A sensitive optical speed sensor has been developed utilizing the transient lateral photoeffect induced in a Si thin-filmp–i–nphotodetector by a moving light spot. The local time dependent illumination changes the lateral photocurrent and modulates the potential barrier across the junction allowing the sensing of the light spot velocity. The performances of an amorphous(a-Si:H)and a microcrystalline(&mgr;c-Si:H)silicon-based sensor will be compared and discussed taking into account the differences in material properties and device geometry. The&mgr;c-p–i–nstructure has the advantage of faster response, high current capability, and excellent stability under illumination when compared with the amorphous counterpart. ©1997 American Institute of Physics.