ASET Scope: Angular Spectrum Evaluation Tool
ASET Scope: Angular Spectrum Evaluation Tool
A colour-coded optical instrument for assessing diamond cut performance and light return
The ASET scope (Angular Spectrum Evaluation Tool) is a handheld or benchtop optical instrument designed to produce a colour-mapped image of a polished diamond's light-handling behaviour. Developed and promoted by the American Gem Society (AGS) as a core component of its proprietary cut-grading methodology, the ASET scope allows gemologists, graders, and informed consumers to evaluate light return, contrast patterning, and optical leakage in a single, rapidly obtained image. It is among the most diagnostically transparent consumer-accessible tools available for assessing cut quality, translating the complex physics of light interaction within a faceted stone into an immediately legible visual display.
Principle of Operation
The ASET scope functions by illuminating a diamond from a hemispherical interior surface lined with three distinct colour zones, each corresponding to a different range of light-entry angles relative to the stone's table. When the diamond is placed face-down against the aperture, reflected light from each angular zone returns through the crown and is visible to the observer above. The colour coding is standardised:
- Red — light originating from low angles (roughly 0–45° from the horizon), representing the primary source of brilliance and the most desirable light return in a well-cut stone.
- Green — light from mid-range angles (approximately 45–75°), contributing to overall brightness but considered secondary to red-zone light.
- Blue — light from steep, near-overhead angles, which manifests as contrast in the face-up appearance; a controlled pattern of blue is desirable, as contrast is essential to the perception of scintillation and pattern.
- Dark or black areas — zones where light exits through the pavilion rather than returning to the observer, indicating optical leakage and, by extension, a loss of brilliance.
An ideal ASET image for a round brilliant diamond typically shows a strong, symmetrical distribution of red, a moderate and evenly placed presence of blue contrast, minimal green, and very little or no dark leakage. Asymmetric patterns, excessive green, or large dark zones indicate proportional or symmetry deficiencies that reduce face-up performance.
Role in AGS Cut Grading
The AGS Laboratories incorporated ASET imagery into its light-performance grading system, which grades round brilliants and certain fancy shapes on a numerical scale from 0 (Ideal) to 10. ASET maps generated from ray-tracing software underpin the AGS Platinum grading reports, and physical ASET scope images are used in laboratory and trade settings to corroborate computer-modelled predictions. The methodology was a significant departure from the proportion-based cut standards historically employed by other major laboratories, shifting the evaluative focus from static measurements to dynamic optical performance.
Practical Use in the Trade
The physical ASET scope is compact and inexpensive relative to most gemmological instruments, making it accessible to dealers, retailers, and technically engaged consumers. Its primary limitation is that interpretation requires some training: the colour map is meaningful only in the context of the stone's shape, and minor variations in viewing angle or ambient light can affect the image. For this reason, ASET images on laboratory reports — generated under controlled, reproducible conditions — are generally more reliable than those obtained with a handheld scope in a retail environment.
In the trade, ASET imagery is most commonly encountered on AGS Platinum reports and in the marketing materials of vendors who specialise in precision-cut diamonds, particularly those targeting the "hearts and arrows" market segment. Several independent online diamond retailers publish ASET images alongside Idealscope and hearts-and-arrows photographs as part of a standard performance documentation package.
Relationship to Other Light-Performance Tools
The ASET scope is closely related to, but distinct from, the Idealscope, an earlier instrument that uses a single red-lined hemisphere to reveal leakage (white) and light return (red) without the angular differentiation that ASET provides. The ASET scope's three-colour system offers greater diagnostic resolution, allowing the observer to distinguish not merely whether light is returned but from which angular range it originates. More sophisticated instruments such as the Sarin DiaMension or GIA's Facetware tool rely on computer modelling rather than direct optical viewing, and are used primarily in laboratory rather than point-of-sale contexts.