# Optical Forums > Ophthalmic Optics >  Question about ANSI standards - specifically PD

## co_mac

Hi all...

I have a job for a patient I'm questioning at work. Power is somewhere around a -4.50; I don't remember what the cyl power is.

The PD is off by about 5 mm. It should come in at 58 but it's measuring at about 63.

This would give at least a diopter of prism per eye, plus more depending on what the cyl power is.

The tolerance chart provided by my company is not very clear on whether the listed tolerances are binocular or monocular. But the chart says for anything over +/-2.75 the PD tolerance is 2.5mm. Is this total, or per lens? I don't know exactly, but it makes no sense to me to allow 5mm, so a total of 2.5mm seems more logical.

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## L.A.B.

Actually, it is 2.25 diopters of total imbalance(if it is a sperical lens). That is total, both lenses. Power X (the amount the P.D. is off, in mm) divided by 10.:)

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## Barry Santini

that, to my knowledge, ANSI does not specify the protocol for quantifying pupillary distance.  Is it:

1. Distance between geometric center of the pupils
2. Distance between the actual (offset from pupillary center) Optical axis of the eye
3. Distance between the apices of the corneal (close to, but not always coincident with the visual axis intersection with same
4. Distance between the the actual lines of sight (va)

And....at what vertex is this measurement to be assumed to be taken at?
1. 13mm
2. actual plane of fitted eyewear
3. other?

If one uses pupillary centers for a very wide (72) PD fitted at a VD of 23mm, and fabricates reading eyewear, there is no doubt that this pair will be significantly out of current ANSI tolerances, no matter how you apply them.

And just how many of us continue to reuse the same, initial adult PD over years for the same client (particularly for monocular set-up of progressives).

I have found that taking a PD with a corneal reflex pupilometer _has dramatic_ variations on the same client, _taken on the same day_, depending on whether I've asked them to assist me with "holding" the pupilometer in a comfortable and stable position vs. taking the measurement all by my lonesome.

Thoughts?

barry

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## Darryl Meister

> PD tolerance is 2.5mm. Is this total, or per lens?


This is total.




> that, to my knowledge, ANSI does not specify the protocol for quantifying pupillary distance. Is it


ANSI Z80.1 defines the interpupillary distance for near vision as:

_The separation between the visual axes of the eyes, at the plane of the spectacle lenses, as the subject fixates on a near object at the intended working distance._

And the interpupillary distance for far vision as:

_The separation between the visual axes of the eyes in their primary position, as the subject fixates on an infinitely distant object._

Keep in mind, however, that the ANSI tolerances apply to the prism reference point location, which is ultimately defined by the optician or optometrist when they specify PD values. Consequently, these tolerances are essentially independent of how you choose to measure or define the wearer's interpupillary distance, as long as you correctly specify how far apart you expect the prism reference points to be located on the lens pair. The person fabricating the eyewear does not need to know how that the distance was actually determined (at least in the vast majority of cases).

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## Barry Santini

Thank you for that factual clarification of the current ANSI standards, Darryl.

Q: What is the most *representative* way of determining the position of the visual axis?

Barry

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## Barry Santini

> Keep in mind, however, that the ANSI tolerances apply to the prism reference point location, which is ultimately defined by the optician or optometrist when they specify PD values. Consequently, these tolerances are essentially independent of how you choose to measure or define the wearer's interpupillary distance, as long as you correctly specify how far apart you expect the prism reference points to be located on the lens pair.


So...When a "Dr." writes down a "PD" on an "Rx", they are, in fact, knowingly or unknowingly, really specifying the MRPs for the finished eyewear to be toleranced against. This then, could be much more sophisticated specification than the commonly-accepted definition of "PD". What does this mean for online ordering of internet eyewear?  Does one give out the desired MRP, or the PD, or....????

Interestingly, with respect to larger, *wrap* eyewear (that has a frame wrap angle of approx 23 degress or greater), I often specify a "PD" that takes account (esp. in minus Rxs) the amount of Base-in prism I would like, plus the desired "blocking" PD for compensating for the narrowing effects of the wrap angle. This also facilitates *not* having the lab call me and tell me the job "won't" cut out.

Getting more complicated, now....

barry

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## HarryChiling

> So...When a "Dr." writes down a "PD" on an "Rx", they are, in fact, knowingly or unknowingly, really specifying the MRPs for the finished eyewear to be toleranced against. This then, could be much more sophisticated specification than the commonly-accepted definition of "PD". What does this mean for online ordering of internet eyewear? Does one give out the desired MRP, or the PD, or....????
> 
> Interestingly, with respect to larger, *wrap* eyewear (that has a frame wrap angle of approx 23 degress or greater), I often specify a "PD" that takes account (esp. in minus Rxs) the amount of Base-in prism I would like, plus the desired "blocking" PD for compensating for the narrowing effects of the wrap angle. This also facilitates *not* having the lab call me and tell me the job "won't" cut out.
> 
> Getting more complicated, now....
> 
> barry


Barry, It's definately fun to watch your mind work. :D

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## Barry Santini

> Barry, It's definately fun to watch your mind work. :D


Thanks, Harry.  My advice is to not watch too closely...:hammer:

Barry

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