This PDF file contains the editorial “Y2K is finally here! What will be our next fear, our future?” for OE Vol. 39 Issue 01

This PDF file contains the editorial “Guest Editorial: Special Section on Optical Methods for Shape Measurement” for OE Vol. 39 Issue 01

We first provide an overview of 3-D shape measurement using various optical methods. Then we focus on structured light techniques where various optical configurations, image acquisition techniques, data postprocessing and analysis methods and advantages and limitations are presented. Several industrial application examples are presented. Important areas requiring further R&D are discussed. Finally, a comprehensive bibliography on 3-D shape measurement is included, although it is not intended to be exhaustive.

We review profile measurement methods using shearography. Traditionally, shearography implies the use of small image shearing, which yields first-order derivatives of displacements or of spatial coordinates. We demonstrate that the use of large image shearing is equivalent to holography, since displacement-related, or spatial coordinate-related, phase fringes are generated. Hence, the technique of shearography can appropriately be perceived as an optical technique that directly measures displacements (or spatial coordinates) and surface strains (or surface slopes). Unlike holography, shearography does not require special vibration isolation since a separate reference beam is not required; hence, it is a practical tool that can be used in the field/ factory environment. Examples of the use of small and large image shearing for surface profiling are given. Finally, a novel method for computing phase derivatives for the determination of curvatures of object surfaces from shearographic measurements is discussed.

A confocal setup based on microlenses for shape investigation, defect analysis and surface topography measurement is presented. The major advantage of this technique is its high light efficiency and the possibility to realize larger object fields without reducing the numerical aperture. Different variations of the setup for different applications are presented. An increased working distance yields a greater variety of its applications. Furthermore, the arrangement of the microlenses on a rotating disk leads to an increased spatial sampling and a high scanning rate. The axial resolution is the same as in a comparable confocal microscope based on a Nipkow disk. The microlens confocal system enables measurements on large field sizes down to microscopic ones. In addition, by using chromatic aberrations it is possible to achieve realtime images with color-coded height information. The topography of the sample can be determined from one color image, which leads to a reduction in measuring time. To reduce measuring times on curved surfaces, for instance, it is useful to adapt the focal lengths of the microlenses to the individual shape of the object. Hence, only the difference between the focal distribution and the real shape must be determined.

Parallel and absolute measurement of surface shape using wavelength scanning interferometry are applied to various objects, including diffuse and milled surfaces having discontinuities such as steps, dips, and protrusions. We employ not only the Michelson setup but also the newly introduced Fizeau setup. The behaviors of interference signals arising from these objects are experimentally compared. Spiky noises are observed from the milled surfaces and from the defocused diffuse surface. In spite of the noise, we succeed in measuring steps and narrow dips on a mirror and a milled surface, as well as a cylindrical protrusion on a diffuse surface. With a dye laser of 4.2 nm tuning range we attain a resolution of 39 µm within a depth of 3 mm.

A method for obtaining 3-D images by illuminating an object with frequency tunable laser light and recording information interfero- metrically is discussed. The instrumentation requirements include a telecentric imaging system and a tunable laser source capable of serially illuminating the object with multiple laser frequencies. Complex-valued image plane data is recorded interferometrically and processed using Fourier transform algorithms, and 3-D images obtained from a variety of objects are presented. These results demonstrate the wide range of measurement applications that can be addressed with this method.

Most optical topolography systems use a single-wavelength laser for projection, using a swept spot, a moving line, or a projected grating. In a typical projected grating system, the gratings are shifted and a series of images is used to recover the 3-D shape of the target. When the series of images is analyzed in the normal phase shift manner, the resulting 2-D phase map typically has phase unwrapping problems due to noise and Nyquist limits. Surfaces with large vertical discontinuities present a problem for phase unwrapping 3-D shape recovery. We look at simultaneously projecting multiple wavelengths onto a surface to help avoid problems in unwrapping the 2-D phase map. Multicolor projection and shape recovery are demonstrated with a white-light Michelson interferometer and with a two-color Mach-Zehnder interferometer. Using multiple wavelengths, it is unnecessary to rotate the interferometer mirror to change the grating pitch and some operations can be done in parallel, which reduces scanning time. Limitations and improvements in the current system are discussed.

