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Quote this post and reply to it Post#1 @ 01-26-02 , 04:57 AM


. Sergey (of MSU) has asked me to show MSU's brightest points alongside those I previously showed for GIA. Here, in blue, is the 'best straight line' through the brightest points of MSU Figure 1 and 4a, which are for light return only.
. This line just skims the 10 limit of head-obstruction for table-to-bezel rays via pavilion mains; this may be significant. I will let Sergey and others comment on this and differences between the MSU and GIA plots.
. Because MSU model has 82% pavilion half-facets, I drew those boundaries (in blue) for consideration of rays via pavilion halves.
. I have just realized that what GIA calls 75% pavilion girdle LENGTH is really about 77% pavilion DEPTH; those borders will be adjusted in the future, for more correct evaluation of the GIA lines.



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Quote this post and reply to it Post#2 @ 01-26-02 , 05:43 AM


Bruce,

Thanks very much.

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Quote this post and reply to it Post#3 @ 01-26-02 , 08:39 AM


. Here's an enlarged view of the graph area.



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Quote this post and reply to it Post#4 @ 01-26-02 , 10:35 AM


Now that MSU line is interesting. (speculating) Looks as though the head obstruction is factored in and shoves the results down when they would have kept increasing if it hadn't been included.

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Quote this post and reply to it Post#5 @ 01-26-02 , 12:18 PM


Jamesd:
. Actually, the reverse is true: the unshaded area is that which is unaffected by 10 head obstruction and would not have influenced the test results. That is why I say that head omission is not a factor on the GIA 'best' tests in this region.
. It is of interest to note (see "Faceting Limits") that the historically-evolved recommended crown/pavilion main combinations for spinel, garnet, corundum, zircon, and diamond (Tolkowsky) are all on the edge of this line. That is - all of the high RI stones commonly cut in Zone A of "FL". There is a truth hidden here!
. Here are the charts for the first 4 materials (see above for diamond) ...


[Edited by beryl on 01-26-02 at 11:32 AM]

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Quote this post and reply to it Post#6 @ 01-26-02 , 12:25 PM


. It is even more curious to note that the historical combinations for topaz, tourmaline, peridot, and spodumene are all on the lower edge of this same head-obstruction band, in Zone B of "FL". This adds even more to the suspicion that there is a truth lurking here!
. Here are the related charts ...



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ACHTUNG !!!
Quote this post and reply to it Post#7 @ 01-26-02 , 12:52 PM


. The black & white charts shown above are taken directly from "Faceting Limits" (GIA, "Gems & Gemology", Fall 1975). The scales are reversed from the first plot, which was made to match GIA and MSU plots: note that horizontal is pavilion and vertical is crown. Hey! I was there first; don't blame me!
. Picture these charts rotated about a diagonal from the lower left. A steep-deep stone is still at the upper right and a low-shallow one is still at the lower left.
. Also note that the crown slope scale is 5 times the pavilion slope scale.
. At the SPINEL chart I found room to add two arrows at the left, indicating increased dispersion and increased brightness (not 'brilliance'); these must be flipped too. For years I regretted this non-factual indication based only on hearsay and observation. Now I am pleased to say that GIA's latest summary (G&G, Fall 2001) confirms it.

[Edited by beryl on 01-26-02 at 11:54 AM]

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Quote this post and reply to it Post#8 @ 01-26-02 , 01:02 PM


I think I used an unfortunate choice of words. By forced down I didn't mean the line on the charts but the brightness results.

The picture I had in mind was steadily improving brightness until the values hit the line, then a tumble as the head obstruction area is reached, leaving the highest value at that point.

Wild speculation on the truth would be wondering what happens to light sources on the other side of the head. Are we seeing peak light from the side of the viewing eye coupled with darkness from the sources obstructed from the other eye, leaving a maximum brightness, some obstruction mix?

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Heads & Eyes
Quote this post and reply to it Post#9 @ 01-26-02 , 01:33 PM


Jamesd: ........ AHA !
. That is a fascinating question. MSU is now working on stereo vision effects and I can't wait to see the results.
. MSU's previous model had one eye in the middle of a head; GIA's has no head.
. My viewer model represents one eye and the same side of the head = 2-1/8" = 10 @ 12" viewing distance. Using 2-1/2" between the eyes (12, see "FL" Fig.8A), this gives 4-5/8" = 21" to the other side of the head or from the other eye to the same side of the head.
. Therefore, with the stone in line with one eye, and the stone cut so that 15 rays are reflected to the eye, that eye will see such rays and the other eye won't. I do not know the psycho-visual implications of this.
. In "FL", "Live" Center, Fig.8, I touched on this subject but it would have burdened the primary message and could have become a complete sequel. I don't even understand what I wrote then and will be happy to wait for MSU's report.

