duochrome

Duochrome refers to something composed of two colors. In ophthalmology, the duochrome test is a subjective refraction technique that uses chromatic aberration to help refine the spherical component of a prescription. The eye disperses light into different wavelengths, and green light tends to focus slightly in front of red light. During the test, letters are presented on red and green backgrounds, and the patient judges which set appears clearer. The clinician adjusts the spherical power until the letters on both colors are equally clear, aiming to balance the refraction around the neutral point. When green letters are consistently clearer, plus power is typically increased; when red letters are clearer, minus power or a reduction of plus is considered. The result is used to fine-tune the spherical equivalent, often to within about 0.25 diopters, and is usually performed after objective measurements and during subjective refraction. The test is most reliable when accommodation is minimal and astigmatism does not dominate the blur.

Limitations of the duochrome test include reliance on chromatic cues, which can be affected by media opacities or color vision deficiencies, and reduced reliability in the presence of significant astigmatism or irregular refractive errors. It provides a rapid, supplementary check rather than a standalone determination of refraction.

Outside ophthalmology, the term duochrome can also describe imaging or printing techniques that use two colors, such as two-color or duotone processes, historically employed to simulate grayscale or add contrast before full-color methods were common.