To determine the coherence of a laser device, the concept of visibility is used and applied to the measured interference patterns produced by this laser light in a specific interferometer. In this case, to determine the coherence of a Diode Pumped Solid State Nd -YAG Laser, a digital image processing procedure was developed and applied to the fringe patterns obtained with a rigid arm Michelson interferometer array in order to obtain the coherence function. A spatial filtered laser beam was used to illuminate the interferometer and to produce a circular fringe pattern. The interference pattern was captured on a spatial calibrated translucent screen and photographed by a digital camera. Afterwards, the images were digitally processed to calibrate the pixel distribution and to produce an intensity function from this calibrated image. Data processing of the intensity function values allows to produce the coherence function and to evaluate the coherence degree of the radiation. The interference patterns, intensity distributions and the coherence calculations of the Nd - YAG laser are presented.