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Ophthalmometer or keratometer is an instrument used to measure the curvature of the anterior corneal surface and assess the extent and axis of astigmatism. Keratometry is the measurement of the curvature of the anterior corneal surface, usually across a fixed chord length of 2 to 3 mm, which falls within the spherical optic zone of the cornea. The principle of keratometry is that the anterior corneal surface behaves as a convex mirror and the size of the image change with the curvature. There are various clinical uses of keratometers, such as corneal astigmatism measurement, contact lens fitting, assessment of refractive error, IOL power calculation, etc. The keratometer readings may not always be perfect because of mistakes in keratometry. The activity deals with keratometers, keratometry principle, type of keratometers, clinical use, errors in keratometry, and clinical significance of keratometers. Objectives: Describe how to perform keratometry. Review the interfering factors for a perfect keratometry. Summarize the indications of keratometry by using a keratometer. Explain the clinical significance of a keratometer. Access free multiple choice questions on this topic.

Keratometer or ophthalmometer is an instrument used to measure the anterior corneal curvature and the axis and extent of astigmatism.[1] The process of these measurements is called keratometry. Hermann von Helmholtz invented the first keratometer, and it came into vogue in 1851.[2] The earlier keratometer was designed by Jesse Ramsden and Everard Home in 1796. In keratometry, the measurements are performed at a fixed chord length of 2 to 3 mm, which falls within the spherical zone of the cornea.[3] The keratometry is based on the principle that the anterior corneal surface behaves like a mirror and the image size varies with the corneal curvature. Hence the radius of curvature of the cornea can be calculated based on image size from the anterior corneal surface.[4] In a keratometer, the relationship between the object, image, radius of refractive surface, and distance between the surface and object is denoted as r = 2ul/ o, where r is the radius of the reflective surface, o is the object size, l is the image size, and u is the distance measured between the reflective surface and object.[5] However, the exact measurement of image size poses a problem since it is difficult to entirely still a living eye while the image is being observed. The various type of manual keratometers can be Helmholtz keratometer, Bausch and Lomb, and Javal Schiotz keratometer.[6] There can be surgical or operating keratometers as well as automated keratometers. The activity describes various types of keratometers, parts of a keratometer, indications of keratometry, limitations, complications, patient safety, education, and clinical significance of a keratometer.[7]

Improper functioning due to dust accumulation Damage to the eyepiece and prisms Friction between instrument parts due to reduced lubrication Damage to the bulb or fuse bulb Damage to on and off button and cable Improper calibration[31]