Received a job back from WileyX and the PD was like 15 off. Sent job back to be fixed and they gave our optician some line about "the job is fine. The PD's read differently than regular lenses."
My digital experience is very limited, but this sounds like total BS to me. What do you think ?
RX is like +2.00 OU so 15 is unacceptable.
They could have prism to compensate for the high base curve. This would look like PDs being off in your lensometer. Check the compensated Rx specs on your tray sheet.
Even with wrap comp the PD should be in no more than about 1.5mm each eye for that power ( .2 to .3 BI per lens). Insist on a remake at 15mm off which would be 3 D horizontal prism total.
this,Originally Posted by cowboytelemark
need the compensated Rx or use the PRP to measure PD
I don't care how much wrap comp you throw at that Rx. 3 D. of prism is too much!
agreed,
however, if there are digital lenses, either they were cut eye for eye, or the frame is super small.
Otherwise the job wouldn't have cut out.
Check and measure from the PRP.
If using a PD stick, I find you get more accurate readings by measuring from the back of the lens.
If using a layout chart, proceed as normal.
Let's separate the base in prism needed to offset wrap angle Rx effects from the need to "block" a wrap job to a "wider" PD to compensate for the fact that blockers normally see only "flat" wrap angles.
B
You would think Wiley X would be used to blocking wraps.
Do you bend the ruler to measure PD?
This is a great point. Prism due to lens tilt is one thing.
Decentration of the OCs outwards on a convex curved surface is actually placing the OCs in front of the eyes. Sounds crazy, but it's true.
I don't think so. You'd probably need to stick a theoretical pin through each OC and look over the top of the wrap frame. Measure how far the pins are apart with a flat ruler just as they emerge from the back side of the lens.
Now, I just made that whole thing up and I really don't know how close that is.
Barry!?!?!?!?
No. Major errors. If all you have is a ruler, measure from the back.
For all lenses, use a centering chart to verify fabrication accuracy.
http://www.luzerneoptical.com/pdf/va...tout-chart.pdf
Then follow up on the face with a penlight/corneal reflex to verify the opticians' measurements.
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.
When Darryl was still alive we discussed this example ad naseum. The lens tilt should be compensated for the PD, we finally agreed on the:
sin(theta) = PD Error / Center of Rotation
as the best method, although my argument was that the error due to the PD error was such a small angle even at great wrap angles that the radian of the angle could be subbed in without much degradation or even the tangent used without any noticeable effect. What I helped enlighten on was the various bevels used by different labs for a wrap job. For instance if a lab were to apply a 60/40 bevel on a lens, then we would have to measure the nasal and temporal thicknesses and then take the difference in bevel placement as a reduction in faceform measurements. This would apply on the vertical meridian as well with PALs being problematic. The angle that would be used to compensate the wrap would be:
sin(theta) = (nasal thickness - temporal thickness) / A Size
The wrap angle would be reduced by this angle when the temporal thickness is greater than the nasal:
Lens Tilt = Frame Tilt - above comp
This is in addition to the PD error not in place of.
Some labs use a shelf bevel in which case the wrap angle would be in full effect, some labs I have seen actually move the bevel all the way to the back of the lens leaving most of the lens substance hanging out of the front, not the most cosmetically appealing but it prevent frame splay and significantly reduces the wrap measure in some high minus cases completely negating any wrap calcs from being necessary.
The blocker prism is an interesting conundrum, I have been cutting my teethe on that one for a few years now. Most blockers now are a camera based system and depending on where the camera is mounted and whether mirrors are used or not the old methods of prism blocking just really don't apply anymore. A newer methods needs to be developed, however the variable is going to be the machine design more than a simple calculation. Of course if you are still using the tried and true simple blockers then the formulas are documented in Understanding Lens Surfacing by Brooks, however be careful his book is filled with typos and errors and his corrections have never went to reprint but you can get a spiral bound from Illinois if interested.
http://www.opticians.cc
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So what you're saying is that "p.d. error" (not the gross fabrication error in the original post, of course) induced by a 20 degree or more frame wrap is one of those errors that are theoretically possible, but practically difficult to offset because it can be affected by other, larger variables?
For example, the minute the frame unwraps itself on a fat head, the error changes?
For example, lens wrap not equaling frame wrap due to beveling preferences making a "cookie cutter calcuation" (such as, 1mm decentration out per lens 20 degrees of wrap) practically worthless?
Yes
That's why I avoid memory metal bridges- the wrap angle tends to be non-existent or negative, and somewhat unstable. Not a problem with +.50's, but I don't see too many of those. We could measure the wrap angle in-situ, with an edge to digital centration devices, although a zero wrap angle is not too hard to observe with just our 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 constantly ask my coworkers (Did the frame fit like this? or did it stretch out?) about the fathead wrap compensation variable curve.
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