---
title: "Head-on light: meaning in gemology"
description: "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…"
url: https://digitalgemology.com/learn/glossary/head-on-light
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Light performance

# Head-on light

Also called pavilion reflecting head-on light, head-on test

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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/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](https://digitalgemology.com/learn/glossary/pavilion-angle): The pavilion angle is the slope of the pavilion mains, measured against the plane of the girdle.
- [Total internal reflection](https://digitalgemology.com/learn/glossary/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.

## Learn more

- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [T73 Round Brilliant Cut](https://digitalgemology.com/cuts/t73-round-brilliant)

[All 328 terms](https://digitalgemology.com/learn/glossary)