For the absolute and shade-free shape measurement of 3-D objects with large height discontinuities, a coaxial optical sensor system with a common image plane for pattern projection and observation is proposed. Experimental results are presented that demonstrate the shape measurements of objects with a deep hole and large discontinuities.

A working handheld 3-D diffraction range finder, nicknamed Moly, is demonstrated. This prototype is distinguished by a far-field magnification feature that is made possible by use of chirped frequency diffraction grating optics that reverse the perspective foreshortening typical of conventional triangulation range finders. This new type of 3-D profilo- meter illuminates its target with a collimated laser projector that produces a rectangle-shaped sheet of light of uniform width at all working distances. Moly also employs dual magnetic wave detectors to facilitate freedom of movement for both the digitizing instrument and the subject. The instrument was designed primarily to digitize human faces and figures for applications in art and medicine.

We present a direct holographic multiwavelength contouring technique for shape measurement of microcomponents with large height steps and/or spatially isolated object areas. The measurement is based on the digital recording and reconstruction of wavefronts using a CCD sensor. Absolute phase measurement is ensured by multiwavelength contouring. Consequently the whole measurement procedure delivers a direct and unambiguous approach to the continuous phase without unwrapping. An algorithm is presented to chose a minimum number of different wavelengths to improve the accuracy of the phase measurement. The results are verified by an experimental setup and some practical examples.

We construct an interferometer for flatness testing of precision-engineered objects such as rigid disk drive platters, pump parts, and fuel injectors. A pair of phase diffraction gratings illuminate an object simultaneously at two different angles of incidence, resulting in an equivalent wavelength of 12.5 µm. This system operates in standard phase-shifting mode for a height resolution of 0.01 µm with up to 150 µm of surface departure. The <1 s measurement speed, 96-mm viewing aperture, simple mechanical part alignment, and 50-mm working distance are consistent with high-volume production testing. A recently developed coherence scanning mode accommodates even larger departures and discontinuous regions such as step heights.

We describe the results of an experimental study to optimize the measurement precision of a whole-field shape measurement system based on projected fringes produced by a spatial light modulator (SLM). Investigated issues include (1) the relative performance of different phase-shifting algorithms (4 frame, 7 frame, and 15 frame); (2) the relative performance of different temporal phase unwrapping algorithms (in particular, the reversed exponential and Fourier transform ranging methods); and (3) the effect of projector defocus. The last point is shown to be a crucial limiting factor in the performance of such systems. Focusing the projector within the measurement volume produces data with high systematic noise content due to the pixel structure of the SLM, whereas focusing the projector a long way beyond the measurement volume introduces significant random noise due to the reduced fringe modulation. At the optimum focal position, a measurement precision of better than 1 part in 20,000 of the field of view is achieved.

Theoretical analyses and experimental results on holographic moire contouring on diffusely reflecting objects are presented. The sensitivity and limitations of the method are discussed. Particular emphasis is put on computer-assisted data retrieval, processing, and recording.

A novel, solid state, structured-light projector is developed that can adjust the spatial phase and period of projected interference fringes with unprecedented speed, repeatability, and accuracy. The projector is applied to video-rate 3-D surface profilometry using accordion fringe interferometry (AFI), a phase-measuring multiresolution active triangulation technique. We present the theory of the technology, the characterization of our prototype, and the 3-D measurement results.

We apply the facet model to extract edge and surface information from computed tomography (CT) images for precision inspection and shape extraction. First we explore the application of the facet model in a three-dimensional (3-D) filter design. A 3-D directional-derivative- based surface detector is developed to extract surface points from the CT images. Subpixel accuracy is achieved by locating the zeros of the 3-D second directional derivative along the estimated gradient direction. Then we develop a precision inspection system to take turbine blade wall width measurements from CT images and compare them to the corresponding optical measurements. Least mean squares (LMS) methods are used to enhance prediction accuracy with the adaptive property of increasing accuracy when additional verifiable data are available. The system accuracy is within 3 mil. Unverified measurements are adjusted based on the verified measurements, and experiments show increasing accuracy of the adjusted measurements as additional verified measurements are available. Also, quantitative analysis of a trapezoid-shaped workpiece is performed. The results indicate that the CT system performance is affected by the structure and size of a workpiece. We also present an algorithm to extract 3-D shapes from detected surface points interactively and use them for visual inspection.

