Hey OB! In the systems for opthalmic dispensing there is a chart that explains the inset for different PDs and powers. For the life of me, I cannot find it online and I leant my systems book out to somebody. Any clue?
Hey OB! In the systems for opthalmic dispensing there is a chart that explains the inset for different PDs and powers. For the life of me, I cannot find it online and I leant my systems book out to somebody. Any clue?
I screwed up guys! It was a reading PD based on RX and distance PD. Hooooooowever…. Should these near PDs be used for bifocal segments also???Originally Posted by Jaketull
link to pic
https://imgur.com/a/JnpIUjx
Here is the page before the one you posted.
"After all of these possibilities, what is the most appropriate
way to determine segment inset? Here are some recommendations
for different situations. The idea is to
provide the best accuracy without making it too diffi cult.
Keep in mind that just using a PD ruler may not be the
most reliable method.
Recommendations for Finding the Correct Segment Inset
• When the working distance is normal (40 cm)
1. Measure the near PD with a pupillometer or a
PD ruler.
2. If the distance lens powers are high, use Table
3-2.
• When the working distance is less than 40 cm
1. Again, measure the near PD with a pupillometer
or PD ruler. Be certain to set the correct
working distance in the pupillometer before
measuring. When measuring with a PD ruler,
the dispenser must be at the shorter working
distance.
2. If the working distance is less than that allowed
for in the pupillometer, use Gerstman’s three quarter
rule (assuming adult PDs between 62 and
68 mm), or use Table 3-1.
• When the distance lens powers are especially high
1. If the working distance is normal (40 cm), use
Table 3-2.
2. If the working distance is closer than 40 cm, use
Ellerbrock’s formula. (Ellerbrock’s formula could
actually be used in any of the above
circumstances, but it is unhandy to work with.)"
If you want the exact inset, use ellerbrock. If you want approximations that work pretty well use the other appropriate method. There is a difference between inset and near PD. Near PD is used for Near only glasses at a specific working distance(sometimes fudged wider for myopes). Inset is used for multifocal lenses and may vary from near PD based on Power, vertex, and working distance.
Are you referring to Donders Table of Presbyopia?
Yup.
More info on segmented multifocals:
https://www.aao.org/bcscsnippetdetai...7-50e55cca7d65
Extra credit:
Note that in hyperopia, base out prism is induced behind the distance Rx as the eyes converge at near. How can the segment position be modified to reduce this very undesirable effect on the visual system?
Best regards,
Robert Martellaro
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.
Hi Robert,
Is this correct? (DPD-NPD) * ( Distance power in the 180 +add power)/10 =x
Add x to grind DPD
Subtract x for NDP and use for finish layout
Note this is a nested operation and should be repeated several times. But as a math teacher once told me that if a boy and girl are three feet apart and decrease their distance by half over and over they will never touch but will be close enough for a kiss. (Yes I know it’s not true but I was a romantic in 7th grade)
Great thread
I mean… according to the chart I just posted you have to move the segment further inwards to counteract the the unwanted prismatic effect, no?
thank you thank you thank you! I had read this so long ago and I knew there was a reason I associated this chart with seg insets. Turns out I just had to read one page before
dima,
We aim to please.
Chris,
How did Walman let you retire? Who does Norb have to talk to now when the going gets tough?
I was thinking along the lines of using excessive inset to create base in prism in the segment to offset the induced prism from the distance power. One could completely offset the distance induced prism, but that's rarely necessary unless prescribed (rarer still, if ever).
Practically speaking, I typically order IPDs for hyperopes with about 1mm more inset per eye- it looks more like 64/58 instead of 64/60. That's good for only .5∆ BI total with a 2.50 add, but it won't hurt, and might provide a tad more comfort for extended close work. Decentering 2mm/eye extra doubles the BI prism, but make sure the vertex distance and panto tilt is optimal to keep the temporal segment edge from infringing on the near field. Or use a wider segment as needed.
Best regards,
Robert Martellaro
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's the rub, though, right? If you get too insetty or outsetty you're going to reduce the binocular field of view through the segment (temporally, nasally, respectively). That is another factor that would have a limit.
Thank goodness for computerized PALs.
No, just to align the segment with the position of the convergent near-eye gaze, eliminating induced prism caused by an improperly positioned segment.
Yeah, but even the better semi-finished PALs had variable in sets by base curve, and if you wanted to, the lens could be turned before surfacing. Besides, a D28 has a huge near field compared to a PAL, with lots a room for positional tweaks. No such luxuries with PALs, which require plenty of skill to function reasonably well even under the most favorable circumstances, especially for those with functional and absolute presbyopia.
