26 and conduct it out of the cylinder fiber 36 is disposed along axis 40 and conducts light from outside the cylinder into the cylinder light that is conducted into the cylinder through fiber 36 is directed towards reflective surface 26 on piston 22 it reflects off piston 22 and returns in a plurality of paths to each of optical fibers 28 these fibers receive the light at substantially the same time and conduct the light out of the cylinder an optical multiplexer combiner 42 is optically coupled to fibers 38 and joins their there individual light beams into a single beam that exits multiplexer combiner 42 in optical fiber 44 thus the light carried by optical coupler 44 is the combination of all the individual beams of light carried by optical fibers 38 referring now to fig1 optical fiber 44 is at its other end connected to optical coupler 46 which directs and focuses the light beam of fiber 44 to photodiode 48 when the light passes through coupler 46 and falls upon photodiode 48 it changes the conductivity of the photodiode causing a change in the current flowing through circuit 50 this change in current or photodiode signal is amplified by photodiode amplifier 52 the output of photodiode amplifier 52 is fed to pulse expansion circuit 54 which increases the width of the.