The Duochrome test relies on which optical phenomenon to refine the spherical component?

• A. Chromatic aberration
• B. Monochromatic light effect
• C. Pupil size changes
• D. Diffraction patterns

Answer: (A)

Chromatic aberration in the eye causes different wavelengths of light to focus at slightly different depths on the retina. Because red and green light are refracted to different focal planes, the image of letters in those colors will come into sharp focus at different spherical powers. The Duochrome test presents red and green targets side by side and asks the patient which side looks sharper. When the spherical correction is just right, both colors look equally sharp. If one color is clearer, the spherical power is adjusted toward that color’s focal position to bring both into balance. This relies on the eye’s dispersion of light, not on pupil size changes or diffraction patterns, and not on monochromatic light effects.