# Optical Forums > Ophthalmic Optics >  Seiko back surface add pal design..

## Texas Ranger

In an industry pub. there is an add tear-out section from Seiko, discussing their new tech, "internal" back surface add design. it's a 1.67 Super Proceed Internal. available from a -10 to a +6.00 and up to a _4.00 cyl, max combined power of -10.00. Now, from an old school guy who started when it was all plus cyls, this is seriuosly confusing; spherical front surface, with up to a -4.00 cyl and the add on the back surface?? Could it be affordable?

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## yzf-r1

Just reading the above post has made me think about a possiblity for a PAL.  I bet someone has already tried it already

If seiko have been able to get a back surface PAL, then we know that is possible.  So why not have a PAL on both surfaces.  That way you can get a +3.00 add by putting approximately +1.50 on each surface, and surely you would get less distortions overall.  or would you?:idea:

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## shanbaum

> *yzf-r1 said:* 
> Just reading the above post has made me think about a possiblity for a PAL.  I bet someone has already tried it already
> 
> If seiko have been able to get a back surface PAL, then we know that is possible.  So why not have a PAL on both surfaces.  That way you can get a +3.00 add by putting approximately +1.50 on each surface, and surely you would get less distortions overall.  or would you?:idea:


Depending on where you live, I think you can once again order these from Johnson & Johnson.

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## yzf-r1

does anyone know how this two-suface progressive lens performs?  were the distortions halved as one would expect?

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## shanbaum

Dunno, I've never seen one - but there's no reason to believe that "distortions" would be _halved_.  To "minimize" in this case probably means a fractional reduction.

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## Steve Machol

Until proved otherwise, optics continues to obey the laws of physics.  Putting one-half the distortion on each surface does not reduce distortion at all.  Put simply:

1/2 + 1/2 = 1

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## yzf-r1

Correct me if i am wrong.

The lower the add, the less the distortions.  Right?

The lower the add, the more peripheral the distortions, Right?

If the above statements are true, (the second one in particular), then half the distortions on the front surface, but peripheral, and half the distortions on the back surface, but peripheral.  

Therefore the distortions remain peripheral, and superimposed over each other giving the patient a greater useable lens area, Right?

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## Steve Machol

No, because the distortions on the front surface are added to the distortions on the back surface.

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## Jeff Trail

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## Steve Machol

Sorry Jeff - I disagree.  Any such effects are minimal and most likely related to the vertex depth of the back surface.  However is Darryl wants to chime in and prove me wrong then I'll happily submit.

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## Jeff Trail

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## shanbaum

Jeff, I find most of what you 've written in this thread to be completely unintelligible ("split the materials?" "type of curves?" "power carried on one side"?); but, since I know what "standard deviation" is, I'd be interested to know, of what?

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## Jeff Trail

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## shanbaum

I didn't call what you wrote "stupid".  I called it "unintelligible".

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## Darryl Meister

I would be inclined to agree with Robert and Steve... Splitting the progressive optics between two surfaces will probably not result in a significant reduction in peripheral astigmatism -- at least for the relatively flat curves of conventional spectacle lenses. Mathematically speaking, the change in power produced by a progressive surface creates a pretty predictable amount of unwanted astigmatism.

For well designed lenses, the maximum level of unwanted astigmatism in the periphery is generally close to the magnitude of the add power. So a +2.00 D lens would create around 2.00 D of unwanted cylinder power. Likewise, as Steve pointed out, two +1.00 D lenses (or surfaces) placed together, each with 1.00 D of cylinder power, would total close to 2.00 D of cylinder power, as well. Sure, you might get a little difference, but I doubt it would be enough to justify the additional manufacturing complexities -- and costs -- involved. I've always been told that for lenses of _reasonable_ curvature, whatever you can accomplish on two surfaces you can generally add together to create one surface.

Though, in all fairness, I've never really given the matter much thought, since there really aren't any commercially _successful_ lenses that do this. I had a look at the old J&J lens (Definity 2?), which split the optics between more than one surface, and was not terribly impressed by it. For instance, if I remember correctly, the corridor was quite long and narrow, and rather truncated looking in the distance zone.

