A Better Understanding

[Hardworlder]

Hey everyone, I'm new to the board, (this is my first post), and wanting to learn a lot more about basically everything optical that I can. I've been working in optical for a little over a year now, most of that time at a large retail chain, but have recently moved to a private practice office.

It seems that everyone I ask has a different take on lens specifications and qualities. I realize that most of this is probably skewed by the amount of money a business is trying to make, and especially in a corporate environment, there is a lot of misleading marketing as well.

Without trying to cover too many things in one post, I'll start here: CR-39 vs Poly

Here's what I (hopefully) know:

Poly is somewhere around 20-30% lighter than CR-39 and more impact resistant. It has a higher refractive index, but a lower Abbe value, so there's distortion around the periphery the more severe a prescription becomes and the larger the lens is. CR-39 is supposed to be optically superior, but the safety issue makes it a better choice for many people (and all minors, of course), and the thickness/weight are also a pretty major concern for a lot of people too.

Poly also tints poorly (this I've seen firsthand), but CR-39 will get as dark as you want it to. As for the durability I mentioned earlier, I have seen CR-39 snap in a semi-rimless frame and have always tried to avoid putting CR-39 into such a frame unless a patient absolutely insisted.

A few conflicting things I've heard:

I was first told that Poly is more scratch resistant than CR-39, and that CR-39 would basically scratch if you breathe on it. I've since read that CR39 is in fact scratch resistant, and that Poly is actually much softer, despite being impact resistant. Now in my retail job, I do know our Poly lenses had a scratch coating applied by the manufacturer (Essilor), and our CR39 was ordered totally uncoated, so maybe that's the difference.

I mentioned that I'm aware of some distortion associated with Poly lenses, and that CR-39 is supposed to be a little superior optically, but I've recently heard a very strong opinion that "People just can't see well out of poly." That seems a little broad, and from what I've seen in dealing with a pretty wide variety of patients, not very accurate, but just throwing that out there as another thing I've been told.


So for now, I guess, if anyone could help shed a little light on the whole scratch resistance thing, and also help me understand exactly how the relationship between refractive indices and Abbe values works (which is more important, and do I have the right idea about the materials' respective values in each?), that'd be really helpful. As I've said before, there are some things I "think" I know, but I'm totally open to hearing something else. I'm wanting to really get an unbiased look at these sorts of technical things (or at least see more than just the same biases I'm used to already haha).

[Fezz]

Welcome to Optiboard!

I am busy tending to my hops farm, but I am sure that some of our helpful members will be along to help you sort it all out!

[edKENdance]

Welcome, and http://en.wikipedia.org/wiki/Correct...Lens_materials

[pseudonym]

HW: I'm kind of new myself but have had my own questions about poly over plastic. The company makes more profit on poly, yet they run constant sales where the lens has to be plastic to "qualify" for the sale price. Otherwisem it's a $40. upgrade.

I had one who I'd forgotten I'd sold CR-39 to before, upgraded her to poly because the company says we have to sell a certain percentage of the stuff (I know I'm going to burn in hell) and she said her vision was sharper with the old pair. The eyesize and lens shape were almost identical, so it was the material.

The real question about CR-39? Is it really the CR-39 of old? I've heard today's plastic is MUCH more brittle than it used to be. Something about Orma vs Durex.

[Robert_S]

I'd like to address this, If I may.

I've recently heard a very strong opinion that "People just can't see well out of poly." That seems a little broad, and from what I've seen in dealing with a pretty wide variety of patients, not very accurate, but just throwing that out there as another thing I've been told.

It's a fact that people, particularly those with a high Rx, will not see well through a polycarbonate lens. The vast majority will see adequately, however, and will never present symptoms associated with poor visual accuity.

However, if you've worked in optics, the first thing you should have realised is that everyone thinks they can see well, until they can actually see well.

How many times will a patient tell you that they have no problem in bright light or with glare, after you tell them the benefits of polarised lenses? The truth is that nobody can see well in all light conditions without polarized lenses, but many, many people think they can.

Or how many times will a patient think that they have excellent vision, and that they don't think they need glasses at all, only to be prescribed a +2.00/-1.00...

They wll tell you that they don't struggle with the dashboard in their bifocals and that they have no peripheral distortion in their [whichever poor quality design] varifocals.

So you can see that whether or not the patient thinks that they can see well really is irrelevent.

The point is that when you are recommending a product to a patient, you are not recommending it on the basis that they will be able to see ok. You are recommending it on the basis that it will give them the best possible vision. The patient may never know the difference, but you will.

[Barry Santini]

The chroma error of poly layers on top of other peripheral errors. Try a good, well corrected FF SV lens like Individual in poly and what u think.

