ON THE GENERATION OF CLUSTERED SPECKLES DUE TO RING-SLIT ILLUMINATION

When a rough surface is illuminated by coherent light passing through a ring-slit aperture, a speckle pattern formed in its Fraunhofer diffr action region presents a clustering or snake-like appearance consistin g of many winding chains of speckle grains. Some properties and origin of such clustered speckles are discussed. From experiments employing a ring-slit aperture as well as some segmented ring-slit apertures, it is shown that clustered speckles have different characteristics depen ding on the type of the apertures, and that the isotropically winding, snake-like Feature of the speckles due to the ring-slit aperture is a ttributed to the circular symmetry of the aperture. An origin of the s nakelike feature is discussed qualitatively on the basis of the random interference of many diffraction patterns. Speckle clustering in part ially developed speckles is also examined. It is shown that a weak dif fuser masked by the ring-slit aperture yields radial speckle chains. w hich turn into isotropic winding chains as the diffuser becomes strong er.