Ok I've searched the forums and still can't come up with a definitive answer as to why a lot of folks recommend diffraction over prism scopes. I understand the spectra scale is evenly spaced with a diffraction scope and elongated in the violet portion as well as condensed in the red with the prism. I've read that prism scopes are brighter (easier to see) and it seems only prism spectrometers have an nm scale. Without a scale you would have to have a lot of experience to know what your looking at and/or a comparison chart to find a match for what you see With a scale the numbers could be matched to discern absorption as well as aiding what you see to match a chart. I can't figure out why the OPL (fixed focus and slit) is recommended over something like a Wollensak (adjustable focus and slit with an nm scale) I did read a post here where Brian wouldn't recommend a Wollensk to a beginner but he didn't say why. I have also read in a gemmology article that better spectroscopes are adjustable and have an nm scale. Seems to me prism is the way to go, being adjustable and with a scale but the recommendations for a diffraction scope have me confused.
Post subject: Re: diffraction vs prism spectroscopes
Posted: Thu Apr 19, 2018 3:14 pm
Site Admin
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Quote:
With a scale the numbers could be matched to discern absorption as well as aiding what you see to match a chart. I can't figure out why the OPL (fixed focus and slit) is recommended over something like a Wollensak (adjustable focus and slit with an nm scale)
Jim, perhaps those with extended experience using hand held spectroscopes prefer the OPL because it is the better instrument.
Post subject: Re: diffraction vs prism spectroscopes
Posted: Thu Apr 19, 2018 5:43 pm
Gold Member
Joined: Mon Apr 16, 2007 12:05 am Posts: 1499
An OPL is the spectroscope best suited to a gemologist's task. And also, it is priced right for a reliable tool.
Gemologists are not interested in the exact wavelength where a dark line or dark band appears, especially since these can vary slightly from one sample to another of the exact same material. So they aren't trying to match their observations to numbers in a table.
Gemologists use the spectroscope for pattern recognition.1 If they look at light through a red gemstone and see a dark band in the yellow, separate from another dark band in the green, that is well separate from a dark region in the blue-violet, and they see some other grey lines sprinkled throughout, then they have one more piece of evidence for what the gem material may be, and also clear evidence for what it isn't.
An evenly-spaced rainbow scale is easier to use for pattern recognition. And adjusting focus and slit width just adds confusion and error possibilities to the process.
As for the Wollensak prism spectrometer and others like it that are made in India or China; they are cheaply produced. I've played with a couple and they both had misaligned optics and irregular knife edges for slits that made them next to useless. If you want a good prism spectrometer, you need to pay up the money and get a Kruss or Eikhorst... or maybe purchase an old Zeiss off ebay.
Finally, I will say that pattern recognition does take experience and/or charts to develop. I first gathered experience using a Kruss prism spectroscope. Even so I'd recommend any beginner start with an OPL.2
1 Either a quote or paraphrase of something Dr. Hanneman once wrote. 2 I have absolutely no affiliation with, or connection to, OPL.
Last edited by Brian on Thu Apr 19, 2018 6:14 pm, edited 2 times in total.
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