Fire Dispersion

[treasurehunter]

Fire or Dispersion is depended on the main crown angle of the diamond or is it depended on the angle and number of individual star and bezel facets relative to the girdle of the diamond ?
Do pavilion angles affect the fire or is it mainly crown angle?

 

[kenny]

Crown angle, pavilion angle, depth % and table % all interact.

That's why, for round brilliant shape, the HCA is so brilliant.

https://www.pricescope.com/tools/hca

It takes those 4 measurements, does the math, and spits out a score.

Reject rounds that score over 2.0, and get an Idealscope image on those scoring under 2.0.
https://www.pricescope.com/tools/ideal-scope

Compare IS image to Reference chart. http://www.ideal-scope.com/1.using_reference_chart.asp



If the shape is not round use an ASET image or scope to evaluate light performance.

http://www.ideal-scope.com/1.using_ASET_scope.asp

 

[Rhino]

Actually when it comes to fire and dispersion the pavilion angle is the key player. Kenny, in the IS chart below the shallow and very shallow examples, while saying "good" and "poor" are referring to the diamonds optics in diffuse lighting. In spot lighting however those 2 would be blazing with fire and dispersion and a very solid IS image.

When you however take into account the optics observable where you are not going to see fire and dispersion, ala diffuse lighting environments THEN those crown angles become more important and what they are is then dependent upon the factors Kenny mentions albeit the HCA technology has it's weakness' as well.

 

[treasurehunter]

Thanks Rhino
I was actually wondering specifically on how the extra crown height of the AVC affects the fire, so what you are saying is it therefore effects the fire more in diffused lighting and has less effect than in spot lighting

 

[treasurehunter]

Is there any chart which references fire?

 

[Paul-Antwerp]

Actually, the idea that one will observe more Fire in a diamond with a higher, steeper crown angle is one of those incorrect, at best incomplete, notions floating around in the diamond-industry for decades now. The sad fact that so many assessment-tools try to give a score for 'Fire', while they cannot.

In fact, more observed Fire depends largely on the precision of the Cut, obviously in a secondary way dependent upon the light return being present. Third major factor is the size of the stone, a bigger stone with exactly the same Cut will show more Fire.

Live long,

 

[teobdl]

[URL='https://www.pricescope.com/community/threads/fire-and-dispersion-techy-help-please.18987/']https://www.pricescope.com/community/threads/fire-and-dispersion-techy-help-please.18987/[/URL]
[URL='https://www.pricescope.com/community/threads/which-is-more-important-fire-or-brightness.190743/']https://www.pricescope.com/community/threads/which-is-more-important-fire-or-brightness.190743/[/URL]

 

[OoohShiny]

https://www.pricescope.com/community/threads/fire-and-dispersion-techy-help-please.18987/
[URL='https://www.pricescope.com/community/threads/which-is-more-important-fire-or-brightness.190743/']https://www.pricescope.com/community/threads/which-is-more-important-fire-or-brightness.190743/[/URL]

Thanks for these links I like the link within the threads that you posted here:

For future readers of the thread, a deleted post led me to find Peter Yantzer and the AGS lab's research on these topics; specifically, how cut proportions affect brilliance, fire, and scintillation. It has been the single most helpful article I've ever read on diamond light performance, and it addressed every discussion point of this thread.

http://www.agslab.com/spie/spie_lo_res.pdf

Given that Serg and Garry received acknowledgement at the end of the article, I'm a bit disappointed that we were not directed to this paper in the first place. It has taken a lot of the mystery out of the fuzzy notions about proportions and light performance that get thrown around here.

It's a great PDF!


The discussion makes my head hurt, though, I will never understand all the technical details! lol

 

[Karl_K]

Actually, the idea that one will observe more Fire in a diamond with a higher, steeper crown angle is one of those incorrect, at best incomplete, notions floating around in the diamond-industry for decades now. The sad fact that so many assessment-tools try to give a score for 'Fire', while they cannot.

