Common-path interferometer with a tri-window

A common-path interferometer is proposed with a tri-window. It is built using a 4f optical system with Ronchi ruling as a spatial filter. The input rectangular aperture is formed by three windows; the central window supports a phase object, and the other two are used for reference beams. Using an appropriate grating period relative to input aperture size, an interferogram containing three patterns can be obtained in the output plane. The object phase can then be reconstructed from the three patterns using just one interferogram. The experiments are carried out to demonstrate the feasibility and reliability of the proposed scheme.     

Talbot interferometer with computer generated gratings

In a Talbot interferometer the self image of a periodic pattern, such as a Ronchi ruling of period d, illuminated by a monochromatic plane wavefront of wavelength λ, is formed at distances which are integral multiples of 2d²/λ. A refracting object placed in front of the Ronchi ruling distorts the self image. A computer generated pattern can be designed, such that, when placed in a self imaging plane, a uniform intensity or a distinct moire pattern characterising a standard object is obtained. A method of computing the required patterns and experimental results for some aspherical objects are described.     

大非球面镜的郎奇检验灵敏度理论研究及仿真分析

为了研究等间距直线条光栅的周期及其他相关参数的变化对大非球面镜朗奇检验法的影响，提出了一种评估郎奇检验灵敏度大小的新方法。其检验灵敏度定义为一个光栅周期所能检测到的镜面偏差大小能力。在朗奇检验的几何原理基础上，利用光线追迹法讨论了镜面偏差与朗奇光栅周期之间的关系，并以此推导出检验灵敏度公式，计算了抛物面镜的朗奇检测灵敏度。分析结果表明，如果给定镜子的规格参数及检验光路，朗奇检验灵敏度分别随着光栅周期间隔和半通光口径的变大而减小。     

Ronchigram simulations for free-form concave reflective surfaces

A new algorithm to simulate ronchigrams and bironchigrams for free-form reflective concave surfaces is reported. In order to know the surface saggita and its partial derivatives at any point within mirror domain a bicubic spline interpolation is done over a finite set of points where the free surface is defined. To simulate the ronchigram irradiances we used ruling with cosinusoidal transmittance instead of Ronchi ruling. Several examples of bironchigrams are shown for spherical surface affected by local and/or radial errors; and surfaces used in ophthalmic progressive lenses.     

Quantitative measurement of large-aperture aspheric surfaces by off-axis Ronchi test

An off-axis Ronchi test based on the phase-shifting technique is presented to measure the aspheric mirror quantitatively. In this paper, slopes of deviations from the mirror's ideal aspheric shape are derived when an off-axis point source is used. By integrating these slopes, the deviations of the shape are obtained and then the tested shape can be reconstructed. Compared with the coaxial Ronchi test, the measurement error caused by combination fringes can be avoided, and this system is more flexible. In the experiment, a marked point is used to guide the phase unwrapping procedure, which improves the accuracy of extracting corresponding phase points. In addition, the method has strong anti-noise ability. The validity of the method has been verified by both computer simulations and a preliminary experiment.     

Production of binary amplitude gratings with arbitrary opening ratio and variable period

A simple double exposure technique is proposed for making binary amplitude gratings with preset opening ratio. Commercially available Ronchi type master ruling is imaged onto the photographic plate by a laser projection system. The controlled lateral displacement of the ruling or the photographic plate between two exposures results in the composite grating with the required opening ratio. A zoom-effect spherical beam projection system is suggested for producing binary gratings with continuously variable period.     

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.     

Surface profile measurement of low-frequency vibrating objects using temporal analysis of fringe pattern

A simple and accurate algorithm (phase scanning method) is proposed for 3D surface contouring and dynamic response determination of a vibrating object. A sinusoidal fringe pattern is projected onto a low-frequency vibrating object by a programmable liquid crystal display projector. The fringe patterns are captured by a high-speed CCD camera with a telecentric gauging lens. Phase values are evaluated point by point using phase scanning method. From the phase values of each point on the object, the contour of the specimen at different instants of vibration can be retrieved. In this paper, a small vibrating coin is used to demonstrate the validity of the method and the experimental results are compared with test results on a stationary coin using four-step phase shifting and fast Fourier transform methods. The technique is especially useful in applications where the vibrating object has a complicated shape.     

