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Thread: measuring prescribed prism ...

  1. #1
    Software Engineer NetPriva.com mirage2k2's Avatar
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    measuring prescribed prism ...

    I'm trying to measure the amount of prescribed prism in my lenses. I'm not second guessing the lenses ... I'm sure they have in them what they should ... just interested to know how to do it myself.

    This is what I have done so far ... found optical centre by determining the part of the lense that has no induced prism and dotted. Now measure the distance between the dots and this tells me how much the prism has decentred the OCs. There is a difference of about 8mm between this measurement and my PD. Now I know this does not mean there is 8d prescribed prism because there is also lots of power correction in the lenses and this must somehow be incorporated in the calculations ... and perhaps prentice's rule might come into it somewhere ...

    so what additional numbers do I throw into the mix to determine the actual amount of prescribed prism

    Any help would be great :bbg:

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    1 mm. decentration = .1 diopter prism per diopter of power.

    1 mm decentration on a 10 diopter lens = 1 diopter of prism.

    10 mm decentration on a 1 diopter lens = 1 diopter of prism.

    10 mm dencentration on an 10 diopter lens = 10 diopters prism.

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    Take a pd stick, dot the lenses at your pd, look at those spots, and that is what prism you are looking through.

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    Software Engineer NetPriva.com mirage2k2's Avatar
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    I don't know what you guys are talking about

    For a start, what is a pd stick?

    The oc's in my lenses have been moved out due to the base out prism. When I dot the lenses (at the point where there is no induced prism) and then measure the distance between the dots, the measurement is 8mm larger than my PD ... this does not equate to 8d prism right ...

    do I just chuck this 8mm of decentration into Prentice rule calculation and that gives the amount of prescribed prism

    Let me see ... the lenses contain 5d base out ... my rx is about 6d so ...
    8 * (0.1 * 6) = 4.8 ... thats close enough. Did I do the right thing?

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    "What's a pd stick?"
    How did you measure the distance between the OC's? How do you measure a PD or seg height? You probably used a pd stick, a ruler measured in millimeters? If your Rx is about +6 diopters and the lenses are decentered out 8mm from your pd, then yes the induced prism MIGHT be about 4.8 diopters base out. You also need to factor in the cylinder power. Depending on it's axis it may come into play or it may not. If it's minus cylinder with an axis of 180, it doesn't matter. If the axis is 90, then add the cylinder to the sphere power to determine what power there is a 180, then apply prentice's rule. This is where you are measuring the prism (at 180). (+6.00 -6.00 x 90 gives you an effective power of plano on the 180 meridian) This is why OCs and PDs are so ineffective at determining horizontal prism unless you have a good understanding of prism, the basics of eyeglasses. The opposite is true if you're measuring prism up and down. Unless you understand prism, you cannot verify a pair of eyeglasses. The horizontal tolerance for a pair of mounted lenses is 2.5mm or 2/3 diopter of prism, which ever is greater. Nothing is more aggravating than a dispenser rejecting a pair of glasses because "the pd is off". That's only half of the test.

    Don't understand prism? Spot your lenses at your pd using whatever measuring device you have, look at those points, and that will give you your prism.

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    Software Engineer NetPriva.com mirage2k2's Avatar
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    I hold the lense over a straight edge and move the lense around until the straight edge runs straight through the lense (from top to bottom) ... then I dot. Once I've dotted both lenses I just measure them with a ruler. I'm only trying to get a rough figure ... and a rough understanding of the process.

    BTW, did you know that I'm not a professional?

    There is cylinder in my lenses but I haven't quite got a grasp of cylinder yet :bbg:
    I know what it does but don't know how it works :bbg:

    Anyway, thanks optixbil, and thanks chip.

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    Sorry, I thought that you might not be a dispenser after my last post. Unless you have a lensometer or similar prism compensating device, you will not be able to verify your lens Rx.

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    Power times decentration (in centimeters) equals prism diopters. You calculate prism for each eye seperatly, so if your rx is -6.00 and your PD is out by 8 mm you should use 4 mm per eye. 4mm is .4 cm so .4x6.00=2.4 prism diopters base unknown since you did'nt specify if your ocs were wide or narrow. You are correct with your figure of 4.8, but you really need to figure the base orientation. Thats why you should calculate for each eye separatly. If you have a cylindrical component, you can induce compound prism by horizontal decentration, that is pd to wide or narrow. Just for fun, oblique axis prism can be specified either as a single amount with the axis given as a degree, or it can be resolved into separate horizontal and vertical components using either mathematics or parallelograms and a rulerand protracter.

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    Software Engineer NetPriva.com mirage2k2's Avatar
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    Quote Originally Posted by Dave Nelson
    ... If you have a cylindrical component ...
    I do, what additional calculations must I do to factor this in as well?

    btw rx is right +5.75 -1.00 a30, left +5.00 -1.25 a180, 5 base out ... I just dug this out from somewhere ... and all of a sudden I realise why you would work the prism out 1 eye at a time ... since the rx is different for each eye ... which leaves me stuck because I don't know what my monocular PDs are, just the binocular PD which is about 67mm
    Last edited by mirage2k2; 04-26-2006 at 09:28 PM.

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    We seem to be on the same subject in 2 different threads. Prentices rule is normally applied to major meridians only, either horizontal 180, or vertical,90. If the rx axis is on a major meridian, then its easy: if you remember cylinders have no power on their stated axis. eg. -5.00 -2.00x180. Power on the 180: -5.00. Power on the 90: -700. It gets pretty complicated when the rx axis is oblique, and there is a formula for it, but most of us run a quick calulation in our heads knowing that the change from sphere to cylinder takes place in exactly 90 degrees. Prism can be difficult for the layperson. Heck even some opticians have a hard time with it. You are partly correct in your observation about different prescriptions for the two eyes, but the real issue is prism imbalance. You can simply tip your own glasses, raise one lens up and one down. Seeing double yet? The lens raised is giving you base up prism, the one lowered base down. This displaces the images opposite to the base orientation, and you see double. Now raise your glasses slightly-may be a bit blurry, but no double vision: you're still getting the same amount of prism, but both base orientations are the same- no imbalance. Now if you apply prentices rule (you should be an expert with this by now.) to someones prescription where the two lenses are dramatically different, you will find a marked difference in the amount of prism induced. It is the DIFFERENCE in the prism that causes the imbalance, since the images are displaced different amounts. Now go back and read all the recent threads about slab offs and imbalances and you may understand the issue better. :cheers:

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