A method and apparatus for making measurements on light fringes such as are produced by interferometers or gratings wherein such measurements can include measurements of position and velocity of an object such as a step and repeat carriage or microscope stageThe fringe pattern is sensed with a linear array detector whose elements are repetitively time multiplexed. The resultant output signal is filtered, yielding an AC signal with a phase indicative of fringe position. Rapid, digital, submicron counting is made possible using a phase lock loop to control the multiplexor clock. The advantages of AC interferometry are realized without optical modulation.