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CarloBartolini

Lens Properties of Varnish System

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I have been wondering about this, but it's still a bit blurry in my mind,

My thinking is that films of different refractive indexes laid one upon each other should have Gradient Index Lens properties.

For a magnifying effect of the wood, should each succeeding film of a different compositions be of a higher index of refraction,? Or are there are better combinations for increasing depth and magnification?

What is the point where you start to loose transparency - because of the different refractive indexes?.

Does the first layer upon the wood cell also has a lens effect increasing DOI (Distinction of Image) etc.? What is the Best RI for that - is it one as close to the wood or one a little bit higher (the wood's RI can be many different values depending on humidity)?

Or is it really not that simple?

Any thoughts on this?

Thank you very much.:)

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The understanding i have is that the first layer on wood should have an RI that reduces the difference in RI between wood and air. Air has an RI of around 1.00 and wood is generally around 1.40. Its all very complicated and when ive researched it ,i just get confused. Would a substance with an RI of less than 1.4 be better or something with an RI of more than that.

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I have a friend who is an optics specialist and I asked him this question-he said yes, theoretically there may be some magnification possible by combining different IR layers but that it would be slight.

Oded

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The understanding i have is that the first layer on wood should have an RI that reduces the difference in RI between wood and air. Air has an RI of around 1.00 and wood is generally around 1.40. Its all very complicated and when ive researched it ,i just get confused. Would a substance with an RI of less than 1.4 be better or something with an RI of more than that.

Thank you fiddlecollector....interesting, so it could the opposite of what I've been thinking...the woods's refractive index and the filling of it's cells get me very confused...these trees are playing a trick on me....I find this stuff very interesting, done some research and now I know less than before...:D

Measurement of the refracting index of wood for microwave radiation

I have a friend who is an optics specialist and I asked him this question-he said yes, theoretically there may be some magnification possible by combining different IR layers but that it would be slight.

Oded

Thank you Oded, I am guessing it may have to do with the films being too thin, or perhaps the need of materials with even higher RI than the ones normally used for varnish.

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"Does anybody know the principle of these kids' cards with a real depth effect?"

If you are referring to the ones with tiny grooves on the surface, it is a modification of the Fresnel lens (q.v.). I have one that is clear, exactly as thick as a credit card, but magnifies like a large glass lens.

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"Does anybody know the principle of these kids' cards with a real depth effect?"

If you are referring to the ones with tiny grooves on the surface, it is a modification of the Fresnel lens (q.v.). I have one that is clear, exactly as thick as a credit card, but magnifies like a large glass lens.

Chet is correct!

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One confusing aspect of varnished surfaces is that most things you read will say a perfectly flat top surface of high RI will give the best appearance of depth .But many old violins have very high appearance of depth despite having a very rough surface (on a micro scale) full of surface cracking and defects (which should increase reflection and so reduce depth) and a matt like surface appearance.

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Does a coating on a photographic filter turn it into a lens? Of course not, and this won't make a lens either. The gradient in a lens is usually radial, i.e. perpendicular to the lens axis. The Wikipedia article you referenced mentions an axial gradient, but that would only work in certain cases, for a thick lens. The i.r. gradient in varnish is parallel to the supposed axis, i.e., axial, and you have a thin coating.

Two people mentioned a slight effect, so I suppose I have to elaborate on that. On a microscopic scale the varnish is not actually a thin layer, but consists of multiple bumps as it fills irregularities in the cells. If you look at the surface through a microscope at a varnished surface, the varnish will distort the view a little if the surface isn't flat. But does it look like you have put little magnifying glasses all over it? Or does it make the fiddle look bigger? I don't think so.

Nice idea, but no.

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One confusing aspect of varnished surfaces is that most things you read will say a perfectly flat top surface of high RI will give the best appearance of depth .But many old violins have very high appearance of depth despite having a very rough surface (on a micro scale) full of surface cracking and defects (which should increase reflection and so reduce depth) and a matt like surface appearance.

This may or may not be the correct description of what's going on, but it makes sense to me:

A bare wood surface is rough... and even at the small microscopic scale, the fibers are rough. That means it scatters light. If you add a thin coating to the fibers that reduces the surface roughness, there will be less scattering. But if the IR's aren't matched, you would still have some scattering going on at the interface between the fibers and the coating.

The wood surface can still look rough, as in some old violins, but as long as the coating matches the fibers at the microscopic (light wavelength) level, the scattering will be reduced. Of course, if the final outer surface was perfectly smooth, that would give the most transparency, but I think the microscopic roughness effect is extremely important.

