Technique for exploring the spatial coherence in a speckle field

Speckle patterns are influenced by several parameters and one of them is th e coherence of illumination source. It is clear and well known from others studies on coherence that interference fringes in general are reduced in th eir visibility by reducing the spatial coherence of the illuminating light. In the present work we study this behaviour by analyzing the relationship between the visibility reduction of the correlation fringes obtained in a d igital speckle pattern interferometer (DSPI) and the spatial coherence degr ee, when one of the beam of the interferometer is spatially incoherent. In this way, the technique represents a digital alternative to explore the var iations in the spatial coherence of speckle field. In practice, the incoher ent beam is produced by focusing the beam on a ground glass which is contin uously shifted in a transversal direction. The area of the focused beam on the ground glass was chosen as parameter for studying the visibility variat ion. We present the theoretical explanation and the corresponding compariso n with the experimental data.