Fire and dispersion techy help please?

[Garry H (Cut Nut)]

I asked Andrey to re-open this 2004 thread and move it across into the new Research forum section.

Bruce wrote and published this relevant article in 2006
https://www.pricescope.com/journal/fractioning_color_gem/

The long and short of this old discussion is that when it comes to fire it does depend if the light is exting the stone close to the critical angle.

But in some movies I was making last week I noticed strong flashes from the table of a deep diamond shown here.
This stone is 90 degrees to camera, so there is no dispersion on exit.

I checked to see the red was not caused by light entering the pavilion.
I then experimented by blocking the light and got to the area shown in yellow - but bumped the stone before a final location.

 

[Garry H (Cut Nut)]

Please start at the last post on page 3!!!!

Next I took the stone modeled 3D scan and sure enough - this is what we get.
Notice that only the red ray can exit from that zone because any blue or other colored rays will be excluded.

I have some long running discussions on this point. If the stone had better light retrun - we would not see this flash because it eminates from a zone that would otherwise be bright in many lighting situations.

It is Master Stone #5 from here http://www.octonus.ru/oct/mss/index.phtml


Unfortunately this does not explain the earlier part of the discussion in this thread - but it does explain something Beryl has been discussing

 

[Garry H (Cut Nut)]

Not happy with the ray trace image - you may have missed the red ray - if all colours of rays entered - only the red would come out where that flash is.

 

[beryl]

. 'Fractioning', as described in my article, is not the same as dispersion. It is the separation of some light from the spectrum by refracting while others are reflected. In that article, degree symbols have been replaced by question marks; it is being fixed.

 

[ChunkyCushionLover]

Garry,

This is a good topic to be revisited.
The issue of forward being different than reverse dispersion seems to be resolved and they are different.

However I cannot match the Reverse fire map created by Sassian's work:



With the Intensity Weighted Dispersion Statistics Diagram (IWDSD) created in DC.


The relative intensities of various regions do not match particularly in regions beside the arrowheads.
Could you or Sergey help explain the differences and the reason for the differences.

 

[beryl]

. In 'Fractioning of Color ...' the degree marks have now been fixed - only one day after I notified Andrey - congratulations, Andrey.
. I notice the word 'anticlockwise', which I have never used; this means that someone is tampering with our artcles without our knowledge. Be warned and check them carefully when they are published.

 

[ChunkyCushionLover]

. In 'Fractioning of Color ...' the degree marks have now been fixed - only one day after I notified Andrey - congratulations, Andrey.
. I notice the word 'anticlockwise', which I have never used; this means that someone is tampering with our artcles without our knowledge. Be warned and check them carefully when they are published.

I'd like to suggest you and Garry provide a proper link and text reference to the original print publication whenever possible in all future articles and threads.

 

[beryl]

. I just checked my original manuscript and it DOES say 'anticlockwise'; I don't know why.
. I should have checked this first before my comments; sincere apologies to Andrey & Garry.

 

[beryl]

. Here is a reproduction of Fig.5 from ‘Fractioning of Light by a Gem’. The yellow is produced by crossing of red and green rays. About 10x the size of the gem away it will cease to exist and a viewer cannot see it. However he can see green and red separately if he moves his head sideways. This could be a problem in close-up photography of gems.

 

[beryl]

. The greatest photo of a diamond I have seen is this one by Mike Cowing. At the time Garry and I both accused him of faking something but he said “No.” He says that it was under a tree with the sunlight coming through the leaves, as a series of tiny spotlights.

 

[michaelgem]

. The greatest photo of a diamond I have seen is this one by Mike Cowing. At the time Garry and I both accused him of faking something but he said “No.” He says that it was under a tree with the sunlight coming through the leaves, as a series of tiny spotlights.

Bruce,

Thanks for remembering. Seems like the good old days now, but it was only a decade ago.

That is my signature shot of a hearts and arrows super ideal taken out in my yard under a blue sky broken up as you recall into "a series of tiny spotlights".

