Cut & Light Performance
Performance resulting from cut quality has been studied and developed since the 1800's. Traditional components of a diamond's light performance include brightness (all light returning to the eye), dispersion (“fire” seen as white light is broken into spectral colors), contrast (the pattern of dark and light areas) and scintillation (sparkle seen as the diamond, the light source or the observer move).
Diamond Performance is elegantly seen in the AGS ASET® and its precursor, the Holloway Ideal-Scope®. These tools are “performance reflectors” developed in factories & laboratories to improve cut performance. The AGS ASET® is the only diamond cut grading tool endorsed by the science community.
How do ASET® and Ideal-Scope® work?
Light entering a well cut diamond will (a) reflect from the pavilion and return to the viewer’s eye or in a poorly cut diamond it will (b) ‘leak’ through the pavilion.
These performance reflectors, when placed over a diamond, allow a viewer to see where light is returning to the eye and where it is leaking, as well as a “contrast pattern” which is critical to scintillation (sparkle). Simple to use, both devices do this naturally and effectively. The ASET® does reveal more detail, making it preferable for fancy shapes.
THE AGS ASET®
The American Gem Society began cut grading diamonds in 1996, after 30 years of research by the AGS Diamond Grading Standards Committee. Their Angular Spectrum Evaluation Theory or ASET, introduced in 2005, is the foundation of their three-dimensional cut grading system and the AGS Ideal light performance grade. Here is the hand-held version of the AGS ASET®, with backlight. The diamond is placed on the backlight and the unit is placed over the diamond.
Light entering the crown (top) of the diamond is now color-coded by the AGS ASET® viewer.
RED is Direct Light (drawn from 45-75 degrees). Red will be the most intense. It comes directly from the source.
GREEN is Reflected Light (drawn from 0-45 degrees). Green has less intensity. It is light reflected from walls, the environment, etc.
BLUE represents light Obscured by the observer (your head blocks this light from reaching the diamond). These areas will light up when the diamond is tilted and other areas will become shaded.
WHITE (if the diamond is backlit, as above) or BLACK (if not) is Leakage. These areas show where pavilion facets are acting as windows rather than mirrors. You see white because those windows allow you to look through the diamond and see the light underneath. White should be minimized.
Different Shapes have Different Standards
Each cut has its own set of light performance standards. Fancy cuts are not held to the same standards as Round Brilliants, which are the best at returning an abundance of RED direct light.
What to look for
In general RED should be maximized. Some BLUE is necessary. Too much GREEN is undesirable. The distribution of the three colors is important. WHITE should be minimized.
Round Brilliants Examples
The performance qualities in round brilliants are predominantly direct light (RED in both tools) contrast pattern (BLUE in ASET, BLACK in IS) and leakage (WHITE in both tools) so both tools are well-suited to analyzing rounds.
Round Brilliants with High Light Performance
In Ideal Cut rounds RED is dominant, broken by an even BLUE contrast pattern. GREEN is present in small, symmetrical quantities. WHITE leakage should be minimized.
Princess Cut Examples
For fancy shapes the ASET® is more suitable for analysis. The Ideal-Scope® images (bottom) do not show the detailed differences that the ASET® does by separating direct light (RED) from reflected light (GREEN).
Princess Cuts with High Light Performance
In high light performance princess cuts RED and GREEN dominate, and the brightest performers have more red. BLUE is present in moderate, even quantities. WHITE (if backlit) or BLACK (if not) may appear but should be minimized.
Why don’t more sellers have these tools?
Because few diamonds achieve top light return. Many diamonds look nice under bright lights and these tools reveal the cutting flaws that are present in the vast majority of the world’s diamonds.
Why aren’t more diamonds cut for Performance?
Because it’s cost-effective to follow the shape of the rough. Even a few tenths of a degree away from the critical angles which produce optimum performance produce less brightness and leakage.
Both tools provide simple, elegant and 100% natural ways to analyze light performance. For interested consumers, a USA distributor selling both of these instruments and a variety of other diamond-related tools is David S. Atlas in Philadelphia.