# Optical Forums > Progressive Lens Discussion Forum >  Measuring PD for progressives

## QDO1

We all know we are supposed to measure Mono PD's for progressives, but how

There seem to be a few schools of thought.. 

Dot the lenses, and measureUse a pupilometer and measure the purkinje reflex, and call that the mono Distance PD's (always seems less than the regular ruler method)Measure mono PD's on a rulerMeasure the near centration with a pupilometer and add 2.5 mm 
What is the best way, right way, and why?

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## Jedi

I do a combination of the first two, with the addition of marking up the existing pair. 
I'll measure with a pupilometer first, apply those measurement to the cull lens and do a mock seg ht. I look at the existing pair and ask about previous satisfaction with the lenses. Adjust markings if necessary and confirm the measurements by have the client try them on again. A few additional seconds of making sure a progressive is fit proper goes a long way.

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## ziggy

All our valid ways tomeasure for a PAL, I think what ever works best for you is the way to go. I only use a PD stick and have very few remakes, but my boss only uses a pupilometer with the same results. One thing I've notice is every one is thinks their right.:bbg:

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## For-Life

> We all know we are supposed to measure Mono PD's for progressives, but how
> 
> There seem to be a few schools of thought.. 
> 
> Dot the lenses, and measureUse a pupilometer and measure the purkinje reflex, and call that the mono Distance PD's (always seems less than the regular ruler method)Measure mono PD's on a rulerMeasure the near centration with a pupilometer and add 2.5 mmWhat is the best way, right way, and why?


I prefer the second option for several reasons.  First you are getting a fair split.  If you do it by dotting the lens or by  mono measure on ruler you will not get an accurate split.  The final way is the absolute worse way, because if you take any PAL and move it right to left even very slightly and you will get distortion.  Therefore, it is extremely important that the distance is set perfectly.

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## QDO1

> All our valid ways tomeasure for a PAL, I think what ever works best for you is the way to go. I only use a PD stick and have very few remakes, but my boss only uses a pupilometer with the same results. One thing I've notice is every one is thinks their right.:bbg:


I think the same. but as an experement I have been using all the methods, and the varience is sometimes up to 3mm per eye..sometimes the reflex on a pupilometer is so far IN it seems unreal compared to the dotted up method. Being a anesiemetripic optician with a lazy left eye - I prefer the pupilometer, especailly as my PD is 68... but being a bit long in the tooth, I can work round my visual failings, and measure with a ruler/dots spot on. some manufacturers prefer the measure the near pd and add 2.5... but this method statistically seems the most off

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## For-Life

> I think the same. but as an experement I have been using all the methods, and the varience is sometimes up to 3mm per eye..sometimes the reflex on a pupilometer is so far IN it seems unreal compared to the dotted up method. Being a anesiemetripic optician with a lazy left eye - I prefer the pupilometer, especailly as my PD is 68... but being a bit long in the tooth, I can work round my visual failings, and measure with a ruler/dots spot on. some manufacturers prefer the measure the near pd and add 2.5... but this method statistically seems the most off


Well we also have to include visual behaviour.  So the way the client is looking at you in both situations may be different and cause some deviation.

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## HarryChiling

My personal way of measuring is to take the near pd and add 3mm to it I know that this is not the most accurate.  The reason I do this is that most progressives seem to have the most distortion on the bottom half of the lenses.  By measuring this way I am insuring that the most accurate is the reading or bottom portion, and that accuracy is more comprimised near the top.  Ultimately my goal is to make sure that the umbilic or corridor is correctly placed.  Just my preference and the reason why I do it.

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## Happylady

I always use a pupilometer and I always dot the lenses where the pupils are. I double check the two measurements against each other.

I always have the patient rest his/her elbows on the table and hold the pupilometer like a pair of binoculars. I make sure it is straight and not tilted.

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## C-10

happylady 

is right I take four measurements with the pupilometer distance mon. occur & bi occur the same with near. Then I split the pupil sitting down and check it standing up. Since I started this, I have not had a remake in pal because on measurements

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## Jedi

> My personal way of measuring is to take the near pd and add 3mm to it I know that this is not the most accurate.  The reason I do this is that most progressives seem to have the most distortion on the bottom half of the lenses.  By measuring this way I am insuring that the most accurate is the reading or bottom portion, and that accuracy is more comprimised near the top.  Ultimately my goal is to make sure that the umbilic or corridor is correctly placed.  Just my preference and the reason why I do it.


Interesting way to fit the lens. What would you do in a case of a variable inset or an inset that is not 3mm?

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## Lynne

Funny you should ask today!  Just had an inservice from Sola, on segmented bifocal to PAL conversion, and they said always, always,  use mono distance PD's, from a Pupilometer!  Which is what I do anyway.  Amazing how many folks are 1 to 3, or more, mm's different from side to side.  With mono PD's don't get any redo's - at least not for that!  Sometimes the doc's Rx!

For seg height, I use the way we were taught, also by a Sola rep, many years ago, have pt. sit naturally, I usually say, as if driving, dot them up, then draw a straight line through the dot, stand, again, naturally for them, look far, and ask is the green line above, below, or through their vision.  Don't tell them what it should be, as they will move their heads to please you, and make it "through"!