It was a breakthrough in diamond photography that several professional jewelry photographers have now figured out and employ. I clip copy cat examples from trade magazines when I see them.

Back around 2000 I developed several similar photographic and lighting techniques to reveal the super ideal cuts potential to exhibit fire in realistic lighting. Some of those realistic lighting and photography techniques light up a dispersive gem like a Christmas tree. The attached photo for Al Gilbertson of his lithium niobate is an example. Unfortunately it looks too good to be believable. However that is how the gem appears in what David Federman first called my "fire friendly" illumination. A single photograph has captured what the eye observes over time as a gem is moved in lighting with the high contrast needed to enable this degree of fire.

That white lighting although broken up to maximize contrast is realistic in that it accounts for the "viewer obstruction" whose importance in facet design was first brought to the GIA and faceters in Gems and Gemology by Bruce way back in the 70's.

The younger among us may not be aware that Bruce was the first to show the necessity of accounting for the "contrast" or "viewer obstruction" caused by the observer's head when choosing combinations of pavilion and crown main angles in facet design.

Michael

 

[beryl]

Michael:
. Great photo; I had not seen this.
. You give me too much credit. Yes, I 'discovered' that the viewer's head causes darkness but didn't know why the historical 'best' cuts were near the EDGES of the darkness zone. It was Garry who reasoned that it resulted in contrast and then we all came to realize that contrast was important to appeal. Our dialog in 'Diamond Talk' was really productive, wasn't it? 'good old days', as you say.
. I will send you a copy of my Antwerp presentation, which discusses this and even shows why RI 1.6-1.7 are cut at slopes which are apparently inconsistent with other RI's (Andrychuk sought but missed this answer). I did not publish it here because these folks are primarily interested in diamonds.

 

[Karl_K]

reminds me of Marty's patented fire viewer system.

http://www.adamasgem.org/7315356.pdf



Also no discussion of fire is complete without discussing human vision which AGS has put a lot of research into. That is "new" since this thread was first started.
Will search for some links later.

 

[beryl]

. Karl: Was this pic taken with just a camera and lighting, as Michael does, or was it taken through some kind of instrument? I am unaware of Marty's efforts in this area.

 

[Karl_K]

. Karl: Was this pic taken with just a camera and lighting, as Michael does, or was it taken through some kind of instrument? I am unaware of Marty's efforts in this area.

Hi Bruce, good to see you back.
I posted the link to the patent for the lighting system/instrument used for the picture.
It explains it in great detail.

http://www.adamasgem.org/7315356.pdf

 

[beryl]

Karl:
. Thanks but I cannot open it. I will need help from my grandchildren. My great-granddaughter is one year old today!
. Marty moved back to Syracuse last year; perhaps you knew.

 

[Garry H (Cut Nut)]

Karl:
. Thanks but I cannot open it. I will need help from my grandchildren. My great-granddaughter is one year old today!
. Marty moved back to Syracuse last year; perhaps you knew.

I had the closest near death from cold experiance in my life at Syracuse II and some of the best pasta from Syracuse I.
I also have a collection of amazing coins from Syracuse I

But i digress.
Here is Marty's pinhole camera patent image Bruce:

 

[michaelgem]

reminds me of Marty's patented fire viewer system.

http://www.adamasgem.org/7315356.pdf



Also no discussion of fire is complete without discussing human vision which AGS has put a lot of research into. That is "new" since this thread was first started.

Re: Back around 2000 I developed several photographic and lighting techniques to reveal the super ideal cuts potential to exhibit fire in realistic lighting. Some of those realistic lighting and photography techniques light up a dispersive gem like a Christmas tree. The photo for Al Gilbertson of his lithium niobate is an example. Unfortunately it looks too good to be believable.

However that is how the gem appears in what David Federman (former senior editor Modern Jeweler) first called my "fire friendly" illumination. A single photograph has captured what the eye observes over time as a gem is moved in lighting with the high contrast needed to enable this degree of fire.

Karl points out that “no discussion of fire is complete without discussing human vision.” This is essential.

