AR coating's quality of FreeForm lenses

[Nikolay Angelov]

From some time i have noticed that the AR on FreeForm lenses is sort of odd, and wanted to share my observations.

When illuminated by a compact fluorescent lamp, the coating looks sort of Newton ringish, like the AR on some cheap lenses (that don't do index matched hard coatings).

I have seen this on the highest class individual lenses on the market (progressives, office progressives and single vision).
Here are some examples (identifying marks avoided ). The lenses are top of the line individual Single Vision and PAL with the best AR coating (made in Europe):

Lens 1:
It also has a sort of circle shaped mark at the center which is not visible on the photo.


Lens 2 (some concentric marks visible temporally, perhaps from the FreeForm generator):


Lens 3 (some concentric marks visible temporally, perhaps from the FreeForm generator):


Lens 4:


Lens 5:



Have someone else seen this or is it just me ?

PS: Why Yes ... i am a myope .

Best regards,
Nikolay Angelov

[LENNY]

Nice pics!

[Nikolay Angelov]

Thanks, just got a new camera and lens

Kitchen lamp + Canon EOS 5D Mark II = nice improvised shots .

[AustinEyewear]

Would you mind taking a photo of a non-free form lens, with all other factors being the same (same lab, same A/R, lens shape, lens material, lighting conditions, camera settings) and post it. Would love to see a controlled environment comparison!

[Nikolay Angelov]

Here is what i did for comparison:

Pictures 1 and 5 are 1,6 (MR8) photochromic lenses (individualized mono focal)

The following pictures are of spherical stock CR39 With AR (Hoya Hilux 1,50 HiVision ViewProtect).
Same shape, RX and centration parameters as Pictures 1 and 5:





I could match your exact requirements if you supply/pay for the non FreeForm lenses .
Thous are sort of expensive lenses .

Since we have other orders i will check the closest match (RX, material and coating) from the same manufacturers and make comparison pics (of the uncut lenses).

PS: Here we don't deal with labs, we place an order with the manufacturer (Hoya, Essilor, OptiSwiss etc.) and receive the product. In which of their labs the lens was produced is their business.

For example here Essilor is Essilor, they may have a million labs or just one for all we know and care (that's their manufacturing).

Best regards,
Nikolay Angelov

[AustinEyewear]

PS: Here we don't deal with labs, we place an order with the manufacturer (Hoya, Essilor, OptiSwiss etc.) and receive the product. In which of their labs the lens was produced is their business.

For example here Essilor is Essilor, they may have a million labs or just one for all we know and care (that's their manufacturing).

Best regards,
Nikolay Angelov

Very interesting indeed! The first thing that comes to mind is that you really couldn't begin to make a judgment against the Freeform, unless it is bring produced at the same lab (Essilor, or who ever) using the same A/R machine (and of course, as many other parameters being the same as you can). Otherwise you've introduced a new variable(s) into the equation. Meaning, maybe there is something wrong with the place where you get the freeform lens, something wrong with their A/R machine, maybe it is coating unevenly, maybe it has a clogged spray nozzle, (I don't claim to know how they work)? I do know one thing, can't wait to find a fluorescent light!

[Nikolay Angelov]

Hmm this are different lenses produced by two big international manufacturers in their home labs.
The lenses are produced with a few months difference.

The only connecting peace that i saw was the FreeForm technology, and the similar oddity in the AR .

[TLG]

Hard to believe it's from freeform processing. Why not have your lab surface you two single vision lenses - one traditional and one freeform - same material, manufacturer and coating and see if there is a difference. If it is the process then it shouldn't matter if its sv or PAL so you could save yourself some dough testing with single vision.

[Nikolay Angelov]

Got a few more lenses for examination
All of the following are from the same supplier:

Lens 7:
Non FreeForm Single Vision, same AR (as lens 1, 2 and 3), material is mid-index photochromic:


So the cause is not the FreeForm technology... as this is a not a FreeForm lens.

Lens 8:
FreeForm Individual PAL, MR8, photochromic:


Lens 9:
FreeForm Individual PAL, MR8, photochromic:


lens 10:
FreeForm PAL CR39, same AR as pic 1, 2 and 3:


Well this is a Perfect AR coat on a FreeForm lens by the same supplier.

