Progressive Lens Terminology

[Pete Hanlin]

I'm putting together a CE entitled Progressive Lens Characteristics, and part of the course will attempt to define the various terms used by manufacturers to describe their designs. I know some of the terms I'm listing here are archaic, but I was hoping to enlist y'all in helping me put together a complete lexicon of PAL terms.

Hard Design- a design where the edges of the channel and reading area are marked by a rapid increase in unwanted astigmatism

Soft Design- a design where the edges of the channel and reading area may be smaller, but are bordered by a less dramatic increase in unwanted astigmatism

Symmetric- a PAL with one, symmetric design for both lenses. Lenses are tilted to create a right or left lens

Asymmetric- a PAL design in which lenses for right and left have different characteristics between the temporal and nasal areas

Mono-design- a PAL that carries the same design throughout all add powers

Multi-design- a PAL that changes in design as the addition power increases


I realize that there are many other terms used to describe PALs. If any of you could
list a few more (or refine the definitions for the terms I have) I would be most appreciative. This will only be a one hour course (and I'm not presenting this one until October), so we won't be spending a huge amount of time on terms- but I'd like to be able to supply a handout. In particular, some help with the terms relating to new designs that attempt to provide an accurate Rx at the retina would be greatly welcomed.

Also, for you manufacturer types. Other than a power grid and a graphic showing the speed and location of the power progression, what other graphics do you feel would be helpful in determining the characteristics of a progressive addition lens.

Just splashing my feet in the Oasis that is this forum,
Pete

[Darris Chambless]

Hello Pete,

Since I'm in a particularly jaunty mood I'll add to your list :-)

Progressives are also known as:

"Those lenses that the bifocal doesn't show": Because I need intermediate viewing power.

"Those kind that transition" (note the small "t" on transitions) Because I have no clue what these are called. That's why I came to you, the expert.

"Lineless bifocals" Well, that's what I've been told they're called

"Linesless trifocals" Refer to above.

"I don't wear bifocals" Little do they know :-)

All joking aside I would have to say you've got it pretty well covered there small to medium guy :-) I say run with it.

Tlak to you later,

Darris C.

[Alan W]

Pete
Another term I coined and use when presenting my training material is"tangential field of view." This is the shape of the near window drawn by the patient as they look through the glasses. It's an incredible tool to determine whether the best possible pantoscopic angle and pantoscopic curve has been reached. I supply a marker and a classified section of the paper. Makes an excellent uniform subject to view with the glasses on. To my knowledge its the only interactive diagnostic tool around. I am delighted to corroborate if you like. If so, I need some time. Lightening struck 100 feet off my bedroom window and fried a couple goodies on my computer.

[Darryl Meister]

Hi Pete,

I have a few suggestions for you.

I wouldn't use the absolute terms "hard" and "soft" but rather the relative terms "harder" and "softer." Few lens designs are entirely "hard" or entirely "soft." Most designs fall somewhere in the middle. Moreover, some designs may have softer distance zones combined with harder near zones.

Progressive lenses are often arbitrarily classified into two broad categories, or design philosophies, by the relative magnitude, distribution, and gradients of their surface astigmatism. (The gradient is the rate of change of power and unwanted astigmatism.) These various characteristics describe the relative hardness of the design. Lenses within each category show broad similarities in the magnitude, distribution, and gradient of their surface astigmatism (or HARDNESS):

HARDER PAL DESIGNS will have higher gradients of astigmatism. On a contour plot, the contour lines will be closely spaced. A harder design concentrates the astigmatic error into smaller areas of the lens surface, thereby expanding the areas of perfectly clear vision at the expense of higher levels of blur and distortion.

SOFTER PAL DESIGNS will have lower gradients of astigmatism. On a contour plot, the contour lines will be widely spaced. A softer design spreads the astigmatic error across larger areas of the lens surface, thereby reducing the overall magnitude of blur at the expense of narrowing the zones of perfectly clear vision. The astigmatic error may even encroach well into the distance zone, but the levels of blur and distortion are generally low.

