An eye doctor examining a patient with a<unk>  Typical presentations of keratoconus as detected through a<unk>  A<unk> , also known as an ophthalmometer, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It was invented by the German physiologist Hermann von Helmholtz in 1851, although an earlier model was developed in 1796 by Jesse Ramsden and Everard Home. A<unk>  uses the relationship between object size (O), image size (I), the distance between the reflective surface and the object (d), and the radius of the reflective surface (R). If three of these variables are known (or fixed), the fourth can be calculated using the formula : There are two distinct variants of determining R; Javal-Schiotz type<unk> s have a fixed image size and are typically 'two position', whereas Bausch and Lomb type<unk> s have a fixed object size and are usually 'one position'.
Keratometer
Esthesiometer
Campimeter
Stadiometer
Perineometer
String galvanometer
Focometer
Dolorimeter