CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sun Jul 24, 2011 2:53 am 
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Again mikko, all you've shown is what I'd expect. And apparently, they show what Gene expects. Um... as expected.

Not to detract too much from the topic, but since Gene mentions 'em, I have one of those diode lasers that fall into "extreme measures". The broad distribution of the diode offers some interesting narrow tunability, but you really have to baby and beg them along. Firstly, those extreme measures increase the cost by a factor of ten to fifty. Then, a single degree change in temperature, or a similar change in humidity (a major problem in Alabama), requires starting all over from the beginning. Narrow-line tunability in these guys is painful to deal with, but when they work, they work. Some peeps livelihood depends on making 'em work, and so they can. But it is painful like no other physical piece I encounter, other than a vacuum leak. Even then, when you have a leak, you know what the problem is. That is not quite the case when a narrow-line diode laser has gone awry.

Of course, even as I write about how difficult they are to work with, I have my eye on a $30k tunable blue diode laser that I have the perfect experiment designed for.


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Mon Jul 25, 2011 6:14 am 
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a Typo that made me jump till i saw the graph below..


argghh :evil: That would have been a steal.

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In contrast to atomic emissions from Sodium or Mercury lamps which are stable many decimal points to the right of what we need. If you discover THOSE things varying then it means there is a black hole drifting nearby distorting space and time.


Or my mother-in-law perhaps :shock:

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Very typical. Always a long extension in the long-wavelength tail.


Looking more carefully the ssef brochure the leds they are using for luminescence setup are not much better. A long tail up to 400nm. I'm still developing similar led booster for UV-VIS transmission measurements where these leds could be used. For luminencense studies it seems to be better to stay in old technology. I just found nice 365nm interference filter from Omega having almost 50% throughput. Used with the low power Hg tube it's not gonna be very intense. At least it's pure IF the specs are to be trusted this time.

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Mon Jul 25, 2011 9:03 am 
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I found something really interesting and it's boggling my mind. Is it possible to make low cost raman for gem ID work.
I would like to hear some educated comments about this article and supplementary recipe.

Two things are especially interesting:

a) Is it really possible to use <$100 4mW laser pointer as source. I thought they should be at least 50mW and little bit more sophisticated than handheld pointers.
b) What is practical minimum quality for notch filter. They have used S grade which means optical density 4 (0.01% transmission).

The parts are not free so this could be totally wasted money. On the other hand if it can be used to get at least some fingerprints it could be nice addition to the toolbox.

Heh :D , probably just crossed the line where you big boys are going to gimme some tarring and feathering :smt018

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Mon Jul 25, 2011 11:26 am 
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The thing that those very nice articles did not point out, is that the analytes which they are taking Raman spectra on, are ones with very strong signals. The Raman spectra of water and benzene are both used to determine the "merit" of monchromators.

The signals you might be looking for gemologically speaking are probably much weaker and might require all the things that are the difference between a "toy" Raman instrument and
a "real" one. Namely a more powerful (and stable) laser to excite the Raman signal, a better laser line isolation filter. (usually fairly expensive but now can be gotten on fleabay from Omega Bob Johnson) and a bigger better more sensitive spectrometer.

The articles use 180 degree (aka as Face Front or FF) sampling geometry using a microscope objective. For Gemology a lower mag , longer working distance objective would be better. Actually for the setup they showed it would be better too because the working distance of a 20x objective is less than the wall thickness of the cuvette they were using. (did I just say that??) You do want to use as high mag as possible because they have higher numerical aperature and gather more signal back.

The setup shown could also be implemented with any number of incident light compound scopes without having to resort to making your own semireflective mirror. And a more efficient method would be to simply bring the laser beam in from the side and let the objective collect all the light without the obstruction of an incident light mirror or prism.

If you do succeed in collecting a Raman spectrum , what you are very likely to have is a wiggly line that is very hard to interpret. One of the first and most obnoxious barriers to interpretation is the fact that Raman spectra are calibrated in wavenumbers. Well so are FTIR and regular IR spectra. So what! you say. But they are in WAVENUMBERS OF RAMAN SHIFT(aka delta). That is they are related to the fundamental frequency of the exciting laser. (As opposed to IR spectra in wavenumbers that can be converted directly to wavelenghts. Raman wavenumbers can't without including a factor for the laser wavelength.)

