Fire and dispersion techy help please?

[Garry H (Cut Nut)]

Beryl Micheal's article is on his website

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 GH EDIT 4 grainer = 1.00ct in human speak, 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

Hi Michael,
Clearly there is a trade off in virtual facet size to number of virtual facets. And we know that different people have different preferences.
In addition to your message above - I would add that if you focus on any dark facet - say a pavilion main, as you rotate the stone (or you move) the facet (if it has light return) the will go from dark, through a rainbow color form the blue or the red end of the spectrum, into brighter white, and then back to black via the opposite spectral color. If the lighting is 'fire friendly, then you will move from say red to orange and yellow before white and then from blue through to violet and back to dark.
The change from these facets works best just into Bruces dark zone caused by head obstruction of light sources.

 

[Garry H (Cut Nut)]

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

I do not seem to be able to upload the lighting file.
Michael it would take you 5 minutes to generate it in DiamCalc.
If thats too hard email and I will send it (to anyone who would like a copy).

 

[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

I do not seem to be able to upload the lighting file.
Michael it would take you 5 minutes to generate it in DiamCalc.
If thats too hard email and I will send it (to anyone who would like a copy).

Thanks Garry,

I and perhaps others would benefit from a lesson from you on how to generate this pinpoint lighting in DiamCalc.

There is so much intelligence and innovation built into DiamCalc that I need to make a major effort just to keep up with all the advances.

Meanwhile I'll email you offline for the copy you offered us.

Michael

PS I apologize if the font size is too large. The Normal font size is too small on my screen, while the next larger size looks too large. How do these font sizes appear to you, and which one, Normal, Large or some in between size does everyone prefer?

 

[Garry H (Cut Nut)]

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

I do not seem to be able to upload the lighting file.
Michael it would take you 5 minutes to generate it in DiamCalc.
If thats too hard email and I will send it (to anyone who would like a copy).

Thanks Garry,

I and perhaps others would benefit from a lesson from you on how to generate this pinpoint lighting in DiamCalc.

There is so much intelligence and innovation built into DiamCalc that I need to make a major effort just to keep up with all the advances.

Meanwhile I'll email you offline for the copy you offered us.

Michael

PS I apologize if the font size is too large. The Normal font size is too small on my screen, while the next larger size looks too large. How do these font sizes appear to you, and which one, Normal, Large or some in between size does everyone prefer?

Font is a bit large Michael.
Here is how:
Options>HDR Settings..>Panorama>apply

 

[Karl_K]

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

I would disagree it depends on the crown/pavilion/table angles/size.
For a modern tolk 34.5/40.8/56 yes 77%-78% lgf is probably a sweet spot.
However in my opinion for a 34/41/56 closer to 80 and even a little over balances it out better.
For a 60/60 I prefer them a bit on the long side also. The more open table increases the area of the mains under the table.
For a shallow pavilion long lgf% helps minimizes contrast issues so there is valid reason to go out to 80 with them also.
Also you get large flashes off the lowers with a longer lgf% that must be taken into account.

http://www.pricescope.com/journal/do_pavilion_mains_drive_light_return_modern_round_brilliant

 

[beryl]

. re: last paragraph on previous page (4); sorry, I had not noticed that p.5 had begun:
. Garry: can you suggest why dispersion is most noticeable here?
. For those who don't know what "Bruce's dark zone" means. it is the region of the pavilion/crown slope chart which represents obstruction by the viewer's head, of table-to-bezel rays (and vice-versa), shown below. This edge represents 10° ray divergence between the viewer's ear and eye on the same side, which was the subject of "Faceting Limits", GIA 'Gems & Gemology', Fall 1975.
. This illustration first appeared in 'Diamond Talk' circa 2002; it was re-oriented, with pavilion scale vertical, to match that of MSU and GIA, and shows only the area pertinent to diamond design. The upper (horizontal) dark zone represents table-to-table rays blocked by the viewer's head. In addition to the zones for the mains, which was all that most studies addressed then, this chart also shows the areas that are blocked for rays reflecting off 75% deep pavilion halves (as seen through the crown mains), which represent the greater component of light return (as noted by Yuri Shelementiev of MSU).

 

[michaelgem]

. re: last paragraph on previous page (4); sorry, I had not noticed that p.5 had begun:
. Garry: can you suggest why dispersion is most noticeable here?
. For those who don't know what "Bruce's dark zone" means. it is the region of the pavilion/crown slope chart which represents obstruction by the viewer's head, of table-to-bezel rays (and vice-versa), shown below. This edge represents 10° ray divergence between the viewer's ear and eye on the same side, which was the subject of "Faceting Limits", GIA 'Gems & Gemology', Fall 1975.
. This illustration first appeared in 'Diamond Talk' circa 2002; it was re-oriented, with pavilion scale vertical, to match that of MSU and GIA, and shows only the area pertinent to diamond design. The upper (horizontal) dark zone represents table-to-table rays blocked by the viewer's head. In addition to the zones for the mains, which was all that most studies addressed then, this chart also shows the areas that are blocked for rays reflecting off 75% deep pavilion halves (as seen through the crown mains), which represent the greater component of light return (as noted by Yuri Shelementiev of MSU).

