A keratometer, also known as an ophthalmometer, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It was invented by the GermanphysiologistHermann von Helmholtz in 1851, although an earlier model was developed in 1796 by Jesse Ramsden and Everard Home.
An eye doctor examining a patient with a keratometerTypical presentations of keratoconus as detected through a keratometerShin Nippon Nvision K-5001 RefkeratometerThe optics inside a Shin Nippon Nvision K-5001 Refkeratometer
A keratometer uses the relationship between object size (O), image size (I), the distance between the reflective surface and the object (d), and the radius of the reflective surface (R). If three of these variables are known (or fixed), the fourth can be calculated using the formula
There are two distinct variants of determining R; Javal-Schiotz type keratometers have a fixed image size and are typically 'two position', whereas Bausch and Lomb type keratometers have a fixed object size and are usually 'one position'.
Javal-Schiotz PrinciplesEdit
The Javal-Schiotz keratometer is a two position instrument which uses a fixed image and doubling size and adjustable object size to determine the radius of curvature of the reflective surface. It uses two self illuminated mires (the object), one a red square, the other a green staircase design, which are held on a circumferential track in order to maintain a fixed distance from the eye. In order to get repeatable, accurate measurements, it is important that the instrument stays focused. It uses the Scheiner principle, common in autofocus devices, in which the converging reflected rays coming towards the eyepiece are viewed through (at least) two separate symmetrical apertures.
Bausch and Lomb principlesEdit
The Bausch and Lomb Keratometer is a one position keratometer that gives readings in dioptric form. It differs from the Javal-Schiotz in that object size is fixed, image size is the manipulable variable. The reflected rays are passed through a Scheiner disc with 4 apertures – As there are two prisms, each aligned perpendicular to the other, the major and minor axis powers can be measured independently without adjusting the orientation of the instrument.
In converting the measurements obtained from the corneal surface into a dioptric value, the B&L keratometer uses the general lens formula (n’-n)/R and assumes an n’ of 1.3375 (compared to the actual corneal refractive index of n’=1.376). This is a fictional value, which includes an allowance for the small, yet significant, negative power of the posterior corneal surface. This allows for a readout in both refractive power (dioptres) and radius of curvature (millimeters).
ReferencesEdit
Gutmark R and Guyton DL. Origins of the Keratometer and its Evolving Role in Ophthalmology. Survey of Ophthalmology 2010; 55(5): 481-497
Javal L, Schiötz H. Un opthalmomètre pratique. Annales d’oculistique, Paris, 1881, 86: 5-21.
October 20, 2023
keratometer, this, article, includes, list, references, related, reading, external, links, sources, remain, unclear, because, lacks, inline, citations, please, help, improve, this, article, introducing, more, precise, citations, august, 2011, learn, when, remo. This article includes a list of references related reading or external links but its sources remain unclear because it lacks inline citations Please help to improve this article by introducing more precise citations August 2011 Learn how and when to remove this template message A keratometer also known as an ophthalmometer is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea particularly for assessing the extent and axis of astigmatism It was invented by the German physiologist Hermann von Helmholtz in 1851 although an earlier model was developed in 1796 by Jesse Ramsden and Everard Home An eye doctor examining a patient with a keratometerTypical presentations of keratoconus as detected through a keratometerShin Nippon Nvision K 5001 RefkeratometerThe optics inside a Shin Nippon Nvision K 5001 RefkeratometerA keratometer uses the relationship between object size O image size I the distance between the reflective surface and the object d and the radius of the reflective surface R If three of these variables are known or fixed the fourth can be calculated using the formula R 2 d I O displaystyle R 2d frac I O There are two distinct variants of determining R Javal Schiotz type keratometers have a fixed image size and are typically two position whereas Bausch and Lomb type keratometers have a fixed object size and are usually one position Javal Schiotz Principles EditThe Javal Schiotz keratometer is a two position instrument which uses a fixed image and doubling size and adjustable object size to determine the radius of curvature of the reflective surface It uses two self illuminated mires the object one a red square the other a green staircase design which are held on a circumferential track in order to maintain a fixed distance from the eye In order to get repeatable accurate measurements it is important that the instrument stays focused It uses the Scheiner principle common in autofocus devices in which the converging reflected rays coming towards the eyepiece are viewed through at least two separate symmetrical apertures Bausch and Lomb principles EditThe Bausch and Lomb Keratometer is a one position keratometer that gives readings in dioptric form It differs from the Javal Schiotz in that object size is fixed image size is the manipulable variable The reflected rays are passed through a Scheiner disc with 4 apertures As there are two prisms each aligned perpendicular to the other the major and minor axis powers can be measured independently without adjusting the orientation of the instrument In converting the measurements obtained from the corneal surface into a dioptric value the B amp L keratometer uses the general lens formula n n R and assumes an n of 1 3375 compared to the actual corneal refractive index of n 1 376 This is a fictional value which includes an allowance for the small yet significant negative power of the posterior corneal surface This allows for a readout in both refractive power dioptres and radius of curvature millimeters References EditGutmark R and Guyton DL Origins of the Keratometer and its Evolving Role in Ophthalmology Survey of Ophthalmology 2010 55 5 481 497 Javal L Schiotz H Un opthalmometre pratique Annales d oculistique Paris 1881 86 5 21 Retrieved from https en wikipedia org w index php title Keratometer amp oldid 1166086325, wikipedia, wiki, book, books, library,
