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What is keratometer ? by Godang Medicalna

Thursday, November 10th 2016.


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 hand the distance d between the two, the radius of curvature r of the cornea can be determined using the approximate formula.

r = 2d (h′/h)

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.

Fig. K2 Schematic diagram of the Reichert keratometer
More recent equipment in the museum tends to be big. Sometimes we can only collect part of it, as in this instance. The object is a computerised videokeratoscope / topographer with laser aperture and would have come originally with an attached computer to enhance its images for easier and more accurate diagnoses. You could use it to collect information about the shape of the cornea and its refractive characteristics. Pictorial records of patient examinations could be stored in the optional Bernoulli drive that was considered to have a huge capacity of 90MB. Made by Computed Anatomy Inc around 1989-1993 the TMS-1 was the first of a popular international range and its mapping function was based on the  Corneal Modelling System (CMS) pioneered at the New York Eye and Ear Infirmary. Even though computer technology has allowed keratoscopy to become more widely accepted and more clinically friendly, the basic principles underlying the new technology are the same as those articulated by Gullstrand a century before. The example illustrated is a late model of its type and you’re looking at the part the operator would have used. The integral monitor was for checking the alignment of the instrument with the patient’s eye whilst the patient attempted to fixate on a blinking white light. A cone reflected 25 concentric rings on to the surface of the eye where curvature measurements would be taken automatically. The cone could be replaced with one containing 31 rings for the purpose of precise fitting of RGP contact lenses. This particular device was used by a Harley Street ophthalmic surgeon. It is to be hoped that his consulting rooms were spacious since practitioners were advised to place the unit well away from other instruments to avoid distractions during an examination. A low power laser beam was used to position the patient’s corneal axis. For safety’s sake the instrument had an automatic cut-out after sixty seconds but it was reckoned that normal examination should require no more than fifteen seconds.