Roughness is one of the most important surface quality parameters in metal sheet processing. Thus contact-free optical methods are highly interesting. Using the innovative approach of spectral speckle correlation (SSC) has the potential to measure the spatial roughness distribution of large surfaces quickly and thus inline. In SSC, speckle patterns are recorded at different wavelengths and correlated with each other to determine the roughness of the sample. We show that the relationship between roughness and correlation coefficient as a function of wavelength difference is valid over a large range. The roughness of larger surfaces can be determined by separate evaluation of sub-images. We present the importance of the sub-image size to get reliable, reproducible measurements relating to existing standards. In this work we show how to measure the roughness parameter Sa for values ranging from 0.59 μm to 7.75 μm with a spatial resolution below 1 mm by SSC. We demonstrate that the theory is valid over a large range of wavelength differences. This is shown using wavelength differences between 0.2 nm and 93 nm in the visible spectrum.
We present digital multi-wavelength holography measurements generated inside a machine tool. The digital holographic sensor HoloTop NX used in the application has a field of view of 12.5 mm × 12.5 mm, each field of view consists of 3008 px × 3008 px. Three separately stabilized diode lasers connected to a fiber switch serve as light sources. The unambiguous measurement range of the holographic sensor defined by the laser wavelengths is approximately 140 μm and the axial reproducibility is ~0.4 μm. The sensor is mounted to a standard 5-axis machine tool Hermle C 32U. An automatically created numerical control (NC) program enables the machine tool to meander the sensor over larger test objects, here 280 mm × 94 mm. For each field of view, a height map with 9 million data points is generated using digital holography. The images are acquired with 2 mm lateral overlap and are then stitched together to achieve a height map of the complete sample with more than 1.4 billion data points. For comparison the shape of the sample was measured with a state-of-the-art coordinate measurement machine (CMM). The datasets from the CMM and the holographic measurement are semi-automatically referenced to each other. The holographic height data are averaged over a rectangular area with radius 50 μm around each of the 360 measurement locations where the tactile CMM measurement data have been acquired. This allows us to evaluate deviations between the holographic and the CMM dataset. It turns out that the root mean square (RMS) error between the two data sets is smaller than 0.4 μm.
Digital holography enables high-precision quality control in machining production and has already been introduced to several multi-axis systems1, 2. To meet the demanding measurement tasks in the quality control of complex components, accuracies in the sub-micrometer range with measurement ranges larger than several centimeters are required. Previous measurements have shown the potential of multiwavelength digital holography to allow unambiguous ranges of few millimeters3 . We present multiwavelength digital holographic measurements using synthetic wavelengths with two meters down to a few micrometers, potentially enabling measurements with meter-scale unambiguity at sub-micrometer accuracy. Measurements on a 10 cm step-height sample have been conducted using the compact digital-holographic sensor HoloTop NX for various multi-axis systems, supplied by an Ondax LMFC single frequency diode laser at 632.852 nm and the tunable laser Hübner C-Wave used in the wavelength range of 480.786 nm – 632.852 nm. The latter offers a frequency stability of 150 MHz on a time scale of several hours. The maximum laser drift during data acquisition was observed to be 0.02 pm. Thus, at the 2 m synthetic wavelength, this results in a maximum synthetic wavelength error of 200 mm. Random noise of 20 mm at the largest used synthetic wavelength of 2 m requires multiple synthetic wavelengths to get down to micrometer precision: Eight nested synthetic wavelengths from 2 µm to 2 m and numerical refocusing of the hologram were used to evaluate a milled sample with multiple step heights, machined on a Hermle C32U machine tool. Ten repetitive measurements confirm a machining uncertainty of 9 µm for this sample at its maximum step height of 10 cm.
