Abstract: Electro-optical apparatus measures the average relative phase of an incident wave fringe pattern. The subject fringe, e.g., an interferometric pattern, passes through three sections of an optical mask, one characterized by fixed transmissivity and the other two by quadrature-displaced spatial fringe patterns. The light passing through each section is separately collected and detected to average the respective incident wave/mask section interactions. The phase of the incident fringe pattern relative to the mask is then determined by arithmetically processing the detected signals.In accordance with one aspect of the present invention, the subject fringe pattern is time modulated and the quadrature-shifted mask signals A-C coupled to obviate the requirement for the third, fixed transmissivity mask section.