Back then I was working on discovering why prior photography had failed to capture the fire that human vision perceived. Solving the problem of getting the camera to “see” what human eyes see led to the photographic and lighting techniques, which in discussions with Eightstar, (for whom I was a research consultant), AGS and others I called “fire demonstration tools.” As you may imagine I was really excited about this discovery.

Here is one among hundreds of my photographs. This one was taken to demonstrate, study and compare the fire in a typical “ideal” cut to the fire in an “optically symmetric” ideal. It was published in the “Consumer Guide to Diamond Cut” in the September 2003 issue of Modern Jeweler Magazine.

Michael D Cowing

 

[Garry H (Cut Nut)]

reminds me of Marty's patented fire viewer system.

http://www.adamasgem.org/7315356.pdf



Also no discussion of fire is complete without discussing human vision which AGS has put a lot of research into. That is "new" since this thread was first started.

Re: Back around 2000 I developed several photographic and lighting techniques to reveal the super ideal cuts potential to exhibit fire in realistic lighting. Some of those realistic lighting and photography techniques light up a dispersive gem like a Christmas tree. The photo for Al Gilbertson of his lithium niobate is an example. Unfortunately it looks too good to be believable.

However that is how the gem appears in what David Federman (former senior editor Modern Jeweler) first called my "fire friendly" illumination. A single photograph has captured what the eye observes over time as a gem is moved in lighting with the high contrast needed to enable this degree of fire.

Karl points out that “no discussion of fire is complete without discussing human vision.” This is essential.

Back then I was working on discovering why prior photography had failed to capture the fire that human vision perceived. Solving the problem of getting the camera to “see” what human eyes see led to the photographic and lighting techniques, which in discussions with Eightstar, (for whom I was a research consultant), AGS and others I called “fire demonstration tools.” As you may imagine I was really excited about this discovery.

Here is one among hundreds of my photographs. This one was taken to demonstrate, study and compare the fire in a typical “ideal” cut to the fire in an “optically symmetric” ideal. It was published in the “Consumer Guide to Diamond Cut” in the September 2003 issue of Modern Jeweler Magazine.

Michael D Cowing

Hi Michael, it is indeed a wonderful way to capture the effects seen under a tree (and multiplied). But I do not believe it proves that better symmetry shows more fire with this technique.
If you take diamonds without a lot of leakage, but lousy symmetry, then you should get very similar results. Maybe even better from the poor symmetry.

 

[Garry H (Cut Nut)]

Garry,

This is a good topic to be revisited.
The issue of forward being different than reverse dispersion seems to be resolved and they are different.

However I cannot match the Reverse fire map created by Sassian's work:



With the Intensity Weighted Dispersion Statistics Diagram (IWDSD) created in DC.


The relative intensities of various regions do not match particularly in regions beside the arrowheads.
Could you or Sergey help explain the differences and the reason for the differences.

CCL Sergey is 're-looking' at DiamCalc fire info - so just for the moment please do not read too much into the DC resluts.

But it is worth noticing that tilting stones or making slow small movement DC movies can show how the fire moves through the spectrum, with generally one end of the spectrum, the white bright bit in the middle, and then the other end of the spectrum. The intensity and brightness of the colored flashes might often be associated with the brightness of the white flash. But brighter white flashes sometimes have shorter coloured flashes.

And BTW I will copy bits of the relevant links or direct readers to the targeted parts

 

[michaelgem]

Hi Michael, it is indeed a wonderful way to capture the effects seen under a tree (and multiplied). But I do not believe it proves that better symmetry shows more fire with this technique.
If you take diamonds without a lot of leakage, but lousy symmetry, then you should get very similar results. Maybe even better from the poor symmetry.

I agree Garry.

At this magnification they both look similarly beautiful, but the benefit of high optical symmetry is larger flashes of fire that are more apparent to the eye than is the fire in the diamond with ideal angles but poor optical symmetry . With diamonds having poor physical as apposed to optical symmetry there is even greater break up of reflections (more but smaller virtual facets) and the fire is considerably less visible in normal viewing.