Bottom line: the coating has to wet the fibers, the dried surface has to be smoother than the wood (at the microscopic scale), and the IR's need to match.

That's all educated guessing, but it seems to work.

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Carlo, you are confusing two, or possibly three, things. To reduce diffuse scattering of white light from the wood surface and bring out the color, the varnish should match the i.r. of the wood. It's really that simple, and that's all you have to know.

But you will still have a reflection from the surface of the varnish. To minimize reflections even from the surface, lens makers apply a carefully-constructed sandwich of two or more thin coatings with specially chosen refractive indices. But these are carefully controlled, extremely thin layers. I don't think you'll have any success doing that with wood.

And forget any notion of constructing a lens. I don't think that's going to get you anywhere.

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)snip)

And forget any notion of constructing a lens. I don't think that's going to get you anywhere.

Hi All - just a thought - would the "corduroy effect + varnish" not create a longitudinal lens?

cheers edi

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when I was a kid I had a fresnel lens. I used to take it out in the sun and melt stuff with it... :D I don't think a thin film of uniform thickness is going to do much magnifying unless it does have well engineered grooves like a fresnel lens.

Hey! I found it. This is the one I had.

http://www.scientificsonline.com/large-fresnel-lens.html?&cm_mmc=Mercent-_-Google-_-NULL-_-3052833&mr:trackingCode=CC6E1735-DB81-DE11-8C0A-000423C27502&mr:referralID=NA

I guess this thread is due to the idea that the cremona finish somehow magnifies the wood grain. Is that even true? or just legend?

Here's a picture and a short video of a recent ground layer and varnish sample. The varnish layer is thin but irregular in texture with convex and concave areas but looking straight down into it I don't see anything that looks like magnification or distortion of the grain. Also in another thread someone was commenting on whether a smooth glossy surface vs. a mat surface is best for bringing out a deep 3D look, I think either one could work well.

http://youtu.be/e6d8JMC5jTg

post-31367-0-41672000-1325910334_thumb.jpg

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Thank you all for the great answers, I really appreciate everyone taking the time to write.;)

Read and re-read all posts, MikeC my thought came from reading about flat GRIN lenses, made by varying the index of refraction within the lens materials, and wondering if it is applicable to varnish coats. But as some have posted above, the effect would be minimal, still I do not understand if it would not be possible using other materials, it appears they make lenses of 0.2mm thickness, way over my head at the moment.:)

But the matching the refractive index of the wood is a very practical and useful information, I am hoping I was overcomplicating and the RI is aprox.1.4 instead of what my limited research appears to show that th RI varies according to the humidity of the wood, the paper I posted above shows a variation of IR of 1.2 to 1.6 in the same species, with different humidity, or is it as this other study shows?

When wood is dried, the interiors of the cells become filled with air. Following previous work on scattering from individual wood fibers that make up paper [saarinen and Muinonen 2001], we assume that the cell walls, despite their complex structure, can be approximated as a homogeneous dielectric material with refractive index 1.55. For convenience we will refer to this material as cellulose, even though it has other constituents. This means that the interior of wood, optically, is a block of cellulose (which behaves much like glass) with parallel air tubes running through it.

Measuring and Modeling the Appearance of Finished Wood

Or, depending on the process/material used to fill the wood cell this RI could change, as the humidity changes RI of wood, so perhaps for sound you'd want the air spaces open, but for looks to have them filled? Or no to both? (air-surface -air surface causes damping)

Thank you once again, this is very interesting subject.:)

Edi - The corduroy effect is an interesting thought, I'll do some reading on it.

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Hey Carlo. how have you been, pff' dumb question, paradise is always paradise, haha'...

To me this is a question that has two roads to go down. 1.To understand this scientific tangent and to wonder if it is controlable, but probably more importantly 2. is it something that has practical appication in the varnishing of violins.

To me there is just to much "thinking" going on. I "think" :lol: its great to get down to the scientific level, and like to stand back and observe others who are more "scienific". I like theorize about things, but really it always just comes down to practicality and is "it", whatever it may be, doing what I want it to and is it serving its purpose.

One thing I would say is that when we talk of refraction levels, the mil build and depth of the varnish comes into play. As layers of solids build on top of each other, often times light interuption will reduce over all refraction.

Again, I suppose this, like any tangent of making or varnishing could be disected to the nano level. But I just search for a "system" that does what I want

1. looks good

2. goes on thin, looks thick

3.wears well

4.is "sonically" invisible, or improves the sound

5. easy to apply and safe to use

other than that, I try not to think about it too much.

