ADAMAS GEMOLOGICAL LABORATORY publishes gemologically oriented software for the jewelry appraiser, jeweler, or student in gemology or mineralogy. The laboratory also provides technical consulting and jewelry appraisal services to the trade and the public and expert witness services to the legal profession.

GIA Raytrace Cut Study Materially Flawed

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Adamas Gemological Laboratory has determined that the GIA raytrace cut study published in Gems & Gemology, (Fall 1998  "Modeling the Appearance of the Round Brilliant Cut Diamond: An Analysis of Brilliance", Hemphill, et al.) has a material flaw in it's selection of a baseline illumination model, which in  this writer's technical opinion,  INVALIDATES ANY CONCLUSIONS which are made or could be made by reference to the GIA article.

GIA Gems & Gemology editorial staff, to their credit, published this writer's technical analysis, given below, in the Winter 1998 "Letters To The Editor" as well as the response by the authors of the study. UNFORTUNATELY, the response by the author's of the article was no more than CLINTONESQUE OBFUSCATION which did NOT ADDRESS this writer's concerns.  Typical of what seems to be GIA's kneejerk reactions in the past, the authors failed in their technical duty and responsibility to the trade (from which they obtain what appears to be misdirected overfunding) to forthrightly respond to the technical critcism presented by this writer.

Additionally, it appears from their reponse, that some of the authors lack the basic understanding of mathematical modeling principles and even basic high school plane and solid geometry, which is an indictment in itself, of their study. To vehemently deny that they did not do a perturbation analysis in an attempt at obsfucation ("In fact, we did no such thing"), with out realizing that, in fact, that is what they have presented, is PLAIN and CLEAR ignorance.  The author's comment "we do not believe that Mr. Haske's geometrical argument applies to (three-dimentional) round brilliant cut diamonds" suggests very strongly that some of them should go back and conduct remedial studies in some basic mathematical fundamentals.

William Boyajian's editorial  "Demystifying Diamond Cut", which prefaced the first part of the study, in one part states "Thus we caution you to read this article fully and carefully, and to refrain from drawing unequivocal conclusions from this initial work"  and at it's end states "Although it is not GIA's role to discredit the concept of an "Ideal" cut, on the basis of our research to date we cannot recommend its use in modern times" are CONTRADICTORY.  Indeed the trade press, in article published in New York Diamonds, JCK, and others, have blindly followed GIA's technically flawed report, to the detriment of the trade.

This writer can PROVE, and has in fact published these proofs, of GIA's continuing "goofs", which are never retracted or even admitted to by GIA, as good science dictates. (Recent examples include the electrical conductivity issue on both synthetic diamonds and moissanite).  It is time for this to change. GIA is NOT GOD IN AMERICA as they would like the trade to believe, they are human beings who make mistakes, and unfortunately, they have made too many recently.

Since GIA was not aware of any SAS2000 two dimensional simulations being published,  I will oblige them and publish data to be found further along in this article, at  Two Dimensional Simulations .  My critique of GIA's hemispherical illumination model can be found at  Analysis . Unfortunately I cannot publish the full text of their response which can be found in Gems & Gemology Winter 1998. My suggestions for a proper illumination model to judge brilliance is at  Suggested Brilliance Model
 
 


ANALYSIS

The long awaited GIA report on the ray trace analysis of round brilliant diamonds has just been published in Gems & Gemology. The writer initially saw this publication as an opportunity to qualitatively validate his own SAS2000 2 dimensional ray trace facility, but soon realized that this report on the brilliance aspect of cut grade analysis, based on GIAs weighted light return metric (WLR), contains what appears to be, an unintentional fatal flaw in its design.  The apparent flaw is in the hemispherical lighting model chosen by GIA for this study, which was supposedly selected to be an unbiased , diffuse illumination model.  This model is unfortunately not an unbiased benign source as used, and as a result probably invalidates any conclusions presented in the article.  The use of any comparisons in the GIA G&G article should be strictly avoided, as the illumination model chosen apparently  (based on limited SAS2000 2D analyses) weighs the analysis results presented towards favoring shallower crown angles and possibly larger tables.

A scientific perturbation analysis such as attempted in GIAs report, is intended to show the effect on some output parameter (such as GIAs defined WLR) as a function of a change in a given parameter or combination of parameters.  The model chosen by GIA in this case, was apparently intended to present a uniformly (diffuse) distributed set of input ray incident angles to the modeled diamond, each emanating from the hemispheres internal surface.  The amount of light entering the diamond (that which can potentially contribute to WLR) is dependent on the angle of incidence and a uniform distribution of this angle of incidence was what was sought.

Figure 1a  shows a ray incident on the table (RO), originating from the hemisphere, with angle of incidence i defined by ROP.  For the table, any ray such as ROP can have an angle of incidence bounded by the 90 degree angles defined by POA and POB where PO is a normal to the table..
 
 







For any other crown facet, such as the crown main shown in Figure Ib, rays originating from the hemisphere, such as RO (defined by the angle ROP) are limited as to angle of incidence i, by the angles defined by POA and POB, where POA is less than the 90 degree angle defined by POB.  PO is a normal to the crown main shown.   With this model, the allowed angular inputs for any facet below the table change as the facet angle changes, hardly the uniform, unchanging, angular distribution applied to the table facet.  Theoretically at least, the allowed angular distribution within a given point in a particular crown facet also changes, the magnitude of the change dependent on the radius chosen for the illuminating hemisphere.  The angle defined by POA is, in the limit defined with an infinite hemispherical radius, equal to 90.0 degrees minus the crown angle.  Likewise, in GIAs three dimensional model, the star and break facets will each have similarly defined angular distribution limits, each having an individual angular cutoff on one side of the distribution.
 
 





What does all this mean?  To the author, it mandates an immediate retraction of the G&G article by GIA and a republication of results with a corrected model.  This is the proper procedure followed when a scientific study is materially flawed and presents misleading analyses such as this.

GIA, should also, in order to eliminate what is a perceived bias in their report, include in an amended Table 3, comparison of GIAs own Make Grade system, which is taught in their own educational courses, regardless of the results shown: good, bad, or indifferent.
 

Credentials applicable to this critique..

The author is a graduate gemologist (GIA), and has both a bachelors and masters of science degree from MIT. The author spent thirty years in the development, analyses, test and mathematical modeling of the most precision electro mechanical and electro optical devices known to man, namely guidance and navigation systems for strategic weapons.

Martin D. Haske
 
 

2 Dimensional Simulations


 


 

Suggested Brilliance Lighting Model

The selection of an illumination model to "rate" the effectiveness of light return for a round brilliant diamond , ie the "brilliance" factor, is not necessarily a subjective tack. It involves both the need in a light model to have the visibility to distinguish amongst the various cutting styles as well as the implied requirement that that model provide objectivity in the results. This model should be reflective of the typical viewing environment, not one that artificially induces some preordained result.

GIA's initial approach was not totally without merit, in fact if not for the error in its' implementation, the diffuse source sought would have formed at least part of a balanced and objective lighting model.  A diffuse illumination source,  the "background" environment per se, should be included in the lighting model, but it alone should not be the basis for determining the realtive "goodness" of a particular cut with respect to the "brilliance" factor.
 
 

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