Keratometry: Principles, Operation, and Recording

What Keratometry Measures

Keratometry is the clinical measurement of the curvature of the anterior corneal surface. Because the cornea provides roughly two-thirds of the eye's total refracting power, knowing its curvature is essential for fitting contact lenses, calculating intraocular lens power, and detecting irregular astigmatism. The keratometer measures two principal meridians—the flattest and the steepest—and reports each as a radius of curvature in millimeters (mm) or as a dioptric power (D) using a standardized refractive index convention.

Units and the Keratometric Index

Keratometers do not measure true refractive power; they convert radius of curvature to diopters using a fixed keratometric index, commonly 1.3375, rather than the actual corneal index. This is a calibration convention, not a physical measurement of tissue properties. The conversion formula is:

K (D) = (n − 1) / r

where n = 1.3375 and r = radius in meters. A radius of 7.80 mm therefore yields approximately 43.27 D. Because every instrument uses this same convention, readings are directly comparable across brands.

Clinical keratometry results are written as:

K flat meridian / K steep meridian × axis of flat meridian

Example: 43.50 / 45.00 × 180

The flat K is always listed first. The axis is the direction of the flat meridian, expressed in degrees from 1° to 180° using standard ANSI notation.

Anatomy of a Manual Keratometer

The classic Bausch & Lomb manual keratometer uses a one-position, one-mire doubling system. Its key parts are:

• Eyepiece and focusing ring – focus the observer's view of the mires
• Mire projection lamp – illuminates the target reflected off the corneal surface
• Doubling prisms – split the mire image so the operator can align the two halves
• Power drums – one for each principal meridian, calibrated in both millimeters and diopters
• Locking ring and joystick – used to position and lock the instrument

The cornea acts as a convex mirror. The keratometer measures the size of the reflected mire image; a steeper (more curved) cornea produces a smaller image, corresponding to a higher dioptric reading.

Operating a Manual Keratometer: Step-by-Step

1. Calibrate first. Place the calibration sphere in position and confirm the instrument reads the sphere's known value (±0.25 D). Never skip calibration.

2. Seat the subject. Adjust chin rest and forehead bar so the eye is level with the alignment mark on the instrument.

3. Focus the eyepiece. While looking at a blank, bright field, rotate the eyepiece until the crosshair or reticle is sharply focused for your eye.

4. Illuminate and align. Turn on the lamp. Using the joystick, center the mire reflection in the patient's cornea. The central circle of mires should appear roughly centered and the instrument axis aligned with the visual axis.

5. Focus the mires. Adjust the instrument's focusing knob until the mire images are sharp—this sets the working distance precisely.

6. Read the horizontal meridian. Rotate the power drum until the horizontal mire segments (the + symbols or semicircles, depending on instrument model) align correctly. Record the power and axis.

7. Read the vertical meridian. Rotate 90° and align the vertical mire pair. Record the power and axis.

8. Repeat. Take at least two readings; they should agree within 0.25 D. Average repeated readings if they differ slightly but remain within acceptable tolerance.

Automated Keratometry and Corneal Topography

Automoated keratometers and corneal topographers acquire readings electronically, often measuring many more points across the corneal surface. Topographers produce a color-coded map showing curvature variation across the entire cornea, which is especially valuable for detecting irregular astigmatism, keratoconus, or post-surgical changes that a single-zone keratometer would miss. When comparing manual K readings to topographic output, differences up to 0.25 D are generally within instrument variability; larger differences should prompt a repeat measurement and a discussion about corneal irregularity or measurement technique error.

Recording in Standard Clinical Notation

Always record:

• Both K values in diopters (to 0.12 or 0.25 D resolution depending on instrument)
• The axis of the flat meridian
• Which eye: OD (right) or OS (left)
• The method (manual vs. automated)
• Date and technician initials

Example chart entry: OD: 43.50 / 45.00 × 180; OS: 43.75 / 44.75 × 175 — manual keratometry, calibration confirmed

Safety and Infection Control

The keratometer does not touch the eye, so direct corneal trauma risk is low. However:

• Never allow the instrument to contact the subject's eye or eyelashes. Maintain safe working distance at all times.
• Disinfect all patient-contact surfaces (chin rest, forehead bar) between subjects using your facility's infection-control protocol.
• Do not proceed if the eye appears acutely red, painful, or if the patient reports recent eye injury or surgery — refer to the supervising optometrist or ophthalmologist.
• Students must work under qualified supervision during all patient contact. Your scope of practice as a student does not include independent patient diagnosis.
• Calibrate before every session. An uncalibrated instrument produces systematically wrong readings that could lead to an improperly fit contact lens, creating a risk of corneal hypoxia or lens discomfort for the patient.