I was asked by the customer today and....

[LENNY]

I was asked by the customer today and I had a tough time answering this.

1. What is more resistant to scratches (in general not depending on manufacturer) uncoated CR39 or Poly?
2. What is more resistant to scratches AR coated (in general not depending on manufacturer) CR39 or Poly?
3. Any other material is more scratch resistant then other without any coatings?

Your thoughts...

[optical24/7]

1. Your not going to get a poly lens without an src. The stuff scratches without it by just looking at it (almost).

2. The harder the substrate, the tougher to scratch. If the src is the same on both CR and poly, poly would scratch easier.

3. CR is the most scratch resistant plastic in un-coated form (it's also the only plastic you can get without an src unless you specifically asked your lab to not apply one on the other plastics out there, which would be a mistake.)

[rbaker]

I was asked by the customer today and I had a tough time answering this.

1. What is more resistant to scratches (in general not depending on manufacturer) uncoated CR39 or Poly? CR39
2. What is more resistant to scratches AR coated (in general not depending on manufacturer) CR39 or Poly? Depends on coating
3. Any other material is more scratch resistant then other without any coatings? Crown glass

Your thoughts...

1. What is more resistant to scratches (in general not depending on manufacturer) uncoated CR39 or Poly? CR39
2. What is more resistant to scratches AR coated (in general not depending on manufacturer) CR39 or Poly? Depends on coating
3. Any other material is more scratch resistant then other without any coatings? Crown glass

[LENNY]

I am aware that all poly and HI come with SR coat.

I am trying to compare uncoated plastic and obviously SR coated Poly with and without basic AR.

[optical24/7]

An src coated poly will scratch less than an un-coated cr lens. How much any src coated lens scratches depends on what type of coating is applied. Lacquer type coatings (like TD2) are the most resistant src's. AR's applied to lacquer coated lenses are the most durable AR coated lenses (other than a crown glass AR'ed lens)

[drk]

I don't let patients get neurotic about scratch resistance or let them shift the burden onto me.

In my experience, the way you treat your glasses has 100X more significance than the difference of angels dancing on the head of a Bayer-abrasion pin.

I tell them they are all scratch resistant, and they are about the same, and that you have to take really good care of the lenses to avoid scratches. Even so, after 2-3 years there are going to be scratches, no matter what. That's about the lifetime of a lens.

[Hayde]

I don't let patients get neurotic about scratch resistance or let them shift the burden onto me.

+1

This conversation only ends with the patient buying TD2 or an AR with a 2 year warranty...or getting their specs elsewhere.

Splitting hairs on scratch resistance of uncoated materials like it should affect their decision is great fun for a retired ECP discovering how much their own optician really knows. For gamers trying to trick us into making an ambiguous sentence to hold over our heads later, no thank you. The unit of measure with patients regarding scratch resistance: duration of the scratch warranty/number of times it can be used in that time frame. Those who are truly worried about a good 'scratch resistance' decision will gladly get the premium SRC every time without a technical quibble.

If they're committed to the TD2 AND are taking these specs into work/the shop, then we can talk about technical virtues of different lens materials for all aspects of their anticipated exposure. (And really, marginal cost difference aside, how often is there a better choice than Trivex?)

[Uilleann]

This sounds exactly like a patient who will scratch ANYthing they wear.

I would be talking about your choice of warranty/policy, and how that relates to the patient more than the perceived merits of a given lens, material, scratch coat, or AR process.

[Paul Smith LDO]

When the conversation does along the least scratch resistant path I discuss polycarbonate and inform them of the merits of glass. Look at it this way, the more a lens can flex the softer the matrix the more it will scratch. The more slick the ARC the more it diverts an objects path, less scratches.

[AngeHamm]

In my experience, the way you treat your glasses has 100X more significance than the difference of angels dancing on the head of a Bayer-abrasion pin.

Truth.

[Quince]

If you are looking at the basics, my understanding is that Trivex is going to be your best option, aside from glass. We don't sell anything uncoated, so I couldn't tell you how UC CR39 holds up.

With basic AR, I would say CRAR and poly AR are neck and neck. CR39 with SRC vs poly comes out about the same.

