Keratometry is the measurement of the anterior corneal curvature and is one of the most clinically important procedures performed by ophthalmic assistants. K readings, as they are commonly called, provide critical data for contact lens fitting, intraocular lens (IOL) power calculations before cataract surgery, and screening for corneal conditions such as keratoconus. The COA exam expects candidates to demonstrate proficiency in manual keratometry technique, understand normal values, and interpret results in clinical context.
The keratometer (also called an ophthalmometer) measures the radius of curvature of the central corneal surface — approximately a 3mm optical zone — in two perpendicular principal meridians. This reveals both the degree and axis of corneal astigmatism, which is the difference in curvature between the flattest and steepest meridians. Readings are typically expressed in millimeters of radius and converted to diopters of refractive power.
This guide covers the complete keratometry skill set for the COA exam: instrument setup, mire alignment technique, normal reference values, the mm-to-diopters conversion formula, interpretation of steep and flat corneas, and the key clinical applications you need to understand for exam success.