Post subject: Visual Optics: A repository for everything related to VO!
Posted: Sat Dec 17, 2016 11:17 pm
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Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
I think your post simply said check the B/D ratio. Now what is a person new to gemology supposed to make of that? Let's do a search and see what we find out:
Post subject: Visual Optics: A repository for everything related to VO!
Posted: Sun Dec 18, 2016 1:08 pm
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Joined: Fri Nov 01, 2013 3:45 pm Posts: 289
Hello madame Voltaire,
Maybe there is a difference in the two specters because they are not the product of the same phenomena. It is just a guess but the one from a OPL is a reflected light. Some colors are absorbed and others are reflected. But in the method in the video, it seems to be the light that goes throught the gem. all the colors are there. It is more about the dispersion and if there is a double band.
Post subject: Visual Optics: A repository for everything related to VO!
Posted: Sun Dec 18, 2016 4:54 pm
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Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Brigitte wrote:
Hello madame Voltaire,
Maybe there is a difference in the two specters because they are not the product of the same phenomena. It is just a guess but the one from a OPL is a reflected light. Some colors are absorbed and others are reflected. But in the method in the video, it seems to be the light that goes throught the gem. all the colors are there. It is more about the dispersion and if there is a double band.
What do you think? Does it make sense?
Best regards
Brigitte
Nope, when you use Visual Optics the spectrum you see with your eyeball will always appear as I posted it for a ruby (natural or synthetic).
Post subject: Visual Optics: A repository for everything related to VO!
Posted: Sun Dec 18, 2016 6:08 pm
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Joined: Fri Nov 01, 2013 3:45 pm Posts: 289
Hello,
I see that the 2 specters for the ruby are not the same. From my understanding, those 2 specters for the ruby are supposed to be different.
To be more precise, the firts image you posted, seems to be for a specter from refraction. We can see the nm written. The other one and the one that we see from the video seem with visual optic.
So the specter with the nm written in for light that is refracted. On that case, some parts of the light (colors) stay in the gem and some other parts of the light are refracted, which give the color we see the gem. So the objective of that particular specter is to evaluate the way a gem refract light. The specter that we see is the specter of the parts of the light that are refracted.
I understand that visual optic has another objective. It is to evaluate the light after it has passed through the gem. In that case, It seems that all colors(parts of the visble light) pass through the gem. But each gem bends the lights differently to make a different diffusion pattern in lenght and precision. We have to add that lot of gems have 2 patterns for a facet because they have uniaxial( 2 RI) or biaxial(3 RI). I understand that the mix of those possibilities make the B:D.
Those are the reasons I think that it is correct that the 2 specters (nm and visual optic) are different. Why do you think that they should be the same?
I am familiar with Visual Optics because Alan Hodkinson came over to my house and taught it to me and a small group of fellow gemmos over dinner and drinks.
Post subject: Visual Optics: A repository for everything related to VO!
Posted: Sun Dec 18, 2016 11:21 pm
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Joined: Fri Nov 01, 2013 3:45 pm Posts: 289
Hello,
I also think that absorption spectrum are measured in nm.
My point is that in Visual optic, we do not see the absorption spectrum but all the spectrum after it went throught a gem. I think that it is the reasons that the 2 ruby spectrums are not the same.
It must have been a interesting experience to learn visual optic directly with mister Hodgkinson. With all your knowledge, you probably had a lot to share with him.
Post subject: Visual Optics: A repository for everything related to VO!
Posted: Mon Dec 19, 2016 12:38 pm
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Joined: Fri Nov 01, 2013 3:45 pm Posts: 289
Hello,
So, from your experience, you see the spectrum with absorbed lines when you use the visual optic method?
I do not have a ruby here, but I have my synthetic sapphire. When I looked into it trying the visual optic method, the spectrum seemed to be continuous. I used a 10 x loupe.
My flash light batteries are low, so I can not use it. I just tried by putting the stone in the sun. The spectrum seemed to me continuous, even with the glasses on and the loupe. Maybe the absorbed spectrum is best viewed that way with a ruby. I will try it when I can.
I do not understand the physics behind that. I will have to read more about it. Do you have references for me, concerning how light travel through a gem and how it affects its spectrum?
Post subject: Re: Tanzanite or Amethyst set in 14K?
Posted: Mon Dec 19, 2016 1:31 pm
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Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
When I learned with Alan, we simply used a candle on the dining table....at night with the candle being the only light source. Remember he developed the technique while sitting in his taxi on a rainy night using an overhead street light.
I've taken the liberty of copying a couple pages with images from his excellent book "Gem Testing Techniques". A must have! Apologies for the quality of my pics. I snapped them on the coffee table.
Post subject: Re: Tanzanite or Amethyst set in 14K?
Posted: Mon Dec 19, 2016 3:50 pm
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Joined: Fri Nov 01, 2013 3:45 pm Posts: 289
Hello,
Thank you so much for your answer. I am delighted.
I had to think why I thought the transmitted spectrum was different fron the refracted spectrum, that the first one did not had absorption lines. And I found the answer. Each time I saw the transmitted spectrum, it was with a prism. So the prism has a full spectrum, and I understand from your answer that it is not the case with everething. I suppose a prism has a full spectrum in both transmitted and refracted light. If so, I suppose it has a full spectrum because it is an isometric system and because it has no element keeping a particular lines(nm) so all the light of the spectrum is transmitted and refracted, so we see the prism with no color.
In a ruby, there is one optic axis and elements in the gem to keep some lines of the specter. So we see the ruby as red.
I hope that I have it right this time.
How will I know where your last post will be transfered? I have other points for which I have been searching answers for days. I am hoping to be able the continue this exchange that is so helpful for me.
Post subject: Re: Visual Optics: A repository for everything related to VO
Posted: Mon Dec 19, 2016 5:13 pm
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Joined: Fri Nov 01, 2013 3:45 pm Posts: 289
Hello madame Voltaire,
You are right, I was confused with all those concepts. I am so glad that I was able to share my confusion with you, so you could help me make sense with all those concepts.
Thank you for the precisions. They are very helpful.
When I wrote the term refracted, I was seeing reflected light but I did not use the good word. Thank you for reminding me.
I will answer your question later. Sorry for the delay. And I will come back with some other questions about this very interesting subject.
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