Fig. 1 illustrates the general setup of a complete multi-wavelength digital holographic measurement system. It consists of a passive sensor head (A) and a multi-laser control electronics system (B). Variations between the different systems in this contribution are marked with an asterisk.
Fig. 1 Sketch of the holographic setup.Multiple laser sources (1) are cycled through by a fiber switch (3), controlled by a computer, inside the control electronics B. Single-frequency laser light is connected to the computer-controlled (5) temporally phase shifting measurement head A by a polarization maintaining fiber (2).
Multiple stabilised lasers (1) (three lasers are shown as examples in Fig. 1) with free-space optical powers of 25–50 mW are used to generate synthetic wavelengths ranging from ~2 µm to 2000 µm. For good interference contrast, the lasers must operate with a single frequency and a single mode. The laser sources (up to eight lasers) are successively coupled into the sensor head using a polarization-maintaining single-mode glass fibre (2) and switched between by a fast micromechanical fibre switch (3). The switch is controlled by an I/O interface (4) which in turn is driven by a PC or embedded system (5) inside the control electronics (B). The switching duration is specified as less than 7 ms.
Inside the sensor head (A), the beam is divided into an object and a reference beam by a polarising beam splitter cube (PBS) (6). One common lens refracts (7) the beams in such a way that they fit the size of the object (8) and the camera chip (9), respectively.
The reference beam is deflected by a mirror mounted on a piezoelectric actuator (10), which is again controlled by the I/O interface of (B). Thus, the phase of the reference beam can be shifted selectively within a single laser wavelength, which is called temporal phase shifting12.
The light scattered by the sample is imaged onto a camera sensor by an objective lens (11), where it is superimposed with the reference beam. At least three phase-shifted interferograms are recorded for each wavelength. In addition, the following polarising optics are used: a polarising filter (12) is used to minimise the influence of laser polarisation fluctuations. The beam ratio between the object and reference beams is set by a half-wave plate (13). The quarter-wave plate (14) and half-wave plate (15) are required for the beams to pass or be deflected at the following PBS. The final polarising filter (16) enables interference between the object and reference beams.
During sensor development, the measurement field and lateral resolution can be adapted to the measuring task by selecting imaging optics and a camera chip. The following example specifications given in the text refer to the HoloTop 65M sensor unless stated otherwise.
A total of nine images (three lasers used for the measurements presented in this paper, three phase steps) at a resolution of 9344 × 7000 pixels were captured within 200 ms and digitised with 10-bit resolution. The acquisition time for a three-laser configuration is determined by the following: nine images must be recorded by the camera, which is capable of a framerate of 71 frames per second; a fibre switch (Leoni 8 × 2-pm-fiberswitch) must perform two switching operations taking 10 ms each; and between each of the images, the piezo-actuator must be moved for temporal phase shifting, which takes less than 1 ms. The resulting acquisition time is thus 156 ms. A USB controller or framegrabber handles the data being transferred to the computer unit.
Using a parallelised numerical reconstruction on the GPU of the control electronics (a PC or, if available, the internal embedded system) and its Compute Unified Device Architecture (CUDA), the complete reconstruction of the phase maps at the different synthetic wavelengths, along with the evaluation of the height map, takes only 100–300 ms, depending on the filter settings. For very large camera images (65 MP) and filter radii (e.g. 7 × 7 pixels), these processes can take up to 800 ms.
This is done in two steps: first, the complex wavefront in the hologram plane is calculated using a method introduced by Cai et al.12 that compensates for vibrations and thus changes in the phase between the object and reference beam during data acquisition. Then, the complex wavefront is propagated to the focal plane without reconstruction of the disturbing twin image and zero diffraction order4, 18. A flowchart of the algorithms used for hologram reconstruction can be found in Ref. 19.
Image acquisition, data evaluation, and feature extraction are done in parallel to meet the requirements of the respective production cycle. For a detailed description and mathematics of the numerical reconstruction procedure of digital multi-wavelength holograms, we refer to3, 4, 12, 20.
