Keratometry is a diagnostic procedure used by eye care professionals to measure the curvature of the clear, dome-shaped front surface of your eye, known as the cornea. This quick and non-invasive test is fundamental for understanding how your eye focuses light. By providing precise measurements of corneal shape, keratometry plays a significant role in assessing overall eye health and vision quality.
What Keratometry Measures
Keratometry measures the curvature of the cornea’s central area. This region is the primary surface responsible for bending and focusing light onto the retina at the back of the eye. The cornea contributes approximately two-thirds of the eye’s total focusing power.
The instrument used, a keratometer, projects a light pattern onto the cornea. It then measures the size and shape of the reflection, which changes based on the cornea’s curvature. This reflection principle allows the device to calculate the radius of curvature and the optical power of the cornea. Irregularities in this central curvature can lead to blurred or distorted vision.
Key Applications of Keratometry
A primary use is in the diagnosis of astigmatism, a common condition where the cornea has an irregular, often football-like, shape rather than a perfectly round one. The test helps identify the degree and axis of this irregular curvature, which causes light to focus improperly on the retina, leading to blurred vision.
Another application is in fitting contact lenses. Accurate corneal measurements ensure that contact lenses sit comfortably and provide clear vision. This is particularly true for toric contact lenses, which are designed to correct astigmatism and require a precise match to the cornea’s unique shape.
Keratometry is also used for calculations involving intraocular lenses (IOLs) used in cataract surgery. By measuring the corneal curvature, eye care professionals can determine the correct power of the artificial lens to implant, aiming for optimal vision after the cataract is removed. An inaccurate keratometry reading can lead to an unexpected refractive outcome after surgery. Furthermore, it is used in the pre-operative assessment for refractive surgeries like LASIK, ensuring the cornea is suitable for the procedure and helping to plan the surgical approach.
The Keratometry Examination
You will sit comfortably with your chin resting on a support and your forehead against a bar to keep your head steady. The eye care professional will then position an instrument called a keratometer in front of your eye.
You will be asked to look straight ahead into the device. The keratometer projects these lights onto your cornea and then measures their reflection. The test is quick and completely painless. No special preparation is needed, though you might be asked to remove contact lenses beforehand as they can temporarily alter corneal shape.
Interpreting Keratometry Readings
The results of a keratometry test are expressed in diopters (D), a unit that measures the optical power of the cornea, or in millimeters for the radius of curvature. A common K reading is around 43 diopters, with higher numbers indicating a steeper curvature and lower numbers indicating a flatter one.
For individuals with astigmatism, the keratometry readings will show two different diopter values, representing the steepest and flattest meridians of the cornea. The difference between these two values, along with their orientation, helps determine the degree and axis of astigmatism. These numerical readings provide objective data about the cornea’s shape and its focusing ability. However, these readings are just one piece of information, and an eye care professional interprets them as part of a comprehensive eye examination to provide a full understanding of your vision.