Polycarbonate

[TEMPLBNDER]

I am having an issue with my staff selling premium products. Does anyone think polycarbonate is a premium product? I say there are too many inherent problems with the product that you don't get with Trivex, 1.56, 1.60, 1.67. Any thoughts would be appreciated.

[drk]

Polycarbonate is "premium" compared to...uncoated CR39.

It's a word game.

I have used it almost exclusively for about 25 years without any real problem, optically.

Truth be known, though, I would consider it "a standard material" but the industry doesn't.

[Robert Martellaro]

I am having an issue with my staff selling premium products. Does anyone think polycarbonate is a premium product? I say there are too many inherent problems with the product that you don't get with Trivex, 1.56, 1.60, 1.67. Any thoughts would be appreciated.

1) Lowest density, lightest weight, ophthalmic lens available.
2) The only ophthalmic lens that doesn't chip or crack.

Need more proof?

Low chromatic aberration, high strength, available in almost all PAL designs.

No disadvantages except for high dioptric values in improperly sized frames.

That makes it premium in my book.

I have no financial interest in PPG or any of its subsidiaries and affiliates.

Best regards,

Robert Martellaro

[Uilleann]

Trivex is still more expensive, and more limited in availability. Though it has slowly improved a bit over the years. The "optics of poly are crap" argument has been well and thoroughly debunked. The powers that it is often used, say +2.00 to -3.00 or so, and cyl under 2.00, iif fit with even a shred of competency 99.999% of patients wouldn't notice any optical difference between glass, CR, poly, Trivex, or 1.60, Abbe values be damned. Add to that the fact that poly is thinner, negating any noticeable weight savings Trivex tries to claim. You'll probably find more than a few pts far more concerned with thickness than optics.

At the end of the day, fit what you like, what works, what is profitable enough to allow you to keep the lights on, and what allows you to keep your remakes to an absolute minimum. There rarely is a "right" or "wrong" answer to these questions, but there is no valid optical or medical reason NOT to offer poly in your quiver of lens materials. When and where you choose to use it is up to you of course.

[msimko]

... The "optics of poly are crap" argument has been well and thoroughly debunked. ...

That's a bit extreme. The optics of CR39 is better than polycarbonate, and crown glass optics is better still for optics.

The ligntness, thinness, UV absorption, and impact resistance make poly a good choice, but prismatic thinning over 2 D with polycarbonate and there will be focus issues. 1.6 with it's lower Abbe is worth considering, but the additional weight (higher specific gravity) becomes a consideration.

"Abbe values be damned" is not the way I approach it.

[Uilleann]

That's a bit extreme. The optics of CR39 is better than polycarbonate, and crown glass optics is better still for optics.

The ligntness, thinness, UV absorption, and impact resistance make poly a good choice, but prismatic thinning over 2 D with polycarbonate and there will be focus issues. 1.6 with it's lower Abbe is worth considering, but the additional weight (higher specific gravity) becomes a consideration.

"Abbe values be damned" is not the way I approach it.

Hardly extreme. The original assertion I mentioned was extreme. Poly simply works, and works extremely for vast percentages of patients worldwide. It's not perfect in every case, but then, neither is crown glass, CR, 1.60, Trivex, or any of the rest. All have pros and cons. My point was that poly has been so viciously maligned for so long, that it's erroneously taken as wrote to be "bad", and should "never be used" by many. It absolutely has it's place, as has been proven by decades of satisfied eyeglass wearers in poly globally.

[drk]

msimko, "optics" are just numbers. Put the material in an application and the "numbers" can be meaningless.

[drk]

OK, brain exercise for me and you: If there is prismatic thinning, does that help, hurt, or is it neutral with dispersion?

Here's the formula:
n1sin(θ1)=n2sin(θ2)

[Alfredo Alaniz]

Poly’s not premium in the traditional sense, but it’s also not trash like people used to say. It’s durable, light, blocks UV, and works great for most scripts. Sure, Trivex has better optics and isn’t as brittle, but it’s pricier and not as widely available. Unless you’re fitting high Rx or super picky patients, poly gets the job done just fine.

It’s more about using the right material for the right person, not whether poly is “premium” by label.

[ChrisBowers]

DRK,
There are three types of prism thinning:

1. 2/3 add power as a prism ground base down on plus lenses only. This was old school and dropped as computers could navigate more complex math problems.
2. 2/3 add power ground base down on both minus and plus lenses. This has added a bunch of unwanted consequences.
3. Optimal prism thinning where prism thinning is computed with regard to lens power, vertical decentration and add power. On plus power the greatest dispersion will be at the PRP. On minus powers of lessor amount the greatest dispersion will be at the movement away from the PRP. On lenses where the refractive power is greater than the prism thinning this dispersion will grow exponentially as movement from the PRP increases. This is also a good reason for higher minus lenses be held to smaller “B” measurements as well as MRP’s be held to smaller vertical movements.

Most prism thinning is held to max 2.00 D so as not to take too much latitude with Dr’s prescribed RX. As a side note I do not remember but a handful of rejections for chromatic aberrations. I never even made a redo code for this.

Chris

[drk]

Wow! Great post.

[AndyOptom]

Agreed, great post on dispersion 👍

Another criteria w.r.t. polycarbonate lenses... for patients truly concerned with the lenses and durability, I do tend to recommend frames positioned slightly closer to the eyes, along with a little more wrap. Why bother with improved impact resistance if the frame doesn't give proper coverage anyway?

Following from that, for such frames, I find it less likely for patients to be able to consistently gaze that far off-axis such that chromatic aberration becomes a major problem. Similar logic to why one would fit shorter corridors for PAL in frames with less vertex distance and/or more pantoscopic tilt, the lens peripheral needs more extreme eye rotation to be accessed.

And of course, in such cases I go for a custom grind polycarbonate lens with POW compensation, so the wrap should be less of an optical problem. Stock lenses don't pass the minimum impact resistance thickness requirements anyway.

[AmandaWithrow]

I prefer trivex over poly because I don't see the issues with delamination and stress fractures that I see with poly - there is the caveat, however, that trivex doesn't tint as dark as poly and there aren't as many options available in trivex as there is in poly.

[SergeTrigger]

huh, I always thought Trivex tinted darker than poly but I think it has more to do with how tintable the scratch coat is more than anything else. You're right about availability though

[AlirezaM]

I’ve worked a lot with polycarbonate, but honestly, I don’t consider it a high-quality lens.
Here, polycarbonate is even more expensive than 1.61 MR8, so I usually go for it only when it doesn’t make financial sense for the customer to choose otherwise.

My reasons:

Polycarbonate is naturally soft and kind of gummy, so if the base curve of the lens doesn’t match the frame perfectly, it distorts.

It has a low Abbe value, which means stronger prescriptions show noticeable chromatic aberration when looking away from the optical center.

Its AR coating usually wears off faster than other lenses, especially in hot climates.

Edging polycarbonate puts a lot of pressure on the machine.

When I need to cut a groove for mounting, it’s often a headache and rarely comes out clean.

And last but not least, polycarbonate isn’t truly unbreakable — under heat stress it cracks over time, and in pressure points it can even react with alcohols or acetone and crack.