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  1. #1
    Master OptiBoarder OptiBoard Silver Supporter ak47's Avatar
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    pd on digital lenses

    On a digital lens, do you measure pd on the front side or back side of the lenses?

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Curious how much difference this could create and would you return them without having the patient try them?

    It seems the minuscule difference would be within tolerance but I'm willing to be corrected with nothing harsher than a wet noodle please.

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    OptiWizard
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    It surely depends on the Rx and base curve. Take an 8 bc lens in wrap frame 54 eye when you will find the front pd larger by 3 or 4 mm than when measured from the back. The question arises that normal practice says measure from the front and you will get push back from everyone. If you have a frame with a 4 bc and you put 8 bc lenses in the frame the frame pd will change and often you will end up out of tolerance when completed. There is a solution anyone want to to take a guess.

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    It doesn't matter, if you do it correctly. Or just do it simply. Line up the 34mm mark on your PD stick with the nasal engraving of the right eye lens, and measure to the nasal engraving of the left eye lens. Or use the zero of your PD stick doing to same procedure and add 34mm to your result. There is little to no wrap to interfere with the measurement between the nasal marks to make more than a hair of difference.

    Edit-added a comma
    Last edited by Kwill212; 03-18-2023 at 03:38 PM.

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    OptiWizard
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    Kwill
    i don’t think that was the question but I could be wrong. You are trying to get a three dimensional reading with a two dimensional measuring device.

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    You don't think the question was

    "On a digital lens, do you measure pd on the front side or back side of the lenses?"

    What do you think the question was if it wasn't exactly what was stated in the question? You seem to be talking about issues that arise in the surfacing layout of wrap lenses, and not how to verify the correct PD of finished spectacles.

  7. #7
    OptiWizard
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    Put a dot 17 mm to right of the nasal dot on both lenses and measure the distance between your result. This is where the eyeball is actually looking.

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    Quote Originally Posted by Lensman11 View Post
    Put a dot 17 mm to right of the nasal dot on both lenses and measure the distance between your result. This is where the eyeball is actually looking.
    Geez it's painful engaining with you. I regret it every time, but it seems like I'm just a glutton for punishment. I guess if we're going to go down this path we might as well go back to the beginning and ask AK47 to state exactly what they mean when they say "digital lens". Then we can go though the next 45 steps of lens making until we get right back to where we were 5 minutes ago.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Quote Originally Posted by ak47 View Post
    On a digital lens, do you measure pd on the front side or back side of the lenses?
    Troublemaker!

    I think we are talking about an etching on the outer surface (convex side) versus the inner (concave) surface where there is no etching, so the apex of the cornea is looking through the optical center based on the middle of the lens thickness. Or not...

    The difference of that thickness affecting the pd would be minuscule imo.

    But I still always put the glasses on the wearer first whether I think they're right or wrong and see if they complain!

    I'd troubleshoot from there where this this would be one of the last arrows from the quiver I would target as the problem.

    Today's digital lenses make me assume if my entry is right the lens power and OC's placement is not wrong.


    Now be nice kwill if I'm wrong.

    Eventually even wet noodles from guru's can hurt...

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    Quote Originally Posted by ak47 View Post
    On a digital lens, do you measure pd on the front side or back side of the lenses?
    For all lenses, for verification, place the manufacturer's mask on the front of the lens, then on a centering/cutout chart (see below).

    The final evaluation is with the adjusted/fitted eyeglasses in front of the eyes using a light/corneal reflex.

    https://www.essilorpro.com/content/d...91_PRO_VAR.pdf

    Hope this helps,

    Robert Martellaro
    Roberts Optical Ltd.
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    www.roberts-optical.com
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  11. #11
    Master OptiBoarder OptiBoard Silver Supporter ak47's Avatar
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    Okay, original poster here, my concern is mainly about wraps in stronger Rx. A couple mm can make all the difference between base in and base out prism. I believe we are supposed to measure digital lenses on the backside, I was really hoping that someone with more knowledge than me could confirm that.

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    Quote Originally Posted by ak47 View Post
    Okay, original poster here, my concern is mainly about wraps in stronger Rx. A couple mm can make all the difference between base in and base out prism. I believe we are supposed to measure digital lenses on the backside, I was really hoping that someone with more knowledge than me could confirm that.
    Powers don't matter, although the fitting point and PRP tolerances are not as tight on a low power SV lens.

    A little more detail from Barry.

    https://www.optiboard.com/forums/showthread.php/55691-How-do-YOU-measure-PDs-on-wrap-sunglasses
    Last edited by Robert Martellaro; 03-22-2023 at 01:06 PM.
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    What's up? drk's Avatar
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    I need an education, here. Please help an old guy out.

    Question #1: What is the practical difference between measuring the distance between two points separated by, say, the thickness of 3 millimeters? I would think measuring two dots on the front of a wrapped frame would be almost exactly the same as measuring the inside.

    Question #2: Is it even practically possible to measure the linear space between two points on a curve? That would be like a chord (from geometry), right? Wouldn't there be parallax error because the flat ruler would hover over the gap between the ruler and the curved-away-from-you lens? (And bending a flexible ruler just takes you into a different, warped-space universe, so that's a no-no.)

    Question #3: Isn't such stuff modified, anyway, with the software when the lenses are made (if you do a specifically POW-compensated lens design)? I mean, you give the (admittedly imperfect) pupillometer readings and they know how to manipulate the optical centers? (Hint, they have to move them out about 1 mm per lens with ~15-20 degrees of wrap, for those who like to do their own wrap-designing.)

