The four windows and their settings
What the four windows show
One trace, read four ways. We follow the light back from your eye, through the stone, to the part of the dome where it started. In ISO, COS and SC2, a white facet returns light from the brightest part of that dome. A black facet returns no light.
Light Return
Where the light comes from:
- Direct, 45° to 75° up
- Indirect, below 45°
- Contrast, above 75°, where your head is
- Leakage
ISO
A dome lit evenly from every direction. This is the most forgiving test: it asks only how much light comes back, not from where.
COS
Bright overhead, fading to black at the horizon. It is hardest on shallow cuts, because they depend on low-angle light.
SC2
A ring of light with its peak at 45°, dark overhead and at the horizon. It is the closest to the light on a stone on a hand.
None of the four windows depends on scene lighting. They model the geometry of the cut, not a real stone. ISO, COS and SC2 use the definitions in the GemRay user guide by Robert W. Strickland. The implementation is our own.
Face-up brightness
The percent of the dome’s available light that the stone returns into the viewing cone, averaged over its face-up image. Whole stone is the full outline. Table only is the area inside the table facet. This figure changes most between a good cut and a poor one. Leakage sits below the rule because it is not a light model. It counts the light paths that came back with no light at all, so it does not change when you change the head shadow.
| Light model | Whole stone | Table only | vs T57 |
|---|---|---|---|
| ISO | |||
| COS | |||
| SC2 | |||
| Leakagelight that never came back |
Brightness against tilt
The stone tilts under a fixed dome, as it does on a hand. Solid lines show the whole stone. Dashed lines show the table only. If the lines of a cut stay high across the sweep, the cut keeps working when you do not look at it straight on.
The chart as a table
What we count
Here, brightness is energy, not only direction. At the crown, a Fresnel share of the light reflects straight off and does not go into the stone. That light still gets to the eye. Face-up, the table’s reflection points straight back at you, and your head’s shadow hides it. It shows only when the stone tilts by more than half the head-shadow angle. The head shadow also causes the dip near zero tilt. A well-cut round sends much of its light back from directly overhead, where your head is. The light that goes into the stone loses a Fresnel share again on the way out. We apply no loss to internal bounces, because almost all of them are total internal reflection, which loses nothing.
This choice is important. We measured these 22 cuts. If we count only direction and not energy, 14 of them change rank on ISO, 13 on COS and 9 on SC2. If we add the exit face to the entry face, 4 change rank on ISO, 3 on COS and 2 on SC2. The model is stable; the direction-only count is the outlier.
What the model leaves out. We do not count the energy lost at partial internal reflections. That is why these figures are higher than those of a full physical simulation. It is also why the dip near zero tilt is shallower here than in GemRay.