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Post#1 @
09-28-01
, 11:00 PM
. GIA Insider, published bi-weekly, just came by e-mail, discussing the effect of dirt, grease on the appearance of diamonds. Odd: we had just been discussing that. . I have been in contact with GIA this week about the effective RI of dirty diamond. I thought I had seen it in old Diamond course. They sent me old reference text but no mention of RI. Apparently this is not known. |
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RI of Dirty Diamond
Post#2 @
09-29-01
, 08:21 AM
Beryl, The material properties of diamond do not change when they get dirty, so we would not expect the refractive index of the diamond to change. What we would see is a "coating" on the diamond which had it's on refractive index which would alternatively change the way light interacts with the diamond. What may be interesting is if we could try and model coatings on the diamond which had their own refractive index which we may be able to find common RI for oils caused by the human body then apply those to the diamond models as well as add in surface roughness (which I suspect is the biggest culprit to change in appearance) and look at computed light return values. -Matt |
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Post#3 @
09-29-01
, 11:44 AM
MattS: . We KNOW that the RI of diamond does not change when it gets dirty. We are talking about 'effective RI'; that is, how much light is refracted through the modified interface. It is like a compound lens: we don't talk about the FL of each component, but of the group acting together, and I am sure that you know that. |
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Well, sorta
Post#4 @
09-29-01
, 12:20 PM
Beryl, When speaking of multiple element lensing systems we most certainly talk of the index of refraction of each element, not an effective index of refraction. With multiple systems, for example a hastings triplet for a loupe, I would look for the power and index of each element to understand the system, not an effective index of refraction. Due to the shape of each surface and the index of the material, even if you were to determine an index of refraction for the system, you need to understand if there are coatings, what are the individual curvatures of the system, etc. My point about a dirty diamond is that the issues occur upon entering the diamond, that is the roughness and scattering properties of the "gunk" will certainly have significant impact on the interaction of the light. Take for example putting a coating of black paint on a diamond. This paint (assuming a paint coating with TiO2 materials) is going to be around 2.7 RI and would have some index change but not a whole lot, but if the paint and it's pigments become black, the absorbing effect of the black paint is huge, even though the refractive index has not changes a whole heck of a lot, now light is primarily abosrbed and not transmitted through the material to the diamond, and any that does has little change of getting back out. I was not trying to point out that any one was wrong, just trying to show were the light interacts. Once the light gets into the diamond it's a very similar story to what we know, but it has to get there first. Another example. Assume your diamond is perfectly clean, no gunk on it, now assume it has REALLY poor polish. The RI is still the same, but less light is going to get in and even less back out in aeas of interest. The surface quality is what drives in this situation. Most makeup and oils from the body have indices of refraction around 1.3-1.5, if you were to apply these compunds in a smooth layer around the diamond, two things would happen, you would get more light loss because the media surrounding the diamond will be closer to the diamond than to air and the critical angle will change to a grater angle thus allowing more light to leave. Now if you assume those compounds are not smooth, but applyed rough, say a fingerprint and soap from washing your hands, at the interface of the diamond and the gunk, you still have a smooth layer and the critical angle is still affected, but at the interface of the air and the compounds you have a rough surface that is very similar to having a very poorly polished facet or, and in most cases, much worse such that it is like applying a mdeium grit diamond sandpaper, it looks rough and the light interacting with that surface is scattered even more both on the way in and on the way out. Just some more thoughts. -Matt |
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Post#5 @
09-29-01
, 06:13 PM
MattS: . I did not say 'effective RI' of a lens system, I said 'effective FL' (focal length), as an analogy. Or effective R-factor of composite insulation, or effective Cv of multiple fluid-flow restrictions, or effective resistance of compound electrical resistors, etc., etc., etc. . We also understand that the irregularity of the diamond and coating are factors. . We already know the things you mention, except the RI of the oils. What is the RI of diamond-to-oil? Is this published? It sounds like you are equipped to measure it. I gave you RI's of diamond; now it's your turn - can you answer this? [Edited by beryl on 09-29-01 at 06:20 PM] |
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Effective RI and RI of oils ...