Vector carrier frequency by interfering the kth harmonic of two rulings rotated polarly

This paper presents a method for introducing carrier fringes inclined at any angle into an interferogram. The setup is built on a 4f optical system consisting of two apertures in the input plane and a Ronchi ruling in the Fourier plane. Additionally, a Ronchi ruling rotated on polar direction is placed at each aperture and two passband filters are placed in the Fourier plane for filtering the kth harmonic of their spectra. We demonstrated that the magnitude and direction of the vector carrier frequency depend on the grating period at the input plane, the polar angles, and the kth harmonic, which gives this method the ability to modulate its magnitude only, or its direction only, or both in a wide range. The theoretical model and experimental results are shown in this paper.     

Optical heterodyne measurement of vibration phase

An optical heterodyne system for measuring a vibration phase, which uses a frequency-shifted and phase-modulated reference beam, is proposed. This system is applicable to a rough surface. By using the light scattered by a standard vibrating point on the object surface as the reference beam (local reference beam technique), effects of the uniform displacement of the object can be eliminated, i.e. only the vibration component is detected with no sensitivity to the uniform displacement. Preliminary experiments demonstrate the validity of the theory.     

某型振动轮噪声数值模拟及优化分析

空中声源的辐射噪声谱包括宽带连续谱和窄带线谱,线谱能量高于连续谱。与水下声源相比,空中声源的运动速度普遍较高,线谱多普勒频移明显,可用于进行水下对空中声源的运动参数估计。首先通过时频分析提取接收信号的瞬时频率,而后利用非线性最小二乘法将瞬时频率提取值与预测值相拟合,进而估计声源的运动参数(声源的运动速度、静止频率、与接收器最小水平距离及经过最近点时刻)。仿真与实验均能较为准确地估计出声源运动参数,同时在实验中实现了水下对空中运动声源的测距和定位,测距误差小于15.8%。在满足一定信噪比和保证足够多普勒信息的情况下,该参数估计方法具有很好的适用性。     

Vibration testing based on impulse response excited by pulsed-laser ablation: Measurement of frequency response function with detection-free input

We have developed a non-contact vibration-measurement system that is based on impulse excitation by laser ablation (i.e. laser excitation) to measure the high-frequency-vibration characteristics of objects. The proposed method makes it possible to analyse the frequency response function just by measuring the output (acceleration response) of a test object excited by pulsed-laser ablation. This technique does not require detection of the input force. Firstly, using a rigid block, the pulsed-laser-ablation force is calibrated via Newton's second law. Secondly, an experiment is conducted in which an object whose natural frequency lies in the high-frequency domain is excited by pulsed-laser ablation. The complex frequency spectrum obtained by Fourier transform of the measured response is then divided by the estimated pulsed-laser-ablation force. Finally, because of the error involved in the trigger position of the response with respect to the impulse arrival time, the phase of the complex Fourier transform is modified by accounting for the response dead time. The result is the frequency response function of the object. The effectiveness of the proposed method is demonstrated by a vibration test of an aluminium block.     

Separation of vibration fringe data from rotating object fringes using pulsed ESPI

In industrial and other types of non-controlled environments, an unbalanced rotating object may present characteristic out-of-plane vibration amplitude at a specific frequency. For this type of cases and as a first step towards a complete evaluation, it is only desired to visualize the effect of the vibration on the rotating object, or vice versa, for instance to achieve object balancing. Real time optical non-intrusive measurement techniques such as pulsed electronic speckle pattern interferometry (ESPI), are well suited to study this rotating-vibrating object. The advantage offered by ESPI is that real-time fringe data is qualitatively analyzed while being observed on a TV monitor. The present paper proposes a qualitative method, based on pulsed ESPI, to separate rotation fringes from fringes solely related to vibration. The method relies on a high precision scheme that synchronizes and fixes an object point during rotation, without the use of an optomechanical object derotator.     

一种朗契检验新方法

为了较快、较高准确度地检验大波差的非球面或球面,将一个液晶显示器作为低频光栅和移相装置被用于朗奇检验,在小液晶显示器上由计算机生成光栅图,代替普通光栅和步进电机.消除了因电机移动引起的移相误差,消除了因转动光栅的角度不能正好达到90°而引起的误差.以一个非球面为例,做了移相测试及波面复原.实验表明液晶显示器可作为一个光栅和移相装置.     

