The Angular Spectrum Evaluation Tool (ASET) from the American Gem Society (AGS) is an ingeniously simple device that quickly provides a wealth of information about how a diamond is handling light. Because diamond beauty derives primarily from its brilliance and fire, understanding details about cut quality and light performance is key to choosing a great diamond.
ASET, in addition to being a simple reflector tool, is also a theoretical framework at the heart of the American Gem Society Laboratories (AGSL) light performance based cut grading system. The grading system which was released by AGS in 2005 is the most advanced and scientifically vetted cut grading system in the diamond market. The AGSL methodology affords direct assessment by several versions of the ASET reflector device, as well as a sophisticated software-based ray tracing analysis which not only provides light performance grading but can also render computer generated ASET images and other types of light maps.
Understanding the ASET Framework
ASET as a concept is very simple. By viewing a diamond through the top of a hemisphere painted with different colors that delineate specific angular ranges, the light that the viewer sees when looking through the ASET tool will appear in different colors depending on where that light is coming from. It will also indicate areas of the diamond that may not be returning light (light leakage). Because light from different ranges above the horizon has different attributes, the ASET tool can provide important insights about the diamond’s specific light handling abilities.
The Angular ranges, as illustrated in the graphic above, consist of 0-45 degrees color coded in green, 45-75 coded in red, and 75-90 degrees coded in blue. Because our world is typically lit from above by either sunlight or overhead artificial light, the most direct light entering a diamond will generally come from the higher angular ranges. Low angle light tends to be that which is reflected off of objects in the environment and is therefore less intense than high angle light. Light from the very highest angular range (directly overhead) is blocked by our head when we are observing a diamond at close range. This light is color coded in blue. Although intuitively one would assume blue is undesirable because it indicates darkness rather than brightness, in the right amounts and distribution head shadow (also referred to as obscuration) creates positive contrast which contributes to the appearance of brilliance and to the on/off blinking of the facets known as scintillation.
ASET views of well cut round brilliant diamonds will show a preponderance of red, a small amount of green, a symmetrical pattern of blue, and very little leakage.
One of the strengths of ASET-based analysis is that it is applicable to any shape diamond, and grading constructs can be developed around each them. To date AGSL has developed grading systems for Round, Princess, Oval, Emerald Cut, and certain Cushion and proprietary shapes. Fancy shape diamonds have different ASET signatures which are more variable. With or without an official lab grade available, viewing an ASET of a fancy shape provides insight into how the diamond is handling light.
There are several different versions of the ASET tool and two distinctly different variations. There are handheld and desktop models as well as many different improvised photo setups. There are backlit versions and versions with dark background; the former will show leakage as white, the latter will show leakage as black.
Below are the desktop, presentation, and the handheld models. You will notice that in the desktop model the color coded hemisphere is inverted. The diamond is placed upside down on the glass to ensure the table of the diamond is perfectly flat. The black cone shaped cover is placed over the diamond to create the black background. The image is bounced off of mirrors inside the device and projected to the eye through the viewer.
Early version Tabletop ASET with cap providing black background
New "Presentation" ASET Viewer
The handheld model is more difficult to use, but with some practice it is a functional, inexpensive, and highly portable tool for inspecting diamonds. The tool can be used with or without the portable light source which enables the stone to be placed in a stationary upright position for inspection and provide backlighting where leakage will appear as white.
ASET Handheld Viewer with Light Source
Below are examples of an ASET for a top quality super ideal contrasted with an ASET of a diamond with compromised light performance.
ASET of a Super Ideal Cut Diamond
ASET with Light Performance Issues
Computer Generated ASET
The ASET framework is fundamental to the AGS light performance grading system and is integrated into the software. The grading process involves sophisticated ray tracing of an accurate 3D model of the diamond, measuring the contribution of every facet of the diamond and evaluating brightness, fire, contrast and leakage. It is also capable of rendering ASET images of the diamond. AGSL reports are available that contain ASET light maps. The Platinum report can be ordered with single or with dual light maps showing both table and pavilion ASET views of the diamond. Dual light maps will be most commonly seen on high precision diamonds to demonstrate hearts and arrows patterning, in addition to overall face-up light performance.
AGSL Platinum Report with Single ASET Light Map
AGSL Platinum Report with Dual ASET Light Map
Computer generated ASET images can also be rendered by standalone AGS software and other programs such as DiamCalc by Octonus
, and are sometimes provided by merchants in lieu of actual images. AGSL provided ASET images, such as the light maps on the reports, are simplified but reliable. Because they are generated by mathematical computation within the exact constructs of the theoretical framework, they can be as useful as skillfully captured actual images, and often more accurate. But results are only as good as the scan from which they are rendered. At AGSL the measuring devices used are regularly calibrated and operators are highly practiced in scanning. With computer generated ASETs as well as actual ASET photos, one must consider the source in terms of accuracy.
AGS has produced charts using composites of ASET images that illustrate how the ASET signatures change with different proportion sets.
Proportion sets along the “cutter’s line” of combinations of parameters known to result in top performance show ASET signatures consistent with that which is expected in terms of amounts of red, green, and blue, thereby providing graphical validation of ASET theory.
ASET theory and applications represent a simple yet powerful way to assess diamond light performance. It is both a mathematical construct that forms the basis of sophisticated computerized ray tracing and cut grading of a 3D model of a diamond, as well as a convenient physical tool for use in the field. It can be used with any shape of diamond to gain insight into just how that diamond is handling light.
One question that is asked frequently pertains to center of the ASET map of round diamonds; sometimes it is green and sometimes red. The very center of the light map is the table reflection and whether red or green it is largely inconsequential. For more on this topic please see our article on ASET table reflection
. This variance can also be seen in the ASET matrices.
The Ideal Scope Reflector
A popular reflector tool which operates on a very similar principle is called the Ideal Scope. All light returning to the eye will appear in red, obscuration in black and light leakage in white.
Ideal Scope Viewer and Image of Ideal Cut Diamond
The ideal scope provides a good indication of light return as well as some symmetry aspects, but not quite as much insight into the quality of light the diamond is returning compared to ASET. Please see this article on practical differences between ASET and Ideal Scope