National Shutdowns have put the lid on most shows.
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Sun Aug 09, 2020 7:37 pm

All times are UTC - 4 hours




Post new topic Reply to topic  [ 24 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: duChaulnes method of determining RI
PostPosted: Sun Mar 01, 2009 1:07 am 
Offline
Platinum Member
User avatar

Joined: Tue Jan 02, 2007 8:20 pm
Posts: 2657
Quote:
Yep, thanks Trace! Finally a use for my old olympus compound scope!
(cause from your explanation I get that a bionoc isn't neccesary, what extra value would a binoc bring over a monoc?)


You don't need a binoc scope. An old monocular would work fine with a searcher objective (2x-4x initial magnification X 10x ocular) and also probably a 10x or 20x objective could be useful.

The higher the mag of the objective the less depth of field and the easier to decide on the zone of focus. On the other hand higher mag objectives have less working distance and above 20x you won't be able to focus on the bottom of the stone unless it is very very tiny.

Some chalk dust on the table facet helps focus on that. or a greasy finger print. Dial indicators from the far east have gotten so cheap as to be almost free nowadays. Even digital electronic ones. Of course Mitutoyo or LS Starrett or Mahr give much more "I got one , you don't!" satisfaction than no name ones. But the no name ones are more than accurate enough for this measurement .

Here are some candidates , the planer and height gauge

Here are some cheap ones

and an inexpensive digital one


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 01, 2009 7:16 am 
Offline
Moderator
User avatar

Joined: Thu Dec 13, 2007 3:42 pm
Posts: 4086
Location: the Netherlands
schweet... :D


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 01, 2009 12:37 pm 
Offline
Gemology Online Veteran
User avatar

Joined: Sun Jan 28, 2007 1:29 pm
Posts: 578
Location: Dallas, Texas
For anyone who is lost, this thread is a spin off from THIS thread.

Doos wrote:
MacGyver wrote:
Tim Spauwen, FGA wrote:
what extra value would a binoc bring over a monoc?)


Tim I've never tried it on a monoc, but the research I did specified a binocular microscope. The only thing I can think of is that we're talking about dealing with depth perception which requires the use of stereo optics to attain the 3-D effect. Otherwise it's like trying to parallel park a car with one eye closed.


Trace,

Thanks.
Yes one of the problems was with getting the table parallel and focusing on it . From the top of my head, I think Webster suggested sprinkling some dust on the table and focus on that.

Never new the technique by this name.


I find that a greasy fingerprint normally works well, at least that is what I normally find for the focal point. I will add that at 160x I find it much easier to place the stone culet up and focus on the culet first. Then focus down through the stone to find the grease on the table. To me this helps solve the problem I mentioned in the previous thread of trying to focus on the outside of the stone from both sides.

As for the binocular reference, Gene would know. The reference I read basically had said, "this can be done with any binocular microscope capable of high magnification." Since most gem microscopes are binocular perhaps it was poorly worded and they were saying, "this can be done with any high power microscope in the gem lab." In other words maybe the statement was saying, "this can be done with any high power binocular scope" and was not saying, "this cannot be done with a mono scope."

_________________
-Trace

I'm only here because my flux capacitor is broken.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 01, 2009 3:22 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1470
MacGyver wrote:
I will add that at 160x I find it much easier to place the stone culet up and focus on the culet first. Then focus down through the stone to find the grease on the table. To me this helps solve the problem I mentioned in the previous thread of trying to focus on the outside of the stone from both sides.


Indeed, setting the gem table-side down, culet-side up would seem to solve some experimental problems, but it calls up a theoretical problem that may invalidate the measurement. But after thinking about it some more, I decided I have to think about it some more.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 01, 2009 6:05 pm 
Offline
Gemology Online Veteran
User avatar

Joined: Sun Jan 28, 2007 1:29 pm
Posts: 578
Location: Dallas, Texas
Edit added after I posted and Brian's original post was gone: DANG IT BRIAN, quit doing that. Some of us REALLY like your longer insightful dissertations!!! :smt105

Wow. As always Brian, thank you. While I was able to understand all that you said, I certainly didn't see it that way before you pointed it out to us.

