# Optical Forums > Ophthalmic Optics >  As-worn optimization

## Darryl Meister

Conventionally, progressive addition lenses have been designed with two power reference locations: a distance reference point (located at the center of the distance verification circle) and a near reference point (at the center of the near verification circle). The distance reference point is equal to the base curve of the lens, while the near reference point is equal to the base curve plus the add power of the lens. Lens designers have ensured that these two points remain relatively spherical, and that surface astigmatism (produced by the progressive change in surface power) is prevented from encroaching into these regions. This was done to ensure that dispensers and optometrists could readily verify the lens with typical vertometers and vertometers, these spherical points allow dispensers to verify the distance and add powers without unwanted cylinder (from the surface astigmatism) affecting the reading.

Although lenses designed like this provide accurate power readings at the distance and near reference points, they dont necessarily deliver the exact intended prescription to the wearer. Because of the effects of oblique refraction through the lens, pantoscopic tilt, prism, and vertex distance, the power actually experienced by the wearer with the lens in position may differ from the power measured by a conventional lensometer and vertometer. Consequently, determining the optical power provided by a progressive lens in the as-worn position is a more accurate measure of what the wearer actually sees. However, this can not be done with conventional lensometers or vertometers without special modification  or by optical analysis using ray tracing software.

It is possible, using asphericity, to optically optimize the distance and near reference points so as to provide an optical power that matches the desired prescription when the lens is actually in position. Unfortunately, designing a lens to provide the desired optics in the as-worn position generally results in a difference between the doctors prescribed prescription and the power measured by a conventional lensometer or vertometer. The distance and near reference points may be designed with a small amount of cylinder, or a change in the sphere power, out of necessity  in order to compensate for the as-worn power of the lens. This added cylinder and/or sphere power makes it difficult to determine whether or not the lens was made accurately. (That is, whether or not the powers were accurately manufactured and/or surfaced.) This is further compounded by the fact that this small cylinder may combine with the doctors prescribed cylinder, producing a crossed cylinder effect. When the prescribed cylinder is also small, this crossed cylinder may have a different axis and cylinder power than the prescribed cylinder.

So, the option becomes: A) Keep the distance and near reference points relatively spherical to ensure ease of verification and quality assurance, or B) Optimize the distance and near reference points to provide a more accurate optical power in the as-worn position. It is also possible to compromise between both; the distance and near reference points can be left relatively spherical, while the region surrounding them is aspherized for as-worn optimization. So Given these alternatives, what would your opinions be?

Best regards,
Darryl

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## shanbaum

Greetings, fellow minion.  Great article.  Another Meisterpiece.

I'd suggest a third possibility - how's 'bout you lens designer types let us software types know what the design assumptions are (e.g., the assumption that's made as to how the surfaces will be tilted in front of the patient's eyes, and to what extent that has been corrected for in the front surface design) so that we could provide the user with a set of Rx parameters that could be used for QA?

BTW, since Steve requested that I do so, I shall introduce myself (in case anyone we don't know actually visits this forum).  I am Robert Shanbaum, Director of Software Development at Gerber Coburn in South Windsor, CT.  You can learn more about Gerber Coburn at www.gerbercoburn.com. 

I reiterate Darryl's challenge - let's get some interesting questions going here, so Darryl can answer them for us!

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## Darris Chambless

Okay Darryl Bob :-)

You have given an interesting take on this problem and asked a very perplexing question. If I understand what you're saying correctly (and I may not, but I think I do)you would have such a wide range of variables to have to consider. 

Here's why: You will have to have as many design settings as you do individuals for optimum results. We both know that this is not feasible by any standard (unless of course you have pockets so deep that you can't see the bottom ;-). Your company was attempting to combine the spherical aspect and the optimization in the Percepta, was it not? If I remember correctly it was and did suceed to an extent.

