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.
In more detail
It is hardest on shallow cuts, because they depend on low-angle light. The definition follows Robert W. Strickland's GemRay user guide; the implementation is our own.
How we measure it
In the Brightness & Tilt Analysis, W = sin e for elevation e. W is 0 inside the head shadow and below the horizon.
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.
- 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.