Keratometry is the measurement of the anterior corneal curvature and is traditionally performed with a manual keratometer. This device, also known as an ophthalmometer.
The two basic keratometers are the Helmholtz type and the Javal-Schiotz type. Both use the relationship between object size, image size, and distance, to calculate corneal curvature. The former is the more familiar to most ophthalmologists. It is a one-position device that uses adjustable image size and consists of aligning plus sign and minus sign mires. The latter is a two-position instrument that uses adjustable object size and requires alignment of a red square and green staircase design.
Keratometers measure the size of an image reflected from two paracentral points on the cornea. The instrument contains doubling prisms to stabilize the image allowing more accurate focusing. The anterior corneal curvature is then obtained from the convex mirror formula and corneal power is calculated empirically using Snell’s law of refraction with simplified optics. The keratometer measures the anterior corneal surface but uses an assumed index of refraction (1.3375 rather than the actual 1.376) to account for the small contribution from the posterior corneal surface, the corneal thickness, and also to allow 45 D to equal 7.5 mm radius of curvature (K (diopters) = 337.5/r).
Alternatives to the traditional keratometer are automated instruments that provide keratometry readings alone or in addition to a number of other functions. These include autorefractors (like the Unicos URK 800F) that measure refraction, corneal topographers (like the Antares from CSO) that map the anterior corneal surface.
The advantage of corneal topography is the ability to measure and quantify irregular astigmatism, which cannot be done with a keratometer. Although topography devices can average numerous corneal curvature measurements over a variety of central optical zone diameters, it is important to remember that the simulated keratometry readings (SimK) that these machines provide are essentially what a manual keratometer would estimate the corneal curvature to be at approximately the 3 mm zone.
We can supply a range of manual keratometers, such as the CSO JVL/1 LED Ophthalmometer, call us for more information on this product and the advantages it could bring to your practice