Light performance is how well a cut stone handles light. We describe it with brilliance (light returned), fire (color) and scintillation (sparkle).
In more detail
It depends on the cut's geometry and on the stone's refractive index and dispersion. A simulation of light performance describes a design, not a real stone. It is not a grading report.
How we measure it
We measure it four ways: the light census, Brightness & Tilt Analysis, Cut Grading Tool and simulator. The census is behind the Cut Performance Wheel and the Index. The four answer different questions, so their numbers are not interchangeable.
Related terms
- Brilliance: Brilliance is the white light that a cut stone returns to the viewer. In our census, it is the share of entering light that leaves through the crown toward the viewer.
- Fire: Fire is the flashes of spectral color that a cut stone sends out. In our census, fire is the share of returned light whose colors leave more than 0.6° apart.
- Scintillation: Scintillation is the pattern of light and dark flashes a stone shows as it, the light or the viewer moves. Our score reads it from the number of facets that reach the surface of the cut.
- Light census: The light census is our ray tracer behind the Cut Performance Wheel, the cut-page scores and the Cut Performance Index. It drops a fixed set of rays on a stone from above and counts where each one leaves.
- 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.
- 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.