A PSD sensor (Position Sensitive Detector) is delivering an electrical signal related to the position of the light spot impacting the surface.
It is a large-sized photodiode including namely a resistive internal layer and 4 electrodes on the sides (in red on the diagram). During illumination, the incident photons create electron/hole pairs which produce electrical currents. X1, X2, Y1, and Y2 current intensities will depend upon the position of the light spot (according to the wheatstone bridge principle) and on its luminous intensity. So, on the diagram, Y2 current will be higher than Y1, since the distance between the spot and the electrode is smaller, and therefore less resistive.
SOLEMS designs « large size » PSD typically 4 x 4 cm, much bigger than commercially available the PSDs.