After reading your post, I made a crude attachmnet to try and rectify part of my problem. In the past, I'd tried to place the stone culet down and allow the iris of my darkfield base to hold the stone evenly. The problem was that this placed the culet down inside the light well and out of the focal/travel range of my scope at 160x. To try and resolve this, I just took a piece of PVC pipe about the same size as my light well and used it to make a raised stage for lack of a better word. I also have a whole box full of black lids that have various sized holes drilled in the center. I use them often on the polariscope to test for ADR as I can place the lid on the scope and place the stone over the one with just the right sized hole and it blocks all light except what passes through the stone. (These are screw on lids so they actually hold the stone up off the polariscope.) I took the lid and set it on the raised stage and it was much easier to hold the stone in a parallel position with the scope objective lens.

_________________
-Trace

I'm only here because my flux capacitor is broken.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 01, 2009 7:12 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1470
Yep, sorry Trace.

I'm not absolutely, positively sure what I was saying was correct. So I decided I should set down this week and just do the ray tracings that will convince me one way or the other.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 01, 2009 7:19 pm 
Offline
Gemology Online Veteran
User avatar

Joined: Sun Jan 28, 2007 1:29 pm
Posts: 578
Location: Dallas, Texas
OK Professor. You're excused. :mrgreen:

_________________
-Trace

I'm only here because my flux capacitor is broken.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Mon Mar 02, 2009 2:16 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1470
Ok, I checked out my ray tracings, confirming my suspicions.

With the table closer to, and the culet further from, the objective... light from the culet passing through the flat table will produce an image of the culet at an apparent height...then the ratio of the object height divided by image height (alternatively, actual height divided by apparent height) corresponds to RI.

But, with the culet closer to, and the table further from, the objective... light from any given point on the table passing through the angled sides of the gem will form an image at some height smaller than the apparent height that gives a true RI reading. The angled sides act like a lens, shifting the image position away from the apparent height that gives true RI reading.

Fun little problem. :)


Last edited by Brian on Tue Mar 03, 2009 12:38 am, edited 1 time in total.

Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Mon Mar 02, 2009 2:23 pm 
Offline
Gemology Online Veteran
User avatar

Joined: Sun Jan 28, 2007 1:29 pm
Posts: 578
Location: Dallas, Texas
Great information Brian. Thanks! :smt033

That explains some odd readings I've gotten in the past. The new elevated stage seems to be working well.

Now if we could just come up with a way to get an actual measurement of some of these nice red crystals we've seen inside diamonds, perhaps you could give us a formula to calculate the apparent depth of that crystal, using the known RI of the diamond.

Just kidding Brian, but it is an interesting thought. ;)

_________________
-Trace

I'm only here because my flux capacitor is broken.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Mon Mar 02, 2009 6:19 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1470
MacGyver wrote:
Now if we could just come up with a way to get an actual measurement of some of these nice red crystals we've seen inside diamonds, perhaps you could give us a formula to calculate the apparent depth of that crystal, using the known RI of the diamond.


Yes, as long as the inclusion is viewed through the table, the whole thing should work exactly the same with the inclusion replacing the culet. Find apparent depth of inclusion, d'. Then actual depth d = n d', where n is refractive index of diamond.

It looks like the example that Brad posted could be measured this way.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 15, 2009 11:22 pm 
Offline
Platinum Member
User avatar

Joined: Tue Jan 02, 2007 8:20 pm
Posts: 2657
I am not sure the method would work on an inclusion. First of all how will you obtain an accurate actual depth of the inclusion. It isn't possible as far I can tell. Unless the stone has parallel sides.

And wouldn't the apparent depth be also distorted by having the inclusion floating in a matrix whose refractive index is different from air. Since the RI of the gemstone is going to be higher than air it will have the effect of
reducing the apparent depth.

So unless you are measuring inclusions in square shaped gemstones both the real and apparent depths are going to be inaccurate. Brian have I missed something here?


