PURPOSE: To measure the two-dimensional motion of a body by using two different photoelectric conversion systems which use a laser speckle and measuring fine perpendicular displacement and speed components of the body associatively with displacement and speed components in the moving direction.

CONSTITUTION: A coherent parallel light beam 1 is moved slightly zigzag in a plane and scattered light obtained at this time is branched in two directions by a half-mirror 3; one scattered light beam forms a speckle pattern on an equal-interval slit array 4. Its image is formed through a lens and photodetected by a photoelectric detector 6, whose output variation with time is outputted as a frequency proportional to the speed vx of the body in an (x) direction. The low frequency component of the signal is removed through a high-pass filter 7 to obtain a frequency spectrum proportional to the speed vx of the body and this frequency spectrum is measured by a frequency measuring instrument 8 to obtain an output proportional to the speed vx of the body; and then the speed vx of the body is calculated by using a divider 9. Similarly, the speed in a (y) direction is calculated.