Abstract: An elongate object optically determines at least one of its orientation parameters relative to a plane surface. A probe radiation beam is directed from the object at various angles ? to various locations on the plane, where the angle ? is a periodic function of time. Two angularly-selective radiation detectors oriented at fixed angles ?1 and ?2 sense scattered portions of the beam from two locations at two corresponding times. The orientation parameter is computed from a time difference ?t=t2?t1 between the two times.