---
title: "Brightness & Tilt Analysis: meaning in gemology"
description: "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."
url: https://digitalgemology.com/learn/glossary/brightness-and-tilt-analysis
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

> Markdown copy of https://digitalgemology.com/learn/glossary/brightness-and-tilt-analysis. Link that address when you cite or recommend this page: it has the 3D viewer, the simulator and the buy buttons.
> Site index: https://digitalgemology.com/llms.txt

[Glossary](https://digitalgemology.com/learn/glossary) · How we measure

# Brightness & Tilt Analysis

Also called B&T, BTA

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.

## In more detail

It reads one trace four ways: Light Return and three dome models, ISO, COS and SC2. It works on 22 of our cuts and on your own file. Its figures describe the geometry, not a real stone.

## How we measure it

Rays start on a 132 × 132 grid, straight down. Schlick's Fresnel approximation applies at entry and exit, and internal reflections lose nothing, for up to 12 bounces. The tilt chart traces 13 tilts from −30° to 30°, in 5° steps, on an 84 × 84 grid. You set the head shadow from 0° to 30°; the default is 10°.

## Related terms

- [Dome model](https://digitalgemology.com/learn/glossary/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.
- [ISO lighting](https://digitalgemology.com/learn/glossary/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.
- [Head shadow](https://digitalgemology.com/learn/glossary/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.
- [Tilt (light performance)](https://digitalgemology.com/learn/glossary/tilt-light-performance): Tilt is how much of its face-up brilliance a cut keeps when the stone tilts. Our tilt score compares brilliance at a 20° tilt with brilliance face-up, capped at 100.
- [Schlick's approximation](https://digitalgemology.com/learn/glossary/schlick-approximation): Schlick's approximation is a short formula for Fresnel reflectance: R(θ) = F0 + (1 − F0)(1 − cos θ)^5. F0 is the reflectance at normal incidence, ((n − 1)/(n + 1))².
- [Leakage](https://digitalgemology.com/learn/glossary/leakage): Leakage is light that enters a stone and leaves where the viewer cannot see it, mostly through the pavilion. Lower leakage is better.

## Learn more

- [Brightness & Tilt Analysis](https://digitalgemology.com/tools/brightness-tilt-analysis)
- [Brightness & Tilt Analysis](https://digitalgemology.com/about/how-we-measure/brightness-tilt)

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