Fire and dispersion techy help please?

[michaelgem]

Post by beryl » 19 Nov 2010 18:14
. What does everyone mean by the term 'virtual facet'? It is new and meaningless to me.

In my answer to Bruce I attributed the term to the GIA and their early diamond course.

Sergey sent me a polite inquiry asking: "What is date for this GIA early diamond course? Could you send link or scan copy?"

After checking, I reported to him that I was unable to find reference to virtual facets in any of the past GIA diamond courses or other literature.

Sergey wrote back explaining the reason why:

"I introduced this term during IDCC1(2004) http://www.gemology.ru/cut/english/conferens-article/7.htm

I developed VF term for ETAS.

Virtual diamond facets

A virtual facet of diamond can be explained by the overlapping the crown and pavilion facets.

A good diamond should have two types of virtual facets: small facets guarantee that at least one sparkle is always visible, and big facets reveal bright sparkles." Sergey

I want to set the record straight on this, as I believe correct attribution is important.

Michael D Cowing

 

[michaelgem]

Post by beryl » 19 Nov 2010 18:14
. What does everyone mean by the term 'virtual facet'? It is new and meaningless to me.

In my answer to Bruce I attributed the term to the GIA and their early diamond course.

Sergey sent me a polite inquiry asking: "What is date for this GIA early diamond course? Could you send link or scan copy?"

After checking, I reported to him that I was unable to find reference to virtual facets in any of the past GIA diamond courses or other literature.

Sergey wrote back explaining the reason why:

"I introduced this term during IDCC1(2004) http://www.gemology.ru/cut/english/conferens-article/7.htm

I developed VF term for ETAS.

Virtual diamond facets

A virtual facet of diamond can be explained by the overlapping the crown and pavilion facets.

A good diamond should have two types of virtual facets: small facets guarantee that at least one sparkle is always visible, and big facets reveal bright sparkles." Sergey

I want to set the record straight on this, as I believe correct attribution is important.

Michael D Cowing

 

[michaelgem]

.

Using back-tracing from the aperture of my pupil or the camera's lens, we find that due to dispersion a virtual facet reflects long wavelengths from slightly different spots than shorter wavelengths. For instance, virtual facets emanating blue-fire are reflecting/picking up the red-to-yellow end of spectral wavelengths from slightly higher angles due to less bending or refraction of longer waves. But that is where there is darkness from my silhouette, so that end of the spectrum is actually clipped off (fractioned) leaving the blue end of the spectrum whose shorter wavelengths are refracted and dispersed to a greater degree. Since the virtual facets are reflecting/picking up the blue end of the spectrum from this greater angle where there is white light the blue end of the spectrum comes to our eye as spectral fire.

This is why simple head obstruction most often produces blue fire. The reverse situation can occur around an ear or shirt collar where instead of a dark-bright transition there is a bright-dark one. Then the blue end of the spectrum is clipped leaving yellow to red fire to be observed.[/color]


Michael D Cowing

Not trying to pick on you Michael, but you know that when we see a spectral effect from an edge, that one side is blue and the other red yellow. Does this effect your point?

With a little explanation it helps make this important point, Garry.

I'm glad you have taken the time to consider it.

You can observe from ETAS that the spectral fanning coming from most virtual facets is away from the observer's head with the blue end fanned/refracted the most. So the contrast edge of the observer obstruction goes from dark to light in the direction of the fan (red to blue). The direction of fanning in your prism is roughly right to left, so the equivalent transition that produces blue fire is the same dark to light transition on the left side of the vertical black bars where you see blue fire. The yellow-red end has been clipped or split off by the "dark-side". This is why simple head obstruction most often produces blue fire, as seen in the top diamond.

The reverse situation can occur around an ear or shirt collar or anywhere that instead of a dark-bright transition there is a bright-dark one. Then the blue end of the spectrum is clipped leaving yellow to red fire to be observed. This is seen in the bottom diamond where, besides the most prevalent blue fire, there is yellow, orange and red. The equivalent transition in your prism example is the contrast edge at the right side of the bars where you have a bright-dark edge transition in the direction of fanning resulting in the blue end of the spectrum being clipped/split off by the dark-side of the contrast edge leaving the yellow to red end of the spectrum.

