Abstract
A collimated light beam from a He-Ne Laser, 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 the two beams, a diverging spherical wave front containing large-sized speckle patterns is scattered. If a smooth surface reference mirror is placed in the vicinity of the focal point of the other beam, the reflected beams have essentially the same shaped wave front. The two beams can be combined to form concentric interference fringes, which are fairly regular. By observing the concentric fringes flowing out or sinking 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.