[LENNY]

If you are looking at the basics, my understanding is that Trivex is going to be your best option, aside from glass. We don't sell anything uncoated, so I couldn't tell you how UC CR39 holds up.

With basic AR, I would say CRAR and poly AR are neck and neck. CR39 with SRC vs poly comes out about the same.

Thanks!

Unfortunately its for my cousin that I made a mistake to employ 10 years ago....

[Paul Smith LDO]

If you are looking at the basics, my understanding is that Trivex is going to be your best option, aside from glass. We don't sell anything uncoated, so I couldn't tell you how UC CR39 holds up.

With basic AR, I would say CRAR and poly AR are neck and neck. CR39 with SRC vs poly comes out about the same.

Quince, try this experiment with a CR-39, Trivex, and a Polycarbonate blank. The ones that flex more are softer and more shatter resistant, hence it will scratch more. The success of an ARC lens is a stable platform/lens material, FHC and the stacking process. I've seen more crazing with polycarbonate than any other lens material. Crazing is the separation of the ARC from the lens. Flex and expansion are the primary causes for crazing.

[Chris Ryser]

3. CR is the most scratch resistant plastic in un-coated form (it's also the only plastic you can get without an src unless you specifically asked your lab to not apply one on the other plastics out there, which would be a mistake.)

If you get and use un-coated CR 39 you get one of the best lenses besides glass.

You can also tint them very easily in store location, and as the last step apply a simple dip, scratch resistant anti static slick coat, same type as used on AR coated lenses, by just dipping into the solution for a few seconds.

That way you just made today's cheapest lens materials into one of the best quality solutions, at the lowest price possible.

[Quince]

Quince, try this experiment with a CR-39, Trivex, and a Polycarbonate blank. The ones that flex more are softer and more shatter resistant, hence it will scratch more. The success of an ARC lens is a stable platform/lens material, FHC and the stacking process. I've seen more crazing with polycarbonate than any other lens material. Crazing is the separation of the ARC from the lens. Flex and expansion are the primary causes for crazing.

That makes sense. I haven't done any testing with Trivex myself, but know that Trivex in combo with the right AR (top tier) is supposed to reach glass scratch resistancy. Is there any truth to this?

I've been exposed to a greater amount of crazing on hi index, but that is usually from heat exposure.

[optical24/7]

That makes sense. I haven't done any testing with Trivex myself, but know that Trivex in combo with the right AR (top tier) is supposed to reach glass scratch resistancy. Is there any truth to this?

No. Trivex is very soft. Soft substrates will scratch easier than a harder substrate. The only way to make an AR more scratch resistant is mainly to use a superior lacquer hard coat. ( once again, like TD2) coupled with a good hydro/olio layer. AR is AR, it's the type of hard coat along with the material (substrate) that will determine durability and adhesion of the AR.

What made Crizal so scratch resistant and durable in the 1st place is when Essilor figured out that a consistent and durable lacquer coating gave it more overall scratch resistancy along with a consistent base coat to apply AR to.

I've been exposed to a greater amount of crazing on hi index, but that is usually from heat exposure.

Heat and thin centers along with flexure in a frame is a main culprit of crazing.

[Paul Smith LDO]

That makes sense. I haven't done any testing with Trivex myself, but know that Trivex in combo with the right AR (top tier) is supposed to reach glass scratch resistancy. Is there any truth to this?

I've been exposed to a greater amount of crazing on hi index, but that is usually from heat exposure.

Again, which material flexes more. Certainly not glass. Glass is the Eliot Ness, of lens materials.

[Chris Ryser]

Again, which material flexes more. Certainly not glass. Glass is the Eliot Ness, of lens materials.

I Beg to differ ..............................

When I was doing my opticians apprenticeship back in Switzerland in the good old days of 2 types of glass only,
crown and flint, we used to bend it.

How would you bend glass .....................?

I learned to how take the leftover circle after cutting a lens into shape on the lens cutter, using a former plate.

Using a chipping plier we learned how brake off the circle around the cut shape into one or two pieces if possible.

Then we heated the middle point of the chipped off circle over the bunsen burner, whose purpose was to heat the frames, at the tip of the small inside flame which is the hottest point within the whole flame.