In fact, more observed Fire depends largely on the precision of the Cut, obviously in a secondary way dependent upon the light return being present. Third major factor is the size of the stone, a bigger stone with exactly the same Cut will show more Fire.

Live long,

define more..
larger vf's of a larger diamond are more likely to show white light than smaller and med. virtual facets across a wide range of lighting on the other hand large vf's produce larger fire events...so saying larger diamonds show more fire is also incomplete.

Cut precision can help because there are fewer ineffective virtual facets however a well cut diamond can be returning white light hiding fire that a less well cut would show in that lighting.

I do agree that our understanding of fire is incomplete and no existing metric works for measuring it.

The real bottom line is lighting lighting lighting!!
You see in some lighting what Paul said is 100% correct more fire events from a larger diamond in others I would say it can reverse.

 

[Paul-Antwerp]

Actually, the idea that one will observe more Fire in a diamond with a higher, steeper crown angle is one of those incorrect, at best incomplete, notions floating around in the diamond-industry for decades now. The sad fact that so many assessment-tools try to give a score for 'Fire', while they cannot.

In fact, more observed Fire depends largely on the precision of the Cut, obviously in a secondary way dependent upon the light return being present. Third major factor is the size of the stone, a bigger stone with exactly the same Cut will show more Fire.

Live long,

define more..
larger vf's of a larger diamond are more likely to show white light than smaller and med. virtual facets across a wide range of lighting on the other hand large vf's produce larger fire events...so saying larger diamonds show more fire is also incomplete.

I wonder on what you base this statement, Karl.

And while talking about this can we please clearly separate the fact of dispersion from the possibility of observing fire?

Live long,

 

[teobdl]

Karl--I don't quite follow the rationale that larger VF's are more likely to show white light than smaller VF's.

But to try, in the scenario you described, is a larger VF more likely to produce white light than a smaller or med vf's because the smaller facet clips a part of the ray of light which only allows certain wavelengths to continue on the path (for example, the red part of the spectrum), so the eye sees a red flash; versus with larger vf's, more wavelengths are allowed to continue in the path toward the eye, so the eye will see the fuller spectrum of visible light: in this case, white light?

 

[Karl_K]

Karl--I don't quite follow the rationale that larger VF's are more likely to show white light than smaller VF's.

Take an rb and view it in a lot of lighting conditions and you will notice fire coming from outside the table and white light return from the center at the same time.
If you go through the pictures on PS you will see this time and time again.
The reason is the light returned from outside the table(small and med vf) is generally bent more than the in and out light from under the table therefore more likely to be split into fire.
This happens with other cuts also.
A long EC will show this effect even more so, fire from the corners and white light return from the center long narrow virtual facets.

 

[Karl_K]

Go through this thread of fire pictures, where is the vast majority of fire coming from on the diamonds?
[URL='https://www.pricescope.com/community/threads/the-fire-thread.77649/']https://www.pricescope.com/community/threads/the-fire-thread.77649/[/URL]

 

[Dancing Fire]

Actually, the idea that one will observe more Fire in a diamond with a higher, steeper crown angle is one of those incorrect, at best incomplete, notions floating around in the diamond-industry for decades now. The sad fact that so many assessment-tools try to give a score for 'Fire', while they cannot.

In fact, more observed Fire depends largely on the precision of the Cut, obviously in a secondary way dependent upon the light return being present. Third major factor is the size of the stone, a bigger stone with exactly the same Cut will show more Fire.

Live long,

Paul
but the most fiery stones I have had seen are the ones with a steeper crown + smaller tables. these type of stones been stuck in my head ever since. Right/wrong I don't know...

 

[Dancing Fire]

Go through this thread of fire pictures, where is the vast majority of fire coming from on the diamonds?
[URL='https://www.pricescope.com/community/threads/the-fire-thread.77649/']https://www.pricescope.com/community/threads/the-fire-thread.77649/[/URL]

Stormy ...where does the majority of fire come from in an Octavia?