Michael D Cowing

 

[adamasgem]

Anisotropic effects are caused by strain, Marty I do not see this in my car windscreen without my polaroid sunny's on? but mostly it is numerous small virtual facets that result in mixing of spectral colors, inside or outside the stone, but it is observed... Possibly interferance colors. Have you ever seen this in any normal lighting situation other than your pinlight model? I never have and never heard anyone report seeing 2nd or 3rd order colors (like those you see in a garage pavement on a wet day) Or doubly refractive stones like this calcite in the smithsonian

Real old pic from archives.. bottom row combined pic with increased saturation. (Better camera doesn't require any saturation increase)

Garry..... When you see white, you are seeing color mixing; that "white" may come from a continuous spectrum or from a RGB system (three primaries)[as in your camera or TV] (your eyeball converts to RGB type and your brain mixes them again].. You see, or better yet, RESOLVE "fire", because there are large areas(angular spread" of one color reaching your eye. You can see the "fire" in a pinpoint white opal (as opposed to broadflash opal) when you are close to it, but when you move away, the colors mix, and you see white.

The 10,000 plus points of white light (8000Kelvin) is an environment that enables you to discriminate the fire, either DIRECTLY, or through a cameras lens. (what you visually see, is what you photographically get) In a diffuse environment, like you describe (your windshield), it would be very difficuly to resolve "fire".

Resolving (seeing) fire is a combination of the cutting AND the envirionment. I just created the envirionment, based on a natural analog, to be able to compare the cuttings ability to generate fire. The results are what they are, the pastels and muddy earthtones were a surprize to me, and I still think about it; all I KNOW, is that there is a direct relationship between the composite angle set combined with optical symmetry, and the ability to visually RESOLVE fire.
Your calcite example, is, my friend, not applicable.

Hi Marty, I stand corrected.
When I increase the number of lights, or as Michael and Bruce suggest, by opening the pupil more, the result in your model does occur.[/quote]

Gary... I have a fixed small pupil (camera iris) size, only illumination is pinpoints. The human eye pupil will change depending on the ambient light level, the higher the ambient light, the smaller the pupil, but the more chance for white ambient reflections to wash out (by combination) any pure hues caught by small pupil, depending on the viewing distance also.

What I've tried to demonstrate is the POTENTIAL for the diamond to produce DISCERNABLE fire. There are many variables, including internal color mixing due to ray paths comingling. Having a quazi fixed envirionment (I can rotate stone and stone size theoretically changes angles of incidence also, but my sources are at least 5 to 11 inches away from the stone since my prototype baffle is cylindrical). Intensity of source points are not equal due to geometry considerations. But , the fixed envirionment, as unaliasing as possible, allows direct comparisons of stones fire protential. And the pictures correlate well with the older (pre ASET) AGS cut classifications, surprisingly.

In my opinion, balanced angle sets and optical symmetry are most important. Virtual facets due to assymetry, SCREW IT UP, PERIOD, no matter what the angle set.

As I said, think broadflash versus pinpoint opal as you move away from the stone. The synthetic white opals, with LARGER "pinpoints of individual hues" are more as to DISCERNABLE fire , than typical white natural pinpoint, as you move away from them; they don't wash out into pure white.

 

[adamasgem]

I want to set the record straight on this, as I believe correct attribution is important.

Michael D Cowing

IT IS ABOUT TIME...

 

[Serg]

re:

And the pictures correlate well with the older (pre ASET) AGS cut classifications, surprisingly.

In my opinion, balanced angle sets and optical symmetry are most important. Virtual facets due to assymetry, SCREW IT UP, PERIOD, no matter what the angle set.

Facet flatness( polishing quality) is very important for quality and number Fire flashes.
Usually RBC with high level symmetry and "Ideal" proportions have better polishing quality( facet flatness) than RBC with "Good" proportions.
This test did not proof what diamond with high symmetry and "Ideal" proportion have best Fire.
If you want evaluate symmetry input you should not change in same time the polishing quality, proportion and cleaning

 

[adamasgem]

re:

And the pictures correlate well with the older (pre ASET) AGS cut classifications, surprisingly.