Bruce,

Re: can you suggest why dispersion is most noticeable here?

Forgive me for jumping in, but this is one of my favorite cut topics. Let me offer an explanation from my article written about a decade ago. (Time goes by so fast at this age)

The short answer is: Fire emanates from "virtual facets" that reflect from a spot in the surrounding illumination where there is light/dark contrast. With slight movement a reflection from a virtual facet transitions from bright to dark and in between exhibits fire. So reflections from high contrast areas such as the boundary created by observer obstruction result in the display of fire in virtual facets reflecting from those areas. So not only is the contrast in the illumination from observer obstruction key to the Ideal's contrast quality of brilliance, it is key to the Ideal's superior fire as well.

That is one reason I say "the same diamond exhibiting superior brilliance in lighting conducive to brilliance will exhibit superior fire in lighting conducive to fire." Observer obstruction is a built in producer of contrast which is key in lighting that is otherwise diffuse like the sky in my silhouette image.

So, one key property of the Ideal cut is that reflections from virtual facets from its mains come from the area where there is contrast in illumination at the outer edge of the observer's head. That fact may be one key reason that first Morse and then subsequent cutters like Tolkowsky and his father empirically found a 41 pavilion and close to a 34 crown to be best.

From the article:

Observe the mosaic pattern of light reflection and dispersion resulting from the interaction of the diamond and its surrounding panorama of light. This pattern of light reflection and dispersion is described in terms of brilliance, fire and scintillation (sparkle with movement). What is the source of the large color patches in this American Ideal cut diamond photo?

The answer is fire, dispersed by the diamond from open sky daylight, which was the source of this diamond's illumination. The daylight was broken up to get this degree of fire in a manner that often happens in natural viewing environments.

The first two photos illustrates the American Ideal cut's beauty with respect to both brilliance and fire. The American Ideal possesses a superior combination of the contrast quality of brilliance and the light return quantity of brilliance in lighting favorable to these aspects of diamond beauty. The Ideal is also superior with respect to fire when the lighting is favorable to this important and often unheralded aspect of diamond beauty.

Notice that the second image has captured the seldom-present green fire as well as the more often observed, blue, yellow and red fire. Blue, yellow and red fire are present in the following, third diamond photograph.

This diamond photograph was taken under a similar open sky as was the first, but with my head and torso providing the break up of the diffuse daylight illumination enabling this display of fire. The other photo shows the sky light, partially obscured by my silhouette, illuminating this diamond.

These three diamond photographs support the importance of the discovery that the American Ideal's combination of pavilion and crown main angles is on the verge of obstruction by the viewer's head resulting in the blocking of table-to-crown-main and crown-main-to-table light rays.

Interacting with the obstruction from the viewer's head and torso, Ideal diamond pavilion and crown angle combinations result in a greater contrast quality of brilliance. Equally important, these Ideal combinations also give rise to much of the fire observed in these diamond photographs especially number three.

The reflections from the pavilion main facets in the American Ideal alternately flash from dark to bright to dark when the diamond is tilted from the normal, face-up view, giving rise to large flash scintillation and contrast brilliance. This mosaic of larger reflections is reminiscent of the larger high contrast sparkle and fire many prefer in Old European and Old Mine cut diamonds from times past.

As seen in these photos, fire flashes from these table-to-crown-main and crown-main-to-table reflections in their transition from dark to bright. The dark-bright, high contrast edges in the illumination results in the fire in these diamond photographs.

Michael D Cowing

 

[beryl]

Michael: I will review your lengthy response later and perhaps comment.
. I came just now to report that I was just in the Russian website and scanned "Rainbow in a Colorless Gem", which I reviewed for Anton after it was converted from Polish to Russian to English. It has lots of math goodies about dispersion. I had forgotten that and will review it closely, then comment here on things I find applicable. Great - like eating ice cream! URL is:
. http://www.gemology.ru/cut/english/rainbow/index.htm

 

[beryl]

Michael:
. Thanks for the explanation; it sounds great.
. What is the article you refer to?

 

[michaelgem]

Michael: I will review your lengthy response later and perhaps comment.
. I came just now to report that I was just in the Russian website and scanned "Rainbow in a Colorless Gem", which I reviewed for Anton after it was converted from Polish to Russian to English. It has lots of math goodies about dispersion. I had forgotten that and will review it closely, then comment here on things I find applicable. Great - like eating ice cream! URL is:
. http://www.gemology.ru/cut/english/rainbow/index.htm

Bruce,

Remember how I referred in my writing to your "seminal" work showing the importance of considering observer obstruction?