 

[Rhino]

Hey Paul,

Not discounting precision and I hear what you're saying about taller crowns however I find that as long as the pavilion angles are either cut proper or even shallow fire will abound.

hi treasurehunter,

No charts that I know of and for good reason too as the chart would have to be akin to the HCA and also include lower half length. You see ... the subject of "fire" itself is relevant to the size of the flash and dispersive light exiting the diamond in the spot lighting environment and is directly tied into the characteristic of sparkle scintillation.

Just taking standard round brilliants as an example. Say you had two diamonds with what we'd consider Superior Optical Symmetry, both had optics (brightness, patterned scintillation, fire and sparkle scintillation) that fall within both GIA Ex's and AGS Ideal's grade (confirmed by their own software algorithms of course) and had ASET's similar to the two diamonds below.



While both of these diamonds have Ex/Ideal optics the nature of their fire and sparkle scintillation differ. The round with the 75% lower halves will show fewer but larger flashes of fire while the round with the 85% lower halves will show flashes of fire not as broad as the other but compensate with more pin fire flash. They represent two personalities of beautiful diamond yet with varying optics.

Then there are diamonds on the market that take these characteristics to their extremes. Ideal cut OEC's or those that have would I would consider ideal optics in the OEC (Old European Cut) will show the largest flashes of fire possible and on the other end of the spectrum there are diamonds like the Star129 which eliminates broad flash but intensifies pin fire flash like no other.

Fire/dispersion and sparkle scintillation are not the easiest to predict but when you've got sufficient data it can be enough to show you the elements that will comprise a diamonds particular personality. Personally I like video myself.

Kind regards,
Rhino

 

[Karl_K]

Go through this thread of fire pictures, where is the vast majority of fire coming from on the diamonds?
[URL='https://www.pricescope.com/community/threads/the-fire-thread.77649/']https://www.pricescope.com/community/threads/the-fire-thread.77649/[/URL]

Stormy ...where does the majority of fire come from in an Octavia?

Well, considering how small the table is I would say 99% outside the table from med VF's.
There is far more to it and its by design but it would be self promotion to get into it here.

 

[Paul-Antwerp]

Karl--I don't quite follow the rationale that larger VF's are more likely to show white light than smaller VF's.

Take an rb and view it in a lot of lighting conditions and you will notice fire coming from outside the table and white light return from the center at the same time.
If you go through the pictures on PS you will see this time and time again.
The reason is the light returned from outside the table(small and med vf) is generally bent more than the in and out light from under the table therefore more likely to be split into fire.
This happens with other cuts also.
A long EC will show this effect even more so, fire from the corners and white light return from the center long narrow virtual facets.

Seriously doubt that this is correct, Karl.

 

[teobdl]

To perceive colored light, there are two essential ingredients: 1. light is dispersed and 2. a portion of the fanned visible light spectrum is clipped so the eye sees just the red part, or yellow, or blue, or a combination (yellow through blue = green). If no portion of the light spectrum is clipped, the eye sees white.

If I'm not mistaken, fire coming from the crown in those photos is much less a function of the size of the vf's (if at all) than it is a function of the degree of angular dispersion allowed by the crown angle vs the table's angle relative to the ray of light being directed back at the eye.

That the amount of dispersion is a function of the angle of incidence can be seen on page 3 here: https://www.cis.rit.edu/class/simg232/lab2-dispersion.pdf

 

[Serg]

Flash Lifetime is very critical for human fire perception .
Usually small VF's produce only short lifetime flashes( not because these VF's are small, but for same reason why these VF's are small)
when Big VF's produce mainly long lifetime flashes.