All sensor heads were simulated during development using the finite element method (FEM) to ensure maximum stiffness and thus natural frequencies below externally occurring excitations. For example, a natural frequency of 605 Hz was realised using the HoloTop 20M21. This is especially important for the HoloPort system working in the machine tool. With excitation frequencies typically well below 300 Hz, the influence of external stimulations could be minimised by mechanical means at a first natural frequency of 323 Hz22.
Table 1 shows the specifications of the three holographic measurement systems developed at Fraunhofer IPM, which are presented with an application in this paper. They differ in terms of sampling and optical resolution, depending on the application they were designed for. One remarkable design consideration in the development of holographic sensors is the fact that speckle patterns generated by rough object surfaces must be sampled well, and slight oversampling yields the best results.
HoloTop 9M HoloTop 65M HoloPort Diffraction limited resolution/µm 13.5 6.4 16.2 Lateral sampling/µm 5.86 1.91 < 7 Magnification −0.94 −1.67 −0.5 Numerical aperture 0.06 0.10 0.04 Camera Sensor CMOSIS CMV12000 GPixel GMAX3265 Sony IMX253 Number of pixels 3072 × 3072 9344 × 7000 4112 × 3008 Single FOV/mm2 18.0 × 18.0 17.8 × 13.4 20 × 20 Acquisition time per point/s 6.4e-6 3e-9 3.3e-8 Acquisition time per FOV/ms ~ 100 ~500 < 500
Table 1. Specifications of selected holographic measuring systems developed at Fraunhofer IPM.
In addition to the systems presented here, there are other systems available as well: systems with a postcard-sized field of view, for measurements on moving objects, and with a microscope objective for particularly high lateral resolution23.
Digital holography in production: an overview
- Light: Advanced Manufacturing , Article number: 15 (2021)
- Received: 23 January 2021
- Revised: 25 March 2021
- Accepted: 22 May 2021 Published online: 18 June 2021
Abstract: Many challenging measurement tasks in production simultaneously have high requirements for accuracy, measurement field size, lateral sampling, and measurement time. In this paper, we provide an overview of the current state of the art in digital holography for surface topography measurements and present three applications from completely different productions with no alternative to digital holography; we describe the HoloTop sensor family, which has been designed specifically for industrial use, and present the most recent results achieved in real-life industrial applications. All applications address measurement tasks that could not be solved until now, either by optical or tactile means. We start with a description of the first-ever inline integration of a digital holographic measurement system that inspects precision turned parts for the automotive industry. We proceed by presenting measurements performed with a compact sensor that can be placed inside a tooling machine and operated fully wirelessly. In this case, the tool holder was used to position the sensor directly. Integration into a tooling machine places high demands on both robustness and reliability. Finally, the quality control of electronic interconnectors such as microbumps with the highest demand for accuracy and repeatability is demonstrated. All of these applications illustrate the major advantages of digital holographic systems: it is possible to measure a relatively large field of view with interferometric precision and very short acquisition times. Additionally, both reflective and matt surfaces can be measured simultaneously. We end this publication with an assessment of the future potential of this technology and the necessary development steps involved.
Digital Holography: The next generation of inline 3D measurement
Over the last decade, digital holography has become one of the fastest and at the same time most accurate methods for surface topography in production lines. The basic measurement principle uses interferometric recording of light waves reflected from component surfaces. By clever use of several lasers with different wavelengths, also macroscopic objects can be measured with sub-micrometer accuracy. Scientists at Fraunhofer Institute for Physical Measurement Techniques now give an overview of industrial applications that demonstrate the unique selling point of the technology for optical inline measurements of macroscopic measurement fields with sub-micron accuracy. Despite the high sensitivity to external influences such as vibrations, the implementation of holographic sensors in production environments has been very successful. Industrial applications in metal processing and chip manufacturing prove this impressively.
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