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 15, 2009 11:30 pm 
Offline
Gemology Online Veteran
User avatar

Joined: Sun Jan 28, 2007 1:29 pm
Posts: 578
Location: Dallas, Texas
Gene I think something got lost in the translation. I was actually making a joke about being able to measure the RI of the crystal/inclusion. I think Brian was talking about measuring the depth of the crystal in which case I saw the same problem you did, which was the issue Brian described regarding measuring from the culet to table instead of table to culet. It seems you'd need parallel sides to affect the measurement.

_________________
-Trace

I'm only here because my flux capacitor is broken.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sun Mar 15, 2009 11:39 pm 
Offline
Platinum Member
User avatar

Joined: Tue Jan 02, 2007 8:20 pm
Posts: 2657
Actually the reason I revisited this thread was to post this link which is from a Bausch and Lomb Reference manual on the StereoZoom series, dated April 1960. Here is the last part of the text.

Initially it merely amused me to see a B&L Stereo with a dial indicator attached. But then I looked at the method and realized that the accuracy could be increased if someone wanted to take the trouble to replicate what they did with the oculars in the method they gave. Their purpose was just accurate depth measurement, but they claimed it is possible to get results to less than half a thousandth of an inch, which is good even with a caliper.
The method involves two reticles, a stage target , and an accurate centering adjustment on one ocular tube.

If you click the plus cursor the image gets bigger , the moire disappears and you can see details of the right ocular tube and the attachment of a good ole AMES made in Massachusetts dial indicator. The scope they show is a fixed mag version. In this reference manual though they already had a StereoZoom Four going.

Yeah I missed the "Just kidding" part. :D


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Mon Mar 16, 2009 1:00 am 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1470
Oops, I missed the additional postings...

G4Lab wrote:
I am not sure the method would work on an inclusion. First of all how will you obtain an accurate actual depth of the inclusion. It isn't possible as far I can tell. Unless the stone has parallel sides.


The question, I think, was how to determine actual depth, d, from a measurement of apparent depth, d', and known refractive index, n.

G4Lab wrote:
And wouldn't the apparent depth be also distorted by having the inclusion floating in a matrix whose refractive index is different from air. Since the RI of the gemstone is going to be higher than air it will have the effect of reducing the apparent depth.


This is what gives the impression of apparent depth in the first place... light from an object inside a transparent medium (the gem) refracting through a flat interface into a second medium with lower refractive index. (the air)

G4Lab wrote:
Brian have I missed something here?


I think so. Let's realize that light from the culet must travel through the gem and refract through the table into the air in order to get to the microscope objective lens. In the same way, light from an inclusion also travels through the gem and refracts through the table into the air in order to get to the microscope objective.

The objective just needs to be looking down through the flat surface of the gem's table, and anything inside the gem will appear at an apparent depth equal to its actual depth divided by the refractive index, d' = d/n, where n is refractive index. Or, to put it another way, if you know the apparent depth d' and you know the refractive index n, then the actual depth of the object will be equal to the apparent depth times the refractive index, d = n d'. Hmmm.... seems like I've written this equation before. :roll:

Note that light from an object inside the gem that leaves through the side of the gem won't be directed up into the objective, so the sides of the gem have no effect on apparent depth.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Mon Mar 16, 2009 10:05 am 
Offline
Platinum Member
User avatar

Joined: Tue Jan 02, 2007 8:20 pm
Posts: 2657
I admittedly missed the part about determining actual depth of an inclusion knowing its RI.

But even assuming that the specimen in this case has parallel side so you could get an actual depth and assuming you are looking straight down on the table:

Would you not at the very least need to to a more complex calculation based on the fact that the light is going through media of three different RIs.
Wouldn't the apparent depth of the inclusion be less when embedded in a gemstone than it would be in air if you etched away the surrounding gemstone and measured the inclusion surrounded by air while holding it in your brand new Levitron anti gravity gem stone holder? Also wouldn't the inclusion need to be parallel in the Z axis or at least have a table facet of its own.

Gene


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 24 posts ]  Go to page 1, 2  Next

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 0 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock