Duochrome (Red-Green) Test: How It Works and RAM GAP

Last updated: October 2026

Short answer: the duochrome (red-green) test checks the sphere at the end of a refraction using chromatic aberration. If letters on red look clearer, the eye is under-minused (add minus); if letters on green look clearer, it is over-minused (add plus). Remember "RAM GAP": Red Add Minus, Green Add Plus. It works even in colour-deficient patients.

How it works

• Because of chromatic aberration, the eye focuses shorter wavelengths (green) slightly in front of longer wavelengths (red).
• With the correct sphere, yellow light focuses on the retina, with green slightly in front and red slightly behind, so both look equally clear.
• If red is clearer, the focus has shifted forward (the eye is relatively myopic), so more minus is needed.
• If green is clearer, the focus has shifted back (too much minus, or the eye is relatively hyperopic), so plus is needed.

How to do it

1. Use it monocularly at the end of refraction, after the cylinder has been refined.
2. Ask: "Are the letters clearer on the red, on the green, or the same?" Ask about the black letters, not the colour brightness.
3. Red clearer: add −0.25 D. Green clearer: add +0.25 D.
4. Aim for balance. If the patient switches from red to green with a single step, many clinicians stop on the red side to avoid over-minusing.

Tips and limitations

• It relies on wavelength, not colour perception, so it works in red-green colour vision deficiency.
• It is less reliable with small pupils, in older patients and when vision is poor (worse than about 6/12).
• Accommodation in young patients can mislead the test; avoid adding minus just because the patient says green looks better.
• Confirm with a final sphere check and binocular balance.

Frequently asked questions

Related: retinoscopy tips, Jackson cross-cylinder and optics lectures.