A keratometer is a specialized optical instrument used by eye care professionals to accurately measure the curvature of the eye’s clear, dome-shaped front surface, known as the cornea. This surface is responsible for most of the eye’s focusing power. This measurement, called keratometry, is a fundamental diagnostic test in both optometry and ophthalmology. Understanding the cornea’s precise shape is necessary for diagnosing vision problems and planning medical procedures.
The Science Behind Corneal Curvature Measurement
The physical principle behind the keratometer’s function relies on the fact that the cornea acts like a convex mirror. The instrument projects a precisely sized pattern of illuminated rings or squares, called mires, onto the corneal surface. The cornea reflects this pattern, creating a virtual image that is then measured by the instrument’s internal optics.
The size of this reflected image is inversely proportional to the curvature of the cornea. A small reflected image indicates a steep, highly curved cornea, while a larger image suggests a flatter, less curved surface. The instrument measures the reflection across the central two to three millimeters of the cornea, which is the most optically significant area for vision.
An internal formula converts the measured image size into a radius of curvature, which is then expressed in diopters (D), known as the K-reading. This conversion uses a standardized refractive index of 1.3375 to estimate the eye’s refractive power.
Essential Uses in Eye Care
Keratometry provides foundational data for several major areas of eye care. The data is typically recorded as K1 and K2, representing the curvature of the flattest and steepest meridians of the cornea. Analyzing these two values allows clinicians to assess the presence and degree of astigmatism, a common condition where the cornea is shaped more like a football than a basketball.
Contact Lens Fitting
One of the most frequent applications is determining the proper parameters for fitting contact lenses. The radius of curvature measurement, often expressed in millimeters, dictates the base curve of the contact lens. A lens that is either too flat or too steep will not fit properly, leading to discomfort, poor vision, and potential damage to the corneal surface. Keratometry is particularly useful when fitting rigid gas permeable lenses, which require a precise curvature match.
Cataract Surgery Planning
Keratometry is also a required step in planning cataract surgery, where a cloudy natural lens is replaced with an artificial intraocular lens (IOL). The K-readings are incorporated into complex biometric formulas to calculate the exact power of the replacement IOL needed to restore clear vision. The accuracy of this measurement is necessary for a successful refractive outcome after surgery.
What Happens During a Keratometry Exam
A keratometry exam is a quick, non-invasive procedure requiring no special preparation. The patient sits comfortably, placing their chin on a rest and their forehead against a support bar. This positioning ensures the eye is stable and correctly aligned with the device’s optical path.
The patient looks into the instrument, similar to peering through a telescope, and focuses on a central light or image target. The clinician adjusts the device until the projected mires are centered and clearly focused on the cornea. The examination is painless and takes only a few seconds per eye.
The instrument used may be a manual or an automated keratometer. A manual device requires the eye care professional to manipulate dials until the reflected mires are perfectly aligned and overlap, at which point the reading is taken. Automated devices, often combined with an autorefractor, project light and use internal sensors to instantly capture and calculate the corneal curvature, providing a fast, computerized measurement.