In addition to multi-designs, you should probably identify:

DESIGN BY RX: This is a SOLA design philosophy that employs a unique progressive lens design for each base curve and add power combination. For instance, steeper base curves will utilize a progressive lens design optimized for the magnification, peripheral aberrations, and additional convergence requirements of plus-powered prescriptions. This provides consistent optical performance for all wearers.

ATORIC DESIGNS: The most common atoric progressive lens design will have a complex, free form -- or "atoric" -- surface ground upon the back of a typical progressive lens blank. This will allow for additional customization of the lens based upon the individual fitting and prescription requirements of the wearer.

AS-WORN (or POSITION OF WEAR) OPTIMIZATION: This is the process of using ray tracing and lens-eye modeling to refine the optical powers as perceived by the wearer with the lenses in their intended positions. This takes into account the influence that oblique aberrations, lens tilt, vertex distance, and other variables affecting viewing conditions have on the optical powers perceived by the wearer.

You should also consider defining things such as the prism, distance, and near reference points; the progressive corridor (or "umbilic"); skew distortion (produced by the unwanted astigmatism of the lens surface); etcetera.

Best regards,
Darryl

[Sara]

Good Darryl,an excellent explaination.
Thanks,
Sara

[Jeff Trail]

Pete,

Another way to go about talking about "hard" and "soft" designs are rooted in the way it came across way back when when we didn't have many choices in "design", I can remember it also used to describe the types of add design years back i.e. a FT was a hard design. You went from distant to addition in one visual jump, no blending of image jump.
Then the new "buzz" words started and the way I remember it is the PAL made it a smoother transition of image jump and less traumatic, TADA softer design.
Then they started improving the transition by adding "steps" of power, say from the old AO's had (thinking was something like 14 steps) to say the premium stuff we have today that is in the high 30 range or better.
This helped reduce image jump and made it more comfortable and compariable to the way the crystiline lens used to perform.
That was the older version of "hard" to "soft" :finger:

Jeff "remembering from the old days when we had no OD/OS PAL's" Trail

[Susan Henault]

Other terms that come to mind (though perhaps they are just too obvious) are "corridor, channel & eyepath" which refer to the intermediate area of progressives -- as do "corridor length, channel length & drop", which refer to how far down the prescribed near occurs in a given design.

I'm surprised no one has mentioned the term "Aspheric". It seems that all the vender (reps anyway) are calling their new lenses "fully aspheric" ... although whether they fully understand exactly what that means is another issue altogether!

Common uses of "aspheric" in progressive design include the following:
1. The fact that all progressives are not spherical due to the curve change from distance to near on the front surface.
2. The use of "flatter" curves for cosmetic purposes which requires aspheric technology to restore the peripheral optics
3. Finally, the most common use of the term "fully aspheric" by manufacture reps refers to super soft designs. Instead of having large "spherical" areas in the distance and near zones, which result in consentrated areas of unwanted astigmatism in the periphery (harder) -- the latest use of "aspheric" technology allows designers to spread the necessasry curve change over the entire lens surface. This generally produces extremely soft designs with virtually no area of "true sphere" anywhere on the progressive lens surface -- but the amount of unwanted astigmatism is so low, it's barely perceivable by the human eye (especially in lower add powers).

As for graphics, I have a terrific illustration that I would be willing to send to you via email that really drives home exactly how progressives work, as compared to conventional multifocals. Drop me a line if you are interested!

[Darryl Meister]

Hi Susan,

Great explanation. You've very nicely distinguished between the various uses of the word "aspheric" in the context of progressive lenses. Your explanation is a lot like how I normally describe the term, myself, in articles and seminars. The term is often misused, if not misunderstood.

Keep in mind though, that the use of asphericity in the distance zone to spread out unwanted astigmatism isn't really new or state-of-the-art technology. Manufacturers have been making "fully aspheric" designs for years now. The Varilux II is an early example of a lens that incorporated noticeable asphericity in the distance zone. The AO Omni is another highly aspheric, ultra soft lens design that has been around awhile. Today, many modern PAL designs represent a more reasonable balance between being super-soft and hard. The amounts and gradients of surface astigmatism spread into the distance zone are more thoughtfully controlled and distributed to afford maximum visual acuity where appropriate with minimum swim and distortion.

