Keratometer/Ophthalmometer: Calibration, Readings & Interpretation
Key Takeaways
- A keratometer measures anterior corneal curvature in diopters via tear-film reflection; standard K readings look like 44.00/45.00 @ 090.
- Convert diopters to radius with r (mm) = 337.5 / D, so 45.00 D = 7.50 mm.
- Average K is ~43.00-44.00 D; the difference between the two principal meridians is the corneal astigmatism.
- With-the-rule astigmatism has the steep meridian near 090; against-the-rule has the steep meridian near 180.
- Keratometry samples only the central 3-4 mm, assumes regular astigmatism, and is calibrated with a steel ball of known curvature.
Keratometry: Calibration, Readings & Interpretation
Quick Answer: A keratometer (ophthalmometer) measures the anterior corneal curvature in diopters by reflecting mires off the tear film. Standard K readings look like 44.00/45.00 @ 090. Convert diopters to radius (mm) with r = 337.5 / D, so 45.00 D = 7.50 mm. Average K is ~43-44 D, and the difference between the two meridians is the corneal astigmatism. Keratometry only samples the central 3-4 mm, so it assumes regular astigmatism and misses peripheral pathology.
Principle
The keratometer treats the cornea as a convex mirror. Mires (target objects) are projected onto the precorneal tear film and the reflected image size is measured. Because image size depends on corneal curvature, the instrument reports curvature in diopters (D) of refractive power and/or millimeters (mm) of radius of curvature. The corneal refractive index used by most keratometers is 1.3375, which is why 337.5 appears in conversion formulas.
Instrument Types
| Type | Features |
|---|---|
| Bausch & Lomb (one-position) | Mires doubled via prism; both meridians read without rotating; most common in US clinics |
| Javal-Schiotz (two-position) | Doubled images via Wollaston prism; instrument rotated 90° between meridians; mires are fixed shape |
| Automatic keratometers | Auto-alignment, digital readout; common in auto-refractors |
Calibration
Calibration is verified with a precision steel ball of known curvature (commonly 42.50 D). Place the ball on the chin-rest cradle, focus the mires, and confirm the reading matches the engraved value. Tolerance is typically ±0.25 D. Mire clarity also confirms instrument focus; distorted mires on the steel ball suggest optical misalignment or a dirty objective.
Measurement Procedure
- Seat the patient comfortably; adjust chair and instrument height.
- Instruct the patient to fixate on the reflected image of their own eye (or the mire target).
- Focus the eyepiece first by turning the eyepiece ring until the mires are sharpest — this corrects for the examiner's refraction.
- Align the mires: superimpose and center the plus/minus signs or circles.
- Read the horizontal meridian, then rotate 90° (one-position instruments read both simultaneously).
- Record as flat K / steep K @ axis of steep meridian.
Reading Format & Interpretation
A K reading is recorded as: 44.00 / 45.00 @ 090.
- 44.00 D = flatter meridian
- 45.00 D = steeper meridian
- 090 = axis of the steeper meridian in degrees
With-the-rule (WTR) astigmatism: steeper vertical meridian (~090), e.g., 44.00/45.00 @ 090. Common in younger patients.
Against-the-rule (ATR) astigmatism: steeper horizontal meridian (~180), e.g., 44.00/45.00 @ 180. Common with age.
Oblique: steep axis near 045 or 135.
Average K is ~43.00-44.00 D. Values >46.00 D should raise suspicion for keratoconus or ectasia, especially when asymmetric between eyes. The amount of corneal astigmatism equals steep K − flat K (here 1.00 D).
Diopter-to-Millimeter Conversion
The relationship: r (mm) = 337.5 / D.
| K (D) | r (mm) |
|---|---|
| 40.50 | 8.33 |
| 42.50 | 7.94 |
| 43.00 | 7.85 |
| 44.00 | 7.67 |
| 45.00 | 7.50 |
| 46.00 | 7.34 |
| 48.00 | 7.03 |
Worked example: a K of 45.00 D → r = 337.5 / 45.00 = 7.50 mm. Conversely, a 7.80 mm radius → D = 337.5 / 7.80 = 43.27 D.
Contact Lens Application
For initial base curve (BC) selection:
- Soft lenses: typically 0.8-1.2 mm flatter than flat K (or near flat K). A common rule is on-K or 0.2-0.4 mm flatter than flat K for corneal soft lenses; modern sag-based designs use sagittal depth instead of K alone.
- RGP corneal lenses: fitted ~0.1 mm flatter than flat K to produce an apical clearance or three-point touch pattern; use trial lenses and over-refract.
- Scleral lenses: vault the cornea entirely; K is only a starting reference, with the scleral landing zone shaped by OCT and topography.
Worked Fitting Example
A patient presents with K readings 43.00 / 44.50 @ 090.
- Astigmatism: 44.50 − 43.00 = 1.50 D WTR.
- Steep meridian radius: r = 337.5 / 44.50 = 7.58 mm.
- Flat meridian radius: r = 337.5 / 43.00 = 7.85 mm.
- Soft lens starting BC: 0.8-1.2 mm flatter than flat K → 8.6-9.1 mm (select an 8.6 or 8.9 mm BC trial).
- RGP starting BC: ~0.1 mm flatter than flat K → ~7.75-7.85 mm trial lens, then evaluate fluorescein pattern.
If the mires appear distorted or broken, suspect an irregular cornea and proceed to topography before any lens selection.
Limitations
- Measures only the central 3-4 mm corneal zone — peripheral pathology is missed.
- Assumes regular astigmatism; irregular corneas (keratoconus, post-surgical) produce distorted mires that may be unmeasurable.
- Tear-film quality affects mire clarity; dry eye distorts readings.
- One-position instruments assume the two principal meridians are exactly 90° apart — not valid in irregular astigmatism.
- Keratometry cannot detect early keratoconus; an apparently normal K pair with asymmetric bow-tie on topography may still represent forme fruste keratoconus.
A patient's K reading is 45.00 / 46.00 @ 090. What is the radius of curvature of the steeper meridian, and what type of astigmatism is present?
Which statement about keratometry is correct?