I would split the difference in a progressive. If they could be 4 mm apart but have been previously wearing them at same height OU, I would go 2mm apart. I would certainly not say anything.Originally Posted by edKENdance
If they came back a year or two later for another set, I would consider increasing further.
Paul,
If you mean without eyeglasses, I'll have to leave that to the vision experts to discuss. But I suspect that it wouldn't be a concern unless there was an injury to the the orbital area causing the eyes to suddenly misalign. If you mean prescribed prism to counter induced VI from improper lens positioning, read on.
It's the optician's job to eliminate VI whenever possible. For example, Rx is +10 sph OU. The right pupil is 2mm higher than the left pupil, and the frame is pre-fit. If we do not align the right OC 2mm higher than the left OC, the wearer will experience 2∆ of VI.
This is pretty close to our fusional reserves, and unless habitual, may present symptoms. I have never seen a prescriber introduce prism, in the this case, 2∆ BU OD, with the assumption that the fitter is unprepared to position the lens correctly.
Best regards,
Robert M.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
That is exactly the issue to what Dan is addressing.
I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain
It's the optician's job to eliminate VI whenever possible. For example, Rx is +10 sph OU. The right pupil is 2mm higher than the left pupil, and the frame is pre-fit. If we do not align the right OC 2mm higher than the left OC, the wearer will experience 2∆ of VI.
Robert, no discord with the above, as I agree that we are here to provide our patients with the best option to maximize their VA. I just don't see the necessity of split seg hts. for most PAL wearers. When fitting patients for eyewear I tend to align the frame to the wearers eyes as they relate to the superior portion of the eyewire and not always their brow. If we have to justify a split seg ht then I use a fixed corridor PAL design, but these are more of an exception to the rule, at our practice.
Cheers
I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain
OK, my turn to play Devil's advocate. Do you use binocular or monocular IPDs? Sauce for the goose...
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
REALLY!!? ...whats an IPD, I use binoculars at the stadium, my side kick LDO uses a monocle, where are you going with this Robert, or should I call you Beelzebub. I use a digital pupilometer to gain the corneal apex reflex for the individuals mono IPD, I also use a, Haag-Streit, distometer for Vertex measurements and to administer a slight static jolt.OK, my turn to play Devil's advocate. Do you use binocular or monocular IPDs? Sauce for the goose...
I didn't attend the funeral, but I sent a nice letter saying I approved of it. Mark Twain
Not fallen, but my halo is plenty bent.
Let me rephrase- if the horizontal meridian deserves the red carpet (monocular measurements), why not the vertical meridian? Don't we want the eyes to track downwards through the corridor equally, encountering the same power at all angles of downgaze? Doesn't this also improve horizontal symmetry, which has been shown to improve binocular function with PALs? Do the advantages overcome the disadvantages, pretty much limited to extra time and effort, and a slight change in spatial perception and prismatic effects, which are temporary and not enduring?
Envision a 7x28 trifocal fit level with a horizontal line tangent to the bottommost part of the pupil. Assuming a differential of 2mm in the height of each eye, using equal seg heights, which eye should be fit 1mm or 2mm above or below the bottom of the pupil, and why might it be better to match the position of the segment line with the position of the eyes?
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I wore a +2.00 for about 2 years in a PAL to vet myself. Even without actively needing a PAL, I know how to fit them and love to make sure patients get the best vision through their lenses. PAL's are a different animal and since they are indistinguishable from a SV lens to the consumer differing seg heights and accurate fitting a good idea to improve the vision, but with lined multifocals COSMETICS must be taken into account, I have been burnt one to many times by trying to be accurate when the client was never experiencing issues due to the imbalance. I find the KISS principle applies well to lined multifocals, often times I find that even wearers with high Rx's that should be wearing a slab have more issues when I give them a slab since they have spent most of their lives suppressing and the minute I clear the lens up they start to experience double vision. As human beings we have ways of compensating for these things. Doctors have enough knowledge and experience to know that they need to often cut the astigmatism or power changes in older adults to avoid non-adapt issues. Opticians learn the same with experience. Your post seemed very raw and vague so I thought I would offer a bit of depth into my earlier explanation.
http://www.opticians.cc
Creator of the industries 1st HTML5 Browser based tracer software.