The most recent use of asphericity would be of the second variety you mentioned. When certain manufacturers started flattening progressive lens designs from their recommended best form base curves, they compensated the peripheral optical performance by using asphericity in the distance zone -- similar to single vision aspherics. SOLA's VIP Gold is an early example of this philosophy. Several lens designs from several manufacturers now incorporate flatter base curves with aspheric optical optimization. Today, the concept has been further extended with as-worn (or position of wear) optimization.

Best regards,
Darryl

[ashish]

use the ruben's ruler for taking height measurements.
its a simple but an excellent piece of ruler.

dont fiddle with old height if the patient is comfortable with it even though it appears wrong to you

keep in mind the profession of the person,for example someone who needs bifocals in sunglasses as a spare spects then I keep the segment lower than usual.

many people wear there spects lower doen on the nose once they start using it . So our segment measurement may error if we take it with the frame sitting right on top

[Sara]

Ashish,
Thank you for your intelligent posting.By the way what does your under the line writing "mera bharat mahan !!!" mean?

Is this an indian or american slang :D

Thanks,
Sara

[hcjilson]

Pete,
You're definitely not old enough to remember this, but if you research the "History" of progressive lenses you will find references to the AO Ultravue lens which if memory serves, was the original progressive lens introduced around 1970.In order to take an "accurate" mesurement of the PD and fitting cross height, you "needed" a Grolman Fitting Unit which they sold you for about $150.It was a midevil looking trial frame which clipped to the frame front and had screw adjustments for ht's and pd's.Demonstrating this device would be an excellent way for you begin your presentation!I happen to have one in "original" condition...(.because it was rarely used )...we didn't find out we didn't need it until AFTER AO sold us the program.If you are interested I'll be happy to loan it to you for that purpose.
Just e-mail me with where to send it!

[Darryl Meister]

Although certainly an old one, the Ultravue wasn't quite the first. A few other lenses, including David Volk's Omnifocal and the original Varilux, were out some time before the Ultravue. Progressives really didn't start moving much in the US until the 70s though, so the earlier ones -- which certainly weren't great by today's standards -- didn't build a lot of awareness for a while.

But how 'bout that Grolman Fitting Device?!? I loved those little targets you had to move around with the knobs, to center over the pupils... ;) My father still has one buried somewhere in his lab.

Best regards,
Darryl

[Pete Hanlin]

Thanks to everyone for your input! As I start actually assembling this course, I'll probably be contacting those of you who offered demonstrations (I have your names, now... ;) ). I've been questioning several of the Opticians here in town concerning their PAL knowledge, and I think one of the things the typical Optician really wants to know is "how are different designs actually different?"

Someone on this board was nice enough to send me power grids for many of the mainstream PAL designs some time ago. I think I'm going to use some of these grids in Powerpoint to illustrate typical examples of different designs... If anyone has examples of power grids for the newer lens designs (SolaMax or Panamic), I'd appreciate having a chance to receive them via mail or internet. Also, if any of you can think of "classic" examples of particular designs (e.g., I've always considered the Sola XL a classic example of a softer design and a VIP an example of a harder design), I'd appreciate your suggestions.

I took a survey, and 7 of the 11 Opticians I asked admitted to remaking a PAL into a different design without changing measurements or Rx because they "couldn't find anything wrong with the fit." When I asked what they include in the term "fit," I found that none of them mentioned pantoscopic tilt or face form- or vertex distance. One of my contentions has been that, while one design may offer certain advantages over another, most PALs will "work" relatively well if properly fit.

Do face form and panto- become more crucial in fitting the latest designs in lenses (such as the SolaMax)? Also, I've always assumed that keeping as small a vertex distance as possible allows the widest possible field of vision through the channel and the reading portions of the PAL.

Thanks for your additional comments. I've always been pretty confident with fitting PALs. However, before I put anything into a presentation, I like to make sure its absolutely true (wouldn't want to disseminate false information... it seems like people will miss every other point other than the one that was false- which they cling to for life ;) ).

