# Optical Forums > Ophthalmic Optics >  patent pending...

## Dave Nelson

Regarding the posts on determining whether a lens is high index or not...or what the index is, heres a thought. Why not use a "wet" cell . A transparent plexiglass "holder" to simply drop the lens into. The cell is filled with water. (n=1.3375) The power before and after is read, the 2 indices are 1.00 and 1.3375, and calculate how much neutralisation has taken place. This should be quite accurate to determine index, and should work even with an asheric front surface. What do you think?

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## Pete Hanlin

I can't see why that wouldn't work (although Darryl may 8^)!  On the same line of thought, if you obtained liquids with different refractive values (equal to the refractive values of different lens materials), you could submerge the lenses in the various liquids until there was no reflection...

Of course, this wouldn't work perfectly with coated lenses (AR and/or SRC)...  actually, the more I think about it, the more I think your idea sounds much more practical!  However, if you could find a clear liquid with an index higher than water (oil?), the test would better differentiate between the indices.

Neat stuff!
Pete

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## Pete Hanlin

Now you got me thinking (and it hurts)...

Hmmm, if you had a container that could accurately measure the volume displacement of a lens... and a very accurate scale... you could figure out the material by its specific gravity... right?

Pete "Like a tune you can't get out of your head" Hanlin

PS Maybe we could list all the different ways indices of materials can be determined...?

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## Shwing

Sorry, guys, already invented.

It is in fact called a wetcell.  It is used to determine the power of a contact lens in conjunction with a lensometer.  Used the same way you describe.

I first heard about it about 6 years ago, but have never seen one.

Helluv an idea though.

Personally, I go by sound.  I have a gold pinky ring, and I can differentiate all materials with a couple of quick taps.  Even 1.6 vs. 1.67

Of course, you can also use the lens clock and the formula, if you know the given rx...



------------------
 Shwing;-}

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## Brad Gelb

Specific gravity would get you in the ballpark, but....

Aren't poly and Ormex both 1.21 (or 1.2 depending on the source)?

Brad "hair splitter" Gelb


(gotta get past that "novice" level)

[This message has been edited by Brad Gelb (edited 06-06-2000).]

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## Darryl Meister

> Originally posted by Dave Nelson:
> *Regarding the posts on determining whether a lens is high index or not...Why not use a "wet" cell . A transparent plexiglass "holder" to simply drop the lens into. The cell is filled with water... This should be quite accurate to determine index, and should work even with an asheric front surface. What do you think?*


Hi Dave,

Unfortunately, I don't think your idea would work with spectacle lenses, since the curvatures of their front and back surfaces complicate things. Also, depending upon the geometry of the "holder," there may not be any neutralization during the first measurement. If the holder has parallel sides, I believe that it won't show any power, no matter what the index of the liquid inside. (Just like looking through a windshield doesn't affect vergence for distant objects.)

If you were able to form your piece of lens material into a parallel slab, you could determine its index with a couple of measurements/experiments. However, this would require destroying the lens to do so.

I, personally, can't think of any simple, yet accurate, nond-destructive means of determining the refractive index of a spectacle lens.

Best regards,
Darryl

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## Darryl Meister

On second thought... If you were to combine Dave's and Pete's ideas, you might have something you could work with. For instance, if you could vary the refractive index of the liquid in the cell (perhaps by adding some chemical additive), you could effectively "neutralize" the lens. That is, you could make the lens disappear (no reflection, no refraction, etc.) once the index of the liquid matches the index of the material.

Now, once you have matched indices you can calculate the refractive index of your liquid -- since it is easier to calculate the index of a liquid than a spectacle lens.

I don't know, however, what chemicals (if any) could be used for this purpose.

Best regards,
Darryl

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## Bill M

If I remember correctly, there was a test for identifing contact lens material. You would drop the contact into different solutions and the lens would float or sink depending on the material. Could the same thing apply to specatacle lenses?

