The present invention relates to using a self calibrating and recalibrating (230, 925) optical sensors piston rod displacement. Self calibration enables field calibration of uncalibrated (230, 925) optical sensors. During operation, recalibration enables detecting and correcting for wear and damage of the (200) piston rod and/or (230, 925) optical sensors. (210) Calibration positions on the surface of the (200) piston rod are imaged by (230) optical sensors using laser or darkfield lenses designed for optical computer mice. Natural surface patterns can be used in locations where (210) calibration positions are required, which reduces or eliminates the need for marked (210) calibration positions. Marked (210) calibration positions are spatially unique encoded sequences used to determine the piston rod absolute position. Storing only the significant features of (210) calibration positions saves significant memory. The reduced memory requirements of each (210) calibration position enable the use of closely spaced or continuous (210) calibration positions. Multiple (210) calibration position features and multiple (230, 925) optical sensors together collectively provide immunity to localized (208) surface damage. Proximity sensors, (925) time of flight sensors and (031) cumulative relative displacement are used to estimate the (200) piston rod absolute displacement and reduce the number of spatially unique (210) calibration positions needed to compare in order to determine the piston rod absolute displacement.