Current trends in the miniaturization of structures require the use of smaller and smaller components. Development of these microcomponents and, in turn, of mechanical microstructures/microsystems requires, in addition to the integrated use of advanced design, analysis, and fabrication methodologies, state-of-the-art test and measurement methodologies. We have developed one such methodology, optoelectronic laser interferometric microscope (OLIM) methodology, with capabilities for rapid characterization of microelectromechanical systems (MEMSs). MEMSs are a unique class of mechanisms created using integrated circuit (IC) fabrication processes. Microgears and microengines, smaller than a gnat's eye, are groups of MEMSs created by such a process. MEMSs offer challenging opportunities for sensor/actuator applications and for improved test methodologies, e.g., shape measurement. This work has produced a new methodology for the characterization of MEMSs. To demonstrate the OLIM methodology, motions of a 64-µm-diameter microgear, driven at speeds up to 360,000 rpm, are investigated. Manufacturing tolerances result in gaps between the microgear and its hub. There gaps allow the microgear to tilt as it rotates. The OLIM methodology permitted measurements of tilt angles of less than 11 mrad, and deflections below 0.5 µm. Knowing how individual microgears behave is important, e.g., when many such gears are combined to form transmissions.

A new algorithm for carrier removal, a key step in the Fourier transform method of fringe pattern analysis, is presented. The accuracy of frequency estimations is critical to carrier removal to avoid potentially significant errors in the recovered phase. A new algorithm based on the Fourier transform and the curve fitting technique is developed. To avoid an ill-conditioned result in solving the least-squares problem, an orthogonal polynomial curve fitting algorithm is developed. A new system that combines projected grating moire (PM) with shadow moire (SM), recently designed and built for both component-level and board-level warpage measurement for the reliability study of electronic packaging materials and structures, is presented and demonstrated.

Two practical computer-aided optical techniques for full-field surface shape measurement are presented, one for diffuse surfaces and the other for specularly reflective surfaces. The former technique is based on projecting a computer-generated fringe pattern onto a diffuse surface, and the latter is based on reflecting the fringe pattern from a specularly reflective surface. The fringe pattern is perturbed in accordance with the object surface with the fringe phase bearing information on the depth/slope of the object surface. The computer-generated fringe pattern conveniently enables the fringe phase to be manipulated, and hence the determination of the phase distribution using a phase extraction algorithm. Instead of deriving the mathematical relationship between the fringe phase distribution and the surface depth/slope, this relationship is obtained by calibration. The techniques described can be easily implemented for rapid measurement of 3-D surface shapes in an industrial setting.

The described 3-D measurement system with fringe projection applies the principle of uniform scale representation by the exclusive use of phase measurement values for the coordinates of each point. The test object is successively illuminated with a grating structure from at least three different directions with a telecentric system, where gray code is combined with five 90-deg phase shifts. A cCd camera records the intensity distribution of the fringes that appear as intersection lines on the surface of the object. This provides the linearly independent absolute phase values that are necessary for the calculation of the coordinates. Note that all coordinates (x,y,z) are determined with the same accuracy. To compensate the influence of specular reflections or shadowed areas up to 15 light projection directions can be used. Moreover, the object can be rotated around a second axis, yielding other views of the object. Thus we acquire different patches of the object that are transformed into a global coordinate system without any interactive user help. During this procedure, correlation methods or special points are not necessary. The calibration of the 3-D orientation of the second axis is realized with a special calibration body.

We report the basic concepts and experimental arrangements of self-calibrating 3-D measurement systems using structured- light illumination (fringe projection), which ensure a high number of object points, rapid data acquisition, and a simultaneous determination of coordinates and system parameters (self-calibration), making the system completely insensitive to environmental changes. Furthermore, it is unnecessary to have any marker on the object surface and a subsequent matching of the single views is not required to obtain a full-body measurement. For this purpose, the test object is successively illuminated with two grating sequences perpendicular to each other from at least two different directions, resulting in surplus phase values for each measurement point. Based on these phase values one can calculate the orientation parameters as well as the 3-D coordinates simultaneously. Different measurement setups that have the ability to measure the entire surface (full-body measurement) are reported. Results are presented showing the power of this concept, for example, by measuring of the complete 3-D shape of specular technical surfaces, whereas the object volumes can differ strongly. Theoretical estimations proven by first measurements show a coordinate measurement accuracy of up to 10-5 of the measurement field size.

