There will be a rush of crazed AR coatings over the next few days ...................

[Chris Ryser]

As the weather will be dipping into the deep freeze temperatures in the NorthEast of this continent over the next few days, there will be loads of AR coatings that will craze.

As Ar coatings consist of SIO2 (which is Glass) applied on lenses made of plastic materials, of which both have different expansion coefficients when exposed to sudden large temperature changes, as when going from the heated interior the the sub freezing temperatures outside or vice versa.

Plastic lenses will expand or contract more and faster than the barely moving glass AR coating on top of the lens surface resulting in cracking and delamination.

[Lelarep]

But doesn't this only happen if the coating is not applied correctly or with this in mind? It should be possible to mitigate this, to a point; at a low enough temperature, it wouldn't be possible to compensate, but people shouldn't encounter temperatures like that usually.

[lensmanmd]

Not necessarily. It has to do more with the hardcoat layer than the actual AR stack. Premium coatings utilize a primer layer to address this. But there are also index matched dip coatings, as well as the more common spin coatings that bypass the primer. Spin coatings tend to fail the most often, but are more cost effective, thusly, more common. Yes, AR coatings, whether applied via box coaters or sputter coaters all rely on SI layers in hi and low index layers. Recipes vary by design and color, but all thin coatings still depend on the HC layer. When ARs craze, typically it is the HC layer that fails.
Now, for the AR stack to fail, there has to be a fairly extreme temperature differential. CR39 has a higher tendency to fail during the thin film application, as the the substrate expands and contracts at a higher rate than the HC layer. This is what creates the issues. This failure is seen more readily in sputter coaters as compared to box coaters due to the proximity of the target and heat levels. Otherwise, the plasma is relatively the same.
Winter temps, regardless of how cold it gets shouldn't cause this problem. Even when coming in from freezing temps, most buildings and homes are not heated above 80 degrees F. Summer is when one needs to worry about crazing. Think about it, people leave their glasses in the car, with the windows rolled up. Temps can reach over 140 degrees, and even above 160 degrees inside a closed vehicle. This is death to AR. Talk about expansion and contraction, when the AC is on at full blast to cool the vehicle back down to 70 degrees. Poly, TVX, CR39, Hi index, it doesn't matter.
Just my 2 pennies.

[Chris Ryser]

Not necessarily. It has to do more with the hardcoat layer than the actual AR stack. Premium coatings utilize a primer layer to address this................................


Winter temps, regardless of how cold it gets shouldn't cause this problem. Even when coming in from freezing temps, most buildings and homes are not heated above 80 degrees F.

Summer is when one needs to worry about crazing.

Talk about expansion and contraction, when the AC is on at full blast to cool the vehicle back down to 70 degrees. Poly, TVX, CR39, Hi index, it doesn't matter.
Just my 2 pennies.

As you say winter is nothing, from sub freezing temperatures into warm home it does not contract and expand, but in summer it does.

I am right now in Montreal at - 17 C degrees and my house is heated to + 21 C, that is a difference 38 C which is huge to make any plastic make expand or contract.

[Tallboy]

Great post lensman. I've always found poly crazes most from poor edging and 1.74 crazes most from life exposure. 1.67. Wish I hadn't been so cocksure and listened to Robert M.'s warnings about essilors 1.74 coatings

[lensmanmd]

Great post lensman. I've always found poly crazes most from poor edging and 1.74 crazes most from life exposure. 1.67. Wish I hadn't been so cocksure and listened to Robert M.'s warnings about essilors 1.74 coatings

A solid point, Tallboy. Most of the Poly crazing issues I find from edging are with FSV when the block is not matched to the BC. SFSV generally does not have this issue, as it is normally thicker than FSV. HIP will also have this problem, but most HIP FSV are on 2BC or lower, so the correct blocks are used more often than not.

There is another problem that I often see. We will have lenses returned as lab remakes on Zyl frames more often than not, mostly on higher myopes. Why? It seems that many of the opticians fail to remove the lenses,, or even protect them while heating the frames during adjustments. I always groan when I see these types of remakes. 1.67 and higher tends to get returned more often, followed by CR39. My mounters and inspectors all know to pre-adjust zyls when mounting AR. We even made special deflectors to spot heat the temple or bridge, just in case.