In my opinion, balanced angle sets and optical symmetry are most important. Virtual facets due to assymetry, SCREW IT UP, PERIOD, no matter what the angle set.

Facet flatness( polishing quality) is very important for quality and number Fire flashes.
Usually RBC with high level symmetry and "Ideal" proportions have better polishing quality( facet flatness) than RBC with "Good" proportions.
This test did not proof what diamond with high symmetry and "Ideal" proportion have best Fire.
If you want evaluate symmetry input you should not change in same time the polishing quality, proportion and cleaning

Hi Serg..
The "test" , if you will, merely correlated my 17 Lazare Kaplan masters with their old estimated AGS cut grade, pre ASET days.
After the holidays, I want to retake the photos with the prototype and new camera.

I have the SRN files on the stones (and would be happy to share with you) and also can get some on EightStar quality cutting regarding the symmetry issues.

You are correct that surface finish will cause some problems, as some run of the mill stones are probably only using maybe a 5 micron polish, but it is hard to model, other than assuming random angle of internal incidence (and exiting refraction), like I used in my work with bruted girdles, much worse conditions than the polished facets. Then of course, each facet has its own polishing direction, so you would have correlated furrows (polish lines, if you will) depending on the grit size used for the cutting.
So you have to make a lot of assumptions.. Not easy to do it "right", if there is such a thing.

For example , how do you model internal strain on a particular diamond, not to mention gletzes. Same type of issues, I believe.
The photos I can take are worth a thousand words, as they clearly show the strain patterns on type IIa HPHT'd IIa's.
I have three I can do, with the improved camera setup.

 

[Garry H (Cut Nut)]

re:

And the pictures correlate well with the older (pre ASET) AGS cut classifications, surprisingly.

In my opinion, balanced angle sets and optical symmetry are most important. Virtual facets due to assymetry, SCREW IT UP, PERIOD, no matter what the angle set.

Facet flatness( polishing quality) is very important for quality and number Fire flashes.
Usually RBC with high level symmetry and "Ideal" proportions have better polishing quality( facet flatness) than RBC with "Good" proportions.
This test did not proof what diamond with high symmetry and "Ideal" proportion have best Fire.
If you want evaluate symmetry input you should not change in same time the polishing quality, proportion and cleaning

Hi Serg..
The "test" , if you will, merely correlated my 17 Lazare Kaplan masters with their old estimated AGS cut grade, pre ASET days.
After the holidays, I want to retake the photos with the prototype and new camera.

I have the SRN files on the stones (and would be happy to share with you) and also can get some on EightStar quality cutting regarding the symmetry issues.

You are correct that surface finish will cause some problems, as some run of the mill stones are probably only using maybe a 5 micron polish, but it is hard to model, other than assuming random angle of internal incidence (and exiting refraction), like I used in my work with bruted girdles, much worse conditions than the polished facets. Then of course, each facet has its own polishing direction, so you would have correlated furrows (polish lines, if you will) depending on the grit size used for the cutting.
So you have to make a lot of assumptions.. Not easy to do it "right", if there is such a thing.

For example , how do you model internal strain on a particular diamond, not to mention gletzes. Same type of issues, I believe.
The photos I can take are worth a thousand words, as they clearly show the strain patterns on type IIa HPHT'd IIa's.
I have three I can do, with the improved camera setup.

Marty do you have access to any polish quality instruments like Avalon (made by HRD's tools division)?

 

[adamasgem]

Marty do you have access to any polish quality instruments like Avalon (made by HRD's tools division)?

Garry.. No automatic stuff, just Gemological Zooms and Leitz (Ultralux, Dialux) Petrographic Trinoc microscopes with very high magnification, as well as filar eyepieces and stage micrometer for measurements.

Actually never bothered looking at the various polish levels at ultra high power. Might be a pain to set up one of the petrographic scopes with a stone holder. I'd have to make a male end to fit the stone holder and use one of the existing holes on the rotating pol stage, to get >1000X with 100X objectives. Small FOV, although the <1 micron polishing would show up.

The absense or presence of cutting or polishing lines and luster, I am told by a well known cutter, ARE NOT necessarily correlated.

 

[beryl]

Sergey:
. What means 'ETAS'? Perhaps I knew but forgot.