The four-dimensional imager (4DI) system, a real-time threedimensional (3-D) imager, is a laser range data sensing system for making continuous geometric measurements of 3-D surfaces. We concentrate on the objective of improving the resolution and accuracy of this system using a camera-turntable arrangement. A fast and inexpensive method to capture dense range data for 360-deg viewing of an object is provided. After the system calibration procedure and parameter estimation operations, the method to improve the data resolution by object rotation is derived. By oversampling the object surface and then using a 3-D smoothing and resampling operation, the accuracy of the data can be improved while maintaining a given spatial resolution. The method of removing the low credibility data points is also discussed.

This paper describes the development of a highly sensitive optical profilometer well suited for the detection of faults with millimeter lateral dimensions and a micrometer depth. The objective is to meet the requirements of the automotive industry. The principle combines the high sensitivity of reflection techniques with the evaluation potential of phase shifted fringe projection. After a mathematical description the paper gives a number of experimental results with both reference surfaces and original sheet metal parts.

Retroreflective metrology has been used to measure the 3-D surface waviness on standard artifacts. An optical configuration was constructed for the measurement, in which the grating image is produced by projection. The three-step algorithm of phase-shifting technology was applied to analyze the image. Two standard artifacts with 8.0- and 16.0-µm peak-to-peak wave amplitudes were tested. The experiments show that retroreflective metrology can successfully recover the contour and amplitude of the surface waviness on the standard artifacts. © 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)02101-2]

This paper describes the development and practical application of a fringe analysis system for the high-speed measurement of human body shape and position. The application for this system is in the measurement of patient surface location and shape during the delivery of radiotherapy treatment for cancer. The system uses a twin-fiber interferometer as the basis for fringe production. The fringes are then projected onto the surface to be measured and captured by a CCD camera before being analyzed using Fourier-transform fringe analysis by a computer system. The novel features of this work are in the way in which the system has been realized, with maximum robustness and speed as primary goals. This has led to the development of a number of new techniques in data preprocessing, use of the algorithm itself, and calibration. Particular features include the larger than customary field of view, the use of noncollimated fringes, toleration of the damaging radiation environment, robustness to the variable color, texture, and profile of the surface, and the ability to operate at high speed with a conventional PC platform. The system is capable of measuring in excess of 32,000 surface points per second. Further developments of the system are also described, which are intended to extend its capabilities further.

This paper focuses on the characteristics and performance of an eye-safe laser range scanner (LARS) with short- and medium-range 3-D sensing capabilities for space applications. This versatile LARS is a precision measurement tool that will complement the current Canadian Space Vision System. The major advantages of the LARS over conventional video-based imaging are its ability to operate with sunlight shining directly into the scanner and its immunity to spurious reflections and shadows, which occur frequently in space. Because the LARS is equipped with two high-speed galvanometers to steer the laser beam, any spatial location within the field of view of the camera can be addressed. This versatility enables the LARS to operate in two basic scan pattern modes: (1) variable-scan-resolution mode and (2) raster-scan mode. In the variable-resolution mode, the LARS can search and track targets and geometrical features on objects located within a field of view of 30 by 30 deg and with corresponding range from about 0.5 to 2000 m. The tracking mode can reach a refresh rate of up to 130 Hz. The raster mode is used primarily for the measurement of registered range and intensity information on large stationary objects. It allows, among other things, target-based measurements, feature-based measurements, and surface-reflectance monitoring. The digitizing and modeling of human subjects, cargo payloads, and environments are also possible with the LARS. Examples illustrating its capabilities are presented.

Laser speckle interferometry as a full-field noncontact measuring technique offers interesting opportunities for strain-stress analysis on components. While its application in material testing and material research has already achieved some acceptance in research and industry, its application to complex industrial components like car bodies, gear boxes, engines, and suspensions has been limited. Basic difficulties have arisen from the relatively large rigid-body movements of components under test, harsh environmental conditions in the real test world, and the often complex shape of the analyzed component, especially in the most interesting areas. The commercial availability of a radically miniaturized 3-D speckle interferometer has led to the new laser-optical measuring device, the MicroStarâ„¢, which can be used for quantitative strain-stress measurement on nearly any industrial component. The device uses 3-D speckle interferometry to measure the shape and the 3-D deformation in the area of interest. The combination of shape and deformation provides all necessary data for quantitative 3-D strain analysis. The principal stresses as well as the bending and tensile components of the strains can be easily determined. In this paper, the principle and applications of this new system are presented.