Crazing from edging is centralized around the block, due to chuck pressure, an incorrect block curve used, and even from deblocking. These are not always 'instantly' noticeable. Crazing form overheating, however, is very noticeable. The entire surface, usually the front surface, have ripples that look like stained glass or even Roman glass. You can't tell me that it left the lab that way. Crazing from edging, I can see getting missed at QA occasionally. Crazing from sheer laziness, that is another story.

[Happylady]

I had a pair of lenses crazed in my checked luggage. I was surprised and when I asked the lab about it, was told it was due to the extreme cold in the cargo hold area of a plane.

Here in Texas we are very aware that extreme heat can craze a coating but I had never thought about extreme cold!

[lensmanmd]

I had a pair of lenses crazed in my checked luggage. I was surprised and when I asked the lab about it, was told it was due to the extreme cold in the cargo hold area of a plane.

Here in Texas we are very aware that extreme heat can craze a coating but I had never thought about extreme cold!

Crazing occurs when different materials expand/contract at different rates. Your situation is a bit different, as two other coefficients were introduced. Pressure and humidity (or lack of). Cargo holds are not pressurized like the passenger cabins are, and it gets very cold way up above the clouds. Sounds like there was rapid expansion as the plane descended. May I assume that the plane landed in a very warm and humid climate?

[Chris Ryser]

In the cold up north where you shovel snow in your driveway at below zero and then get into the house that is heated to 72F the temperature difference will do harm to your AR coating.

[rbaker]

OK ! You have all convinced me. Between "crazing" and unwanted coloring, no more crappy AR lenses for me.

[Robert Martellaro]

As lensmanmd pointed out, heat, especially above 120°F, is the most detrimental, compared to the cold, or to thermal stress. I'm reasonably sure that thermal cycling is one of the tests that first-rate antireflection coatings must pass to get a high rating.

http://www.2020mag.com/story/43918

[Happylady]

Crazing occurs when different materials expand/contract at different rates. Your situation is a bit different, as two other coefficients were introduced. Pressure and humidity (or lack of). Cargo holds are not pressurized like the passenger cabins are, and it gets very cold way up above the clouds. Sounds like there was rapid expansion as the plane descended. May I assume that the plane landed in a very warm and humid climate?

It happened when the plane flew from Dallas to Portland in August.

[Chris Ryser]

As lensmanmd pointed out, heat, especially above 120°F, is the most detrimental, compared to the cold, or to thermal stress. I'm reasonably sure that thermal cycling is one of the tests that first-rate antireflection coatings must pass to get a high rating.

http://www.2020mag.com/story/43918

I went on the link, and here is what wee need:



The Lens

One of the most important things to understand related to this topic is that lenses have what is scientifically referred to as a coefficient of thermal expansion. This is a fancy way of saying that the lenses expand and contract with temperature.

When the lens expands and contracts at a different rate than the AR coating, the AR coating can, and very often will, crack. (Keep in mind the AR coating is very, very thin relative to the lens.)
\
The highest coefficient of thermal expansion in lenses belongs to Hi Index, then CR39 and Mid Index. Poly and Trivex have relatively little thermal expansion. If we are going to see temperature related cracking, we will typically see it first in Hi Index and then follow it down. One hardly ever sees temperature related cracking in Poly.

[lensmanmd]

Chris, I too, viewed this article and an interesting quote was "I would suggest applying your AR coating on a Sola CR39 -2.00 lens and putting it through the test" Now, we all know that the Sola FSV that is used for the salt water boil has, well, a ridiculously soft factory HC. This is the exact same test, with the same materials that we conduct our shock test with. Routinely, the lenses score 4s and 5s. When compared to the UV87 or even the AST, the Sola Permaguard is akin to the lizard oil that we use to treat CR39 with and overcharged the patients. Dip coating is a better method, but can be fairly cost prohibitive for a small lab, hence the spin coating that most labs use. That said, today's spin coatings are so much more durable as compared to what was available even 5 years ago. They are harder and protect the lenses better. This may be the primary issue with temperature related crazing. The article did point this out. For AR applications without a primer layer, softer HCs may be the answer.