Hey, I know, you could buy a bunch of lupes' and pop out the lenses and then paste them on the fiddle, and then overcoat them :lol:

How'd the boat turn out?...Fantastic I'm sure B)

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Jezzupe, great to hear from you, long time, hope it's paradise over there too, after the storm there is always the wonderful calm, even more appreciated than always calm.

I'm overcomplicating what I don't understand, lucky no exotermic reactions in my mind yet.

My original simpler thought was, would I get a better view of the wood details if the overlapping coats where of increasing RI?, and what would be the best RI to start with?......but I'm new at this so have not reached the stage of simplicity as you maestros have.

Overthinking can get us into trouble, is this tuna fish in the fridge still good to eat? mmmm maybe I've put it there x days ago, do some math, research on the net - EAT IT - OH NO it was BAD....run to the toilet - a cat would not think, would simply smell an not eat it.:D

As for the boat, I'm starting the saloon, most important part, that's why I'm doing my last thinking and research.

Back to the samples - reality.

Thanks!

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Jezzupe, great to hear from you, long time, hope it's paradise over there too, after the storm there is always the wonderful calm, even more appreciated than always calm.

I'm overcomplicating what I don't understand, lucky no exotermic reactions in my mind yet.

My original simpler thought was, would I get a better view of the wood details if the overlapping coats where of increasing RI?, and what would be the best RI to start with?......but I'm new at this so have not reached the stage of simplicity as you maestros have.

Overthinking can get us into trouble, is this tuna fish in the fridge still good to eat? mmmm maybe I've put it there x days ago, do some math, research on the - EAT IT - OH NO it was BAD....run to the toilet - a cat would no think, would simply smell an not eat it.:D

As for the boat, I'm starting the saloon, most important part, that's why I'm doing my last thinking and research.

Back to the samples - reality.

Thanks!

Yes, samples will be the best way. I think you'll find that this is not something you can really exploit to any degree that would serve you. Just think thin, durable,easy to clean, looks nice.

Still, sailing the storm in uncharted water...But hey, what fun would life be if it was all smooth sailing?

Be well.

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A simple balsam ground, is balsam the right word for it? a resin dissolved in turpentine? anyway that seems to have a close IR match to wood. It gives the wood that 'wet' look even when dry. Brings out the grain really well. You mention air spaces in the cells. Even if you fill the upper cells with a sealer like a balsam ground, the cells underneath will still be air filled so maybe there could be an IR mismatch there. That could increase reflectivity maybe? hmmm

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Nice paper. The quote that I found most applicable to violin varnishing was:

When wood is planed and sanded to produce a flat surface, the

result is a layer of partial (and damaged) cells on the surface, with

the intact structure below the surface. The first interface presented

to incident light is therefore irregular, and much of the light is reflected

diffusely. This white surface scattering is the reason that

the color of unfinished wood is light and unsaturated. When varnish

or other clear finish is applied to the wood, this first surface is

effectively eliminated because the refractive index of the finish is

close to that of the wood.

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You mention air spaces in the cells. Even if you fill the upper cells with a sealer like a balsam ground, the cells underneath will still be air filled so maybe there could be an IR mismatch there. That could increase reflectivity maybe? hmmm

According to the paper, it is precisely this mismatch that causes the internal reflections in the wood that we WANT to see. We just want to get rid of the scattering at the first surface.

If we filled up the air spaces in the wood with IR-matching stuff, you might not need to use mirrors or look thru the endpin hole to set a soundpost. Just look straight through the top.

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I guess this thread is due to the idea that the cremona finish somehow magnifies the wood grain. Is that even true?

No, as you discovered.

Hi All - just a thought - would the "corduroy effect + varnish" not create a longitudinal lens?

No.

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Hi All - just a thought - would the "corduroy effect + varnish" not create a longitudinal lens?

cheers edi

If the varnish is letting the texture of the wood show through then the varnish surface is following the surface of the wood. This isn't very conducive to changing how the light propogates. Things that strongly bend light, like a lens or prism, don't have parallel surfaces. Of course there are special lenses that look like a thin flat sheet of plastic but if you look closely enough at them you will see that they are made from very small prisms that are wedge shaped in crossection.

Now if you were to completely fill in the corduroy texture of the spruce with varnish so that the outside surface were as flat and smooth as a new car finish then there would be some lens type effects happening. I haven't tried doing this so I can't say how much you would get, if it would look good, or if you could even notice a difference without first noticing how thick the finish looked. Maybe this is one way our eyes detect a finish that looks too thick?

