Snellen visual acuity was developed at a time when the only possible refractive correction of the optical system of the eye was spherocylindrical glasses and the surgical techniques of treating eye diseases were less advanced. The 20/20 Snellen visual acuity was considered normal vision and the goal of treatments and surgeries. This measurement of visual function does not suffice for a full understanding of quality of vision, nor is it adequate for the current clinical practice of refractive surgery. Many people have visual acuity that is better than 20/20, and real-world visual performance also includes low-contrast objects under low or high illumination.
Wavefront analysis technology has enabled physicians to measure higher-order optical aberrations in addition to spherocylinders in clinical practice. These measurements can be used to describe the optical properties of the eye and to detect optical irregularities of the visual system that contribute to visual quality.