So the odd effect is on higher than 1,50 index photochromic lenses.
It does not look like Newton rings caused by index mismatch between the hard coat and the lens.

It sort of looks like the hard coat is uneven.


Best regards,
Nikolay Angelov

[Speed]

There are different types of A/R. Could you determine if this is the traditional A/R of the chemat type? That may be an interestion comparison.

[Nikolay Angelov]

According to the information from the manufacturer, the coating structure is:

1. Lens
2. adhesion improving undercoat
3. hard coat (heat cured AFAIK)
4. broadband multilayer AR stack
5. easy cleaning layer

Application method: plasma assisted deposition in high vacuum.

I will check for more info on lenses 4 and 5 as they are from a different manufacturer. What i know now is that they are AR coated in high vacuum.
These are "Highest Class" type of AR coatings so i don't think they would be related to the chemat type.

Lenses 1, 2, 3, 7 and 10 are the same type/brand of AR coating.

Btw, today i saw another lens by a third manufacturer with the same odd AR (conventional "not FreeForm" PAL, midle-index photochromic).

[LENNY]

Are all opticals in Varna so advanced as yours?

[Nikolay Angelov]

Thank you for the kind words .

In terms of equipment, yes there are many advanced opticals.
When it comes to knowledge we all have a long way to go (textbooks are being translated, so they can be used by our staff etc.).

Best regards,
Nikolay Angelov

[AustinEyewear]

According to the information from the manufacturer, the coating structure is:

1. Lens
2. adhesion improving undercoat
3. hard coat (heat cured AFAIK)
4. broadband multilayer AR stack
5. easy cleaning layer

Application method: plasma assisted deposition in high vacuum.

I will check for more info on lenses 4 and 5 as they are from a different manufacturer. What i know now is that they are AR coated in high vacuum.
These are "Highest Class" type of AR coatings so i don't think they would be related to the chemat type.

Lenses 1, 2, 3, 7 and 10 are the same type/brand of AR coating.

Btw, today i saw another lens by a third manufacturer with the same odd AR (conventional "not FreeForm" PAL, midle-index photochromic).

Very interesting. Was looking around to see exactly what plasma assisted means, happened across this : http://www.lesker.com/newweb/Vacuum_...tems_notes.cfm

Are you seeing this effect when the Freeform lens is NOT A/R coated? It crossed my mind that freeform technology is capable of producing pretty much unlimited variable topography within the lens that it could just be the way the light is catching the topography in the lens, but, the CR39 example you posted sort of rules that out. It seems you would expect the pattern to be tilted 90 degrees from what it is in your first post. That last CR39 photo, which seems perfect, makes the issue appear material dependent?

[Nikolay Angelov]

Thanks for the link, it has a lot of interesting information .

it could just be the way the light is catching the topography in the lens

At first i was thinking the same thing, but the spherical (not FreeForm) lens has the same oddity . And the CR39 FreeForm PAL shows an excellent coating.

There is something else as well:
the visible patterns neither change in shape nor shift in position when the illumination direction changes.

Considering the surface quality of the FreeForm process ... i am not thrilled:
There is a center round artifact on almost all the lenses.
On some you can see the rough concentric circles that the CNC generator made. The circles are about 0,5 - 0,8 millimeters apart. An example of this are lenses 2 and 3 (look temporally, upper corner).
Today I was looking at another MR8 individual FreeForm PAL, that has even more pronounced concentric circles (and the center round thingy).
I will take some pics tomorrow.

The visible tool-marks sort of point to a lens/hard coat index mismatch:
If the lens and the hard coat had the same index these FreeForm grinding tool-marks would have vanished in the smooth hard coat (as they apparently do on the CR39 lens).
The coated lens surface is perfectly smooth meaning that the hard coat smoothed out the tool-marks very well ... but the tool-marks are visible on mid and high index lenses.

So the image of the tool-marks must be created by the light reflected at the lens/hard coating transition point.

