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 Post subject: Brewster Angle Refractometer
PostPosted: Sat Dec 15, 2007 12:08 pm 
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Does anyone actually own a workable Brewster angle refractometer that is accurate to .01 RI units? I've read about them in the literature for years but have never actually seen one.

Are there any commercial units currently for sale that anyone knows about?

Thanks,

Bruce


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PostPosted: Sat Dec 15, 2007 12:59 pm 
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The late Cornelius Hurlburt patented a unit and a few prototypes were built
and are in the collection of a list member who shall remain nameless. (Not me :cry: ) They were beautifully made and one was modified in conjunction with GIA in attempt to make a product but it was too expensive. Professor Hurlburt passed away and the project has languished.

Our list member jleb mentioned a prototype RI meter that sounded like
a Brewster Angle unit a while back but has gone silent on that project as well.

The English Gemmologist Peter G. Read , originally trained as an engineer,
developed a Brewster Angle Refractometer which until recently was offered for sale on UK GAGTL website. Last I looked their webstore was down. His unit was about $2,000. Probably hand built by him. I have heard he may have retired from things.

He also mentioned a very clever unit made by someone named Yu in his book Gemmological Instruments. That one never was made commercially as far as I know.


If one has a table spectrometer it is fairly easy to add the necessary items to convert it to an ellipsometer and measure RI by the Brewster angle method. The materials list consists of a 589 nm light source (and ble and red ones if you want to measure dispersion) polariziers (Two a polarizer and an analyzer) and a quarter wave plate (1/4 wave at 589 and the other wavelengths if desired) You can do the Brewster angle method without the quarter wave plate but it increases the sensitivity.

Google ellipsometry for better explanations.


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PostPosted: Sat Dec 15, 2007 1:17 pm 
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Hello Gene,

I just happen to have a table spectrometer but it is a painstaking process to get any data out of it. When I have lots of time on my hands I might try your suggestion. Thanks.

My understanding is that the problem with the BAR is that there is difficulty in determining the angle of zero reflection with the older models. The eye is not sensitive enough to see the difference between zero and near zero, so visual methods are impractical.

Elsewhere on the forum Marty posted photos of Connie Hurlbut's unit. If anyone could solve the problems associated with it, he probably could.

BTW, do you have a source for bandpass filters corresponding to the B&G lines for dispersion determinations?

Thanks.


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PostPosted: Sat Dec 15, 2007 4:08 pm 
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Quote:
just happen to have a table spectrometer but it is a painstaking process to get any data out of it. When I have lots of time on my hands I might try your suggestion. Thanks.


I haven't tried the Brewster Angle method yet but I suspect a table spectrometer is MORE useful for that than anything else.

Quote:
My understanding is that the problem with the BAR is that there is difficulty in determining the angle of zero reflection with the older models. The eye is not sensitive enough to see the difference between zero and near zero, so visual methods are impractical.


Most ellipsometers have a little reflex swing mirror that allows you to switch between an eyepiece (which the newer ones don't even bother with)
and a very sensitive photomultiplier sensor. It would be fairly simple to hang such a thing on your table spectrometer. I even have some I could send you.
Quote:
Elsewhere on the forum Marty posted photos of Connie Hurlbut's unit. If anyone could solve the problems associated with it, he probably could.

That unit used red LEDs Marty's first suggestion was to use 590 LEDs which were new back them but totally old hat now. The firm of Leitz
addressed the problem of asymptotic approach to the Brewster angle
in a patent for an automated refractometer.


Quote:
BTW, do you have a source for bandpass filters corresponding to the B&G lines for dispersion determinations?

The 687 line is extremely difficult to find maybe impossible. I have a bunch of red interference filters that are close. The 431 line is a little easier.

Search interference filters on ebay and you will get two major returns. One is buy right auctions who liquidate filters and if they have it you can buy it.

The other is a guy whose handle is something like bjomejag which stands for Bob Johnson Omega (his interference filter company) and Jaguar which is what HE does with his disposable income. He can make ANYTHING in filters and will for you. For a reasonable price too. He has a webstore and sells "gift certificates." What he has up varies But he has a very busy company and has extras of all kinds of laser lines and analytical lines. He makes filters for astromoney , spectrophotometric instruments , and is well known in the fluorescence microscopy bidness. He is the go to guy for filters. I have never seen a 687 nm filter. Maybe we should get together and order ten of them.

Of course you can always use a monochromator.


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PostPosted: Sun Dec 16, 2007 1:47 am 
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G4Lab wrote:

The firm of Leitz addressed the problem of asymptotic approach to the Brewster angle in a patent for an automated refractometer.


This is the sticking point on this type measurement. I hate locating minimum signals. As you rotate the turntable, intensity decreases to a minimum at some angle, then rises again. By definition then, rate of change of intensity is zero at that angle, and so you are going to get a flat region of very similar readings around that angle. It seems to me that, without a great deal of trial and error, measurement by hand won't give the precision that Bruce seeks.

This sticking point is probably the reason why you don't see much in the way of commercial refractometers based on Brewster angle measurement.

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The other is a guy whose handle is something like bjomejag which stands for Bob Johnson Omega (his interference filter company)


Omega Optical can make most any interference filter you can dream up... for a price. As can a couple other companies: CVI Laser Optics and Chroma Technology. I've purchased custom filters from both Omega and Chroma. But Gene's suggestion might get you the filters you want at a discount price.


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PostPosted: Sun Dec 23, 2007 12:28 am 
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http://homepage.ntlworld.com/w.amos2/Br ... ewster.pdf

Here is a link to Dr. Brad's article on Brewster conoscopy which should be mentioned in this thread for completeness.


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PostPosted: Tue Feb 05, 2008 4:54 pm 
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Apologies if you have tried this link and it did not work. Try
http://homepage.ntlworld.com/w.amos2/Br ... page4.html
or just search under 'Brad Amos Cambridge Birefringence'.
With care, you can get down to 0.02, but 0.01 is tricky.
Brad


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