A comparison of digital holographic microscopy and on-axis phase-shifting interferometry for surface profiling

This paper presents a quantitative comparison between off-axis digital holographic microscopy (DHM) and on-axis phase-shifting interferometry (PSI) for surface micro topography measurement. The comparison has been applied on an object of a 1.34 μm nominal step height. The experimental results show that single shot, dual-wavelength, off-axis DHM surpasses on dual-wavelength, on-axis PSI in terms of accuracy and repeatability.     

Comparison of FRF measurements and mode shapes determined using optically image based, laser, and accelerometer measurements

Today, accelerometers and laser Doppler vibrometers are widely accepted as valid measurement tools for structural dynamic measurements. However, limitations of these transducers prevent the accurate measurement of some phenomena. For example, accelerometers typically measure motion at a limited number of discrete points and can mass load a structure. Scanning laser vibrometers have a very wide frequency range and can measure many points without mass-loading, but are sensitive to large displacements and can have lengthy acquisition times due to sequential measurements. Image-based stereo-photogrammetry techniques provide additional measurement capabilities that compliment the current array of measurement systems by providing an alternative that favors high-displacement and low-frequency vibrations typically difficult to measure with accelerometers and laser vibrometers. Within this paper, digital image correlation, three-dimensional (3D) point-tracking, 3D laser vibrometry, and accelerometer measurements are all used to measure the dynamics of a structure to compare each of the techniques. Each approach has its benefits and drawbacks, so comparative measurements are made using these approaches to show some of the strengths and weaknesses of each technique. Additionally, the displacements determined using 3D point-tracking are used to calculate frequency response functions, from which mode shapes are extracted. The image-based frequency response functions (FRFs) are compared to those obtained by collocated accelerometers. Extracted mode shapes are then compared to those of a previously validated finite element model (FEM) of the test structure and are shown to have excellent agreement between the FEM and the conventional measurement approaches when compared using the Modal Assurance Criterion (MAC) and Pseudo-Orthogonality Check (POC).     

Nanoscale surface deformation inspection using FFT and phase-shifting combined interferometry

In this paper, a flexible optical interferometer incorporated with both fast Fourier transform (FFT) and phase-shifting method is developed for three-dimensional (3D) testing of micro-components. Using light interference, microscopic optics, piezoelectric transducer (PZT) nanoscanning and a CCD camera, the proposed system can detect deformation and surface contour in the order of nanometers. An application of the proposed technique is demonstrated using two micro-components: a micro-beam in an accelerometer and a micromirror. The resulting interference fringes that are related to the deformation and surface contour are analyzed using FFT method or three-step phase-shifting method depending on the test surface features. Experimental results show the feasibility of the proposed method for 3D deformation and surface contour measurement of micro-components.     

Efficient work measurement system of manufacturing cells using speech recognition and digital image processing technology

Work measurement methods previously proposed require considerable time and effort by time study analysts because they have to measure the required time through direct observations. In this study, however, we propose a method which efficiently measures the standard times without involving human analysts by using speech recognition and digital image processing techniques. First, we implement a prototype system which can acquire the status of manufacturing cells through a speech recognition system. Second, using image processing, we suggest a method which consists of two main steps: motion representation and cycle segmentation. In the motion representation step, we first detect the motion of any object distinct from its background by differencing two consecutive images separated by a constant time interval. The images thus obtained then pass through an edge detector filter. Finally, the mean values of coordinates of significant pixels of the edge image are obtained. Through these processes, the motions of the observed worker are represented by two time series of data of worker location in horizontal and vertical axes. In the cycle segmentation step, we extract the frames which have maximum or minimum coordinates in one cycle, store them in a stack, and calculate each cycle time using these frames. In this step we also consider methods for detecting work delays due to unexpected events such as an operator’s movement out of the work area, or interruptions. To conclude, the experimental results show that the proposed method is very cost-effective and useful for measuring time standards for various work environments.     

Linear spatio-temporal characterization of a UV microscope objective for nonlinear imaging and spectroscopy by using two-dimensional spectral interferometry

Two-dimensional Fourier transform spectral interferometry is used to characterize the spatio-temporal aberrations of a UV microscope objective. The spatial and temporal profiles of a 420 nm, 38 fs pulse at the focus of a 0.32 NA UV objective are then deduced using a wave propagation code incorporating the measured aberrations.     

Measurement of radial expansion and tumbling motion of a high-speed rotor using an optical sensor system

In order to investigate the load capacity and the strength properties of high-speed rotors, dynamic deformation and vibration measurements are of importance, in particular at lightweight composite devices which cannot be simulated reliably. This is a challenging task in metrology since non-contact inspection techniques are required which offer micron uncertainties and high temporal resolution simultaneously, also under vacuum conditions. In order to meet these requirements, a non-incremental laser Doppler distance sensor system was developed using fiber and diffractive optics. In this paper we present for the first time high-speed deformation measurements of a cylindrical steel rotor using this novel sensor system. The radial rotor expansion of only some microns was determined despite the presence of an unsteady tumbling motion of the rotor, which was measured simultaneously. Future prospects are discussed including the possibility to measure non-metallic devices such as fiber-reinforced composites.