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Thread: bicentric grinding

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    OptiBoard Professional skirk1975's Avatar
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    Confused bicentric grinding

    Can anyone explain the bicentric grinding concept and its uses?

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    registeredoptician Refractingoptician.com's Avatar
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    Last edited by Refractingoptician.com; 02-23-2007 at 11:20 PM.

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    Master OptiBoarder Jeff Trail's Avatar
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    Re: bicentric (slab-off) grinding

    Gary said:
    In a bifocal, if there is a significant difference in power measured in the 90 degree axis, between the right and left lenses, then the patient will not be able to read properly due to a condition called "vertical imbalance"

    Vertical imbalance is corrected by a special grinding technique called "slab-off" or bicentric grinding.

    Vertical imbalance only occurs in bifocals !
    Gary did a good job but left out the "reason" why you have that problem, on any lens that has a seg, the seg sets at a different "angle" and magnification from the full field of the lens, in in that seg you have a seperate optical center from the distant opticatal center... and when the OC's maybe alligned in the distant portion as you travel downward to the seg that oc will cause it to appear to seperate images do to the amount of total power..induce prism :-) also referrred to as vertical imbalance..
    That is why , as mentioned by gary, using dissimular seg types works..as an example a FT28 the seg O.C. is 5 mm below the seg line while the rd 22 it is 11 mm below the seg line.. so it makes a natural "slab" effect.
    Good job Gary :-)

    Jeff "grind'em if ya got'em" Trail

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Re: bicentric (slab-off) grinding

    Gary said:
    Vertical imbalance only occurs in bifocals !
    Well, that's certainly untrue; vertical imbalance can most certainly occur in single vision lenses, whenever the patient's gaze passes above or below the horizontal meridian on which the prism reference point lies, given dissimilar powers in his two lenses.

    I've seen single vision Rx's for slab-offs; presumably, patients who are naturally inclined (or for some reason required) to drop their gazes, as opposed to their chins, to read, might require these.

    The presence, or absence, of a segment has nothing to do with the inducement of the prism; which is to say, slaboffs can be every bit as necessary for patients fit with progressives as those fit with flat-tops or other "segmented" multifocals, though it may make less sense cosmetically (and therefore, economically).

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    OptiBoard Professional skirk1975's Avatar
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    Smilie thanks

    So it is safe to say that bicentric grinding is another name for a slab-off prism?

    Steve

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    Bi-centric grinding is another name for slab-off prism.:)

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    registeredoptician Refractingoptician.com's Avatar
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    Last edited by Refractingoptician.com; 02-23-2007 at 11:20 PM.

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    registeredoptician Refractingoptician.com's Avatar
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    Last edited by Refractingoptician.com; 02-23-2007 at 11:21 PM.

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    Forever Liz's Dad Steve Machol's Avatar
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    Gary said:
    How do you guys &( girls) make that pink box thingy appear at the top of the reply with the previous quote inside it ?
    Click on the 'quote' button.


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    registeredoptician Refractingoptician.com's Avatar
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    Last edited by Refractingoptician.com; 02-23-2007 at 11:21 PM.

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    As long as we are on this subject, how often do you check a S-V job from the lab for vertical imbalace? I have been told that half a diopter is too much, but I see a lot of jobs come from very good labs with more than this. Sure most opticians check the P.D. for horizontal imbalance (which is not nearly so devastating to the patient) but how many check for verticle imbalance?

    Chip

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    Master OptiBoarder LENNY's Avatar
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    We never check for vertical imbalance but if you do check the power of the lenses and do it wright you will see the vertical imbalance right away in the lensometer!

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    Last edited by Refractingoptician.com; 02-23-2007 at 11:21 PM.

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    Master OptiBoarder Jeff Trail's Avatar
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    Re: V.I.

    Gary said:
    We check EVERY job...horizontally and vertically...and we reject them regularly.

    Be very careful about that rejection rate :-) I have had on a number of occasions the optician send me back (as well as OD's) jobs telling me "no good, vertical imbalance" because they check every job exactly the same instead of going from the most plus to least plus or dominate to weak instead of right to left all the time.. so say I have a lens with hardly any power in the 180 degree merridian and check that lens first.. I could run it up and down the lens and the optical cross hardly moves... than cross it over to the other that has power and tada...it "appears" as vertical imbalance when technically it is not :-)

    Then again maybe you do have a bad problem, which NO one should have with it being so easy to spot a lens OC and block it correctly...if so it's just sloppy pathetic finsihing...

    Jeff "sometimes my fellow lab rats make me feel so ashamed" Trail

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    Master OptiBoarder Jeff Trail's Avatar
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    Bev Heishman said:
    Bi-centric grinding is another name for slab-off prism.:)

    Bev,

    To make it even more "fun" now he gets to figure out working slab or reverse slabs as well :-)


    Gary,

    Slabbing a SV is something that is far and few, in my couple of decades in lab I probably could count the number of SV slabs I've done on both hands and still have some fingers left over.. :-) Most often it went down to a restricted motion of the person and they could not move their head was the culprit for making us slab them in SV

    Jeff"boy, what a can of worms" Trail

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