The Keratometer Argo, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism.
The Keratometer Argo 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 R = 2dI/O.
There are two distinct variants of determining R; Javal-Schiotz type keratometers have a fixed image size and are typically 'two position', whereas Bausch and Lomb type keratometers have a fixed object size and are usually 'one position'.