J.-A. Beraldin’s research while affiliated with National Research Council Canada and other places

Recent interest in portable coordinate measuring systems (CMS) has attracted the attention of industrial users and the technical community at large. Articulated-arm CMM and Optically-Tracked CMM (OTCMM) systems have emerged from being nice-to-have to must-have 3D equipment on the shop floor. Portability and flexibility for in-process product measurements and verification are displacing more traditional methods that require fixed gantries, especially when OTCMM are combined with tracking devices that can compensate for the vibration levels of a typical shop floor. The current set of recognized contact CMS standards produced by ISO, VDI, ASME organisations cover fixed-gantry CMM. However, these standards cover one class of OTCMM i.e. laser trackers and neglect the class of OTCMM that include spherical-stylus touch probe tracked by contrast targets-based optical triangulation methods. It is within this context that we propose a harmonisation scheme for terminology, discuss test methods complemented with experimental data, and reference artefacts for the purpose of ensuring comparability of CMS and helping in pre-normative documentary/physical standards definition.

One of the strongest limiting factors in close range photogrammetry (CRP) is the depth of field (DOF), especially at very small object distance. When using standard digital cameras and lens, for a specific camera – lens combination, the only way to control the extent of the zone of sharp focus in object space is to reduce the aperture of the lens. However, this strategy is often not sufficient; moreover, in many cases it is not fully advisable. In fact, when the aperture is closed down, images lose sharpness because of diffraction. Furthermore, the exposure time must be lowered (susceptibility to vibrations) and the ISO increased (electronic noise may increase). In order to adapt the shape of the DOF to the subject of interest, the Scheimpflug rule is to be applied, requiring that the optical axis must be no longer perpendicular to the image plane. Nowadays, specific lenses exist that allow inclining the optical axis to modify the DOF: they are called tilt-shift lenses. In this paper, an investigation on the applicability of the classic photogrammetric model (pinhole camera coupled with Brown’s distortion model) to these lenses is presented. Tests were carried out in an environmentally controlled metrology laboratory at the National Research Council (NRC) Canada and the results are hereafter described in detail.

One of the strongest limiting factors in close range photogrammetry (CRP) is the depth of field (DOF), especially at very small object distance. When using standard digital cameras and lens, for a specific camera – lens combination, the only way to control the extent of the zone of sharp focus in object space is to reduce the aperture of the lens. However, this strategy is often not sufficient; moreover, in many cases it is not fully advisable. In fact, when the aperture is closed down, images lose sharpness because of diffraction. Furthermore, the exposure time must be lowered (susceptibility to vibrations) and the ISO increased (electronic noise may increase). In order to adapt the shape of the DOF to the subject of interest, the Scheimpflug rule is to be applied, requiring that the optical axis must be no longer perpendicular to the image plane. Nowadays, specific lenses exist that allow inclining the optical axis to modify the DOF: they are called tilt-shift lenses. In this paper, an investigation on the applicability of the classic photogrammetric model (pinhole camera coupled with Brown’s distortion model) to these lenses is presented. Tests were carried out in an environmentally controlled metrology laboratory at the National Research Council (NRC) Canada and the results are hereafter described in detail.

This paper discusses a methodology to evaluate the accuracy of recently developed image-based 3D modelling techniques. So far, the emergence of these novel methods has not been supported by the definition of an internationally recognized standard which is fundamental for user confidence and market growth. In order to provide an element of reflection and solution to the different communities involved in 3D imaging, a promising approach is presented in this paper for the assessment of both metric quality and limitations of an open-source suite of tools (Apero/MicMac), developed for the extraction of dense 3D point clouds from a set of unordered 2D images. The proposed procedural workflow is performed within a metrological context, through inter-comparisons with "reference" data acquired with two hemispherical laser scanners, one total station, and one laser tracker. The methodology is applied to two case studies, designed in order to analyse the software performances in dealing with both outdoor and environmentally controlled conditions, i.e. the main entrance of Cathédrale de la Major (Marseille, France) and a custom-made scene located at National Research Council of Canada 3D imaging Metrology Laboratory (Ottawa). Comparative data and accuracy evidence produced for both tests allow the study of some key factors affecting 3D model accuracy.

A set of test procedures and known artifacts to characterize the capability of short-range triangulation-based 3-D imaging systems are presented. The approach consists of scanning metallic and coated glass artifacts characterized by measurements with known uncertainties which are much less than the measurement uncertainties produced by the System Under Test (SUT). The artifacts were grouped on the same plate for portability purpose. To define a set of test procedures that is practical, simple to perform and easy to understand, we decided to use a terminology that is already well-known in the manufacturing field, i.e., Geometric dimensioning and tolerancing (GD&T). In its current version, the NRC Portable Characterization Target (NRC-PCT) is specifically designed for the characterization of systems with depths of field from 50 mm to 500 mm. A number of tests were performed to validate the capability of the NRCPCT. These results, along with some basic information on 3-D imaging systems, will be presented in the paper.

Written by a team of international experts, this book provides a comprehensive overview of the major applications of airborne and terrestrial laser scanning. It focuses on principles and methods and presents an integrated treatment of airborne and terrestrial laser scanning technology. After consideration of the technology and processing methods, the book turns to applications, such as engineering, forestry, cultural heritage, extraction of 3D building models, and mobile mapping. This book brings together the various facets of the subject in a coherent text that will be relevant for advanced students, academics and practitioners.

The deployment of 3D optical scanning technologies for measuring three-dimensional shapes of sculptures of high morphological complexity, have seen recently a development so noticeable that it emerges as one of the most promising methods for the analysis and preservation of the Cultural Heritage. In spite of an undoubted richness of information produced with this technique, the method for generating a digital model from single 3D acquisitions involves errors propagation that may deteriorate the actual metric accuracy attainable with such procedure. The activity reported in this paper describes a recent experiment in 3D scanning of a highly complex sculpture: the wooden statue "Maddalena" by Donatello, kept in the museum of the "Opera del Duomo", in Florence. The acquisitions, taken with a commercial system based on fringe projection, give a local measurement uncertainty of about 70m, but when the complete model is generated by automatic alignment of the raw 3D images in a common coordinate system, possible scale variations might be involved, especially along the height of the statue. With this preliminary work, the authors want to verify the metric reliability of the three-dimensional model, obtained through iterative alignments of single 3D acquisitions. For this purpose, a measurement over well-identified features of the sculpture have been performed and the distances between couples of significant points have been calculated with photogrammetric operations. By repeating the same measurements on the 3D model, a corresponding set of point-to-point distances have been evaluated and compared with the photogrammetric results.

The National Research Council in Canada (NRCC) has created a framework for gleaning information from very large TLS and imagery datasets, an evolution demonstrated in the Erechtheion project. Cultural heritage has been a central factor in the NRCC research and development program in 3D technologies. Erechtheion is on the Acropolis, next to the Parthenon in Athens, Greece, whos refurbishment began in 1987. The 'Porch of the Maidens' is on the north side and consists of six draped female figures. NRCC has created algorithms for 3D-image processing, management and real-time visualization as part of Atelier3D.ca, a general framework evolved for acquisition, processing, modeling, analysis and visualization of very large 2D/3D datasets created from 3D point-clouds and imagery. Atelier 3D.ca allows view-dependent, real-time visualization of multi-resolution models. Algorithms have been created for displaying these datasets interactively at full resolution on inexpensive laptops or desktop computers.