A method and apparatus are provided for monitoring motion of a scanning probe microscope (SPM) actuator using an improved optical displacement sensor (ODS) including a light emitter, an intensifier, and a photodetector. One of these three devices, most preferably the intensifier, is preferably movable with the actuator and relative to the other two devices. The intensifier intensifies the light incident on the detecting face of the photodetector, increasing the sensitivity of the ODS and improving the ability of the SPM to translate the actuator in the desired manner. The photodetector preferably is a split diode or a quad diode photodetector. The light emitter includes at least a light source such as an LED and may additionally include an aperture that limits the light passing therethrough to that which contributes to the displacement signal, thereby reducing shot noise and increasing the signal-to-noise ratio.