There _might_ be some sort of advantage to having the progressive zone on the back surface, but none come to mind at the moment. I'll look into it, though. You could argue that the progressive zones would be closer to the pupil, and hence provide a wider field of view, but this would also require a greater amount of eye movement to get through the progressive corridor.

This discussion excludes progressive lens designs with customized atoric or complex surfaces on the back, which simply "tweak" the optical performance provided by the progressive front surface for a specific wearer. These designs do improve the performance of the lens slightly by tailoring it for the individual wearer.

Best regards,
Darryl

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## Jeff Trail

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## Jeff Trail

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## Jeff Trail

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## Jeff Trail

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## Darryl Meister

Hi Jeff,

I didn't quite understand your example, but I can say that you really do not have _that_ much conrol over the axis of the surface astigmatism in a progressive lens if you want the lens to increase in plus power as you look down. But, as I stated before, even if you could manipulate the surface astigmatism on each individual side to gain some sort of net advantage, you could still achieve the same thing on just one surface (again, unless you are dealing with especially steep lenses). Any two spherocylindrical powers can be combined into a single, resultant (or _crossed_) spherocylindrical power. So having 0.25 x 090 on the back surface and 0.25 x 180 on the front surface is not appreciably different from having a 0.25 sphere on the front and nothing on the back.

Best regards,
Darryl

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## Jeff Trail

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## shanbaum

I mentioned it because the axis of the cylinder correction does have a direct effect on the maximum astigmatic distortion and it's location.. 


OK, I can (for once) parse that much of your post, and it's untrue.  That astigmatism in a progressive lens which results from its being a progressive lens - that is, which results from the necessary transition from one desired curvature in the distance to another in the near- is not related to the Rx cylinder or its axis.  If you can show mathematically how this is true, please do so.  If you can't, please stop posting nonsense in the (loosely-worn) guise of information.

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## Jeff Trail

First, it is NOT untrue, I think your main problem is you, for whatever reason, you just do not like me, which shows by the posts, you follow one of my post with a condescending and or just a down right insult posting.. hey that's up to you.. You can go back  through a number of the threads and easily find this out
  But more or less you called my posts stupid in this thread, untrue and nonsense.. well I can and do know the formula to prove the last post, I have a ton of references in text on the formula's as well as the process to plot this.. and had two separate optical engineers who DID read the posts and they said that the "technical" side (describing the zones, power shifts, criteria to verify I posted etc., etc.) was all correct and they had no trouble following the posts..
   If you want the simplified way to see how power ground into a PAL actually changes the plotting in distortion all anyone needs to do is get any of the "plot" sheets all the lens companies put out, usually comparing distortion fields from their design to the competition .. look at the "power" used in the lens... than get two or three of them that used different powers in the SAME exact design and you can easily see the shifts in the distortion field...
   But than again.. I guess this post is "stupid, untrue, unintelligible and nonsensical" as the rest.. so forget it...
   I'll keep posting and you keep posting your condescending follow ups and it's back to normal.. I'm just amazed you haven't sunken to even correcting my spelling and grammar as well  :Rolleyes:  Lets save that for "special" occasions, shall we?.. :cheers:

Jeff "whatever" Trail

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## shanbaum

Jeff, I don't dislike you, but I have a problem with your making assertions supported only by incoherent lists of technical, or technical-sounding terms ("standard of deviation"?), as though your ability to list these lends  credence to your assertions.

This is a technical forum, and I'm one of the moderators, and as such, I try to the best of my ability to ensure that the technical content is reasonably well-grounded in optics, so as not to mislead those who look to this forum as a technical resource.

And while I try not to be too overbearing in correcting people's grammar and spelling, I think it's important that professional participants on this board (most of whom are opticians) take care to express themselves as clearly and accurately as possible, for this reason: anyone can view the postings, and poor expression and incorrect spelling might well diminsh the regard in which opticians are held by members of the public and members of the other two O's.