Expansion and contraction is a known thin film killer, be it AR or Mirror. Expansion occurs more rapidly than contraction. That much is known. The question here is, which is more damaging? Expansion then contraction, or contraction then expansion? My hypothesis is expansion to contraction, as it is a more rapid and destructive process. If you take a thin water glass, heat it to 160F then shock it with cold (ground temp) water, chances are, the glass will crack. However, if you take that same water glass, freeze it and then shock it with the same ground temp water, the glass will survive. This is what most of us in the real world deal with, so this would make sense. Now, we are not talking about extremes here.

Winter climates north of the border are a bit more extreme, granted. Here in the Mid-Atlantic region, our winters are not as extreme, though there are days when it can get that way. My experience with AR crazing in this region during the winter months is basically nil. However, I see a lot more issues in the summer months, as temps can average between 90F and 100F with fairly high humidity very frequently. Again, this happens more to those that are lazy and leave their 'clear' pair in the car for extended periods of time.

Back to the article, it does not mention pressure and its effects. Happylady had an issue with her AR. Hers was an issue that included heat to cold and back to heat, but also introduce pressure variables into the equation. An odd thing about this, thin films are applied in a vacuum which approximates the cruising altitude of a jet, but a much shorter duration of time. A room temperature lens is introduced to a vacuum and a hot plasma, then the heated lens with the thin film applied is reintroduced to cooler ambient temps and 'sea level' pressure, generally without any side effects. Happylady's lenses were a more extreme example, and did create a crazing issue.

If there are any of you out there that can shed light on this, I would greatly appreciate it. One always learns from smarter persons.

[Chris Ryser]

Congratulations on your post lensmanmd, well written and full of good information.

You also have taken a lot of your free time to write and post it, and if you continue to do so, you will be very much appreciated by the readers of OptiBoard post's.

Right now at 2.05 am, "There are currently 319 users online. 2 members and 317 guests".

So there are manyfold more people looking at your posts than the presently registered members, and they can be located in the USA, or any other place as Russia, Japan or Europe.

[Chris Ryser]

Expansion and contraction is a known thin film killer, be it AR or Mirror. Expansion occurs more rapidly than contraction. That much is known. The question here is, which is more damaging? Expansion then contraction, or contraction then expansion?

..........Winter climates north of the border are a bit more extreme, granted.

If there are any of you out there that can shed light on this, I would greatly appreciate it. One always learns from smarter persons.

Temperatures not only north of the border, also all the USA states closer to the northern border in North America, the Rockies and northern parts all over the world from Europe to Asia can be affected.

Also winter turns around to the southern hemisphere when we have summer time and it get cold from the South Pole direction North.

Wherever it is extremely cold people stay inside and watch TV or scan the internet for anything interesting to them.

[ak47]

Okay, it makes sense that a softer hard coat (a term kind of like "jumbo shrimp") will reduce the likelihood of temperature-related crazing. Which easily cleanable AR have the best of the soft hard coats? I have a patient who is a baker (constantly sticking their head in and out of a hot oven), other than using glass, which AR is least likely to craze and is not impossible to keep clean?

Chris, I too, viewed this article and an interesting quote was "I would suggest applying your AR coating on a Sola CR39 -2.00 lens and putting it through the test" Now, we all know that the Sola FSV that is used for the salt water boil has, well, a ridiculously soft factory HC. This is the exact same test, with the same materials that we conduct our shock test with. Routinely, the lenses score 4s and 5s. When compared to the UV87 or even the AST, the Sola Permaguard is akin to the lizard oil that we use to treat CR39 with and overcharged the patients. Dip coating is a better method, but can be fairly cost prohibitive for a small lab, hence the spin coating that most labs use. That said, today's spin coatings are so much more durable as compared to what was available even 5 years ago. They are harder and protect the lenses better. This may be the primary issue with temperature related crazing. The article did point this out. For AR applications without a primer layer, softer HCs may be the answer.

Expansion and contraction is a known thin film killer, be it AR or Mirror. Expansion occurs more rapidly than contraction. That much is known. The question here is, which is more damaging? Expansion then contraction, or contraction then expansion? My hypothesis is expansion to contraction, as it is a more rapid and destructive process. If you take a thin water glass, heat it to 160F then shock it with cold (ground temp) water, chances are, the glass will crack. However, if you take that same water glass, freeze it and then shock it with the same ground temp water, the glass will survive. This is what most of us in the real world deal with, so this would make sense. Now, we are not talking about extremes here.

