The need for simultaneous measurement of more-than-one degree-of-freedom (DOF) motions can be found in numerous applications such as robotic assembly, precision machining, optical tracking, wrist actuators, and active joysticks. Conventional encoders, though they are able to provide very high-resolution measurements (linear or angular), are limited to single-DOF sensing in motion control. The use of these single-DOF encoders for measuring 3-DOF motions in real time often requires additional mechanical linkages that often introduce frictions and backlashes. We present here a non-contact optical sensor for measuring multi-DOF motions. This paper begins with the operational principle of a microscopic-surface-based optical sensor. The design concept and theory of a dual-sensor system capable of measuring a 3-DOF planar motion in real time is then presented. Along with a detailed analysis, the concept feasibility of a prototype 3-DOF dual-sensor system for measuring the instantaneous center of rotation and the angular displacement of a moving surface is demonstrated experimentally. It is expected that the analysis will serve as a basis for optimizing key design parameters that significantly influence the sensor performance.