Scheiner scales, also known as Scheiner's disks, are optical instruments used for measuring the refractive error of an eye. Sometimes known as the "duo-chrome test," these scales use a pair of colored disks that are viewed through a small aperture. The disks are placed at different distances from the eye, and the observer is asked to report whether the two colors are seen as separate or blended together. Synonyms for Scheiner scales might include "refraction testing equipment," "ocular refractometers," or "eye exam tools." Other related terms might include "visual acuity test," "eye chart," or "retinoscope." No matter what they're called, these tools are an essential part of the eye examination process.