from the inside or outside???
from the inside or outside???
I think you mean "verify" PD. Good Question!
B
Exactly :)
What is your method? and why do you favor one over the other?
For all eyeglasses, I start with a centering chart...
http://www.variluxusa.com/SiteCollec...hart-Final.pdf
...or its equivalent.
But the final evaluation is with the adjusted/fitted eyeglasses in front of the eyes with a light/corneal reflex.
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.
Allow me to clarify.Originally Posted by Robert Martellaro
Using a ruler,how do you verify the PD measurement of a wrap Sun Rx job coming from the lab before dispensing to a patient?
From the inside, if all you have is a ruler.
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 have a flexie ruler.
Ha! Good for girth, or maybe mirth!
Jonah,
You'll get a an incorrect measurement (on the high side) if you bend the ruler across the front of the lens, if the front of the lens is curved.
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.
Yup, on 60 eyesize 25 degree wraps you'll err as much as 5mm per eye with the ruler bent against the lens rather than "boxed" straight.
if I could piggy back on this topic... say you have a PD for a daily pair and they get a Sun pr. with a high wrap, I know there is a default pd offset due to the wrap but I can never remember what it was. like you add 1mm from regular pair or subtract. Im guessing you add.
Hi Robert! Thanks for the replies.
I never suggested bending a ruler, as the inaccuracy is inevitable.
I still did not hear any clear answers on how to measure the PD (without bending of course)
Rule of thumb for wrap PD:
10 degrees: Decenter each lens 0.5mm out in the blocker
14 degrees: Decenter each lens 1.0mm out in the blocker
18 degrees: Decenter each lens 1.5mm out in the blocker
21 degress: Decenter each lens 2.0mm out in the blocker
Barry
PS - NOTE: Assumes blocker is NOT in PD compensation mode
Last edited by Barry Santini; 09-13-2013 at 03:00 PM.
Printed and Hung on wall! Thanks Barry.
How much base in prism do you decenter for parabolic wrap?
Your welcome!
I didn't know you didn't know! (I meant to say that I didn't know that you knew, so now I know!)I never suggested bending a ruler, as the inaccuracy is inevitable.
Post #4. Don't use a ruler.I still did not hear any clear answers on how to measure the PD (without bending of course)
Barry,
Is that the same as the BI prism from the POW Rx compensation?
Last edited by Robert Martellaro; 09-12-2013 at 05:42 PM.
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.
Wait, I need help on this.
1. Do you measure and supply the normal p.d. on wrap frames? Yes, right?
2. When the lens is tilted mounted in a frame, the optical center moves "in", right? It's moving "back" in an arc relative to the flat spectacle plane. So, the "effective position of the optical centers" becomes squeezed, or inset, right? Hence, Barry's formula to "project them forwards" or to "flatten it back out" by outsetting.
3. If you use a straight ruler, you are inaccurately measuring a 3-D world with a 2-D tool. But when you bend a ruler, you're still getting a true measurement, I'd think, right? If the maker's OC compensation is correct, doesn't that make the bendy ruler correct, too?
I think the outset of the OCs would be negated by the bending (shortening) of the scale.
IT'S LIKE EINSTEIN'S THEORY OF SPECIAL RELATIVITY! <Head exploding>
Ok. It has occurred to me that a bendy ruler will measure the compensated p.d., not the specified p.d.
neither
b
No. this to compensate only the edging/blocking PD in a blocker that DOES NOT or is not set for compensating wrap PDs
B
What PD do you use when ordering the lenses? Do you consider that u block out?
You'd have to know if the lab compensates or not, right Lenny?
if you are not edging yourself, always supply the real, unmodified PD
B
Barry, when you said "neither" above, what did it refer to???
So there are "smart" blockers these days? I remember doing it manually with a Viewmaster or something. Man, I need to visit a lab sometime.
okay......
So after 24 posts I still do not have an accurate method for verifying PDs.
I think my question may have been misinterpreted...
After dotting the FINISHED pair of wrap Rx in the lensometer, how EXACTLY do you verify the PD (O.C to O.C).
I've heard not to bend the ruler, however, that leaves me with what NOT TO do, not what TO do.
Without bending the ruler there is still a discrepancy of a few mm when measuring front vs. back.
So??
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