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.