Keratometry uses for determining the curvature of the anterior surface of the cornea, particularly for examining the degree and axis of astigmatism. It was created by the German physiologist Hermann von Helmholtz in 1851, although an earlier model was established in 1796 by Jesse Ramsden and Everard Home.
How Does Keratometry Work?
A keratometer uses the relationship between object size (O), image size (I), the distance between the reflective surface and the things (d), and the radius of the reflective surface area (R). If 3 of these variables are understood (or repaired), the fourth can be calculated using the formula
There are two unique variations of identifying R; Javal-Schiotz type keratometers have a fixed image size and are generally ‘two position’, whereas Bausch and Lomb type keratometers have a fixed item size and are usually ‘one position’.
Keratometry and Javal-Schiotz Principles
One of the types of keratometry uses Javal-Schiotz Principles. The Javal-Schiotz keratometer is a two position instrument which uses a set image and doubling size and adjustable object size to determine the radius of curvature of the reflective surface. It uses two self brightened mires (the things), one a red square, the other a green staircase style, which are held on a circumferential track in order to preserve a repaired distance from the eye. In order to get repeatable, precise measurements, it is important that the instrument stays focused. It uses the Scheiner concept, common in autofocus devices, in which the converging shown rays coming towards the eyepiece are viewed through (at least) two different balanced apertures.
Keratometry and Bausch and Lomb Principles
The Bausch and Lomb Keratometer is a one position keratometer that gives readings in dioptric form. It differs from the Javal-Schiotz in that item size is fixed, image size is the manipulable variable. The reflected rays are gone through a Scheiner disc with 4 apertures – As there are two prisms, each lined up perpendicular to the other, the major and small axis powers can be measured individually without changing the orientation of the instrument.
In transforming the measurements gotten from the corneal surface area into a dioptric worth, the B&L keratometer uses the basic lens formula (n’-n)/ R and presumes an n’ of 1.3375 (compared to the actual corneal refractive index of n’=1.376). This is an imaginary worth, that includes an allowance for the small, yet significant, negative power of the posterior corneal surface. This permits a readout in both refractive power (dioptres) and radius of curvature (millimeters).
