ASET (Angular Spectrum Evaluation Tool) images have become as popular as Idealscope images for evaluating the light performance of a diamond. This is certainly also due to the fact that AGS (one of the most reputable grading labs) uses ASET images to determine the cut grade of a diamond.

If you look into a regular AGS grading report you will always find the following ASET image:

Of course AGS does not only use the ASET image to determine the cut grade but it does have some influence.

One of the main advantages of an ASET image is that it can provide more information than an Idealscope image. Or better said it can provide different information which we will get into in a moment. This is especially useful for fancy shaped diamonds, though. For usual round cut diamonds an ASET image does not necessarily provide more data than an Idealscope image would.

In this article I want to have a close look with you at ASET images so that you will be able to evaluate them on your own!

How to evaluate an ASET image?

The ASET was developed to indicate how well a diamond is making use of the light around it.

The ASET does so by showing what range or angle of light the diamond is using. It specifically uses different colors to represent a differing angle range of available light. Thus, unlike the Idealscope the ASET is not showing direct areas of light leakage but much rather from which angles the diamond is taking light in.

The blue color represents light ranges of 75° to 90°. The red color represents ranges of 45° to 75°. Finally, the green color represents ranges of 0° to 45°.

Now, just imagine you are looking at this diamond from above as is the case when you perform an ASET analysis. Logically, you will have the diamond taking in light from many different angles.

What range of light a diamond can use naturally also depends on its surroundings. Very often it is your own head that is obscuring some of the available light to the diamond.

But assuming that the lighting conditions are perfect a really ideal cut diamond will be able to use much more light of the 45° to 75° range than a poor cut diamond.

The red color represents the light of the 45° to 75° range and this is also the light that is reflected the brightest. The more red color you can see in an ASET image the brighter the diamond will appear.

The green color represents the light that strikes the diamond from the 0° to 45°. These areas are perceived as less bright but nonetheless still bright. Every diamond has these less brighter areas to a certain degree. They also serve in providing a contrast and contrasts also create a brightness effect.

The blue color represens areas of obstruction, that is to say light the diamond could not take in due to your head or body for instance. These areas provide for the most contrast needed to create a really bright diamond.

Finally, black/white areas represent areas of light leakage which are the most undesirable.

Basically, you can take the following examples as a general ASET reference chart:

The average diamond displays white areas of light leakage and many green areas of less light return. The above average diamond on the other hand has much fewer areas of light leakage but still quite some green areas of diminished light return. The Excellent cut diamond on the other hand has the most red areas of bright light return.

Apart from only looking at ASET images it is equally important to have a real look at the diamond itself from all angles. Online vendors like James Allen, Whiteflash and Brian Gavin are among the very few who will let you have access to ASET images and 360° videos of all their diamonds.

Should the center dot of an ASET image be red or green?

Another issue that very often causes confusion among people is the fact that you will find ASET images of diamonds with a green dot in the middle and with a red dot in the middle:

Not surprisingly, I get asked a lot by people what kind of diamond is to be preferred.

The answer is: It does not matter.

The dot in the middle is also referred to as the table reflection. Because in the end the dot is just the reflection of the table facet by the pavilion angles. Now, depending on the exact pavilion angle the area is either receiving light from above 45° or below 45°. And this means that the centre will either be green or red:

Scientists from AGS have found out that when the pavilion angle is above 40.768° it will receive the red light from above 45°. When the pavilion angle is below 40.768° it will receive green light from the below 45° spectrum. Obviously there is also a twilight zone in which the centre can be somewhere in between red and green.

As a perfect pavilion angle can range anywhere between 40.6° and 41.0° the centre of a super ideal cut diamond in an ASET image can be either green or red. So don’t get intimidated by that!

Let us practice ASET image evaluation

Now, let us simply have a look at several examples of ASET images to determine which diamond has a better light performance:

In the case above it is pretty obvious that the diamond on the left side exhibits more green areas. Which means that the diamond on the left side will have more areas of less light return. In this case the diamond on the right side would be the better choice in terms of the light performance.

Let us have a look at another example. Which of the following three diamonds do you think has a better light performance just based on the ASET image?

As you can see the style of all ASET images is somewhat different. And this is also what I wanted to demonstate to you with this example.

You can see that the left ASET image looks rather orange, while the one in the middle looks rather red. The red ASET image is completely reddened out.

Cases like this are mostly due to inconsistent practices of taking the ASET images. The ASET images might have been taken with different ASET Scopes and in differing lighting conditions.

The ASET image on the right looks far too perfect and I think it is unreliable.

In a case where the ASET images have been taken in different conditions it is difficult to form a conclusive opinion.
Still, I think that in this case the left diamond would be the best choice as it is the one with the fewest white[r] areas of very slight light leakage.

Now, with the help of ASET images you should definitely be able to filter out low performing diamonds. However even two super ideal cut diamonds that are perfectly fine can look different on an ASET image because they simply have a different sparkling style. In such a case you would need to know what kind of sparkling style you prefer in a diamond.

Thus, if you have any doubts or difficulties I would be glad to help you out with my diamond experience. Please contact me and I will be more than happy to help you evaluate some diamonds or answer any other question you might have.