ABSTRACT
There are several three-dimensional (3-D) imaging methods based on interferometry that allow the measurement of minute displacements and surface profiles. Methods such as holographic interferometry and speckle metrology can provide full-field noncontact information about coordinates, deformations, strains, stresses, and vibrations. The basic principle of these methods is the transformation of phase changes into recordable intensity changes [1]. Because of the high spatial frequency of these intensity fluctuations, the registration of a hologram requires a light-sensitive medium with adequate spatial resolution. Therefore, special photographic emulsions have dominated holographic technologies for a long period. However, the recording of holograms on electronic sensors and their numerical reconstruction is almost as old as holography itself. First ideas and implementations came up already in the 1960s and 1970s [2-4]. But only in the 1990s did the progress in high-resolution camera technology and computer hardware make it possible to record holograms directly on the chargecoupled device (CCD) target of a camera and to reconstruct the wavefront in a reasonable time [5]. But digital holography (DH) is much more than an elegant recording technique. In contrast to conventional approaches, DH allows the direct calculation of both parts of the complex wavefront, phase and intensity, by numerical solution of the diffraction problem in the computer. Several advantages for the measurement process result from this new development. Besides electronic processing and direct access to the phase, some further benefits recommend DH for the solution of numerous imaging, inspection, and measurement problems [6]. We will return to this point later. Here, only some obvious advantages will be mentioned: certain aberrations can be avoided and the remaining aberrations can be corrected numerically, different object states can be stored independently, the number of necessary holograms for quantitative evaluations can be decreased considerably, and the size of holographic sensors can be reduced drastically.
