C.J. Tay

Shape measurement of small objects using LCD fringe projection with phase shifting

This paper describes the use of optical fringe projection method for three-dimensional shape measurement of small objects. In this method, sinusoidal linear fringes are projected on the object surface by a programmable liquid crystal display (LCD) projector and a long working distance microscope (LDM). The image of the fringe pattern is captured by another LDM and a CCD camera and processed by phase-shifting technique. A simple procedure is described which enables calibration of the optical setup for subsequent quantitative measurement of unknown object shapes. The method developed can also be applied to the measurement of the warp of a small component under thermal loading. This method is relatively simple and accurate, and is capable of conducting fully automated measurements.

Microscopic surface contouring by fringe projection method

Abstract This paper describes the use of optical fringe projection method for 3D surface profile and deformation measurement of micro-components. In this method, sinusoidal linear fringes are projected on a micro-component surface by a grating phase shifting projector and a long working distance microscope (LWDM). The image of the fringe pattern is captured by a high-resolution CCD camera and another LWDM and processed by phase-shifting technique. A simple procedure is described which enables calibration of the optical set-up for subsequent quantitative measurement of micro-components of unknown shapes. This method is relatively simple and accurate, and is capable of conducting fully automated measurements. In this paper, two micro-components, a micro-mirror ( 0.1 mm ×0.1 mm ) and a micro-electrode pad are used to demonstrate deformation measurement and microscopic surface contouring.

In situ surface roughness measurement using a laser scattering method

Abstract In this paper, the design and development of an optical probe for in situ measurement of surface roughness are discussed. Based on this light scattering principle, the probe which consists of a laser diode, measuring lens and a linear photodiode array, is designed to capture the scattered light from a test surface with a relatively large scattering angle ϕ (=28°). This capability increases the measuring range and enhances repeatability of the results. The coaxial arrangement that incorporates a dual-laser beam and a constant compressed air stream renders the proposed system insensitive to movement or vibration of the test surface as well as surface conditions. Tests were conducted on workpieces which were mounted on a turning machine that operates with different cutting speeds. Test specimens which underwent different machining processes and of different surface finish were also studied. The results obtained demonstrate the feasibility of surface roughness measurement using the proposed method.

Multiple-image encryption by space multiplexing based on compressive sensing and the double-random phase-encoding technique

In this paper, a new multiple-image encryption and decryption technique that utilizes the compressive sensing (CS) concept along with a double-random phase encryption (DRPE) has been proposed. The space multiplexing method is employed for integrating multiple-image data. The method, which results in a nonlinear encryption system, is able to overcome the vulnerability of classical DRPE. The CS technique and space multiplexing are able to provide additional key space in the proposed method. A numerical experiment of the proposed method is implemented and the results show that the proposed method has good accuracy and is more robust than classical DRPE. The proposed system is also employed against chosen-plaintext attacks and it is found that the inclusion of compressive sensing enhances robustness against the attacks.

Whole-field determination of surface roughness by speckle correlation

A whole-field method of double-exposure speckle photography is employed to determine metal surface roughness by correlation between two speckle patterns. A movable rectangular aperture that is mounted before an image lens is shifted between the exposures, which results in a decrease in the contrast of the reconstructed Youngs fringes with increasing roughness. The technique permits evaluation of the roughness of particular points on a surface as well as the average roughness of an entire surface. Four sets of random surfaces that were prepared by different machine-finishing processes and with roughnesses ranging from 0.6 to 13 µm have been tested. Different methods have been carried out to process the test data, and a practical method for the evaluation of surface roughness is proposed.

Whole field surface roughness measurement by laser speckle correlation technique

Abstract This paper describes a speckle correlation technique for the determination of surface roughness, ranging from 1.6 to 50 μm . Instead of moving the laser beam, the specimen is rotated to achieve angular speckle correlation (ASC) in the far-field plane. The technique is simple and requires minimum optical alignment. The experimental results show a good agreement with the standard specimen of known roughness. An error analysis on the experiment has been carried out. Together with the theoretical curves, the roughness values can be easily related to the change of incidence angle at a particular visibility of the correlation fringes between two speckle patterns.

Contour measurement by fibre optic fringe projection and Fourier transform analysis

This paper describes an optical technique for contour measurement based on the fibre optic fringe projection and Fourier transform analysis. Sinusoidal fringes are produced by the interference of two spherical wavefronts emitted from the output fibre of a directional coupler. The fringe pattern is projected on an object surface and the deformed grating image is captured by a CCD camera for subsequent analysis by a microcomputer. In contrast to the conventional moire method, this method is relatively simple and accurate, and is capable of conducting fully automatic measurement.

Flaw detection in composites using time-average shearography

Abstract Results on flaw detection of glass fibre reinforced plastic beams using time- average shearography are presented here. Detection and sizing of flaws such as debonds or delaminations are successfully carried out using this technique. For easy detection of flaws, the component has to be excited at the resonance frequencies of the flaws. As the flaw gets smaller, a higher frequency is required. For flaws of the same exterior size, a deeper one will also require a higher frequency.

Deformation and profile measurement using the digital projection grating method

Abstract This paper presents a digital projection grating method for full field measurement of out-of-plane deformation and shape of an object. Two grating patterns on an object before and after deformation are captured by a CCD camera and stored in a computer. With the aid of Fast Fourier Transform (FFT) and signal demodulating techniques, a wrapped phase map is generated. The phases are expanded in the range of 0–2 π and compared with the resulting moire pattern. An unwrapping procedure is used to obtain a continuous phase. In addition, a digital method for fractional fringe multiplication is also developed. Results on deformation and object profile measurements are presented.

Surface roughness measurement of semi-conductor wafers using a modified total integrated scattering model

Summary Light scattering is a non-contact technique which can be used for characterizing the topography of smooth reflecting surfaces. A proposed technique which incorporates a modified Total Integrated Scattering (TIS) model for surface roughness measurement of semi-conductor wafers has been developed. The technique employs a low power He-Ne laser and incorporates conventional optical components to record surface roughness in the nanometer range ( R a