Described is a device and method of measuring the linear position of a piston 002 movable within a hydraulic or pneumatic cylinder barrel 001. The measuring device includes a photo optical sensing apparatus 011 mounted at the cylinder head. The photo optical sensing apparatus 011 can be located inside or outside of the cylinder 001. The sensing apparatus 011 design utilizes a typical optical sensing apparatus, and optional functional modules for determining absolute displacement, and communication. Calibration locations, which are used to obtain absolute displacement measurements, are determined by calibration images or separate sensors indicating their presence.