What Is a Keratometer?
A keratometer (also called an ophthalmometer) measures the curvature of the central anterior corneal surface. It provides measurements in both diopters of corneal power and millimeters of radius of curvature. These measurements, commonly called K readings, are essential for contact lens fitting, astigmatism assessment, and monitoring corneal conditions.
How the Keratometer Works
The keratometer projects illuminated targets (mires) onto the cornea and uses the size of the reflected images to calculate the corneal curvature. A steeper cornea (smaller radius) produces a smaller reflected image, while a flatter cornea (larger radius) produces a larger image.
The instrument measures the two principal meridians of the cornea, which are the steepest and flattest meridians, oriented 90 degrees apart. The power difference between these meridians indicates the amount of corneal astigmatism.
K Readings Explained
K readings are expressed as:
- Power in diopters: The refractive power of the corneal surface (e.g., 43.50 D @ 180 / 44.75 D @ 090)
- Radius of curvature in millimeters: The physical measurement of the curve (e.g., 7.76 mm @ 180 / 7.53 mm @ 090)
| K Reading | Classification | Approximate Radius |
|---|---|---|
| Below 41.00 D | Flat cornea | Above 8.23 mm |
| 41.00 to 45.00 D | Average cornea | 7.50 to 8.23 mm |
| Above 45.00 D | Steep cornea | Below 7.50 mm |
Clinical Applications
Contact Lens Fitting
K readings are fundamental for selecting the initial base curve of a contact lens, especially rigid gas permeable (RGP) lenses. The base curve of an RGP lens is typically selected in relation to the flattest K reading:
- On-K fit: Base curve matches the flattest K
- Flatter than K: Base curve is flatter (larger radius) than the flattest K
- Steeper than K: Base curve is steeper (smaller radius) than the flattest K
Soft contact lens fitting relies less heavily on K readings since soft lenses drape over the cornea, but K values still help identify patients with unusual corneal curvature.
Corneal Astigmatism Assessment
The difference between the two K readings indicates the amount of corneal astigmatism. For example, K readings of 43.00 @ 180 and 45.00 @ 090 show 2.00 D of with-the-rule corneal astigmatism (the steeper meridian is vertical).
Corneal Monitoring
Serial K readings track corneal changes over time, useful for:
- Monitoring keratoconus progression (increasing corneal steepening and astigmatism)
- Post-surgical follow-up after corneal procedures
- Detecting corneal warpage from contact lens wear
Types of Keratometers
- Manual keratometers: The operator aligns the mires visually and reads the measurements from calibrated scales. The Bausch & Lomb and Javal-Schiotz designs are the two classic types.
- Automated keratometers: Measure corneal curvature electronically with minimal operator input. Often integrated into auto-refractor/keratometer combination instruments.
Mire Quality
Beyond the numerical readings, the quality of the reflected mires provides important clinical information:
- Clear, sharp mires: Indicate a smooth, regular corneal surface
- Distorted mires: Suggest corneal irregularity (keratoconus, scarring, dry eye)
- Doubled or split mires: May indicate significant corneal irregularity
Clinical Relevance
The keratometer provides essential data for contact lens fitting, particularly for RGP lenses where the base curve must closely match the corneal curvature. It also helps differentiate between corneal and lenticular astigmatism, which affects the choice between toric and spherical contact lenses. Understanding K readings is a core competency for opticians involved in contact lens dispensing.
Key Takeaways
- The keratometer measures central corneal curvature in diopters and millimeters
- K readings provide the two principal meridians and the corneal astigmatism
- K values guide RGP contact lens base curve selection
- The instrument measures only the central 3 to 4 mm of the cornea
- Mire quality indicates corneal surface regularity
- Corneal astigmatism is only one component of total refractive astigmatism