small VF's produce color flashes more often but with shorter life time than bigger VF's.( small VF's usually have faster then big VF's)

if VF's are too small and too fasts then Human has not enough time to recognise color flash and to receive positive emotions.
If VF's too big and slow then consumer see Color flashes rarely and he is again unhappy.
certain size and speed of VF's are important to produce optimal quantity flashes with certain lifetime( to receive best subjective perception , to receive biggest Illusion phenomena of increasing number and lifetime for color flashes: If flash big , bright enough then brain see it after it disappear in diamond)

 

[Karl_K]

Flash Lifetime is very critical for human fire perception .
Usually small VF's produce only short lifetime flashes( not because these VF's are small, but for same reason why these VF's are small)
when Big VF's produce mainly long lifetime flashes.

small VF's produce color flashes more often but with shorter life time than bigger VF's.( small VF's usually have faster then big VF's)

if VF's are too small and too fasts then Human has not enough time to recognise color flash and to receive positive emotions.
If VF's too big and slow then consumer see Color flashes rarely and he is again unhappy.
certain size and speed of VF's are important to produce optimal quantity flashes with certain lifetime( to receive best subjective perception , to receive biggest Illusion phenomena of increasing number and lifetime for color flashes: If flash big , bright enough then brain see it after it disappear in diamond)

100% agree.

 

[Karl_K]

To perceive colored light, there are two essential ingredients: 1. light is dispersed and 2. a portion of the fanned visible light spectrum is clipped so the eye sees just the red part, or yellow, or blue, or a combination (yellow through blue = green). If no portion of the light spectrum is clipped, the eye sees white.

If I'm not mistaken, fire coming from the crown in those photos is much less a function of the size of the vf's (if at all) than it is a function of the degree of angular dispersion allowed by the crown angle vs the table's angle relative to the ray of light being directed back at the eye.

That the amount of dispersion is a function of the angle of incidence can be seen on page 3 here: https://www.cis.rit.edu/class/simg232/lab2-dispersion.pdf

This is hard to see on an image that will fit the board trace the light from where it enters on the left to where it exits on the right.
notice the difference in how they are bent.
From there you have to understand how VF's are created, they are an interaction of the real facets, small and medium virtual facets have more interactions and more light paths.

 

[Paul-Antwerp]

To perceive colored light, there are two essential ingredients: 1. light is dispersed and 2. a portion of the fanned visible light spectrum is clipped so the eye sees just the red part, or yellow, or blue, or a combination (yellow through blue = green). If no portion of the light spectrum is clipped, the eye sees white.

If I'm not mistaken, fire coming from the crown in those photos is much less a function of the size of the vf's (if at all) than it is a function of the degree of angular dispersion allowed by the crown angle vs the table's angle relative to the ray of light being directed back at the eye.

That the amount of dispersion is a function of the angle of incidence can be seen on page 3 here: https://www.cis.rit.edu/class/simg232/lab2-dispersion.pdf

This is hard to see on an image that will fit the board trace the light from where it enters on the left to where it exits on the right.
notice the difference in how they are bent.
From there you have to understand how VF's are created, they are an interaction of the real facets, small and medium virtual facets have more interactions and more light paths.

Does this still make sense if we do not consider the extraordinary starting-position of light entering perpendicular to the table?

Live long,

 

[teobdl]

AGS has already done the work.

http://www.agslab.com/spie/spie_lo_res.pdf
Page 15, section 6.2 Fire Maps
"Figures 27 and 28 show matrices for reverse and forward fire maps, respectively. The reverse fire map shows the potential to observe a light source given that the chromatic flare (the spread of light into a spectrum) covers a larger area in the hemisphere, which increases the probability of intersecting a given light source. The forward fire map shows the potential to observe fire given that the larger the chromatic flare, the larger the probability is that the eye’s pupil would clip and capture light from a facet. Forward fire maps are considered the main tool to assess the gem’s intrinsic fire potential (without considering the source distribution and observer).
Both reverse and forward fire maps maintain the trade- off between crown and pavilion angles along the cutter’s lines. The stones that exhibit significant fire are along the main cutter’s line. These stones have the property of exhibiting relatively high dispersion in the table. Other stones that have high dispersion are in the lower-left and top-right corners of the fire matrices. However, these stones suffer from not having high dispersion in the table.