When the glass rim was all red hot, with an other plier at the other end holding the rim, we waited until the red hot area became soft, and then started to pull at it. At the right temperature the glass would become all soft and you could gently pull at it, and a thin thread was produced. The idea was to be able to make the longest and thinnest thread possible.

If you were able to get a length of about 2 to 2 1/2 feet and a diameter of less than 1 mm, you could actually bend it into a half circle without breaking it.

Glass can be bent, I have done it already a long time ago.


A superb website on glass, in general, worth looking at for any optician, can be seen at:

https://www.pilkington.com/en-gb/uk/...istry-of-glass

[Paul Smith LDO]

Chris, take a glass lens, put it between your thumb and index finger. Try to make it bend without the application of heat Dude, or burning yourself. Under the right amount of pressure and the correct geological time required, a lump of coal becomes a diamond. THUMP!! That's me dropping the mike, I'm out. The pressure's too much.

[Chris Ryser]

Chris, take a glass lens, put it between your thumb and index finger. Try to make it bend without the application of heat Dude, or burning yourself.

Paul, that is what we did, after pulling the glass into a thread as thin as possible we could bend it in a cooled off condition and it bends in a cold state. It has become flxible to a ceratin degree.

[Paul Smith LDO]

Yes Chris, that is the plasticity of glass along with anything else that can be changed with temperature; warm or cold, and pressure. Because in the physical world nothing is truly a solid; everything is formed by molecules, atoms, and sub atomic particles. There is space between all of these sub structures that make up your oak desk top. Hence your desk top has plasticity. We acknowledge these things but break them into more simplistic tangible categories for the sake of comparisons,solid,liquid,steam etc. The Earth can change from a solid form into liquid by convective and advective heat transference. I can rub two sticks together, creating combustion and make fire. Back to the question at hand, which lens material has more inherent flex.

[rbaker]

In the configuration of a spectacle lens at between 0 and 100 degrees Celsius all of the glass lenses used on both ophthalmic and precision optics are pretty darn stable over their useful life, far more resistant to bending or warping than "plastic" lenses.

However glass can be easily bent when drawn into thin fibers as in telecommunications and endoscopes for which you can thank your luck stars the next time you go in for a colonoscopy or some other procedure where they shove an imaging device up one of your bodily orifices. Glass, being a semi amorphous substance does indeed "flow". You can observe this in old window panes where after hundreds of years the bottom of the pane is perhaps .010 thicker than the top. You can also visually see the resulting distortion due to this flow.

Glass is neat stuff. It exhibits many interesting characteristics under different conditions and with different elements and compounds added. However, in Mary Jane's -1.00 D Spheres it ain't going to bend or warp during the life of the prescription. All polymers will to one degree or another.

[Chris Ryser]

Thank you Dick for that excellent post.

Maybe in a few years somebody, I know who, is going to re-invent glass lenses as the new high class and optically perfect lens material for normal, not to heavy prescriptions.

However they will be much more expensive than ever, as there is a lot more work involved to surface each of them.

[Browman]

Thank you Dick for that excellent post.

Maybe in a few years somebody, I know who, is going to re-invent glass lenses as the new high class and optically perfect lens material for normal, not to heavy prescriptions.

However they will be much more expensive than ever, as there is a lot more work involved to surface each of them.

At which point the industry will have to overcome decades of "poly or Trivex only or it'll shatter in your eyes" programming.

A CE I read recently advocated for a return to glass, and with dispensaries steadily gravitating towards a boutique experience, I can definitely see it becoming the norm again in brick and mortar retailers. The question is, how do we "reprogram" patients who've heard all their lives that if they get into a dangerous situation with anything other than poly (or more recently, trivex), they'll be blinded or suffer eye injury? Even with my experience in the field, my OCD won't let me dispense CR39 simply because "you'll blind your patient" was so brutally hammered into me in my early years.

[rbaker]

One of the most effective marketing tools is fear. Fear will trump science, common sense and actual experience in the mind of the consumer. The expression "put out an eye" effectively killed the joy of running with scissors or owning a BB gun. And it killed glass lenses.

Perhaps glass lenses could regain market share if we apply the good old, time tested, fear factor. Seriously, who wants to put diethyleneglycol bis allylcarbonate and diisopropyl peroxydicarbonate just a few millimeters in front of their eye.