Final answer:
The slide rule in Ortho-K uses keratometric readings, not the methods listed, to assist in selecting an initial Ortho-K lens by measuring corneal curvature with a keratometer.
Step-by-step explanation:
The slide rule in Ortho-K (orthokeratology) does not function by any of the listed options (calculating corneal curvature with precision, estimating corneal elevation maps, determining tear film quality, or assessing corneal thickness variations). Instead, it aids in selecting an initial lens by using keratometric readings and corneal diameter measurements to approximate the fitting parameters for Ortho-K lenses.
A keratometer is used to measure the curvature of the cornea, which is crucial for fitting Ortho-K contact lenses. When light is reflected off the cornea, behaving as a convex mirror, a keratometer assesses the magnification of the image to determine the radius of curvature. This information is then utilized to select an appropriately curved Ortho-K lens.