---
title: "Dome model: meaning in gemology"
description: "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:…"
url: https://digitalgemology.com/learn/glossary/dome-model
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Dome model

Also called light model, dome lighting

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.

## In more detail

A dome model asks only where the light comes from, not what scene or color it has. ISO weights every elevation equally, and COS favors light from overhead. SC2 favors a ring of light near 45°. The definitions follow Robert W. Strickland's GemRay user guide; the implementation is our own.

## How we measure it

We follow each path back from the eye to the direction its light came from. Its elevation e sets the weight: W = 1 (ISO), sin e (COS) or 2 sin e cos e (SC2). W is 0 below the horizon and inside the head shadow. The Cut Grading Tool's brilliance uses the ISO dome with a 23° head shadow.

## Related terms

- [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.
- [COS lighting](https://digitalgemology.com/learn/glossary/cos-lighting): COS is a dome model in which light is brightest overhead and fades to zero at the horizon. Each direction is weighted by the sine of its elevation.
- [SC2 lighting](https://digitalgemology.com/learn/glossary/sc2-lighting): SC2 is a dome model with a ring of light that peaks at 45° elevation. It is dark overhead and at the horizon.
- [Brightness & Tilt Analysis](https://digitalgemology.com/learn/glossary/brightness-and-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.
- [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.
- [GemRay](https://digitalgemology.com/learn/glossary/gemray): GemRay is Robert Strickland's ray tracer for GemCad designs, for Windows. It shows how a finished stone will look and how it darkens as it tilts.

## Learn more

- [Brightness & Tilt Analysis](https://digitalgemology.com/about/how-we-measure/brightness-tilt)
- [Brightness & Tilt Analysis](https://digitalgemology.com/tools/brightness-tilt-analysis)
- [Four light measures, side by side](https://digitalgemology.com/about/how-we-measure/light-measures)

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