Characterization of surface shape and deformation is of primary importance in a number of testing and metrology applications related to the functionality, performance, and integrity of components. In this paper, a unique, compact, and versatile state-of-the-art fiber-optic- based optoelectronic holography (OEH) methodology is described. This description addresses apparatus and analysis algorithms, especially developed to perform measurements of both absolute surface shape and deformation. The OEH can be arranged in multiple configurations, which include the three-camera, three-illumination, and in-plane speckle correlation setups. With the OEH apparatus and analysis algorithms, absolute shape measurements can be made, using present setup, with a spatial resolution and accuracy of better than 30 and 10 µm, respectively, for volumes characterized by a 300-mm length. Optimizing the experimental setup and incorporating equipment, as it becomes available, having superior capabilities to the ones utilized in the present investigations can further increase resolution and accuracy in the measurements. The particular feature of this methodology is its capability to export the measurements data directly into CAD environments for subsequent processing, analysis, and definition of CAD/CAE models.

A new concept for 3-D shape measurement of complex objects is proposed, which is based on photogrammetry and fringe projection. The shape of a partial view is determined by a combination of the phase-shift method for fringe evaluation and a photogrammetric triangulation to calculate the 3-D coordinates related to the sensor coordinate system. For the measurement of complex objects, partial views are taken from different sensor positions. The problem of matching the partial views into each other is solved by transforming each sensor position and the relating point cloud of the shape in a global coordinate system by photogrammetric matching of reference targets.

Holographic interferometry makes it possible to measure high-precision displacement data in the range of the wavelength of the laser light used. However, for a precise determination of 3-D displacement vectors of complex objects the 3-D shape of the surface is required. Modern optical shape measurement technologies enable a very effective approach to finding the Cartesian coordinates of complex surfaces. These data are used to calculate the spatially variable sensitivity vectors for the displacement measurement. The shape data are measured with white-light fringe projection using a multiwavelength technique to acquire absolute phase values. To make the shape data available for 3-D displacement measurement they have to be transferred into a reference coordinate system of the interferometric setup, where the deformation of the object caused by operational load is measured precisely. For this purpose a registration procedure is applied. For engineering applications it is useful to make the data available for computer-aided engineering systems. The object surface has to be approximated analytically from the measured point cloud to generate a surface mesh. The displacement vectors can be assigned to the nodes of this surface mesh for visualization of the deformation of the object under test. They also can be compared with the results of finite-element calculations or can be used as boundary conditions for further numerical investigations. The described procedure is demonstrated on an automotive component. Thus more accurate and effective measurement techniques make it possible to bring experimental and numerical displacement analysis closer together.

A new concept for 3-D shape measurement of complex objects is proposed, which is based on photogrammetry and fringe projection. The shape of a partial view is determined by a combination of the phase-shift method for fringe evaluation and a photogrammetric triangulation to calculate the 3-D coordinates related to the sensor coordinate system. For the measurement of complex objects, partial views are taken from different sensor positions. The problem of matching the partial views into each other is solved by transforming each sensor position and the relating point cloud of the shape in a global coordinate system by photogrammetric matching of reference targets.

Branching stochastic processes are used to describe random systems such as nuclear chain reactions, population development, and gene propagation. We show that the creation of a scanning-electron- microscope signal can be described as a branching stochastic process. A statistical model is described step by step, as a function of the physical parameters of the process. Using the model, we propose a method for determining the unknown probability distribution of the secondary electron emission. Using this method, a lognormal distribution is shown to approximate the secondary electron emission well, and a Poisson distribution is shown to do so poorly.

We introduce a multichannel radio-frequency (RF) correlator capable of correlating a received RF signal simultaneously against a library of known reference waveforms stored as angularly multiplexed holograms in a photorefractive crystal. The system's operation is inherently two-dimensional and may yield a compact, robust, and efficient device through integrated optical techniques, in contrast to previous optical multichannel schemes. Operation of a 16-channel system is experimentally demonstrated.

