A Snellen chart is a tool used by eye care professionals to measure visual acuity, which is the clarity or sharpness of vision. It typically consists of rows of letters that decrease in size from top to bottom. Patients stand a specific distance away from the chart, usually 20 feet, and read the letters aloud to determine their vision level. The chart was developed by Herman Snellen, a Dutch ophthalmologist, in the 1860s. It is commonly used in eye exams to help diagnose vision problems and to assess the effectiveness of treatments or corrective lenses.