keratometer

keratometer

 [ker″ah-tom´ĕ-ter]

an instrument for measuring the curves of the cornea.

Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

ker·a·tom·e·ter

(ker'ă-tom'ĕ-tĕr),

An instrument for measuring the curvature of the anterior corneal surface.

Synonym(s): ophthalmometer

[kerato- + G. metron, measure]

Farlex Partner Medical Dictionary © Farlex 2012

ker·a·tom·e·ter

(ker'ă-tom'ĕ-tĕr)

An instrument for measuring the curvature of the anterior corneal surface.
Synonym(s): ophthalmometer.

[kerato- + G. metron, measure]

Medical Dictionary for the Health Professions and Nursing © Farlex 2012

keratometer

An optical instrument used to measure the radius of curvature of the cornea. See also KERATOMETRY.

Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005

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

Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann