January 24 Through February 4—TUCSON, ARIZONA: Annual show
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 Post subject: Spectroscopes - Diffraction Grating V. Prism
PostPosted: Tue Apr 10, 2007 6:47 pm 
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I use a diffraction grating spectroscope but lately seem to be having a bit more trouble focusing. I'm fighting it but it may be time for glasses.

Oh well, my quesiton is do those of you who have a experience with both prefer the diffraction grating or prism scopes? I really want to stay portable so I'm not leaning toward a new desktop unit.

Thanks.


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PostPosted: Wed Apr 11, 2007 12:08 pm 
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I looked at both types this year at Tucson, and ended up buying the Eickhorst prisim model with the light base. I couldn't really picture using it with out the adjustable light base. Many time only a small amount of light is needed to see bands in one area, and then much more light is needed on the same stone to define bands in another region.
I also like the scale in the prisim, but I can see where the uncompressed red spectum would be nice.

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PostPosted: Wed Apr 18, 2007 2:40 pm 
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I have a Kruss 1506 (with all the bells and whistles) that I like very much. You might be more interested in the Kruss 1504 (with variable slit and wavelength scale) or the Kruss 1502 (with variable slit only). I wouldn't bother with the Kruss 1501, because I don't think it comes with focusing capability.

The Kruss optics are first-rate, and I can tell you more information about that than you'd want to know. Focusing on the wavelength scale, or focusing on lines in emission spectra, they have very sharp edges. And as you probably already know, any prism spectroscope is going to create a much brighter spectrum than a diffraction-grating 'scope, because a grating can only transmit a fraction (usually much less than half) of the light entering it to the first order spectrum appearing in the 'scope.

From my comparisons with known wavelengths in atomic emission spectra, the wavelength scale is quite accurate. The slits can be closed to the degree that emission lines are narrower than the wavelength scale lines. Of course the 'scope doesn't have resolution to separate the sodium D-line doublet (which are separated by 0.6 nm), but it does have enough resolution to separate the yellow doublet in mercury emission (which are separated by 2 nm). Thus, in yellow up through violet, you can close the slits enough for 2 nm resolution, and you can read any wavelength with an accuracy of +/-2 nm. In the red, the resolution is more like 5 nm with a wavelength accuracy of +/-5 nm.

From slits wide open all the way to slits closed, I observe no vignetting (black horizontal lines striped across the spectrum caused by defects along slit edge).

Mechanical adjustments are very smooth. Slit width is adjustable with a smoothly rotating dial, concentric with the tube. The dial has an autostop to prevent banging the slit edges together. There is no marking on the barrel and knob to tell you the slit width, and there is no way to set the slit width to prevent an accidental bump of the dial from changing the width. Focusing of the wavelength scale and spectra are done by sliding in or our the respective tubes, but you need a small screwdriver to unloosen or set the screws that hold the focuses. Calibrating the wavelength scale to the sodium D-lines is done with a knurled thumb-screw that is set with a knurled locking nut.

I only learned about the Eickhorst 'scope from reading this forum. From studying their website, it appears the Kruss and Eickhorst optics performance is similar, but there's always the matter of mechanical fit and finish. From my enquiry, the Eickhorst costs about double what the comparable Kruss cost. I'll be saving my pennies to see how the Eickhorst compares with the Kruss.

For full and complete disclosure, I also have a made-in-India "Wollensak" spectroscope from Surplus Shed, but I never use it.


Last edited by Brian on Wed Apr 18, 2007 5:31 pm, edited 3 times in total.

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PostPosted: Wed Apr 18, 2007 3:09 pm 
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Thanks so much for taking the time to write all that Brian.

I have been curious about the Kruss instruments for sometime, but have not had the opportunity to really play with one.


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PostPosted: Wed Apr 18, 2007 3:38 pm 
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Yep, sorry MacGyver, pedagogy and over-explanation is a hazard of my trade.


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PostPosted: Wed Apr 18, 2007 3:48 pm 
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Absolutely no apology necessary. I enjoyed the useful information. Thanks again!


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PostPosted: Wed Apr 18, 2007 5:37 pm 
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Brian , how does the one from surplusshed actually perform? I know its hard to use something from India when you have a German original there. I have been tempted by surplus sheds goodies from India because I have bought stuff from them before and they are cheep. But I have enough in the way of hand specs. Can they be recommended for beginners??


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PostPosted: Wed Apr 18, 2007 11:01 pm 
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G4Lab, the short answers are "not so well," and "no, I wouldn't recommend one to a beginner."


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PostPosted: Thu Apr 19, 2007 1:59 am 
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G4Lab, the short answers are "not so well," and "no, I wouldn't recommend one to a beginner."


If you get a chance I'd like to know what the problems are. I have mentioned those units on this forum without having played with one , essentially just because I have had some OK experiences with some far east microscopes and refractometers. At $100 they would be a bargain if they worked. The next step up in a scale unit is at least $500 and maybe $700.

There is an english DG scope with a fixed slit and fixed focus. I was prepared to not like it. I am 56 and 2.5d presbyopic. But this OPL is perfectly designed for gemology and worthy of its good reputation. it sells for about $100
www.oplspectra.com if you arent familiar with it. I had a chance to play with it at Alan Hodgkinsons Houston seminar. I brought a Wavelength Kruess and a Beck DG unit (Big pain to use , MAY have some lube issues. The DG is on a tangent micrometer mount with a 0-100 logging scale.) There was no question that the OPL was the easiest to use. I think the designer Colin Winter has converged all the design points into the optimum positions. No fiddling you just look and see what you are supposed to see.
I had no trouble using it but one listmember with worse eyes couldnt see anything through it.
Gene


Last edited by G4Lab on Sat Apr 21, 2007 2:04 am, edited 1 time in total.

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PostPosted: Thu Apr 19, 2007 9:59 am 
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Whoa Betty!

Comparing the OPL to prism spectroscopes is like comparing apples to oranges... or diamonds to emeralds, maybe. Even though they perform similar tasks, they are significantly different technologies. And which one to choose is pretty much a matter of personal bias... do you like to tweak settings, or would you rather leave well enough alone?... do you see information in the colors or in the wavelength scale?...etc.

Even though I have no direct experience with the OPL, I have extensive experience with diffraction gratings in general (I have holographic gratings, ruled gratings, 590 lines/mm, 600 lines/mm, 830 lines/mm, 1200 lines/mm, flat gratings, Rowland gratings, just sitting around collecting dust most of the time). I also have a collection of dispersion prisms made of SF18 and SFL11 glass (very low Abbe numbers for each of these) also gathering dust (not really, all this glass and gratings are in protective casings... the cases are gathering dust).

From reading discussions here about how well the OPL has been optimized, I think it is by far the best choice for a beginning gemologist. And for most gem people, it is probably the only spectroscope they ever need. Firstly, no adjustments means you can't mess it up. And no adjustments also means standardization... everyone looking through a similar spectroscope should be presented with essentially the same picture. Secondly, with the linear dispersion of the DG, people who spend extensive effort studying color are rewarded with a relatively easy method to convert color to wavelength. With a little practice, I'm sure a beginner could identify the wavelength position of an absorption band to within +/- 10 nm, and with a lot of practice ... to within +/- 5 nm, perhaps much better.

You can, however, make a rough comparison of the cost to make an OPL and a prism spectrometer of the same optical quality by looking at their construction. The OPL has a couple optic elements (DG and lens) that have to be precisely positioned. A prism spectrometer has about five times as many optic elements (beginning with the 5 prisms that disperse the light) that require precise positioning. And from experience, the difficulty of aligning an optics train grows geometrically with the number of elements in the train. And then, the prism spectroscope has lots of moving parts that have to work without disturbing alignments. Comparing the components that make up each, and the effort required to position these components, it should be clear that a variable-slit-with-wavelength-scale prism spectrometer of same quality should cost between 5 and 10 times as much as a fixed-slit DG spectrometer with no wavelength scale.

Now personally, I don't find much use for the OPL. This is from background and training. I design and build a fair number of experimental optical systems, so I like having as much control as possible over an optical system...

