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.
In more detail
A well-cut round sends much of its light back from straight overhead. So the head shadow causes a dip in return near face-up, and it draws the contrast pattern. Each tool uses its own head shadow, which is one reason their figures differ.
How we measure it
Simulator: no light within 6°, fading back in by 10.5°. Brightness & Tilt Analysis: 0° to 30°, which you set; the default is 10°. Cut Grading Tool brilliance: 23°, after Hemphill et al. (1998) and Moses et al. (2004). Light census and Cut Grading Tool light maps: none.
Related terms
- Contrast (Light Return): Contrast is the part of a face-up stone whose light comes from 75° or higher above the girdle plane. This zone lies within 15° of the line of sight, where the viewer's head is.
- 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.
- 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.
- 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.
- 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.