Keratometry

A keratometer does not touch the cornea and does not photograph it. It treats the tear-covered front surface as a convex mirror and measures the size of the reflected image of an illuminated object of known size held at a known distance. A steeper mirror makes a smaller reflection; a flatter mirror makes a larger one. From that image size the instrument derives the radius of curvature, and everything else is arithmetic. A doubling system splits the image in two so the examiner aligns one image against the other rather than chasing a single image across a constantly micro-moving eye.

The keratometric index. The cornea is really a two-surface lens: a strongly plus anterior surface (stroma, index 1.376) and a weakly minus posterior surface facing aqueous (1.336). A keratometer sees only the front. To make one anterior measurement stand in for the whole cornea, the instrument is calibrated to a fictitious index of 1.3375, which discounts the front-surface power by roughly the amount the back surface takes away. Two consequences follow, and both are tested. First, the diopters a keratometer reports are keratometric diopters, an estimate of total corneal power, not a directly measured refractive power. Second, the estimate is only as good as its assumption that the front-to-back ratio is normal — an assumption that corneal refractive surgery destroys.