一种新的非球面零朗奇检测法

提出一种新的零朗奇检测法用于测量大口径非球面镜的面形。利用光线追迹和正弦条纹的相位信息设计补偿正弦光栅。使用透射液晶显示屏显示补偿正弦光栅并作为相移装置。一个离轴点光源发出的光被镜面反射后通过补偿正弦光栅,摄像机记录携带镜面偏差信息的相移条纹图。通过对相移条纹图的分析确定被测镜面的实际横向像差以及对应的理想横向像差,然后基于朗奇检测的几何原理得到被测镜面的偏差梯度,对其积分获得被测镜面的偏差,进而重建被测镜面的三维面形。与传统的零朗奇检测法相比,这种方法可以消除补偿光栅上每个条纹带边缘的锯齿形状,而且可以获得镜面上足够多的待测点信息。计算机模拟和初步实验验证了该方法的可行性。     

Contouring of artwork surface by fringe projection and FFT analysis

In the present work, we propose a simple optical method to perform profilometry on works of art. The method is based on the projection of a Ronchi grating onto the surface to be analyzed. When viewed at an angle different from the projection angle, the grid pattern appears deformed by the surface shape. This pattern is digitized, by a high-resolution CCD camera, and then processed using a Fourier transform analysis. The technique is free from the errors caused by higher harmonic components of the grating pattern. Furthermore, the method relies on very simple equipment and it is therefore suitable for in situ measurements. Theoretical details and examples of the technique in operation are given.     

Non-destructive collimation technique for measuring glass constants using a Ronchi grating shearing interferometer

A non-destructive liquid immersion collimation technique in combination with a Ronchi grating shearing interferometer has been used for determining the constants of a transparent glass material, especially when it is in the form of a lens. Due to its simplicity and the fact that it will work equally well at any wavelength, the technique is superior to existing methods for determining the various glass constants (refractive index, Abbe number or v-value, partial dispersion factor etc).The shearing fringes appearing inside a Ronchi-gram are adequate to locate the exact position of the point source which has been treated as the target in the present communication. The fringe-free space in the overlapping region between the two laterally displaced spherical wavefronts indicates the sharply focused position of the target which corresponds to zero defocusing. In this way the target displacements with different liquids are determined.     

Ronchi test formulae : 1. Theory

The Ronchi test of the quality of optical systems and components is very widely used but is usually regarded as a qualitative test. In an attempt to make this very simple technique at least semi-quantitative, we have derived easy to use analytical formulae for some basic but important types of aberration. A companion paper will describe their implementation and their experimental verification.     

物面单反射镜波前分割三光束全息干涉计量研究

提出了一种用于分析物体三维位移场的全息干涉计量新方法。该方法将一个小平面反射镜贴于被测物体的表面，用三束呈空间分布的发散光波，在干版的三个不同区域或同一部位，记录被测物体的三个独立的双曝光干涉图。这些干涉图被由小平面反射镜运动造成的参考光虚点光源的位移所调制。基于对这种调制的理论分析，导出计算参考光虚点光源和被测物体三维位移的二个线性方程组。     

Digital ultrasonic speckle phase-shifting method

The digital ultrasonic speckle phase-shifting method (USPM), which is introduced in this paper, can be applied to the measurement of small displacement that is smaller than speckle size at the test point compared to traditional ultrasonic speckle correlation method (USCM). Using USPM, a digital ultrasonic reference signal is introduced to interfere with the ultrasonic speckle signal, which is picked up at the test point on an object surface and is referred to as the object signal. As the phase of the reference signal is shifted several times using the software and then they superimpose with the object signal respectively, the phase of the object signal can be calculated according to the intensities of the superimposed signals. If the object surface moves a small distance, the phase variation of the object speckle can be detected by the same process. As a result, the displacement of the object surface can be measured. Based on the feature of ultrasonic speckles, inner surface displacement of an object can be measured using this proposed method. In this case, the effect of outer surface roughness to the measurement accuracy of USPM is examined experimentally. The experimental results show that the measurement is successful when the displacement is smaller than half of the speckle size at the test point and the roughness parameter R_a of the outer surface of the specimen is less than about 5.47 μm.