AGS ASET 40.768 Pavilion = ''On the ledge''

[JohnQuixote]

A little FYI for those following the ASET image peculiarity.

Yesterday I asked Brian Gavin to look at images and the chart for 57 table that shows the consistent change from green to red in the eye of table at 40.768. Upon seeing it he accurately assessed it as a table reflection dynamic. He also pointed out the fact that GIA has long used this area to assess LGF construction, and that cutters routinely use this area when cutting to assess other optical properties. (I should have asked him in the first place).

AGS sent a very helpful graphic that confirmed this: At a pavilion angle of 40.768 degrees the area in the center of the diamond (the ''eye'' as we''ve called it) is receiving light from 45 degrees. When the pavilion angle decreases to less than 40.768 degrees the table reflection area now receives its light from below 45 degrees which is the green area of the ASET. When increased to above 40.768 the light is received from the red area which is 45 to 75 degrees in the ASET.

What you are noticing is a pavilion combination that is transitioning at the 45 degree mark. James Caudill calls these stones "on the ledge."

(Copyright, AGS 2005 - Used with permission)

 

[JohnQuixote]

Micro example of ASET table for 57% table, illustrating where the 'ledge' occurs.

(Copyright AGS 2005, used with permission)

 

[JohnQuixote]

Examples to show the difference in the ''eye'' for diamonds on either side of the ledge. These are not matched diamonds in any sense - the only purpose here is visible differences in the eye.

(Copyright AGS 2005, used with permission)

 

[JohnQuixote]

For sake of argument (actually, to save argument) I want to strongly express this:

As stated previously, green is not a negative, depending on the size and distribution of areas. In many cases, as in this one, it is one of two vehicles that enhances contrast - the other being escape, or ‘leakage.’ Additionally, AGS is noting many very intriguing things taking place in this area that have to do with dispersion.

So, what's the point John?

In a positive or negative sense, nada. I do think we will revisit this as AGS moves forward in the development of their baseline for a meaningful measure of scintillation.

For now I just think the 'ledge' factoid is way cool.

 

[oldminer]

John: Very intriguing stuff.

Is there a quantitative measure of green or red being performed, or is the amount of each being interpreted by AGA or other users subjectively?

How does one determine the point where green becomes a definite negative, or red a definite positive?

Thanks

 

[strmrdr]

What is the difference in appearance between 2 diamonds close but on opposite sides of the red/green split in common light conditions?

 

[JohnQuixote]

Date: 6/16/2005 2:54:05 PM
Author: oldminer
John: Very intriguing stuff.

Is there a quantitative measure of green or red being performed, or is the amount of each being interpreted by AGA or other users subjectively?

How does one determine the point where green becomes a definite negative, or red a definite positive?

Thanks

Dave, we'll be able to discuss it more intelligently once the standardized desktop ASET begins hitting our offices.

Some of us have come up with wizardry to take photos through the hand held ASET, but it's not suited for that, so we're seeing a lot of variation in imagery and distrubution of colors, depending on diamond placement (which is peculiar to each photographer's method). The desktop unit should provide a unified placement platform. At that point we can arrive at better conclusions.

No doubt AGS will assist with interpretation. For now we are making a practice of comparing ASET views to ideal-scope views. There is much similarity and I find handshaking occuring in the two views with regard to excellent return and balanced contrast.

 

[JohnQuixote]

Date: 6/16/2005 2:59:53 PM
Author: strmrdr
What is the difference in appearance between 2 diamonds close but on opposite sides of the red/green split in common light conditions?

Strm, given identical attributes - except for a dif between 40.7 pavilion avg and 40.8 - no difference.

ETA: Having a bit more fun, I pulled about a dozen ideal-scope images on diamonds below and above the ledge. There seems to be negligable difference in the eyes of those IS images versus their ASET companions. I viewed images of diamonds with PA averages running from 40.6 - 40.9 in 55, 56 and 57% table sizes.

The color change is an interesting phenomenon that means little in these static images.

Down the road, as AGS moves forward with their studies of dynamic fire and dynamic contrast (baseline for scintillation) this may become more meaningful.