This paper presents three computational visual distinctness measures, computed from image representational models based on selective filtering, statistical features, and visual patterns, respectively. They are applied to quantify the visual distinctness of targets in complex natural scenes. The measure that applies a simple decision rule to the distances between segregated visual patterns is shown (1) to predict human observer performance in search and detection tasks on complex natural imagery, and (2) to correlate strongly with visual target distinctness estimated by human observers.

A new general registration method for images of different nature is presented. As gray levels or textures cannot be used for the registration of images from separate spectral bands, an edge-based method has been developed. Edge images are processed to extract straight linear segments, which are then grouped to form triangles. A set of candidate transformations is determined by matching triangles from the source and destination images. The transformations are then evaluated by matching the transformed set of source segments to the set of destination segments. As the coincidence of vertices or edge overlapping cannot be assumed in the registration of images of different nature, a new function for evaluating the matching quality between source and destination segments that does not rely on overlapping measures is proposed. Results and subjective evaluation of the registration of visual and thermal infrared images are presented.

We report on the investigation of refractive microlens arrays with diffractive grating surfaces in the context of a microspectrometer array system. The elements fabricated combine the fairly large dimensions of the refractive microlens (990-µm diam, 60-µm height) with the submicron features of the diffraction grating (1-µm grating period) on one transmitting surface. The fabrication process of these elements was studied, as well as their performance with respect to resolution and stray-light suppression. The maximum resolution was 3 nm, and the stray-light suppression 25 dB. We present a concept for a system of miniaturized spectrometer arrays for chemical analysis.

This paper covers the design and operation of a new mode scrambler for polymer optical fibers. This scrambler enables the rapid achievement of the equilibrium mode distribution without incurring significant insertion loss. Its main other features are small size and a costeffective design, making it very easy to build, configure, and operate. A numerical model based on geometrical optics is developed to understand the behavior of the scrambler, assess its overall performance, and help optimize its configuration. Results obtained through numerical simulation are then validated through experiments.

The long trace profiler (LTP) has become the instrument of choice for surface figure testing and slope error measurement of mirrors used for synchrotron radiation and x-ray astronomy optics. In order to achieve highly accurate measurements with the LTP, systematic errors need to be reduced by precise angle calibration and accurate focal plane position adjustment. A self-scanning method is presented to adjust the focal plane position of the detector with high precision by use of a pentaprism scanning technique. The focal plane position can be set to better than 0.25 mm for a 1250-mm-focal-length Fourier-transform lens using this technique. The use of a 0.03-arcsec-resolution theodolite combined with the sensitivity of the LTP detector system can be used to calibrate the angular linearity error very precisely. Some suggestions are introduced for reducing the system error. With these precision calibration techniques, accuracy in the measurement of figure and slope error on meter-long mirrors is now at a level of about 1 µrad rms over the whole testing range of the LTP.

A liquid sensor based on surface plasma wave (SPW) excitation was fabricated by the deposition of an optimized thickness of gold metal film on a prism. This sensor was applied to measure the reflectivity versus resonant angle of the SPW excitation using water, methyl alcohol, and ethyl alcohol of various concentrations as dielectric. An attempt was also made to theoretically estimate the position of the dip, and the result is in good agreement with experiment. The measured positions of dips, which correspond to the resonant angle of the SPW mode, varied with the species of dielectric, making it easy to distinguish methyl alcohol from ethyl alcohol.

The construction and optical performance of Fabry-Perot eta- lons for spaceflight are considered. Optical mounts with sufficient mechanical firmness to handle intense vibration loads cause significant distortion of etalon plates, degrading the etalon resolving power and peak transmittance. A mount is described that combines mechanical robustness with very low plate distortion. An etalon mounted in this manner survived spaceflight-level vibration testing, in two axes, without damage or degradation. The defect finesse of this etalon, measured after the vibration tests, was 46, at a wavelength of 532 nm, about three times the values achieved with previous Fabry-Perot etalons for spaceflight.

Advances in high-repetition-rate high-average-power ultrashort-pulse laser systems1–3 are expected to be utilized in many applications, such as higher-harmonic generation4 and material processing,5 that require high average flux. However, the average power of such systems is still limited to several watts because the average power of pump sources has been limited to tens of watts.