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The red green test may be used to study spherical correction that has already been refined to within 1 D of emmetropia. The test is based on chromatic aberration; red light (longer wavelength) is focused behind green light (shorter wavelength) when a pencil of light is rendered convergent by a plus lens. While the eyes are fogged with a plus sphere, the patient gazes at letters on a half red, half green background. Letters on the red side should appear clearer. Plus sphere is removed in 0.25 D steps until the two backgrounds appear equally clear. If the letters on the green side appear clearer, more plus correction is indicated. Red green color blindness does not interfere with this test.