The red-green duochrome test is based on a monocular endpoint in which each eye is tested separately. It is a subjective test that requires responses from the patient and is used to refine the spherical endpoint. Chromatic aberration, the basis of the test, occurs because different wavelengths of light are bent to a different extent. The longer wavelength (red) is refracted less than the shorter (green). If the letters on the red side stand out more, add minus power; if the letters on the green side stand out more, add plus power. Neutrality is reached when the letters on both backgrounds appear equally distinct.