 

[strmrdr]

kewl thanks for the info.

 

[Paul-Antwerp]

I think that it is very important to note the following.

The handheld ASET-scope or the desktop ASET-scope is just one part of the equation. This is a straight (no tilt) view of a diamond with 30° obscuration, if I am not mistaking.

The AGS-cut-grade does not stop at this point. That takes into account the same head-on view with 40° obscuration, and then both views, tilted in 4 directions, north, west, south, east. So, the cut-grade depends on 10 ways of observing the stone, of which the ASET-scope only offers one.

Live long,

 

[JohnQuixote]

Paul - there are other elements, but AGS is very high on static ASET evaluation as a strong indicator of performance, and a provisional indicator of cut grade. I recall that Light Performance (as per ASET) is 4 of those 11 components.

"You can use the ASET to get an ‘at a glance’ assessment of light performance. You don’t need to take any measurements or know any proportions. This instrument can be used to both buy and sell diamonds. And, the ASET works on most mounted goods."

Also

"The ASET will soon include charts. Compare the ASET image you see to the charts. The closest image will put you ‘in the ballpark’ for light performance. AGS Members will be able to assign provisional cut grades using this method. Charts for the Round Brilliant and the Square Princess Cut will be released soon. Other shapes to follow."

As we are seeing in our offices, the value of this tool is akin to what ideal-scope has been telling us for some time. It's not the absolute, but it's highly valuable and tells a lot.

The fact that AGS will endorse members assigning provisional cut grades using the ASET and charts is testament to the value they are placing on this 'one part of the equation.'

 

[JohnQuixote]

Paul, another question... In the first iteration of the 11 components in the new system AGS included tilt with it's judgments on proportion factors, not light performance.

"Light Performance: Contrast, Brightness, Dispersion, Leakage
Proportion Factors: Tilt, Durability, Weight Ratio, Girdle Thickness, Culet Size
Finish: Polish, Symmetry"

On the new DQD explanation girdle, culet, weight ratio, durability and tilt are included in "Proportion Factors." To that end, in the latest iteration of information, AGS included a tilt chart showing table size and minimum PA (attached). My understanding is that diamonds falling short of this criteria will be penalized.

I don't see any indication that tilt is included in Light Performance (listed as brightness, dispersion, leakage and contrast). Is tilt evaluation to fall within the "Light Performance" analysis now? If so, how are they measuring it - or is it being evaluated according to this chart only?

(Copyright AGS 2005, used with permission)

 

[Garry H (Cut Nut)]

Date: 6/16/2005 2:38:29 PM
Author:JohnQuixote

A little FYI for those following the ASET image peculiarity.

Yesterday I asked Brian Gavin to look at images and the chart for 57 table that shows the consistent change from green to red in the eye of table at 40.768. Upon seeing it he accurately assessed it as a table reflection dynamic. He also pointed out the fact that GIA has long used this area to assess LGF construction, and that cutters routinely use this area when cutting to assess other optical properties. (I should have asked him in the first place).

AGS sent a very helpful graphic that confirmed this: At a pavilion angle of 40.768 degrees the area in the center of the diamond (the ''eye'' as we''ve called it) is receiving light from 45 degrees. When the pavilion angle decreases to less than 40.768 degrees the table reflection area now receives its light from below 45 degrees which is the green area of the ASET. When increased to above 40.768 the light is received from the red area which is 45 to 75 degrees in the ASET.

What you are noticing is a pavilion combination that is transitioning at the 45 degree mark. James Caudill calls these stones ''on the ledge.''

(Copyright, AGS 2005 - Used with permission)

Excellent coverage guys, excellent.

Just a small point of graphic detail - the light rays through the table reflection go off to the opposite side that they enter as shown here.

 

[Garry H (Cut Nut)]

Date: 6/16/2005 4:42:14 PM
Author: JohnQuixote

Paul, another question... In the first iteration of the 11 components in the new system AGS included tilt with it''s judgments on proportion factors, not light performance.

