Many optical measurement techniques provide fringe patterns as their results. The decodification processes that employ one or several fringe patterns to automatically retrieve the phase are generally designated as phase-evaluation methods. In this work, an overview of these methods will be schematically presented. Their particular performances will be compared, stressing their main advantages and drawbacks. An important group of these methods employs the well-known phase-shifting algorithms as a tool for calculating the phase. In the general form of these algorithms, the principal value of the optical phase is computed by an inverse trigonometric function whose argument is a combination of phase-shifted intensity values, provided by the modulation of one or several fringe patterns. These algorithms will be also studied in the general context of the phase-evaluation methods.

https://iopscience.iop.org/article/10.1088/0957-0233/10/3/005/pdf

Phase-evaluation methods in whole-field optical measurement techniques

Digital speckle correlation method (DSCM) has been widely used in experimental mechanics to obtain the surface deformation fields One of the challenges in practical applications is how to obtain the high accuracy with far less computation complexity To determine the subpixel registration of the DSCM, a high efficient gradient-based algorithm is developed in this paper The principle is described and four different modes of the algorithm are given Based on computer-simulated images, the optimal mode of the algorithm is verified through the comparison of computation time, optimal subset-region size and sensitivity of the four modes The influences of speckle-granule size and speckle-granule density on accuracy are studied and a quantitative estimation of the optimal speckle-granule size range is obtained As the applications of this method, the practical deformation measurements with the rigid body translation and rotation as well as an experiment on biomechanics are presented to certify the feasibility and the validity of the algorithm

Application of an improved subpixel registration algorithm on digital speckle correlation measurement

A simple and repeatable speckle creation method based on water transfer printing (WTP) is proposed to reduce artificial measurement error for digital image correlation (DIC). This technique requires water, brush, and a piece of transfer paper that is made of prefabricated decal paper, a protected sheet, and printed speckle patterns. The speckle patterns are generated and optimized via computer simulations, and then printed on the decal paper. During the experiments, operators can moisten the basement with water and the brush, so that digital patterns can be simply transferred to the carriers' surfaces. Tensile experiments with an extended three-dimensional (3D) DIC system are performed to test and verify the validity of WTP patterns. It is shown that by comparing with a strain gage, the strain error is less than 50μe in a uniform tensile test. From five carbon steel tensile experiments, Luders bands in both WTP patterns and spray paint patterns are demonstrated to propagate symmetrically. In the necking part where the strain is up to 66%, WTP patterns are proved to adhere to the specimens well. Hence, WTP patterns are capable of maintaining coherence and adherence to the specimen surface. The transfer paper, working as the role of strain gage in the electrometric method, will contribute to speckle creation.

http://iopscience.iop.org/article/10.1088/0957-0233/26/9/095201/pdf

A method to transfer speckle patterns for digital image correlation

Soft tissues of the body are composite, typically being made up of collagen and elastin fibers with high water contents. The strain measurement in soft tissues has proven to be a difficult task. The digital speckle method, combined with the image processing technique, has many advantages such as full field, noncontact, and real time. We focus on the use of an improved digital speckle correlation method (DSCM) and time-sequence electric speckle pattern interferometry (TSESPI) to noninvasively obtain continual strain measurements on cartilage and vessel tissues. Monoaxial tensile experiments are well designed and performed under constant temperature and the necessary humidity with smart sensors. Mechanical behaviors such as the tensile modulus and Poisson ratio of specimens are extracted based on the deformation information. A comparison of the advantages and the disadvantages of these techniques as well as some problems concerning strain measurements in soft tissues are also discussed.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-10/issue-3/034021/Strain-and-mechanical-behavior-measurements-of-soft-tissues-with-digital/10.1117/1.1895185.pdf

Strain and mechanical behavior measurements of soft tissues with digital speckle method.

We propose a new computer-controlled phase-shifting method based on computer-generated holograms (CGHs) displayed on a spatial light modulator (SLM). In this method the accurate phase shifts required in phase-shifting digital holography or interferometry are induced by a suitable transformation of the encoding patterns of the CGH displayed on a SLM. Both the theoretical analysis and the experimental results demonstrate the feasibility of this approach. We also discuss possible applications of this method in the field of interferometric null testing of aspheres.

Phase-shifting with computer-generated holograms written on a spatial light modulator.

A new computer-controlled phase-shifting apparatus based on polarization techniques is developed. This apparatus changes phase only by rotating a polarizer using a precise step motor and a specially designed decelerator. The precision of this method can be higher than other methods because of the accurate control of the rotating angle. Four applications of this technique to optical interlerometry and 3-D shape measurements are presented.

Applications of a novel phase-shift method using a computer-controlled polarization mechanism

A simple new method of automatically reconstructing the 3-D surface of an object from 2-D moire patterns is proposed. The method can automatically and accurately obtain phase value at every pixel point without assigning fringe orders and interpreting data in the regions between fringes by using a discrete cosine transform based algorithm, which is free from errors caused by the higher harmonic components, the variation of dc light level, and the modulations of amplitudes on a fringe pattern. The method is capable of fully automatic distinction between depression and elevation on an object surface. Theoretical analysis and experimental verification are presented.

Automated Moire Contouring Of Diffuse Surfaces

An electronic speckle-pattern interferometer with a polarization phase-shift technique for the deformation measurement of a diffuse surface is proposed A common-path optical phase-shift arrangement is
adopted in the interlerometer to improve the stability of the optical system A polarization phase-shift technique is used to obtain precisely phase-shifted interferograms A phase map of a fringe pattern, which is capable of distinguishing surface depressions from elevations, can be automatically and accurately obtained from four interferograms by the computer data processing The numerical data representing the deformation over the entire field can be easily extracted from the phase map An example of deformation measurement using this interferometer is presented

Guan-chang Jin

Papers

Applications of a novel phase-shift method using a computer-controlled polarization mechanism

Automated Moire Contouring Of Diffuse Surfaces

Electronic speckle pattern interferometer with polarization phase-shift technique

Surface detection and 3D profilometry for microstructure using optical metrology

Application of nondestructive testing methods to electronic industry using computer-aided optical metrology