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## For-Life

I would bet that one of the higher reasons why there is PAL failure in the industry is the lack of mono and the use of bino.

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## Darryl Meister

There is also a lengthy discussion of PD measurement for progressive lenses at: PAL / PD Issue Thread.

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## rbaker

The Gold Standard for measuring the interpupillary distance used to be the PD rule and muscle light to determine the Purkinje reflex for both distance and near gaze. I dont think anything has changed.

As QDO1 stated _I think the same. but as an experiment I have been using all the methods, and the variance is sometimes up to 3mm per eye._ These findings can be resolved by the method above. After all, there is only one correct distance PD measurement.

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## HarryChiling

> Interesting way to fit the lens. What would you do in a case of a variable inset or an inset that is not 3mm?


Jedi to answer your question it is not a very accurate way of doing things, but I have found that I have better luck if my near pd is accurate then when my far pd is accurate.  The umbilic in the lens is set at a specific inset anyway and The only control I would have over that is the accuracy of the far pd or the accuracy of the near pd, if my near pd is accurate I have a higher adaption rate.  Everyone else in my office thinks I am stupid for doing it this way and what they do is take the distant pd then subtract 3mm from it and thats what they call their near pd.  In essence it is the same thing that I am doing but they are focusing the accuracy on the distant part of the Rx.  My goal is to make sure that the proggressive corridor is lined up as accurately as possible to give my patients the best reading out of the lens.  Ultimately it would be nice to measure both the near and distant and match that up with a lens that has the corridor at that specific inset, but I have good luck with the way I am doing it so I will just stick to it, until I find a better way.

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## QDO1

> The Gold Standard for measuring the interpupillary distance used to be the PD rule and muscle light to determine the Purkinje reflex for both distance and near gaze. I dont think anything has changed.
> 
> As QDO1 stated _I think the same. but as an experiment I have been using all the methods, and the variance is sometimes up to 3mm per eye._ These findings can be resolved by the method above. After all, there is only one correct distance PD measurement.


MMM not sure.. one could call the pupil centre the correct PD or the reflex the correct PD.  Experience tells me the reflex is often IN from the Pupil centre.  Looking at a diagram of the eye, the pupil centre ought to be the correct distance PD, unless the macula is offset when the eye is fixated on infinity

Harry's method makes sense, if the distance power is low comarative to the near power - for example +1.00 Ds + 3.00 Add, but makes less sense for the folowing RX -3.00DS +3.00 Add

what is fasinating about this little debate is

based on the varying measurment methods, what on earth are we supposed to order for a variable inset lens?why is there so many interpretations of what we do on a day to day basis?We were taught that we are lining up the Optical centre (for a distance lens) to the optical axis of the eye... that is why we compensate the height measurements for frontal angle etc.  the real question should be..

When taking a measurement - where is the optical axis?

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## chip anderson

Many years ago Joe Soper and Louis Girard, M.D. found that with extensive keratometry and finding the apex of the optical cap of donor eyes, using this as the center for the corneal transplant, visual acuity post transplant was trememdously enhanced.   Possibly this is the clue to where the optical center/visual axis really is.   Anybody out there with a lot of money, time and staff with a progressive manufacturer willing to do research?  Pete?  Darryl?

Chip

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## HarryChiling

For a variable inset lens you would measure both the near and far pd and supply that to the manufacturer otherwise their would be no benifits to a variable inset lens. But for a regular proggressive the reason for me measuring the way that I do is due to the location of the most distortion in the lens and that seems to be in the lower half or itermediate and reading area of the lens.  I have seen all the ways discussed on this board for measuring used and all seem to work for the optician using it.  I think the most important points to remember are that all progressives require mono pd measurements.

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## QDO1

> For a variable inset lens you would measure both the near and far pd and supply that to the manufacturer otherwise their would be no benifits to a variable inset lens.


yes - that assumes we can get the right measurements - that was partially why i asked the question

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## Darryl Meister

> Experience tells me the reflex is often IN from the Pupil centre. Looking at a diagram of the eye, the pupil centre ought to be the correct distance PD, unless the macula is offset when the eye is fixated on infinity


Oddly enough, the macula _is_ offset from the optical axis of the eye, and the eye must rotate out 2 to 3 degrees to compensate for this, which is why the corneal reflex is displaced nasally. (The difference is sometimes referred to as _Angle Alpha_.)Though there is also some argument that, because the center of the entrance pupil of the eye (corresponding to the center of the iris) is also offset nasally by slightly smaller amount, the CRP may still be slightly too narrow.

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## QDO1

Perhaps the most accurate method would then be halfway between the CRP and the pupil centre... Amazing how something so simple and standard is so out of kilter when we explore it a little

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## Bobie

1.We use monocular P.D. from pupilometer.

2. We use near monocular P.D. + 2.5 mm and concider with " near point down gaze convergence P.D." for Single Inset PALs.

3. We use far monocular P.D. for Variable Inset by Ametropia and Addition PALs.

4. We use far & near monocular P.D. to order Free Form PALs.

5. We only recommended Free Form PALs for customer at below ;

5.1 Anisometropia over 1.25D

5.2 Cyl. over -2.00D

5.3 Axis of Cyl. conflict 90 degree

5.4 no more binocular vision function or blind one eye.

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