The fire matrices show that the old diamond-cutting industry rule of increasing the crown angle to achieve more fire is not necessarily true. It only significantly applies to stones at or near the main cutter’s line or about lines passing by other regions such as the left-bottom and top-right corners.

Fire results from higher dispersion and this mainly happens in the stone bezel. Table fire is rare and stones with significant bezel and table fire are considered superior. Some stones that exhibit significant fire are designed with a small table to maximize the bezel area and therefore fire.

Figures 29 and 30 show some matrices for reverse and forward fire maps upon secondary refraction, respectively. The matrix for reverse fire exhibits significantly more dispersion than the matrix for forward fire. However, in both matrices the stones near or at the main cutter’s line appear with significant fire potential."

Figure 50 also shows a real life example, which may be easier to grasp.

I think Paul's point about Karl's model w/ light entering solely perpendicular to the table is key. AGS's model considers the aggregate result of a distributed source, and it clearly shows that, on the whole, more fire is observed coming from the crown facets, not from under the table Karl's more limited model suggests.

I see Serg's point about lifetime of "fire" events, though, and also agree.

But I still don't quite understand how the bigger size of a vf would create less dispersion ("chromatic flare"). Is this what you are saying? or is your point that smaller vf's will show more "fire events", as Sergey wrote?

 

[Karl_K]

Does this still make sense if we do not consider the extraordinary starting-position of light entering perpendicular to the table?

Live long,

Even more so Paul.
Here is a strong off axis fire flash as seen in many of the pics in the thread above.

 

[Karl_K]

But I still don't quite understand how the bigger size of a vf would create less dispersion ("chromatic flare"). Is this what you are saying? or is your point that smaller vf's will show more "fire events", as Sergey wrote?

There are a ton of holes in the ags fire metric, it has been discussed before and is to much to go into right now. There is no fire metric that actually works in the real world.

Larger VF's will return white light at times when smaller VF's are returning fire.
They also show quicker fire events. ie: they flash faster.

The more interactions and greater light bending of small and med VF's are more likely to display fire.
For the mains in a near tolk. to flash fire the light type and location has to be just right which is why you rarely see it in the show the fire thread linked above.

 

[Serg]

Angular dispersion for ray depends from angle of prism

For Tolkowsky round cut, angle of prism is 18,5 degree for rays Crown-Table( table-crown) and 16 degree for table-table
https://www.dropbox.com/s/rddqhidjl10cyzo/Screenshot 2014-05-24 23.11.29.png

increasing of Pavilion angle increases angle of Table-Table prism and decreases angle of Crown-Table prism.
to increase ability Crown-Table prism create fire you have to increase crown angle.

 

[Rhino]

Angular dispersion for ray depends from angle of prism

For Tolkowsky round cut, angle of prism is 18,5 degree for rays Crown-Table( table-crown) and 16 degree for table-table
https://www.dropbox.com/s/rddqhidjl10cyzo/Screenshot 2014-05-24 23.11.29.png

increasing of Pavilion angle increases angle of Table-Table prism and decreases angle of Crown-Table prism.
to increase ability Crown-Table prism create fire you have to increase crown angle.

This looks like a Pacman.

But yes ... a +1 on everything you've said Serg.

 

[teobdl]

That's a great graphic. Hadn't ever thought to represent light rays like that. Very intuitive.

 

[Paul-Antwerp]

Does this still make sense if we do not consider the extraordinary starting-position of light entering perpendicular to the table?

Live long,

Even more so Paul.
Here is a strong off axis fire flash as seen in many of the pics in the thread above.

But does your choice now for light entering perpendicular to the crown make this statement more valid? This direction of light is just as extraordinary as the often-used perpendicular to the table.

Live long,