 

[beryl]

Marty:
. When I was in microfinish business (1960) we measured surface finish below 1 microinch with Newtonian rings which were 11 microinches wide and estimated the height of the bumps as 11ths of bandwidth in the oblique light (sorry I forget the conversion to metric units - it is not clear anyway: there are 4 microfinish standards, often confused; I knew them all once). At 1/10 microinch finish everything appeared black because there was no dispersion of light in the reflection - all or nothing was reflected (you may have recognized this effect in the chrome on a new car).
. We also found thay 'lay' was important (direction of grooves). I asked a faceting machine manufacturer to make laps with grooves (ref my tests for DuPont on diamond polish) and he sent me some with circular grooves instead of radial (I had not thought to specify that). The result was humorous - the ridges were reproduced as grooves in the only facet I polished with one.
. I don't know if this has anything to do with what you and Serg are saying; perhaps it may help.

 

[beryl]

Marty:
. When I was in microfinish business (1960) we measured surface finish below 1 microinch with Newtonian rings which were 11 microinches wide and estimated the height of the bumps as 11ths of bandwidth in the oblique light (sorry I forget the conversion to metric units - it is not clear anyway: there are 4 microfinish standards, often confused; I knew them all once). At 1/10 microinch finish everything appeared black because there was no dispersion of light in the reflection - all or nothing was reflected (you may have recognized this effect in the chrome on a new car).
. We also found thay 'lay' was important (direction of grooves). I asked a faceting machine manufacturer to make laps with grooves (ref my tests for DuPont on diamond polish) and he sent me some with circular grooves instead of radial (I had not thought to specify that). The result was humorous - the ridges were reproduced as grooves in the only facet I polished with one.
. I don't know if this has anything to do with what you and Serg are saying; perhaps it may help.

SORRY FOR DUPLICATION; I AM GETTING STRANGE RESPONSE.

 

[Serg]

Hi Bruce,


"ETAS

A part form Basic Light Responses that are strongly dependant on illumination conditions we introduce a new type of BLR not dependent on illumination but describing fundamental cut properties - the possibility of a cut diamond to increase the visible part of space.

A diamond is an optical instrument that redirects light from illumination sources and distributes it into surrounding space. A diamond splits a light beam into several smaller (secondary) beams; their effective total angular size (ETAS) may be greater or smaller than that of the primary one.

An increment in number of virtual facets can result in an increase or a decrease of ETAS. While the number of facets is small enough, ETAS grows with the number.

A decrease of ETAS begins when the size of virtual facets becomes so small that blinding reduces the brightness of primary source to the level comparable to that of secondary sources.

It is important to evaluate light response of diamonds as they are perceived by the human mind. Two important effects are that light rays are not simple parallel beams, the simplification used by scientists and most ray tracing software (see fig. 7). Secondly, as we have shown, the eye is not an objective measuring device. But not only does it suffer (or enjoy) perceptual variations, it also has its own geometry of lens and receptors."
please check below link
http://www.gemology.ru/cut/english/conference_posters/4.htm

 

[Serg]

Pavilion main facets has width around 0.4mm for 1ct round diamond.
even non flatness 2microns gives curvature 0.3 degree what increase beam divergence on 0.6 degree

we see more pure spectral colors in Diamonds Fire if Dispersion angle is bigger and Beam divergence angle is less.
if Dispersion angle is less than Beam divergence angle then we can not see green color.

Usually these angles are around 0.5-3 degree. So additional 0.6 degree could significantly reduce pure color in diamond Fire

 

[ChunkyCushionLover]

Pavilion main facets has width around 0.4mm for 1ct round diamond.
even non flatness 2microns gives curvature 0.3 degree what increase beam divergence on 0.6 degree

we see more pure spectral colors in Diamonds Fire if Dispersion angle is bigger and Beam divergence angle is less.
if Dispersion angle is less than Beam divergence angle then we can not see green color.

Usually these angles are around 0.5-3 degree. So additional 0.6 degree could significantly reduce pure color in diamond Fire

Serg,

I was hoping you could answer a couple of questions:

1) Do you have examples to isolate the variable of polish in comparison diamonds under fire friendly lighting conditions that could confirm your approximations?