About the visible oddities in the AR, maybe they are caused by an uneven layer (undercoat ?). Normally the lenses are drawn slowly from the liquid hard coating dip tank.
It is like the lens was pulled 3 mm from the dip tank, paused and pulled another 3mm and so on, making the coating uneven. This would explain the lines that are parallel to one another.

Are you seeing this effect when the Freeform lens is NOT A/R coated?

We have never ordered an FreeForm lens without AR, so no idea. Most manufacturers here don't offer their FreeForm lenses without AR.
Come to think of it, it is generally more hassle to get a lens without an AR.

Best regards,
Nikolay Angelov

[rickyforever]

Canon EOS.. nice camera and great pics

[Mauro.Airoldi]

I feel there is some confusion, Newton's rings are NOT connected with the geometry of the lens but only with the difference in refractive index between the lens itself and the layer of primer and hard coat (and it is related with the not uniformity distribution of the HC), if you use a lens 1ndex 1.6 and a hard coat 1.49 you see the Newton rings, if you use a cr 39 index 1.49 and a HC index 1.49... non wave.
The AR has not effect because the thickness of the layers is not enough to determinate this effect.
REMEMBER every difference bigger than 0.04-0.006 of index betwen lens and HC give you this effect.
The bigger producer use different HC index in his productions but the small laboratory use only HC 1.49 not colorable and or colorable.
Stock lens (big production) use every time the right HC and you don't see Newton rings.

[bhess25]

well the concentric rings around the entire surface of the lens is exactly what you thought, generator marks. The problem ive noticed with free form designs (especially from essilor) is that they go straight from the generator to polish, with no fining. now i understand why this process is more convenient, it provides faster processing times (because the generator takes forever to cut the lens so smooth). What im not so sure of is whether or not the time on polish is long enough. I believe that free form technology will continue to improve and those generator marks you see in nearly all FF designs will fade away into the past. Maybe slightly more coarse polish or longer times on polish will be necessary to eliminate these rings, who knows. Im sure someone will grow enough of a brain at some point to do something about the shawdy workmanship of the most expensive lenses available. They may even do something super complicated like increase the polish times.

[olispur]

Newton rings, "as they are called" are the result of the hardcoating machine not the AR machine. If you do not have high index coating in your hardcoat set up then the AR machine will produce the rings as a default. Some are not noticeable at all but the higher the index of the lens the more it is noticed.

[olispur]

Hmmmm i know what your saying and i have the answer (more or less) but cant quote on here as it may come back and haunt me.

[olispur]

sorry i was replying to bhess25

[cocoisland58]

I see this phenomena on my own free form lenses as well as all that I sell. I assumed it was just the AR effect on the particular lens design since it does not show up on any non free forms. What I really wanted to say is...it has no effect whatsoever on visual acuity or the durability of the coating...if that was a concern.

[AustinEyewear]

Newton rings, "as they are called" are the result of the hardcoating machine not the AR machine. If you do not have high index coating in your hardcoat set up then the AR machine will produce the rings as a default. Some are not noticeable at all but the higher the index of the lens the more it is noticed.

Can you explain this in more detail? I'm unclear because an A/R coating should decrease reflectivity, right? Due to this, wouldn't it would decrease Newton rings? Unclear on what you mean in your references to the hardcoat setup?

Interesting info on Newton rings over here:

http://physicsed.buffalostate.edu/pu...ing/rings.html

And nice info on A/R over hear:

http://www.edmundoptics.com/technica...tion-coatings/

[Georg Mayer]