Well for me "Rainbow in a Colorless Gem" by Anton Vasilief was the seminal work that filled in all the blanks in my understanding of human observation of fire in diamonds and other dispersive gemstones. His brilliance, especially in this area, was not lost on other scientists, as I later learned from writing by Vladimir Oneschuk and Sergay.

You are right. Learning from the works of Anton is like eating ice cream, and its better for both mind and body.

Michael

 

[Garry H (Cut Nut)]

. re: last paragraph on previous page (4); sorry, I had not noticed that p.5 had begun:
. Garry: can you suggest why dispersion is most noticeable here?
. For those who don't know what "Bruce's dark zone" means. it is the region of the pavilion/crown slope chart which represents obstruction by the viewer's head, of table-to-bezel rays (and vice-versa), shown below. This edge represents 10° ray divergence between the viewer's ear and eye on the same side, which was the subject of "Faceting Limits", GIA 'Gems & Gemology', Fall 1975.
. This illustration first appeared in 'Diamond Talk' circa 2002; it was re-oriented, with pavilion scale vertical, to match that of MSU and GIA, and shows only the area pertinent to diamond design. The upper (horizontal) dark zone represents table-to-table rays blocked by the viewer's head. In addition to the zones for the mains, which was all that most studies addressed then, this chart also shows the areas that are blocked for rays reflecting off 75% deep pavilion halves (as seen through the crown mains), which represent the greater component of light return (as noted by Yuri Shelementiev of MSU).

Bruce,

Re: can you suggest why dispersion is most noticeable here?

Forgive me for jumping in, but this is one of my favorite cut topics. Let me offer an explanation from my article written about a decade ago. (Time goes by so fast at this age)

The short answer is: Fire emanates from "virtual facets" that reflect from a spot in the surrounding illumination where there is light/dark contrast. With slight movement a reflection from a virtual facet transitions from bright to dark and in between exhibits fire. So reflections from high contrast areas such as the boundary created by observer obstruction result in the display of fire in virtual facets reflecting from those areas. So not only is the contrast in the illumination from observer obstruction key to the Ideal's contrast quality of brilliance, it is key to the Ideal's superior fire as well.

That is one reason I say "the same diamond exhibiting superior brilliance in lighting conducive to brilliance will exhibit superior fire in lighting conducive to fire." Observer obstruction is a built in producer of contrast which is key in lighting that is otherwise diffuse like the sky in my silhouette image.

So, one key property of the Ideal cut is that reflections from virtual facets from its mains come from the area where there is contrast in illumination at the outer edge of the observer's head. That fact may be one key reason that first Morse and then subsequent cutters like Tolkowsky and his father empirically found a 41 pavilion and close to a 34 crown to be best.

From the article:

Observe the mosaic pattern of light reflection and dispersion resulting from the interaction of the diamond and its surrounding panorama of light. This pattern of light reflection and dispersion is described in terms of brilliance, fire and scintillation (sparkle with movement). What is the source of the large color patches in this American Ideal cut diamond photo?

The answer is fire, dispersed by the diamond from open sky daylight, which was the source of this diamond's illumination. The daylight was broken up to get this degree of fire in a manner that often happens in natural viewing environments.

The first two photos illustrates the American Ideal cut's beauty with respect to both brilliance and fire. The American Ideal possesses a superior combination of the contrast quality of brilliance and the light return quantity of brilliance in lighting favorable to these aspects of diamond beauty. The Ideal is also superior with respect to fire when the lighting is favorable to this important and often unheralded aspect of diamond beauty.

Notice that the second image has captured the seldom-present green fire as well as the more often observed, blue, yellow and red fire. Blue, yellow and red fire are present in the following, third diamond photograph.

This diamond photograph was taken under a similar open sky as was the first, but with my head and torso providing the break up of the diffuse daylight illumination enabling this display of fire. The other photo shows the sky light, partially obscured by my silhouette, illuminating this diamond.

These three diamond photographs support the importance of the discovery that the American Ideal's combination of pavilion and crown main angles is on the verge of obstruction by the viewer's head resulting in the blocking of table-to-crown-main and crown-main-to-table light rays.

Interacting with the obstruction from the viewer's head and torso, Ideal diamond pavilion and crown angle combinations result in a greater contrast quality of brilliance. Equally important, these Ideal combinations also give rise to much of the fire observed in these diamond photographs especially number three.

The reflections from the pavilion main facets in the American Ideal alternately flash from dark to bright to dark when the diamond is tilted from the normal, face-up view, giving rise to large flash scintillation and contrast brilliance. This mosaic of larger reflections is reminiscent of the larger high contrast sparkle and fire many prefer in Old European and Old Mine cut diamonds from times past.

As seen in these photos, fire flashes from these table-to-crown-main and crown-main-to-table reflections in their transition from dark to bright. The dark-bright, high contrast edges in the illumination results in the fire in these diamond photographs.