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## Dave Nelson

wow what a response. Im familiar with the wet cell used in contacts. Its used to measure power in a soft lens:but not index. This all came about because of a paper I wrote "fluid neutralization of optical surfaces." In the paper, written to allow contact lens students to better understance the effect of the tear-fluid lens on the net power of a rigid contact lens, I describe how to measure the index of any given fluid.(sewing machine oil is n=1.45 approx.) using the variables power, index, and radius.one surface is always plano, and thick lens formulas are excluded. The short version is this... a plano meniscus lens is filled with enough water to form a small circular pool in the lens, and the power is read. since the back surface of the fluid lens is plano, the resulting power is from the interface curve-It will therefore read as a plus lens, with the index known. (1.3375).With the power and index known, the radius can be calculated. Now the "unknown" fluid replaces the water, and the power is read. The unknown fluid will assume the same radius as the water, and with the new power now known, the one unknown variable can be calculated... the index. I used this to confirm that ciba tear gel is indeed the 1.3375 they claim it is. Now, the question before us is...can the method be "reversed" or re-worked so the index of a solid can be determined. I think so. Ill check more on this and let you know what I find out-regardless of how embarrasing the results...

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## harry a saake

Darryl, since we know that the speed of light passing through a medium changes the speed and direction, could not the index of a material be determined by that means?

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## Shwing

Dave, ehllo!  I recall you being from 100 Mile House, or William's Lake, correct??  If so, do you remember a family of opticians by the last name of Martens??  Moved to the lower mainland...

Anyway, what you describe sounds very similar to how a radiuscope works. Sorta... 

Regardless, I would be VERY interested in seeing a copy of your paper.  I teach @ NAIT part of the Contact Lens course, and am always looking for new, bettter or alternate ideas/ explainations.

For now,



------------------
 Shwing;-}

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## Darryl Meister

> Originally posted by Bill M:
> *If I remember correctly, there was a test for identifing contact lens material. You would drop the contact into different solutions and the lens would float or sink depending on the material. Could the same thing apply to specatacle lenses?*


Hi Bill,

This is actually another application of that involves determining the material's density (see Pete's earlier posting). An object will only float to the top in a liquid if it has a lower density. If it has a higher density, it will sink. If the densities are equal it will remain suspended at any level. Once you've determined the density (assuming that you can do so to any degree of accuracy), you can guesstimate the particular lens material and refractive index. This method will only work well, though, if you have a variety of fluids with the appropriate densities for testing.

Best regards,
Darryl

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## Darryl Meister

> Originally posted by harry a saake:
> *Darryl, since we know that the speed of light passing through a medium changes the speed and direction, could not the index of a material be determined by that means?*


Hi Harry,

Determining the speed of light in a lens material would be a challenge. Also, the speed of light any a lens material is still very fast (200,000 km/s in CR), which would be difficult to measure with the kind of equipment that we have access to.

Determining the refraction of light would work. Unfortunately, you can only use this method if you can determine the exact angles of incidence and refraction at a given surface. For a flat slab of lens material, this would be relatively straightforward... However, for curved spectacle lenses the problem becomes more complicated (as mentioned earlier).

Best regards,
Darryl

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## Pete Hanlin

Dave,
I would like to see the paper as well... is there any way you could post it to the "downloadable files" section of OptiBoard?

Pete

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## Pete Hanlin

Okay, one more shot at a process of determining index...  CRITICAL ANGLE!  If the critical angle of the material could be determined with preciseness, the index could be determined.  Whattya think, Darryl?

Pete

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## Darryl Meister

> Originally posted by Pete Hanlin:
> *Okay, one more shot at a process of determining index...  CRITICAL ANGLE!  If the critical angle of the material could be determined with preciseness, the index could be determined.  Whattya think, Darryl?
> 
> Pete*



Yep... But it would be difficult to determine with a curved spectacle lens. If it were a flat slab, we could do all kinds of stuff... The easiest would probably be to use a microscope. You can bring the microscope into focus with and without the lens slab in place, recording the difference in microscope heights. Then, simply compare this to the actual thickness of the lens to determine the "reduced" thickness of the lens material. Once you have done this, you can use a simple forumla to calculate the index (index = actual thickness / reduced thickness).

