Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.
In more detail
Leakage rises when pavilion facets are too shallow for the stone's critical angle. So stones with a refractive index below 2 usually leak more than diamond in the same cut. Each of our tools counts leakage in its own way. Compare leakage figures only within one tool and one gemstone.
How we measure it
Light census (Cut Performance Wheel, cut pages and Index): the share of entering rays that leave through the pavilion. The Wheel's method page also counts rays that leave through the crown heading down. Brightness & Tilt Analysis: rays that leave below the horizon, find no exit, or stay inside after 12 bounces. That figure does not change with the head shadow. Simulator light map: light out through the pavilion, from below the horizon, or still inside after 18 bounces.
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.
- 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.
- Critical angle: The critical angle is the angle from a facet's normal beyond which light inside a stone reflects totally. It follows from the refractive index: θc = arcsin(1/n).
- Windowing: A window is a face-up area through which you can see straight through a stone. It forms where pavilion facets are too shallow to reflect light, so light passes out of the back.
- Overall score: The overall score is one figure out of 100 that combines our light-performance scores for one cut in one gemstone. The Cut Performance Wheel calls it Global.
- Census ray classes: In our light census, every ray that enters a stone ends in one of five classes. The classes are useful, blink, leak, grazing and lost.