---
title: "First-order red plate: meaning in gemology"
description: "A first-order red plate is a retarder in a polariscope that shows which of a stone's two rays is slower. It turns parts of the interference figure blue or…"
url: https://digitalgemology.com/learn/glossary/red-plate
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Gem testing

# First-order red plate

Also called red plate, compensator, full-wave plate

A first-order red plate is a retarder in a polariscope that shows which of a stone's two rays is slower. It turns parts of the interference figure blue or yellow, and that gives the optic sign.

## In more detail

Where the stone's slow ray lines up with the slow direction of the plate, the color climbs to blue. Where they cross, it falls to yellow.

## How we measure it

Our Polariscope's red plate is a 550 nm linear retarder, with its slow direction from northeast to southwest. The page reads the sign from the figure's interference colors alone.

## Related terms

- [Optic sign](https://digitalgemology.com/learn/glossary/optic-sign): The optic sign of a uniaxial stone tells which of its two rays is slower. It is positive when the extraordinary index is higher than the ordinary one, and negative when it is lower.
- [Conoscope](https://digitalgemology.com/learn/glossary/conoscope): A conoscope is a strain-free glass sphere set on a stone in a polariscope. It gathers light from many directions at once, so the eyepiece shows an interference figure.
- [Interference figure](https://digitalgemology.com/learn/glossary/interference-figure): An interference figure is the pattern of colors and dark bands that a doubly refractive stone shows through a conoscope. Each point of the figure is one direction through the stone.
- [Polariscope](https://digitalgemology.com/learn/glossary/polariscope): A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.

## Learn more

- [Polariscope](https://digitalgemology.com/about/how-we-measure/polariscope)

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