I apologize if I appear to assume an air of condescension when I dispute your assertions.  The best remedy for this would be for you to prove me wrong, by providing actual support for your assertions, as I suggested.  You say you "know the formula" and "have a ton of references" to prove your point, so - let's see the formula, and the references.

And, by the way, you misspelled "whatever".

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## Jeff Trail

> *shanbaum said:* 
> Jeff, I don't dislike you, but I have a problem with your making assertions supported only by incoherent lists of technical, or technical-sounding terms ("standard of deviation"?), as though your ability to list these lends  credence to your assertions.


   I "did" not go into detail about all the formula's and processes because I figured most of us in this thread probably already knew the zones of a PAL... I did not place formula's because they would be a VERY LONG posting and very complicated ..but if that is what you want..........I just figured if anyone wanted to know something they would ask about the parts they did not understand, not expecting it to be called nonsense, unintelligible, untrue, etc., etc.






> I apologize if I appear to assume an air of condescension when I dispute your assertions.  The best remedy for this would be for you to prove me wrong, by providing actual support for your assertions, as I suggested.  You say you "know the formula" and "have a ton of references" to prove your point, so - let's see the formula, and the references.
> 
> And, by the way, you misspelled "whatever".


     So what you stated as the maximum astigmatic distortion, which "every one knows" .. is only based on the distortion induced by the design of the lens (addition on the front) that "hour glass shape" every one is so quick to reference, which also could technically be referred to as induced oblique astigmatism by its location in relation to the lens surface.. BUT what changes that, as I stated in the one you said was "untrue" is the influence of the back curves and inducing oblique power, shifting that power in over lapping points along and or around or over the
contour plot of the "distortion" induced by the PAL design..the "hour glass shaped" distortion is a set plot of distortion so if for instance you have a script with a 90 degree axis, we know the power is actually at 180 which is the OPPOSITE "axis" of distortion due to design.. there by you get a reduced field of vision by having "over lapped distortion on opposite planes..i.e smaller MAD area..so claiming "we already know the distortion and it will be the same" is NOT correct.  I also mentioned the "power" as well as "axis" influencing the different field in shape size and "power".. so you can toss in barrel distortion and pincushion distortion as well depending on the RX of course..basically what you are calling the disortion we all know about is the radial astigmatism that is induced from the front surface WHICH is not reduced by the back surface once we grind it but is added to depending on the RX.. so now we have a shift in the contour plotting closer to the MRP, not even taking into account if we use "prism thinning process" or not.. so if you did do a MAD test the NUMBERS would show a wider variation, NOT just that "hour glass" plot that gets rolled out as the "only" answer. There by it would not "always" be close to or "same" as the add power.
    All this will have a direct effect on EVERY thing I listed or as you referred to as "incoherent lists of technical, or technical-sounding terms" ... OR I probably just am making "technical-sounding terms ("standard of deviation"?), as though your ability to list these lends  credence to your assertions." again.. :Rolleyes: 
    BTW a list of formula's to use to figure out all this would include
power in the oblique meridian (Martins formula) circle of least confusion and how it effects "over lapping contour plots (i.e. interval of sturm) Since we have two sets of distortion you can also use Thompsons formula to help as well, also know about spherocylindrical and deviation (tada, oh my there is that "silly" standard of deviation again popping up :))
     I can keep going, list a bunch of the criteria for the testing of that "incoherent list" as well as probably 7 or 8 references (text) if needed..BUT than again maybe I just made all this up as well, "ability to list these lends  credence to your assertions."
;)  Than again I don't think it really matters one way or the other, if I said it was "black" you would tell me how wrong I was and say it was "white"

Jeff "stick a fork in this thread..I'm done" Trail

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## shanbaum

Well, I couldn't ask for a better case in point.  I guess you just really don't know when what you write is unreadable.

I did notice one brief moment of something like clarity, however:


the "hour glass shaped" distortion is a set plot of distortion 


...which I think may be Jeff-ese for exactly what Steve, Darryl and I have been saying right along.  The question in the first place was, can "the distortion" be reduced by putting half the add on the back of a progressive lens instead of putting all of it on the front?