My suggestion is this, if you can design a lens that will optimize direct focus (e.g. same plane of view through every quadrant in the lens) without any sagital or tangential astigmatic aberration that would be the goal. Granted I also want the world so when you get finished let me know I want one :-)

I am also a little curious about how keeping the distance and near reference points spherical will affect coma in the lens? The reason I ask is because of the aspheric changes in the front surface outside the spherical capsule (so to speak); can this increase the the aberration of oblique rays? Or will it lessen the effects of it due to the combining of the two potential solutions given? It seems to me that when you have an aspheric front surface as in current progressives light will tend to "break up" a little more and actually reduce some of this effect, but it does create more marginal astigmatism. Of course that's in my opinion which is based on practical application and not as much on mathematics.

The lesser of the two evils. Which will it be? I can't say what would be better over all for my patients nor do I know what they would accept as to one over the other without actually trying each design on a test group and determining its acceptability to the patients. In others words without trying them there is no real way to know.

I would be interested in the answers to my questions. With them I might be able to sway more one way over the other and give you a better take on it.

Let me know,

Darris "I want the world. I want the whole world. I want to lock it all up in my pocket, its my bar of chocolate. So give it to me now!" Chambless

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## Pete Hanlin

Hi All,
Hmmm, I notice we now have four Demonic Minions in this string... maybe that's why my cursor is currently in the shape of a pentagram!

No, really Darryl... I would like a PAL that has a distance verification point optimized for verification and a near point optimized for the wearer's vision.  Unless I'm mistaken (which I may be ;^), the benefits of optimizing the distance verification point aren't nearly as great for the wearer as is the case for the near point (I assume this because the visual axis usually intersects the distance point close almost perpendicularly).  In addition (pun intended), most Opticians recognize that verifying the near power of a PAL is "touch and go" at best anyway (that's why you give us those laser engravings ;^).  If the engraving says the add is +2.50, hey, I believe ya... but I want the distance to read "as ordered."  I've also noticed that near points optimized for the wearer do perform perceptibly better (we use the 1.66 Pentax PAL quite often, and patients seem to really like it).

Let me know if my assumption about the distance is incorrect (I always like to learn :^).
Pete

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## Darryl Meister

> Originally posted by shanbaum:
> *
> I'd suggest a third possibility - how's 'bout you lens designer types let us software types know what the design assumptions are (e.g., the assumption that's made as to how the surfaces will be tilted in front of the patient's eyes, and to what extent that has been corrected for in the front surface design) so that we could provide the user with a set of Rx parameters that could be used for QA?
> *


Good to see you on board, Robert! Always a pleasure. That is an excellent point... And at least one manufacturer has already begun doing just that for their direct customers. However, since the surfacing lab would be the one grinding the Rx, the lab would also have to calculate for themselves what the resultant Rx would be. This includes calculating the crossed cylinder effect and such, and would also vary with the lens power and thickness they chose for the job. (Though, we could probably offer a program to do that kind of stuff for them.)

Best regards,
Darryl

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## Darryl Meister

> Originally posted by Darris Chambless:
> *
> I am also a little curious about how keeping the distance and near reference points spherical will affect coma in the lens? The reason I ask is because of the aspheric changes in the front surface outside the spherical capsule (so to speak); can this increase the the aberration of oblique rays?*


Hi Darris,

Well, even single vision aspheric lenses are relatively "spherical" at the design center -- or pole -- of the asphericity. This point is referred to as the vertex curvature, and is generally located at the geometric center of the lens blank. The lens gradually becomes less and less spherical (i.e., more aspherical) away from this center. But, for most aspherics for low to moderate powers, there is a region surrounging the design center that can be considered spherical -- because the departure from a sphere is so small. Consequently, it is possible to have a relatively spherical region (albeit not necessarily perfectly spherical) in the distance zone of a progressive that still becomes aspheric away from the center of that zone.