B

[uncut]

The chroma error of poly layers on top of other peripheral errors. Try a good, well corrected FF SV lens like Individual in poly and what u think.

B

There's got to be an app for this, Barry.............LOL

Expanding on Barry's explanation:

The usual visual result should be expected. If you move a person, to a higher index of lens, than the one currently being worn, and the power change is nil, then expect a reduced excursion gaze, and an increase in chromatic aberration. Also, expect an increase in reflectance.

A untreated CR39 lens versus an untreated polycarbonate lens will result in the CR39 lens winning the hardness(scratch resistant) contest, but..................

most of the lenses dispensed usually have at least one side factory scratch resistance coated.

Remember that brand hype, production hype is rampant............just because you get a wow..........perhaps someone is just impressed by their new RX, and your skill, plus a nice clean, scratch-free set of lenses.

[kat]

Welcome to Optiboard!
Do you have an option for Trivex? Great VA, stronger than Poly.

[Hardworlder]

There's got to be an app for this, Barry.............LOL

Expanding on Barry's explanation:

The usual visual result should be expected. If you move a person, to a higher index of lens, than the one currently being worn, and the power change is nil, then expect a reduced excursion gaze, and an increase in chromatic aberration. Also, expect an increase in reflectance.

A untreated CR39 lens versus an untreated polycarbonate lens will result in the CR39 lens winning the hardness(scratch resistant) contest, but..................

most of the lenses dispensed usually have at least one side factory scratch resistance coated.

Remember that brand hype, production hype is rampant............just because you get a wow..........perhaps someone is just impressed by their new RX, and your skill, plus a nice clean, scratch-free set of lenses.

Ok, this is the sort of stuff I was hoping to get into. I'll look into "excursion gaze" and see what that refers to, and I think I already know a tiny bit about chromatic aberration, but I'll read up on it some more as well.

So what you're saying is that the higher the refractive index, these things are bound to happen regardless? So even a high index lens (1.67, for instance, which I've heard at both my jobs described as "the best vision possible") actually has more drawbacks aside from just being a less durable and more expensive lens?

And CR39 is more scratch resistant, but poly is still safer overall. . .is this independent of scratch coatings? At my new job, all of our lenses have at least a scratch coat by default, but before, our CR39 was described in distribution center ordering lists as "uncoated CR39."

[Hardworlder]

Ok, so I attempted to search for "excursion gaze" and came up with nothing except for a link to another post on this board, but it was just mentioned in passing, so nothing I could really gather from the context in which it was used :P

[doctorjmjb]

I'd like to address this, If I may.



It's a fact that people, particularly those with a high Rx, will not see well through a polycarbonate lens. The vast majority will see adequately, however, and will never present symptoms associated with poor visual accuity.

However, if you've worked in optics, the first thing you should have realised is that everyone thinks they can see well, until they can actually see well.

How many times will a patient tell you that they have no problem in bright light or with glare, after you tell them the benefits of polarised lenses? The truth is that nobody can see well in all light conditions without polarized lenses, but many, many people think they can.

Or how many times will a patient think that they have excellent vision, and that they don't think they need glasses at all, only to be prescribed a +2.00/-1.00...

They wll tell you that they don't struggle with the dashboard in their bifocals and that they have no peripheral distortion in their [whichever poor quality design] varifocals.

So you can see that whether or not the patient thinks that they can see well really is irrelevent.

The point is that when you are recommending a product to a patient, you are not recommending it on the basis that they will be able to see ok. You are recommending it on the basis that it will give them the best possible vision. The patient may never know the difference, but you will.

Perception is everything. You are spot on about people who feel that they have no problem. I have on numerous occasions, had patients tell me that they have no problems with their vision and then I test them at less than 20/40. They wouldn't even be able to pass the driver's license test.

[uncut]

Excellent point, doctorjmjb, and that is why we should pick lenses we use for eyewear with care......the effect, and the performance can be skewed towards the negative.

[uncut]

Ok, so I attempted to search for "excursion gaze" and came up with nothing except for a link to another post on this board, but it was just mentioned in passing, so nothing I could really gather from the context in which it was used :P

Our optical world is divided into two types of wearers, head turners, and eye turners, according to some of the gurus. Some tasks and/or occupations may require a head turner or eye turner behavior in order to accomplish the task. Some peoples health, and/or the power of their required lenses automatically categorize them.

From the patient's point of view..............if you either densify/compact their lens, or add asphericity into the product, deliberately or inadvertently, you reduce their ability to eye turn.

