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Thread: Prism Thinning problems

  1. #1
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    Prism Thinning problems

    Hi my name is Rafael i`m 28 and i am from germany. I have found this awesome bord surfing the web.
    Here in germany we donīt have such a website, forgive me if but my english is not the best.

    I have a problem with progressive lenses and prism thinning

    For examle i have this RX:

    R: sph -2,75 cyl +0,5 A: 0 Add 2,25 Prism 0,71cm/m Bas:315
    L: sph -2,25 cyl +0,25 A:175 Add 2,25 Prism 0,71 Bas:135


    Equipmend:

    LOH R-Xpert calculating software
    LOH PRA Blocker (Axis calibration is done)
    LOH V-Pro Generator (Rms Prozess)
    LoH Toro X for polishing


    The problem is the result prism, calculated from the software is not matching with the job card.
    Itīs a problem to verify this.

    So i make a few test.

    1 Test: I deactivate the prism thinning from the calculating software.
    I have finishe then the lens with the new calculation without prism thinning.

    I should verify this results:
    (Blocking was done at PRP Point, fitting high to Cross was 4mm)

    For the Right Lens I must habe 0,71 cm/m Bas:315 or 0,5 cm/m Base 0
    0,5 cm/m Base 270



    For the Left Lens I must habe 0,71 cm/m Bas:135 or 0,5 cm/m Base 180
    0,5 cm/m Base 90

    But after the Lenses Was finished i get this:

    For the Right Lens I get:0,5 cm/m Base 0
    1,5 cm/m Base 270 so resulting prism is 1,58 cm/m Base 288



    For the Left Lens I get: 0,5 cm/m Base 180
    0,5 cm/m Base 270 so resulting prism is 0,7 cm/m Base 225


    I was going crazy, so i have take the same half-finished Blancs an take look what they measured
    at the PRP befor i will surfacing them. I was suprized because i have measured a vertical prism
    for about 1,0 cm/m base down. Both lenses right and left have for the Add of 2.25 a base down prism
    of 1,0 cm/m. So i took a look for more lenses of this Add and figured out there was about 1,0 - 1,2 prism
    Base down on them.


    So i make an next test:


    Test 2: I have calculatet the same RX with the RX Calculating software and put in the field Thinprism,
    the 1,0 cm/m Base down, that I have measured in the PRP point on the half finished blancs.
    I must put this with -1,00 in the software.

    My Idea was do get a prism at the PRP point to 0, so i have only the Prism that the client would have.

    SO i have finished them get this:

    For the Right Lens I get:0,52 cm/m Base 0
    0,50 cm/m Base 270 so resulting prism is 0,72 cm/m Base 316


    For the Left Lens I get: 0,50 cm/m Base 180
    0,49 cm/m Base 270 so resulting prism is 0,70 cm/m Base 136


    The Results was very fine for me so, but what do you think about. Is this the right way.
    Ok the lens thickness is not looking good, the lenses are thicker, but I have in the
    lenses the results that the client will have. And the co-worker have no problems with controlling.

    But is this Base down prism on the PRP normal in the half-finished Lens?
    If it should be normal, any ideas i can modify the calculation to get this compensated?


    I hope you can help me


    Rafael
    Last edited by Rafael; 09-03-2005 at 12:20 AM.

  2. #2
    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Virtually all progressive lenses will tilt in the blocking ring, towards the reading portion of the lens - that is, the steeper curve. Up to 1/2 diopter base down is not unusual, and some lenses tilt much more than that.

    I'm not sure what the limitations of your software are. Ours, for instance, lets you set up a table containing the unwanted prism that results from the tilt in the blocking ring (for each lens), and we take that into account in the amount of prism we block or generate. It sounds as though you're doing this manually by entering RX prism opposite to the unwanted prism you're finding.

    You may have no other solution available with that software.

  3. #3
    OptiBoardaholic
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    Progressive and Prism

    When blcking a progressive press the distance portion of the lens against the blocking ring. this way the calculated prism thinning will work properly. at least it does when I surface on my Coburn 113MX generator and Coburn RXP-II software

    Ed

  4. #4
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    I usually block everything by holding the fitting cross with my thumb. usually if i have a down prism, i keep the down prism axis down, and up for up. make sure you apply the same pressure to each lens. make sure the thickness adds up. usually what i do if im cutting the thickness to be: 3@3, i go 3@2.75 so in case if i have prism i can block it back up and stick it in the generator to knock it out. when my lab server was down i had to do the job of calculating cuts and it can be a pain. there's some formula to use. i have to find it again and i'll post it. but with progressives, press and hold the fitting cross and you should be in good shape.

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