''Light Performance: Contrast, Brightness, Dispersion, Leakage
Proportion Factors: Tilt, Durability, Weight Ratio, Girdle Thickness, Culet Size
Finish: Polish, Symmetry''

On the new DQD explanation girdle, culet, weight ratio, durability and tilt are included in ''Proportion Factors.'' To that end, in the latest iteration of information, AGS included a tilt chart showing table size and minimum PA (attached). My understanding is that diamonds falling short of this criteria will be penalized.

I don''t see any indication that tilt is included in Light Performance (listed as brightness, dispersion, leakage and contrast). Is tilt evaluation to fall within the ''Light Performance'' analysis now? If so, how are they measuring it - or is it being evaluated according to this chart only?

(Copyright AGS 2005, used with permission)

I do not know if AGS''s software reports tilt exactly the same way thatDiamCalc does, but I would assumethat it does.
This is the amount of tilt required to reach a position that enables you to see a fisheye (the opposite side girdle just inside the table).
Tilt does indeed come under the proportion factors.
There have been no published data to say what a stone with 4.5degrees of tilt would rate - but it might also get a lowering of score for being too large a spread or other combined factors.
Remeber too that proportion scores and light performance scores are added together - so a fish eye will pprobably take a double hit - I suspect AGS4 or below is where you would be looking.

 

[Mara]

john what is the T placed on the 34.4/40.8 diamond in your second image up top?

 

[belle]

Date: 6/16/2005 5:34:36 PM
Author: Mara
john what is the T placed on the 34.4/40.8 diamond in your second image up top?

tolkowsky?

 

[denverappraiser]

John,

Has AGS released any documentation on interpreting these images? Both borders of the green areas strikes me as highly questionable but the pink/blue line is a bit less affected by the way the image is made. I agree that the tabletop version helps some of the issues simply by being bigger but it doesn’t completely eliminate the issues. Improvement is good although the price tag still galls me. It would sure be nice to have some more information about what they think we can, and can’t conclude from these images. The ledge strikes me as something that is mathematically there but is in practice not there. Here are two pictures of the same stone taken with an identical setup using Garry’s light trays. The only difference is the selection of which tray was used resulting of a variation of about a 1mm in the relative position of the stone to the cone.

Yeah, I know, they aren’t particularly level. That''s not the point.

So, is this stone over the ledge?

Neil Beaty
GG(GIA) ISA NAJA
Independent Appraisals in Denver

 

[JohnQuixote]

Date: 6/16/2005 5:21:40 PM
Author: Garry H (Cut Nut)

I do not know if AGS''s software reports tilt exactly the same way thatDiamCalc does, but I would assumethat it does.
This is the amount of tilt required to reach a position that enables you to see a fisheye (the opposite side girdle just inside the table).
Tilt does indeed come under the proportion factors.
There have been no published data to say what a stone with 4.5degrees of tilt would rate - but it might also get a lowering of score for being too large a spread or other combined factors.
Remeber too that proportion scores and light performance scores are added together - so a fish eye will pprobably take a double hit - I suspect AGS4 or below is where you would be looking.

I think that''s a safe assumption (DC).

A well made round brilliant can stand a larger amount of tilt before girdle reflection occurs. AGS studies (based in part on George Kaplan''s work) of a diamond with a 65% table withstood only 10 degrees until you could see girdle reflection, whereas the 55% table Tolkowsky model withstood up to 18 degrees.

For the grading system they wanted to relax the Tolkowsky number while maintaining the same look, and arrived at 14 degrees as the baseline. That corresponds to a 59% table, btw.

(Copyright AGS 2005, used with permission)

 

[JohnQuixote]

Date: 6/16/2005 5:34:36 PM
Author: Mara
john what is the T placed on the 34.4/40.8 diamond in your second image up top?

T''s for Tolkowsky.
B is for Belle.
She got the answer -
Isn''t that swell?

 

[strmrdr]

Date: 6/16/2005 6:03:28 PM
Author: JohnQuixote

Date: 6/16/2005 5:34:36 PM

Author: Mara

john what is the T placed on the 34.4/40.8 diamond in your second image up top?

T''s for Tolkowsky.

B is for Belle.

She got the answer -

Isn''t that swell?

