Calling all "Meisters"....

[Barry Santini]

Here's a question my lame brain can't compute, and I need all optiboarder's help:

At what minus RX (sphere) power does the *edge* thickness of the following two materials end up being equivalent?

Essilor Ultra Litestyle (1.1mm center thickness) Aspheric Polycarbonate

vs.

1.67 Aspheric (center thickness 1.6mm)?

These are the two material choices i have on my plate when *surfacing* a minus Rx, as opposed to finished lenses where the 1.67 centers are often found to be approx 1.3mm.

Brainiacs...let's go!

Barry

[Jacqui]

I cheat ;) , I have a program that I got from Jack at Optispares (OB member) that calculates it for me. :bbg:

[MarcE]

I got -4.75 using the Opticampus calculator. I suppose this could vary 0.25D either way depending on the asphericity and base curve of the lens, which could be different for different lens material and manufacturers.

However, I wouldn't pay for a 1.67 lens with a 1.6mm CT. My stock lenses come in at 1.1mm and want my surfaced lenses at 1.4mm

[AWTECH]

Why is the 1.67 center thickness 1.6mm on a minus lens. What is the logic behind your question? Just curious.

You could calculate these as both spherical to get an idea. Then you would not be considering different aspheric designs.

1.6mm center thickness?????

[Barry Santini]

1.6mm centers on a 1.67 surfaced lens...because that's the best my lab will consitently do. And I'm only speaking on the *thickness -equality-threshold" here.

And I'm not a big fan to argue about 0.3mm on *any* lens thickness..for me..it just doesn't matter that much.

With this info, I think I'll visit the Opticampus calculator and try it out for myself.

Thanks

Barry

[HarryChiling]

The asphericity is what throws off this problem, but since you are asking at what Power the thicknesses are the same? Just make the thickness formulas equal to each other.

((diameter/2)^2*Power_poly)/(2000(0.586))=((diameter/2)^2*Power_hiindex)/(2000(0.67))

[MarcE]

And I'm not a big fan to argue about 0.3mm on *any* lens thickness..for me..it just doesn't matter that much.

I think it should matter. The difference of 0.3mm is about the same as the difference between a 1.60 and a 1.67 lens. Or the difference between a 1.67 and a 1.70 lens. If you accept 1.6mm, you are cheating your patient or yourself.
0.3mm cost quite a bit of money. I would challenge your lab to do a better job.
Maybe your calipers need to be calibrated;)

[MarcE]

That last post sounded too harsh. I know where you are coming from. The lab shoots for 1.5mm and will accept 1.6mm CT. However I want my lab to shoot for 1.4mm and I don't WANT to accept 1.5mm, but I will.
That's why I try my best to use a stock lens for any SV, especially High index.
I ordered a 1.70 stock once and the CT was 1.05mm. Complete with AR it was half the price of a surfaced 1.67 lens at 1.5mm CT. The difference on the edge would have been about 0.75mm. That is very visably significant. The patient was impressed comparing them to her 'old' high-index lenses.
The factory-applied coatings on stock lenses are more consistant too.

This isn't news to any of you experienced guys, but the newbies might have learned something.

[Darryl Meister]

Just make the thickness formulas equal to each other.

If you're using that sagitta approximation, you can actually just drop much of it, and then rearrange the powers as a function of each other:

Power1 = Power2 * (Index1 - 1) / (Index2 - 1)

where Power1 is the first power made using Index1 and Power2 is the second power made using Index2. However, this approach wouldn't work if you're using different center thickness values. You'd have to use something slightly more complicated:

Power1 = Power2 * (Index1 - 1) / (Index2 - 1) + Center2 * (Index1 - 1) - Center1

where Center1 and Center2 are the minimum center thickness values of Power1/Index1 and Power2/Index2.

If you accept 1.6mm, you are cheating your patient or yourself.
0.3mm cost quite a bit of money.

Keep in mind that the ANSI thickness tolerance is actually 0.3 mm, so insisting on a zero tolerance may cost you quite a bit of money on your lab bill, as well.

[Barry Santini]

My previous posts did indicate *surfaced* lenses...where the target thicknesses and tolerances are different.

I agree with Darryl...insisting on tight tolernaces for center thickness for surfaced lenses is, well...misplaced attention/focus in my opinion.

