David K. MacKinnon

GD&T-Based Characterization of Short-Range Non-contact 3D Imaging Systems

We present a series of test metrics, artifacts, and procedures for characterizing and verifying the operating limits of a short-range non-contact three-dimensional imaging system. These metrics have been designed to correspond to dimensioning and tolerancing metrics that are widely used in industry (e.g. automotive, aerospace, etc.). We introduce operating limit metrics that correspond with the geometric dimensioning and tolerancing (GD&T) metrics of Form (Flatness and Circularity), Orientation (Angularity), Location (Sphere, Corner, and Hole Position Errors), and Size (Diameter, Sphere-spacing, Plane-spacing and Angle Errors). An example is presented to illustrate how these metrics, artifacts, and associated test procedures can be used in practice.

Adaptive Laser Range Scanning using Quality Metrics

We present an approach to laser range scanning in which quality metrics are used to automatically reduce the number of measurements acquired from a scanner viewpoint in order to guide a minimally trained operator through the scanning process. As part of this approach we present improved versions of the orientation and reflectivity quality metrics, and introduce six new within-scan quality metrics: outlier, enclosed, resolvability, planarity, integration, and aliasing. These metrics are combined to generate a total within-scan quality metric for each measurement in the scan. The orientation, resolvability, reflectivity, and planarity quality metrics are used to divide the total field of view into regions based on their likelihood to produce useful measurements. A series of small high-density raster scans is then automatically generated to cover regions automatically identified as having a significant likelihood to produce useful measurements. All scans are then merged to generate a composite range image. The total number of measurements in the composite range image is minimized by merging statistically close measurements using a minimum variance estimator weighted by the total within-scan quality of each measurement.

Lateral resolution challenges for triangulation-based three-dimensional imaging systems

Lateral resolution is a particularly challenging concept to quantify in triangulation-based three-dimensional (3-D) imaging systems. We present these challenges, then describe an artifact-based methodology for evaluating the lateral resolution of a triangulation-based 3-D imaging system that uses laser spots or laser lines. In particular, the response of a 3-D imaging system to a spatial discontinuity (step edge) has traditionally been modeled as a first-order linear system. We model the response of a triangulation-based laser imaging system to a spatial step edge from first principles and demonstrate that the response should be modeled as a non linear system. This model is then used as a basis for evaluating the lateral (structural) resolution of a triangulation-based laser imaging system.

Hierarchical characterization procedures for dimensional metrology

We present a series of dimensional metrology procedures for evaluating the geometrical performance of a 3D imaging system that have either been designed or modified from existing procedures to ensure, where possible, statistical traceability of each characteristic value from the certified reference surface to the certifying laboratory. Because there are currently no internationally-accepted standards for characterizing 3D imaging systems, these procedures have been designed to avoid using characteristic values provided by the vendors of 3D imaging systems. For this paper, we focus only on characteristics related to geometric surface properties, dividing them into surface form precision and surface fit trueness. These characteristics have been selected to be familiar to operators of 3D imaging systems that use Geometrical Dimensioning and Tolerancing (GD&T). The procedures for generating characteristic values would form the basis of either a volumetric or application-specific analysis of the characteristic profile of a 3D imaging system. We use a hierarchical approach in which each procedure builds on either certified reference values or previously-generated characteristic values. Starting from one of three classes of surface forms, we demonstrate how procedures for quantifying for flatness, roundness, angularity, diameter error, angle error, sphere-spacing error, and unidirectional and bidirectional plane-spacing error are built upon each other. We demonstrate how these procedures can be used as part of a process for characterizing the geometrical performance of a 3D imaging system.

Evaluating laser range scanner lateral resolution in 3D metrology

In this study, laser range scanner lateral resolution is investigated for laser range scanners. A standardized method is proposed and demonstrated for quantifying the lateral surface resolvability of a laser range scanner through the use of an appropriately-designed artefact. A new metric for lateral surface resolution, the limit of surface resolvability, is presented and is obtained using what is referred to as the wedge test. The results of applying this metrics using this test method to laser range scanners is also presented.

