Surface shape measurements using digital holography with a fringe projection system and partially coherent illumination

Abstract

In recent years several methods based on digital holography to combine shape and deformation analysis in the same setup have been proposed. For measuring the object deformation in the most cases two holograms with a certain wavelength have to be recorded for different object states. For shape measurement the object has to remain unchanged while two holograms with slightly different wavelengths or slightly different illumination points are recorded. In the paper a different, simple and inexpensive method for shape and deformation measurements of rough surfaces is proposed. The method uses fringe projection combined with digital holography. First complex amplitude from surface under test is registered using digital holography without any optical system. Next, the fringe image is reconstructed from the hologram as an intensity distribution what allows determination of the surface shape. Small displacements are measured using reconstructed object phase distribution and digital holography interferometry, larger ones by comparing the shapes before and after deformation. Because only one hologram for the shape and two for the displacement investigation are necessary, quite fast measurements are possible. Light detector used in the arrangement mainly restricts measurements time and methods precision. Unfortunately an application of coherent radiation introduces high contrast coherent noise into the holographic imaging. To reduce it the partially coherent illumination is used instead. The coherence degree of illumination beam is chosen to limit spatial bandwidth of holographic fringes according to used detector.