Creator of the industries 1st Mac tracer software.
Creator of the industries 1st Linux tracer software.
I don't believe that it's necessary that we wear the lens design before it can be fit and designed it properly. Education and training are necessary though.
Consider a switch to PALs if cosmetics are the priority.
If the client is new to me, or the change from equal to dissimilar segment heights is new, I cover my backside by telling my clients why I'm making the change, leaving no doubt as to the appearance of the lenses at dispense. Insets should also be handled in the same fashion: equal, or unequal, as needed, aligned with the eyes, compromising only when the disparities are large- where undercorrecting may be necessary to keep the nasal edge of the segment from being significantly truncated by the bridge. Never, ever, overcorrect.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Spot on, just because someone has asymmetric pupil heights, does not mean they have diplopia.
Just like tilting your head doesn't cause diplopia.
My biggest problem with non symmetrical seg/fitting heights is we are assuming that the accommodative/convergent pupil heights are the same as they are at infinity, which is not always true.
also, if you use an outside lab, they can be off by 1mm per eye. So if you are starting at a 2mm difference, the lab can cut it with a 4mm difference, and it technically will still pass final inspection.
This has to be quite rare! Do we call this nonconcomitant vertical tropia? Regardless, they probably won't be wearing multifocals.
The tolerance is ±1mm for each eye, but the difference in vertical fitting point height should be no more than 1mm. I'm getting zero difference for fitting heights when specified as such, and less than one degree off-axis segment line and reference marks from Walman, Oak Creek WI.also, if you use an outside lab, they can be off by 1mm per eye. So if you are starting at a 2mm difference, the lab can cut it with a 4mm difference, and it technically will still pass final inspection.
http://www.opticampus.com/tools/ansi.php
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
haha, I'm not sure I would call it rare. Just not common among PAL wearers, or not easily identifiable given the type of equipment opticians use on a regular basis for PAL fitting.
Maybe most closely related to convergence disorders.
I actually had to look up the z80.1 tolerances, and not just the summary, to make sure I am remembering things correctly.
As I read it, Vertical imbalance/prism only applies to binocular seg/fitting heights.
"6.2.3.2 Progressive Addition Lenses
- The vertical location (or height) of the fitting point for each progressive addition lens shall be within ±1.0 mm of specification. In addition, the difference between fitting point heights for the mounted pair shall not exceed 1.0 mm of specification. Measurement shall be made using the method in 8.7.
The horizontal fitting point location in progressive addition lenses shall be within ± 1.0 mm of the specified monocular interpupillary distance for that lens. Measurement shall be made using the method in 8.7. "
Not that I personally would pass a pal with segments being off 1mm in each direction.
But it does not say 1mm sum of differences from each specified height(perhaps it should?).
I can only read that only one way. Here are a few examples. If I'm off-base, please give me a heads-up before the Google bots write it in stone.
If we order a fitting height of 20mm OU, 21/21, 21/20, 20/19, 19/19, 19/20, and 20/21 are all within tolerance. 21/19, 19/21 exceeds the 1mm difference and should be rejected.
If we ordered unequal heights of OD 22mm and OS 20mm, 23/20, 22/19 and so on would pass, but 23/19, 21/21 and so on would fail.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
after rereading your quote of my quote of 6.2.3.2
It could, and should be interpreted as sum of differences.
I understood your implications from the beginning.
But I know and have worked with techs who would pass over half of your examples.
Simply because that quote is not present on most ANSI z80.1 spec summaries/tables.
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
Reply With Quote
Bookmarks