ophthalmometer

oph·thal·mom·e·ter (fthl-mm-tr, -thl-, p-)

n.

See keratometer.

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ophthalmometer

an instrument used in ophthalmometry.

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keratometer

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.

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

Table K2 Extended keratometer range
with 21.00 D				with 11.25 D
actual drum reading (D)		extended value (D)		actual drum reading (D)		extended value (D)
36.00		30.87		45.00		52.46
36.50		31.30		45.50		53.05
37.00		31.73		46.00		53.63
37.50		32.16		46.50		54.21
38.00		32.59		47.00		54.80
38.50		33.02		47.50		55.38
39.00		33.45		48.00		55.96
39.50		33.88		48.50		56.55
40.00		34.30		49.00		57.13
40.50		34.73		49.50		57.71
41.00		35.16		50.00		58.30
41.50		35.59		50.50		58.88
				51.00		59.46
				51.50		60.04
				52.00		60.63