Application of the structured illumination method to study the topography of the sole of the foot during a walk

The structured illumination technique consists of projecting a fringe system onto a 3D object from a well defined space point, which results in a pattern that depends on the characteristic of the projected fringes, the viewpoint and the illuminated object morphology. Therefore, the structured illumination method enables to determine the topography of 3D objects. To implement this technique we set up an optoelectronic array that allows studying the sole of the foot during a walk. The method consists of projecting a Ronchi grid and capturing the images generated on the foot by a CCD camera. These obtained images are graphically processed and fringes converted into vectors. A depending algorithm on the experimental setup allows obtaining, from those vectors data, cotes for a discrete profile plotting of the studied object. The method enables the quantitative determination of the sole topography during the walk. Qualitatively, it can be used for diagnosis and control of deformation and injuries caused by accidents or illnesses. It can be introduced in the primary attention health system to study a great number of patients, due to its simplicity and low cost installation, and for being a non-invasive technique.     

Fracture detection by grating moiré and in-plane ESPI techniques

Optical interferometry techniques have been used for high-precision displacement measuring. Commonly, in-plane sensitive arrangements use two symmetrical collimated wavefronts for object surface illumination. However, this is a limitation when large object surface, has to be analyzed. In this case spherical illumination is needed. As a consequence of using non-collimated symmetrical dual-beams the sensitivity vector varies with the local position on the surface target. Then, this kind of illumination is also capable of detecting a lightly and systematic out-of-plane component of deformation. In this paper a theoretical analysis of the sensitivity vector components behavior is made. Each component of the sensitivity vector to minimize the required displacement component uncertainty is calculated. This study is important in the stage of planning any interferometric measurement experiment, particularly, for moiré grating interferometric technique, which has been used only in collimated illumination. By using a spherical dual-beam optical setup, the present work shows results of fracture measuring by using moiré and speckle interferometric methods. As a result, advantages and disadvantages of both techniques are discussed and an accuracy study is reported.     

预成像数字全息测量大物体三维形貌

为了扩大数字全息三维形貌测量的视场,提出了一种预成像的方法.物体先被成像镜头成一缩小的实像,再使用离轴菲涅耳数字全息对此实像进行全息记录.采用倾斜照明光的方法使两次数字全息的再现像之间产生相位差,通过相位去噪和去包裹处理从相位差中提取物体的三维形貌数据.实验证明,预成像方法使测量视场的大小得到了至少一个数量级的提高,采用离轴菲涅耳数字全息降低了测量系统对环境稳定性的要求.     

Speckle noise reduction in digital holography by use of multiple polarization holograms

A novel speckle reduction technique for digital holography is proposed. Multiple off-axis holograms are recorded using a circularly polarized illumination beam and a rotating linearly polarized reference beam. The speckle noise in the reconstructed images is suppressed by averaging these fields. We demonstrate the effectiveness of this technique experimentally and conduct additional statistical evaluation.     

基于传像束的小视场双目结构光三维形貌恢复

提出基于传像束传像和双目结构光相结合的三维测量系统,实现对无法直接成像场景,如爆轰、冲击等环境下的小视场物体三维面形测量.测量时,结构光直接投影到待测物体表面,变形条纹图像首先被成像到传像束前端面,再被传递到传像束后端面,并最终被成像到相机像平面,再由三频相位展开技术计算出对应相位分布后,即可根据双目匹配计算出待测物体...     

Essay Review-Scholarly froth and engineering skeletons

Holography is a two-stage process of photography using coherent light from a laser to illuminate the scene. In the first stage a hologram is formed by combining the light scattered from the object and the direct laser beam on a photographic plate. In the second stage a three-dimensional image is reconstructed without the use of lenses, by directing the laser beam through the hologram. Applications of holography include information storage, recording of images in depth, the use of holograms as optical elements, and as a means of performing precise interferometric measurements on three-dimensional objects of any shape and surface finish.     

Improvement of speckle noise suppression in digital holography by rotating linear polarization state

An improved polarization recording approach to reduce speckle noise in digital holography is proposed. Multiple off-axis holograms are obtained by rotating the linear polarization state of both illumination and reference wave simultaneously. By averaging the intensity fields, the speckle noise in the reconstructed images is well suppressed. Statistical evaluation of the experimental results shows the effectiveness and improvement of the proposed method.     

