Robert M has answered thus question:
Use a protractor and/ or a penlite insitu
B
Originally Posted by jonah
Robert M has answered thus question:
Use a protractor and/ or a penlite insitu
B
Is this what you mean??
I'm starting to get this.
We're all "X-Y coordinate" about this stuff: P.d. rulers. Lensometers. Coplanar ocular entrance pupils. Ophthalmic frame fronts.
We routinely ignore that there is a "Z" dimension: Lenses tilt around a horizontal and vertical axis (and so do eyes, but let's not get stupid). We can't measure a three-dimensional effect with two-dimensional tools.
That leads me to think I understand the "ray tracing" concept: it's a 3-D computer-animated tool that shows the effect of lens design in virtual space.
So, the simple answer, jonah, is that you have to trust the labs to compensate properly.
"To trust in the labs" can only go so far.
What will happen when there is a non adapt? your troubleshooting won't be able to consider the PDs as the culprit.....
There needs to be a set in stone way that both labs and ECP can agree on, otherwise you will be arguing with the lab all day.
Don't get really nuts over all this. Centration to an ultra-precise degree only gets important with progressive intermediate zones,and in adds over 1.50 in wraps
b
My question was posed only because of real scenario that took place where we ordered a PD of 30/30 on a PAL for a wrap, and when verified from behind we found a 32/32. Measured with a ruler on the front we got 30/30
Is it within tolerance or not?
Tolerance is determined by the resultant prism, not by MM until very high powers that are probably out of range for most wrap frames.
My method:
*Dot lenses like normal
*Check with layout chart looking through the backside
*If there is a difference dot at desired PD with a pen
*Check the prism at this reference point.
Use normal tolerance chart from there.
I'm curious to hear from some wholesale lab folks how they verify before sending out to their clients...
Again, there are two ways to verify: 1 Use a progressive verification protractor and sight the marks made on the lenses from a bird's-eye position above the lenses, from the rear, with the eyewear place and centered on the protractor. This verifies if the eyewear is made according to the measurements supplied.
2.Use a penlight and sight the pupil of the wearer with the eyewear in place. align your sighting eye directly in front of the wearer's pupil, and instruction the wearer to fixate on your pupil. This verifies if the eyewear is aligned to the wearer's pupil.
Barry
For starters, did the order originate from you. If so, you have the IPD that was taken. Spot the OC's, using your "flexy" PD ruler measure the distance between the OC's. If you are selling wrap product I am assuming that you are utilizing someones panoramic angle measuring tool/chart, measure the lens pano angle. Using the chart, Barry so kindly provided you with, determine the additional amount of recommended horizontal decentration. Subtract that amount from the measured PD and you should be able to come pretty damn close to the given PD; provided the lens blank used was able to work within the context of the frame, Patients RX, and allowed decentration.
To answer OP's question. Id put a ruler starting on center of bridge and 90* to the arm (perpendicular). then hold it up and line it up to your eye and eye ball it, no pun intended.
OK Jonah, develop a pin size suction cup/dart looking thing that you can place right over the OC, place the top of the frame down on the table and walla there you go. Now use Prentice's rule to calculate the amount +/- to induce the appropriate amount of base-in prism compensation, and verify the PD. Or put them on the patient and ask them "How many of me do you see?"
Measure the PD from outside (from inside it is also possible, but it is much complicated). Mark an optical centers in the focimeter, put the frame on the ruller (that lies on the table) and by "perpedicular" look from the top read the value.
By wrap lenses you need to be carefull where the optical center really is - when the lenses are compensated, in the "optical center" you may have some value of prism, that is being made from calculation. The value of prism should be given by lens producer.
As Barry pointed out, some companies (or their's blockers) use the Rule of thumb for wrap PD so you should lower the measured PD by given values.
On some types of focimeters is a function of PD measurement, but i do not trust the results at all.
The design of progressive lenses is a bit different from design of SV lenses, so i think that amnout of prism that occured by decentration by progressive lenses cannot be compared to the same situation as with SV lenses (intermediate zone will suffer). Rule of thumb for wrap PD says, when the pantoscopic tilt is 21 degress, you are in tolerance.
Last edited by essegn; 10-05-2013 at 04:03 AM.
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