---
title: "Light census: meaning in gemology"
description: "The light census is our ray tracer behind the Cut Performance Wheel, the cut-page scores and the Cut Performance Index. It drops a fixed set of rays on a…"
url: https://digitalgemology.com/learn/glossary/light-census
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Light census

Also called census, face-up light census

The light census is our ray tracer behind the Cut Performance Wheel, the cut-page scores and the Cut Performance Index. It drops a fixed set of rays on a stone from above and counts where each one leaves.

## In more detail

The census is deterministic: the rays come from a fixed seed, so the same cut always gets the same score. It leaves out Fresnel loss, absorption, color, inclusions and polish. So a dark colored stone looks darker than its score.

## How we measure it

We scale each cut to unit size in plan. For each gemstone, 3,200 rays fall within 0.12 rad (6.9°) of vertical, cosine-weighted, over a disc larger than the stone. Rays that miss do not count. Inside, a ray reflects totally or leaves through the first facet that lets it out, for up to 12 bounces. A second run tilts the stone 20°, with 2,200 rays.

## Related terms

- [Census ray classes](https://digitalgemology.com/learn/glossary/census-ray-classes): In our light census, every ray that enters a stone ends in one of five classes. The classes are useful, blink, leak, grazing and lost.
- [Ray tracing](https://digitalgemology.com/learn/glossary/ray-tracing): Ray tracing follows straight lines of light through a model of a stone. At each facet it applies refraction and reflection, and it records where the light comes from or where it leaves.
- [Fire spread (Δ)](https://digitalgemology.com/learn/glossary/fire-spread): Fire spread Δ is the index difference that our light census uses to split a ray into two colors. It is about 0.86 of each stone's published dispersion.
- [Fixed settings](https://digitalgemology.com/learn/glossary/fixed-settings): Fixed settings are the constants of our scoring method that we chose rather than measured. They are the same for every cut, so the scores can rank cuts within one gemstone.
- [Cut Performance Index](https://digitalgemology.com/learn/glossary/cut-performance-index): The Cut Performance Index is Digital Gemology's open table of light-performance scores for the cuts we sell, in seven gemstones. It gives each cut brilliance, fire, scintillation, tilt, symmetry, spread, shape-norm, leakage and an overall score.

## Learn more

- [Cut Performance Wheel and cut-page scores](https://digitalgemology.com/about/how-we-measure/performance-wheel)
- [Digital Gemology Cut Performance Index](https://digitalgemology.com/learn/cut-performance-index)

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