基于多角度无透镜傅里叶变换数字全息的散斑噪声抑制成像研究

在数字全息成像中,散斑噪声严重影响了再现像的信噪比和成像分辨率,因此为了提高数字全息成像质量,迫切需要抑制再现像的散斑噪声.分析并给出了矩形散射光斑的强度协方差,定量计算了特定实验条件下产生退相关散斑图样的最小角度差.结合透镜的性质设计并搭建了多角度无透镜傅里叶变换数字全息成像系统,利用光纤端面在透镜焦平面的二维机械移动代替传统反射镜的旋转,使照明光束在不改变照明位置和大小的同时,可任意改变光束方向.移动光纤端面使多角度照明满足最小角度差,获取了81幅数字全息图.利用单次快速傅里叶逆变换实现数值再现,对多幅再现像的强度像求平均,实验结果表明散斑对比度降低为单幅再现像的14.6%,使图像信噪比提高了6.9倍.该抑制散斑的多角度数字全息成像方法有效的抑制了散斑噪声,且成像光路结构简单,可操作性强.     

基于相移结构光照明的浮雕成像技术研究

详细介绍了一种基于相移结构光照明的浮雕成像技术,对该成像技术的原理和成像特性进行了实验研究,并采用相移结构光照明的方法实现彩色浮雕成像。实验中,将多幅相移正弦条纹光场斜投射到被拍摄物体上,数码相机依次拍摄被照明的物体,再利用拍摄到的多幅相移图像,通过计算的方法重建出所需要的图像。理论分析和实验结果表明,相移结构光照明成像技术能有效地消去环境不均匀光照的影响,获取的反射率分布图像只与结构光照明有关,突显了斜投射照明所形成的阴影,图像的明暗变化反映出物体表面的法线变化,因而在视觉上形成了明显的浮雕效果。     

Digital spatially incoherent Fresnel holography

We present a new method for recording digital holograms under incoherent illumination. Light is reflected from a 3D object, propagates through a diffractive optical element (DOE), and is recorded by a digital camera. Three holograms are recorded sequentially, each for a different phase factor of the DOE. The three holograms are superposed in the computer, such that the result is a complex-valued Fresnel hologram. When this hologram is reconstructed in the computer, the 3D properties of the object are revealed.     

Illumination-angle-scanning digital interference holography for optical section imaging

We propose and experimentally demonstrate a new (to our knowledge) digital holographic method to reconstruct section images of objects with wavelength-dependent reflectivity. A number of holograms of an object are taken as the illumination angle of the laser beam with a specific wavelength is changed in regular intervals. The complex object fields reconstructed from the holograms are numerically superposed to show the image of a sliced section of the object, whose position and thickness can be chosen arbitrarily. By changing the wavelength of the illumination beam, wavelength-dependent section images can be obtained with our method.     

Gerchberg迭代法在三维面形测量中的应用

在基于结构光照明的三维面形测量中,针对有孤立区域物体的条纹图像存在不规则缺陷的问题,提出先对其进行Gerchberg迭代,消除或减少条纹缺陷,获取较完善的迭代条纹,然后再利用傅里叶变换轮廓术(FTP)重建面形。设计了迭代流程,并分析了影响迭代精度的因素,解决了光学加工用具磨盘表面不完善条纹Gerchberg迭代过程中存在的问题,成功地重建了磨盘的三维面形。该工作为表面存在孤立区域的被测物体三维面形测量提供了可供参考的技术方案,也给可实时形变的应力盘三维面形动态测量奠定了基础。     

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

一种分析薄膜振动模式的结构光条纹时间平均法

提出了一种用于振动物体表面快速测量的光学测试系统和方法。该系统采用结构光三维传感技术,利用时间平均法来实现对振动表面的测量和振动模式分析。用低帧频商用CCD相机记录由光栅投影到振动物体表面上形成的一系列变形正弦条纹,对获取的一帧变形条纹经过二维傅里叶变换、频谱滤波、逆傅里叶变换等处理得到调制物体振幅的零级贝塞尔函数分布。给出了该方法的理论分析,推导了相应的计算公式。计算机模拟和实验验证了该方法的可行性。实验证明,该方法具有数据获取速度快,全场非接触测量以及实验装置简单等优点。     

Some investigations in holographic microscopic interferometry with respect to the estimation of stress and strain in micro-opto-electro-mechanical systems (MOEMS)

Holographic microscopy with conjugate reconstruction for the interferometric determination of three-dimensional displacement was used for the investigation of the mechanical behaviour of micromechanic and microelectronic components. An experimental set-up for the exposure of the holographic interferograms is described for the application of the spatial heterodyne technique, for the application of phase shifting, and for electro-optic holography. Three holograms for different illumination directions recorded on one holographic plate were reconstructed conjugately, and spatial-heterodyne technique as well as phase-shift technique were used to evaluate the interferograms. Only by conjugated reconstruction, it is possible to obtain a perfectly optimised interferometer for the static evaluation method. The evaluation of interferograms, which are strongly disturbed by speckle noise, can be performed successfully. A comparison of the results of the application of these techniques is given. The influence of the speckle effect on the resolution was investigated.     

