Head-on light is light that enters a stone straight down along its axis. Our head-on test gives the share of the pavilion whose facets are steep enough to reflect that light.
In more detail
A facet reflects head-on light only if it lies at or above the critical angle from the girdle plane. The test checks the first reflection only; it does not trace where the light goes next.
How we measure it
Cut pages give “Pavilion reflecting head-on light” as a share of the pavilion, by projected area. The verdict reads “Holds head-on light” at 99% or more, and “Minor windowing” from 90%. Below 90% it reads “Steepen the pavilion”. A culet flat counts as not reflecting, and a cut with no table is not measured.
Related terms
- Critical angle: The critical angle is the angle from a facet's normal beyond which light inside a stone reflects totally. It follows from the refractive index: θc = arcsin(1/n).
- Windowing: A window is a face-up area through which you can see straight through a stone. It forms where pavilion facets are too shallow to reflect light, so light passes out of the back.
- Guide minimum pavilion angle: The guide minimum pavilion angle is the shallowest pavilion at which a round still returns head-on light through the table. It is the larger of the critical angle and (180° − critical angle) ÷ 4.
- Pavilion angle: The pavilion angle is the slope of the pavilion mains, measured against the plane of the girdle.
- Total internal reflection: Total internal reflection happens when light inside a stone meets a facet beyond the critical angle. The facet then acts as a perfect mirror, and all the light reflects back into the stone.