Therefore, my fitting assumptions for PALs are:
Monocular pds
Fitting cross centered in the pupil (for the average fit)
Pantoscopic Tilt of about 4-8 degrees (since the optical center of a PAL is ground about 2-4mm below the fitting cross in most designs)
Smallest vertex distance practiceable
For hyperopes, prism thinning equal to 2/3rds the add power


I use a rather simplistic method for fitting... I dot the lens with a marker and note the proximity to the pupil, then redot if necessary. Then, I compare the results to the pupillometer and the heights I've taken with my p.d. stick. It's always seemed to work well for me, but I appreciate any other "tricks of the trade."

Pete

[Darryl Meister]

Hi Pete,

I would suggest a minimum of 10 to 12 deg of panto. Refer to our discussion on pantoscopic tilt for low-powered lenses. It really won't make a noticeable difference in most cases, but the extra pantoscopic tilt with increase the wearers field of view at near (the keyhole effect you mentioned). Besides, the optical center of many progressives could be just about anywhere, particularly with prism-thinning (which is quite popular now).

Also, don't specify the amount of prism-thinning to use. Both plus and minus lenses can be prism-thinned (though the advantages are certainly more dramatic on plus lenses). Sophisticated software will determine the exact amount of prism-thinning necessary to produce the thinnest, best looking job. This may be more or less than 2/3 the add power. The 2/3 rule should be used by labs without such software. That's not the kind of thing that you want dispensers dictating, either -- unless they are matching one lens of a pair. Just let them know that prism-thinning is a good thing, why it is used, how to verify it, how much to expect, how to check for prism-thinning in current spectacles, etcetera.

By the way, I don't know about the kind of charts you're looking for, but feel to e-mail me if you'd like to discuss some contour plots.

Best regards,
Darryl

[Pete Hanlin]

Thanks, Darryl...

Prism thinning seems to generate (pun intended ;) ) a lot of confusion. In my course on Ophthalmic Applications of Prism, I spend a good amount of time explaining the mechanics of prism thinning. I guess I figured I would give them the basic formula for determining prism thinning so they could have a clue to how the amount is determined.

Come to think of it, however, that is just the sort of thing that someone would latch on to and demand from their lab! Having started optical life as a lab rat, I can appreciate the consequences... As you mentioned, the main thing is making sure they check for prism thinning when only one lens is being replaced (kinda like the need for checking OC decentration above seg in a multifocal lens that is only being replaced in one eye...).

The biggest misunderstanding I see with PALs, however, concerns the prism verification point! Over and over again, I would see lenses that had been sent back to the lab for reprocessing for "prism imbalance."

Perhaps you could list your "Top 5" (or 10, or whatever) misunderstood aspects of PALs? I think the best thing that comes out of a CEC is a piece of knowledge about something that was previously misunderstood...

Thanks for all the input and advice!
Pete

[chip anderson]

I got this from Varilux and it has served me well, almost without fail. To measure segs: Cover the patient's left eye and close your right eye. Have patient look you in the eye and dot lens at pupil. Repeat the process for the fellow eye. Then draw a line across the lense at height of dots.

Next have patient stand up, and look at a mark on the wall aproximately head high. Tell the patient not to stand at attention, or prop up on the table (you want the normal laid-back stance). See if the line is across the patient's pupils. If it is this is the height you send to the lab. I should mention that this should only be done after frame has been alighned.

Chip

[Bev Heishman]

Pete,
I just compiled a lot of info on this and blended it together and presented it. I would be happy to share with you my power point.
Bev

[Pete Hanlin]

Bev,

By all means fire it on over to my email address! Thanks a bunch! Coincidentally, I had a patient grill me for about 30 minutes yesterday on the differences between SolaMax and Panamic. Today's consumer is definitely information hungry!

Pete

[Pete Hanlin]

Just "bumping" this old post back to the top because one, it was a great string (IMHO), and two, I am now finishing up my course on progressives and would really appreciate it if those of you with graphics or other information could either post it in this string or send it to my email (eyesite@nettally.com).

Thanks to all of you for the information. All of my lectures end with a presentation of OptiBoard for this very reason- namely, the posters here represent a valuable source of information!

Pete