This article deals with a theoretical analysis of two allied laser speckle interferometric techniques for measuring surface displacement and strain. One technique uses a Double Aperture Speckle Camera, and the other uses a similar interferometer called a Double Aperture Speckle Shearing Camera. These techniques, although developed elsewhere, had not been analysed for the cases when a generalized deformation of the surface under study takes place. Such an analysis was done and is presented here in an abbreviated form. It results in ‘new’ equations governing the formation of fringes by the two cameras. Various solution methods of these equations are outlined and discussed.