nI am interested in small structural deformations in optical systems. nIs there a way to measure just the relative displacement of the two points in the z-axis instead of their absolute displacement?nThe difference in the absolute displacements of the two points is the relative displacement.nOptical systems are sometimes connected to a frame using kinematic mounts. The benefit of a kinematic mount is that deformation in the frame does not induce deformation in the optical elements. There may be rigid body motion of the optical elements, but no stress. Four fixed points is not a kinematic mount. That means deformation in the frame (that you have not shown) can have an effect on the distance between the two points.n