A movable fixation target attached to the front of a keratometer, used in fitting contact lenses to measure the curvatures of the cornea in its peripheral zones.
[topo- + G. gonia, angle, + metron, measure]


Etymology: Gk, topos, place, gonia, angle, metron, measure
a movable fixation target attached to an instrument for measuring the radius of curvature of the cornea. It is used in fitting contact lenses of correct curvature.


A device attached to a keratometer that allows a measurement of the curvature of the cornea off the visual axis. It consists of an illuminated fixation light that can be moved along two axes, both of which are perpendicular to the axis of the keratometer. Scales are provided with the device to indicate, in millimetres, the amount of decentration of the visual axis from the optical axis of the keratometer at the corneal surface. This device helps in the fitting of contact lenses, by providing an estimation of the flattening of the peripheral cornea and of the position of the corneal apex. This instrument has been superseded by computerized instruments. See corneal topography; videokerato-scope.