Best regards,
Darryl

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## Darryl Meister

> Originally posted by Pete Hanlin:
> *Hi All,
> I would like a PAL that has a distance verification point optimized for verification and a near point optimized for the wearer's vision... Let me know if my assumption about the distance is incorrect (I always like to learn :^).
> Pete*


I agree on both points, Pete. That would be an effective approach, that would preclude both problems to some extent (assuming that optometrists, opticians, and labs won't get too carried away while measuring the add power).

Best regards,
Darryl

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## shanbaum

> Originally posted by Pete Hanlin:
> *Hi All,
> No, really Darryl... I would like a PAL that has a distance verification point optimized for verification and a near point optimized for the wearer's vision.  Pete*


Unless one were to create a lensmeter that could measure lenses in an as-worn attitude, the effective power as-worn will differ from that as-measured, for prism-thinned progressives - or for that matter, any lens with prescribed prism.  Lensmeters hold the back surfaces of lenses normal to the target; that's rarely the attitude of the lenses as worn.

I think this is really a problem in the way lensmeters work, as opposed to being of serious consequence to end-users.  That is, I think the effects of the surface tilting due to prism are minimized on the patient and maximized in the lensmeter.

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## harry a saake

In reply to all of the previous posts, is it not true that we create problems because of the way we sell and dispense. It would seem in most cases if we were to sell and properly fit the frame first and then had the doc do the exam,the doctor would know what degree of panto or retro and adjust the phoropter accordingly, and he would also know the vertex depth for that paticular frame. time consuming,probably, anyway just a thought..harry

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## Brad Gelb

Has anyone (Rotlex, TOPCON, Etc.) released equipment to easily determine as-worn vs. through-power measurements? I was told years ago they were working on it. My concern is not only measuring at the surfacing level, but manufacturing as well. We all know there is a large difference between ideal design and the lens that pops out of the molds (especially poly!). An as-worn optimization is great, but how do you verify this at manufacture?

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## Susan Henault

Maybe I'm way off base, (tell me if I am) but I wonder if there is a way to design "power/prism rings" for use with conventional lensometers, that would off-set the adjustments needed to optimize the near zone for "as worn" viewing? Assuming that the design modifications needed could be limited to, say 6 different "designs" that would cover the standard BC and add ranges, new "power/prism rings" would allow manufacturer, lab & optician to verify the near easily and accurately. I believe that the only reason ECP's have trouble verifying PAL adds is because they do not (know to) use prism rings to counter balance effect of verifying the near power so far off the OC.

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## Pete Hanlin

Actually, many lensometers come equipped with an adjustable prism offset (adjustable for power and axis) that allows you to bring the cross into the center of the target for verifying power and/or prism strength.  

The problem Darryl is speaking of is the trend among lens manufacturers towards modifying the power of the near and distance verification zones to allow for optimized viewing by the patient.  

Especially at near, the patient is looking at the lens at an angle, so the manufacturers are compensating their near add design.  Unfortunately, some folks get upset when the near power doesn't read what it is "supposed to," and invariably complain to their labs (who invariably complain to the manufacturer ;^).  While a set of "rings" to compensate for the manufacturer's modifications is probably unfeasible (now watch Darryl tell us they've already got em ;^), I don't understand why we can't just tilt the lens in the lensometer to match the fit experienced by the patient (which should yield the near Rx we ordered, I think...).

Pete

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## Raanan Bavli

Dear all,
If I understand correctly the problem of as-worn power versus lensometer measured power, manufacturers (designers) hands are tied. This is mainly because they don't know the thickness of the finished lens. Can someone tell me whether I have a point here ?

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## Barry Santini

I think all this atention to lens design and theory is a waste when the refraction methods and techniques coould be improved and rethought.  The concept should be not how the lens i worn different from the refraction, but rather how the refraction could be done to be more representative of the glasses orn in situ.

It may not have been anything you have considered before.