I believe, and I think that Steve and Darryl believed, that the "distortion" to which the inquiry referred is the astigmatism that you want to call, the "hour glass shaped distortion".

The question was not, to try to recast it as something that would be related to the topic you seemed to want to write about, "is there anything that can happen on the back surface of a progressive lens that will affect the 'maxmum astigmatic distortion' measured anywhere on the lens?"

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## Texas Ranger

Wow, I queeried a reasonable question of how do you get a
 -4.00 cylinder AND the add power on the back surface of a progressive lens? and after everyone says I dunno, it degenerates into some bad feelings about splitting adds front and back, etc. so, who do we get these Seiko lenses from? Has anyone tried them? Are they quite expensive?

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## Jeff Trail

> *Texas Ranger said:* 
>  who do we get these Seiko lenses from? Has anyone tried them? Are they quite expensive?



   TXR,

       I have seen them,  prices are a no-no on here (moderators do not like prices mentioned. Hmm..think what a Varilux comfort is and just double it and you are in the ball park :) .. If you contact Seiko and request it they have a number of labs that handle it.. I think it's five or so labs.. but they order it from oversea's and than they get it sent to them.. one of the labs that I know of is Lea over in Tampa/St.Pete area.. IF you do decide to do it I would really advise you to get it edged at the lab.. I seen these lens and they are pretty tricky to edge, and considering you have a 4 week or so order time just to get the lens I would not take the chance...
      BTW I don't have any "hard feelings" one way or the other, I just more or less am going into "ignore" mode.. :)

       If you get a chance to sell this lens and find someone willing to go for the price and waiting it is a good lens.. you will sure like the look and the optics.. I know I did.. If you want I can find Lea # and you can give them a call and get some info.. I know they had a post in the Job board for a finishing tech a few weeks ago and you could get an E-M from there..

Jeff "no qoute, shoot Robert would only claim I was using techno babble anyway" Trail :)

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## Darryl Meister

> Wow, I queeried a reasonable question of how do you get a -4.00 cylinder AND the add power on the back surface of a progressive lens?


It's actually related to the discussion on splitting the add power between two surfaces. Just as you can create a single progressive surface from adding together two separate progressive surfaces, you can also create a single progressive with cylinder power by adding together a progressive surface and a toric surface. Here is an easy way to think about... Imagine a plain cylinder, which is flat along its length and curved around its perimeter. Now, take a progressive surface and "wrap" it around the cylinder so that it conforms to it. Finally, pull the progressive surface away from the cylinder. You now have a progressive surface with cylinder power.

Best regards,
Darryl

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## Pete Hanlin

I didn't read the entirety of all the posts that have preceded (and wow, they look like they may have been interesting), but here is my $0.02...

To me, the whole concept of "splitting power" (of any kind) comes down to this- you can create the exact same outcome without splitting the power.  You can create the same optics without complicating the process (which, as I understand it, involves using a grind-to-polish generator and then a soft lap polish).

Why complicate things if it isn't required?

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## shanbaum

> *Pete Hanlin said:* 
> To me, the whole concept of "splitting power" (of any kind) comes down to this- you can create the exact same outcome without splitting the power.


That was precisely Darryl's point (one of his points, anyway) - anything you can do on two surfaces, you can do on one.

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## shanbaum

> *shanbaum said:* 
> That was precisely Darryl's point (one of his points, anyway) - anything you can do on two surfaces, you can do on one.


Though I should point out, while the above may be true from a theoretical standpoint, there may be some practical limitations to implementing some feature on a single surface - i.e., manufacturing constraints - which might be relaxed by utilizing two.

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## yzf-r1

i am so glad i posted this question on the board because i have one point pretty clear in my mind.  i.e. The point made independantly by darryl and pete.  "Whatever you can accomplish on two surfaces you can generally add together to create one surface."

This was a point that was bugging me for months, but i just kept putting off actually posting.

I dont think such a healthy discussion as this could have taken place on any other board anywhere on the web.

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