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Quote this post and reply to it Post#10 @ 01-27-02 , 06:51 AM


Garry:
. Too much here to answer well quickly; I spot a few trick questions (horses & apples). I will print this out and respond carefully.
. I think that GIA has accomplished much, as has MSU. I think they have both provided the same general answers; the differences are less important than the similarities. There is more information to be found, but is it worthwhile?
. Going visiting in pegmatite country today - will take you next time you visit (if weather appropriate).

[Edited by beryl on 01-27-02 at 06:07 AM]

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Re: Hmmm
Quote this post and reply to it Post#11 @ 01-28-02 , 07:18 AM


Quote:
Originally posted by Garry Holloway

1. Beryl and others - just because the GIA's brightest stone results happen to fall in the no head obstruction zone, how does that result relate to the fact that the WLR for all the stones along that line falls as the pavilion gets shallower and the crown steeper?

. Who knows? One has nothing to do with the other. It is a phenomenon that exists and has been observed by everybody and proved by MSU and GIA.
. It might have something to do with "internal reflections from the bezel", as discussed with Fig.6 in FL.

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Re: Hmmm
Quote this post and reply to it Post#12 @ 01-28-02 , 07:30 AM


Quote:
Originally posted by Garry Holloway

2. What if you applied the same theory in FL to reflected light (the so called "glare" that GIA wrongly, in my opinion, choose to ignore as either a source of light return or brilliance

. What 'theory' in FL would you apply to surface reflection? (there are no theories, just facts).
. The major factor, head obstruction, pertains to rays which have 10 external divergence - that is, 10 between incident and returned rays. In the case of external reflection, this means that a facet is tipped within 5 off-perpendicular to the line of vision.
. Most typically, this is the table, in which case, having no glare is good because then you can see the internal reflections, contrast, and fire better through the table.

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Re: Re: Hmmm
Quote this post and reply to it Post#13 @ 01-28-02 , 07:32 AM


Quote:
Originally posted by beryl
Quote:
Originally posted by Garry Holloway

1. Beryl and others - just because the GIA's brightest stone results happen to fall in the no head obstruction zone, how does that result relate to the fact that the WLR for all the stones along that line falls as the pavilion gets shallower and the crown steeper?

. Who knows? One has nothing to do with the other. It is a phenomenon that exists and has been observed by everybody and proved by MSU and GIA.
. It might have something to do with "internal reflections from the bezel", as discussed with Fig.6 in FL.


Any index which measures any type of LR will show this phenomenon

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Re: Hmmm
Quote this post and reply to it Post#14 @ 01-28-02 , 07:40 AM


Quote:
Originally posted by Garry Holloway
Since reflected light has less 'opportunity' to become colored it is of more value in colorless diamonds and a less valued commodity in colored gems (which incedently have lower RI's and therefore reflect more light).

, Reflected light has NO opportunity to become colored if the surface is smooth, because there is no dispersion or diffraction.
. Yes, colored gems have lower RI, but for this reason they reflect LESS light directly from a surface (check Fresnel)!
Check out 'adamantine' luster of diamond and its definition - it reflects MORE light from its surface than any other gem.
. Where are you getting this input? It sounds like you are asking someone else's questions.

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Re: Hmmm
Quote this post and reply to it Post#15 @ 01-28-02 , 07:43 AM


Quote:
Originally posted by Garry Holloway

Perhaps Beryl you could ascertain the most reflective crown facet combinations for diamond - and the least reflective for colored gems?

. I see no reason to try.

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Re: Hmmm
Quote this post and reply to it Post#16 @ 01-28-02 , 07:56 AM


Quote:
Originally posted by Garry Holloway
3. Since FL describes advantageous light return by minimising obstruction the 'ideal' stone has little contrast. Assuming contrast is desirable - How would you suppose to define the best or ideal amount of contrast?

