A unit of measurement used by eye doctors and opticians to communicate the degree of refractive error with respect to nearsightedness, farsightedness and astigmatism. A +1-2.00 diopter reading indicates a mild form of refractive error. A +1-6.00 on the diopter scale indicates a moderate to severe condition, depending on the nature of the refractive problem. Readings can go as high as +1-10.00 or more, indicating very severe conditions.