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.