    Question #4: In my world, there's no sense in "verifying" measurements when you order a POW lens. The powers won't match. The optical centers won't match. It's a thing I just take on faith.

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    What's up? drk's Avatar
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    Robert, you're always coming up with something interesting!

    So, you are introducing a method that works on aligning the pupils' optical axis (I think that's what you find from a corneal reflex) and the lens' front and back surface reflections? (The back reflection would be of variable size depending, I'd imagine.)

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    Quote Originally Posted by drk View Post
    Robert, you're always coming up with something interesting!

    So, you are introducing a method that works on aligning the pupils' optical axis (I think that's what you find from a corneal reflex) and the lens' front and back surface reflections? (The back reflection would be of variable size depending, I'd imagine.)
    The corneal reflex and a reference point on the front of the lens. Also used for measuring IPDs. I used the device below to assist with determining the IPD using a light for the corneal reflex. I would transfer that data to the lens on the fitted frame (before the lab work) to determine IPDs and fitting points. Essentially the verification process in reverse.

    Hint: when using a centering chart and/or the pd meter shown below, occlude the fellow eye and compensate for parallax error when there are significant differences between the fitting point value and the measurer's IPD.
    Attached Thumbnails Attached Thumbnails PD Meter.jpg  
    Roberts Optical Ltd.
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    OptiWizard
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    The device is called a pd light and meter never widely accepted but very accurate. In the days before implants we had to make post cataract lenses with +12 thru +20 front curves. The measured pd would always be wide on the convex side. No one was interested in the back side measurement. We actually had metal pd rulers with different base curves like +12 +14 +16 to measure the frame so you decenter correctly at least it satisfied our customer. I don’t if it is optically correct but that is what was done.

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    Quote Originally Posted by Lensman11 View Post
    The device is called a pd light and meter
    Here's the actual light that came with it. Made by a Minneapolis Co. Precision-Cosmet.

    https://americanhistory.si.edu/colle...ILNMAHTL_37149

    I had fun using it on kids after Men in Black came out- I would tell them "this won't hurt a bit... and even if it did you won't remember it".

    Best regards,

    Robert Martellaro
    Attached Thumbnails Attached Thumbnails grolman.jpg  
    Roberts Optical Ltd.
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    Experience is the hardest teacher. She gives the test before the lesson.



  18. #18
    What's up? drk's Avatar
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    Robert, I heard about "the Grolman" in school about 1987 and it was obsolete even then! You're talking something that's probably 45 years old!

    In essence, you have a corneal reflex pupillometer without the box, right?

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    OptiWizard
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    The pd light and meter that I have is at least 60 years old. It was made by precision cosmet but if my memory holds if was patented by House of Vision a retail optical chain.

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    Quote Originally Posted by drk View Post
    Robert, I heard about "the Grolman" in school about 1987 and it was obsolete even then! You're talking something that's probably 45 years old!

    In essence, you have a corneal reflex pupillometer without the box, right?
    Right, minus the mirrors and the associated increased functionality. I used this in the 70's- very few frames were glazed at the time, making it a much better system for positional measurements than tape and plastic devices attached to the frame fronts.

    https://www.visionmonday.com/scene-a...gressive-idea/

    Quote Originally Posted by Lensman11 View Post
    The pd light and meter that I have is at least 60 years old. It was made by precision cosmet but if my memory holds if was patented by House of Vision a retail optical chain.
    Not sure of patents but Benson Optical had one in all or most of their offices. Bensons acquired House of Vision in 1982. HOV did have their own distometer.

    https://imgur.com/a/uDZwmtu

    Both companies were based in Minnetonka and owned by Revlon before they went into GE medical pension, then acquired in 1992 by Martin Franklin, yes, that Marty Franklin. Bensons was essentially bankrupt, and was on the brink of losing the building housing the lab for 180 locations and their home office. Story goes that Franklin just sat down, pulled out his checkbook, and wrote an 8 mil check to buy the building outright.

    The next 4 years were gold for the very earliest investors, but the last year was a living hell for the employees. About 50 of us (out of probably hundreds) acquired legal representation to retrieve back pay and benefits.



    Roberts Optical Ltd.
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    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.



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    Does the optical center come into existence in the universe due to the front curve, the back curve, or both curves working together in harmonious optical centrion birthing?

    Perhaps a more entertaining answer would be found from the question; How do the optical centers differ on a digital lens that would inspire one to ask this weird question?

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    Measure the distance between the pupils of your right and left eyes starting with the right one. The measurement you need is the millimeter that corresponds to your left pupil. Your PD is represented by that number. Note it down.

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    Quote Originally Posted by Rodney Elmer View Post
    Measure the distance between the pupils of your right and left eyes starting with the right one. The measurement you need is the millimeter that corresponds to your left pupil. Your PD is represented by that number. Note it down.
    Sir, this is a Wendy's.

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    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by Rodney Elmer View Post
    Measure the distance between the pupils of your right and left eyes starting with the right one. The measurement you need is the millimeter that corresponds to your left pupil. Your PD is represented by that number. Note it down.
    When I want to measure my own pd I just run, eyes wide open into my sliding glass door and measure the difference between the center of the splat’s my eyeballs left on it. ( I do measure from right to left too!) no need to note it down though….I just don’t ever wash my glass door. I leave the splat marks on it as future reference when needed. ( It also reminds me not to do that again…)

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    What's up? drk's Avatar
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    I just do this:

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