Post#6 @
09-30-01
, 11:58 AM
Here is why I thought you said effective RI ... quote "We are talking about 'effective RI'; that is," Sorry if I misunderstood and took it to mean diamonds and by example a lens system. I also do not understand what you meant in your last post which said "What is the RI of diamond-to-oil?" Is that to mean that RI chages next to diffeent materials? In my post I mentioned that most make-ups and oils (meaning body oils) are 1.3-1.5 RI. I am not trying to argue with anyone or tell anyone something they already know, I am just trying to communicate and discuss and get a feel for what the general knowledge is and learn from it. Sorry if I come off otherwise. -Matt |
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Post#7 @
09-30-01
, 07:38 PM
MattS: . In the GIA and/or GAGB courses, when discussing the principle of the contact refractometer (such as GIA or Rayner) they point out the function of the liquid between the gem and the 'hemisphere'. This involves materials of 3 RI's in a certain relationship, which I forget, but the instrument is limited because of available limits on liquid RI (methylene iodide tinctured with sulfur being the highest and subject to degradation). I will review that material (21+ yrs later) when I get a chance. . In the meantime, we will ask Sergey & Yuri; I expect they will know how to approach it. I am doing this for Garry; "frankly, Scarlett, I don't give a damn." . EDIT: misspelled word & typo. [Edited by beryl on 09-30-01 at 08:39 PM] |
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Dirt on diamonds
Post#8 @
10-01-01
, 12:41 PM
Hi gentlemen, the refractive index of the dirt on a pavilion of a diamond is greater then 1, (vacuum or air for gemmological purposes) If it is greater then one, the critcal angle of the new system dirt/diamond is bigger then the critical angle of air/diamond system, so greater is the refractive index of dirt, greater will be the critical angle of the dirt/diamond system. The properties of the diamond cannot, obviously, change, but now all calculations must be done taking into account the new dirt/diamond system. The air/Diamond system has a very small critical angle so very little light exits the diamond, other air/gemstones systems have a much bigger critical angle so in these sytems more light is leaked out. The effect of dirt on a diamond is that causes a bigger critical angle of the diamond system and the light goes from the diamond through the dirt easly first and then out of it into the air, scattered, causing a lot of light to leak out of the diamond and therefore it is not brilliant anymore. It can be simply demonstrated. critacl angle of air/diamond 24° 26' 20'' critical angle of dirt/diamond 32° 32' 10'' I used 1.3 as refractive index of the dirt. Fleimstaler |
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Post#9 @
10-01-01
, 01:13 PM
Fleimstaler: . Thank you, thank you - you are on my wavelength. How did you arrive at the critical angle - and with such precision? Garry: . The corresponding 'effective RI' is 1/sin(critical angle) = 1.86, therefore you would design using "Faceting Limits" chart for zircon or YAG, for which RI = 1.85. If you have Anton Vasiliev's program of this, you can adjust the 'effective RI' to anything and it gives you the corresponding chart immediately and precisely (he made some improvements to "FL").I think the zircon chart is close enough. |
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How an oil layer changes the diamond design
Post#10 @
10-02-01
, 12:41 PM
Some thoughts on how a layer of oil would change the diamond design: Some caveats: The film changes the optimum pavilion angle. Also, there are edge and corner effects. This post ignores both of these factors. Also, this post rounds off numbers per the editor's notes to Diamond Design, not per Tolkowsky's rounding errors. 1) A smooth layer of oil does not change which rays can get in a smooth diamond surface. If the incident angles are between 0° and 90°, their sines are between 0 and 1. By the time they penetrate through the oil and into the diamond, their sines are still between 0 and 1/2.417, and their refracted angles are still between 0° and arcsin(1/2.417) = 24° 26'. (This should be rounded off even more.) 2) A smooth layer of oil does not change which rays can get out of a smooth diamond surface. If the refracted angle is between 0° and 24° 26', the ray will manage to get out. 3) The smooth layer of oil does absorb some of the rays that would otherwise have been internally reflected. These rays have refracted sines between 1/2.417 and Noil/2.417. For Noil = 1.3, this gives 24° 26' to arcsin(1.3/2.417) = 32° 32'. For Noil = 1.31, the angle = 32° 49', which is a magic angle. 