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.
In more detail
A dome model asks only where the light comes from, not what scene or color it has. ISO weights every elevation equally, and COS favors light from overhead. SC2 favors a ring of light near 45°. The definitions follow Robert W. Strickland's GemRay user guide; the implementation is our own.
How we measure it
We follow each path back from the eye to the direction its light came from. Its elevation e sets the weight: W = 1 (ISO), sin e (COS) or 2 sin e cos e (SC2). W is 0 below the horizon and inside the head shadow. The Cut Grading Tool's brilliance uses the ISO dome with a 23° head shadow.
Related terms
- 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.
- COS lighting: COS is a dome model in which light is brightest overhead and fades to zero at the horizon. Each direction is weighted by the sine of its elevation.
- SC2 lighting: SC2 is a dome model with a ring of light that peaks at 45° elevation. It is dark overhead and at the horizon.
- Brightness & Tilt Analysis: 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.
- 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.
- GemRay: GemRay is Robert Strickland's ray tracer for GemCad designs, for Windows. It shows how a finished stone will look and how it darkens as it tilts.