For example, the slit setting of the OPL may be the best general choice possible for viewing absorption bands, but I find tweaking a variable slit can help improve contrast in separate regions of the spectrum... a little more closed in the green, a little more open in the violet, and so on.

Also I spent a long time looking at light spectra way outside the visible range long before I ever began to study spectra in the visible range. So for me, color seen in the spectroscope is just a nice little perk. But I am as comfortable with, or perhaps even more comfortable with, looking at the graphs that Ux4 is publishing in the research section of this forum.

I like the superimposed wavelength scale, because when I record a wavelength measurement, I want to know just how good that measurement is likely to be... is the uncertainty in the measurement +/- 2 nm, or +/- 5 nm. The wavelength scale on the prism spectroscope allows me to quantify not just my measurement, but the uncertainty in the measurement.

So, with my personal biases, it is not difficult to see why I'd prefer to work with a prism 'scope.

Finally, on some webpage I googled up before joining this forum, I read an interview with Colin Winter, where he said at one time there was an OPL that came with a wavelength scale. The wavelength scale was discontinued because it was economically unfeasible. Which goes back to my earlier point, gemologists who are the target consumer for this 'scope are just as comfortable working without a wavelength scale.


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PostPosted: Thu Apr 19, 2007 12:32 pm 
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Suddenly, I feel so cheap :oops: :wink: .

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PostPosted: Thu Apr 19, 2007 2:50 pm 
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Modo

Your too classy to ever be cheap.

Brian, I can see you and Gene are going to have fun speaking to each other. Instrument obsessives are such fun, :P

For my own part (and I'm sure for many others here) the scale is not a neccesity...Though it'd sure be handy sometimes. I make no real attempt to designate a nanometer reading to any line or band unless it's a diagnostic line(s) which you have learned (ie 450 complex in blue sapphire). For the most part I am looking at the overall pattern. I can tell a ruby from a spinel from a red garnet without any problem. There are a finite number of gemstones which have usable spectra and these aren't too hard to learn.

I have both a DG and a prism scope though the prism has a fixed slit. I'd like to have a play with a variable slit model just to see how much difference it made but on the whole I want something easy to use without too much fuss which will give me quick and easy answers.

Please continue to educate us on the various instruments you use though it's all very interesting stuff. Up till now Gene has had to do it all

Be well

Frank


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PostPosted: Thu Apr 19, 2007 6:54 pm 
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hi Frank,

Thanks again for the welcome. Are you the Frank responsible for the Dictionary of Gem Terms? If so, I was hoping you could define a term I read here on the boards... "dichroic". This is one of my favorite things, and I want to see if it is the same gemologically speaking.

Gene is most helpful, indeed.

You guys talk a lot of physics on this board, and for example, I'd never heard of a conoscope. It was fun piecing together what it did from the comments in the thread.

Thanks,
Brian


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PostPosted: Thu Apr 19, 2007 7:26 pm 
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Frank, you are a gentleman and a scholar :D .

Perhaps MacGyver and I are in the same age range ... I, too, have trouble with the spectroscope. I have the OPL hand-held and have trouble seeing the spectrum sometimes, let alone the absorption lines! :lol: Would a desk-top model be helpful to me?

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PostPosted: Thu Apr 19, 2007 9:42 pm 
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I don't know about you Morning Dove, (and I'm too much of a gentleman to ask) but I just crossed over into the 40s. I've never even had an eye exam other than what is given during a flight physical or something like that. I've just never seemed to have any issues rather it's landing a plane or grading a diamond. I do read an awful lot though, and over the past 6-8 months I've noticed that if something is very close (a few inches away) I have trouble focusing on it. I know that's not usually an issue, but I didn't have that problem a year or so back so it was my first sign. That got me to wondering if perhaps as difficult as some lines are to see in a spectroscope if there were maybe some things there I was missing.

Do you remember what it was like the first time you tried to look in a monocular style scientific microscope? How the shadow would get in your way until you trained yourself to look through it correctly? I sometimes experience that with my sepctroscope when I'm tired and have had my head in a loupe or microscope all day.


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