Linear displacement sensor using a position sensitive photodetector

A position sensing device comprises a light source that radiates diffuse source light along an optical axis direction to a detector, and a moving aperture arrangement positioned between the source and detector, to move perpendicular to the optical axis direction. The moving aperture arrangement comprises first and second limiting apertures, which angularly filter and transmit the diffuse source light to form a measurement spot on the detector. At least one signal output by the detector is indicative of a position of the movable member along a measurement axis. The angular intensity distribution of the angularly filtered rays of light which form the measurement spot is more consistent as a function of position along the measurement axis than the angular intensity distribution of the source light. The resulting consistent intensity distribution within the measurement spot throughout the measurement range enhances measurement linearity and accuracy.