AGS cut grading & the ASET

[denverappraiser]

As I understand it, with the new AGS grading, there are 10 theoretical observations made of the stone, one of which is represented in the ASET scope. The others are similar with the stone tilted towards the four compass points and a similar set with a bigger blue area. I have a question about the tilt. Is the stone being lighted the same way and just observed from a different direction? In effect, is the peephole in the cone moving, is the entire blue section representing the observers head moving, or is the entire cone tilting?

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

 

[Rhino]

From what I understand only the diamond is being tilted, not the cone.

 

[denverappraiser]

Rhino,

If the stone is tilted in relationship to the cone then the profile of the light entering the stone will be drastically changed in addition to viewing from a position that is not perpendicular to the table. If the peephole is moved from the center then the stone will be lighted in the same fashion but the viewer will be seeing the light returning from from the stone from a tilted position. They may both be useful information but they are quite different.

I'm not good enough at photoshop to modify the diagram but hopefully this makes sense.

Neil

 

[Rhino]

Yea... and as you tilt the stone all the angles of entry change as well as obscuration. Genius isn''t it?

 

[Garry H (Cut Nut)]

Calm down guys.

I think you will find the tilting analysis is done within the ray tracing program.

Which also raises Marty''s question - in what orientation is the stone?
Star pattern North South?
OR is it random?
Same for Princess cut - the results from corner to corner tilt vs side to side can be quite different.

 

[JohnQuixote]

I'm rather sure it is done within the ray-tracing program. I have queries in to AGS about orientation for rounds as well as degree of tilt. It's been said that for princess cuts the orientation is side to side. My guess is that lighting, viewer perspective and cone position will be static. Obscuration will be set at 30 degrees, then at 40 degrees. I suspect this evaluation may be along the same lines as girdle reflection judgment.

More interesting, it may be the basis for study of dynamic contrast and dynamic fire (forthcoming from AGS as they develop a metric for scintillation).

 

[denverappraiser]

I realize that AGS does this entirely in a simulation and that this is something that is difficult to do at home. I''m just trying to understand what it is that the program is simulating.

Minor details in the rules can have profound effects on the outcome.

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

 

[JohnQuixote]

From AGS regarding tilt: In addition to light return, the program is also calculating the dispersion potential for each zone of the diamond using a virtual tilting algorithm. Tilt is being averaged, which means the static view is weighted more (as we suspected).

Also, tilt to each compass point is indeed being handled similarly to girdle reflection judgment; the same measure of 15 degrees is applied (here is a prior post explaining the reasoning). Diamonds that perform the best have the ability to maintain certain patterns and values during tilt, reinforcing their ‘elite’ status. Outside of the 0 range the diamond starts to crescent (see prior post), and once the girdle shows in the table area values suffer. This all makes an abundance of sense to me relative to George Kaplan’s ‘cone of beauty’ concept & well-made diamonds.

Neil, you might be interested in this: Though it has no influence on the evaluation - during physical photographic quantification the actual diamond is also tilted to ensure the environment is maintained (I assume for verification). This is expressly a side note. Let’s not make it another green/red table reflection brouhaha...

/

 

[Garry H (Cut Nut)]

the program is ray tracing the first group, the scanner results are examined for the second and the third is done by a human

AGS 0 Cut Grades must pass each of these criteria. One strike and the grade is lowered:
1. Light Performance
• Brightness
• Fire (dispersion)
• Leakage
• Contrast
2. Proportion Factors
• Girdle thickness
• Culet size
• Weight or Spread
• Durability (no crown angles below 30°)
• Tilt (fish-eye)
3. Finish
• Symmetry
• Polish

 

[JohnQuixote]

Date: 7/5/2005 8:33:47 PM
Author: Garry H (Cut Nut)
the program is ray tracing the first group, the scanner results are examined for the second and the third is done by a human

AGS 0 Cut Grades must pass each of these criteria. One strike and the grade is lowered:
1. Light Performance
• Brightness
• Fire (dispersion)
• Leakage
• Contrast
2. Proportion Factors
• Girdle thickness
• Culet size
• Weight or Spread
• Durability (no crown angles below 30°)
• Tilt (fish-eye)
3. Finish
• Symmetry
• Polish

Thanks for the list Garry.

All: Two different judgments made using the same degree of tilt. One ray tracing, one proportions-based.

1. Light Performance

Tilt analysis here relates to the measure of "Fire (dispersion)" - as discussed in this thread.

as opposed to

2. Proportion Factors

Tilt estimation here relates to the measure of "Tilt (fish-eye)" - or, girdle reflection.

 

[Garry H (Cut Nut)]

Date: 7/5/2005 8:53:30 PM
Author: JohnQuixote

Date: 7/5/2005 8:33:47 PM
Author: Garry H (Cut Nut)
the program is ray tracing the first group, the scanner results are examined for the second and the third is done by a human

AGS 0 Cut Grades must pass each of these criteria. One strike and the grade is lowered:
1. Light Performance
• Brightness
• Fire (dispersion)
• Leakage
• Contrast
2. Proportion Factors
• Girdle thickness
• Culet size
• Weight or Spread
• Durability (no crown angles below 30°)
• Tilt (fish-eye)
3. Finish
• Symmetry
• Polish

Thanks for the list Garry.

All: Note the two different judgments made using the same degree of tilt.

1. Light Performance

Tilt analysis here relates to the measure of ''Fire (dispersion)'' - as discussed in this thread.

as opposed to

2. Proportion Factors

Tilt estimation here relates to the measure of ''Tilt (fish-eye)'' - or, girdle reflection.

I seem to be being arguementive - and really I am not. Because we are splitting hairs anyway that dont much matter.

This is verbatim from Peter Yantzers presentation John.

Copyright 2005 American Gem Society
Tilt Examples It’s easy to make tilt charts for the standard round brilliant. We started with the Tolkowsky model and verified what it looked like at 18 degrees of tilt. In order to build a grading system, we needed to ‘relax’that number while still maintaining the same ‘look’. 14 degrees of tilt seems to be realistic.

So what i wroote above is as i believe it to be.

I will be seeing Peter in India next week, if it is an issue that needs clarification - let me know.

 

[JohnQuixote]

Garry - Actually my intent was to reinforce what you were saying, by way of explaining that it's two different looks.

The tilt example you are citing is the analysis of where the girdle reflection starts (part of the Proportions grade). I described it in this post and think you'll find we're on the same page.

However Garry, AGS is also using a separate tilt analysis with ASET/ray-tracing. This virtual tilt algorithm is a measure of the dispersion potential for each zone of the diamond and is factored into the Light Performance grade.

This was my interp after speaking to AGS, but I am not infallible you know

 

[Garry H (Cut Nut)]

Yes John we are on the same page - but the 14 degree tilt is not the same as 15 degree tilt.
The software for tilt calculation is I believe the same as the DiamCalc Fish eye under the advanced tab and is the equivalent of the 5 degree factor.