The argument has been made that slight differences in polish or symmetry do not contribute significantly to differences in fire, but I have seen no comprehensive study exploring the significant/insignificant border.

2) What is the correlation (if any) between the Intensity Weighted Dispersion Statistics Diagram and the observation of fire?

 

[ChunkyCushionLover]

Gary... I have a fixed small pupil (camera iris) size, only illumination is pinpoints. The human eye pupil will change depending on the ambient light level, the higher the ambient light, the smaller the pupil, but the more chance for white ambient reflections to wash out (by combination) any pure hues caught by small pupil, depending on the viewing distance also.

What I've tried to demonstrate is the POTENTIAL for the diamond to produce DISCERNABLE fire. There are many variables, including internal color mixing due to ray paths comingling. Having a quazi fixed envirionment (I can rotate stone and stone size theoretically changes angles of incidence also, but my sources are at least 5 to 11 inches away from the stone since my prototype baffle is cylindrical). Intensity of source points are not equal due to geometry considerations. But , the fixed envirionment, as unaliasing as possible, allows direct comparisons of stones fire protential.

And the pictures correlate well with the older (pre ASET) AGS cut classifications, surprisingly.

Pretty similar to these as well.



Peter Yantzer and Jason Quick, April 2010 All About 'Fire' Presentation

If there was a diamcalc .hdr or .dmc with this lighting I'd love to model diamonds I have been tweaking under this lighting.

In my opinion, balanced angle sets and optical symmetry are most important. Virtual facets due to assymetry, SCREW IT UP, PERIOD, no matter what the angle set.

Could you please explain this correlation between fire and optical symmetry. (Please define fire as well so we are on the same page).

I haven't seen proof of a direct correlation between optical symmetry and dispersion. But it does have an impact on the size, shape and pattern of the Virtual Facets. Please clarify.

 

[ChunkyCushionLover]

• Fire is not the same as dispersion. Fire is the
appearance of colored flashes (chromatic flares).
• Light rays of different colors, coming in at the
same point and direction, can take different paths
through a polished diamond.
• Fire is influenced by a diamond’s proportions in
four ways:
ο The angle that light enters a diamond
ο The angle that light exits a diamond
ο The number of facet interactions (bounces)
the light has inside the diamond
ο The number of times that light rays spread
across facet junctions

http://www.gia.edu/research-resources/cut-microsite-pdfs/diamond-optics-part-2.pdf

Anyone agree or disagree with the above?

AGSL beleives that Fan Clipping at facet junctions has a negligible effect on the observation of fire in an 'Ideal' Tolk Round but it is as of yet not proven by strong factual support.

 

[beryl]

CCL:
. What mean the f-no's under the pics? Are those camera f-stops?

 

[ChunkyCushionLover]

Yes they are camera f stops which should correspond to the lense diameter underneath when used on a full frame camera. (I never checked this though)

 

[Serg]

Pavilion main facets has width around 0.4mm for 1ct round diamond.
even non flatness 2microns gives curvature 0.3 degree what increase beam divergence on 0.6 degree

we see more pure spectral colors in Diamonds Fire if Dispersion angle is bigger and Beam divergence angle is less.
if Dispersion angle is less than Beam divergence angle then we can not see green color.

Usually these angles are around 0.5-3 degree. So additional 0.6 degree could significantly reduce pure color in diamond Fire

Serg,

I was hoping you could answer a couple of questions:

1) Do you have examples to isolate the variable of polish in comparison diamonds under fire friendly lighting conditions that could confirm your approximations?

The argument has been made that slight differences in polish or symmetry do not contribute significantly to differences in fire, but I have seen no comprehensive study exploring the significant/insignificant border.

2) What is the correlation (if any) between the Intensity Weighted Dispersion Statistics Diagram and the observation of fire?

CCL,
reo you have examples to isolate the variable of polish in comparison diamonds under fire friendly lighting conditions that could confirm your approximations?

