Keratometry
Corneal curvature measurement — mires, calibration, and readings for contact lenses and IOL calculations.
Reading the Cornea's Curves
What it measures — the curvature of the central ~3 mm of the anterior cornea, expressed in diopters (or mm of radius); higher diopters = steeper cornea = shorter radius.
The mires — focus and align the reflected rings, then read the two principal meridians. The difference between them is corneal astigmatism; meridians 90° apart make it regular (with-the-rule when the vertical meridian is steeper).
Irregular mires — distorted, unfocusable rings point to keratoconus, scarring, or a poor tear film; have the patient blink, then re-measure before concluding anything.
Where the numbers go — contact lens base-curve selection and IOL power calculation, which is why calibration checks against the reference balls matter.