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Thread: Prism Thinning

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    Redhot Jumper Prism Thinning

    So; can I get some feedback on Prism Thinning? We order with a lab called Precision Optics. We have had several prism induced jobs come in. Now it is my understanding that for instance if you have a Progressive Lens that is marked up and you neutralize the RX and get 1.00 BASE UP OU that it passes inspection because Prism is equal. If you have OD with 1.00 BASE UP and OS with 1.00 BASE DOWN does it fail? We had 15 jobs come in this week that we failed because of Prism induced in different directions. The Lab says this is due to Prism Thinning. Which I understand but if Prism is in different directions I would think that it does not pass inspection. Any input appreciated.

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    Correct. Prism thinning should be symmetrical. Get a new lab.

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    Just to be sure- Are you using the prism reference point on the lens or the circle on the tape when you read it. Also what powers are you dealing with? Prism thinning is usually done on moderate to higher plus powers and as a rule of thumb is generally two thirds of the add power.

    Welcome to Optiboard!!!

    Have you read this great tutorial by Darryl Meister?

    http://www.opticampus.com/cecourse.p...rism_thinning/
    Last edited by Uncle Fester; 06-16-2015 at 10:24 AM. Reason: tweak...

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    The ANSI tolerance for unwanted vertically induced prism in progressive lenses is .33 Diopters, up or down. So that is about as much as any prism thinning should be "off".

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    It depends on where you are reading for prism. If you have unequal powers and you don't read on the MRP you will not have the same prism.

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    15 jobs though? How are you checking the Prism? Is it an Auto Lensmeter? Can you verify those reading with a manual lensometer? 15 jobs failing to unwanted prism is unbelievable from a professional lab, is this the Precision Optics in Minnesota? This is hard to believe.

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    Quote Originally Posted by Don Gilman View Post
    It depends on where you are reading for prism. If you have unequal powers and you don't read on the MRP you will not have the same prism.
    Thanks for all of the responses. Prism is being read on the Prism Reference Point. Is this correct?

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    Quote Originally Posted by VA View Post
    Thanks for all of the responses. Prism is being read on the Prism Reference Point. Is this correct?
    MRP and PRP are the same thing. It's midway between the mfg's engraved symbols or often shown as the dot(.) usually 4mm below the tapes T or cross.

    FWIW- That's also where the cylinder axis should be read.

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    Yes, which is either 2mm or 4mm below the fitting cross depending on the lens design.

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    Quote Originally Posted by Don Gilman View Post
    Yes, which is either 2mm or 4mm below the fitting cross depending on the lens design.
    Unless its a Zeiss lens, it can be as much as 6mm then, because Germans Be CRAZY.

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    Quote Originally Posted by VA View Post
    Thanks for all of the responses. Prism is being read on the Prism Reference Point. Is this correct?
    Any prescibed prism must be read from here as well.

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    Quote Originally Posted by Tallboy View Post
    The ANSI tolerance for unwanted vertically induced prism in progressive lenses is .33 Diopters, up or down. So that is about as much as any prism thinning should be "off".
    That's 0.33 imbalance, so the prism should not differ by more than that. I don't think there is anything about yoked prism limits, and I think 1+ diopter yoked prism thinning is pretty common.

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    Quote Originally Posted by DanLiv View Post
    That's 0.33 imbalance, so the prism should not differ by more than that. I don't think there is anything about yoked prism limits, and I think 1+ diopter yoked prism thinning is pretty common.
    Yes thank you, that is what I was trying to say. I was speaking in regards to his finding 1 diopter base up on one lens and 1 diopter base down on the other. I'm still having a hard time wrapping my head around receiving 15 jobs like this in a week - I would probably ask to speak to the owner of the lab personally if this happened to me. I would be getting my lensometer checked also.

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    I second the question about the auto-lens meter. I have seen many cases where the optician is clearly reading power in the corridor, or checking prism off the PRP. I have no beef with auto-lens meters. You just need to be certain (with auto or manual) that you are actually on the PRP.

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    Sounds to me like a lot of folks need a seminar on this at your next convention. The prism reference point is right in between the engraved markings. Match it there and you should be good to go.

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