What you see in the ASET.

The Angular Spectrum Evaluation Tool (or ASET) is used in the new AGS grading system for light performance assessment. Light performance assessment is 1 of 3 sub-grades in the overall cut grade category.

Before making summary judgments it is important to note the weighting of the ASET and the single view it offers:

Using the desktop or handheld model you see just one static view of a diamond. AGS will be allowing members to assign provisional grades with this one view, so it figures prominently into the assessment matrix, but ‘back at the lab’ there will be further paces: Ray-tracing software will calculate light return by way of the ASET model using 5 positions of the diamond relative to the observer (straight and tilted to 4 compass points). These assessments will be calculated at both 30 and 40 degrees of obscuration for a total of 10 slightly different positions/looks.

The ASET is a device that gives a color-coded map of light usage by a diamond. The two configurations are handheld and desktop.

The handheld unit is providing interesting cursory images and is useful even on mounted goods.  However, a lack of standardization/consistency prevents detailed comparisons for grading purposes at this time. Therefore, my comments here pertain to the forthcoming desktop configuration, not the handheld.

The centering arm gets the diamond to its correct position and the glass platform provides automatic leveling and standardized placement – though I am curious about this placement fundamental being relative to diamonds’ tables rather than their girdles. The glass has dual anti-glare treatment on the bottom side 2mm thick and single treatment on the top.  This treatment also stops UV rays. In the next few months AGS will be shipping it to many users.

(Copyright AGS 2005, used with permission)

Here is a good stone

Here is a bad stone

Interpreting Images

Blue areas are the dark contrast areas you observe in a diamond due to obscuration. In the ASET configuration it is intended to represent light coming from 75 degrees to 90 degrees. The amount and distribution of these areas are important. In the AGS metric well made round brilliants have about 18% blue content. A princess cut runs less than this.

Red is the most desirable color to have in the image. This is intended to be light coming from 45 to 75 degrees: It is not obscured so it is producing brightness via the most direct light.

Green light comes from the horizon to 45 degrees. It is usually reflected light and of lower quality. It should be minimized if possible with this important exclusion: Undesirable in large quantities, green is not altogether bad depending on the size and distribution of the areas in the image. It is one of two vehicles whereby brightness contrast effects are produced - the other is leakage.

Black (or white if using white backlighting) represents areas of non-reflection referred to as leakage. Depending on how you define it, this is ‘escape’ (a better scientific word than leakage) or simply non-returning facets.

(Copyright AGS 2005, used with permission)

Here is a CosSq chart for 30 degrees of obscuration.

Distance

One important determination AGS had to make was that of distance selection. The foundation identifies it as designed around ‘a close viewing distance.’ This distance is 25 cm (just short of 10”), which was chosen for many reasons having to do with standardization in other visual fields and physical/optical properties of human vision (the near point of accommodation for typical humans over age 40 moves to >20 cm).

Also key was the fact that the ability to discern cut quality decreases with distance. Most importantly, the effect of the observer’s head decreases with distance – and a stone of fine make next to a stone of inferior make are much closer in appearance without obscuration providing contrast.

Even without those reasons, it seems practical to me. If you notice a sparkly diamond and ask someone to show it to you I would suspect that the general tendency is to view it at about 10 inches away - perhaps just slightly closer or further (depending on your eyesight and how well you know the person).

The point has been made that this is not a metric for every possible distance, environment, etc. However, AGS is representing it as a practical baseline.

The above info is intended in reference to the standardized desktop ASET,
Below:  is the handheld version of the ASET along with light tray 

An AGS chart showing angular spectrum images for practical combinations of crown and pavilion angles (increments of .2) for 57% table and 30 degrees obscuration. These charts are really something. It’s interesting to see where small shifts in angles become ‘breaking points’ for different light return behaviors.

(Copyright AGS 2005, used with permission)

Contrast is a tremendous part of the foundation of this system, and deservedly so. Identifying contrast in static views is a primary reason for going to this multi-colored system.

As an example of the difference contrast via obscuration makes, here are cosine squared images of 2 stones. They are identical, except that a 30 degrees cone of obscuration, equivalent to an observer’s head, is present in the one on the right. During total grading analysis performance at both 30% and 40% obscuration is assessed by AGS.

(Copyright AGS 2005, used with permission)

This is a zoomed example of one section of the chart with 30 degrees obscuration.  Tolkowsky is indicated at 40.8 because, in increments of 0.2 = 40.75 is closer to 40.8 than 40.6.

You can see a phenomenon that occurs due to table reflection dynamics between 40.7 and 40.8 PA across the board – a shift in ‘eye’ color within the table from greento red. This is interesting but insignificant.

(Copyright AGS 2005, used with permission)

Using it without the light  tray but with background lighting