Also, the sorry state of HMO-managed refractions has resulted in more comprimised vision/proressive lens performance, IMHO, than all the recent market-position- based lens design "advancements"

Daryl, i respect the work going into these new lenses, but I think the industry is definitly "afraid" to confront the outdated and sloppy refractive techniques so prevelant today.

If the theoriticians would consider aboning the 100+ year old Hemholtz theory of physiogical accomodation, then why are we so afraid to rethink the procedures used in refraction?

What do you all think??

Barry Santini
ABOM

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## Darryl Meister

> Originally posted by Barry Santini:
> _...The concept should be not how the lens i worn different from the refraction, but rather how the refraction could be done to be more representative of the glasses orn in situ._


This would certainly be one approach. Keep in mind though, that there are a lot of variables involved when fitting the prescription in some arbitrary frame (which would be the as-worn position). Although "in situ" refraction may obviate the need for prescription compensations in one particular fitting instance (the one used while refracting), it wouldn't really be representative of other fitting situations. Providing a "reference refraction" -- free from the effects of tilt and other variables -- allows the prescription to be used for any fitting situation, with only small compensatory adjustments made to the prescription. Good points, though.




> _...I think the industry is definitly "afraid" to confront the outdated and sloppy refractive techniques so prevelant today... If the theoriticians would consider aboning the 100+ year old Hemholtz theory of physiogical accomodation, then why are we so afraid to rethink the procedures used in refraction..._


My guess: Abandoning the Hemholtz theory doesn't require a new investment in refracting equipment and additional training.   ;)

Best regards,
Darryl

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## Cameron

Darryl Meister,

Or anyone else!!

A question for you.

I am a consumer who has a new set of Zeiss progressive lenses and I have better distance acuity from the intermediate portion of the lens than from the distance portion of the lens.

My optometrist is convinced that the problem is the refractive index of the glass (1.67) and is about to reorder the lenses in 1.6.

From what I can understand of what you have been saying, the problem may not be related necessarily to the refractive index of the lens. So, my new glasses may have the same problem.

My distance Rx is -10.25 in my right eye, and 10.0 in the left.

Can you shed any light on this?

Cameron Reddy
creddy@cameronreddy.com

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## Darryl Meister

> Originally posted by Cameron:
> _I am a consumer who has a new set of Zeiss progressive lenses and I have better distance acuity from the intermediate portion of the lens than from the distance portion of the lens.
> ...My distance Rx is -10.25 in my right eye, and 10.0 in the left._


Hi Cameron,

At first glass, it sounds like you're getting too much minus in the distance portion. Looking through the intermediate zone of the lens increases the plus power (or reduces the minus power) of your prescription. In your case, looking through the intermediate zone would have the effect of reducing your distance prescription. You could be getting too much minus in your distance portion for a number of reasons: Lenses not made with the correct prescription, lenses sitting too close to the eyes, lenses too tilted, lenses fit too low, doctor's refraction slightly off, etc. Because of the high strength of your lenses, the fit of the eyewear can have a significant impact on your vision.

The only odd factor here is that you would have to be significantly over-corrected in the distance portion to experience this effect, or have little or no accommodation left. I say this because the eye, itself, should be able to accommodate a bit -- which is the process of increasing the plus power of the eye -- for your distance vision, if you needed to reduce excess minus power. If your add power is less than +2.50 D, I would question question this -- since you should probably have some accommodation left. In any case, if you cannot resolve the problem, I would have the doctor verify that the lenses were made correctly and then verify his/her refraction -- just to rule that out.

It is also possible -- though a lot less likely -- that your lenses have some sort of aberration in the distance portion (from either the manufacturing or grinding processes).

You might also check to see whether your vision is worse in one eye over the other. Do you have your old prescription? Has the doctor prescribed cylinder power for you in the past?