4) Tolkowsky ignores rays that are internally reflected from the crown and table -- he assumes that they are lost or unusual. 5) Thus, a smooth layer of oil on the crown and table has no effect on Tolkowsky's analysis. Only the oil on the pavilion matters. 6) Just using a different refractive index over-corrects for the problem, because it will change the angles of the rays entering and exiting the crown and table. 7) Tolkowsky emphasizes two sets of rays: Rays vertically incident to the table, and oblique rays that cross the middle of the diamond before entering the table. 8) Vertical rays: 1/3 of the light that enters the table (with - 1/3 < incident sine < 1/3, or incident angle < 19° 28. The sine of the refracted angle < (1/3/2.417), so the refracted angle < 7° 56. 9) If Noil < 1.31, the vertical rays are not directly affected by the oil. They will be doubly reflected (despite the oil), and exit through the crown and table. (These rays can be indirectly affected by the oil, if the oil changes the optimum pavilion angle. But that is a story of its own.) 10) Tolkowsky pays close attention to oblique rays (that cross the middle of the stone) that are lost through the pavilion. And a smooth layer of oil on the pavilion can cause all of them to be lost. 11) Tolkowsky says that of the oblique rays, only the ones between 19° 28' and an incident "effective angle" are "effective". The refracted effective angle = (the pavilion angle) - (minimum angle of total reflection). 12) If Noil = 1.31, the minimum angle of total reflection is increased from 24° 26' to 32° 49'. For a pavilion of 40° 45', this reduces the refracted effective angle from 16° 19' to 7° 56'. 13) Note 12 explains why Noil = 1.31 is magical: It eliminates all of the oblique rays (that cross the middle of the stone), without affecting the vertical rays. 14) Also, note 12 suggests that steeper pavilions should be less affected by oil -- or at least require Noil to be greater before the diamond is affected as much. 15) Tolkowsky's logic depends on the oblique rays to determine the crown angle. A thorough coating of oil on the pavilion forces us to use a different rationale for the crown angle and table size. (This post does not solve that problem, either.) 16) What happens if the oil affects just a small percentage of the pavilion? Suppose just 10% of the pavilion surface is covered with oil, with Noil = 1.31. Then Tolkowsky's weighting for the singly reflected rays will be reduced by 10%. 17) As shown in my girdle article, reducing the weight of the singly reflected rays changes the optimum crown. It reduces the optimum crown angle, increases the optimum table size, and reduces the optimum crown thickness. 18) If anyone is interested, I could modify my GirdleSection.class program to estimate these effects. But this would not be a complete analysis. To be valid, we would need to adjust the pavilion angle and the slopes of the minor facets. -- Jasper [Edited by Jasper on 10-02-01 at 12:59 PM] |
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dirty diamonds
Post#11 @
10-03-01
, 01:35 PM
Hi Beryl, I have to apologize for the delay in answering but I was away. Thanks for the appreciation of my post. Jasper has answered in detail on how I arrived at the critical angle of the dirt/diamond system Great job Jasper. Rgds. Fleimstaler |
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"Diamond Cross Section" calculator program
I have posted my "Diamond Cross-Section" calculator program at: http://www.folds.net/diamond The "Tolkowsky made interactive" thread (#11080) discusses this program. This program can be used to adjust for the effects of slightly dirty diamonds. I would be happy to post details (and caveats) in the current thread (#10178), if anyone is interested. -- Jasper [Edited by Jasper on 11-12-01 at 02:01 PM] |
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