Essilor & Trivex

[Pete Hanlin]

Robert,

Very good observations...

1.) As you note, high index lenses run into the same challenges as polycarbonate lenses.

2.) As you mention, by bringing the distance power of a PAL up to +4.00, with an add power of +2.50, you end up with about 7 diopters of prism at near (figuring on a reading distance 14mm below OC and subtracting out the prism thinning). With a polycarbonate lens, this results in about 0.25 diopters of C.A.

No one material is going to be well-suited to every Rx. Using your example, polycarbonate and high-index are both going to compromise near vision to a slight degree (although we also have to take into account that the wearer isn't really viewing 14mm away from the OC of the +6.50 power, so the C.A. will actually be a bit less than 0.25 diopters).

If anything, I believe the example shows how little abbe value effects the optical quality of the lens. After all, the vast majority of Rx lenses fall between +2.00 and -4.00. At these relatively mild powers, it will be hard to encounter enough prism to create much C.A.- especially since PALs with minus powered distance Rx will tend to have reading zones with less prism power than commonly encountered in similar portions of a SV lens.

Anyway, it is refreshing to discuss realistic effects of lens properties- instead of aberrations that exist only in marketing materials. Meslin & Obrecht have an interesting chart that measures relative visual acuity through different amounts of prism and different materials. I forget what publication it appears in, but it gives an interesting perspective on just how much acuity is lost to chromatic aberration.

[David Wilson]

Pete,
I have been away for a bit and have just caught up with this post. I am not aware of the Meslin and Obrecht chart but I think that the Abbe number and the transverse chromatic aberration 'problem' has been a bit overplayed. If only we paid as much attention to other aberrations. Dan Torgersen of the United States and Mo Jalie of England have both written excellent articles giving the clinical significance of TCA, and it is relatively insignificant, particularly for powers below 4.00 D. As you also point out other high index materials (from all companies including your own) have Abbe values which are not much higher that poly but receive little criticism. My own lenses have an Abbe value of 36 and my Rx is +7.00 D with a +1.50 D add yet I am not troubled with TCA. Sure, I can find it if I look hard enough.

I think that its fair to say that all of the companies produce excellent materials and designs. Some practitioners will prefer Hoya, some Essilor , some Sola etc, and each of those companies will highlight what they consider their advantages over others. I think that it may be a bit of an oversimplification to suggest that one company prefers marketing and another research. Both of the companies mentioned have extensive R and D departments and both market exceptionally well. It is also true that mantufacturers of Trivex and other higher Abbe number materials have highlighted this in comparison with poly.

Poly has had to defend itself from a rather poor start (as did CR-39 in the early days). I think that it is better to merely consider it a part of the suite of lens materials available, sometimes it will be ther material for the job and sometimes it won't, just like all other materials.

Regards
David

[Jim G]

Oh my, where do I start?

First, a disclaimer...I'm retired and, while prejudiced, have absolutely NO vested interest in anything (well, almost nothing!).

Probably first, the discussion above should be used as a primer by every dispenser here. Namely, NOT EVERY PRODUCT IS BEST FOR EVERY PATIENT. Learn and use appropriately.

Second, maybe first for y'all, the availability of alternatives and the INFORMED recommendtions by dispensers is the reason YOU EXIST! If poly was best for all, or Trivex, or Vx, or Hoya, we wouldn't need educated, informed dispensers. You could access an ATM-like device, enter your Rx, and get the result spit out.

Inasmuch as this seems to be a poly/Trivex debate, let's look:

Cost: poly-lower; Trivex-higher.
Styles: poly-extensive; Trivex-limited.
Power range: poly-slight advantage
Weight: poly-slightly more; Trivex-slightly lower (poly can generally be surfaced thinner and is available in 1.0 finished while Trivex has a lower weight)
Drill-mount: given = quality equipment, equal; given otherwise, advantage Trivex.
Surfacing: both more difficult than CR-39 but, a small advantage to Trivex.
Aberration: theoretical advantage to Trivex; practical below +/- 4.00, equal; above that, advantage Trivex.
Impact: essentially equal. Any impact that would break either would probably place the lens firmly in the back of the wearer's head.

