Angular Spectrum Evaluation Tool (ASET): What is it?

The Angular Spectrum Evaluation Tool, abbreviated as ASET, refers to a scientific tool designed by the American Gemological Society (AGS) to investigate the light-handling properties of a diamond. The device examines how a diamond absorbs light from the hemisphere above. It then indicates to what extent light returns to the viewer’s eyes.

There are two key aspects to consider while determining a diamond's fire and brilliance. First, whether the diamond absorbs light in the first place and how much light it collects. Second, how the stone handles the collected light, a diamond may absorb light properly, but the light may reflect on the observer’s eyes or leak through.

According to many buyers, a diamond’s ability to absorb and reflect light depends on its color. But this is not entirely true. As we shall find out, the diamond cut plays a more significant role.

The ASET tool gathers as much information on a diamond’s brilliance as possible. The data is indicated on a diamond certification. The onus is on the diamond shopper to check with the retailer that their certifications contain ASET details.

When the AGS created ASET in 2005, it was not meant to be used only as a reflector tool. Instead, the device would offer the theoretical basis that the American Gem Society Laboratories (AGSL) uses for cut grading and light performance analyses. Since its creation, the tool has significantly changed the landscape regarding diamond grading. The device uses scientifically advanced grading systems and collates, analyzes, and presents data through computer-generated imaging.

The basic concept behind ASET is establishing crucial details regarding a diamond’s light-handling properties.

First, the diamond is viewed from the top of a hemisphere. But before that, the hemisphere must be painted with different colors to delineate the specific angular ranges. As light travels through the diamond, it will assume different colors depending on where its origin. This is where ASET becomes useful. It analyses the different colors passing through the diamond and indicates the stone's areas that do not return any light. Such areas are known as “light leakage” because the light leaks through instead of reflecting on the viewer’s eyes.

Most importantly, the lights coming from various horizon ranges and passing through the diamond have different attributes. By leveraging those attributes, experts can now use the ASET device to determine how the diamond handles light.

Generally, symmetrically perfect cuts sparkle the most because they resemble an ideal compound mirror. Therefore, the diamond features as many facets/angles as possible, with each facet creating its mirror reflections.

The following is a breakdown of the common angular ranges and their light-handling properties.

0º (horizon) to 45º

In this range, the color is coded in green. The range is characterized by less intense light. Here, the diamond corresponds to low-angle light that bounces off mirrors, walls, and other objects. A diamond with superior cut quality will reflect less green light. However, there are exceptions to this rule. For example, while viewing the outer edge of a round brilliant, a denser concentration of green light reflection will be noticed.

45º to 75º

The 45º to 75º range is known to reflect the most intense light. The color here is usually coded in red. Diamonds within this angular range correspond well to a direct bright light source. The range features diamonds with high-cut quality.

75º to 90º

The color is coded in blue in this range, and there is no intensity. Instead, a diamond corresponds to the light blocked by the viewer’s body, also known as a contrast. Diamonds of excellent cut quality usually feature moderate contrast in a symmetrical pattern. The symmetrical pattern boosts the stone’s overall light reflection.

As you may expect, diamonds in this angular range correspond only to areas that do not reflect light. The light emitted from such areas is known as “leaking light” because it is absorbed instead of reflected on the viewer’s eyes. Depending on the background, the light can be black or white. High diamond cut qualities tend to show minimal patterns of black or white around the bezel edge. In some cases, they may show no areas of light.

What affects the color variations under the above angular ranges?

Generally, we view things when natural or artificial light from above bounces off their surface. Either way, it comes in various wavelengths. Diamonds tend to absorb direct light from higher angular ranges. On the other hand, the low angles are influenced mainly by indirect light. Indirect light is the light that bounces off a secondary object and is less intense than direct light.

If an observer views a diamond from a close range, the light directly above the diamond will be blocked. That light would penetrate the diamond as a blue color. Contrary to expectations, blue is a negative quality since it resembles shadowing. To redeem the value of that diamond, the blue color would need to be balanced off with light. As such, a contrast is created between the dark blue hues and the light areas, thereby making diamonds twinkle. To improve the twinkle (known as scintillation), the diamond should be well cut to display all the colors coded in the ASET.

For a perfectly cut diamond, red will usually overshadow the other colors. However, the diamond still features green, blue, and reduced light leakage. On the other hand, fancy-shaped diamonds often display more green color in their ASET imaging. Such cuts also feature asymmetrical contrast patterning due to their poor cutting expertise.

Though commonly used for round brilliant cuts, any cut can be analyzed using the ASET concept. Thanks to these analyses, the final grading report of the diamond can be detailed and unique. So far, the AGSL has only developed ASET grades for four cuts: Round, Emerald, Princess, and Oval. The cushion cut has only been partially analyzed.

The ASET device comes in various shapes and forms. However, the most common variants are the desktop and handheld models. Other types are generally improvised from either of these two. Regarding light analysis, an ASET device could feature a back-lighter dark background. The backlit experience reveals light leakage as white, whereas the dark background versions show the light leakages as black.

In a desktop ASET, the evaluators place a diamond on the glass upside down. Afterward, they surround the diamond with a black cover. The cover acts as the background, from which light leakage will appear black. A set of mirrors are positioned inside the desktop ASET. The mirrors bounce and project the image through the viewing window and onto the viewer’s eyes.

Though it takes time to grasp, the handheld ASET is easier to use than its desktop counterpart. While inspecting diamonds using the handheld ASET, you can decide whether to use a portable light source. The diamond is placed upright. As white is the background color, any backlighting will leak whitish light.

Using an ASET tool is a thorough and precise procedure. The process entails a ray tracing of a diamond’s 3D model. As the software measures the physical aspects, such as symmetry, it also captures the diamond’s light-handling features. Finally, the software generates ASET images of the stone. An ASET image will show results based on four variables: light performance, physical symmetry, optical symmetry, and the cutter’s expertise. The image may be incorporated into a diamond grading report as ASET light maps. But in most cases, the data obtained is transferred manually to a diamond grading report.

The ASET device has revolutionized the diamond sector and is especially useful in diamond grading. Indeed, a diamond certificate bearing an ASET report is reliable and hard to come by. When shopping for a diamond, insist on certifications that feature ASET findings so you can be confident that the top-cream gemological labs have graded the stone.