Michael D Cowing

Michael in a face up scenario I agree with you. However in the set up that you used in your examples, there is not much contrast / obstruction at all (I tilted 40 degrees away and 5 degrees to one side) from an observer.
In fact in this position a pavilion angle of 45.75 has a lot more fire (and blue in ASET).

 

[beryl]

. .Bruce,

Re: can you suggest why dispersion is most noticeable here?

Forgive me for jumping in, but this is one of my favorite cut topics. Let me offer an explanation from my article written about a decade ago.

Michael:
. You did not answer 'what article' ?

 

[michaelgem]

Bruce,
Re: can you suggest why dispersion is most noticeable here?
Forgive me for jumping in, but this is one of my favorite cut topics. Let me offer an explanation from my article written about a decade ago.

Michael:
. You did not answer 'what article' ?

There are two articles that I have referred to that you are asking about. The one dealing with observer obstruction and its relationship to fire and the contrast quality of brilliance is the older of the two found on acagemlab.com at:

http://www.acagemlab.com/articles/FirePower.htm

The newer article discussing the "Central Ideal" and the "sweet spot" range around it that is considered Ideal or Excellent by AGS, GIA, cutter's of Ideal and myself was published in the Gem-A's Journal of Gemmology in 2007. A pdf of that article can be downloaded from:

http://www.acagemlab.com/news/JoG07305.pdf

and a web version is at:

http://www.acagemlab.com/articles/sweetspot/index.html

This accordance article is where I make the argument as to why the early cutters of H&A Ideals logically may have settled on 77% lower half length as the best compromise between the area of the main reflections and the area of the halves. "This is the center of the sweet spot range of lower half length from 75% to 80% that retains the large flash sparkle and fire and at the same time provides a greater amount of scintillation" than the early ideal.

I also noted "The range of possible GIA ‘Excellent’ lower girdle facet lengths is 70% to 85%. Both ranges have the same 77.5% as the centre of the sweet spot of lower half length."

Another article , which appeared in Gems and Jewellery titled "The Central Ideal" can be found at:

http://www.acagemlab.com/temp/CentralIdeal093.pdf

This short summary of the “accordance in round brilliant diamond cutting” work explains the Ideal "sweet spot" for cutters in what I think is a compelling 3D graphical manner showing the positions of both the Morse and Tolkowsky's angle combinations and pointing out the key observation that:

"According to the 3-D graph of GIA-AGS light performance, a diamond cutter could obtain equivalent light performance by moving to the right on the plateau to (41.5°, 32°). Not only is equivalent light performance indicated by both GIA and AGS, but cutting to either this combination or the ‘Central Ideal’ combination of (41°, 34°) enables greater weight retention from typical diamond rough than is obtained when cutting to Tolkowsky’s theoretical angles.

Top performance with greater weight retention from the rough crystal is a big win-win in diamond cutting.”

After running into a lot of 41, 34-35 Ideals It seems that many cutters of Ideals no longer waste rough cutting the pavilion down to Tolkowsky's 40.75. That is, unless their clients are requiring exact adherence to his theoretical angles under the now questionable belief in their superiority.

Michael D Cowing

 

[michaelgem]

Re: can you suggest why dispersion is most noticeable here?

Forgive me for jumping in, but this is one of my favorite cut topics. Let me offer an explanation from my article written about a decade ago. (Time goes by so fast at this age)

The short answer is: Fire emanates from "virtual facets" that reflect from a spot in the surrounding illumination where there is light/dark contrast. With slight movement a reflection from a virtual facet transitions from bright to dark and in between exhibits fire. So reflections from high contrast areas such as the boundary created by observer obstruction result in the display of fire in virtual facets reflecting from those areas. So not only is the contrast in the illumination from observer obstruction key to the Ideal's contrast quality of brilliance, it is key to the Ideal's superior fire as well.

That is one reason I say "the same diamond exhibiting superior brilliance in lighting conducive to brilliance will exhibit superior fire in lighting conducive to fire." Observer obstruction is a built in producer of contrast which is key in lighting that is otherwise diffuse like the sky in my silhouette image.

So, one key property of the Ideal cut is that reflections from virtual facets from its mains come from the area where there is contrast in illumination at the outer edge of the observer's head. That fact may be one key reason that first Morse and then subsequent cutters like Tolkowsky and his father empirically found a 41 pavilion and close to a 34 crown to be best.

These three diamond photographs support the importance of the discovery that the American Ideal's combination of pavilion and crown main angles is on the verge of obstruction by the viewer's head resulting in the blocking of table-to-crown-main and crown-main-to-table light rays.

Interacting with the obstruction from the viewer's head and torso, Ideal diamond pavilion and crown angle combinations result in a greater contrast quality of brilliance. Equally important, these Ideal combinations also give rise to much of the fire observed in these diamond photographs especially number three.