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Read and re-read all posts, MikeC my thought came from reading about flat GRIN lenses, made by varying the index of refraction within the lens materials, and wondering if it is applicable to varnish coats. But as some have posted above, the effect would be minimal, still I do not understand if it would not be possible using other materials, it appears they make lenses of 0.2mm thickness, way over my head at the moment.:)

For these lenses the index of refraction doesn't change with the thickness of the material, it changes across the width of the lens and that is what causes the focusing. Since violins are varnished with 'sheets' of varnish there won't be a changing index of refraction in the correct dirrection for a lens.

If you were to combine layers of different varnishes with different indices of refraction you probably wouldn't like the results. The result would be increased reflection of the incident light which would cause the layered finish to have a hazy or milky appearance compared to layers of varnish which all have the same index of refraction.

It is possible to combine materials of different indices of refraction and with just the right thicknesses to cause the amount of reflection to increase or even nearly vanish. However, using varnishing tools you won't have the control needed to produce these uniform thickness coatings.

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Thank you all, for the answers and great information, I guess some of the practical things here are what fiddlecollector and Don Noon have posted, the IR matching , light scattering etc., I've read many times Wm. Johnston very interesting thread The Index of Refraction, Both of Them from 2009.

And if I may quote your past post (I hope you don't mind, it's great info).

In the past there has been a good deal of discussion here about the index of refraction of varnishes and particularly of grounds. The thing that I remember hearing the most are the comments about trying to match the index of refraction of the ground to the wood's. There are a couple problems that I have had with trying to do this, conceptually.

First, the index of refraction isn't just a constant for a material. It varies with the wavelength of the light. Unless you have the equipment to measure the index of refraction of your wood and ground it will be very hard to match them to each other at all visible wavelengths.

Second, the index of refraction is not just one number. The index of refraction is one complex number which is really two numbers! See this, http://en.wikipedia....efractive_index go down to the dispersion and absorption section. The 'real' part of the index of refraction is the number often quoted as the 'index of refraction' but it's really just half of it. The so called 'imaginary' part of the index of refraction gives rise to the absorption of light. The imaginary part of the index of refraction is directly related to the absortion coefficient that I have brought up in other varnish discussions. The imaginary part of the index of refraction gives rise to the color and opacity present in varnishes.

Now an important thing to keep in mind at wood/varnish or varnish/air interfaces is that if you want to deal with reflection and transmission at that boundary you should use the complex index of refraction not just the single number often quoted (if you are plugging into the Fresnel equations). This means that if you want to minimize the reflection at the ground/wood interface you need to match both the real and imaginary parts of the index of refraction at all wavelengths! That's a lot to do and in someways it's obvious that the result wouldn't look right. If you did this the wood/ground interface would be invisible and you would just see the ground instead of the wood, so this isn't a really good thing to do.

Now another interesting thing about the real and imaginary parts of the index of refraction is that they are not independent of each other. They are related through the Kramer-Kronig relations. If you are trying to match the index of refraction of two things this should be your friend. These relations show that if you know the real part of the index of refraction then you know the imaginary part if you do the math. Also if you only know the imaginary part then you know the real part also. In fact experiments typically only measure one of these numbers, typically the imaginary part since absorption is easy to measure and then the other index is calculated by a computer.

Now don't let this deter you from trying to do anything with the index of refraction because this suggests a very simple way to deal with the real part of the index of refraction. If you want to match the index of refraction of your ground and wood then really all you have to do is get the ground to match the color and opacity of the wood, if the color and opacity are the same for a given thickness then the imaginary and real indices of refraction should be the same at all wavelengths! Color is a much easier thing to deal with for a hands on home tinkerer than the real part of the index of refraction. As a practical matter I think it's better for the ground to have the same color of the wood but less opacity. I've found that for the best look I have to add a little opacity to my ground rather than aim for maximum transparency. The wood seems to sparkle better with a somewhat cloudy ground than a perfectly clear one, strange huh?

Of course as usual there is a catch with this as well. Your eyes don't really see color by detecting the individual wavelengths of light. This means that simply matching colors might not be enough to actually match the spectrum of two things. Still it's a lot easier to try to do than deal with the real part of the index of refraction. Never hurts to try.

Anyways the main point of this is to put some information out there to give people ideas to play with.

I have more thoughts on this subject but I've typed enough for now.

So, adding a little opacity - extremely interesting, in the form of pumice perhaps? Or maybe something such as Kremer's Rock Crystals 0 - 63 µ?

And the color matching maple being almost white, perhaps the color matching and the opacity could be somewhat accomplished with the same Rock Crystals + pigments? Or if I'm working with wood such as mahogany, should the ground be red/brown?

No cigar?

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