A detailed analysis is made of a system for fringe evaluation in hologram interferometry in which a conjugate reconstruction beam scans the hologram, and the resulting fringe motion is observed in the real image of the object. The factors decisive for best use of the system are studied, especially the sensitivity and accuracy of the measurements and their dependence upon the geometry of the setup. For the relation between the geometry and the sensitivity, the concept of the K sphere is introduced. The effect of speckle noise due to the limited apertures of the system is also discussed. It is shown that the speckles determine the displacement measuring range together with suitable aperture sizes of the system.