In the 1970s a Japanese scientist named Okuda developed ways to study diamond light performance using colored reflectors in magnified scopes. This method of assessment became popular on the internet with the “ideal-scope,” a simple magnifying tube containing a red reflector that creates a structured light environment. The Ideal-Scope was developed by Garry Holloway FGAA, DipDT, JAA Appraiser.
An ideal-scope photograph shows what areas of a diamond return light to the viewer’s eye and what areas do not. In 2005 the AGS introduced a similar scope, the Angular Spectrum Evaluation Tool. Based on the same principal, the ASET shows light return using multiple colors. The AGS grades light performance as part of cut on a scale of 0 through 10. Diamonds earning the top grade of 0 are considered “Ideal.” Only a fraction of the world’s diamonds will earn this grade.
In these scopes, light returning at extremely high angles will be dark since that light normally comes from directly above and is blocked by the camera. In an optically symmetrical diamond these dark areas should look like matching arrows radiating outward from the center of the diamond. Light returning at optimum angles outside of the obscured area will be red. Less optimum angles will look light pink in the ideal-scope or green in the ASET. Light that is escaping (or leaking) from the diamond will appear white. Small, triangular patterns of white along the edges (girdle), and at meet point junctions outside of the table are acceptable and enhance a diamond’s contrast, but large patches of white indicate leakage that is detrimental to the diamond’s face-up appearance.
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