Position Sensing Devices
A position-sensing device (PSD) is a photosensor (photodiode or phototransistor) which is able to differentiate between the position where incident light strikes the sensing surface. There are uniaxial sensors which are only able to distinguish position along a single axis, and duo-lateral or tetra-lateral sensors which are able to distinguish position along two axes. All of these sensors provide currents on the output leads which is proportional to the overall intensity of light striking the sensing surface as well as to the distance between the output terminal and the location where the light struck the sensor. The sensors act as current sources, because the photoelectric effect dislodges electrons, which drives a current, so more light produces more current. The distance from the output terminal to the incident point is proportional to the resistance the current experiences, resulting in different currents at different distances.