Keratometer simulator

How a keratometer works, in plain words

The front of the cornea acts like a curved mirror. A keratometer shines a lit ring onto the cornea; the mire is that lighted target pattern reflected off the cornea. The principal meridians are the steepest and flattest directions of the cornea, 90° apart. The keratometer measures the size of the mire: a steeper cornea makes a smaller image. Prisms inside double the image, and the drums change the doubling until the signs on the copies overlap. Each drum then reads the curvature of one meridian, in diopters (K readings). It measures a small central zone, roughly 3 mm across, not the whole cornea.

Steps

1. Focus the eyepiece. Turn the eyepiece fully counterclockwise until the crosshair blurs, then slowly clockwise until it is just sharp. Keep both eyes open and don't rock back and forth.
2. Position the patient. Seat the patient with chin in the rest and forehead against the band. Swing the occluder over the patient's other eye and have them look into the instrument at the reflection of their own eye. Line up the patient's outer canthus with the leveling sight and set the height with the elevating knob.
3. Center and focus. Aim the instrument until the mire reflection is on the cornea. Center the crosshair in the lower-right (focusing) circle, then focus until its doubled image becomes single.
4. Lock the instrument. Lock the instrument with the locking knob so it cannot drift while you measure.
5. Find the principal meridians. Rotate the instrument until the two plus signs line up with no step between them (the horizontal bars of the plus signs form one continuous line), then read the axis on the axis scale. The minus signs are then on the other principal meridian, 90° away.
6. Superimpose the plus signs. Turn the horizontal measuring drum (left) until the plus signs overlap into one. Read that drum: this is the K in the plus-sign meridian.
7. Superimpose the minus signs. Turn the vertical measuring drum (right) until the minus signs overlap. Read that drum for the other principal meridian.
8. Record. Write flat K and steep K with their meridians, for example 42.00 @ 180 / 43.00 @ 090. The difference is the corneal astigmatism.

Diopters and millimeters

Keratometers assume a keratometric index of 1.3375, so radius (mm) = 337.5 ÷ K (D). For example, 43.00 D = 7.85 mm and 45.00 D = 7.50 mm. Higher diopters mean a smaller radius (a steeper curve). Contact lens base curves are usually written in mm, so this conversion comes up often.

The drums normally cover about 36–52 D. Holding an auxiliary +1.25 D lens in front of the opening extends the range to about 61 D for steep corneas, and a −1.00 D lens extends it to about 30 D for flat ones (StatPearls: Keratometer).

Corneal astigmatism

• With-the-rule: the vertical meridian (60–120°) is steeper.
• Against-the-rule: the horizontal meridian (0–30° or 150–180°) is steeper.
• Oblique: the steep meridian falls between those ranges.

The amount is steep K minus flat K. Example: 42.00 @ 180 / 43.00 @ 090 means 1.00 D of with-the-rule corneal cylinder; written as minus cylinder that is −1.00 × 180 (the minus-cylinder axis is the flat meridian).

Want it all in one place? The full NCLE course has 13 chapters with quizzes and explanations plus two full-length timed mock exams, $59 for 6 months of access. Get the NCLE course — $59 →

K readings and contact lens base curves

K readings are a starting point for choosing a contact lens base curve, especially for rigid gas permeable (RGP) lenses, which are often fitted relative to flat K. The exact rule depends on the lens design and the manufacturer's fitting guide, and the fit is always confirmed on the eye. Large corneal astigmatism may point toward a toric design.

Common mistakes

• Skipping the eyepiece focus.
• Reading before the doubled mire images are focused into one.
• Turning the drums before rotating the instrument onto the principal meridians, so the plus signs still have a step.
• Swapping flat and steep K, or forgetting the meridian.
• Converting the wrong way: mm = 337.5 ÷ D, and D = 337.5 ÷ mm.
• Patient blinking or a dry tear film distorting the mires.

Studying for contact lens or ophthalmic assistant certification? See the NCLE certification course and the COA certification course.

Read the keratometry basics guide, or try the lensometer simulator and the transposition calculator.