Developing of Diamond Cut Grading System by MSU (OctoNus and GC MSU) Computer Tools
Thanks to Garry Holloway, FGAA, DipDT, Australia
for help during preparing this material
Table of Contents
- Introduction
- Evolution stages of knowledge on diamond cuts - solution of the first, or direct, task
- Factors of human perception of diamond appearance
- Classification of components of diamond perception
- MSU methodology of cost reduction for creating cut quality grading system
- Some features of OctoNus Ltd. software products
- Example: Understdanding the Bowtie effect in diamonds
Introduction
There are two tasks in diamond cut study:
- First, or direct, task. Creating a system of appraising the cut quality.
- Second, or reverse, task. Creating an instrument or a methodology to grade a cut of real stones according to the above system by the cheapest and the most reliable way. The second task cannot be solved before the first one.
Some organizations are trying to solve the second task first so the market uses their appraising system and their instrument. It is not the way we want to follow.
Different methods of solving the reverse task presented at the market:
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Human-guided
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Automatic
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| Analysis of diamond image taken with a structured lighting |
FireScope
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BrillianceScope
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| Measuring diamond proportions |
Proportionscope
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Sarin
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The main part of this paper is devoted to analysis of possible solutions of the first task.
Evolution stages of knowledge on diamond cuts - solution of the first, or direct, task
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Author
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Year
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Method
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Achievements
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Drawbacks
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Cutters |
Before 1919 |
Empirical |
Pattern created, proportions found |
High price of diamonds makes impossible a complete study by one company (even for a single pattern). |
|
Tolkowsky |
1919 |
Statistical review + ray tracing calculation Tolkowsky - First page |
Selected one of all sets of parameters |
Other sets of parameters were not found. Scintillation was not taken into account. The methodology of statistical researches is not described. |
|
Other calculations (Harding, Dodson, Eppler, Eulitz, Vasiljev, Varshavsky etc.) |
ХХ century |
Ray tracing calculation by different methods |
Algorithms, software, instruments created for researches on this subject |
Researches did not leave the theoretical stage. Their work had little impact on the market. |
|
Gemologists + traders |
middle-end of ХХ century |
Two triads of notions are formed: |
Technicians find the first triad measurable. There is no methodology to evaluate the second triad. |
|
|
Devices for appraising (Firescope, Brilliancescope etc) |
From 1984 |
Real diamonds are analyzed with specific fixed lighting |
Repeatable results on real stones |
Lighting conditions in the devices differ from those in real world. Factors of different nature (proportions, polish, absorption, inclusions) are mixed together. |
|
Device for measuring cut proportions (Sarin) |
1991 |
Scanning and creating a model of a real diamond |
Main parameters are measured quickly and precisely enough |
Only measures parameters. |
|
AGS |
1996 |
System of appraising cut of a round brilliant cut |
Method created for appraising diamond beauty by measuring main parameters of the cut (features) |
Assumption that separate each feature affects diamond beauty independently. |
|
GIA |
1998 |
Light return coefficient is determined on the base of computer 3D model of a diamond and illumination |
Light return graphs for ranges of main cut parameters |
Incorrect illumination used. Only light return is analyzed. Selected light return model not related to percieved reality. |
|
Garry Holloway |
1984-2000 |
Empirical methods |
In 1984 – 1990 he observed (using a Firescope™) a link between crown and pavilion angles. |
The serious progress in empirical findings of beautiful diamonds have been achieved from Tolkowsky ages. The correlation for beautiful stones not precisely describes areas of diamond parameters with equivalent appearance. |
|
MSU |
1999 |
Computer model: illumination - diamond - observation Analytical instruments for appraising cut quality |
Different viewing conditions are taken into account. Modeling and calculations in real-time. Possibility to scan real diamonds and to perform calculations on their 3D models. |
Preliminary results promising but experimental testing of good and bad zones is needed. |
Factors of human perception of diamond appearance
Factors of human perception of diamond appearance that determine cut quality can be divided into three main groups:
| Table legend: | ||
| Green color denotes features already implemented in Bril software | Yellow marks directions in which there is some progress | Red is for features that are not yet implemented |
| 1. Parameters of light sources that characterize the illumination: |
| 1.1. Source type (its spectrum, luminance, and size) |
| 1.2. Position of light source |
| 1.3. The number of light sources |
| 2. Optical effects exhibited by the diamond itself: |
| 2.1. Light return - the capability of the diamond to return a fraction of the incident light to the observer's eye |
| 2.2. Fire - the capability of the diamond to disperse a white light into iridescent colors in a direction able to be perceived by the observer |
| 2.3. Color - the result of the dependence of the light absorption factor of the gem upon the wavelength of the incident light |
| 2.4. The pattern of facets and highlights within the diamond and their symmetry |
| 2.5. "Dynamic Contrast" - the rate of change of the diamond appearance while stone is rotated (either light source or the observer is moving) |
| 2.6. Clarity - the prefered absence of inclusions that absorb or scatter light |
| 2.7. Surface lustre, flatness of facets, quality of polish, and surface smoothness |
| 3. Viewing conditions: |
| 3.1. Position of the observer relatively to the diamond |
| 3.2. Psycho-physiological particularities of the observer : |
| 3.2.1. Stereoscopy of vision |
3.2.2. Pupil motion  |
| 3.2.3. Eye sensitivity |
| 3.2.4. Eye adaptation |
| 3.2.5. Color perception, effect of the background |
| 3.2.6. Eye resolution |
| 3.2.7. Spatial and temporal contrast |
| 3.3. Psychological criteria of diamond beauty perception. |
| 3.4. The existing stereotypes, fashion on the market. |
Classification of components of diamond perception
Triad: illumination, diamond, eye.
