Patrick Sandoz

Wavelet transform as a processing tool in white-light interferometry

Results of the application of wavelet transform for signal processing in white-light interferometry are reported. The mother wavelet frequency is chosen to be the light-source correlogram 1s, and accurate phase measurements are obtained from simple correlation computations. The fringe envelope is also addressed and permits a complete analysis of coherence-limited fringe patterns. Miscalibrations of phase shift and mean wavelength are also considered.

Optical implementation of frequency domain analysis for white-light interferometry

The purpose of recent developments of profilometry by using white light interferometry is to provide new tools for the analysis of rough samples which when studied by monochromatic phase-shifting interferometry, may cause phase calculation ambiguities. The usual way to perform depth measurements by white light interferometry is to analyze the coherence-limited interference fringes while the optical path difference is scanned. The method proposed here does not use optical path difference scanning. A spectroscopic device is used instead to separate the interference intensities associated to each spectral component of the light source. Phase variations due to wavelength change are proportional to the optical path difference and allows depth measurement to be performed without axial scanning. The profile of one line of the inspected sample is obtained from only one 2D interferogram. In this 2D interferogram one direction corresponds to the inspected direction of the surface while the other one is the chromatic axis which allows phase to change with wavelength. Experimental results show the ability of the proposed method to obtain the profile of 1D surface with nanometric resolution.

Phase-sensitive vision technique for high accuracy position measurement of moving targets

This paper presents a vision technique for position and displacement measurement of moving targets. An adapted phase reference pattern is fixed to the object to be controlled and allows the location of the object in the scene observed by a static camera with an accuracy better than 10/sup -2/ pixel. The phase reference pattern is scaled as a function of the desired field of observation and position accuracy. Then the system measurement performances can be chosen from the nanometer to the millimeter ranges (or even larger). Furthermore, the method is self calibrating in length, since the phase reference pattern includes its own length reference. Drifts in vision system magnification therefore do not affect the measurement accuracy. Applications can be used in different fields, for instance, the position control of masks and wafers in photolithography processes or the servo-control of micro-robots.

Processing of white-light correlograms: simultaneous phase and envelope measurements by wavelet transformation

A common procedure of profilometry by means of white light interferometry is to scan one interferometer arm step by step. In this way, the intensity detected for each surface point reproduces the autocorrelogram of the light source, which is used for the determination of the absolute phase between a reference position and the zero optical path difference position. Phase changes due to reflection on the inspected surface produces a shift of the interference fringes with respect to the coherence envelope. If those phase changes vary from points to points, artifacts can be introduced in the profile reconstruction. We propose to measure simultaneously the interferometric phase and the shift of the interference fringes with respect to the coherence envelope. That processing is based on a wavelet transformation of the sampled light source correlograms and leads to complementary information that describes more completely the optical behavior of surfaces.

Surface profiling by means of double spectral modulation

Double modulation-in frequency and intensity-of the power spectral distribution of a light beam is proposed for interferometric profilometry. The procedure is based on two facts: (1) the continuous spectrum of a light source is frequency modulated by the path difference in an interferometric device, (2) the continuous spectrum of a light source is intensity modulated by the transparency of an object placed in the exit plane of a spectroscopic device. Both procedures can be used to measure the profile of a surface with high precision. Moreover, phase shifting is automatically performed by the continuous wavelength variation along the spectrum, so that no piezoelectric transducers are necessary. The method is adaptable for the analysis of remote surfaces through optical fibers.

Phase-shifting methods for interferometers using laser-diode frequency-modulation

Laser diode interferometers are of particular interest for metrological applications since the optical-frequency dependence on the injection current can be used to produce modulations in the interferometric phase. By this way, the optical path difference can be evaluated accurately without being modulated. However, injection-current variations induce output intensity changes which prevent the direct application of phase-shifting algorithms. In this paper, we show that a corrected fringe pattern can be reconstructed, by considering pairs of output intensities with a relative phase-shift multiple of 2π. Subsequently, the assumption of constant background intensity and fringe visibility is satisfied and any classical phase-shifting algorithm can be applied. Only larger sets of phase-shifted intensities need be considered. The proposed method is validated by the application to experimental data.

