A compact, robust and versatile moiré interferometer

A moiré interferometer was designed and constructed based on a general system design using a reflective crossed-line diffraction grating to produce the four beams of light necessary for moiré interferometry. The design concept, basic design and tuning procedures are discussed. The important features of the interferometer, i.e. compactness, versatility, polarization insensitivity, relaxed collimation requirements, low laser power and remote optics, are addressed. Several such interferometers have been constructed and successfully applied to engineering problems. These include examining the displacement fields surrounding drilled and preformed holes in composite laminates loaded in tension, and the evaluation of nonhomogeneous behavior in textile composites.     

Realization of high sensitivity displacement field from moiré interferometer with rough phase shifting mechanism and pattern matching technique

In this study, phase shifting method is used to modify moiré system into micro moiré interferometer which can measure displacement field with highly improved sensitivity. Apart from existing micro moiré technique, a low cost and less precise translation stage with rough resolution (10 μm resolution) is adapted for the phase shifter. Least square algorithm is applied to estimate the arbitrary phase shifted amount and to minimize the errors induced by lowering the cost. Moreover, specimen grating is phase shifted instead of reference grating which enables simple construction from given moiré system. To compensate for rigid body in-plane translation of specimen that may occur during phase shifting, pattern matching algorithm is put into practice to ensure pixel correspondence for each phase shifted images. To verify the newly constructed micro moiré technique, local displacement fields of Fine pitch Ball Grid Array package and Wafer Level Chip Size Package with elevated sensitivity up to 26 nm per fringe was acquired.     

Analysis of moiré fringes for measuring the focal length of lenses

We describe a technique based on fast Fourier transform and least-squares fit for automated analysis of moiré fringe patterns for accurately measuring the focal length of lenses. An interference fringe pattern is produced by a reflective diffraction grating interferometer and illuminates the test lens. Moiré fringes are generated by digitally superimposing the magnified fringe pattern imaged by the lens on the interference fringe pattern without the test lens. We analyze two common ways to generate the digital moiré effect: multiplication and subtraction with successive rectification of the two digitally stored intensity distributions, and we show that in the latter case by means of the described technique it is possible to determine the moiré spatial frequency by a simple and fully automated procedure.     

Automatic fringe analysis for moiré topography

Two digital systems are developed for the automatic measurement of 3-D shapes using moiré techniques—an automatic 3-D shape measuring system using the scanning moiré method and an interactive fringe analysing system for moiré fringe photographs. In the automatic 3-D shape measuring system, a deformed grating is scanned and sampled with an electronic image scanning device so that moiré fringes are generated in a computer. This technique of electronic fringe generation eliminates ambiguity with regard to the sign of the moiré fringes. The interactive fringe analyser provides a flexible and versatile tool for moiré fringe analysis. Medical applications of these analysing systems are discussed.     

New large-aperture and high- accuracy lateral shearing interferometer

The principle of a new large-aperture high-accuracy interferometer for flow visualization is presented. The interferometer uses a Twyman-Green interferometer as a beam-splitter, a grating as a detector, and a real-time moiré fringe method to eliminate systematic errors. The interferometer has been used to measure the flow fields in a shock tunnel and obtained satisfactory results.     

Phase shifting nano-moiré method with scanning tunneling microscope

In this paper, a new nano-moiré method using scanning tunneling microscope (STM) is proposed. This method is capable of measuring nanoscopic deformation of matter. The formation mechanism of the STM moiré fringe and the phase shifting technique used in STM moiré fringes are explained in details. Typical experiments are conducted with the crystal lattices of freshly cleaved highly oriented pyrolytic graphite, are used as specimen grating, to generate STM moiré fringe patterns. Phase shifting is realized in four steps from 0 to 2π by controlling the PZT in the STM system to shift the specimen in the vertical direction. This method provides a new way for disposal of moiré fringes pattern in the nano-moiré measurement.     

Computer Aided Moiré Methods

Computer aided moiré methods (CAMM) use the versatility of the computer to manipulate gratings rapidly, the basis of all moiré methods. Application of CAMM to moiré teaching, in-plane moiré, shadow and projection moiré and reflection moiré are reviewed in this paper.     

Fringe formula for projection type moiré topography

The rays that form moiré fringes in projection type moiré topography are discussed. It is shown that the nodal-ray formula conventionally used in projection type moiré topography gives erroneous results for an object located close to the lenses. An alternative formula based on principal rays is proposed and compared with the conventional nodal ray formula. The principal ray formula proposed can be applied to a wide class of optical systems, including a telecentric system.     

Analysis of CCD moiré pattern for micro-range measurements using the wavelet transform

A new approach based on the moiré theory and wavelet transform (WT) is proposed for measuring the micro-range distance between a charge-couple-device (CCD) camera and a two-dimensional reference grating. The micro-range distance is determined by measuring the pitch of the moiré pattern image, which is digitized by a CCD camera. A one-dimensional WT algorithm is applied to estimate the pitch of the moiré pattern. Experimental results prove that this technique is very efficient and highly accurate. The moiré range finder is an economic technique for measuring a micro-range distance.     

Moiré topography: alternative technique in health care

The instrumentation and application of moiré topography in the diagnosis of corporal asymmetries associated to different illnesses of the osseous-muscular system in early ages is presented in this paper. A sample comprising 203 adolescents ranging between 12 and 15 years were studied. Some moiré topography analysis of human legs and the sole of the foot are also given as examples of moiré applications. The results show the advantages of using this technique in the health care and treatment of different illnesses in adolescents. Some of these advantages are provided by the easy diagnosis and the low cost of the moiré technique.     