Best regards,
Robert Martellaro
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.
Since we have time to count angels dancing on heads of pins, I think it's like this:
Say we have a +4.00 distance power and a +3.00 add.
When someone converges through the distance portion (ignore the add for a minute), they'd be getting BO prism. OUCH! They'd have to converge a lot to keep the image single.
Through the add, however, the additional plus power TECHNICALLY shouldn't add any more convergence BO prism strain, because the geometric optical centers of the add are "in perfect position" because they've been insetted properly for the convergence for near, based on pupillometry/geometry (which is done without Rx correction/prism confounding of measurement).
BUT:
TEMPORAL to the geometric lateral center of the segment, the segment will add some additional BI prism (which cancels out some of the distance portion BI) which is bueno
ON-geometric center of the segment: no additional prism
NASAL to the geometric center of the segment, even more BO prism added (muy malo!).
This is an exercise in combined prism.
So what would a patient like that do? They'd automatically go to where there is the least amount of prism so they can converge less, and they will choose to be looking somewhere temporal to the geometric center of the segment. That is, they'd be looking somewhere between the outer edge of the seg and the dead center of the seg.
Hey, let's help! Let's decenter the seg outwards to give them a better width of view and keep their eyes away from the outer edges of the segs! Well, that won't work, because in doing so you move the optical center further out and it's a vicious circle.
Then hey, let's use a larger seg! Well, correct me if I'm wrong, but that exacerbates it, too, because it's larger/thicker and it will add more prism effect.
So I don't think (other than the minimal tweak) much can be accomplished. High power plus is always going to see the outer edges of the segs (relatively) and high power minus will be the opposite.
Please proof this!
Last edited by drk; 04-14-2022 at 03:33 PM.
Now, Robert, you're saying something entirely different, and I can never get this right. You're talking about the patient's view, or the apparent position of, say, a penny on a desktop, and how the seg is moved to be centered where the penny appears to be.
We've gone around with this regarding downgaze-induced BU and BD prism on PALs and whether the corridor should be raised or lowered.
We're doing the same problem horizontally, in the thread above.
I still don't know which idea is right: adjust the segment for the apparent location of the target, or they offset each other in a combined prism sorta way.
The size of the seg or the thickness of the seg will not alter the prismatic effect it is a function of the power. We used to have more problems of this type when executive type lenses were popular. The correct way to process those lenses was to place the distance o.c. 1 to 2 mm out from the near o.c. And move the distance oc as you would a sv lens for pd placement. The problem arises when the blanks were to small to decenter so the request was made to grind prism in the distance to maximize the size of the blank and the oc in the near portion would not move as much because the power is greater the same prism would have less effect on the decentration. Worse yet if the distance was minus and the total reading power was plus the near oc was so far out that the glasses were for the most part unwearable.
Lensman, you may be right, but there is such a thing as a plano prism...controlled entirely by thickness, of course. Right?
Prism angle, not thickness.
https://www.2020mag.com/ce/the-spectrum-of-prism-part
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.
A Plano prism is not controlled by thickness but by prism angle or by not having parallel surfaces but that does not come into play in this case as the segment is optically centered and contains no prism at the geometric center. Let’s not for forget about prism segs they could solve this problem when it comes up but expensive and do not have a large field of view.
Yes, the angle, got it (proportional to differential edge thickness between apex and base)(I'm used to looking at my bar prisms.)
But if you have a bulging D-35 with a fat ledge (thick at midpoint/optical center) and 17.5 mm away zero edge thickness (at the "seam"), compared to:
A not-so-bulging D-28 with a trim ledge and 14 mm away zero edge thickness...
...would you say they have equal prism effect?
Let's do it!
Suppose it's a +3.00.
For D-35@ 17.5 mm:
deviation = F x D (cm)
deviation = +3.00 x 1.75 cm
deviation = 5.25^
For D-28 @ 14 mm:
deviation = +3.00 x 1.4 cm
deviation = 4.2^
So...more prism for the thicker-ledged seg, right?
If I understand what you are doing you are moving a +3.00 two different amounts and correctly get two different amounts of prism. This has nothing to do with thickness only the fact that the larger seg gives you the ability to move it more and still give the patient some lens to look thru. Try this with an Exec where you have no size limitations as the reading is all across the lens.
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
Reply With Quote
Bookmarks