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.
In more detail
It is the most forgiving of the three dome models. The definition follows Robert W. Strickland's GemRay user guide; the implementation is our own.
How we measure it
In the Brightness & Tilt Analysis, ISO gives every direction above the horizon the weight W = 1. The weight is 0 inside the head shadow. The Cut Grading Tool's brilliance is the ISO figure with a 23° head shadow, averaged over 13 tilts.
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.
- 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.
- Cut Grading Tool: The Cut Grading Tool is a free Digital Gemology tool that grades a round brilliant diamond's cut from its proportions. It reads GIA's published cut grade estimation tables; it is for education and does not give a GIA grade.