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PostPosted: Sat May 09, 2009 10:34 am 
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When you lower your expectations a little, you suddenly start to see more


Let that be a fact! You see what you see, not what you should see. Very dangerous conclusions come from events where a specific spectrum is 'thought' to be seen, or better is 'wanted' to be seen.

But I'm still curious... are you just lining up a point and shoot with your spectroscope JB? I don't come close to capturing a spectrum that way... (but want to :D )


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PostPosted: Sat May 09, 2009 11:22 am 
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Hi Tim,

I found another image in my picture file that may help. The main thing is to be able to secure the spectroscope in position and adjusted to the image you want to shoot. It's adjusted when you look through the scope and see the best image of the subject being photographed, proper light intensity, distance from subject etc.

Then its a matter of working hands free with your camera to zoom in through the viewing tube of your spectroscope. I use a Canon Powershot A95.

Back to another image to demonstrate. The photo below will show the same Reveal spectra, but this time you'll see that it's not zoomed in on the spectra. The white is the actual Reveal bulb. Tweezers are holding the spectroscope (third hand tool) and you will see the spectra in the viewing tube of the spectroscope. Actually in this photo at that distance, the spectra is a little more distinct than in the comparison image I posted before.

At this point, just zoom in and watch the LCD monitor on your camera. When you think you are close to a good image, click away. Digital pictures are cheap. Download to your picture file, view and crop the good ones.

Image

I'll repost the get-up for the gem spectra photograpy in a minute. Got to go find the picture.


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PostPosted: Sat May 09, 2009 11:41 am 
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Hi JB and thanks for your additions of images one could expect to see with standard gemological testing tools.
It would be useful to coordinate Brian's images with your images, IMHO.


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PostPosted: Sat May 09, 2009 11:52 am 
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For shooting the gem spectras it's a pretty simple set-up.

Image

You'll see the "third hand tool" I mentioned above holding the spectroscope. I think it's actually a Jewelers tool for holding pieces to be soldered. It has a weighted base and ball swivel bearing that the tweezers attach to making for easy movement adjustments.

Btw: You can also hold a dichroscope or conoscope with this if you want to shoot through those as well. Same principles apply.

The business end is the lighted base. Halogen light, rheostat and iris diaphragm. Usually, I adjust the iris to accept the pavilion of the gem or if it's a cab just place over the adjusted iris opening. Adjust the spectroscope using the third hand tool and then it's lights, cameras, action. :)

You can probably see part of the attached prism scope that came with the light base. It's mounted on an adjustable levered arm for positioning over the stone. I have it pushed back out of the way in this photo so I can hover the camera above the DG scope.


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PostPosted: Sat May 09, 2009 1:31 pm 
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The Reveal bubb that JB shows above is a regular tungsten filament lamp with a Neodymium glass (Didymium ) envelope.

These have a complicated rare earth spectrum that has lots of peaks and valleys. For example it very sharply filters the sodium doublet so this same glass is used for glass blowers goggles because red hot glass throws off lots of sodium into the gas flame making alot of sodium light glare.
The glass exhibits color change appearing blue under daylight and fluorescent light and purple under tungsten light.

Didymium filters also are used to calibrate spectrophotometers because their spectrum is very constant. Holmium Oxide glass is too.

Here is the spectrum of Didymium glass.


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PostPosted: Sat May 09, 2009 4:51 pm 
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JB wrote:
When you lower your expectations a little, you suddenly start to see more. :)


This is exactly why these little photographs are useful. Plus, I can do little mashups like this...

Image

JB, I apologize in advance for mangling your photo.


Gene, the spectrum of that didymium glass is pretty cool. Gotta love those rare earths; they make things exciting in the vis.


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PostPosted: Sun May 10, 2009 7:55 am 
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JB, I apologize in advance for mangling your photo.


Brian that's precisely what I hoped you would or could do. :)
It's helpful for those of us that don't have access or training with more advanced lab equipment.

Something else that would be helpful would be a graph of a broad spectrum light source we use with our handheld spectroscope.

This would be the "ground zero" graph, if you will. Neatly posted directly above the garnet mashup above, we could see what happens to the graph from ground zero point and the changes that occur with a gem spectrum.

I could take a photo of the broad spectrum produced from my light source if needed.
____________________________________________________________

On the neodymium subject, many of you are familiar with "Zandrite." It's a popular color change glass Gem simulant. The color change is a result of the neodymium in the formula.

There's a pink to green version and a purple to blue version, like the glass Gene mentions above.

The spectrum of this gem simulant is very interesting. I have a photo of it, but think I'll wait to post until I do a reshoot. I think I can improve on it.

Anyway, in the Zandrite spectrum with the handheld, the entire yellow region is entirely absorbed, I'm talking a jet black band in this region. it continues slightly although fainter into the red.

Is this a result of the neodymium doping?


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PostPosted: Sun May 10, 2009 9:57 pm 
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JB wrote:
Something else that would be helpful would be a graph of a broad spectrum light source we use with our handheld spectroscope.

This would be the "ground zero" graph, if you will. Neatly posted directly above the garnet mashup above, we could see what happens to the graph from ground zero point and the changes that occur with a gem spectrum.

I could take a photo of the broad spectrum produced from my light source if needed.


Ah yes, this is another thing that makes it tricky to compare visual readings from the spectroscope with machine readings from the spectrometer. In the machine measurements, one routinely divides out the dependence on light source... so that the graphs represent the fraction of whatever light is available. Of course there is no way to divide out the eye's dependence on light source. And there isn't an affordable "perfect" light source to exactly match the sun, or to just provide a flat response at all wavelengths.