To create a grading system with repeatable results it is necessary to fix (standardize) illumination and observation conditions, distance to the diamond, mono or stereo mode of viewing, dynamics of stone movement relatively to the observer and of illumination. Also, relative input of scintillation, brilliancy and fire to diamond's beauty should be evaluated.
MSU methodology of cost reduction for creating cut quality grading system
1. Selecting "bad" zones in automatic mode for the whole range of parameters using light return analysis, fisheye, "kozibe", etc. (a method was created for round cut diamonds and can be accommodated for fancy shapes)
- Light Return Picture To other page (illustration from "Diamond Cut Study in MSU"),
- "Kozibe" + Fisheye + Light Return Picture To other page
Fire and scintillation calculations (Round 1 Iteractive model, Round 2 Iteractive model To see illustration, Marquise Iteractive model) for the whole range of cut parameters. Selecting zones of equivalent cut quality for given parameters. Merging such zones with those of valid parameters that were determined on the first stage.
2. Selecting borderline and definitive diamonds (control stones for cutting).
3. Cutting of experimental stones
4. Scanning of resulting diamonds, creating their 3D models To other page.
5. Visual testing of control diamonds. Testing how a human evaluates their photorealistic images and synthesized movies.
6. Grading of control stones. Correction if necessary for Fire estimation formulas To other page and new formulas for Scintillation should be developed (Bril).
Some features of OctoNus Ltd. software products
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BRIL |
3D-BOOK |
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Software for cut study inside a company |
Commercial software for modeling diamonds with built-in systems of grading cuts |
Interactive learning software on diamonds |
Hardware and software package for marking large rough diamonds |
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Creating realistic images of diamonds, ray tracing, stereo mode, calculation of diamond weight |
|||
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Calculating parameters of a cut quality by automatic search |
Creating video movies to demonstrate scintillation. |
A course on diamond grading |
One of features is scanning and building a model of real diamonds |
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Calculating a “photorealistic” (realistic photo-like) image of a diamond for different illumination conditions, shapes and absorption spectra. |
Loading and editing different shapes created by GemCad software |
Interactive tutorials |
In addition to polished diamonds, it works with rough and partially polished diamonds |
|
Demonstration of distribution of light leaving the diamond (can be used for quantitative evaluation of scintillation) |
Possibility to work with 3D models of real diamonds that are created by PaCor software |
Photo galleries |
Possibility to determine inclusions’ positions |
Example: Understdanding the Bowtie effect in diamonds
There is no real diamond for which Bowtie effect is present due to leakage of light through the pavilion, as all Bowtie effects (there are only two main types of them) appear due to partial shielding of a light source by a human head. We know two types of Bowtie effect:
1. First type occurs when a pavilion angle is about 45°. This case is not very interesting, as it rarely happens in practice
2. Second type appears when a pavilion angle is 39-40°, a crown angle depends on it (for example, when a pavilion angle is 40°, then a crown angle should be in 29-36° range. If a pavilion angle is 39°, a crown angle should be 36-42°. The ranges of a crown angle are specified for a photo camera with 14° angular size). The second type is more common in practice, as it is closer to round diamond proportions.
Conclusions for Bowtie effect of the second type:
1. If a pavilion angle is fixed (less than 43°), one can avoid Bowtie effect by selecting a proper crown angle. It is important both for cut grading and for cut prognosis
2. A notion that the Bowtie effect depends only on a pavilion angle is false. It can be seen especially clearly for a 39-40° pavilion angle
3. When the observer's head is moving further from the diamond, the Bowtie effect can diminish up to complete disappearance. If we think that the look of a diamond from a larger distance is more important than from a close range, the presence and value of the Bowtie effect should not affect our estimation of a diamond (for the second type of the effect).
The reasoning above illustrates possibilities of MSU technologies with application to appraising fancy shapes on example of the well-known Bowtie effect.