And if the angles and proportions deviate from the ideal sweet spot range, resulting in greater leakage or other decreases in light performance, the fire image is significantly degraded from what you see in these two diamonds.

Michael

 

[Garry H (Cut Nut)]

Hi Michael, it is indeed a wonderful way to capture the effects seen under a tree (and multiplied). But I do not believe it proves that better symmetry shows more fire with this technique.
If you take diamonds without a lot of leakage, but lousy symmetry, then you should get very similar results. Maybe even better from the poor symmetry.

I agree Garry.

At this magnification they both look similarly beautiful, but the benefit of high optical symmetry is larger flashes of fire that are more apparent to the eye than is the fire in the diamond with ideal angles but poor optical symmetry . With diamonds having poor physical as apposed to optical symmetry there is even greater break up of reflections (more but smaller virtual facets) and the fire is considerably less visible in normal viewing.

And if the angles and proportions deviate from the ideal sweet spot range, resulting in greater leakage or other decreases in light performance, the fire image is significantly degraded from what you see in these two diamonds.

Michael

Michael do you have the parameters or 3D model for the stone on the right?
It appears to have a lot of leakage and if so - it is not a relevant example.

 

[michaelgem]

Hi Michael, it is indeed a wonderful way to capture the effects seen under a tree (and multiplied). But I do not believe it proves that better symmetry shows more fire with this technique.
If you take diamonds without a lot of leakage, but lousy symmetry, then you should get very similar results. Maybe even better from the poor symmetry.

I agree Garry.

At this magnification they both look similarly beautiful, but the benefit of high optical symmetry is larger flashes of fire that are more apparent to the eye than is the fire in the diamond with ideal angles but poor optical symmetry . With diamonds having poor physical as apposed to optical symmetry there is even greater break up of reflections (more but smaller virtual facets) and the fire is considerably less visible in normal viewing.

And if the angles and proportions deviate from the ideal sweet spot range, resulting in greater leakage or other decreases in light performance, the fire image is significantly degraded from what you see in these two diamonds.

Michael

Michael do you have the parameters or 3D model for the stone on the right?
It appears to have a lot of leakage and if so - it is not a relevant example.

What we are seeing here is the light performance of two diamonds both with average Tolkowsky/Morse crown and pavilion angles, the right diamond having less physical symmetry and little optical symmetry . Neither is exhibiting much light leakage, at least not the middle ring leakage seen in the steep-deep cuts, which would show through with a tone like the background color.

Most of the parameters can be measured or estimated in this face up view using techniques taught in the GIA diamond grading course. The table is 55.3%, the table reflection indicates 41 degree mains plus or minus a few tenths degree, the roughly two to one magnification of the pavilion main arrows in the crown mains indicates crown angles close to Tolkowsky's 34.5, the star length is about 55% and the half length around 80%.

The comparison here primarily demonstrates the greater break up of reflections (more but smaller virtual facets) in the diamond with less optical symmetry causing smaller flashes of fire that are considerably less visible in normal viewing.

Michael

 

[Garry H (Cut Nut)]

Hi Michael, it is indeed a wonderful way to capture the effects seen under a tree (and multiplied). But I do not believe it proves that better symmetry shows more fire with this technique.
If you take diamonds without a lot of leakage, but lousy symmetry, then you should get very similar results. Maybe even better from the poor symmetry.

I agree Garry.

At this magnification they both look similarly beautiful, but the benefit of high optical symmetry is larger flashes of fire that are more apparent to the eye than is the fire in the diamond with ideal angles but poor optical symmetry . With diamonds having poor physical as apposed to optical symmetry there is even greater break up of reflections (more but smaller virtual facets) and the fire is considerably less visible in normal viewing.

And if the angles and proportions deviate from the ideal sweet spot range, resulting in greater leakage or other decreases in light performance, the fire image is significantly degraded from what you see in these two diamonds.

Michael

Michael do you have the parameters or 3D model for the stone on the right?
It appears to have a lot of leakage and if so - it is not a relevant example.