THEN you get to start trying to make sense out of the wiggly line. If the analyte or fingerprint you are looking for has a strong and distinctive signal you can see it.

But in big boy Raman and FTIR gear the software takes the spectrum you just shot and compares it to a library and picks out a variable number of candidate library spectra for you to look at. Then you get the pleasure of trying to figure out how the wavelength calibration compares (ie did they use the same laser you did) and the X axis stretch (how many pixels were there on THEIR detector compared to yours.

Or if you actually got your PhD in Raman spectra in analytical chemistry then you too can look at that wiggly line and say what you are seeing. The bosses of the guys that come around selling these instruments are like that.

As I have said since I started fooling around with them FTIR and Ramn spectroscopy are not beginners techniques. And by this I am not trying to discourage you from becoming a confirmed spectroscopy addict. :twisted: :twisted: you need to buy much much more equipment....


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Fri Nov 04, 2011 6:53 pm 
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Just wanted to report some progress of the project. We have shared lot's of ideas and knowledge with Albe lately (eternal thanks pal!) and preliminary results suggest a low cost DIY raman & PL spectrometer is possible to build. While it doesn't identify minerals automatically it can still be used as serious "last word" for ID work.

Here is 50mW 532nm DPSS laser installed to the spectrometer setup.
Image

I'm still waiting for shipment for decent longpass filter. Currently a piece of orange glass tries to serve for the purpose. It has bit annoying fluorescence as can be seen from the spectras. The laser does have IR filter but small artifact at 808nm is still visible. Despite of these "problems" some raman signals are easily detectable riding on the top of fluorescence.

First example shows raman spectras of diamond and some of it's simulants. Please forgive me mislabeled x axis. These are not nanometers but raman shift in wavenumbers. Peaks at 2500 cm-1 and 6400cm-1 are artifacts.

Image

Second example is tanzanite. It matches with the spectras at RRUFF database.

Image

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Fri Nov 04, 2011 11:59 pm 
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Sorry, I haven't kept up with this thread. mikko come visit my lab; I have plenty of toys you can play with. ;)

mikko wrote:

I just found nice 365nm interference filter from Omega having almost 50% throughput. Used with the low power Hg tube it's not gonna be very intense. At least it's pure IF the specs are to be trusted this time.


I have one of these filters, and they work just as advertised.

But on to the Raman spectroscopy. Good job there! Good choice to go with a DPSS laser; they are much more wavelength-stable than diode lasers (and have nice circular cross-section, compared to the highly elliptical diode laser cross-sections). Although, as you are already seeing and so probably know, the green wavelength excites much more fluorescence than does a 780 nm laser.

Again, even though I've made some outlandish claims and made 'em work, I wouldn't have tried Raman without a collection lens. So really good job collecting spectra that aren't complete junk.


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Nov 05, 2011 1:33 am 
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That's F.amazing!


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Nov 05, 2011 4:05 am 
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So really good job collecting spectra that aren't complete junk.


Thank You :D

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Again, even though I've made some outlandish claims and made 'em work, I wouldn't have tried Raman without a collection lens.


Me either! The real purpose of the setup is for studying crystallographic defects in diamond. I just couldn't believe my eyes the diamond raman peak was always there.

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Nov 05, 2011 8:30 am 
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mikko wrote:
Just wanted to report some progress of the project. We have shared lot's of ideas and knowledge with Albe lately (eternal thanks pal!) and preliminary results suggest a low cost DIY raman & PL spectrometer is possible to build. While it doesn't identify minerals automatically it can still be used as serious "last word" for ID work.


Yeah, it was indeed my pleasure and it was definitely a good example of collaboration, thank you too for your help and insights :D . the results speaks for themselfs, spotting Raman peaks with a DIY low cost spectrometer coupled to a very cheap chinese made laser, almost incredible what a motivated man can do..... =D> =D> =D>

ciao
albé

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