[chip anderson]

Just a how come: How come so many opticians on Optiboards don't know that every lens except those made of glass or quartz are plastic? This includes CR-39, Polycarbonate, Polymethylmethacrarylate, Phoenix, as 150, 170, 167, 171 and every lens not made of glass, quartz or water, or a few precious gemstones.

Chip

[RIMLESS]

Welcome to Optiboard!

I am busy tending to my hops farm, but I am sure that some of our helpful members will be along to help you sort it all out!

Fezz, do you really grow your own hops???

[Hardworlder]

Our optical world is divided into two types of wearers, head turners, and eye turners, according to some of the gurus. Some tasks and/or occupations may require a head turner or eye turner behavior in order to accomplish the task. Some peoples health, and/or the power of their required lenses automatically categorize them.

From the patient's point of view..............if you either densify/compact their lens, or add asphericity into the product, deliberately or inadvertently, you reduce their ability to eye turn.

Ok, I think I see what you're saying. So a patient with progressive lenses doesn't have the excursion gaze that someone in a single vision lens would, right?

Thanks for the help!

[Hardworlder]

Just a how come: How come so many opticians on Optiboards don't know that every lens except those made of glass or quartz are plastic? This includes CR-39, Polycarbonate, Polymethylmethacrarylate, Phoenix, as 150, 170, 167, 171 and every lens not made of glass, quartz or water, or a few precious gemstones.

Chip

Hopefully you're not referring to me, but if so, I am aware that Poly is a form of plastic, which is why I've been careful to specifically say CR-39 and not call it plastic, at least when I'm talking specifics in a thread like this.

[Hardworlder]

If you move a person, to a higher index of lens, than the one currently being worn, and the power change is nil, then expect a reduced excursion gaze, and an increase in chromatic aberration. Also, expect an increase in reflectance

So even a high-index lens suffers from reduced excursion gaze? (which, if I understood your last response, is the specific term for the peripheral distortion you get with poly). What then, aside from being thinner, is better about high index?

At my old job, we would only push for high index if a patient was already in it, (and so would freak out about their lenses being thick in anything else), or if they had a prescription higher than a -6.00 or -7.00. At my new place, I was told by someone that if they're above -2.00 diopters they ought to go ahead and get high index (which sounds crazy to me).

[Jubilee]

Part of that answer may be, what does "high index" mean to them. The standard definition is 1.66 or higher, though I have heard of poly, spectralite, and trivex as being "high index" from some folks. The "higher the index" the longer it takes for light to go through the lens. This affect is what contributes to chromatic abberations. Light also gets scattered, decreasing the amount getting to the eye, and increasing the amount of reflections. That is why a good anti-reflective coating is needed on those lenses.

The excursion gaze is partially based upon the material, but also design. In a best form conventional lens, the curve on the front surface is uniform through out. The problem with this is when the gaze shifts to something other than the optical center of the lens, incremental amounts of abberations/distortion is introduced. This is more pronounced in larger scripts and those with cylinder.

Aspherics "flatten" the lens form to improve the cosmetics. The lenses do not offer any better or worse cosmetics, but offer similar vision as a traditional lens while reducing magnification/minification, etc. The issue with a poor Aspheric design is the departure from the best form is too sudden, then those who rely more on their periphery, aka.. the eye turners, will notice the abberations showing up in their field of vision.

Atorics use two different non spherical curves to open up the field of view for astigmatic patients.

Cosmetics is a huge part of what people look for in their glasses. Why just focus on function if you can look good and see good at the same time? ;) High index can reduce weight, helping to alievate pressure on the nose. The combination of a high index aspheric can allow you to adjust the vertex distance, which will help with optical clarity as well.

For some really good information and the technical detail.. see our own Darryl Meister's OptiCampus.
http://www.opticampus.com/cecourse.p...=high_powered/
http://www.opticampus.com/cecourse.p...sign/#aspheric

[bryanquinn1981]

I'd like to address this, If I may.

However, if you've worked in optics, the first thing you should have realised is that everyone thinks they can see well, until they can actually see well...

The point is that when you are recommending a product to a patient, you are not recommending it on the basis that they will be able to see ok. You are recommending it on the basis that it will give them the best possible vision. The patient may never know the difference, but you will.

That's about the best thing I've read on OptiBoard about optics. That's a beautifully succinct statement. I'm going to incorporate some form of it in the things I share with my patients and customers.

[FL-Opt]

May I suggest a program that would help you out practically and professionally in numerous ways? The Ophthalmic Career Progression Program by the National Academy of Opticianry. It's great for those new to the field and looking for ways to become educated without having to enroll in an optical school. If you're interested, you can find out more information on their website at: http://www.nao.org/?page_id=2357#toggle-id-1