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Thread: Lens Clock

  1. #1
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    Lens Clock

    Is it true that lens clocks are designed to accurately read materials with an index of refraction of 1.53, and that a lens clock reading of a material > 1.53 would read too low? If that's the case, then how would I accurately match someone's base curve in a higher index material?

  2. #2
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    No, a lens clock measures only the curvature on the lens surface. It is essentially a sag guage writ small. The calibration is set to read curves in 1.53 index, it is up to you, the optician to decypher what those curves relate to in the actual lens material. And lens clocks are notoriously inaccurate.

    Use the lens clock to measure the existing base curve, then select the base curve of the new material that you want to use from the manufacturers selection. Base curves are all measured in 1.53 index regardless of the index of refraction.

  3. #3
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    I would measure the existing base curve with the typical tool, with alacrity, and inform the lab of the measurement. They, in turn, would understand, convert and punt.
    Eyes wide open

  4. #4
    Rochester Optical WFruit's Avatar
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    Keeping in mind that the curve measured with a lens clock, and the true curve of a lens are never (well, exceedingly rarely) the same thing. The best analogy would be that a lens clock, on a good day, measures the nominal curve of the lens, not the true curve (I've seen the nominal and true curve differ by over 2 diopters, usually on high index, high base curve lenses).

    As uncut said, measure, specify base, specify index, and we'll figure it out on our end.
    There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.

  5. #5
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    It is quite easy to convert SAGITTA value (the value you measure with sphero or torometer) to lens power in the following program:
    http://www.opticampus.com/tools/surface.php

    The results will be accurate only if you use torometer with 50mm distance rods and proper lens material will be choosen.

    If you have the distance different than 50mm, you will find here exact formulas to calculate it:
    http://www.optiboard.com/forums/show...needed-for-sag
    Last edited by essegn; 07-21-2012 at 10:00 AM.

  6. #6
    Master OptiBoarder Darryl Meister's Avatar
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    If you have the distance different that 50mm
    Not too long ago, I added an option on the OptiCampus.com Surface Conversion form that allows you to specify the sagitta at any diameter, once you click on the Sagitta option, although 50 mm is still the default.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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