Light Return and Light Leakage Explained

Diamond light performance is a hot topic, especially with the rise of e-commerce sales. Fortunately, diamond imaging to demonstrate a stone’s light performance is possible.

Light entering a diamond will do one of two things. It will either reflect and return to the viewer’s eyes as positive beauty components like brightness, dispersion, and sparkle, or it will leak through the diamond’s pavilion (also known as windowing), creating dull or dark areas. Light return is good. Light leakage is bad.

Light leakage can be defeated by overwhelming the diamond with light. Most jewelry stores use bright overhead spotlights to do that, so you won’t see leakage at the sales counter.

PriceScope Pointer: When examining diamonds in stores ask to compare them in several different lighting conditions, including a dimly lit or shaded area, so you can analyze how appearance holds up, place to place.

The Beauty of Structured Light Environments (SLEs)

To analyze diamond light performance, a translucent or reflective hemisphere can be used to color-code light entering a diamond from different angles. Research gemologists and optical scientists have used SLEs since the 1970s to analyze different aspects of light behavior, including overall light performance (light return versus light leakage) and precision of craftsmanship (reflective optical symmetry).

Performance SLEs vs Precision SLEs

Some SLEs are designed to interpret performance-only. Some SLEs are designed to interpret precision-only. It is extremely important to know what a particular SLE is designed to show.

Light Return Analysis

Why Light Return Analysis is Important

Most cut grading systems do not differentiate at a high level, making further diamond light performance analysis necessary for those who want a diamond with little or no light leakage. Undeniably, the most important light performance component is light return. The light which returns to the viewer will be seen as brightness, dispersed light (fire) and scintillation events (sparkle).

The SLEs below, are the simplest, most repeatable, and unbiased ways to analyze light behavior within a diamond. The only scientifically peer-reviewed system for grading diamond light performance was established using the ASET SLE.

Ideal-Scope

• Strengths: Simple and decisive
• Limitations: Less useful for fancy shapes
• Popularity: Used by researchers and a few e-commerce sellers
• Learn more: PriceScope education page covering Ideal-Scope

Ideal-Scope Diamond Imaging Examples

Note: In diamonds with the highest light return, small, symmetrical white triangles at girdle and facet meet points are normal.

ASET®

• Strengths: Useful for all shapes
• Limitations: Learning curve, hard to standardize
• Popularity: Used by AGS Laboratories, researchers, and a few e-commerce sellers
• Learn more: PriceScope education page covering ASET

ASET® Diamond Imaging Examples

Note: In diamonds with the highest light return, small, symmetrical black triangles at girdle and facet meet points are normal.

Optical Symmetry Analysis

Why Optical Symmetry Analysis is Important

Arguably, optical symmetry analysis is not essential. The vast majority of diamonds are produced without regard for fine-tuning the facets so that the internal facet reflections overlap perfectly with their opposites.

The nuanced answer: When combined with strong light return and sharp, balanced contrast, diamonds with the highest level of reflective optical symmetry can produce more intense scintillation and more visible fire, due to larger internal compound mirrors. Diamonds with truly superior optical symmetry are not casually or accidentally made.

Hearts & Arrows Viewers

• Strengths: The simplest Optical Symmetry tool
• Limitations: It tells you nothing about Light Return or Light Leakage
• Popularity: By far, the most well-known SLE, although many don’t understand its limitations
• Learn more: PriceScope education page covering Hearts and Arrows

Hearts & Arrows Diamond Imaging Examples

Warning: Computer Generated Images (CGI)

Photographs versus CGI

Some sellers provide light performance proof by taking real photographs, made in various SLEs. This is highly desirable, as long as the environment is set up properly, because it permits extremely detailed analysis. 

When photography is not practical, various software programs can produce CGI of diamonds in various SLEs. This can be useful – or misleading – depending on how those images are produced.