Keratometer Principles and Use: Mire Alignment, K Readings, and Calibration

What Is a Keratometer?

A keratometer (also called an ophthalmometer) is an optical instrument that measures the curvature of the anterior corneal surface. It works by treating the cornea as a convex mirror and measuring the size of reflected images to calculate the radius of curvature. Keratometry is one of the most fundamental measurements in contact lens fitting, providing the starting point for base curve selection and detecting corneal astigmatism.

How the Keratometer Works

The keratometer projects illuminated targets called mires onto the corneal surface. Because the cornea acts like a convex mirror, it reflects a virtual image of these mires. The instrument measures the size of this reflected image, and from that measurement, it calculates the corneal curvature using a fixed relationship between object size, image size, and reflective surface curvature.

The principle is based on the fact that a steeper (more curved) surface produces a smaller reflected image, while a flatter surface produces a larger reflected image. By measuring the image size in two perpendicular meridians, the keratometer determines both the amount and axis of corneal astigmatism.

Mire Alignment

The operator views the mires through the eyepiece and adjusts the instrument to achieve proper alignment. Different keratometer models (Bausch & Lomb, Javal-Schiotz) use different mire designs, but the principle is the same: you adjust the instrument until the mire images are perfectly focused, centered, and aligned.

With a Bausch and Lomb-type keratometer:

1. Focus the eyepiece to eliminate refractive error of the examiner
2. Position the patient and align the instrument with the corneal reflex
3. Focus the mires until they are sharp
4. Rotate the instrument to align with the principal meridians (indicated by the mires aligning horizontally or vertically with their reference marks)
5. Turn the power drums to superimpose the plus signs (+) and minus signs (-) of the mires
6. Read the power in diopters and the axis from the instrument scale

Reading and Interpreting K Values

Keratometry readings can be expressed in two formats:

• Diopters: The refractive power of the corneal surface (e.g., 43.00 D). Higher diopter values indicate steeper curvature
• Millimeters of radius: The radius of curvature in mm (e.g., 7.85mm). Smaller radius values indicate steeper curvature

These two values are inversely related: a higher diopter value corresponds to a smaller (shorter) radius. The conversion uses the formula: D = 337.5 / r, where D is diopters and r is radius in millimeters (using the keratometric index of 1.3375).

Normal K readings typically fall between 41.00 and 46.00 diopters (7.30 to 8.20mm radius). Values outside this range may indicate corneal conditions such as keratoconus (very steep readings) or previous refractive surgery (very flat readings).

Limitations of Keratometry

Standard keratometry has important limitations:

• Measures only the central 3mm: The keratometer samples only the central portion of the cornea, missing peripheral irregularities
• Assumes the cornea is spherical: Between the two measured meridians, the instrument interpolates a regular surface
• Cannot detect irregular astigmatism: Distorted or irregular mires suggest irregularity but cannot quantify it (corneal topography is needed)
• Measures only the anterior surface: The posterior corneal surface contributes to total corneal power but is not measured by standard keratometry

Calibration

Regular calibration ensures the keratometer provides accurate measurements. Calibration is performed using a steel calibration ball (also called a test sphere) with a known radius of curvature.

The calibration process:

1. Mount the steel ball on the keratometer's chin rest or holder
2. Focus and align the mires on the ball surface
3. Read the measured value
4. Compare the measured value to the known value of the ball
5. If they differ, the instrument needs adjustment or service

Calibration should be performed regularly (many practices do it monthly or whenever readings seem inconsistent) and documented. An uncalibrated keratometer can lead to incorrect base curve selection and poorly fitting contact lenses.

Clinical Applications

Keratometry values are used in multiple clinical contexts:

• Contact lens base curve selection: K readings are the starting point for selecting the initial trial lens base curve, particularly for RGP lenses
• Detecting corneal astigmatism: The difference between the two K readings quantifies corneal astigmatism
• Monitoring corneal changes: Serial K readings can track corneal warpage from contact lenses or progression of keratoconus
• IOL power calculation: K readings are essential inputs for intraocular lens power formulas in cataract surgery
• Comparing corneal and refractive astigmatism: Used to calculate lenticular astigmatism

Key Takeaways

• The keratometer measures central corneal curvature by analyzing reflected mire images
• K readings are expressed in diopters (higher = steeper) or mm of radius (smaller = steeper)
• Normal K values range from 41.00 to 46.00 D (7.30 to 8.20mm)
• Standard keratometry measures only the central 3mm and cannot quantify irregular astigmatism
• Calibration with a steel test sphere ensures measurement accuracy
• Distorted mires suggest irregular astigmatism and warrant corneal topography