Subpixelic Measurement of Large 1D Displacements: Principle, Processing Algorithms, Performances and Software

This paper presents a visual measurement method able to sense 1D rigid body displacements with very high resolutions, large ranges and high processing rates. Sub-pixelic resolution is obtained thanks to a structured pattern placed on the target. The pattern is made of twin periodic grids with slightly different periods. The periodic frames are suited for Fourier-like phase calculations—leading to high resolution—while the period difference allows the removal of phase ambiguity and thus a high range-to-resolution ratio. The paper presents the measurement principle as well as the processing algorithms (source files are provided as supplementary materials). The theoretical and experimental performances are also discussed. The processing time is around 3 μs for a line of 780 pixels, which means that the measurement rate is mostly limited by the image acquisition frame rate. A 3-σ repeatability of 5 nm is experimentally demonstrated which has to be compared with the 168 μm measurement range.

Accuracy Quantification and Improvement of Serial Micropositioning Robots for In-Plane Motions

High positioning accuracy with micropositioning robots (MPRs) is required to successfully perform many complex tasks, such as microassembly, manipulation, and characterization of biological tissues and minimally invasive inspection and surgery. Despite the widespread use of high-resolution micro- and nanopositioning robots, there is very little knowledge about the real positioning accuracy that can be obtained and what the main influential factors are. Indeed, very few notable methods are available to measure multi-degree-of-freedom motions with adapted range, resolution, and dynamic capabilities. The main objective of this paper is to quantify the positioning accuracy of serial MPRs and to identify the main influential factors (a typical XY Θ serial robot is chosen as a case study). To reach this goal, a measuring system that combines vision and pseudoperiodic patterns with an extremely large range-to-resolution ratio is introduced as a new way to quantify the positioning accuracy of MPRs for in-plane motions. Then, an open-loop control approach based on MPR calibration is chosen for several reasons: the use of different models to identify influential factors, the quantification of the positioning accuracy, and the necessity of the method when sensor integration is too complex. Experiments using five different calibration models were conducted to classify factors influencing the positioning accuracy of MPRs. The results show that positioning accuracy can be improved by more than 35 times from 96 μ with no imperfection compensation to 2.5 μ by compensating for geometric, position-dependent, and angle-dependent errors through the MPR calibration approach.

Efferocytosis of apoptotic human papillomavirus‐positive cervical cancer cells by human primary fibroblasts

Efficient clearance of apoptotic cells, named efferocytosis, is a fundamental physiological process for tissue development and homeostasis. The contribution of non‐professional phagocytes like fibroblasts to efferocytosis has been established, although the underlying mechanisms are not well understood. We recently demonstrated that horizontal DNA transfer can occur through the uptake of apoptotic human papillomavirus‐positive cancer cells by human primary fibroblasts leading to their transformation. The aim of this present study was to analyse the cellular and molecular mechanisms that drive the phagocytic activity of human primary fibroblasts in the context of apoptotic cervical cancer cell removal.

Development of a microrobot-based micropositioning station: the microrobot and its position and orientation measurement method

The paper deals with the development of a micropositioning station that can be used in manufacturing systems, microfactories, AFM or SNOM microscopes. The central element of the station is a standing waves ultrasonic linear motor. It is a jump-stick-slip driving mechanism that can make longitudinal and lateral shiftings, and rotations in the horizontal plane. Its main merits are its ability to perform microscale displacements and to support heavy pre-loads. An adequate drive-amplifier is developed, allowing the control of the system. Even, a phase sensitive vision method is developed to sense the position and the heading angle with an accuracy better than 0.2 pixel i.e. 12 micrometers with an observed field of about 4 x 3 cm2. A personnel computer controls the whole system.