Coming back to the original question and the original pictures shown I am pretty sure that this effect has nothing to do withthe fact that these lenses are produced on digital equipment, be it spehrical, torical or freeform but with this particular coating Lab using a Hardcoat with different refractiv index of the base lens.
A very simple test would be to go outdoors (natural sunlight) or using an old fashioned Edison type bulb and the structures are gone. If you can see them only
with fluorescent light tubes it is index mismatch = Newton fringing/pattern. This pattern is different from L to R, form Job to job and looks like "Free in Form"
but would have nothing to do with the way the lens is generated and polished.
Next point.
AR usually highlights the Newton fringing in an unpleasing way, the shiny surface of the naked SRC would hide in the white glare/reflection of approx. 5% the
pale colors of the fringing to a certain degree, you take that glare away, there it is, same as with fingerprints.
Finally
Regarding the second subject mid down, not beeing thrilled about the surface quality of some FreeForm lenses, I can wholeheartedly agree. There is so much
FreeForm digital surfacing crap around there that in such cases a good conventionally produced progressive beats the hell out of a badly surfaced (visible turning structures & center knob) and badly polished ( Design completley and utterly destroyed ) FF lens. The challenge is to find those Labs which put an effort into making good quality FreeForm lenses, where the patient has a real visual experience and we can justifiably charge good money from our customers.
We have not qualified any partner LAB worldwide without ensuring via an online surface scanner that reference lenses are checked after surfacing and after polishing every few hours of every generator line to make sure what comes off those lines is what we intended it to be.

Georg Mayer
International Lab support
Rodenstock - Munich
Germany

[Nikolay Angelov]

First a Big Thank you to Georg Mayer, for the expert information.

Some new development

As Mr. Mayer pointed out these are probably "newton rings" -> interference patterns.

The hard coat appears to be index matched. The images of non-index matched hard coated lenses appear different (from the article "Index matched coatings for MR substrates" at this issue of MAFO):
http://www.calameo.com/read/000126065ab55b67d0a6f

Even when the coating is index matched, interference pattern can still appear when the coated surface has:
1. Zones with slightly different refractive index (slight variation of the index of the coating)
2. Zones with slight difference in the hard coat thickness.

How is the lens hard coated:
The cleaned lens is dipped in the coating tank, held by three metal hooks at the sides:


The lens is pulled out of the tank slowly (computer controlled), to not leave streaks of coating.
Next the hard coating is hardened by heat (baked) or UV light.

The metal "hooks" holding the lens leave rough marks on the edges, like the one here:


I noticed that the patterns are not so random and that they approximate parallel lines:

Judging by the parallel interference lines, the effect appears to be created when the lens is pulled out of the tank.

How is high-index hard coating created:
The normal hard coat has a index of refraction of about 1.5 and to increase it higher index particles are mixed with it.

The coating mix must be very homogeneous. If there are zones with more high-index particles the coating there has higher refractive index (and if there are zones with less high-index "dust" the coating there will have lower index). Keeping the mixture homogeneous requires constant mixing and filtration. (the MAFO article states that the mix must be constantly filtered at a high filtration rate).

What could create zones of different thickness and/or different concentration of high-index particles in the wet hard coating layer? Some wild guesses:

1. Marangoni effect: "the mass transfer along an interface between two fluids due to surface tension gradient. In the case of temperature dependence, this phenomenon may be called thermo-capillary convection.": https://en.wikipedia.org/wiki/Marangoni_effect

2. Gravitational convection [is] a type of natural convection induced by buoyancy variations resulting from material properties other than temperature. Typically this is caused by a variable composition of the fluid. If the varying property is a concentration gradient, it is known as solutal convection. https://en.wikipedia.org/wiki/Convec...ant_convection

3. ???

Why does it not appear on 1.5 lenses? I don't know, but some possibilities i could think of:
The coating has the same index of refraction and there are no homogeneity problems since there are no high-index particles to worry about.
The coating environment could be causing this as well (air humidity, temperature, hard coating viscosity etc.)

A prism coupler could measure the coating thickness and refractive index:
http://www.metricon.com/basic.html

Here is an Essilor patent application for high-index hard coat. It contains good and understandable information on some of the problems and challenges:
ABRASION-RESISTANT OPTICAL ARTICLE AND PROCESS FOR MANUFACTURING THEREOF
http://www.freepatentsonline.com/pdf...0100016463.pdf

Some unanswered so far questions:
What causes the interferences patterns? Variations in coating thickness OR variations in coating index of refraction.
Why does it not occur on 1.5 index lenses (CR-39)?

PS: The pattern here appears to be a streak of hard coat that dripped from the lens holding mechanism:


I observed it on other lenses by the same supplier and the streak correlates with a "mark" at the lens edge. It is perpendicular to the interference patterns (which were likely parallel to the surface of the liquid coating material in the tank).