Single-plane versus three-plane methods for relative range error evaluation of medium-range 3D imaging systems

Within the context of the ASTM E57 working group WK12373, we compare the two methods that had been initially proposed for calculating the relative range error of medium-range (2 m to 150 m) optical non-contact 3D imaging systems: the first is based on a single plane (single-plane assembly) and the second on an assembly of three mutually non-orthogonal planes (three-plane assembly). Both methods are evaluated for their utility in generating a metric to quantify the relative range error of medium-range optical non-contact 3D imaging systems. We conclude that the three-plane assembly is comparable to the single-plane assembly with regard to quantification of relative range error while eliminating the requirement to isolate the edges of the target plate face.

Characterization of Triangulation-Based 3D Imaging Systems Using Certified Artifacts

Abstract: A set of test procedures and certified artifacts to characterize the capability of short-range triangulation-based three-dimensional (3D) imaging systems are presented. The approach consists of scanning metallic and coated-glass certified artifacts in which the uncertainties in the associated characteristic reference values are smaller than the measurement uncertainties produced by the system under test (SUT). The artifacts were grouped on the same plate for portability. To define a set of test procedures that is practical, simple to perform and easy to understand, we utilized a terminology that is well-known in the manufacturing field, i.e., geometric dimensioning and tolerancing (GD&T). The National Research Council Portable Characterization Target (NRC-PCT) is specifically designed for the characterization of systems with depths of field from 50 mm to 500 mm. Tests were performed to validate the capability of the NRC-PCT. This paper presents these results, along with some basic information on 3D imaging systems.

Proposed traceable structural resolution protocols for 3D imaging systems

A protocol for determining structural resolution using a potentially-traceable reference material is proposed. Where possible, terminology was selected to conform to those published in ISO JCGM 200:2008 (VIM) and ASTM E 2544-08 documents. The concepts of resolvability and edge width are introduced to more completely describe the ability of an optical non-contact 3D imaging system to resolve small features. A distinction is made between 3D range cameras, that obtain spatial data from the total field of view at once, and 3D range scanners, that accumulate spatial data for the total field of view over time. The protocol is presented through the evaluation of a 3D laser line range scanner.

Measurement quality metrics for rapid laser range scanning

Quality metrics quantify by how much some aspect of a measurement deviates from a predefined standard. Measurement quality evaluations of laser range scanner data are used to perform range image registration, merging measurements, and view planning. We develop a scanning method that uses laser range scanner quality metrics to both reduce the time required to obtain a complete range image from a single viewpoint and the number of measurements obtained during the scanning process. This approach requires a laser range scanner capable of varying both the area and sampling density of individual scans, but can be combined with view planning methods to reduce the total time required to obtain a complete surface map of an object. Several new quality metrics are introduced: outlier, resolvability, planarity, integration, return, and enclosed quality metrics. These metrics are used as part of a quality-based merge method, referred to here as a quality-weighted modified Kalman minimum variance (weighted-MKMV) estimation method. Experimental evidence is presented confirming that this approach can significantly reduce the total scanning time. This approach could be particularly useful for rapidly generating CAD models of real-world objects.

A Methodology for Creating Large Scale Reference Models with Known Uncertainty for Evaluating Imaging Solution

We propose a methodology for acquiring reference models with known uncertainty of complex building-sized objects. Those can be used to quantitatively evaluate the performance of passive 3D reconstruction when working at large scale. The proposed methodology combines the use of a time-of-flight scanner, a laser tracker, spherical artifacts and contrast targets. To demonstrate the soundness of the proposed approach, we built a reference model composed of a 3D model of exterior walls and courtyards of a 130m × 55m × 20m Building. The expanded uncertainty of the 3D reference model and the spatial resolution were calculated.