Contrast Brilliance in diamonds.

[strmrdr]

Note dont take the comments in this post as facts Im mearly throwing out my understanding of the issue for discussion :}

Contrast Brilliance is the difference between the dark areas of a diamond and the bright areas. The more contrast the brighter the diamond appears for any given amount of total light return.
The darker the dark areas are compared to the light areas the brighter it appears.

Diamonds achieve contrast is 3 ways.

Leakage:
Examples of this are asschers that use leakage to define the patterns the diamonds are famous for showing.
Crushed ice looking radiants are another example.
Near the girdle area of h&a diamonds not specifically cut to eliminate it are another example.

Light redirection:
The light is returned at a different angle than the main light return.
This can be seen as blacks in idealscope images of h&a diamonds and is even more obvious in the new AGS aset scope images.
The arrows in h&a diamonds are examples of light redirection contrast.

"AGS has an Angular Spectrum Evaluation Tool or ASET (they pronounce it ‘asset’). The images are much like IS but use different colors to show light returned at different angles. White is leakage, green is from 0 to 45 degrees from the horizon, red is from 45 to 75 degrees, and blue is 75 to 90 degrees."


Mirror:
The third way of achieving contrast is to act as a flat mirror and reflect back the object that is in front of it for the dark areas while returning light for the bright areas.
“head shadow” that is talked about often here is an example of this.
Often times light redirection will appear as acting as a mirror the primary light return is directed away but the secondary reflection show up as a reflection of the environment around the diamond.

 

[Rhino]

Funny you started this thread cause I was about to.

I would like to engage a deeper discussion on the subject and clarify what's on my mind as a research gemologist along with Garry to get his input as I know he studies (and has studied) these issues.

For clarification my position on the subject has been this (what I'm about to write) and I can point to postings/web pages where I have stated such. Perhaps I have not clarified it to this degree. Note: This is picking up on the conversation started by Garry, John and myself from this thread on the subject of contrast brilliance.

I would state that the *primary* causes of contrast and that which *primarly* impacts brightness within a diamond is that contrast between facets that are reflecting back the proper balance of lights/darks ... ie. white suffuse light vs facets reflecting back things obstructing light.

To a lesser extent leakage can contribute to this phenomena but only where minor points of leakage are occuring. Ie. around the upper girdles.

Where I believe I part ways with the MSU/Holloway principal (and Garry, please correct me if I'm wrong), is when excessive leakage is taking place within the diamond. If Garry agrees with the results reflected in MSU scores, the MSU rewards diamonds with a higher rating for contrast when excessive leakage is taking place. THIS CONTRAST between excessive leakage vs facets reflecting back light is what I consider to be *negative* contrast.

Let me illustrate with this graphic (we are working on a photographic setup to demonstrate these principals). This diamond that we called in had upper girdle facets cut over 44 degrees causing *more leakage* around the perimeter of the stone. There is indeed a *contrast* to be observed here but it is what I would consider to be negative contrast. Excessive leakage, IMO should not be rewarded with higher contrast scores.

While this graphic illustrates excessive leakage around the upper girdles I hold the same opinion when excessive leakage is taking place under the table especially.

Do you guys concur or differ in your opinion? I am interested to know. Garry ... John or anyone interested in partcipating?

 

[strmrdr]

Date: 5/21/2005 1:03:15 PM
Author: Rhino
To a lesser extent leakage can contribute to this phenomena but only where minor points of leakage are occuring.

In RB''s maybe/maybe not but asschers use it to provide a ton of contrast for the center patterns.
I think leakage contrast provides the most clearly defined and predictable contrast points under most conditions.
Take the lone star cut where the star is cut as windows that allow the background to come thru in a predictable and strong way.

 

[Rhino]

Hey senor!

I would concur on the Asscher but would add this qualifier ... as long as those areas within the Asscher that are returning light are doing so at the proper intensity. Ie. if you take an Asscher with steps of leakage and they are alongside steps returning back light at a weak intensity, this would provide for a dull stone. If the areas within are reflecting back light at strong to very strong intensity, fine.