As seen in these photos, fire flashes from these table-to-crown-main and crown-main-to-table reflections in their transition from dark to bright. The dark-bright, high contrast edges in the illumination results in the fire in these diamond photographs.

Michael D Cowing

Michael in a face up scenario I agree with you. However in the set up that you used in your examples, there is not much contrast / obstruction at all (I tilted 40 degrees away and 5 degrees to one side) from an observer.
In fact in this position a pavilion angle of 45.75 has a lot more fire (and blue in ASET).

Garry,

The illumination of the diamonds in the first two photos had a lot more contrast than just my silhouette, and consequently much more fire than appears in the last photo where only my silhouette and that of the camera provided the contrast in otherwise overcast diffuse sky light.

Michael

 

[Garry H (Cut Nut)]

Michael in a face up scenario I agree with you. However in the set up that you used in your examples, there is not much contrast / obstruction at all (I tilted 40 degrees away and 5 degrees to one side) from an observer.
In fact in this position a pavilion angle of 45.75 has a lot more fire (and blue in ASET).

Garry,

The illumination of the diamonds in the first two photos had a lot more contrast than just my silhouette, and consequently much more fire than appears in the last photo where only my silhouette and that of the camera provided the contrast in otherwise overcast diffuse sky light.

Michael

So?

 

[beryl]

. Nice articles, Michael; well-written and nicely illustrated. Thank you for the references; I have obviously been 'out-of-touch' for several years.
. You fellows are telling me 'what'; I want to know 'why'.
. Perhaps Sergey can help here.

 

[Paul-Antwerp]

Bruce,

It is a combination of reverse and forward ray-tracing which explains why the contrast-areas are important producers of observed fire.

Live long,

 

[michaelgem]

Michael in a face up scenario I agree with you. However in the set up that you used in your examples, there is not much contrast / obstruction at all (I tilted 40 degrees away and 5 degrees to one side) from an observer.
In fact in this position a pavilion angle of 45.75 has a lot more fire (and blue in ASET).

Garry,

The illumination of the diamonds in the first two photos had a lot more contrast than just my silhouette, and consequently much more fire than appears in the last photo where only my silhouette and that of the camera provided the contrast in otherwise overcast diffuse sky light.

Michael

So?

Just pointing out that, as you note, there is not much contrast/obstruction from the observer and thus not much fire in the tilted diamond when it is diffusely illuminated as in this example. When viewed face up with only slight tilting, the virtual facets pick up more contrast from observer obstruction and the diamond exhibits more fire.

Michael

 

[michaelgem]

. Nice articles, Michael; well-written and nicely illustrated. Thank you for the references; I have obviously been 'out-of-touch' for several years.
. You fellows are telling me 'what'; I want to know 'why'.
. Perhaps Sergey can help here.

The why follows from all this ‘what’ with a little science of optics from sources like “Seeing the light” page 63 combined with Anton’s article and Sergey’s posts on page 3 of this thread.

Paul is “right on” in saying: It is a combination of reverse and forward ray-tracing which explains why the contrast-areas are important producers of observed fire. I would beef up his statement and say: “virtually all observed fire results from contrast in the illumination.”

When discussing this with Bob Long (“Long and Steele, pioneers in ray-tracing and meet-point-faceting”), I was unable to convince him that the fractioning, (that Bruce wrote an elegant article describing), is at best a minor player in the production of observed fire in a gemstone. This is largely due to the fact that what Bruce has called fractioning has always been faceting "orthodoxy" concerning fire, and only heretics challenge orthodoxy.

However, Garry’s keen observations support this important point:

“I have spent 1 hour looking at my CZ’s in my usual café

The left side main facet – from the left side of facet only –first came pale red, orange then the most intensity (because the halogen globe 2.5M above the table is making the entire facet brightly illuminated) then (greenish) blue through to blue and finally fainter violet (because now the dispersion is only coming from the right side of the facet).

But if I move my head vertically, starting from the brightest central position, then as I move my head up and down, the same thing happens – red at top, bright in middle, paler blue at lowest head position.

This means there is a cone of fire emanating from this facet!
Not a strip of fire, but a cone of fire, with a bright middle section.”

If the fire Garry was observing were due to fractioning then part of the complete spectrum of the fire emanating from that virtual facet would be missing having been deflected in another direction.

The fact that virtually all the fire I have observed in diamonds, like Garry has observed, changes hue over the entire spectrum with slight movement tells us that:

"The mechanism of fire is the aperture of the human pupil intercepting and focusing on our retina particular parts of the 3D cone of spectral fanning caused by the gemstone dispersing light from spot lights especially those of small angular size, and by dispersion from other high contrast edges in illumination such as those caused by observer obstruction."

So, observation of fire is dependent not only on the dispersive property (fanning of white light into the spectrum) of the gemstone, but also on other key factors especially the angular size of the light source and the angular size of the viewers pupil.