Of course, if you have an Abbe refractometer you could determine it to.

Best regards,
Darryl

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## Steve Machol

> Originally posted by Pete Hanlin:
> _Dave,
> I would like to see the paper as well... is there any way you could post it to the "downloadable files" section of OptiBoard?
> 
> Pete_


Dave,

If you're willing and able to share this, email the file to me and I'll put it in the file directory.


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Steve
OptiBoard Administrator

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## Dave Nelson

Hi all. Shwing, I've never been to 100 mile house or williams lake in my life. I've lived in this area of the Fraser Valley for many years... There was a fella posting on the board some time ago who was from 100 mile, a retired engineer,turned optician, named Bob. I'm certainly not an engineer, and I'm many, many years away from retirement. Is this the guy you're thinking of? Any way, I would be happy to submit the paper, but it is still a rough draft, and needs some cleaning up. I hav'nt done any work on it for some time due to time constraints.(just ask my wife who won't let me forget I promised to build a family room in the basement 5 years ago...)I was able to "test" the fluid/index idea today. I found an old case for a fresnel prism. perfect. small, clear case, watertight, and open at the top. I found a pair of lenses, both -6.00, one reg plastic, the other high index glass. When immersed in water, the plastic one read about -3.00, and as expected, the high index glass read -4.00.  A diopter differnce, with an index variable of about .2. (1.7-1.49)The power reduction from immersion in a medium .3375 grater than air was about 50% and 33%. I need to work the math some more, but I assume the index of a lens could be determined to within .1.(1.5,1.6,1.7ect)maybe better, if I can figure out a quick formula. I will certainly keep you posted.

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## Darris Chambless

> Originally posted by Pete Hanlin:
> *Okay, one more shot at a process of determining index...  CRITICAL ANGLE!  If the critical angle of the material could be determined with preciseness, the index could be determined.  Whattya think, Darryl?
> 
> Pete*


Hi Pete,

Too many variables involved. Back to the drawing board, Dude :-)

Darryl,

One would have to use a constant to start with concerning the fluid and have a compatible liquid or other additive that had an index predetermined. The only thing I can think of that might be plausible would be water and predetermined amounts of geletin
This should (hypothetically speaking) increase the density of the water without completely distorting its composition for reference purposes. But how much change would be produced is beyond my mental scope not to mention equipment availability :-) AND since I have no idea what the index of refraction of geletin is at any state of solidity it's probably a stupid idea, but it's the only thing I can think of that wouldn't require several different mediums of fluid. (That sounds pretty rank doesn't it? :-)

Well, I'll keep thinking ;-)

Darris "Hey! Watch this." Chambless

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## Jeff Trail

&gt;&gt;&gt;&gt;The only thing I can think of that might be plausible would be water and predetermined amounts of geletin
This should (hypothetically speaking) increase the density of the water without completely distorting its composition for reference purposes&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;

  Darris.....

    Hmmm... isn't this just another "fancy" discription of a LAVA lamp? :-)....do we need the "black light"..the posters..oh and a bead curtain between the retail and the "back room" portion in the store ?..

Jeff "one of the "Brady's" T ...oh that Marsha...hubba hubba...:-)

[This message has been edited by Jeff Trail (edited 06-08-2000).]

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## Darris Chambless

> Originally posted by Jeff Trail:
> *&gt;&gt;&gt;&gt;The only thing I can think of that might be plausible would be water and predetermined amounts of gelatin
> This should (hypothetically speaking) increase the density of the water without completely distorting its composition for reference purposes&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;
> 
>   Darris.....
> 
>     Hmmm... isn't this just another "fancy" description of a LAVA lamp? :-)....do we need the "black light"..the posters..oh and a bead curtain between the retail and the "back room" portion in the store ?..
> 
> Jeff "one of the "Brady's" T ...oh that Marsha...hubba hubba...:-)
> ...


