Hyperspectral interferometry for single-shot profilometry and depth-resolved displacement field measurement (original) (raw)
Related papers
2011
A new approach to the absolute measurement of two-dimensional optical path differences is presented in this thesis. The method, which incorporates a white light interferometer and a hyperspectral imaging system, is referred to as Hyperspectral Interferometry. A prototype of the Hyperspectral Interferometry (HSI) system has been designed, constructed and tested for two types of measurement: for surface profilometry and for depth-resolved displacement measurement, both of which have been implemented so as to achieve single shot data acquisition. The prototype has been shown to be capable of performing a single-shot 3-D shape measurement of an optically-flat step-height sample, with less than 5% difference from the result obtained by a standard optical (microscope) based method. The HSI prototype has been demonstrated to be able to perform single-shot measurement with an unambiguous 352 (m depth range and a rms measurement error of around 80 nm. The prototype has also been tested to perform measurements on optically rough surfaces. The rms error of these measurements was found to increase to around 4× that of the smooth surface. For the depth-resolved displacement field measurements, an experimental setup was designed and constructed in which a weakly-scattering sample underwent simple compression with a PZT actuator. Depth-resolved displacement fields were reconstructed from pairs of hyperspectral interferograms. However, the experimental results did not show the expected result of linear phase variation with depth. Analysis of several possible causes has been carried out with the most plausible reasons being excessive scattering particle density inside the sample and the possibility of insignificant deformation of the sample due to insufficient physical contact between the transducer and the sample
Epj Web of Conferences, 2010
We propose a method that we call Hyperspectral Interferometry (HSI) to resolve the 2π phase unwrapping problem in the analysis of interferograms recorded with a narrow-band light source. By using a broad-band light source and hyperspectral imaging system, a set of interferograms at different wavenumbers are recorded simultaneously on a high resolution image sensor. These are then assembled to form a three-dimensional intensity distribution. By Fourier transformation along the wavenumber axis, an absolute optical path difference is obtained for each pixel independently of the other pixels in the field of view. As a result, interferograms with spatially distinct regions are analysed as easily as continuous ones. The approach is illustrated with a HSI system to measure 3-D profiles of optically smooth or rough surfaces. Compared to existing profilometers able to measure absolute path differences, the single shot nature of the approach provides greater immunity from environmental disturbance.
Measuring full displacement fields on scattering surfaces by a portable low-cost interferometer
2009
The paper presents a device for measuring full displacement fields working on the principle of speckle interferometry. The proposed interferometer was designed with the aim of obtaining a lightweight selfcontained device, easy to assemble with cheap and standard equipment. The investigated area is illuminated by three laser diodes emitting 50 mW at 660 nm, and observed by a B&W firewire CCD camera. The accuracy of the measurements is improved by means of a PZT actuator realized on purpose and driven by a control electronics able to provide a very accurate supply voltage. Furthermore by the control electronics it is possible to turn on and off the diodes and to control the single light intensity of each laser. The paper describes in details all the parts of the interferometer and the systems implemented for optimizing the working conditions of the device. Furthermore all the steps necessary for performing the measurements are reported, and typical sets of experimental data are shown.
Overview of optical techniques that measure displacements: Murray lecture
Experimental Mechanics, 2003
Optical techniques that measure displacements play a very important role in current experimental mechanics, material sciences and metrology. This paper presents a survey of developments in these techniques from a personal experience point of view. Three main aspects are considered. Mathematical and numerical models used in the interpretation of fringe information and the corresponding data processing techniques. Optical and electro-optical developments that have taken place to improve the sensitivity, and the efficiency of these methods to make them competitive with purely numerical methods. Applications that have arisen from the synergy between advanced computational capabilities and optics are also presented.
Frontiers in Mechanical Engineering
The paper presents the impact of lighting type and direction on measurements of surface asperities using focus-variation microscopy. Particular attention was paid to the direction of lighting when using a light ring. It was pointed out that the lighting direction directly affects the values of the parameters Rt, Rz, and Rc. The article also presents the impact of a light polarizer on the surface topography parameters. It has been shown that the positioning of a sample with a regular and directed structure relative to the optical axis of the light polarizer affects the accuracy of mapping surface asperities. The largest differences were observed for Rz and Rt parameters. A method of using an external polarizer mounted on a focus variation microscope lens was also presented.
Open questions in surface topography measurement: a roadmap
Surface Topography: Metrology and Properties, 2015
Control of surface topography has always been of vital importance for manufacturing and many other engineering and scientific disciplines. However, despite over one hundred years of quantitative surface topography measurement, there are still many open questions. At the top of the list of questions is 'Are we getting the right answer?' This begs the obvious question 'How would we know?' There are many other questions relating to applications, the appropriateness of a technique for a given scenario, or the relationship between a particular analysis and the function of the surface. In this first 'open questions' article we have gathered together some experts in surface topography measurement and asked them to address timely, unresolved questions about the subject. We hope that their responses will go some way to answer these questions, address areas where further research is required, and look at the future of the subject. The first section 'Spatial content characterization for precision surfaces' addresses the need to characterise the spatial content of precision surfaces. Whilst we have been manufacturing optics for centuries, there still isn't a consensus on how to specify the surface for manufacture. The most common three methods for spatial characterisation are reviewed and compared, and the need for further work on quantifying measurement uncertainties is highlighted. The article is focussed on optical surfaces, but the ideas are more pervasive. Different communities refer to 'figure, mid-spatial frequencies, and finish' and 'form, waviness, and roughness', but the mathematics are identical. The second section 'Light scattering methods' is focussed on light scattering techniques; an important topic with in-line metrology becoming essential in many manufacturing scenarios. The potential of scattering methods has long been recognized; in the 'smooth surface limit' functionally significant relationships can be derived from first principles for statistically stationary, random surfaces. For rougher surfaces, correlations can be found experimentally for specific manufacturing processes. Improvements in computational methods encourage us to revisit light scattering as a powerful and versatile tool to investigate surface and thin film topographies, potentially providing information on both topography and defects over large areas at high speed. Future scattering techniques will be applied for complex film systems and for sub-surface damage measurement, but more research is required to quantify and standardise such measurements. A fundamental limitation of all topography measurement systems is their finite spatial bandwidth, which limits the slopes that they can detect. The third section 'Optical measurements of surfaces containing high slope angles' discusses this limitation and potential methods to overcome it. In some cases, a rough surface can allow measurement of slopes outside the classical optics limit, but more research is needed to fully understand this process.
Optical Engineering, 2020
We report on the application of an interferometric system based on the low-coherence interferometry technique to the dimensional characterization of large opaque mechanical parts as well as microdeformations experienced by them. The implemented scheme is capable of simultaneously measuring very small deformations and relatively large dimensions or thicknesses (several centimeters) of the sample. By applying the chirp Fourier transform algorithm, it was possible to measure changes in thickness with an uncertainty of 0.35 μm when a 7-cm-thick sample was measured. The measurement scheme was implemented in optical fiber, which makes it highly adaptable to industrial conditions. It employs a tunable light source and a Sagnac-Michelson configuration of the interferometric system that allows the thickness of the opaque sample and the topography of both faces to be obtained simultaneously. The developed system can be used to perform profilometry of opaque samples and to analyze the dimensional behavior of mechanical pieces in production lines or under mechanical efforts capable of introducing some deformations on them. This feature enables the system to perform quality control in manufacturing processes.