. FL does not suggest an 'ideal' stone, nor did it consider contrast. You pointed out to me, a year ago, that perhaps the traditional cuts are on the line because it creates contrast between the mains and halves and that such contrast was desirable.
. On this basis, cuts along (like MSU), or just below the line (see question 4, below) would be best for combined contrast and brightness.
. I cannot define 'contrast' quantitatively. Therefore I cannot define 'best contrast'. Indeed, this is in the eye of the beholder; e.g.: I prefer the 'bang-bang' contrast of the 'old miner'.

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Re: Hmmm
Quote this post and reply to it Post#17 @ 01-28-02 , 08:09 AM


Quote:
Originally posted by Garry Holloway
4. Using the common proportions of the most respected brands - ACA, SC and 8* it would appear that many fall into the bezel - table obstruction zone. Any comments about this in light of point 3?

. I do not know the 'common proportions' of ACA, SC, and 8*. You say here that they fall into the bezel-table head-obstruction zone.
. Nore that the upper diagonal boundary is for main facets, the lower one is for 75% pavilion halves. I do know that ACA places great emphasis on the secondary facets and that GIA's latest report shows that they have significant effect.
. Recall last year when I could not make FL match MSU results until Yuri pointed-out that, in todays stones, the pavilion halves, not the mains, are the primary reflector. I changed the plot accordlingly and the match became very good. This proved to me that the halves are, indeed, the primary reflector and therefore control brilliance.
. Therefore, a stone below the mains limit and above the halves limit would show the most contrast between mains and halves, being brightest near the mains border.

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Re: Hmmm
Quote this post and reply to it Post#18 @ 01-28-02 , 08:10 AM


Quote:
Originally posted by Garry Holloway
5. What factor has the most importance - light loss via leakage out the pavilion or lack of light return because of obstruction?

. You tell me. I think you are trying to.

[Edited by beryl on 01-28-02 at 07:11 AM]

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Re: Hmmm
Quote this post and reply to it Post#19 @ 01-28-02 , 08:17 AM


Quote:
Originally posted by Garry Holloway
I have often pondered why the GIA chose the methods it did to study diamond cut. What would have happened if they had have chosen to study leakage and head obstruction together - rather than light return?

Certainly their two reports thus far from their +decade long study have prompted more questions about the study than answers or solutions to the problems of cut grading that they set out to discover.
Perhaps their results thus far would be considered as a failure if they were operating in a profit based business?
Perhaps they will pull a rabbit out of a hat as someone suggested?

. 1st paragraph: You will have to ask GIA.
MSU already did that, with their 'nailhead' study - according to their definition of 'nailhead'.
. 2nd paragraph: I think they have demonstrated a lot. Even you said that their 1998 results proved your 4:1 theory.

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Re: Not good enough Beryl
Quote this post and reply to it Post#20 @ 01-28-02 , 08:24 AM


Quote:
Originally posted by Garry Holloway
We need to understand WHY it is so!
This is very important.
why do some good proportion combinations leak more?
Sure it is so, but why?
It is a simple thing - so why can we not say why in simple words?

. That's as good as you're going to get from me. Ask Sergey (he already replied).
. You said, in e-mail, that you're asking, rather than telling, to be polite. So be impolite and tell us all.

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Quote this post and reply to it Post#21 @ 01-28-02 , 09:24 AM


Quote:
Originally posted by Jamesd
The picture I had in mind was steadily improving brightness until the values hit the line, then a tumble as the head obstruction area is reached, leaving the highest value at that point.

. This statement has intrigued me since you made it - a very probable conclusion. Finally, I realized that, if this were true, then the MSU chart's value lines would be very close on that side - falling-off rapidly like a cliff on a topographic map.
. This is not the case: the 'slope of the hill' is the same to either side of the peak.
. It is important to note, however, that the limit I have shown is for 10 external diversion of an internally reflected ray (table-bezel & vice-versa). It moves parallelly(?) down-left as this angle is reduced (as by smaller head or by farther viewing distance).
. See white lines of FL on MSU Q-factor chart at http://www.diamondring.com/forums/showthread.php?t=4603 "MSU Chart Overlays", post#39976. 02-05-01 16:34. The central line is for 0 head obstruction, the next 2 are for 5, and the outer 2 are for 10. Since the MSU study corresponds to about 10, only the outer lines apply to this chart.
. I am convinced that a woman with lots of hair must see the same stone differently than a man with normal hair (or none) at the same distance, so it is possible that they would not both prefer the same stone.


