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 .sigma. to various locations on the plane, where the angle .sigma. is a periodic function of time. Two angularly-selective radiation detectors oriented at fixed angles .tau..sub.1 and .tau..sub.2 sense scattered portions of the beam from two locations at two corresponding times. The orientation parameter is computed from a time difference .DELTA.t=t.sub.2-t.sub.1 between the two times.