The red-green test (also called the duochrome test) is a subjective test used at the end of refraction to fine-tune the spherical value of the optical correction.

This test checks whether the image is formed precisely on the retina. In other words, whether the optical correction is properly adjusted.

The patient reads letters displayed on two different background colors: red on one side, green on the other.
Due to the eye’s chromatic aberration :

• Green light (short wavelengths) focuses in front of the retina.

• Red light (long wavelengths) focuses behind the retina.

• Sharper on the green side → The image is focused too far forward → overcorrection → decrease the power (reduce the +).

• Sharper on the red side → The image is focused too far back → undercorrection → increase the power (add more +).

• Equally sharp on both sides → The image is focused on the retina → the correction is accurate.

• At the very end of the refraction process, to fine-tune the sphere before finalizing the prescription.
• It is especially useful when the patient is less sensitive to small changes in spherical power, as it relies on chromatic aberration rather than purely on perceived sharpness.