The Brightness & Tilt Analysis is a free Digital Gemology tool. It measures how much dome light a cut returns face-up, and how this changes with tilt.
In more detail
It reads one trace four ways: Light Return and three dome models, ISO, COS and SC2. It works on 22 of our cuts and on your own file. Its figures describe the geometry, not a real stone.
How we measure it
Rays start on a 132 × 132 grid, straight down. Schlick's Fresnel approximation applies at entry and exit, and internal reflections lose nothing, for up to 12 bounces. The tilt chart traces 13 tilts from −30° to 30°, in 5° steps, on an 84 × 84 grid. You set the head shadow from 0° to 30°; the default is 10°.
Related terms
- Dome model: A dome model is an idealized light source: a dome over the stone that weights light by its elevation. Our Brightness & Tilt Analysis uses three dome models: ISO, COS and SC2.
- ISO lighting: ISO is a dome model that lights the stone evenly from every direction above the horizon. It asks only how much light comes back, not from where.
- Head shadow: The head shadow is the part of the overhead light that the viewer's own head blocks. In our tools, it is a cone around the line of sight in which no light counts.
- Tilt (light performance): Tilt is how much of its face-up brilliance a cut keeps when the stone tilts. Our tilt score compares brilliance at a 20° tilt with brilliance face-up, capped at 100.
- Schlick's approximation: Schlick's approximation is a short formula for Fresnel reflectance: R(θ) = F0 + (1 − F0)(1 − cos θ)^5. F0 is the reflectance at normal incidence, ((n − 1)/(n + 1))².
- Leakage: Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.