The first keratometer
Jesse Ramsden was the first English optician to produce a keratometer, expressly for the purpose of ‘proving’ Kepler’s theory that accommodation of the eye was due to a change in corneal curvature. Ramsden threw it away when he discovered the sad truth. His instrument was apparently based on Ole Rømer’s heliotrope, a split mirrored device for measuring the sun. The first Keratometer however was produced in Paris in 1728 though it could measure various other dimensions of the eye (for instance the anterior chamber) so it was truly an ‘ophthalmometer’ in a way that many of the keratometers that subsequently bore the ophthalmometer name were not.
A keratometer provides readings of corneal curvature in dioptrical values, but since contact lenses were ordered and manufactured on the basis of the radius curvature (in millimetres), a conversion was always needed.
Optical instrument for measuring the radius of curvature of the cornea in any meridian. By measuring along the two principal meridians, corneal astigmatism can be deduced. The principle is based on the reflection by the anterior surface of a luminous pattern of mires in the centre of the cornea in an area of about 3.6 mm in diameter. Knowing the size of the pattern h and measuring that of the reflected image h′ and the distance d between the two, the radius of curvature r of the cornea can be determined using the approximate formula.
In addition, a doubling system (e.g. a bi-prism) is also integrated into the instrument in order to mitigate the effect of eye movements, as well as a microscope in order to magnify the small image reflected by the cornea. This instrument is used in the fitting of contact lenses and the monitoring of corneal changes occurring as a result of contact lens wear (Fig. K2). The range of the instrument can be extended approximately 9 D by placing a +1.25 D lens in front of the objective to measure steeper corneas. The range in the other direction can be extended by approximately 6 D using a −1.00 D lens to measure flatter corneas. Syn. ophthalmometer. See fitted on K; keratoscope; liquid lens; photokeratoscopy; Wollaston prism; Javal’s rule; Topogometer; videokeratoscope.