Abstract: A device for detecting the position of an information recording or reproducing head relative to a recording medium; the head is relatively rotatable around a first axis with respect to the recording medium, and rotatable around a second axis parallel to the first axis. The device includes an autocollimator for measuring a relative positional relationship of the head substantially along the direction of the rotation around the second axis, a rotating table on which either the combination of the recording medium and the head or the autocollimator is mounted to activate the rotational movement substantially around the second axis, and a control unit. The control unit rotates the rotating table in response to the obtained position of the head with respect to the recording medium. According to the measurement result by the autocollimator, the control unit accomplishes a relative positional controlling in which a relative positional relationship between the autocollimator and the head is kept constant.

Abstract: It is an object of the present invention to provide a speckle interference apparatus capable of selecting given phase information at a speed higher than speckle interference fringe patterns with a higher precision. For a speckle interference apparatus in which a light beam from a light source is divided into plural light beams, and one of the light beams is passed through a measuring surface and the other light beam through a reference surface to be superposed respectively for the formation of speckle interference fringe patterns, there are arranged in the optical paths spatial filters having windows for sampling the area of a given phase among the speckle interference fringe patterns. Hence, with such filters, speckle interference fringe patterns of the same phase are produced.