Vladimir and Sergey have written and explained various aspects of all this, as has Anton, and my understanding comes from putting it all together. I was writing a simple way to view this using back-tracing (reverse ray-tracing) in a book on diamond cut and optics that I started before 2000.

My understanding was just beginning to gel way back then, and I have yet to finish the section on fire. Everyone contributing to this thread is helping.

Thanks to all,

Michael D Cowing

 

[michaelgem]

I hasten to add to my previous post that the fractioning that Bruce so clearly and succinctly demonstrates in his article at:

https://www.pricescope.com/journal/fractioning_color_gem/

can and sometimes does contribute to the observation of fire in a gem as Bruce, Garry and others have shown. It is just not the important contributor that those in the faceting community have believed and most still believe.

In my unpublished writing in the section on fire I cover diamond optics on fire that provides an explanation why none of the fire in my example photographs is aided by or is a result of fractioning.

Admittedly, this is hard for many to believe, but it is clear to me that Vladimir, Sergey and Anton would back me up on this.

The argument is along the lines of my comments about Garry's observations of fire in his CZ's. Problem is the argument needs the set up from previous sections concerning the use of back-tracing.

Perhaps others, maybe Sergey, will beat me to a better exposition on this. Come to think of it, if you read his previous posts carefully, in an indirect way with his ETAS images, he already has. It just takes time and effort to put it all together.

Michael The USFG Heretic

 

[Garry H (Cut Nut)]

If the light source is a strip like a thin fluoro tube and the angle of rotation or movement is prependicular to that which the virtual facet will 'see' (which is not always the same direction - it can vary by as much as 45 degrees) then we usually see blue - white - red.
If you find a bright facet and move it in a perp direction (or you) you should see blue or red before the facet 'switches off'.
If you move the stone parallel to the light the bright facet will stay bright. The colors only appear when the facet gets to the end of the tube.
So in this case the light geometry does not result in a cone of colour.

I also feel that a point light source will not create a cone of light - my CZ experiment was not that accurate Michael.

If you paly with DiamCalc and a prism you can also see how a simple faceting structure will work. You can make the prism any length and depth you wish and turn off lights or make your own.

 

[michaelgem]

If the light source is a strip like a thin fluoro tube and the angle of rotation or movement is prependicular to that which the virtual facet will 'see' (which is not always the same direction - it can vary by as much as 45 degrees) then we usually see blue - white - red.

Yes, this is clear from diagrams such as the ones you posted from Octonus and from p61 of "Seeing the Light" except that you more often see yellow rather than red at the long wavelength side of the spectrum. If you pull back the thin fluoro tube from the diamond, at a sufficient distance greater than five times its length, that source of light will have the thin angular size of a slit and the entire spectrum with no white in the middle will be observed in that virtual facet as it is tilted .

If you find a bright facet and move it in a perp direction (or you) you should see blue or red before the facet 'switches off'.
If you move the stone parallel to the light the bright facet will stay bright. The colors only appear when the facet gets to the end of the tube.
So in this case the light geometry does not result in a cone of colour.

I also feel that a point light source will not create a cone of light - my CZ experiment was not that accurate Michael.

You are right Garry, rather than a cone of colour, what is produced is more a strip of spectral colours often with white in the middle.

As seen in any of the diamond images, the shape of the flash of fire or white light emanating from and reflecting to our eyes has the shape of that virtual facet. That is not what Garry and I are discussing.

Think of the shape of the spectral fan of colours as an "aura" around a light source like the fluorescent tube, which is observed as Garry did by slight tilting in all directions around the tube-shaped white center. As Garry points out the colours only appear when the virtual facet gets to the high contrast edges of the tube, just as the colours appear at the high contrast edge between light and dark caused by observer obstruction.

Think of the relatively black silhouette in an identical manner as the relatively black surround of a round light bulb or a fluorescent tube. Both produce high contrast edges that are the occasion of fire.

Now, a key point is that when the light source, no matter what its shape, is sufficiently distant that its angular size is essentially zero then what is seen is the entire spectrum from red to violet with no white center. The direction of movement causing you to observe each spectral colour in turn has to do with the planes and angles of incidence and refraction/reflection into and out of the diamond.

So I now need to amend my statement to read:

"The mechanism of fire is the aperture of the human pupil intercepting and focusing on our retina particular parts of the spectral fanning caused by the gemstone dispersing light from spot lights especially those of small angular size, and by dispersion from other high contrast edges in illumination such as those caused by observer obstruction."

So, observation of fire is dependent not only on the dispersive property (fanning of white light into the spectrum) of the gemstone, but also on other key factors especially the angular size of the light source and the angular size of the viewers pupil.

Now I think I need something to relax my brain tension, maybe meditation or a drink.