The only place is physiological optical that 0.3mm is known to be significant is in the axial length of the human eye...

0.3mm change in axial focal length = 1.00D (approx).

Now that's somethin' to be focussed on.... (hee hee!)

Barry Santini, ABOM

[MarcE]

I know you guys have more experience than me, so I won't convince you otherwise, but there are some lurkers here that may read and learn something.
A patient can look at the edge thickness of a lens and see a 0.3mm difference without a ruler.
According to you Barry and Darryl, I should just accept a 1.67 lens that comes from the lab with a 1.8mm CT. Because the standard CT of a 1.67 surfaced lens is 1.5mm plus the ANSI tolerance of 0.3mm =1.8mm
First you need to remember that the 1.5mm CT is not MY specified thickness. It is the lab's specified thickness based on processing limitations. 1.67 material will meet the FDA std. at something less than 1.1mm, so that is the CT that I want based on the price that I paid. (even though I know I can't get it in a surfaced lens)

I put a -7.00D sph lens (A=50, bridge=18, PD=62) in the Opticampus calculator and this is what I got for poly at 1.1mm CT and 1.67 at 1.8CT.

Edge thickness of 1.67 @ 1.8mm CT = 5.8mm
Edge thickness of poly @ 1.1mm CT = 5.7mm

Someone got ripped off by paying extra for High-index material.

I would never accept a 1.67 lens from my lab that had a 1.8mm CT. You say it's within tolerance, so it's good to go. It's not my tolerance. I wouldn't want to wear it. I won't pay for it.

[HarryChiling]

I wonder how many here even specify center thickness when ordering. They only have to meet the center thickness tolerances, when you specify it.

[LENNY]

Why are we talking surfaced when you can get it stock!
And if it is -14.50-3.50X180 I dont think the center thickness will make that much of a difference as lond as you choose a right frame!

[Barry Santini]

I know you guys have more experience than me, so I won't convince you otherwise, but there are some lurkers here that may read and learn something.
A patient can look at the edge thickness of a lens and see a 0.3mm difference without a ruler.
According to you Barry and Darryl, I should just accept a 1.67 lens that comes from the lab with a 1.8mm CT. Because the standard CT of a 1.67 surfaced lens is 1.5mm plus the ANSI tolerance of 0.3mm =1.8mm
First you need to remember that the 1.5mm CT is not MY specified thickness. It is the lab's specified thickness based on processing limitations. 1.67 material will meet the FDA std. at something less than 1.1mm, so that is the CT that I want based on the price that I paid. (even though I know I can't get it in a surfaced lens)

I put a -7.00D sph lens (A=50, bridge=18, PD=62) in the Opticampus calculator and this is what I got for poly at 1.1mm CT and 1.67 at 1.8CT.

Edge thickness of 1.67 @ 1.8mm CT = 5.8mm
Edge thickness of poly @ 1.1mm CT = 5.7mm

Someone got ripped off by paying extra for High-index material.

I would never accept a 1.67 lens from my lab that had a 1.8mm CT. You say it's within tolerance, so it's good to go. It's not my tolerance. I wouldn't want to wear it. I won't pay for it.

Good! And now I know not to think 1.67 (surfaced) at least until -6.00D.

Marc,

I think you are not fully informed about the variables that contribute to the ANSI thickness tolerance/standard in a laboratory. If the lab were to reject all the lenses using *your* tolernace...you & I would be paying ALOT more for surfaced lenses.

Don't get caught up in 1/10ths of a mm...look at the entire pair of glasses as a finished product. My 33 years of experience says that if you continue to set your personal *bar* as high as you are implying, then you will either be very disappointed...or you will burn out from frustration.

Step back and look at the bigger picture...0.3mm simply doesn't matter...for *any* pair of eyeglasses.

My 2 cents

Barry

[RT]

First you need to remember that the 1.5mm CT is not MY specified thickness. It is the lab's specified thickness based on processing limitations. 1.67 material will meet the FDA std. at something less than 1.1mm, so that is the CT that I want based on the price that I paid.

MarcE:

1.67 materials only pass FDA dropball requirements at 1.1 mm if they have thermal cured hardcoats designed to enhance impact resistance. Most labs use semi-finished blanks with front side factory hard coat, then apply a UV cure spin coat to the backside after surfacing. In that configuration, the minimum thickness to pass FDA would be 1.5 mm.

