Abstract
A method of displacement measurement of a moving object with a diffusing surface has been developed already. This paper presents, an automatic method of the displacement mesurement. The automatic method consists of a feedback loop synchronized with the movement of an object, making fringes using speckle pattern interferometry. This is done by processing the input image of the fringes, including extraction of circles and detection of the difference by moving the reference PZT. The method of displacement measurement is as follows. A collimated laser light beam is transformed into two beams by using a concave lens and a beam-splitter cube, in a Twyman-Green-type interferometer. When a diffusing surface is placed at the focal point of one of two beams, a diverging spherical wave front containing large-sized speckle patterns is scattered. If a smooth surface reference mirror with PZT is placed in the vicinity of the focal point of the other beam, the reflected forms of the two beams have essentially a wave front, with identical shape. The two beams can be combined to form concentric interference fringes which are fairly regular. By observing whether the concentric fringes which flow out or sink in, it is shown that the normal component of the displacement of the diffused surface can be measured with an accuracy of a fraction of a half wavelength.