The refractive index, itself, won't really have much effect upon vision. However, as the refractive index of the material increases, the "Abbe number" generally decreases. This is a measure of the color dispersion produced by the lens; lower Abbe numbers produce more color aberration. Unless your lenses have been ground with a great deal of prism, you really shouldn't be noticing the effects of this color aberration through the center of your distance zone. And it would be even worse through your intermediate zone.

In any case, best of luck to you

Best regards,
Darryl

[This message has been edited by Darryl Meister (edited 06-30-2000).]

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## azthoth

This is in response to some of the comments about 'as worn optimization'. I can only comment on the Pentax AF progressive. When a particular power is ordered from a lab that is able to surface the AF a disk is provided that gives what the neutralized power should be based on that prescription. This 'adjusted power' is based on certain design assumptions, which are panto=8-10 degrees,and vertex= 10-12 mm. There is, from my experience quite a bit of uncertainty about this design, but I believe the optical theory is sound, and I have dispensed quite a few. The response has been resoundingly positive.

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## Cameron

Darrel,

Thanks for your reply, and allow me to apologize for taking so long to thank you.

The head Optometrist now thinks, as you suggest, that there is too much add in the distance portion of the lens. However, it appears that he also thinks backing it off by .5 to 1.0 D will do the trick. And as you said, if I understand, such a minimal over correction would be something to which my eyes would adjust. So, I'm still not convinced this is going to work.

To answer one of your questions, the problem is virtually the same in both eyes. 

Also, I have my old prescription, which, according to the original Optometrist, is exactly the same as the prescription ordered (and verified on the lensometer) on my  Zeiss Gradal pal.

What is cylinder power, and how would it impact this situation?

I'm going back in tomorrow so we will see.

Thanks again. You folks have a great site here!

Cameron Reddy
creddy@cameronreddy.com

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## Darris Chambless

Hello Cameron,

First let me assure you that this is all Darryl's fault because he won't design us the perfect lenses. How stingy is that? :-)

Some things to understand are that 1) An eye examination is very subjective. 2) Educated (and sometimes not so educated) guesses are more of what you get in this day and time. 3) Accommodation will vary from individual to individual and can rarely be assessed during the initial exam.Your doctor made an educated guess and missed by a little bit.

backing your script up .5 to 1.0 is not really what would be considered "a little" change. Half to one full diopter of change is a "booboo" although like I said before exams are very subjective therefore leaving variables that can and do get missed.

Cylinder power is a focus power that is predetermined and preset in your script. Cylinder isn't really a magnification power, it simply sharpens your focus and is ground into your script on the same plane as the irregularity of the cornea. Think of it this way (this isn't the best analogy but it will have to do :-) you're looking through  binoculars but the image you see is a little blurry. In order to get the image to come in sharply you have to turn the adjustment wheel until the image clears. This is basically the same thing the doctor does with the phoroptor but the magnification and focus power are preset for your eyes at a specific distance.

I will make a suggestion to you if I may? If you continue to have a problem with your script and nothing can be found by the doctor as to the cause I would suggest that you find a good optician that understands many of the aspects of high minus lenses and progressive powers. Your doctor may have a very competent optician or tech, but there is always room for a second opinion.

Take care and remember it's all Darryl's fault :-)

Darris "I'd take the blame but we all know it's Darryls fault" Chambless

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## Cameron

Thanks Darris!

Since .5 to 1.0 D is a "boo boo" (I love the term!), what does the guy--whose fault this all is--mean when he says the add power would have to be +2.5 D for me to be able to see better through the intermediate column? Or, did "Mr. Fault" cause me to misunderstand what he wrote? :-)

I presume the add power is the amount of reduced minus that is ground into the intermediate and reading areas of the lens. So, my lens would be something like -10.0 distance, -7.5 intermediate, and ??? reading? Again, however, I agree that my misunderstandings are Darryl's fault. :-)

And I also apologize (for Darryl since it's his fault) that I am cluttering up this area of the forum with layperson questions. You guys are just so helpful, Darryl has caused me to be unable to help myself! 


Cameron

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