Its Color Digital Holographic System Detection Applied Materials

With the development of domestic economy as well as the advance of science and technology, more and more materials are needed in every trade and widening areas. Not only is the demand of material enlarging, but also the requirement toward performance of materials is improving. Starting from the economic interest of materials usage, how to select a correct material that performs excellent operation until its limited invalidation is one of important subject in material science and engineering. When heated or pressed, material will deform which reflect the mechanical intension and extension of structure. For the structure composed of complex stuff or shape, it is hard to evaluate its mechanical parameters via theoretical calculation. In this case, experiment test toward the structure or its model is a good solution.Non-invasion inspection technology can be used to detect the mechanical parameters of material such as micro-deformation; meanwhile the material will not be damaged. Holographic measurement is an important technique in laser non-invasion inspection. With the promotion of computer technology and charge coupled device (CCD), digital holographic measurement is becoming a focus due to the CCD substitution for traditional photosensitive plate. However, the sensitive size and resolution of CCD are less than traditional plate, for a large object image CCD can not totally record the information contained by object wave. Thus, an imaging optical system is usually inserted between object and CCD to convert the object image so as to be recorded completely by CCD. If a hologram is recorded in this condition, the object wavefront reconstruction will concern with the transformation of wavefront through the optical system. Once some complex mechanical parameters are measured, such as deformation or strain, three dimensional shifts in space should be detected along different direction. Color digital holographic measurement could satisfy this kind of detection, because three wavelengths are used in the operation. An imaging optical system is usually made of several sorts of lenses, chromatic aberration always exist in the recording and reconstruction of a color digital hologram, which will affect the measurement. Therefore, how to design an optical system used to record a color digital hologram of large size object is a challenge subject. Especially, the elimination of chromatic aberration in color reconstruction of a hologram will make the measurement be more accurate. In this work, these two problems are studied deeply based on color digital holographic measurement, and detail contains are as follows:1. Optical wave conversion executed by an axial symmetry optical system in paraxial approximation is expressed through Collins formula. A wave conversion optical system with complex structure is identical to a single lens with certain focal length that is placed at a special position. Starting from the Collins formula and angular spectrum theory about wave diffraction, an optical system used to record the color digital hologram of large size object is designed and optimized. The color digital holographic optical system and reconstruction of the color hologram is researched. It is concluded that, utilizing an identical concave lens to record the hologram of large size object will make the wave conversion system be more compact in operating range.2. During the measurement of surface micro-deformation toward an object, three shift vectors with respect to observation point should be determined on three different planes, because three wavelengths of laser are used in color digital holographic detection. The influence of chromatic aberration caused by lens between object and CCD is studied. Based on single-FFT off-focus reconstruction method, not only mono-chromatic reconstruction is given in terms of Collins formula toward same object, but also more practicable and simpler color reconstruction is presented by using focus length depth in coherent imaging.3. When Collins formula is used to reconstruct an object wavefront, each element in the conversion matrix should be known, which is decided by the parameters of optics in imaging system. A wave conversion system is usually composed of several optical units. It is difficult to accurately confirm their optical parameters. For conventional application, optical system could be regarded as a black box whose internal structure would not need to know. Thus, the elements of optical conversion matrix can be measured and carried out by means of point source hologram.4. When an optical system is configured between object and CCD to record a hologram, its reconstruction can be achieved by Collins formula. Theoretical study demonstrates that, even though the imaged wave is transformed from object wave by optical system, it still contains the sufficient information of object. In this way, the image of object could be referred as another new object with respect to holographic recording system. From this view, the digital reconstruction will be performed in traditional method according to the distance between the image and CCD. So the reconstruction procedure will be simplified toward the object wavefront converted by an optical system. Although the imaging plane can be determined through the structure and parameters of the optical system, in practice the position error exist in some extent really. A new ideal is proposed that, it is not necessary to know the detail structure and parameters of optical system, the imaging plane poison could be determined through point source hologram. From this ideal, three different imaging plane positions under three operating wavelength is tested, comparing theoretical calculation to experimental result prove this method feasible. Further study shows that, rigors wavefront reconstruction in image space is miscellaneous much, because the imaging plane position varies with the operating wavelength. After studied the focus length depth of coherent imaging system, it is found that the average position of three imaging planes can be used as the reconstructing plane position. Using well-rounded several methods of reconstruction that does not exist a conversion system between object and CCD, the image wavefront can be reconstructed.5. Based on these conclusions mentioned above, the detection study about micro-deformation of materials is conducted under the single-FFT off-focus reconstruction as well as scaling reconstruction, which the size of CCD photosensitive plane is less than the recorded object size. A color digital holographic measuring system is designed to detect the micro-deformation toward two kinds of opaque materials whose size is lager than the size of CCD photosensitive plane. The first experimental study is carried out to test the micro-deformation of a dumbbell-shaped aluminum alloy plate employed in aerospace, which is pulled through moire fringe meter. The detection of three dimensional micro deformations toward this plate is completed in the proposed color digital holographic system. The second experimental study is made to measure the micro expansion (also another micro deformation) of compound materials that is heated through an electrical heating system. A straight bar of45steel is used as a testing object, whose three dimensional micro expansion is measured in the proposed color digital holographic system. In these two experiments, a real-time micro deformation under heating or pulling is obtained by proposed detecting system. so as to make quantitative calculation toward mechanical parameters of material. Finally, the future prospect about this research is depicted in the end of paper.