Minimum diameter of the read-out laser beam in double-exposure speckle photography

A simple model is proposed for a speckle pattern produced by a diffuse surf ace and intercepted by the plane of a photographic plate. In this model, al l the spots have the same form but are randomly distributed. During the tra nslation, rotation, and strain of the diffuse surface, it is assumed that t he shape of each spot remains the same. The transmittance of the processed plate that would be produced when exposed before and after the displacement of the diffuse surface, is calculated. An equation is obtained for the int ensity of the diffracted light when the plate is illuminated with a circula r interrogation beam. The lower limit that the read-out laser beam diameter should have so that the Young fringes are visible is calculated and a crit erion is proposed. The theory is checked with laboratory experiments. (C) 1 999 Elsevier Science B.V. All rights reserved.