You can do next simple tests.
Take diamond with perfect polishing and CZ with bad polishing.
compare sun reflection on paper ( caustics )for these objects. If distance between paper and object small then Fire flashes have similar brightness for diamond and CZ.
if distance between paper and Objects become big( 1meter) then CZ color flashes become much more dull than Diamond flashes( because flashes become bigger ).
I see two reason for this phenomena: CZ has bigger dispersion and worse facet flatness, what increase beam divergence and reduce flash illumination

try do same test for two diamonds with good and bad diamonds.

 

[ChunkyCushionLover]

Pavilion main facets has width around 0.4mm for 1ct round diamond.
even non flatness 2microns gives curvature 0.3 degree what increase beam divergence on 0.6 degree

we see more pure spectral colors in Diamonds Fire if Dispersion angle is bigger and Beam divergence angle is less.
if Dispersion angle is less than Beam divergence angle then we can not see green color.

Usually these angles are around 0.5-3 degree. So additional 0.6 degree could significantly reduce pure color in diamond Fire

Serg,

I was hoping you could answer a couple of questions:

1) Do you have examples to isolate the variable of polish in comparison diamonds under fire friendly lighting conditions that could confirm your approximations?

The argument has been made that slight differences in polish or symmetry do not contribute significantly to differences in fire, but I have seen no comprehensive study exploring the significant/insignificant border.

2) What is the correlation (if any) between the Intensity Weighted Dispersion Statistics Diagram and the observation of fire?

CCL,
reo you have examples to isolate the variable of polish in comparison diamonds under fire friendly lighting conditions that could confirm your approximations?

You can do next simple tests.
Take diamond with perfect polishing and CZ with bad polishing.
compare sun reflection on paper ( caustics )for these objects. If distance between paper and object small then Fire flashes have similar brightness for diamond and CZ.
if distance between paper and Objects become big( 1meter) then CZ color flashes become much more dull than Diamond flashes( because flashes become bigger ).
I see two reason for this phenomena: CZ has bigger dispersion and worse facet flatness, what increase beam divergence and reduce flash illumination

try do same test for two diamonds with good and bad diamonds.

Serg your test does not isolate polish (RI difference in the materials, cut difference etc) and is not robust enough to be applicable for consideration of the finer nuances between two real diamonds. Your explanation above is plausible but the proof you have not provided.

You also ignored the second question which is unrelated and is a question about DC.

 

[Serg]

CCL,
seems you did not understand my explanation. my be it was bad explanation. Just try and think about reasons

 

[Serg]

reYou also ignored the second question which is unrelated and is a question about DC.

I do not comment new options in DC beta versions. In my new DC version result is different but it is not answer to your question about DC.

 

[Serg]

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.

AGS Dispersion Diagram does not account Beam intensity . VF what has highest dispersion but low intensity have highest rate in AGS Diagram
In DC "ntensity Weighted Dispersion"diagram such VF has low rate.

 

[adamasgem]

Gary... I have a fixed small pupil (camera iris) size, only illumination is pinpoints. The human eye pupil will change depending on the ambient light level, the higher the ambient light, the smaller the pupil, but the more chance for white ambient reflections to wash out (by combination) any pure hues caught by small pupil, depending on the viewing distance also.

What I've tried to demonstrate is the POTENTIAL for the diamond to produce DISCERNABLE fire. There are many variables, including internal color mixing due to ray paths comingling. Having a quazi fixed envirionment (I can rotate stone and stone size theoretically changes angles of incidence also, but my sources are at least 5 to 11 inches away from the stone since my prototype baffle is cylindrical). Intensity of source points are not equal due to geometry considerations. But , the fixed envirionment, as unaliasing as possible, allows direct comparisons of stones fire protential.

And the pictures correlate well with the older (pre ASET) AGS cut classifications, surprisingly.

Pretty similar to these as well.



Peter Yantzer and Jason Quick, April 2010 All About 'Fire' Presentation

If there was a diamcalc .hdr or .dmc with this lighting I'd love to model diamonds I have been tweaking under this lighting.

In my opinion, balanced angle sets and optical symmetry are most important. Virtual facets due to assymetry, SCREW IT UP, PERIOD, no matter what the angle set.

Could you please explain this correlation between fire and optical symmetry. (Please define fire as well so we are on the same page).

I haven't seen proof of a direct correlation between optical symmetry and dispersion. But it does have an impact on the size, shape and pattern of the Virtual Facets. Please clarify.