It is for this reason that 1.67 stock lenses (which have a factory-applied hard coat) and 1.67 lenses with an integrated AR/hardcoat (which use naked blanks and then apply a thermal cure hard coat base) are supplied at a lower CT than 1.5mm for minus Rx's.

But I'm happy that you provided the example you did, because it echoes the point I've tried to make many times here--that higher index does not always equal thinner lenses. The moral of the story is that you have to know what you're ordering, since differences in configuration will make a difference in the final product.

[Darryl Meister]

First you need to remember that the 1.5mm CT is not MY specified thickness. It is the lab's specified thickness based on processing limitations.

And the 0.3 mm tolerance is the lab's specified tolerance based on process capabilities. Expecting a zero tolerance on any kind of manufacturing process like this is unreasonable, at least without paying a lot more for the product.

Secondly, while polycarbonate may pass FDA at a 1 mm center, that doesn't necessarily mean that it's a particularly good idea to have it processed so thin, especially if you're selling it to your patient as a "highly impact-resistant" lens material. You compromise the flexural strength, tensile strength, and impact resistance of the material as you reduce the minimum thickness. This increases the likelihood of processing aberrations and breakage during the surfacing and finishing stages. This is why many labs charge more to process poly to a 1 mm center, and why not all lens manufacturers even test it down to a 1 mm center.

Further, since impact resistance decreases significantly with a reduction in center thickness, you are also compromising that particular benefit of the material to some extent (granted, with poly, you have a lot of room to play with in terms of impact resistance). With thin-centered finished lenses, lens manufacturers often add a primer coating to maintain impact resistance.

[Chris Bowers]

The question is missing some vital information. We would need to know the diameter of the lens or its greatest radii and the described aspheric design. Truth is either would be thinner at a specific diameter or lens size.

[Darryl Meister]

Truth is either would be thinner at a specific diameter or lens size.

Yes, indeed.

[Slug]

Another consideration on the 1.0 Poly is that it can affect how secure lenses are in a frame. In frames with more wrap and small "B" measurements, lenses with higher minus correction will flex more because there is significantly less thickness in the vertical meridian.

[SunnyNC]

I ordered a Poly 1.0, Panamic, Rx -1.50 -0.50 x095 add +2.25. Eyesize 50.

The patient doesn't like the edge thickness. When I check it I get a center thickness of 2.5 mm and an edge thickness of 3.25-3.50 mm. I cannot find a 1.0 mm thickness anywhere on the glasses. How does the add effect this thickness. Can anyone explain why these lenses seem thicker than they should. It seems to me that they may be regular poly. ??????

Thanks for your help.

Sunny

[drk]

This is a very rough, empirically-devised, averaged-out, rule of thumb that I use:

For cosmetically noticeable differences:

plano to -4.00 use whatever (poly IMO)

-4.00 to -8.00, use 1.60 or poly

-8.00 to -12.00 use 1.67

-12.00 up use 1.74

[AWTECH]

I ordered a Poly 1.0, Panamic, Rx -1.50 -0.50 x095 add +2.25. Eyesize 50.

The patient doesn't like the edge thickness. When I check it I get a center thickness of 2.5 mm and an edge thickness of 3.25-3.50 mm. I cannot find a 1.0 mm thickness anywhere on the glasses. How does the add effect this thickness. Can anyone explain why these lenses seem thicker than they should. It seems to me that they may be regular poly. ??????

Thanks for your help.

Sunny

The progressive design is spread out over the entire lens blank. To make the Rx work for both distance and near vision the back spherical curves require a certain minimum thickness which in your case is 2.5mm. If as an example we had made the lenses using the ICE-TECH individualized design. I did a quick calculation with our software and got a 1.6mm center with a maximum 3.2mm edge and minimum edge thickness of 1.7mm. The actual lens you would need may vary by 0.2 due to PD, B measurement etc.

This is one of the reasons an individualized lens is so superior in many cases. The PAL is designed for the frame not a 75mm premade lens blank.

[Darryl Meister]

Keep in mind that both free-form and conventional surfacing are subject to thickness constraints. With sufficient information, a laboratory can surface this job to just about any reasonable center thickness.