What did I miss? Oh, innovation. 1985? Poly. 2001? Trivex.

Your choice.

[Chris Ryser]

Jim G said:

What did I miss? Oh, innovation. 1985? Poly. 2001? Trivex.

Your choice.

Attended lectures on Polycarb lenses in 1976

[Pete Hanlin]

David & Jim,

Excellent comments! I really liked the objective comparison of the two materials, and I suspect you've pretty much summed it up.

Regarding the date of origin for polycarbonate, I'm not sure when polycarbonate was first used in ophthalmic applications, but in my opinion the material was "born again" in 1978. It was in this year that Polygram (a division of Philips) determined that polycarbonate would be the best material for the Compact Disc. The huge demand this created for the material resulted in the high quality raw polycarbonate lens manufacturers utilize today.

Anyway, as Jim points out, no one material is "the" answer for everyone. It would be a mighty boring profession if there were such a universally applicable material. For my money, however, polycarbonate is a very handy material for most Rx needs!

[Jim G]

I reached into the recesses of the (rapidly fading) mind recently on the origins of poly.

I first ran across it in the 70's with Hermann ("the German"--his sobriquet) Weber while at a company owned by Jacque Cousteau (was it Air Liquide???). He later resurfaced as TechSight, also in the 70's. I believe that TechSight was later purchased by AO. Gentex also originated in that time period, originally with 3.0 and subsequent 2.0.

Both companies started finished 2.0 in the early 80's. Semi-finished followed shortly. Vision Ease started Poly Optics, their polycarbonate division, in 1985 with semi-finished SV, D28 and, maybe a D35.

[Jim G]

Just realized I needed to add something on this.

All of us have credentials or experience of some sort...that's why we're posting.

David Wilson is a professor of optics--or something directly related--in Austrailia. When he posts on optics or optical theory, we all best listen up.

We're lucky to have him offering advice, direction or counseling.

[Jimbob]

i agree that all lenses have their place, but in all honesty poly is not terribly welcome in my practice, we charge the same for trilogy, and airwear... simply due to the prices available from our suppliers being too close...

trivex is tintable, has a much higher ABBE and does drill better, while keeping less stress through the lens. chemical resistance closer to glass

poly is available with teflon (just to show this is not a hit on essilor), has a higher index, hmm thats it

poly has high base curves from what i've seen, a very low ABBE, is untintable, and is quite prone to damage... oooo look out it acetone, lens gone.

Germany has quite strict laws regarding optiks, and what can be dispensed, and to the best of my knowledge poly is only available for safety specs to be worn in a workplace, ie not in a car. but contact lenses are made of material similar to polycarbonate, and have abbes to match... did anyone else notice the halos?? i did, and they were found i my great minimal x ... (don't worry kids i have changed my lenses)

sure some may question the relevance of ABBE's... the eyes optical system only has an ABBE of 38, so why would people care about the trivex having only 42, cr39 having 4something, and glass having 48.... who cares.. but why would you reduce the human eyes natural system to and ABBE as low at poly???


essilor/sola and younger all have money in polycarbonate, but essilor (to the best of my knowledge) have the largest amount of money invested in polycarbonate, and this is a problem... when buying the raw materials for poly it is bought in bulk, ex japan, and is quite often contracted.... to anyone who has too much polycarbonate.. sucks to be you, maybe you could sell it to sony...

anyway my personal choices in lenses are to go for trivex, or go to hoya for 1.6 if high index is required ( Australia does not yet have phoenix available in anything BUT stock single vision.... sucks to be here)



James



pete i understand you work for a poly company, and you do appear to be quite unbiased, and respecting of dispensers own decision, and for that i applaud you