A keratometer is a medical instrument that eye care professionals use to measure the curvature and reflection of the anterior surface of the cornea. A keratometer, also sometimes called an ophthalmometer, is primarily used to diagnose the presence of astigmatism and to determine the degree and treatment of astigmatism. Astigmatism is a condition of the eye in which the cornea or lens is misshapen and can result in vision problems. It was invented by the German physiologist Hermann von Helmholtz in 1880, although an earlier model was developed in 1796 by Jesse Ramsden and Everard Home.