That should be "isnt she swell?" and the answer would be yes :}

 

[JohnQuixote]

Date: 6/16/2005 5:39:27 PM
Author: denverappraiser

John,

Has AGS released any documentation on interpreting these images? Both borders of the green areas strikes me as highly questionable but the pink/blue line is a bit less affected by the way the image is made. I agree that the tabletop version helps some of the issues simply by being bigger but it doesn’t completely eliminate the issues. Improvement is good although the price tag still galls me. It would sure be nice to have some more information about what they think we can, and can’t conclude from these images. The ledge strikes me as something that is mathematically there but is in practice not there. Here are two pictures of the same stone taken with an identical setup using Garry’s light trays. The only difference is the selection of which tray was used resulting of a variation of about a 1mm in the relative position of the stone to the cone.

Yeah, I know, they aren’t particularly level. That's not the point.

So, is this stone over the ledge?

Neil Beaty
GG(GIA) ISA NAJA
Independent Appraisals in Denver

Neil, I think this goes to my point, above. I suggest we wait until the desktop ASET comes out. The promised charts will come with those and that will help with our analysis. In the next few months I suspect we will come a long way.

Many of us have mucked about with these homemade setups - I have a number of photos we've attempted in our studio - but the handheld ASET was just not designed for it. The aperture is much less friendly than IS. We may need to adjust our thinking or build a better mousetrap for hand held images. The desktop is pricey, but I imagine part of that tag is the consistency built into it. It's definitely easier to use.

One thing that I like about all of these images relative to ideal-scope is that it is giving the same information from a light return versus light escape POV: Leakage is leakage and appears the same in IS and ASET.

As for the photos you posted: The image on the right appears more viable to me. Better consistency, north to south and east to west. The watermelon seeds look more congruent and construction under all of the star facets seems more realistic (no superfluous reflection). No leakage to speak of. Nice diamond, but this photo is not anywhere near the image quality I hope we'll see in the future.

I would say it is below the ledge (PA <70.77 degrees).

 

[Richard Sherwood]

Excellent info John. Thanks.

How much is the desktop unit, anyway?

 

[JohnQuixote]

It is $495 for members. $995 for non-members as I recall, Rich.

 

[Garry H (Cut Nut)]

Guys there is something basically wrong with this 40.768 thing.

(and do not forget that Tolkowsky stones have a green center - that "T" on the chart is on the 40.8 - not the 40.75)

2/3rds of the potential AGS 0's targets that have 56% table sizes have green centers / or are below this number. that every one seems fixated on.

Forget it - you are going bonkers over the wrong thing.

Repeat for the slow readers

2/3rds of AGS 0's targets with 56% table sizes should have green centers.

 

[JohnQuixote]

Date: 6/16/2005 11:29:34 PM
Author: Garry H (Cut Nut)
Guys there is something basically wrong with this 40.768 thing.

(and do not forget that Tolkowsky stones have a green center - that 'T' on the chart is on the 40.8 - not the 40.75)

2/3rds of the potential AGS 0's targets that have 56% table sizes have green centers / or are below this number. that every one seems fixated on.

Forget it - you are going bonkers over the wrong thing.

Repeat for the slow readers

2/3rds of AGS 0's targets with 56% table sizes should have green centers.

LOL Garry. I don't perceive 'bonkers' as a DEFCON 1 in this thread, but I do think there has been a rush to judgment about the color green when this phenomenon has been discussed.

Thus my caution in this post, above.

I'm also curious about the Tolkowsky PA, which I have ever understood to be 40.75. Garry, you may recall in the photo from JCK that Jim was pointing at the 40.8 as well (??).

 

[Paul-Antwerp]

Date: 6/16/2005 4:42:14 PM
Author: JohnQuixote

Paul, another question... In the first iteration of the 11 components in the new system AGS included tilt with it''s judgments on proportion factors, not light performance.