Michael

 

[beryl]

. Words, words, words. Garry and Michael have shown that it happens but I'm still waiting for a scientific explanation (diagrams, calculations, etc.) as to WHY an edge obstruction enhances 'fire'.
. Yes, I know that 'Fractioning of Light' is not the only source of color separation; indeed, it is probably a minor one but I thought it was interesting and had never seen it discussed (ref article).
. I think Sergey's work about width of spectral beam vs width of pupil and its position was significant, but that does not explain the 'edge effect' being discussed here (not the same as the 'edge effect' discussed in "Seeing the Light").
. My brain needs a rest. I'm out of here for a while; perhaps I will check back later to see what's happened.

 

[michaelgem]

Words, words, words. Garry and Michael have shown that it happens but I'm still waiting for a scientific explanation (diagrams, calculations, etc.) as to WHY an edge obstruction enhances 'fire'.

I think Sergey's work about width of spectral beam vs width of pupil and its position was significant, but that does not explain the 'edge effect' being discussed here (not the same as the 'edge effect' discussed in "Seeing the Light").

Once one sees the light, one will see that the observer obstruction edge effect is the same mechanism as that discussed in "Seeing the Light" and Sergey's colorful diagrams.

Bruce,

I wish my words were as clear and succinct as your writing, but let me have another go at it.

Those words contain the scientific explanation, not that light/dark edges enhance fire, but rather that they are a necessary condition, along with dispersion for there to be what our vision perceives as fire in a diamond.

No light/dark contrast edges in the illumination environment, no fire, period. And this sweeping statement includes the concept of fractioning. Over ten years ago I demonstrated this by photographing a diamond inside a white hemisphere. The diamond almost disappeared and displayed no fire.

After some consideration it will be seen that there is no essential difference between the light/dark contrast between a circle, tube, or any shape light or a point of light, (One with < 1 degree of angular size such as the sun or a light bulb filament), and their relatively dark surroundings, or the light/dark contrast at the edge between the relatively dark observer's head and the bright surrounding daylight. Both types of contrast edges in the illumination result in fire if the virtual facet being observed is reflecting, refracting and transmitting from that edge in the illumination to the observer's eye.

The high contrast edge between the black silhouette and the bright sky causes fire in the same manner as does the relatively black surround at the edge of a round light bulb or a fluorescent tube. Both are high contrast edges in the illumination environment that are the occasion of fire in a dispersive gem.

As we learn from Anton's paper "Rainbow in a Colorless Gem" the only remaining condition for the observation of fire is for the fan area to exceed the pupil size at the eye location. Then only a part of the 'fan' that the pupil cuts-off would be focused on the retina giving rise to perception of a dominant spectral hue.

"Since the angular size of the sun, a lamp or a candle does not exceed 0.5 degree, the fire depends primarily on the pupil angular size. The fire of colorless gems is perfect in the sunlight and even better in illumination coming from multiple point sources, such as a chandelier with many lamps without matted shades or a candelabrum with candles. Then the number of color "fans" from a gem will increase by, as many times as there will be the sources of light. (One the other hand…) The fire from a faceted gem will be completely lost in the light of a dull day under cloudy sky."


It may not be recognized as such, but this is the scientific explanation.

Michael D Cowing

 

[Garry H (Cut Nut)]

Words, words, words. Garry and Michael have shown that it happens but I'm still waiting for a scientific explanation (diagrams, calculations, etc.) as to WHY an edge obstruction enhances 'fire'.

I think Sergey's work about width of spectral beam vs width of pupil and its position was significant, but that does not explain the 'edge effect' being discussed here (not the same as the 'edge effect' discussed in "Seeing the Light").

Once one sees the light, one will see that the observer obstruction edge effect is the same mechanism as that discussed in "Seeing the Light" and Sergey's colorful diagrams.

Bruce,

I wish my words were as clear and succinct as your writing, but let me have another go at it.

Those words contain the scientific explanation, not that light/dark edges enhance fire, but rather that they are a necessary condition, along with dispersion for there to be what our vision perceives as fire in a diamond.

No light/dark contrast edges in the illumination environment, no fire, period. And this sweeping statement includes the concept of fractioning. Over ten years ago I demonstrated this by photographing a diamond inside a white hemisphere. The diamond almost disappeared and displayed no fire.

After some consideration it will be seen that there is no essential difference between the light/dark contrast between a circle, tube, or any shape light or a point of light, (One with < 1 degree of angular size such as the sun or a light bulb filament), and their relatively dark surroundings, or the light/dark contrast at the edge between the relatively dark observer's head and the bright surrounding daylight. Both types of contrast edges in the illumination result in fire if the virtual facet being observed is reflecting, refracting and transmitting from that edge in the illumination to the observer's eye.

The high contrast edge between the black silhouette and the bright sky causes fire in the same manner as does the relatively black surround at the edge of a round light bulb or a fluorescent tube. Both are high contrast edges in the illumination environment that are the occasion of fire in a dispersive gem.

