AR Coating and residual reflections...

[Pete Hanlin]

I'm preparing a CEC on AR coatings, and it is curious to me that- every time I've ever read information on ARC lenses- there is a statement akin to the following: "Although a lens with close to zero residual reflections could be created, the effect is not cosmetically appealing... and so on."

Has anyone on the Board ever actually observed such a lens? Can one be ordered from a manufacturer just for demonstration (although I suppose a complete batch would be required)?

On a slightly different topic, although I have often read information on applied color coatings (most often applied to glass lenses), I am admittably unfamiliar with the process or resulting quality of the lens. Anyone out there with experience in applied tint surface coatings?

Thanks,
Pete "ever curious" Hanlin

[Blake]

Pete,
It's my understanding that the residual reflection is related to the thickness of the coating. For instance, a coating that is 550nm thick will reflect light with a wavelength of 550nm, resulting in a greenish colored reflection. The amount of residual reflection is reduced by multiple layers of ARC. I'm not sure what a lens with almost no residual reflection looks like, but I can't see why it would be cosmetically unappealing (unless you have ugly eyes), since any reflections would be very faint and probably only noticeable at an angle.

Blake

[Pete Hanlin]

You are correct... according to Borish, AR coating is controlled by two factors path and index. The path is controlled by making the coating an "odd 1/4" of the wavelength of the light to be transmitted (e.g., 1/4; 3/4; 5/4...). The index of the coating must also be equal to the square root of the substrate as well...

I, too, am curious as to what a lens with no residual reflections would look like. Hopefully, someone can either tell me where to get one or explain things in fuller detail to me (and now, "us" :) ).

Pete

[Bev Heishman]

Hi Pete! I think it may involve the index of refraction and the base curve and thickness. It has been some time since I have opened my texts on mechanical and geometric optics. Generally the higher the index of refraction the greater the reflection.

I too will be curious to see what the guru's have to say.

[This message has been edited by Bev Heishman (edited 07-27-2000).]

[Steve Machol]

Pete,

I've been doing optical thin film coating for 8 years and I have never seen a design - real or theoretical - that will produce 0% reflection over the entire visible spectrum. Is it possible? Perhaps, but not very likely.

The reason is that such a design would be incredibly complex, and would possibly require materials with indices that don't exist in the real world. It might also require many more layers than would be feasible in a typical manufacturing environment. Bottom line - don't hold your breath!

As for your second question, applied tint coatings can (and often do) use the same vacuum processes and materials that are used to produce anti-reflective and mirror coatings. It just a matter of changing the sequence, number and thicknesses of the individual layers.

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Steve
OptiBoard Administrator

[Pete Hanlin]

Steve said,
"The reason is that such a design would be incredibly complex, and would possibly require materials with indices that don't exist in the real world.

...and you're going to let a minor thing like that stop you from doing it??? How disappointing! :) ;) :)

Really, though, I have read in numerous texts that "a completely reflection-free ARC, while possible, leaves a gray cast on the lens which is considered cosmetically undesireable... therefore, manufacturers leave a residual color on the lens..." and so on.

Oh well, thanks for the info, Steve.

Pete "I guess this is another item that has to join the clear polarizing filter in my 'great ideas' trashcan in my office" Hanlin

[jim c]

yes 0% reflections do exist(.1% reflection or less)...at certain wavelengths.(sorry Steve got it right when he mentioned indicies that don't exist The coating designs used are 12 to 16 layers of optimized thicknesses. The more layers , the wider the wavelength coverage. When theses coatings are centered in the visible spectrum they appear grey or neutral. The use of tio2 in these films is essential for a sufficiently high index. Zinc sulphide also works. For a low index material Mgf2 would be the choice. These coating are generally for military use on glass optics.

For opthalmic coatings, Satis sells a neutral coating that is programed into my Satis 900. It is grey, its less than 1/2% ref across the visible....but no one wants to buy it... cosmetic appeal for sure.

There are also coatings developed for ar use underwater(subs)...the thicknesses are different because the index of water is taken into account...

[Steve Machol]

Jim,

I wasn't aware of the Satis coating. Do you have a reflection spectrum on it? I'd be interested in seeing it.

You're right, of course, that one can achieve 0% reflection at certain wavelengths. However I've yet to see a stack design that would produced 0% R over the entire visible spectrum (roughly 400-720nm.)

