What is Angular Spectrum Evaluation Tool (ASET)

What is Angular Spectrum Evaluation Tool (ASET)

What is Angular Spectrum Evaluation Tool (ASET)

Posted by Sharif Khan on 17th Dec 2019

Angular Spectrum Evaluation Tool (ASET)

It doesn’t matter how experienced a jewelry shopper you think you are. You can never fully grasp the various elements that determine diamond quality. That’s partly due to the intricacies involved in assigning diamonds their value, and partly because industry specialists are constantly modifying the mechanisms used in grading diamonds. 

All said, diamonds are forever, and you desire a stone that you’ll remain attached to for eternity. Therefore, it pays to review and understand the gem from all possible spectrums. Of particular interest should be the cut of that diamond.

You probably are already familiar with the many diamond cuts out there. But do you understand how those cuts impact the stone’s light performance? How a diamond sparkle, also known as its fire and brilliance, have everything to do with its cut and polish. You can understand that further by exploring an ingenious device known as Angular Spectrum Evaluation Tool.

James Allen

What Is Angular Spectrum Evaluation Tool?

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

Now, there are two key aspects to consider here when determining the fire and brilliance of a diamond. First is whether the diamond absorbs light in the first place, and how much of the light it collects. Second is how the stone handles the collected light. A diamond may absorb light alright. But the light may either reflect on your eyes or simply leak through.

To most diamond buyers, the extent to which a diamond absorbs and reflects light has all to do with its color. Well, that’s not entirely true. And as we shall find out, the diamond cut plays a much more significant role.

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

When AGS created ASET in 2005, the intention wasn’t only to use it as a reflector tool. Instead, the device would also offer the theoretical basis that the American Gem Society Laboratories (AGSL) uses for their cut grading and light performance analyses. Since its creation, the tool has significantly changed the landscape in terms of diamond grading. The device uses scientifically advancedgrading systems. It collates, analyzes and presents data through computer-generated imaging.


How Does ASET Work?

The basic concept behind ASET is to establish crucial and particular 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. That’s done to delineate the specific angular ranges. As light travels through the diamond, it will assume different colors depending on where it’s coming from. And this is where ASET now becomes really useful. It not only analyses the different colors passing through the diamond. It also indicates the specific areas of the stone that don’t return any light. Such areas are known as “light leakage”, as the light literally leaks through instead of reflecting on the viewer’s eyes.

Most importantly, the lights coming from various ranges of the horizon 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 tend to have the most sparkle. That’s because such cuts resemble an ideal compound mirror. Therefore, the diamond features as many facets/angles as possible, with each facet creating its own 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 normally 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. If a diamond has superior cut quality, it will reflect less green light. However, there are exceptions to that rule. For example, if you view the outer edge of a round brilliant, you’ll notice a denser concentration of green light reflection.

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 that fall within this angular range correspond well to a direct source of bright light. The range features diamonds with high cut quality.

75º to 90º

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


As you may expect, diamonds in this angular range correspond only to areas that don’t reflect light. The light that’s emitted from such areas is known as “leaking light”. That’s because it’s absorbed but not reflected on the viewer’s eyes. Depending on the kind of background, the light can either 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 at all.

So, what affects the color variations under the above angular ranges?

Generally, we visually perceive things when the light from above bounces off their surface. That light could either be natural or artificial. 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 mostly influenced by indirect light. Indirect light refers to the light that bounces off a secondary object. As such, it’s less intense than direct light.

If you’re viewing a diamond from a close range, you’d normally block the light that’s directly above the diamond. That light would penetrate the diamond as a blue color. And contrary to what you may think, the blue color is considered 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. And that’s what makes diamonds twinkle. Now, to improve the twinkle, also known as scintillation, the diamond will need to be well cut. That way, it can display all the colors that are coded in the ASET.

For a perfectly cutdiamond, red will usually overshadow the other colors. However, the diamond will still feature some 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. That’s primarily due to their rather poor cutting expertize.

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

Versions of the ASET Device

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. In terms of light analysis, an ASET device could either feature a back-litor dark background. The backlitbackground reveals light leakage as white. On the other hand, the dark background versions show the light leakages as black.

In a desktop ASET, the evaluators place a diamond on the glass upside down. Then, 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.

The handheld ASET is easier to use than its desktop counterpart. However, it takes quite some time to grasp. While inspecting diamonds using the handheld ASET, you can choose to use a portable light source or not. The diamond is placed upright. As white is the background color, any backlighting will result in whitish light leakage.

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 like symmetry, it also captures the diamond’s light-handling features. Finally, the software generates ASET images of the stone. Generally, an ASET image will show results based on four variables. The variables include; light performance, physical symmetry, optical symmetry, and the cutter’s expertise. The image may be incorporated into a diamond grading report as is, in the form of ASET light maps. But in most cases, the data obtained is transferred manually to a diamond grading report.

The ASET device has truly revolutionized the diamond sector. It’s especially useful in diamond grading. Indeed, a diamond certificate bearing an ASET report is highly reliable and hard to come by. But whenever you’re shopping for diamonds, insist on certifications that feature ASET findings. Then, you’re sure the top-cream gemological labs graded the stone.