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Thread: Equal thinning prism with Prism rx

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    OptiBoard Novice eye1u2see2020's Avatar
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    Equal thinning prism with Prism rx

    Recently there has been a discussion as which way is correct when verifying Prescribed prism in a progressive lens. We will use the Rx of OD -2.00 sph Os -2.50 sph and OD 1 prism diopter base down and OS 1 prism diopter Base up. Upon reading the Rx, the power is correct and in the od the 1 diopter prism is there however on the os the lensometer shows that the prism is at plano. The lead tech says that we have to take into consideration equal thinning prism. However most of us disagree and think that the prism should not be at plano, and should be at the 1 diopter ring. Which is correct? Please email me back at eye1u2see2020@msn.com. Thank you for your assistance.

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    Read the prism at the OC, located below where you check the distance Rx.
    Look at the layout card, and find the dot at the 180 line. Check it there.

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    Master OptiBoarder optical24/7's Avatar
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    obx is right. The spot to check for prism on most progressives is at a spot along the 180 right where the laser ingraved 180 indicator spots are. You may have prism thinning but you should still see 2 degrees difference.

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    OptiBoard Novice eye1u2see2020's Avatar
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    That is what I thought. I have been doing optics for 13 years and the lab manager (30+ years experience) tried to tell me that equal thinning will totally change the prescribed prism. He had passed the previous referred job as being all within standards. I knew that it was wrong because no matter what there was just 1 prism diopter total, and the prescribed prism should have been 2 total.
    Thank you for your help.

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    You leed Tech is right, you must take the equal thinning prism into account.

    You said R Lens: 1cm/m B270
    L Lens: 1cm/m B90

    So for example you have a equal thining prism of 1@B270

    For the R lens you should messure at PRP 2cm/m B270
    For the L lens you should messure at PRP 0 cm/m

    I have write a small excel sheet where you can check it.

    Use the Excel Sheet as follow:
    1) look at you prescribed Prism where the base is located (0-90, 90-180, 180-270,270-360) and go ther for data input.

    2)Input R or L for Right or Left Lens

    3) Put the prescribed prism and base direction in the yellow marked field.

    4) Put the the Thinning Prism into the Sheet

    The Results you can see in Resulting Prism @ PRP
    Attached Files Attached Files

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    Huh? Isn't any prism thinning the same in both eyes, resulting in no prism. The only change I can perceive is where in the retical the prism will register, with a difference of the prescribed amount. Lab guys??

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    Master OptiBoarder optical24/7's Avatar
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    From what I can make out of Rafael's spreadsheet, he still comes out with 2 degrees difference between OD and OS.

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    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by eye1u2see2020 View Post
    Recently there has been a discussion as which way is correct when verifying Prescribed prism in a progressive lens. We will use the Rx of OD -2.00 sph Os -2.50 sph and OD 1 prism diopter base down and OS 1 prism diopter Base up. Upon reading the Rx, the power is correct and in the od the 1 diopter prism is there however on the os the lensometer shows that the prism is at plano. The lead tech says that we have to take into consideration equal thinning prism. However most of us disagree and think that the prism should not be at plano, and should be at the 1 diopter ring. Which is correct? Please email me back at eye1u2see2020@msn.com. Thank you for your assistance.
    Are you saying that you have OD 1^ BD and OS 1^BU prescribed? If so, then you need to have 1^ BD and 1^ BU or (According to ANSI) be within 0.33^ of vertical imbalance.
    For example you can have 0.75^BD in OD and 1^BU in OS or 1^BD OD and 1.25^BU OS as long as the combination does not exceed 0.33^ from specified.

    If this Rx does not have any prescribed prism then you have a total of 2^ of imbalance and (According to ANSI) you are only allowed 0.33^ of total imbalance.

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    Equal thinng prism

    Hallo Lensgrinder,



    I´m not sure now if im right or not.

    If i have a job with a prescribed prism for the
    right Lens of 1.0cm/m Base 270

    and

    left lens of 1.0cm/m Base 90


    and i put a prism thinng of 1,0 Base 270. (prism thinning of 1.0B270 for both lenses)

    Can you tell me what i will measure at the Prp with my Humphrey?
    Is my Excel sheet right or not?
    And are you running Jobs with prescribed prism with or without prism thinning?

    Thank you

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    OptiBoard Novice eye1u2see2020's Avatar
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    Quote Originally Posted by lensgrinder View Post
    Are you saying that you have OD 1^ BD and OS 1^BU prescribed? If so, then you need to have 1^ BD and 1^ BU or (According to ANSI) be within 0.33^ of vertical imbalance.
    For example you can have 0.75^BD in OD and 1^BU in OS or 1^BD OD and 1.25^BU OS as long as the combination does not exceed 0.33^ from specified.

