Which colors are used in the standard duochrome test for chromatic comparison?

Which colors are used in the standard duochrome test for chromatic comparison?

• A. Red and blue
• B. Red and green
• C. Blue and yellow
• D. Green and violet

Answer: (B)

The test hinges on longitudinal chromatic aberration in the eye, which makes different wavelengths focus at different depths on the retina. Red light has a longer wavelength and green light a shorter one, so they come to focus at slightly different retinal planes. When the eye is correctly refracted, letters seen on red and on green backgrounds appear equally sharp. If one background appears clearer, you adjust the spherical value until the two appear equally clear. Red and green are used because their wavelengths produce a clear, reliable difference in focal positions and provide a strong, easily perceived cue for balancing the refraction.

The test hinges on longitudinal chromatic aberration in the eye, which makes different wavelengths focus at different depths on the retina. Red light has a longer wavelength and green light a shorter one, so they come to focus at slightly different retinal planes. When the eye is correctly refracted, letters seen on red and on green backgrounds appear equally sharp. If one background appears clearer, you adjust the spherical value until the two appear equally clear. Red and green are used because their wavelengths produce a clear, reliable difference in focal positions and provide a strong, easily perceived cue for balancing the refraction.