What we are seeing here is the light performance of two diamonds both with average Tolkowsky/Morse crown and pavilion angles, the right diamond having less physical symmetry and little optical symmetry . Neither is exhibiting much light leakage, at least not the middle ring leakage seen in the steep-deep cuts, which would show through with a tone like the background color.

Most of the parameters can be measured or estimated in this face up view using techniques taught in the GIA diamond grading course. The table is 55.3%, the table reflection indicates 41 degree mains plus or minus a few tenths degree, the roughly two to one magnification of the pavilion main arrows in the crown mains indicates crown angles close to Tolkowsky's 34.5, the star length is about 55% and the half length around 80%.

The comparison here primarily demonstrates the greater break up of reflections (more but smaller virtual facets) in the diamond with less optical symmetry causing smaller flashes of fire that are considerably less visible in normal viewing.

Michael

Michael you do not have the 3D model? Or at least the stones proportions?
If so I question its validity as an example.

Here are reproducable results.
On the left is an 8* from a Helium scan. In the center is a symmetrical stone modeled to the same main parameters as the stone on the right which is a very poorly cut stone with very poor symmetry. All 3 have been adjusted to 1.00ct.
I tilted all 3 2 degrees down and 2 to one side to try to match the angle to camera of the bad example you used. This also removes some table glare from this illumination.
I will also post the Gem Adviser file of the bad stone.

I think it is possible to argue that symmetry has some benefits - but I am unconvinced that enhanced fire is one of them.

 

[Garry H (Cut Nut)]

Here is the Gem file for the stone (actually 2.02ct) I used complete with its illumination.

 

[Karl_K]

I think it is possible to argue that symmetry has some benefits - but I am unconvinced that enhanced fire is one of them.

I agree and I disagree when it comes to fire.
It depends on the diamond size and the degree of non-symmetry.
A .5ct is going to give different results than a 3ct because of the physical size difference of the first and second order virtual facets due to diamond size.
Normalized to 1ct I agree that the differences with minor optical symmetry deviations on fire is very small and real world irrelevant.

If there is a difference other than contrast patterns/balance it will be in scintillation, which I think there is but haven't come up with a good way to prove it yet.

 

[beryl]

Michael:
. Recall that my favorite is an 'old miner' cut to today's slopes and table size (no culet). I said I liked it because of the 'bang-bang' effect of its dominant mains going on-off. I guess you could call me a 'chunky cushion lover' (how'bout that CCL?).
. Also I believe I reported here that I had made a Portuguese cut quartz (to test the repeatability of my reconditioned machine) and I didn't like it because it had too many too small reflections; it 'fizzed' as you turned it, not 'snap-snap' as with an 8-point round brilliant.
. Recall these quotes from "Seeing the Light" (Falk, Brill & Stork, Wiley 1986)*:
. "... the visual system only reports the exciting news, the changes that it sees." (section 7.2),
. "... the overall light level is less important than the local variations in it." (7.3),
. "... your brain is less interested in the overall illumination than it is in the relative light intensity ..." (7.4),
. "... an edge, where the light intensity changes rapidly from brighter to darker, is made more noticeable ..." (7.4),
. "... Your retina ... responds only if there is a change in stimulation in time ..." (7.7).
Items 1 and 4 pertain best to your discussion above, but I show them all for benefit of those who have not seen them before. I agree that more, smaller, reflections are undesirable. However, I apologize that this has nothing to do with 'fire' or dispersion (are they the same or different?).

* It was you who told me about this book in 1999; I thank you for that, also for the gift of "Color Science" by Wyszecki & Stiles, Wiley, 2000), and for remembering our 1975 dialog and introducing me to the 'Diamond Talk' group in 2000 (I had given up gemology and devoted all my effort on gardening by then!). I have just learned of another book on visual response from 'osu-eyedoc' and may report on that in the future.

