---
title: "COS lighting: meaning in gemology"
description: "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."
url: https://digitalgemology.com/learn/glossary/cos-lighting
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# COS lighting

Also called COS dome, cosine dome

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.

## In more detail

It is hardest on shallow cuts, because they depend on low-angle light. The definition follows Robert W. Strickland's GemRay user guide; the implementation is our own.

## How we measure it

In the Brightness & Tilt Analysis, W = sin e for elevation e. W is 0 inside the head shadow and below the horizon.

## 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.
- [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.

## Learn more

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

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