彩色数字全息系统及其材料检测应用研究

【作者】 楼宇丽；

【导师】 李俊昌；

【作者基本信息】 昆明理工大学 ， 材料学， 2013， 博士

【摘要】 随着科技进步及国民经济的快速发展,各行业对材料的需求量增多,使用范围扩大,性能要求逐步提高。综合考虑经济等多种因素,如何在品种繁多的材料中,针对具体的要求选择合适的材料,使其能较稳定地服役于失效极限范围之内,是材料科学与材料工程的重要内容之一。材料在受力或受热时会发生变形,该信息部分反映了材料构件的强度、延性等力学性能。对于形状或材质较复杂的材料构件,很难通过理论计算获得反映力学性能的相关参数。此时,可以对模型或者试件进行实验测量从而获取所需参量。采用无损检测技术可以在非损伤情况下对材料微形变等物理量进行检测。全息检测是激光无损检测技术的一个重要方法,随着计算机及电荷耦合器件(CCD)技术的进步,用CCD代替传统全息感光板的数字全息检测正成为一个研究热点由于CCD靶面尺寸及分辨率显著小于传统全息干板,当CCD靶面尺寸与物体的投影尺寸有较大差异时,在物体和CCD间设计不同形式的光学变换系统,对物光场的横向放大率进行变换,让CCD充分接收物光信息,是目前应用研究中通常采用的措施。在这种情况下,涉及物光通过光学系统后到达CCD的波前重建问题。当测量物体的形变一类复杂物理量时,通常需要测量物体表面沿空间不共面的三个方向的位移,这时涉及同时使用三种不同波长的光波照明下的彩色数字全息检测。由于光学变换系统通常由不同形式的透镜组成,当进行多波长照明的彩色数字全息研究时,透镜的色差作用显著增加了用同一物理尺寸统一重建物光场的困难。因而,当物体投影尺寸与CCD靶面尺寸有较大差异时,如何设计光学系统对物光场进行变换,在同一物理尺寸下统一不同色光重建物光场,实现对大尺寸物体表面微形变的检测,是一个值得研究的课题。本文的研究工作围绕这一课题展开,具体内容如下：1.由于光波通过轴对称傍轴光学系统的衍射可以用柯林斯公式描述,并且,任意结构的透镜变换系统可以用空间某特定位置放置一个特定焦距的透镜来模拟。基于衍射的角谱理论及柯林斯公式,本文以大尺寸物体数字全息记录系统的优化设计为例,对彩色数字全息光学系统及物光波前重建进行了研究。研究结果表明,当使用单一透镜对物光场进行变换时,使用负透镜可以使数字全息记录系统更加紧凑。2.在测量物体表面微形变这一类物理量时,需要对观测点实时检测不共面的三个位移矢量,通常使用三种不同波长的激光进行彩色数字全息检测。我们对物体与CCD间光学系统内透镜色差的影响进行了研究,基于1-FFT离焦重建法,不但建立了利用柯林斯公式重建同一物理尺寸不同色光物光场的方法,而且,基于相干光成像焦深的研究,导出较为简易适用的彩色图像重建方法。3.当采用柯林斯公式重建物光场时,光学系统的光学矩阵元素是须知的参数,确定系统的光学矩阵时必须准确知道每一元件的光学参数及几何位置。光学系统可能由多个元件组成,实际上准确确定每一元件的光学参数比较困难。为便于实际应用,将光学系统视为不必知道内部结构的“黑箱”,提出了利用点源全息图对光学矩阵元素进行检测的方法。4.当物体与CCD间置入光学系统时,虽然可以通过柯林斯公式进行物光场的波面重建,然而理论研究表明,物体的像光场通常能够充分包含物光场信息,可以将物体的像在像空间中视为一个新的物体,这时只要知道像到CCD的距离,则能利用数字全息图对物体的像进行重建。这就可能进一步简化物光通过光学系统的波前重建工作。虽然,给定照明光波长后,可以基于光学系统的结构及透镜元件的折射率,从理论上确定系统的像平面。然而,这种方法繁琐且误差较大。我们提出不必详细知道组成光学系统结构,利用点源全息图确定光学系统像平面位置的方法,测定了三种波长光对应的像平面,并用理论计算与实验测量的比较验证了测量方法的可行性。研究结果表明,在像空间中不同色光的物体像平面位置不同,严格的像空间波前重建比较繁杂。但是,基于相干光成像焦深的研究发现,在焦深所规定的范围内,可以用像平面位置的平均值作为三种色光像平面位置,利用已经比较成熟的物体与CCD间无光学变换系统的多种重建方法进行像光场重建。5.基于上述研究结果,利用1-FFT离焦重建法及可变放大重建法,对投影尺寸大于CCD靶面尺寸的材料进行微形变的检测研究。对投影尺寸大于CCD靶面尺寸的两种非透明材料,设计合适的彩色数字全息检测系统,对其微形变进行实验研究。实验之一,为了研究受力情况下的微形变,通过载荷传感器对一片加工成哑铃形的航用铝合金片施加拉力,采用彩色数字全息检测系统对该铝合金片在受力的情况下的三维微形变进行检测研究；实验之二,为了研究材料加热情况下微膨胀(也可视为微形变),设计一电路系统对一45号钢直棒进行加热,采用彩色数字全息检测系统对样品在受热情况F的三维微膨胀进行检测研究。在实验中,实时获得了受力及受热情况下的三维微形变信息,并进行了定量计算。基于实验研究,对下一步的工作进行了预期及展望。更多还原

【Abstract】 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.更多还原