder lensgrinder's Avatar
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    Darryl, Great answer! I missed the vertex power allowance. I guess that is why you should read the whole question.

    I do have one question for you though. You said "you would not refer to the back surface as the "Base curve" for a plus cylinder lens." In the case of a toric lens ground on the front of a lens, would you not refer to the base curve as the back surface? Because it is the curve from which the other curves are calculated.

    Thanks,
    Brent

  7. #7
    Master OptiBoarder Darryl Meister's Avatar
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    It would really depend on the context I guess, and what you're trying to communicate, since there are quite a few working defintions of the term. The factory-finished surface is certainly one definition. However, when dealing with toric surfaces, the flattest curve of the toric surface is also known as the "Base" curve. If you're referring to the specifics of the toric surface, I would refer to the flattest curve as the Base curve. If you're referring to the lens in general though, you would probably refer to the factory-finished side (assuming there is one).
    Darryl J. Meister, ABOM

  8. #8
    ATO Member HarryChiling's Avatar
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    I just assumed that the base would be the front. :hammer:
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  9. #9
    Banned Jim Stone's Avatar
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    Now help me. Why would one need to do such calculations?

  10. #10
    Master OptiBoarder Darryl Meister's Avatar
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    To make lenses.
    Darryl J. Meister, ABOM

  11. #11
    Master OptiBoarder ikon44's Avatar
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    this person is probably a student doing the ABDO distance learning course for overseas students. The question is typical of the problems that are set for them.
    To find out what,s happening in the UK optical market:
    http://theOptom.com

  12. #12
    Old Optician to New OD Aarlan's Avatar
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    Quote Originally Posted by Darryl Meister View Post
    However, when dealing with toric surfaces, the flattest curve of the toric surface is also known as the "Base" curve...If you're referring to the specifics of the toric surface, I would refer to the flattest curve as the Base curve.
    Darryl,

    This is something that's been bothering me a bit recently after thinking about it. Using the definition of Base curve as the 'flattest' is somewhat incorrect. Hear me out.

    If you have a -1.00 -2.00 x 180, and the front curve is a 5 base, the grinding curves would be (ignoring index & thickness) a -6.00 @ 180 (the base curve) and a -8.00 @ 90 (the cross curve). In this case the base curve is the flattest.

    Now, keeping the base curve at 5 D, let's change the Rx to +6.00 -2.00 x 180. The grinding curves would be +1.00 @ 180, and -1.00 @ 90. Neither is 'flatter', but a surfacing ticket would consider the 180 to be the 'base curve', and the 90 to be the cross curve.

    Lastly, let us keep the 5 D base curve, but now the Rx is a +8.00 -2.00 x 180. Now the grinding curves are +3.00 @ 180, and +1.00 @ 90. The 'flatter' of the two is the 90 meridian, but it is still the cross curve on a generator ticket. The base curve would be at 180, since the generator wouldn't just flip the BC but 90 degrees once the 180 meridian began to get steeper.

    Using the def of flattest is not as accurate as defining it as the meridian that corresponds with the axis (when the Rx is written in minus cyl form, of course). Or we could use least minus/most plus back curve is the Base Curve.

    Sorry for such a long post on such a hideously useless point, but I was arguing w/ another optical professional about this earlier...and I don't want to study, so this is more fun.

    AA

  13. #13
    Master OptiBoarder Darryl Meister's Avatar
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    This is something that's been bothering me a bit recently after thinking about it. Using the definition of Base curve as the 'flattest' is somewhat incorrect. Hear me out.
    The situation gets a bit tricky with the type of surface you're describing, since this isn't a traditional toric surface. Traditional concave toric surfaces are "tire" type surfaces, and the flattest curve is produced by the primary cutting arc of the generator diamond wheel (for traditional generators). The cross curve is then produced by tilting the diamond wheel as it sweeps across this cutting arc.

    However, toric surfaces with a principal convex curve and a principal concave curve are referred to as "capstan" type surfaces. Although I'd have to put some thought into it to decide for certain, my guess is that the convex curve (on the back surface) would be produced by the main generating arc, which would arguably make this curve the "Base" curve with traditional generators. Nevertheless, the lab software may very well consider the curve containing the actual sphere power of the prescription in minus cylinder form the "Base curve" of the lens, which would also be consistent with the flattest curve of traditional minus cylinder lenses. (This would be consistent with your +8.00 -2.00 x 180 example.)

    At the end of the day though, the notion of a "Base" curve doesn't mean much to a generator, since it must simply cut a surface with two principal curvatures within whatever constraints the cutting tool must operate. Modern generators, in particular, which frequently utilize single point cutting, do not produce surfaces by rotating a circular cutting tool about a set radius, so the choice of a "Base curve" is rather arbitrary. These machines just require a mathematical description of the height of the surface at any point.
    Darryl J. Meister, ABOM

  14. #14
    Master OptiBoarder lensgrinder's Avatar
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    The definition of a toric lens is obviously one surface being toric and the lens having a curved form (convex and concave surface). So in this case the flattest surface is the base curve. You can have bi-convex torics or bi-concave torics. An older generator can only cut in plus cylinder(front surface) or minus cylinder (back surface) which keeps the definition of the typical toric lens.
    The surface you describe is bi-convex with curved form in the cross portion of the lens and I do not know what that would be called. Can this form be produced with "free form"?

  15. #15
    Master OptiBoarder Darryl Meister's Avatar
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    The surface you describe is bi-convex with curved form in the cross portion of the lens and I do not know what that would be called. Can this form be produced with "free form"?
    It's called a capstan or saddleback toric surface.
    Darryl J. Meister, ABOM

  16. #16
    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by Darryl Meister View Post
    It's called a capstan or saddleback toric surface.
    Which is what you said in the above post.:D
    Next time I will read before I post.

  17. #17
    Master OptiBoarder Darryl Meister's Avatar
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    No worries. ;)
    Darryl J. Meister, ABOM

  18. #18
    Old Optician to New OD Aarlan's Avatar
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    Quote Originally Posted by lensgrinder View Post
    The surface you describe is bi-convex with curved form in the cross portion of the lens and I do not know what that would be called. Can this form be produced with "free form"?
    You can surface it on a Gerber (Used to use an SGX), and also cut foam laps that are concave to go with it.

    AA

  19. #19
    Master OptiBoarder
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    Quote Originally Posted by Jim Stone View Post
    Now help me. Why would one need to do such calculations?
    :D I can't help but laugh at that, because it looks awefully similar to my own homework!

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