    If this Rx does not have any prescribed prism then you have a total of 2^ of imbalance and (According to ANSI) you are only allowed 0.33^ of total imbalance.
    Hi Lens grinder, yes the od and os ^ is prescribed, however upon reading the Rx the od has the 1^BD and the os has no ^ at the mrp. So, according to how I read the ansi standards this job has to be rejected due to being 1^ prism off. Am I correct or does the Equal thinning prism have some effect on the total prism?

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Rafael View Post
    Hallo Lensgrinder,



    I´m not sure now if im right or not.

    If i have a job with a prescribed prism for the
    right Lens of 1.0cm/m Base 270

    and

    left lens of 1.0cm/m Base 90


    and i put a prism thinng of 1,0 Base 270. (prism thinning of 1.0B270 for both lenses)

    Can you tell me what i will measure at the Prp with my Humphrey?
    Is my Excel sheet right or not?
    And are you running Jobs with prescribed prism with or without prism thinning?

    Thank you
    You should get:

    OD: 2B@270
    OS: 0B@090
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    Master OptiBoarder optical24/7's Avatar
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    again....

    Quote Originally Posted by optical24/7 View Post
    obx is right. The spot to check for prism on most progressives is at a spot along the 180 right where the laser ingraved 180 indicator spots are. You may have prism thinning but you should still see 2 degrees difference.
    :cheers:

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by eye1u2see2020 View Post
    Hi Lens grinder, yes the od and os ^ is prescribed, however upon reading the Rx the od has the 1^BD and the os has no ^ at the mrp. So, according to how I read the ansi standards this job has to be rejected due to being 1^ prism off. Am I correct or does the Equal thinning prism have some effect on the total prism?
    MRP or PRP?

    If you are refering to the MRP as the DRP then no this is not where you check the prism power, if you are refering to the MRP as the PRP then this is where the power should be checked and the imbalance should be equal to 2D I don't believe that MRP is a term used any longer, but may be wrong.

    Here's a link to the lens description standard. http://www.visionsite.org/s_vision/p...scStandard.pdf
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    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by Rafael View Post
    Can you tell me what i will measure at the Prp with my Humphrey?
    Is my Excel sheet right or not?


    Thank you
    I would grind the prescribed prism and not put thinning in these lenses.

    Quote Originally Posted by eye1u2see2020 View Post
    Hi Lens grinder, yes the od and os ^ is prescribed, however upon reading the Rx the od has the 1^BD and the os has no ^ at the mrp. So, according to how I read the ansi standards this job has to be rejected due to being 1^ prism off. Am I correct or does the Equal thinning prism have some effect on the total prism?
    You are correct you are only allowed to have 0.33^ of vertical imbalance.

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    ATO Member HarryChiling's Avatar
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    here's a great article by Darryl in the downloads section.

    http://www.optiboard.com/forums/showthread.php?t=12641
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    I´m grinding at the moment lenses with prescribed prism without prism thinning.

    I have done this because my calculation software give me not out on the ticket the new amount (prescribed+(-)prism thinning) of the prism the customer will see in his focimeter.

    But the lenses cosmeticaly looks not good without prism thinng.

    So I have wrote this small excel sheet, to write the new amout of prism down on the Jobticket manuely and produce then all lenses with prescribed prism in the futer with prism thinning.

    Harry can you please check the excel sheet, if its ok so far.

    Thank you

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    ATO Member HarryChiling's Avatar
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    Rafael,

    Why don't you programatically determine which quadrant the axis lies on and manipulate the data rather than using seperate sheets. Also how are you detrmining prism thinning? Some like to use 2/3 of the add, you could also use the b measure, seg hght, progressive parameters, and add to determine the amount of prism thinning. Also when determining prism thinning the Rx for both eyes should be taken into consideration as often you may have to make slight comprimises. Other than that it looks goo so far.
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    Harry i can´t programming, thats why i make a excel sheet.
    My Calculation software determines it automaticly. takes into account b mesurement, sag high....
    The only problem is that the software gives me only out the amout of prism on the job ticket wich is for the blockers. So its on the machine reference point, and even not in prism dioptries.
    So thats why i calculate its this way.

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Quote Originally Posted by HarryChiling View Post
    MRP or PRP?

    If you are refering to the MRP as the DRP then no this is not where you check the prism power, if you are refering to the MRP as the PRP then this is where the power should be checked and the imbalance should be equal to 2D I don't believe that MRP is a term used any longer, but may be wrong.

    Here's a link to the lens description standard. http://www.visionsite.org/s_vision/p...scStandard.pdf


    Yes, please, let's retire MRP. It was used to describe a few different things, so its meaning is now ambiguous, and therefore, it's not really useful.

    The LDS defines terms for each of the important features on any lens - distance, near, layout, and prism reference points.

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by shanbaum View Post
    Yes, please, let's retire MRP. It was used to describe a few different things, so its meaning is now ambiguous, and therefore, it's not really useful.

    The LDS defines terms for each of the important features on any lens - distance, near, layout, and prism reference points.
    I still hear a bunch of people say MRP when they mean DRP, LRP, or PRP. Your right it has gotten to be ambiguous and often it causes a bunch of confusion.
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