Micro parts are increasingly found in a number of industrial products. They often have complex geometrical features in the millimeter to micrometer range which are not accessible or difficult to measure by conventional coordinate measuring machines or by optical microscopy techniques. In the last years, several concepts of tactile micro coordinate measuring machines have been developed in research laboratories and were partly commercialized by industry. The major challenges were related to the development of innovative micro probes, to the requirements for traceability and to the performance assessment at reduced measurement uncertainty. This paper presents a review on state of the art developments of micro coordinate measuring machines and 3D micro probes in the last 20 years, as far as these were qualified in a comparable way, with a special emphasis on research conducted by the Federal Institute of Metrology METAS in this field. It outlines the accuracy limitations for the probe head including the probing element and for the geometrical errors of the machine axes. Finally, the achieved performances are summarized and the challenges for further research are addressed.

Accurate traceable measurement systems often use laser interferometers for position measurements in one or more dimensions. Since interferometers provide only incremental information, they are often combined with index sensors to provide a stable reference starting point. Straightness measurements are important for machine axis correction and for systems having several degrees of freedom. In this paper, we investigate the accuracy of an optical two-dimensional (2D) index sensor, which can also be used in a straightness measurement system, based on a fiber-coupled, collimated laser beam pointing onto an image sensor. Additionally, the sensor can directly determine a 2D position over a range of a few millimeters. The device is based on a simple and low-cost complementary metal–oxide–semiconductor (CMOS) image sensor chip and provides sub-micrometer accuracy. The system is an interesting alternative to standard techniques and can even be implemented on machines for real-time corrections. This paper presents the developed sensor properties for various applications and introduces a novel error separation method for straightness measurements.

X-Ray Computed tomography (XCT) has become an important non-destructive quality assurance technique in industry. Consequently, standards for quality insurance of XCT and on its performance are required to support industrial XCT users for reliable production. This performance is determined by analysis of the quality of the images produced and by the dimensional measurement accuracy achieved for a given XCT parameter setting. Until recently, standards assessed image quality solely in terms of contrast sensitivity and spatial resolution. Detection limits could not be predicted until now. A new term is introduced: The Detail Detection Sensitivity (DDS). It depends on the contrast sensitivity as a function of contrast and noise, and on the spatial resolution. The spatial frequency needs to be implemented into the analysis to consider sensitivity as a function of the size of an indication. The contrast sensitivity is quantified by the Contrast Discrimination Function (CDF) and the spatial resolution by the Modulation Transfer Function (MTF). The numerical DDS is determined for air flaws from the Contrast Detection Diagram (CDD) at 100% contrast. However, some XCT operators prefer visual determinations rather than numerical ones. To face this need, the SensMonCTII project proposes a new Image Quality Indicator (IQI), consisting of a disk with holes of different sizes for visual DDS determination. The project aims to produce a new ISO standard draft providing a practice to evaluate numerically the XCT image quality via MTF, CDF, CDD and DDS, as well as to evaluate visually the DDS from the hole visibility of a disk IQI. The paper does not address the performance of XCT in terms of dimensional measurement accuracy, but focuses on the performance of XCT in terms of image quality. It describes the methodology to evaluate the image quality, including DDS for the first time.

The realisation of the metre definition is still today mostly performed using iodine frequency-stabilized HeNe lasers. Unfortunately, they deliver little optical power. Instead of migrating to other expensive laser technologies, the use of an injection locked semiconductor laser is proposed in order to boost the optical output of the legacy HeNe laser primary reference. The characterisation of the system and its validation through a comparison measurement is reported whereby no impact on the frequency and the stability of the HeNe reference laser could be observed.