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Thread: Matching Prism Thinning

  1. #1
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    Question Matching Prism Thinning

    This probably is a very simple problem to solve by many people smarter than myself:):

    We just began using a Gerber-Coburn SGX generator and Innovations lab software. When I measure the vertical prism at the PRP for our surfaced lenses I don't always agree with what the software says was used but net vertical prism imbalance is always well within ANSI tolerance.

    If I enter the measured vertical prism at the PRP in the software and disable prism thinning for a one lens job I get too much vertical prism at the PRP. I have no other problems with prism in SV or flat-top lenses. Horizontal and vertical prism are usually within 0.01 to 0.04 prism diopters of that specified for these lenses.

    Is there something I'm doing wrong with either my measurement of the amount of thinning prism used or with what I'm doing with the software and the SGX?

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Question prism & progressives

    Virtually all progressive lenses will tilt unpredictably in the blocking ring at the surface blocker. This produces some amount of unwanted prism, which produces some amount of edge thickness error. Imbalance shouldn't be much of a problem at all, unless you're using dissimilar adds and/or base curves on the two sides, since the amount tilt is related to those two characteristics. Some imbalance may appear, for two reasons: one, progressives vary a bit from lens to lens, and two, many actually rock in the blocking ring, so that even nearly identical parts can tilt differently.

    These errors are generally insignificant in total. It can make doing a single lens a bit of a pain. You'll probably find that your lenses will end up with between 0.25D and 0.50D more base down prism than the software specifies (that is, if zero were specified, you'd end up with 0.25D-.50D). When you specify an amount of base down prism, thus disabling the automatic prism thinning calculation, you need to take that into account.

    If you're seeing excess prism greater than this, make sure that when blocking, you secure the lens against the blocking ring in the distance portion of the lens. If you're using a Step One blocker, be sure that the clamping fixture (a/k/a the "suckball") is in the lower of the two positions, which places it over the distance portion of the lens.

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    OptiBoard Professional RT's Avatar
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    RREADER: If you're using the Step One blocker, another effect concerns the shrinkage of the wax. The wax shrinking will tip up to 0.10 diopter extra base down prism, depending upon add power. This won't happen with symmetrical lenses (i.e. SV, ST, etc.) Again, however, this should not contribute to any vertical imbalance, but is something to remember when the total base down prism doesn't match what you want.

    The 3 point contact on the top of the Step One chill ring was specifically designed to minimize tipping of the progressive blank due to asphericity. The 0.25 to 0.50 that Shanbaum quotes is pretty standard for non-Step One blocking, but typically, desired prism on progressives is within 0.10 to 0.25 on progressives, even with the wax shrinkage factor.

    As Shanbaum mentions, the biggest problem with unintended base down prism on progressives is when remaking a single lens. Once you learn how much you are typically off, just manually modify the amount of prism you generate in the SGX to compensate.

    Shanbaum: I coined the term "Suckball". I insist on royalties if you're going to use it!
    RT

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Big Smile Suckball! Suckball!

    Sorry, but on the innumerable occasions on which you have used the disputed term, you have never until now reserved any rights of authorship. This, together with the fact that the term has been in wide use in everyday conversation throughout the industry for these eight years, places it clearly in the Public Domain.

    Why just the other day, at the dinner table, my wife and I were talking about the Step One, and she inquired as to whether the "suckball" was still performing adequately.

    This does clear up one thing for me, though - now I understand why you were on the patent.

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    Forever Liz's Dad Steve Machol's Avatar
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    And here's Rick patent:

    http://www.delphion.com/details?pn=US05505654__

    No mention of 'suckball' though! :p


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    Thanks very much for the replies:)

    We use an alloy blocker at the present time. I will do some trials to see if we are consistently off by a fixed amount. It also seemed like the prism amount specified on the job ticket and the amount actually sent to the SGX from the software were slightly different (job ticket 0.89 PD @ 270 degrees - SGX 0.74 PD @ 270 degrees). I'll let everyone know how it turns out.

    Thanks!

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    OptiBoard Professional RT's Avatar
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    Big Smile

    Its clear that the only intellectual property I could possibly have contributed to the project was the term "Suckball" and several other terms that probably aren't printable. The fact that the company lawyer couldn't comprehend the implications of such a beautifully descriptive term and include it in the patent does not diminsh its worthiness. If your wife is using the term "Suckball" conversationally, and she is not actually referring to ME, that's probably all I can hope for :D

    RREADER: In conversations I've had in the past with lens designers from several different companies, they always seem fairly nonchalant about your ability to get exactly the desired amount of base down prism in progressives. All they care about is lack of vertical imbalance. The fact that there isn't a regular surface anywhere on the front of the lens that can be used to align the lens in a "normal" fashion on any chill ring is the root cause of the problem you're seeing. That doesn't help you, however, when dealing with that crusty old doc who says "If I wanted base down prism on that lens, I'd have ordered it with base down prism!" Sometimes you have to make lenses the same way I play golf--aim way left and it might hit the fairway.
    RT

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Exclamation Difference

    Originally posted by RREADER
    Thanks very much for the replies:)

    It also seemed like the prism amount specified on the job ticket and the amount actually sent to the SGX from the software were slightly different (job ticket 0.89 PD @ 270 degrees - SGX 0.74 PD @ 270 degrees).

    That can happen. By default, Innovations sends prism to the SGX in the index of the material, but it can send it in any index. It's important that the index in which the software sends matches what the SGX is set to receive; the index is not sent along with the other data. The value on the workticket is by default expressed in the "prism ring index" (specified in each material's properties) but could be in degrees, or could be compensated for the off-centration caused by the use of prism rings or prism blocking.

    Contact our courteous and friendly support staff for further information (or press F1 for Help).

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    Bad address email on file John R's Avatar
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    Arrow

    I'll throw in a couple of other points as well..
    As you are using alloy, is there any flashing on the buttons thus causing the lens not to sit flat in the sgx chuck? and try when blocking, filling the lens half way let the alloy set for a few secs befor filling the rest, but never over fill always aim to under fill..

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