Periodic Reporting for period 1 - InOGeM (Indirect optical geometry measurement)
Reporting period: 2022-09-01 to 2025-02-28
Optical metrology is a growth driver in our society and has had an ever greater impact on manufacturing, mobility, medicine and basic research over the last decade due to advancing developments in the camera sector and the field of artificial intelligence. In particular, the increasing number of publications in this area, as well as the microscopy approach that was awarded the Nobel Prize in 2014, with which nanometer structures can be resolved below the diffraction limit, underline the potential of optical metrology. Although optical methods enable fast and precise geometry measurements, they have the shortcoming that they are dependent on optically co-operative surfaces and only work if sufficient light energy is reflected from the object surface to the detection unit. For this reason, specific measurement methods had to be developed for each type of surface, e.g. deflectometry for highly reflective surfaces.
Indirect geometry measurement (InOGeM) will introduce a paradigm shift here and develop a universal measurement method that can be applied to any surface: instead of measuring the surface of the object, the ‘imprint’ of the geometry in the surrounding gas volume is measured in an inverse process. The surrounding gas medium is therefore enriched with fluorescent microparticles or molecules and detected with a scanning confocal microscope. The area in which the fluorescence signal disappears defines the surface position of the measured object as a boundary layer. With the help of model-based signal processing, resolutions in the sub-micrometer range can be achieved. This breaks new ground for the assessment of additively manufactured components and lightweight components made of fiber composites, as the indirect measurement is less sensitive to the varying optical properties of the surface and the material of the measurement object. In addition, indirect optical geometry measurements on highly curved or translucent objects are also possible through limited access, which was previously considered impossible. Such difficult conditions occur, for example, in gears and additively manufactured parts, so InOGeM has great potential for low-noise gears and fuel cells. InOGeM enables fast geometry measurements with a precision below the classical limits in the nanometer range for a variety of applications that is unattainable today. By developing a new class of measuring devices, InOGeM takes the field of optical geometry measurement to a new level.
Indirect geometry measurement (InOGeM) will introduce a paradigm shift here and develop a universal measurement method that can be applied to any surface: instead of measuring the surface of the object, the ‘imprint’ of the geometry in the surrounding gas volume is measured in an inverse process. The surrounding gas medium is therefore enriched with fluorescent microparticles or molecules and detected with a scanning confocal microscope. The area in which the fluorescence signal disappears defines the surface position of the measured object as a boundary layer. With the help of model-based signal processing, resolutions in the sub-micrometer range can be achieved. This breaks new ground for the assessment of additively manufactured components and lightweight components made of fiber composites, as the indirect measurement is less sensitive to the varying optical properties of the surface and the material of the measurement object. In addition, indirect optical geometry measurements on highly curved or translucent objects are also possible through limited access, which was previously considered impossible. Such difficult conditions occur, for example, in gears and additively manufactured parts, so InOGeM has great potential for low-noise gears and fuel cells. InOGeM enables fast geometry measurements with a precision below the classical limits in the nanometer range for a variety of applications that is unattainable today. By developing a new class of measuring devices, InOGeM takes the field of optical geometry measurement to a new level.
Two confocal fluorescence microscopes were set up so that several people could work in parallel on researching and validating the indirect optical geometry measurement. The measurement object geometry is recorded by analyzing its imprint in the surrounding air using a confocal fluorescence microscope. For this purpose, a topas aerosol generator enriches the air with fluorescent aerosol particles (Ø ≈ 1 µm) made of fluorescent dye (pyrromethene 567) dissolved in DEHS. The dye is excited with a laser at a wavelength of 532 nm, and the fluorescence signal is filtered out with a cut-off filter (> 550 nm) in the detector's beam path. To capture the surface, the confocal microscope scans the space around the measurement object with the xyz-stage units. The pinhole in front of the fiber coupler ensures that only fluorescent light from the area of the confocal volume is detected with the avalanche photodetector. The surface position of the measurement object, i.e. the z-position, for the respective (x,y)-position at which the fluorescence signal disappears, is determined using model-based signal processing. Here, various models were analyzed. In the end, an approach could be applied that only uses the number of detected particle events. By applying an outlier detection algorithm based on the Grubbs test, these rare particle events can be filtered out of the raw signal, which is overlaid by random background noise and possible fluorescent surface contamination. Starting from zero detectable particle events within the object, the number of detectable events increases as the confocal volume enters the aerosol stream. The number of particle events is accumulated to generate the particle signal that is used for the geometry evaluation. It was shown that the particle signal has the form of an error function, which is used in the signal model to determine the surface position. While the current signal model resulted in a measurement uncertainty of a few micrometers, resolutions in the sub-micrometer range are considered to be feasible with high-resolution lenses and a sufficiently large number of observed particles.
According to the current state of research, only liquids have been used for indirect geometry measurement up to now. The aim in each case was to enable in-situ or in-process measurement in a manufacturing process using process fluids enriched with fluorescent dye. So far, the aim has been to enable optical geometry measurements that are obscured by an interfering liquid - whereby contamination of the measurement object was naturally inherent. The InOGeM project should make it possible to characterize objects in ambient air with as little contamination as possible. In particular, it will focus on geometries that are optically uncooperative due to their object structure or surface properties and for which optical geometry measurement has therefore not been possible so far.
Prime examples of optically uncooperative surfaces are geometries with steps, highly curved surfaces or materials that ensure that only little light reaches the receiver unit of the measuring system. As part of the project's ongoing work, it was shown that, in contrast to the white light interferometric reference measurement method, it is also possible to measure highly tilted (>80°) surfaces. The measured 200 µm high step of a metal micro object's geometry agrees with the calculated values within the measurement uncertainty. The InOGeM approach can also be used to measure step standards made of different, optically uncooperative polymer materials, such as low-reflective PMMA, and translucent PTFE or PE. Here too, the measured step heights agree with the reference value within the scope of the measurement uncertainty of a few micrometers. In contrast to the white light interferometric reference measurement method, however, no measurement artefacts such as batwings (overshoots) were detected at the edges of the step with the indirect measurement approach, which also illustrates the potential of the new InOGeM method.
Prime examples of optically uncooperative surfaces are geometries with steps, highly curved surfaces or materials that ensure that only little light reaches the receiver unit of the measuring system. As part of the project's ongoing work, it was shown that, in contrast to the white light interferometric reference measurement method, it is also possible to measure highly tilted (>80°) surfaces. The measured 200 µm high step of a metal micro object's geometry agrees with the calculated values within the measurement uncertainty. The InOGeM approach can also be used to measure step standards made of different, optically uncooperative polymer materials, such as low-reflective PMMA, and translucent PTFE or PE. Here too, the measured step heights agree with the reference value within the scope of the measurement uncertainty of a few micrometers. In contrast to the white light interferometric reference measurement method, however, no measurement artefacts such as batwings (overshoots) were detected at the edges of the step with the indirect measurement approach, which also illustrates the potential of the new InOGeM method.