Light Return sorts each point of a face-up stone by the direction its light came from. Its four classes, direct, indirect, contrast and leakage, together cover the whole face-up stone.
In more detail
The shares depend on geometry alone: there is no Fresnel loss, no dispersion and no absorption. The light map draws the four classes in color. Industry maps such as the ASET map work on the same idea.
How we measure it
The simulator traces the map face-up and orthographic. It classes each ray by the elevation its light came from, above the girdle plane. Contrast is 75° and up, direct 45° to 75°, and indirect below 45°. The simulator's shares come from a 160 × 160 render with one ray per pixel, rounded to whole percent. The Brightness & Tilt Analysis and the Cut Grading Tool use the same zones.
Related terms
- Light map: A light map is a face-up picture of a cut stone, colored by the direction each point's light came from. Red is direct light, green is indirect light, blue is contrast, and a dark color is leakage.
- Direct light (Light Return): Direct light is light that reaches the viewer from 45° to 75° above the girdle plane. Our light map shows it in red.
- Indirect light (Light Return): Indirect light is light that reaches the viewer from below 45° above the girdle plane. Our light map shows it in green.
- Contrast (Light Return): Contrast is the part of a face-up stone whose light comes from 75° or higher above the girdle plane. This zone lies within 15° of the line of sight, where the viewer's head is.
- Leakage: Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.
- ASET map: An ASET map is a four-color picture that shows from which directions a stone's face-up light comes. It can be part of the AGS Ideal Report by GIA.