Comparative accuracy of holographic interferometry and speckle photography for out-of-plane deformation measurement

Out-of-plane deformation of a surface can be measured either by recording a double exposure hologram, or by taking a double exposure speckle photograph with the camera deliberately de-focused. In the latter case the local surface tilt is measured, and the profile change subsequently obtained by integration. It is shown that errors can arise in speckle photography due to focusing the camera incorrectly, whereas the holographic method is not affected instrumentally. The sensitivity of the speckle method can approach that of holographic interferometry, but without requiring the same interferometric stability of recording apparatus.