Digital holography has a limited possibility to reconstruct object details because of an insufficient pixel number of
commercially available CCD cameras. Software hologram apodization introduces an additional damping of registered
object information and, in general, it can be considered as a disadvantageous process. However, for the reconstruction
of phase objects in digital holography the unwrapping procedures are indispensable. Moreover, for surfaces with
considerable roughness depth there are large phase changes between neighbor CCD camera pixels. Therefore
unwrapping procedures can introduce essential errors to reconstructed phase images. In the paper, besides general
considerations on the apodization in digital holography, experimental and computer simulation results of the hologram
apodization for surface shape measurements are presented.
The study of digital holographic system with partially spatially coherent illumination is presented. The analysis concerns digital lensless Fresnel holography with stationary object illumination. We propose a criterion for selection of optimal partially coherent beam to get maximum noise reduction while maintaining full system resolution. The theoretical results are applied to 3D simulation and a laboratory experiment.
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.
KEYWORDS: Digital holography, Holograms, Holographic interferometry, 3D image reconstruction, CCD cameras, Fringe analysis, Interferometry, Holography, Time metrology, Data processing
Optical measurement methods based on digital holography are highly effective for shape and deformation investigation of microcomponents. One of the most commonly discussed methods is digital holographic contouring connected with digital holographic interferometry. Unfortunately, this approach requires much time and it is difficult to apply to real time measurements. In the paper a novel, simple and inexpensive setup for shape and deformation measurements is proposed. The method uses fringe projection combined with digital holography. First a fringe image projected onto the surface under the test is registered using digital holography without any optical system. Next, the fringe image is reconstructed from the hologram what allows the determination of the surface shape. Small displacements are measured with digital holographic interferometry and larger ones by comparing the shapes for two object states. Because only one hologram for the shape and two for the displacement investigation are needed, quite fast measurements are possible. However, the CCD camera used in the arrangement restricts the measurement time. The comparison of both methods, particularly related to the measurement precision, will be given. The results obtained by experimental means are presented together with a discussion of the limitations and further possibilities of this method.
Interference patterns imaged by an optical system and registered by CCD camera are applied in automatic systems for surface shape and deformation measurements. The digital holographic registration of fringe patterns is proposed to avoid the influence of the optical imaging system. Theoretical relationships and numerical simulation results are presented.
In digital holography the imaging process consists of physical registration using CCD camera and digital image reconstruction with specialized software. Amplitude gradients of the reconstruction field at the hologram plane influence significantly the phase reconstruction quality. The field truncation by hologram edges is of particular concern. The image reconstruction process can be software repeated several times using various apodization functions. Basing on phase imaging changes introduced by different apodization functions a decrease of phase errors can be achieved for some object types. Basing on the results received from experimental measurements and computer simulations for some object examples the advantages and disadvantages of the proposed method will be presented.
A new measurement method based on digital holography is proposed. The holographic registration of interferometric fringes generated on a surface under study using CCD camera and their reconstruction allow the determination of the surface shape with the aid of a computer program. Problems related to limited resolution of CCD camera devices are considered. The results obtained by numerical means are presented. The discussion on further possibilities is given.
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