Presented herein is a non-contact torque sensing apparatus and method for measuring the instantaneous torque, or torsional stress/strain, transmitted through an elongated power transmission member such as a rotatable shaft. Polarized light is directed along a measurement light path in a cavity of a shaft where it intercepts a polarizing filter. The polarizing filter is operable to alter the polarization angle of the light according to torsional twisting of the shaft. A measurement device measures the change in the polarization angle of the light to obtain the shaft twist angle. Shaft torque is then calculated from the twist angle.