Fire is, to me, the ability of the stone to separate pure spectral hues such that they are easily resolved by the viewer. This, of course. is environment dependent.

Virtual facets tend to make the areas of pure spectral hues smaller, and thus less resolvable by the viewer.

The virtual facet pattern will be directly a function of the refractive index of the stone at a particular wavelength. You need dispersion in a stone (the difference in refractive index between two specified Fraunhofer lines, mineralogical and gemological definitions are different) to create the separation of spectral hues (in a non absorbing medium).

Here is a tiny technical rub to ponder.. the cutter of a hearts and arrow pattern type stone tries to optimize the pattern for HIS VIEWER, therefore, technically the SAME pattern produced with a RED contrast viewer will result in a PHYSICALLY stone different than one produced with a BLUE contrast viewer (caveat of both stones having the same diameter and same viewer diameter). This is due to dispersion, the difference between the refractive index at one end of the visable spectrum, versus the other end of the spectrum.

With a fixed size viewer of the same color contrast, two stones of different diameters, having the same visable pattern, will have different angle sets.

Nothing is simple..

 

[Garry H (Cut Nut)]

Fire is, to me, the ability of the stone to separate pure spectral hues such that they are easily resolved by the viewer. This, of course. is environment dependent.

Virtual facets tend to make the areas of pure spectral hues smaller, and thus less resolvable by the viewer.

Hi Marty,
I just did some experiments.
There appears to be less very bright fire seen from main facets - most of the activity comes from the upper girdle facets when (Master Stones 7 and 12) are viewed from about 25cm in LBox with X 5 degrees and Y 3 degrees of rotation.
The viewing environment is one that slightly exaggerates fire with thin slits of light from 5 fluoro tubes with a dark ceiling.

I examined several LBox movies (as they palyed as well as the individual photo jpegs) and it seems from the camera distance of about 20cm that my observations are well supported.
From 60cm away it is harder to resolve where the fire is coming from, but it seems to be more balanced between table and crown facets.

 

[Paul-Antwerp]

Stone-problem, maybe?

 

[ChunkyCushionLover]

Fire is, to me, the ability of the stone to separate pure spectral hues such that they are easily resolved by the viewer. This, of course. is environment dependent.

Virtual facets tend to make the areas of pure spectral hues smaller, and thus less resolvable by the viewer.

Hi Marty,
I just did some experiments.
There appears to be less very bright fire seen from main facets - most of the activity comes from the upper girdle facets when (Master Stones 7 and 12) are viewed from about 25cm in LBox with X 5 degrees and Y 3 degrees of rotation.
The viewing environment is one that slightly exaggerates fire with thin slits of light from 5 fluoro tubes with a dark ceiling.

I examined several LBox movies (as they palyed as well as the individual photo jpegs) and it seems from the camera distance of about 20cm that my observations are well supported.
From 60cm away it is harder to resolve where the fire is coming from, but it seems to be more balanced between table and crown facets.

That observation is exactly what I would expect.

A lot of focus has been on the mains, but the mains of a Tolk even upon tilt show very little intrinsic dispersion. (See dispersion diagrams above). I also would expect to see greater frequency and intensity of fire from a facet that can at most viewing environments still gather and return light instead of only at some positions.

The Tolk round has its own intrinsic potential for displaying fire, it doesn't require external "Beam Splitting" factors like your head partially clipping a light source in order to show colored flashes.

I would expect the potential for fire is much greater on the UGFs or Bezels(various pavilion sources) or from the LGFs up through the table.

The focus may be on the mains as they are a large VF and produce a large flash and when colored light hits them they can return big colorful flashes but I see the potential for a greater frequency of fire being available from other facets.

 

[beryl]

CCL:
. The subject of 'beam-splitting', if you mean 'spectrum splitting' (which I had called 'fractioning'), has nothing to do with viewer's head clipping off part of a light beam, which is a different subject Michael Cowing brought up.

 

[Garry H (Cut Nut)]

Stone-problem, maybe?

http://www.octonus.com/oct/mss/diam12.phtml
http://www.octonus.com/oct/mss/diam7.phtml
The same results for both Paul