Winter climates north of the border are a bit more extreme, granted. Here in the Mid-Atlantic region, our winters are not as extreme, though there are days when it can get that way. My experience with AR crazing in this region during the winter months is basically nil. However, I see a lot more issues in the summer months, as temps can average between 90F and 100F with fairly high humidity very frequently. Again, this happens more to those that are lazy and leave their 'clear' pair in the car for extended periods of time.

Back to the article, it does not mention pressure and its effects. Happylady had an issue with her AR. Hers was an issue that included heat to cold and back to heat, but also introduce pressure variables into the equation. An odd thing about this, thin films are applied in a vacuum which approximates the cruising altitude of a jet, but a much shorter duration of time. A room temperature lens is introduced to a vacuum and a hot plasma, then the heated lens with the thin film applied is reintroduced to cooler ambient temps and 'sea level' pressure, generally without any side effects. Happylady's lenses were a more extreme example, and did create a crazing issue.

If there are any of you out there that can shed light on this, I would greatly appreciate it. One always learns from smarter persons.

[King of the Lab]

Winter is coming....

[lensmanmd]

Winter is coming....

With Viserion as the reborn ice dragon breathing his fury!

[lensmanmd]

Okay, it makes sense that a softer hard coat (a term kind of like "jumbo shrimp") will reduce the likelihood of temperature-related crazing. Which easily cleanable AR have the best of the soft hard coats? I have a patient who is a baker (constantly sticking their head in and out of a hot oven), other than using glass, which AR is least likely to craze and is not impossible to keep clean?

This, I would not advise. To counter the expansion/contraction issue, any high end AR would be better. A thermal hard coat with a primer layer would be the best bet. The primer layer will shield the SI layer by absorbing the material shift and keep the AR layer in tact. Since your patient is a baker, perhaps a non-AR pair would be best during baking time. Just stay away from CR39 or 1.67. If AR is necessary, then good old fashioned glass would be best.

[ak47]

Thanks, but which manufacturers or labs use the best thermal hard coats and primer layers?

[lensmanmd]

Zeiss Duravison, Hoya EX3 and Crizal all come to mind. Not sure about independents.

[ak47]

I'm in the desert and have seen lots of Crizal lenses with heat crazing...I think in most cases it was because they were left in the car even though some claim they never do that..

[lensmanmd]

I'm in the desert and have seen lots of Crizal lenses with heat crazing...I think in most cases it was because they were left in the car even though some claim they never do that..

Extended baking will kill any type of coating, even with a primer layer. Imagine putting the lens in a frame warmer for an hour.

We stopped Hoya, but when we did do business with them, their EX3 coating was amazing. Spent several weeks in Honduras and Puerto Vallarta. Heat, humidity, sand, name it, and not a scratch. Sad to say, that was the only amazing thing about Hoya. Service and turn times were ridiculous.

We are a Zeiss qualified independent, but too small to install a solvent based coater and box coater, so we still spin. Our AR is applied via sputter with an excellent hydro. Boil tests consistently rate 4s and 5s all the way through. Tape pull around 3-4 grams. Reflectivity around 1.1%. However, as a spin lab, we rely on MFR front hardcoats, which vary. All of my suns have mirror and BSAR. None have crazed in over a year, and they are all kept in my glove box regardless of season. This is without a primer, nor dip coat.

I am working on an expansion project and if approved, I will add both and work towards Zeiss coating qualification as well. The Duravision is relatively new, but I have the Blue and Platinum. Both are good so far and have survived baking in the humid Maryland oven called my car. Yup, do as I say, not as I do.

We don't provide Crizal, so my experience with it is nil. Just going off of what my peers are saying here on OB.

Not sure if any of these coatings would survive baking in an Arizona car, though. But if you have access to a Zeiss lab, I would consider trying the DV Platinum. My rep tells me that it has a Bayer rating close to 12. Then again, it's the rep's word, to be taken with a grain of salt.

I still say that your baker should consider glass if AR is needed, or go with a work pair without AR, just in case. Even with warranties, you will have additional work and time spent replacing them.

[Quince]

I believe the reason Hoya's ARs hold up better is due to the substrate matching. I assume what that means in relation to this context, is that the chemical changes in the different ARs are set to match the flexibility of the lens material so that it all stays together during expansion/ contracting. Hoya is our only 'high end' AR that is used regularly so I can't speak for other quality options. I do know that I personally see the most crazing issues with Transitions lenses.