Geometric micron-moiré

A high spatial resolution and high-sensitivity geometric micron-moiré method is presented. The method is based upon classical geometric moiré method and microscopic principle, the frequency of the specimen and reference gratings used in this method can be higher than 1000 lines/mm. By a microscopic moiré system, the high-frequency gratings could be magnified to a low-frequency level, so geometric moiré patterns can be formed. Experiments show that both the spatial resolution and the sensitivity can reach or exceed micron-level.     

Contribution to analysis of the reflection grating method

The reflection grating method is a special case of the well-known reflection moiré principle. Whereas this moiré effect is suitable for measurement of the slope and curvature of deformed object surfaces by superposition of at least two grating images, the same results can be determined using only one reflected grating image without moiré lines.Some fundamental relationships between grating lines, optical setup and the desired deformation are developed and compared with the simplifications that are known from the moiré principle. The theory is tested using a circular plate, uniformly loaded and clamped at the circumference.     

A study on the digital moiré technique with circular and radial gratings

A digital moiré technique with circular and radial gratings is proposed to measure in-plane displacement and strain distributions of soft materials in a polar coordinate system. By introducing the coordinate-transform technique, the concentric circles and radial lines in the Cartesian coordinate system are converted to cross gratings in a polar coordinate system. Then, the digital moiré fringes are generated using logical and filter operations on the one-bit binary images. Finally, the in-plane displacement distributions are obtained by means of the four step phase-shifting procedure. Also, the measurement principles and the basic procedures of the new digital moiré technique are explained in detail. An application for rubber sheet with a single-edge crack is studied, and the experimental results show that the presented method is feasible.     

Localisation of fringes in moiré topography

The moiré pattern produced by two separate gratings is characterised by a fringe localisation phenomenon. A theory based on the geometric shadow of light can account for the spatial location of moiré fringes in topographical applications. The fringe location depends on the optical arrangement of the system and also on the surface slope of the specimen. Experimental results are introduced into practical usage.     

Holographic implementation of holographic moiré inspection

Implementation of holographic moiré inspection is demonstrated using one hologram to project the reference grating and a further hologram to produce moiré patterns. Well contrasted moiré fringes are obtained. Results of calibration tests are reported and an example of object contouring is presented.     

Flow visualization by an improved double exposure method in holography

Some optical differential interferometric techniques were investigated in order to measure the density distribution in the air jet flow of a small scale flow cell while excluding the effects of the optical distortion of its glass windows. A technique using moiré fringe formation by rotating the reference wave in one exposure of the double exposure method was found to be most suitable for this purpose. High contrast and multiplication of moiré fringes can be obtained by a moiré fringe sharpening and multiplication method.     

Phase shifting SEM moiré method

A new phase shifting scanning electron microscope (SEM) moiré method is proposed in this paper. The phase shifting technique is realized in four steps from 0 to 2π by shifting electron beam in the y-axis direction controlled by the SEM system. It is successfully applied to determine the residual strain of a deformed holographic grating with a frequency of 1200 lines/mm in an electronic package. As a further application, it is used to measure the virtual strain of a MEMS structure with a 5000-line/mm grating and to determine the phase distribution of a SEM moiré formed with a 6000-line/mm grating fabricated by electron beam lithography. The experiments show the feasibility of this method. It provides a new way for disposal of fringes pattern in sub-micro moiré method.     

Experimental tire mechanics by moiré

Tires and their component parts are made from various low modulus rubbers and cord/rubber laminates. In service they typically undergo large deformations of about 1–30%. The accurate measurement of these deformations by either pointwise (strain gage) or full-field (optical) techniques is rendered difficult in these circumstances. Ordinary strain gages reinforce these soft structures and inhibit their deformation, whereas most optical methods are too sensitive to measure large motions. These difficulties can be overcome by the judicious use of various moiré methods: moiré interferometry, geometric moiré and shadow moiré.Several applications of moiré methods to tire mechanics problems are presented. The emphasis is on the range of problems that has been successfully approached and on the results obtained. Examples are drawn from work on belt-edge strains, tread groove strains, sidewall strains, dynamic sidewall deformations and material properties determination. Proprietary details of technique are not included.     

Diffraction grating interferometers for mechanical characterisations of advanced fabric laminates

It is demonstrated that two grating interferometers with high spatial resolutions can successfully be applied for the mechanical characterisation of the advanced fabric composite materials. Based on these two techniques, the mechanical properties of two kinds of fabric laminates are obtained without assumption of uniform strain fields to be used in the characterisation approaches using the local strain sensors. The degree of the yarn crimp effects of the two laminates is compared in terms of the out-of-plane displacement derivatives. Especially, it is shown that the grating shearing interferometer is appropriate for the crimped fabric structure requiring a three-dimensional analysis. The modification from moiré interferometer to grating shearing interferometer is performed by introducing a Michelson interferometer modified for image shearing.     

Investigation of dynamic displacements of lithographic press rubber roller by time average geometric moiré

Geometric moiré fringe formation method is a classical well-established experimental technique with numerous practical applications. This paper proposes the application of time-average geometric moiré analysis for the determination of dynamic displacements of the lithographic press rubber roller. This optical measurement technique is a natural extension of double-exposure geometric moiré for the identification of dynamic displacements of vibrating elastic structures. Experimental investigations prove the validity and effective practical applicability of the method.