Hello Jeff,

You obviously don't cook do you? ;-) The gelatin that I'm referring to comes in dried sheets and has to be dissolved in water; it can either be used to "thicken" or "solidify" a dish depending on the consistency you want and is clear. The consistency is dependant upon how much is used and is used mainly in French cooking. So there! Neayyyaahh! :-)

Lava Lamps are oil suspensions which will rise when the heat from the bulb causes it to change density or specific gravity from ion exchanges and molecular restructuring. Each petroleum molecule will absorb and expand with the heat exchange and create a lower specific gravity hence causing a "lightening" effect on said molecules (or weightlessness in the surrounding environment) Since liquid dies are not compatible with oil a dry pigment must be added to the petroleum in order to get the vast color array that is the proverbial "Lava Lamp" :-)

For more about inane information please write to "What You Didn't Know But Always Thought "Why Bother?" c/o Darris C. Mr. Extraordinary, P.O. Box 1 (because I was the first ;-) Exceptional, Texas 76904 :-)

By the way we won't be in N.O. until next weekend ;-)

Darris C.

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## Darryl Meister

> Originally posted by Darris Chambless:
> *The only thing I can think of that might be plausible would be water and predetermined amounts of geletin
> This should (hypothetically speaking) increase the density of the water without completely distorting its composition for reference purposes... AND since I have no idea what the index of refraction of geletin is at any state of solidity it's probably a stupid idea...*


Hi Darris,

Well, the good news is that you wouldn't have to know what the index of the gelatin is, or how much change it produces. Once you optically neutralize the lens within the solution, you can remove the lens and measure the refractive index of the solution using the methods we've been discussing.

However, you have to be certain that adding gelatin will increase the refractive index of the solution, not just its density. Also, the gelatin has to completely dissolve in the solution of water (the solvent). If not, it won't create a homogenous solution, and may scatter the light you attempt to measure through the container.

Best regards,
Darryl

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## Darryl Meister

> Originally posted by Dave Nelson:
> * The short version is this... a plano meniscus lens is filled with enough water to form a small circular pool in the lens, and the power is read...*


Hi Dave,

The idea certainly has a lot of potential for liquids.

You know, it seems to me that the depth of the fluid pool on the lens surface would have to be accurately determined, as well, before you could compute the refractive index. You can think of the pool as another lens, in contact with the first. Since the front surface of this lens -- which is equal to the back surface of the actual plano-concave lens, is curved -- the vergence through the pool will be altered by the thickness or depth of the pool. (If you compute the vergence at each interface.) This is one of the complications of the wet-cell idea. If you design everything just right, the errors might be small enough to guesstimate the refractive index within an acceptable level of precision.

One convenient fact is that you can keep the thickness of the fluid pretty small. And, if you know the sag of the back lens surface at some diameter, you can predict the exact thickness by simply filling the back surface up until the fluid pool has the same diameter.

Best regards,
Darryl

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## Dave Nelson

Hi Darryl. As noted in one of my posts on this subject, I did'nt use thick lens formulas in my paper for simplicity. The accuracy was'nt really an issue, since determining an index for a fluid was only an exercise to demonstrate how the fluid lens influences the refractive value of both toric and non-toric surfaces. As you're aware, F=n-n'divided by r will determine the power of a single surface only, while multiple surfaces, to be accurate, require thickness to be taken into account because of the vergence inside the lens. Where n-n' is the index of the material minus the index of the surrounding medium, predicting the power of a single surface should be easy: a glass lens in air: 1.53-1 divided by radius=power, or in water:1.53-1.3375 divided by r=power,or F. Now, F1+F2=the predicted power, excluding the thickness. You correctly point out that excluding the thickness will cause enough error to make an accurate determination of the index immpossible. My thinking is this: the thickness is a constant, as is the radius. the variables are power and index of the surroundin medium.Since the new net power can be determined, and the index of the surrounding medium is known, the thickness, and vergence effect is the same, and one should only need how much index change would effect the power change. Further, most high index lenses are minus, and should have little variation in thickness at the oc, where the power is measured. I could be all wet (no pun intended) with all this and I really appreciate all the input from the "board" If this whole subject has only succeeded in provoking a little thought its time well spent.

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