Duochrome test
A duochrome test is a quick eye test used during refraction to fine-tune the final lens power. It uses the eye’s chromatic aberration: green light focuses a little in front of red light. The person compares which set of letters—green or red—looks clearer. If the green side looks clearer, the prescription is increased by 0.25 diopters (+0.25 D). If the red side looks clearer, the prescription is decreased by 0.25 diopters (−0.25 D). When both halves look equally clear, the spherical correction is balanced. The test relies on focus, not color vision, so it can be used even if color vision is reduced. If the eye accommodates strongly, extra relaxation (cycloplegia) may be needed for accurate results. It isn’t used if vision is worse than 20/30, because the small difference between the two sides would be hard to detect.
This page was last edited on 3 February 2026, at 20:16 (CET).