These instruments measure the radius of curvature of the central part of the cornea in 2 main meridians, based on the light reflected from the corneal surface. Based on the value obtained, and knowing the refractive index of the refractive medium, the refractive power of the cornea can be calculated. The instrument can determine the radius of curvature of the cornea, its shape, refractive errors, and the axis of astigmatism in a fraction of a second.