frequency is the number of fringes per unit length whereas the direction of the frequency refers to the direction of the spacing fringes can be more closely spaced along one direction and less closely spaced along another it is the directional property of the spatial frequency of the speckle patterns which allows the separation of the mixed information recorded by the four lens system of fig4 this statement is true because the direction of the spatial frequency of the speckles recorded by a dual lens system depends on the orientation of the separation of the lenses if the two lenses are separated in the vertical direction the frequency direction cf the speckles produced by the interference of the two images will have a dominant component in the vertical direction thus to read out a fringe pattern recorded by a vertically separated lens pair the lens of the video camera 37 should be positioned vertically away from the image point in frequency plane 34 of fig5 i e the camera should be displaced along vertical line 32 likewise the camera lens should be displaced along horizontal line 38 if the recording lenses are separated horizontally in general the interference pattern generated by a pair of lenses should be viewed along an axis corresponding to the axis of separation between the pair of lenses in the real time method of viewing interference patterns a photographic plate in the image plane of fig1 is exposed while the object is not deformed and then developed in place or developed and then returned to its original position with the object illuminated by laser light and when the object is deformed a 34 live 34 fringe pattern will be observed behind.