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.
In more detail
Red is light from 45° to 75° above the girdle plane, and green is light from below 45°. Blue is light from 75° and up, within 15° of the line of sight, where the viewer's head is. Leakage is light that leaves through the back of the stone, comes from below the horizon, or stays inside. The simulator draws leakage in black; the Light Return images on our gemstone cut pages draw it in gray.
How we measure it
We trace the map face-up and orthographic, with no Fresnel loss, no dispersion and no absorption. So it shows the geometry alone, with no lighting scene. The simulator shows it with its Light map option. The Brightness & Tilt Analysis shows it in its Light Return window. The Cut Grading Tool shows 815 light maps traced on the T73, with no head shadow.
Related terms
- Light Return: 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.
- 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.