As we learn from Anton's paper "Rainbow in a Colorless Gem" the only remaining condition for the observation of fire is for the fan area to exceed the pupil size at the eye location. Then only a part of the 'fan' that the pupil cuts-off would be focused on the retina giving rise to perception of a dominant spectral hue.

"Since the angular size of the sun, a lamp or a candle does not exceed 0.5 degree, the fire depends primarily on the pupil angular size. The fire of colorless gems is perfect in the sunlight and even better in illumination coming from multiple point sources, such as a chandelier with many lamps without matted shades or a candelabrum with candles. Then the number of color "fans" from a gem will increase by, as many times as there will be the sources of light. (One the other hand…) The fire from a faceted gem will be completely lost in the light of a dull day under cloudy sky."


It may not be recognized as such, but this is the scientific explanation.

Michael D Cowing

Or said another way

"briliance is the enemy of fire"

When there very large light sources the diamond will become very bright.
On observers head blocks light, near the boundaries that Bruce defined, the darkness enables one to potentially see some fire.

In this shot I used DiamCalc Ideal-scope and turned the red light to daylight, and when we tilt the stone we can see fire in the semi dark zones that result from the dark lens area in the ideal-scope.

It was the same comment that I made regarding Michaels photo, but it must not have been clear because no one 'got it'.

 

[Garry H (Cut Nut)]

Or put another way

"in the beginning, there was darkness (from obstruction or leakage) and then there was fire, and then briliance, and then fire, and then darkness (from obstruction or leakage)"

This is what happens when you watch any facet that shows fire.
(Not all manage brightness)
Fire never comes from a bright>fire>bright from any single light source (it can from many very small sources like Marty's opin lights)

 

[Karl_K]

I do agree that virtual facets on the edge of dark zones produce observable fire but that contrast can be the lighting, head shadow, environment, and virtual facet light draw direction or leakage.
The smaller the angular size of the light source the more likely fire will be observed from the light coming from that light source returned from the diamond.
This is one reason that Garry's patented florescent tube trick increases fire and scintillation it makes the effective angular size of the light source smaller.
The tubes are interesting in that it has a smaller angular size one direction than the other and a diamond will react differently depending on which way you tilt the diamond or move your head.

I am not convinced however that no fire will exist with no external contrast because the diamond can create its own contrast based on the virtual facets, in particular leakage and virtual facet angle draw.
Even in an evenly lighted dome you will still have contrast.

 

[Karl_K]

Here is an asscher producing fire in DC gemcad iso no head.



It is more obvious in step cuts because they produce more internal contrast than an RB.
They produce fire/scintillation in very flat lighting that makes RB's look dead.
I have confirmed this both in DC and actual observations.

 

[michaelgem]

No light/dark contrast edges in the illumination environment, no fire, period. And this sweeping statement includes the concept of fractioning. Over ten years ago I demonstrated this by photographing a diamond inside a white hemisphere. The diamond almost disappeared and displayed no fire.

Anton: "Since the angular size of the sun, a lamp or a candle does not exceed 0.5 degree, the fire depends primarily on the pupil angular size. The fire of colorless gems is perfect in the sunlight and even better in illumination coming from multiple point sources, such as a chandelier with many lamps without matted shades or a candelabrum with candles. Then the number of color "fans" from a gem will increase by, as many times as there will be the sources of light. (One the other hand…) The fire from a faceted gem will be completely lost in the light of a dull day under cloudy sky."

Even though Anton's words help in my explanation, he missed the key point that even in the light of a dull day under cloudy sky I still observe fire in an Ideal cut diamond whose virtual facets pick up the contrast edge in the illumination environment due to the ever-present observer obstruction.

Michael D Cowing

 

[beryl]

. This extract from the following quote of “Rainbow in a Colorless Gem” is a point we discussed here long ago ...
“... a ray begins to disperse upon entering a prism, but this is much less than at exit.”

. Here is a quote from “Rainbow ...”, p.4 (slightly modified for easier reading; I edited it in 2002) ...
”... consider the observer’s eye focused at the surface of a prism nearest it; that is, looking at a facet. The image of that facet is formed on the eye’s retina, each point on the facet’s surface being mapped at a retina point. ... The beam leaving the prism forms a fan-shaped bundle of rays of various colors.
“... Should ALL of this fan enter the pupil of the eye, the eye’s lens will focus it into a white dot on the retina.
“... if the fan width exceeds pupil size at the eye, then only part of the fan cut off by the pupil would be focused on a dot on the retina.
“The greater the dispersion fan angle, and the smaller the pupil, the narrower is the wavelength band cut out from the complete spectrum; this results in higher saturation of the colors observed.”

. The subject of spectrum segments is discussed in detail at Fig.2 and shows why green does not appear if seeing wide sectors of the spectrum.
. The beam angle vs pupil size is nicely illustrated/discussed in later works by Sergey Sivovolenko.