A Method for Quantitative Evaluation of Scintillation in Round Brilliant Diamond

The main visual effects of diamond upon illumination are brilliance, fire, and scintillation.In scintillation, the fire pattern changes dynamically and flashes of white light are perceived across the crown facets of the stone while the stone, the observer, or the illumination being in relative movement.Typically the observer tilts the stone back and forth to observe scintillation.The authors took 6 photographs of a testing diamond using illumination with different incident angles to evaluate scintillation of the diamond.The illumination is composed of LED lights arranged in 6 circles with the incident angles being 14.5°, 18.5°, 21.5°, 25.5°, 28.5°and 32.5°, respectively.The authors developed a software to capture the RGB colour of each spot in the bitmap image.Subsequent analysis requires converting the RGB readings into CIE XYZ colour space and further into L^*a^*b^* colour space.Additionally, the software calculates the chromatic aberration of the same pixel point in two images.If the value of chromatic aberration is more than 12 NBS, the pixel point flashes, otherwise the pixel point does not flash, then add up the difference of all pixel points that flashes in two images.The authors quantified scintillation of a diamond being the ratio of the sum of chromatic aberration divided by the total number of the illuminated pixel points of that diamond.
There are 48 diamond samples, whose table facet is 56%±1%, star facet is 53%±5%, lower girdle facet is 76%±6%,girdle is 3%±0.5%, the crown angle ranges from 27°-40°, and the pavilion angle ranges from 38°-43°.The value of scintillation of all diamond samples are 41-134.
The authors classified scintillation grade of a diamond "F" when its value of scintillation was lesser than 45, "G" when its value of scintillation was between 45 and 75, "VG" when its value of scintillation was between 75 and 105, and "EX" when its value of scintillation was more than 105.To verify these classifications with human observer, the authors made 2-second long GIF images of each diamond, and two professional diamond graders were asked to observe the GIF images of 48 diamonds and evaluate the scintillation of diamonds.As a result of diamond graders, 34 diamonds are in the same scintillation grade, and 14 diamonds are adjacent.About measuring and observation of diamond graders, the grade of 43 diamonds are the same, and the grade of 5 diamonds are adjacent.
In conclusion, the diamonds with pavilion angle of 38° or 39° can display better in scintillation; the relations among the scintillation, crown angle, and pavilion angle are nonlinear.With proper instrument and computer model, this paper shows a method that is possible to evaluate scintillations in round brilliant cut diamonds.