Autorefractors/ARK, Keratometers, Tonometers

An autorefractor/ARK offers the functionality of a car refractor and keratometer. A refractor measures a patient’s refractive error, while a keratometer, also called an ophthalmometer, is an optical diagnostic instrument for measuring the curvature of the anterior surface of the cornea.


Keratometry is the measurement of the anterior corneal curvature and is traditionally performed with an information keratometer. This revolutionary product is also called an ophthalmometer.


Both basic auto refractors and keratometers  would be 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 more familiar to many ophthalmologists. It is a one-position device that uses adjustable image size and aligns plus sign and minus sign mires. The latter is a two-position instrument that uses adjustable object size and requires 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 fully for the small contribution from the posterior corneal surface, the corneal thickness, and also to permit 45 D to be equal to 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 along with numerous other functions. These generally include autorefractors (like the Unicos URK 800F) that measure refraction, corneal topographers (like the Antares from CSO) that map the anterior corneal surface.


The benefit of corneal topography is the ability to measure and quantify irregular astigmatism, which can not be done with a keratometer. Although topography devices can average numerous corneal curvature measurements over various central optical zone diameters, it is essential to remember that the simulated keratometry readings (SimK) these machines provide are essentially an information keratometer estimate the corneal curvature to be at approximately 3 mm zone.


We could supply a range of manual keratometers like the CSO JVL/1 LED Ophthalmometer.

Ocular tonometry is completed using a tonometer and is the process of determining IOP, or intraocular pressure, that will be fluid pressure in the patient’s eye. A tonometer performs this test included in a glaucoma screening. Tonometers are usually calibrated to assess and measure mmHg or pressure by millimeters of mercury.


A tonometer is an important little bit of instrumentation for optometrists—there many different types and solutions for tonometers.


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