''Light Performance: Contrast, Brightness, Dispersion, Leakage
Proportion Factors: Tilt, Durability, Weight Ratio, Girdle Thickness, Culet Size
Finish: Polish, Symmetry''

On the new DQD explanation girdle, culet, weight ratio, durability and tilt are included in ''Proportion Factors.'' To that end, in the latest iteration of information, AGS included a tilt chart showing table size and minimum PA (attached). My understanding is that diamonds falling short of this criteria will be penalized.

I don''t see any indication that tilt is included in Light Performance (listed as brightness, dispersion, leakage and contrast). Is tilt evaluation to fall within the ''Light Performance'' analysis now? If so, how are they measuring it - or is it being evaluated according to this chart only?

(Copyright AGS 2005, used with permission)

John,

You completely misunderstood my point, and Garry has taken you way off topic later on. Although your discussion on tilt is interesting.

I am talking about the way AGS grades light performance, more specifically which calculations their software makes. After and before the software simulation, they have a visual check and final grade on the stone, I suppose, but that is not what I am talking about either.

The ray-tracing software calculates light return (in the way of the ASET-model) in 10 different positions. These positions are with 30° obscuration and with 40° obscuration (thus 2 basic ways of obscuration) multiplied with 5 positions of the stone relative to the observer (straight and tilted in 4 different directions north-west-south-east). 5 times 2 is 10.

The ASET-scope gives us just one of these 10 positions, being straight with an obscuration of 30°. Therefore, using the ASET on the one hand only gives you an assessment of the grade, while on the other hand, the whole discussion about green or red table-reflection is mute, since it is a phenomenon in only 1 of the 10 observation-criteria.

Live long,

 

[Garry H (Cut Nut)]

Paul there would be a weigthing that should place the main emphasis more on face up.

But you have a good point.

 

[JohnQuixote]

Date: 6/17/2005 4:14:18 AM
Author: Paul-Antwerp

John,

You completely misunderstood my point, and Garry has taken you way off topic later on. Although your discussion on tilt is interesting.

I am talking about the way AGS grades light performance, more specifically which calculations their software makes. After and before the software simulation, they have a visual check and final grade on the stone, I suppose, but that is not what I am talking about either.

The ray-tracing software calculates light return (in the way of the ASET-model) in 10 different positions. These positions are with 30° obscuration and with 40° obscuration (thus 2 basic ways of obscuration) multiplied with 5 positions of the stone relative to the observer (straight and tilted in 4 different directions north-west-south-east). 5 times 2 is 10.

The ASET-scope gives us just one of these 10 positions, being straight with an obscuration of 30°. Therefore, using the ASET on the one hand only gives you an assessment of the grade, while on the other hand, the whole discussion about green or red table-reflection is mute, since it is a phenomenon in only 1 of the 10 observation-criteria.

Live long,

Thank you for clarifying Paul. I did misunderstand. I didn't realize your comments were actually quite on-topic as regards the green-red table reflection. What you are saying lends itself to the overall intent of this post, which is one of the first in this thread.

Regarding your explanation of the ray-tracing, I would also like to know the weighting of the static view. It seems strange that AGS would allow members to assign provisional grades with only 10% of the picture (?) Do you know any more about this? Another question: Do you know the degree of tilt for those 4 compass point observations you spoke of?

 

[denverappraiser]

Date: 6/17/2005 11:35:56 AM
Author: JohnQuixote

Do you know the degree of tilt for those 4 compass point observations you spoke of?

Also, in the case of princess cut and other fancy cut stones, are the 4 compass points towards the sides or corners of the stone?

Neil Beaty
GG(GIA) ISA NAJA
Independent Appraisals in Denver

 

[Paul-Antwerp]

Date: 6/17/2005 11:44:15 AM
Author: denverappraiser

Date: 6/17/2005 11:35:56 AM
Author: JohnQuixote

Do you know the degree of tilt for those 4 compass point observations you spoke of?

Also, in the case of princess cut and other fancy cut stones, are the 4 compass points towards the sides or corners of the stone?

Neil Beaty
GG(GIA) ISA NAJA
Independent Appraisals in Denver

Sorry for the late reply, guys. Here, we have a combination of a heatwave and too much work.

I do not remember the degree of tilt used, and I think that the compass points are towards the side.

Live long,