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Quote this post and reply to it Post#22 @ 01-28-02 , 11:37 AM


beryl,

I'm not convinced but I need to study their lighting model again to see what I expect from it.

Without doing that, I'm not surprised that the Q factor doesn't fall off sharply. Darkness isn't necessarily bad if you accept that darkness is the palying field for fire. Since Q includes both it seems reasonable to expect fire to rise in the obstruction zone and to some extent offset a decline in brilliance.



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Quote this post and reply to it Post#23 @ 01-28-02 , 08:44 PM


Jamesd:
. You will see the same thing on the MSU LR-charts = Fig. 1 & 4a: same rate of drop-off on both sides of the ridge.

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Tolkowsky's consideration of 'lustre'
Quote this post and reply to it Post#24 @ 01-29-02 , 02:20 AM


Quote:
Originally posted by Garry Holloway
It is a contributing factor to brilliance and I guessed it would be similar mathematics to FL?
It would be good to know. I expect about 15-20% of light return is from surface reflection. What if shallow crowns reflect more light than steep crowns?
Would Tolkowsky's study have [gotten] the wrong result then?


Garry and Beryl,

Thank you for your notes on luster.

Tolkowsky seems to write that a diamond's 'lustre' is important, but not much affected by the cut (if the cut is any good at all.)

This is because he normalizes the size of the diamond using the cross-sectional area at the girdle, NOT by using the carat weight.

He treats the 'lustre' as a constant in the optimization, which simplifies his analysis.


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Re: Tolkowsky's consideration of 'lustre'
Quote this post and reply to it Post#25 @ 01-29-02 , 02:32 AM


Tolkowsky writes:
http://www.folds.net/diamond_design/index.html#optical

Quote:
Quotation from Tolkowsky's Diamond Design

It is to light, the play of light, its reflection and its refraction, that a gem owes its brilliancy, its fire, its colour. We have therefore to study these optical properties in order to be able to apply them to the problem we have now before us : the calculation of the shape and proportions of a perfectly cut diamond.

Of the total amount of light that falls upon a material, part is returned or reflected ; the remainder penetrates into it, and crosses it or is absorbed by it. The first part of the light produces what is termed the " lustre " of the material. The second part is completely absorbed if the material is black. If it is partly absorbed the material will appear coloured, and if transmitted unaltered it will appear colourless.

The diamonds used as gems are generally colourless or only faintly coloured ; it can be taken that all the light that passes into the stones passes out again. The lustre of the diamond is peculiar to that gem, and is called adamantine for that reason. It is not found in any other gem, although zircon and demantoid or olivine have a lustre approaching somewhat to the adamantine.

In gem stones of the same kind and of the same grade of polish, we may take it that the lustre only varies with the area of the gem stone exposed to the light, and that it is independent of the type of cut or of the proportions given to the gem (in so far as they do not affect the area) ; this is why gems where the amount of light that is reflected upon striking the surface is great, or where much of the light that penetrates into the stone is absorbed and does not pass out again, are frequently cut in such shapes as the cabochon (fig. 12), so as to get as large an area as possible, and in that way take full advantage of the lustre.

In a diamond, the amount of light reflected from the surface is much smaller than that penetrating into the stone ; moreover, a diamond is practically perfectly transparent, so that all the light that passes into the stone has to pass out again. This is why lustre may be ignored in the working out of the correct shape for a diamond, and why any variation in the amount of light reflected from the exposed surface due to a change in that surface may be considered as negligible in the calculations.

The brilliancy or, as it is sometimes termed, the " fire " or the " life " of a gem thus depends entirely upon the play of light in the gem, upon the path of rays of light in the gem. If a gem is so cut or designed that every ray of light passing into it follows the best path possible for producing pleasing effects upon the eye, then the gem is perfectly cut. The whole art of the lapidary consists in proportioning his stone and disposing his facets so as to ensure this result.

If we want to design a gem or to calculate its best shape and proportions, it is clear that we must have sufficient knowledge to be able to work out the path of any ray of light passing through it. This knowledge comprises the essential part of optics, and the laws which have to be made use of are the three fundamental ones of reflection, refraction, and dispersion.


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