 

[strmrdr]

Asscher example.
Notice the leakage areas providing the contrast for the center pattern.
Also there is a black in the lightscope image high angle return area that is also contributing to the pattern.

The end result is that both act in a simular manner to produce the pattern.

 

[Garry H (Cut Nut)]

Of course that is too much leakage Rhino.
http://www.ideal-scope.com/newsletters_issue003.asp this link shows the contrast contribution in the photo here. The right side image is the head obstruction contrast (H&A''s star). The left side shows how the small edge leakage fills in the pattern.

Sergey and i have had many arguements about what you call MSU''s contrast Rhino - i understand the math algorithm but do not like its inability to account for easily identifyable distribution that causes big visible blothes etc.
I got a SMS Text message while he was travelling in a foreign country a few weeks ago that he has sovled the problem. He is implimenting it and it wil be available in the next release.

And By the way Rhino, I see you have put your GIA article up live. There is a journal publiction process called peer review that is usually conducted behind the scenes and used to iron out all the glitches. I have bothered to read through it all for you, and send you material. You have chosen to publish it before corrections are made. I think that is very sad.

You had best start all over again because many of your stone models are simply wrong. GIGO

Get the correct data from the the Fire article where the numbers were not rounded, and use DiamCalc to more accurately model the stones to match the photo''s and then try again.

 

[Rhino]

Hi Garry,

Thanks for the input. Isn't GIA rounding and not using specific measurements on each facet? This is one principal I thought that was going to seperate the difference between the GIA and AGS systems. I'll make corrections as I see fit but the models are based on the data that GIA has provided.

I had thought you and MSU were on the same page regarding how the Gem Advisor measured contrast as I know you were somehow involved in the making of the program. If this is not true and if MSU is going to be changing their metric entirely for contrast I will certainly make the necessary corrections. I was just going by what I had during the time of writing.

So then you and I are in agreement regarding the issues of contrast eh? At least till this point.

I do have some other questions for you on the subject though.

 

[JohnQuixote]

Date: 5/21/2005 7:44:35 PM
Author: Rhino
Hi Garry,

Thanks for the input. Isn't GIA rounding and not using specific measurements on each facet? This is one principal I thought that was going to seperate the difference between the GIA and AGS systems. I'll make corrections as I see fit but the models are based on the data that GIA has provided.

Jonathan - I'm still fuming about this whole business of rounding the minors by GIA. Stoopid.

We are taking major steps forward due to studies of leakage and contrast by our fellows here, the MSU guys, Jim Caudill, Michael Cowing and company. These two areas are as important to quality aspects of a diamond’s performance as brilliance and dispersion, and they hold the key to our FINALLY placing a foot on the unexplored lunar surface of scintillation quanification!

I believe one of the reasons AGS is including contrast (and leakage) in the light performance analysis on their reports is that it is a legitimate and repeatable way of quantifying a fundamental aspect of the scintillation quality of the diamond - or at least the scintillation potential: Let’s review… The quality of light return in scintillation is the sharp, on-off, bright-dark flashes of light from the diamond’s crown. The level of dramatic effect viewed during scintillation depends on contrast. So…a measure of contrast brilliance may be thought of as one frame in the moving picture of scintillation.

Past technologies have attempted some measure of scintillation by analyzing light performance through movement in a box. I have offered my opinion on this many times; Even if some random number of pixels is counted and a measure is acquired it’s just not possible to replicate a diamond’s panorama of illumination in an enclosed chamber, or to account for an incalculable number of real life illumination scenarios. Now this is not just me, as you well know. The scientific community (and even strong supporters of such technologies) have long pish-poshed this attempted measurement (“We are unaware of current devices that can analyze a diamond through a range of tilting, or use many small distant light sources to correctly estimate scintillation” – Sergey Sivovolenko, 2004).

So, by including contrast, AGS is bravely incorporating a neat way of changing the playing field: Instead of trying to quantify scintillation by measuring active sparkles while the diamond is moving, the measure of contrast analyzes the potential for such performance in any illumination scenario by analyzing the inherent level of contrast brilliance (and its balance with leakage).