Grazing holographic projection of object-adapted fringes for shape measurements with enhanced sensitivity

A new method of fringe projection for shape measurement is presented. We use a combination of object-adapted fringes and holographic grazing projection to determine shape deviations of a turbine blade. If the shape of a test object is correct, a fringe grid with constant period is detected by the camera. The object-adapted fringes are projected coherently onto the master object, and a hologram is taken. If the hologram is reconstructed, grazing incidence of object-adapted fringes can be realized. Grazing holographic projection allows for higher triangulation angles and therefore increased sensitivity. Even small shape deviations can be detected and quantitatively evaluated with fast algorithms. The method can be extended to multiple exposed holograms to examine deep objects.     

基于单幅数字散斑投影及图像相关的离面振动测量

采用单幅数字散斑图投影及高速数字图像采集技术,研究了动态离面位移的测量。采用商用液晶投影仪将计算机产生的模拟散斑图投影到待测动态变形物体表面,由高速数字图像采集设备摄取并保存变形散斑图,采用时间序列数字图像相关软件计算出物体表面各点随时间变化的离面位移。这种方法用于振动分析时不仅可以获得振型分布,而且还可以获得各点的振幅值。与现有的激光频闪照相测振及激光多普勒测振等方法相比,具有光学系统简单,可全场定量测量。悬臂梁振动实验结果证实了该方法的有效性。     

Hologravure as a computer-generated and laser engraved scratch hologram

We presented “hologravure” as the generalization of a scratch hologram, generated by computer from a three-dimensional (3D) model by using an appropriate software, and laser-drawing with a conventional engraver-laser system. For making this type of computer-generated hologram, neither diffractive optics, nor mask pattern or photo-reduction of a transparency onto a high resolution film plate is needed. The shape of a 3D virtual object is encoded in a two-dimensional (2D) multiple circular-scratch drawing. A computer program was created for generating the holograms. Hologravures representing several 3D models are engraved on different materials, and the quality of the lines traced by the CO₂ laser is considered. Sheets of acrylic, polycarbonate, glass and thermo-resistant pieces of acetate were tested. Reconstructed images of all this computer-generated and laser engraved holograms and also of computer assisted and hand-drawn scratch holograms, synthesized by means of the same software are shown.     

Off-axis phase-only holograms of 3D objects using accelerated point-based Fresnel diffraction algorithm

A method for calculating off-axis phase-only holograms of three-dimensional (3D) object using accelerated point-based Fresnel diffraction algorithm (PB-FDA) is proposed. The complex amplitude of the object points on the z-axis in hologram plane is calculated using Fresnel diffraction formula, called principal complex amplitudes (PCAs). The complex amplitudes of those off-axis object points of the same depth can be obtained by 2D shifting of PCAs. In order to improve the calculating speed of the PB-FDA, the convolution operation based on fast Fourier transform (FFT) is used to calculate the holograms rather than using the point-by-point spatial 2D shifting of the PCAs. The shortest recording distance of the PB-FDA is analyzed in order to remove the influence of multiple-order images in reconstructed images. The optimal recording distance of the PB-FDA is also analyzed to improve the quality of reconstructed images. Numerical reconstructions and optical reconstructions with a phase-only spatial light modulator (SLM) show that holographic 3D display is feasible with the proposed algorithm. The proposed PB-FDA can also avoid the influence of the zero-order image introduced by SLM in optical reconstructed images.     

长工作距离显微成像数字全息合成孔径方法(英文)

提出了一种用于长工作距离下显微成像的数字全息合成孔径方法。首先,在不同斜入射照明条件下记录多幅包含不同物光频谱范围的数字全息图。然后,利用每一幅全息图通过数字方法重构并放大物体强度像。最后,通过非相干叠加这些强度像得到分辨力提高且散斑噪声减小的合成物体强度像。理论分析和实验结果表明,此方法可应用于长工作距离下的原位显微观测。