When calibrating a keratometer, which target provides the most reliable result?

The use of a set of steel balls of known radii when calibrating a keratometer is considered the most reliable method because it provides a consistent, accurate reference point. Steel balls are manufactured with precise radii, allowing for the calibration to be based on known measurements. This helps ensure that the keratometer provides accurate readings for various corneal curvatures, which is essential for fitting contact lenses appropriately.

Using a perfectly flat highly polished mirror can offer clarity in reflection but does not provide a curvilinear surface necessary for keratometry. A flat surface may not accurately simulate the curvatures of the cornea, potentially leading to miscalculations in measurement.

Likewise, the single-cut anterior surface of a high-plus lens may introduce distortion in the measurements due to its curvature, and its optical properties could influence the results. Similarly, the unfinished posterior surface of a low-minus lens may not provide consistent measurements either, as it can have irregularities that affect the accuracy of the keratometer.

In contrast, the steel balls present a controlled, uniform curvature that allows for a standard reference, leading to reliable calibration of the keratometer, ensuring that it can accurately measure the cornea in clinical practice.