As has been expressed, there are different ways a diamond can have contrast. With regard to those that incorporate leakage: In order to benefit, the leakage must occur at facet junctions and on the girdle (not under the table). In every instance, the quality of the contrast is achieved through a harmony with leakage, just as qualities of brilliance (WLR) and dispersion/fire (DCLR) may find harmony vis-à-vis minor facet construction. Eightstar and New Line ACA are examples of diamonds that rely on thick pavilion mains to provide contrast character without leakage. Almost all light of the light is returned to the viewer and the precision of the cut (along with those thicker pavilion mains) harmonizes the balance of return through the angular spectrum. Alternately, diamonds with thinner mains sometimes acquire contrast with leakage-pinpoints. Both types of round diamond, well-cut, are beautiful. The 8*/New Line footprint has a fluid and colorful character to scintillation while classic cuts with what I call “pinpoint” leakage (uniform girdle and facet junction - not under the table) have a more snappy, bright-dark, on-off character to the scintillation. I think it's important to remember that obscuration is ALWAYS in play for every diamond… The amount of contrast visible with obscuration depends on the factors above.

Garry, can you check my math on the above paragraph?

Josh Rioux did a thread about such cutting techniques and their resultant visual effects a while ago.

 

[Garry H (Cut Nut)]

Date: 5/21/2005 7:44:35 PM
Author: Rhino
Hi Garry,

Thanks for the input. Isn''t GIA rounding and not using specific measurements on each facet? This is one principal I thought that was going to seperate the difference between the GIA and AGS systems. I''ll make corrections as I see fit but the models are based on the data that GIA has provided. GIA have published 2 lists of data, you used the rounded ones and got very different results for HCA.

I had thought you and MSU were on the same page regarding how the Gem Advisor measured contrast as I know you were somehow involved in the making of the program. If this is not true and if MSU is going to be changing their metric entirely for contrast I will certainly make the necessary corrections. I was just going by what I had during the time of writing. You read too little and obviously seldom follow links to explanations

So then you and I are in agreement regarding the issues of contrast eh? At least till this point. whoopee do, I get 10/10 for you agreeing with me. gee thx

I do have some other questions for you on the subject though.

 

[Rhino]

Date: 5/21/2005 9:33:59 PM
Author: Garry H (Cut Nut)

GIA have published 2 lists of data, you used the rounded ones and got very different results for HCA.

I was not aware that GIA has published 2 seperate lists of data. The data I used for the article was from their officlal magazine to the trade, Gems & Gemology. Have they issued any statement saying that using those measurements were wrong or inaccurate OR do the 2 sets of measurements they published contradict each other in any way? If so, point it out to me and I''ll find out from GIA which set of numbers is the more accurate and adjust my tutorial as necessary.

If not and they are right, I believe I am being accurate in using the averages they provide because they themselves will be basing their grade on those averages as well. At the same time I believe it is also accurate in using the averages to compare HCA scores because the HCA itself is based on averages except the HCA does not take into account the minor facets which GIA in fact does (although we''re not all happy that they are rounding the minor facet measurements).

Date: 5/21/2005 9:33:59 PM
Author: Garry H (Cut Nut)

So then you and I are in agreement regarding the issues of contrast eh? At least till this point. whoopee do, I get 10/10 for you agreeing with me. gee thx

Please don''t be sardonic. I was under the impression that you and MSU were in agreement regarding their grading of contrast. I''ll be sure to remove your name from any reference on that subject. If MSU will be changing their grading as reflected in their software I will update it as well. I am glad we are discussing these things.

Kind regards,
Jonathan

 

[Garry H (Cut Nut)]

Gems & Gemology Fall 2001 table 2 page 179.
It has been discussed here on Pricescope, especially by Marty Haske, many times.

I clearly refferred to it in my Journal article here on Pricescope where both sets of data are given.
Averages have nothing to do with it, only rounding.

 

[strmrdr]

Im starting to really think that its too early to do an in depth analysis of the GIA cut grading system.
They simply havent released enough information and have released multiple sets of data that dont all agree with each other.