Interestingly enough, I once created a coating that had a peak reflection of 0.2% and a photopic reflectance of around 0.1%. However this coating was the result of a malfunction in my ion gun and I was never able to duplcate it! :(


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Steve
OptiBoard Administrator

[jim c]

By the way guys,
In the past I have used sio2< silicon dioxide (the black stuff)> to achive off indexes. By regulating the rate and O2 flow this material will give you a different index.
The faster you go the higher the index, from
1.48 to 1.85 or so. At the higher index absorbtion becomes a problem.
Once upon a time I used Sio2 at 2 different indexes to achive "extremely durable" v-coats
on plastic,jet cockpit,instrument covers.

he he ...got another post

[jim c]

Steve,
I've been there...if only we could re-live the moment.
I have run the coating once in 2 years (frankly by operator error) but I'll try to dig up a curve...or run it again.

Jim

[Darryl Meister]

Hi Jim and Steve,

Correct me if I'm wrong, guys, but my understanding is that coatings with low reflectances and neutral colors are also harder to control the consistency and appearance of... That even slight shifts in layer thickness (perhaps even from the position of the lens in the chamber) can cause noticeable changes to the reflex color. It was also my understanding that green reflex colors are more "forgiving" in this regard -- and are not quite as susceptible to the effects of thickness.

Best regards,
Darryl

[Steve Machol]

Darryl,

You're absolutely right. Neutral and low reflectance coatings are much more difficult to produce consistently - exactly for the reasons you outline.

However there's two schools of thought on whether the green color is more forgiving or not. On one hand, since green is in the center of the visible spectrum, slight shifts are unlikely to produce significant color variations. On the other hand, photopic sensitivity is greatest at the green wavelengths which may mean that the eye is more likely to observe such color differences.

I tend to side with the former explanation over the latter.



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Steve
OptiBoard Administrator

[yzf-r1]

I see, quite often actually, coatings that show multiple colors on a single lens. e.g predominantly green, but areas of pink. Is that anything to do with lens curvature, or would i be correct in assuming what i usually do, and that is the coating has not been correctly applied.

I would be interested in finding out what causes this. Is it different compounds depositing on different parts of the lens, or simply uneven deposition of the coating. I have heard it suggested that it is more likely to be the latter, but fail to understand how, since AR coatings are applied by a vacuum process, so surely the deposition is even throughout, or is it?

Many thanks

[Steve Machol]

The most common reason for this is an index mismatch between the hard coating and the lens substrate. This can be corrected by either using a hard coating with the same index as the substrate or by increasing the thickness of the hard coatings. Neither option is alway feasible because of issues with coating durability.

[Bev Heishman]

I have found some great information that I use in my CEC on Lens Coating Technology at www.cerac.com Look for Technical Publications as they write about the many faces of thin film coating technology.

[Pete Hanlin]

I'm reading the initial post in this thread, and I'm thinking to myself "Yeah, I've often wondered about that, too!"

Then I look over and realize it was my own post from two years ago (now that I'm closing in on 2,000 I suppose its okay to forget one or two of them)!
:p
Thanks for the website, Bev!

[yzf-r1]

Steve Machol said:
The most common reason for this is an index mismatch between the hard coating and the lens substrate.

Steve,
That makes everything so much clearer now, especially after i saw a pair of specs today as i was checking the jobs from the factory. These lenses had the most even AR coat i have ever seen, and upon further examination, they were glass lenses so no hard coat, hence no mismatch!

By the way, would you consider the lenses to be faulty if the residual reflections were 'non-uniform', and how would you react if you got sent back lenses as factory error for that reason? (just curious coz i send loads of these back):eek:

yahya

[sarahr]

I made a mistake in sending a pair of lenses back to Essilor (I think 1.8 glass) because they appeared , not to have been coated. I have since been educated that this is a coating with no colour bloom and is standard issue on this lens. How does this happen, and why is it advantageous?:idea:

[yzf-r1]

Sarah,

I know very little about this topic, so i thought i'll get an expert to answer, well, i will quote what he said:D

Pete Hanlin said:
Steve said,


...and you're going to let a minor thing like that stop you from doing it??? How disappointing! :) ;) :)

Really, though, I have read in numerous texts that "a completely reflection-free ARC, while possible, leaves a gray cast on the lens which is considered cosmetically undesireable... therefore, manufacturers leave a residual color on the lens..." and so on.

Oh well, thanks for the info, Steve.

Pete "I guess this is another item that has to join the clear polarizing filter in my 'great ideas' trashcan in my office" Hanlin

[Steve Machol]

As an ex-AR designer myself, let me add my two cents (or $0.0365 Canadian!)

While neutral AR designs are possible, they are very difficult to manufacture and still maintain hue consistency from run-to-run and lens-to-lens. Therefore to do this with any degree of accuracy you need to allow for a bit more overall reflection than is typical with the AR coatings of today (<1.0% reflection.) Most neutral coatings I've seen tend to fall in the 2-4% reflectance range.

Furthermore designing a neutral coating for 1.8 glass is significantly easier because of its relatively high index of refraction. Designing and manufacturing for lower index substrates in much more difficult.