Materials with magnetic shape memory (MSM) are promising candidates for application in next generation devices, such as actuators and switching valves. They exhibit elongation and contraction in a magnetic field and allow to achieve fast switching times in the order of milliseconds while maintaining high positioning precision over millions of cycles. Studying and developing applications using these materials creates a need for fast and accurate methods for analyzing their shape and deformations. We present a technology that utilizes capabilities of two interferometric methods - digital holography (DH) and electronic speckle pattern interferometry (ESPI). While digital holography enables high-precision 3D measurement of the object surface, electronic speckle pattern interferometry provides data on high-frequency deformations with nanometer accuracy. Combining both techniques allows to obtain comprehensive information about the morphology and dynamics of samples.
Using digital holography in camera-based interferometers, 3D surface topography can be measured extremely quickly and with sub-wavelength precision. Using spatial phase-shifting, a single camera image is sufficient to reconstruct complexvalued wavefronts for multiple wavelengths. Recently, measurements on moving objects were demonstrated using setups with 1× magnification. Increasing the lateral resolution by implementing larger magnification in a microscopic setup would open up new application fields, but the larger numerical apertures (NA) of microscope objectives make the acquisition even more sensitive to motion. In this work, we show the first microscopic setup, measuring objects moving at several mm/s using two-wavelength holography. Despite the large NA of 0.42 of the 10×-objective in our setup, measurements can be acquired at 75 mm/s and beyond. Using two lasers emitting slightly different wavelengths (637.76 nm and 632.87 nm), a maximum height difference of 41.3 μm can be detected unambiguously. One single image covers a lineshaped measurement area of 3.7 mm × 0.2 mm with a lateral pixel pitch of 0.47 μm. In order to inspect larger objects, single frames can be stitched together, permitting an infinite measurement area in the direction of motion. Gap-free stitched measurements are limited to 75 mm/s due to the framerate of the camera. Measurements of the groove depth averaged over several pixels on a groove standard show a repeatability exceeding 10 nm at 35 mm/s and 20 nm at 75 mm/s.
Multiwavelength digital holography enables precise and fast 3D height measurements of rough surfaces. To inspect objects during motion would enlarge the range of applications enormously. In this work the limits of this technique with respect to velocity and inclination angles are studied for linearly moving as well as for rotating objects. We demonstrate measurements on surfaces with inclination angles of up to 40° , moving linearly with a velocity of 2 mm/s, providing 2 μm accuracy, and on a rotating cylinder with circumferential speed of 10 mm/s, we achieve 1.1 μm precision. All measurements are conducted with less than 1 mW of continuous-wave laser light, so the object moves several micrometers during exposure time.
Multiwavelength digital holography on moving objects enables fast and precise inline-measurements of surface pro files. Due to the use of multiple wavelengths, optically rough surfaces with structure heights in the micrometer range can be mapped unambiguously. In this work we explore the influence of the object velocity on height measurements on inclined surfaces. We show measurements using spatial-phase-shifting holography employing two wavelengths and object velocities of up to 90 mm/s with eye-safe cw-lasers with less than 1 mW of laser light. Despite motion blur exceeding the mean speckle size, reliable height measurements can be conducted at these velocities. The height map of a metal cone with two different slope angles (1° , 10° ) is measured at an exposure time of 2 ms. Using line shaped illumination, each frame yields a height map of approximately 2 x 17 mm2. The overlap between the frames allows averaging as the image is put together, improving data quality. The mean repeatability of the height information in the investigated setup is better than 4.5 µm at a synthetic wavelength of 214 µm.
Broadband infrared spectroscopy employing optical parametric oscillation in bow-tie cavities, including a periodically- poled lithium niobate (PPLN) crystal, is well known. We demonstrate, however, that such spectroscopy is also possible using 2-mm-size monolithic whispering gallery resonators (WGRs). This is achieved in a radially-poled WGR by controlling wavelength tuning despite triple resonance of pump, signal, and idler light. Simulated and measured tuning characteristics of the Type-0 OPOs, pumped at about 1 μm wavelength, coincide. Tuning branches, which are crossed or curved at degeneracy, are present over a spectral range of up to 0.9 µm. As a proof-of-principle experiment, we show that all spectroscopic features of ethanol can be resolved using the idler light between 2.2 and 2.55 μm.
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