 

[michaelgem]

Michael:
. Recall that my favorite is an 'old miner' cut to today's slopes and table size (no culet). I said I liked it because of the 'bang-bang' effect of its dominant mains going on-off. I guess you could call me a 'chunky cushion lover' (how'bout that CCL?).
. Also I believe I reported here that I had made a Portuguese cut quartz (to test the repeatability of my reconditioned machine) and I didn't like it because it had too many too small reflections; it 'fizzed' as you turned it, not 'snap-snap' as with an 8-point round brilliant.
. Recall these quotes from "Seeing the Light" (Falk, Brill & Stork, Wiley 1986)*:
. "... the visual system only reports the exciting news, the changes that it sees." (section 7.2),
. "... the overall light level is less important than the local variations in it." (7.3),
. "... your brain is less interested in the overall illumination than it is in the relative light intensity ..." (7.4),
. "... an edge, where the light intensity changes rapidly from brighter to darker, is made more noticeable ..." (7.4),
. "... Your retina ... responds only if there is a change in stimulation in time ..." (7.7).
Items 1 and 4 pertain best to your discussion above, but I show them all for benefit of those who have not seen them before. I agree that more, smaller, reflections are undesirable. However, I apologize that this has nothing to do with 'fire' or dispersion (are they the same or different?).


Bruce,
For the sake of discussion fire is the observation of colored light in a diamond due to incident white light. Dispersion is a necessary but not sufficient property of a gemstone for there to be fire. In addition, the virtual facets that emanate fire must be large enough and not overwhelmed by brilliance in order for a diamond to exhibit fire.

Garry often points out that “brilliance is the enemy of fire”.

I like to say that "the same diamond exhibiting superior brilliance in lighting conducive to brilliance will exhibit superior fire in lighting conducive to fire." An example is the pinpoint lighting Garry and Sergey have provided in DiamCalc.

Your comments and insight go to the heart of the matter, because the old mine cut diamonds with their large virtual facets due to chunky, dominant mains, not only results in the "bang-bang" contrast quality of brilliance in usual lighting, but also results in the large flashes of fire characteristic of those cuts when seen in "fire friendly" lighting.

The optically symmetric super ideal that I call the "Central Ideal" has 77% lower half length which is a compromise between large flash brilliance and fire and increased scintillation from more numerous but smaller "virtual facets" as the half length is increased.

In my article, “Accordance in Round Brilliant Diamond Cutting,” I showed the attached photo of an old mine cut compared in the same lighting to a H&A super ideal with 77% lower half length.

In that article I commented: “During the 20th century, the pavilion halves were further increased in length with consequent increase in their area and influence on the diamond’s beauty. The motivation for this increase in the length of the halves was the increased amount of sparkle or scintillation brought about by larger halves. However, a consequence of the increase in the halves in order to favor scintillation was a decrease in the size of the mains. This brought an accompanying reduction of the desirable properties of large flash sparkle and fire that result from larger mains. This large flash fire and sparkle was a fundamental aspect of the appeal of the early round brilliant from Tolkowsky’s era and the prior eras of the Old Mine Cut and the Old European Cut.

Figure 4 contains a photograph of a 2.38ct Old Mine Cut diamond with shorter pavilion halves compared to a four grainer, Ideal round brilliant cut with roughly 77% lower halves. Both were photographed in the same “fire friendly”, high contrast, spot illumination, which is lighting favorable to display of fire. Both display the diamond’s ability to break up white light into colors of the spectrum. However, larger flashes of fire, due principally to larger mains, are apparent in the Old Mine Cut compared to the more numerous but smaller flashes of fire in the Ideal Cut.

The cutters of H&A ideals for the Japanese market cut 77% half lengths and I believe many H&A cutters still do. Much cutting today goes too far in my opinion to around 80% or greater. However this is just my opinion, and perhaps the opinion of others who, like you, favor the larger flashes of brilliance and fire from large mains. Maybe cutters on this forum would be willing to comment on this subject.

Michael D Cowing

 

[michaelgem]

Here is the Gem file for the stone (actually 2.02ct) I used complete with its illumination.

Garry,

I downloaded and ran your .gem file with pinpoint lighting, but it just looked like disco lighting. What do I have to set to get images like the ones in your post?

Michael

 

[beryl]

Michael:
. Where do I find article "Accordance in Round Brilliant Cutting"?