I can come up with spectra for some white light sources, but I'm not sure how useful the information. I was talking with Frank, and he was mentioning that he could finally see some absorption in the blue using a white LED. And I'm sure he is reporting correctly... common halogen has very little blue and almost no violet, whereas a white LED overemphasizes blue. But for either of these cases, this variation isn't exactly noticeable in the spectroscope.


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PostPosted: Mon May 11, 2009 10:32 am 
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Ok time for some unknowns, of a sort.

First up...

There was this box in the original garnet-poor collection with hundreds of little balls that was labeled "pyrope" from North Carolina. These little balls were quite black and smaller than ... what? ... I dunno, they were small.

I collected spectra from a couple, and the spectrum from the most transparent one is shown below, plotted against an absolute transmission fraction scale.

Image

Comparing with previous spectra in this thread, clearly this spectrum arises primarily from almandine. But as we discussed earlier, a pyrope-almandine mix is going to exhibit an almandine spectrum. The unknown here is "what is the mix?" ... mostly pyrope, or mostly almandine, or what?

I don't know how the original labeling of the box came about. I know there are ways to determine the pyrope-almandine mix, but these little balls aren't really worth the trouble. I just thought it would be nice to show that the color seen (here under normal lighting, the red is so deep, it is black) arises from the almandine portion. It seems to me that since the color is so dense, probably the almandine proportion is large.


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PostPosted: Mon May 11, 2009 11:34 am 
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Brian wrote:
Ok time for some unknowns, of a sort.
The unknown here is "what is the mix?" ... mostly pyrope, or mostly almandine, or what?

Games are fun, but if you really want to know the answer, measure the RI. :D :D


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PostPosted: Mon May 11, 2009 1:15 pm 
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With my apologies to Brian for jumping around a bit in this thread, here is an improved spectrum photo of the Zandrite mentioned a few posts back.

I think it's as close as I'm going to come with my camera and set-up with what you will actually view in person with a handheld DG spectroscope.

The same neodymium question as before.

Image


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PostPosted: Mon May 11, 2009 1:55 pm 
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Bill Hanneman wrote:
Brian wrote:
Ok time for some unknowns, of a sort.
The unknown here is "what is the mix?" ... mostly pyrope, or mostly almandine, or what?

Games are fun, but if you really want to know the answer, measure the RI. :D :D


Yes, that is the point. The spectrum can't be used to distinguish between mostly pyrope or mostly almandine.


Last edited by Brian on Mon May 11, 2009 3:04 pm, edited 1 time in total.

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PostPosted: Mon May 11, 2009 2:03 pm 
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Ok, a small interlude to follow up on what I was thinking of doing with the chrome pyrope. Seems like long ago now, but back then I was musing on whether or not I could use my green 532 nm laser to excite the Cr3+ fluorescence line(s) in the chrome pyrope. Well, indeed I can...

Image

Notice that the wavelength scale is covering a narrow range that extends slightly into the NIR, 640 nm to 740 nm. I'm using the low-resolution spectrometer, so probably there are two unresolved lines there. It is just too much trouble to set up the high-res spectrometer and wait twenty-five times as long to collect a spectrum. But anyways, the low-res spectrometer produces a signature for chromium fluorescence in garnet, which is nice.


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PostPosted: Mon May 11, 2009 2:42 pm 
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And now on to another unknown.

Tim sent me some green garnets, the mystery green garnets from Mt. Garnet, FNQ Australia. He tells me that these garnet are either green andradite Ca3Fe2(Si O4)3, otherwise called demantoid, or grossular Ca3Al2(Si O4)3 that becomes green due to the presence of the chromium ion, Cr3+.

He further mentions that RI is over his limit and SG is no help. Maybe, just maybe, some kind of spectroscopy will come to the rescue.

It seems to me, if I can identify the presence of chromium ion then it is probably grossular, and if chromium seems to be absent then it is probably andradite. Let's look at transmission spectra of three samples: one plotted in black, another plotted in blue, and another plotted in green.

Image

Hmm... ok... First I look for evidence of chromium lines around 694 nm. Curves for two of the samples don't show anything in that region, but there is a hint of an absorption at the right edge of the green curve of the third sample. This is a problem, because as can be seen in ruby (see this post), the chromium "line" may be primarily fluorescence or primarily absorption or both canceling each other out.

So lets think... maybe chromium ion creates green in grossular, just as it creates green in emerald. Lets compare a brown hessonite grossular transmission spectrum to an unknown green garnet's transmission spectrum. The brown curve is the spectrum produced by a hessonite grossular shown in a previous post in this thread. The green curve is the same as the green curve shown in the previous graph.

Image

Ok, so I could imagine adding the spectral curve of emerald (see this thread) to the hessonite spectral curve and ending up with the unknown green garnet's spectrum.

But like Bill says, "Games are fun..."


Last edited by Brian on Fri Oct 02, 2009 1:02 am, edited 2 times in total.

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PostPosted: Mon May 11, 2009 3:33 pm 
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JB wrote:
Anyway, in the Zandrite spectrum with the handheld, the entire yellow region is entirely absorbed, I'm talking a jet black band in this region. it continues slightly although fainter into the red.

Is this a result of the neodymium doping?


In Gene's post, he gives a link to the didymium absorption spectrum. This spectrum shows strong, strong absorption in about the 570-600 nm region, which is yellow. So the answer is yes, the neodymium causes the black band that erases yellow.

And it also causes the dark line seen in the green. And also the line seen in the blue. And also the line seen in violet. All those lines correspond to absorption peaks in Gene's posted spectrum. Beware, this kind of spectrum is an upside-down version of the transmission spectra I am posting here. Peaks in that spectrum become dips in mine.


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