Scanning White Light Research Articles

A robust white-light interference signal leakage sampling correction method based on wavelet transform

White-light scanning interferometry has become a well-established tool for measuring the microscopic three-dimensional morphology of objects. However, it is sensitive to external disturbances and suffers from leakage sampling, which compromises the accuracy of the measurements. To avoid the error, a robust correction method is proposed, based on wavelet transform, to find leakage sampling point (LSP), and the cubic spline interpolation method is used to complete the interference signal to achieve the signal correction. Further, phase evaluation at each pixel provides a high vertical resolution to characterize the microstructure. In addition, the proposed method exhibits a good denoising performance. Simulations and comparison experiments demonstrate the effectiveness of the proposed method.

Hilbert transform-based envelope substitution method for non-uniform sampling signal correction in white-light interferometry

White-light scanning interferometry is an effective and widely used technology for measuring the microscopic three-dimensional morphology of an object. However, it is easily affected by external disturbances and appears to have a non-uniform sampling problem, which reduces the measurement accuracy. In this study, an effective correction algorithm is presented, in which a Hilbert transform and a correlation analysis of the white light interference envelope curves, as well as the simulated ideal interference signal envelope, are employed for a robust and high precision signal correction. In addition, the proposed method is at least 4 times as accurate as a traditional method and achieves a high repeatability, which is analyzed through a simulation and contrast experiments.

Interference-based laser-induced micro-plasma ablation of glass

AbstractGlass is one of the most important technical surfaces for numerous applications in automotive, optical, and consumer industries. In addition, by producing textured surfaces with periodic features in the micrometre range, new functions can be created. Although laser-based methods have shown to be capable to produce structured materials in a wide amount of materials, due to its transparency large bandgap dielectrics can be only processed in a controlled manner by employing high-power ultra-short pulsed lasers, thus limiting the employable laser sources. In this article, an interference-based method for the texturing of soda-lime glass using a 15 ns pulsed (1 kHz repetition rate) infrared (1053 nm) laser is proposed, which allows fabricating different periodic patterns with micrometre resolution. This method consists on irradiating a metallic absorber (stainless steel) put in direct contact with the glass sample and inducing locally an etching process on the backside of the glass. Then, the produced plasma at the interference maxima positions leads to the local fabrication of well-defined periodic line-like and dot-like surface patterns. The produced patterns are characterised using white light interferometry and scanning electron microscopy.

A study ofthe surface microstructure and tool wear of titanium alloys after ultrasonic longitudinal-torsional milling

In order to obtain a better surface micro-morphology on the difficult-to-machine materials such as titanium alloys, the influence of ultrasonic parameters and cutting parameters on the tool path was analyzed theoretically under the ultrasonic longitudinal-torsional vibration. Ultrasonic longitudinal-torsional assisted milling (ULTAM) of titanium alloy was performed, based on a 20° slope. The effects of the ultrasonic parameters and cucuttintting parameters were studied on the surface morphology and surface roughness. The machined surface was examined using scanning electron microscopy (SEM) and white light interferometry. The characterization parameters of the three-dimensional surface micro-morphology including the arithmetic mean height (Sa), the surface skewness (Ssk), and the density of summits (Sds), were obtained under the corresponding cutting parameters. It was found that the ultrasonic amplitude exhibited a significant influence on the surface micro-morphology, followed by the spindle speed n, while the effect of the depth of cut (ap), and the feed amount per tooth (fz) was relatively less. Under the same cutting parameters for the surface, a comparative analysis for the friction and wear testing was performed between the ultrasonic milling and conventional milling (CM). The results showed that the ULTAM transformed the original ravines and anisotropic surface imperfections into an uniform and orderly scaly micro-texture, thus forming an isotropic surface texture in all directions. Furthermore, ULTAM transformed the surface into a fish-scale micro-texture, and thereby improved the surface quality. The fine and smooth surface morphology was conducive by shortening the running-in time, allowing the stable state of friction to be reached quickly, and thereby offering a higher stability in the friction process. `Compared to CM, the tool wear was reduced by 30%, due to the unique tool-work separation characteristics of the ultrasonic longitudinal-torsional assisted milling. Moreover, the unique intermittent cutting characteristics of the ultrasonic longitudinal and torsional milling not only reduced the cutting force and tool wear, but also inhibited the generation of the burr, thus improving the surface quality.

Fast template matching method in white-light scanning interferometry for 3D micro-profile measurement.

White-light scanning interferometry (WLSI) is an important measurement technique that has been widely used in three-dimensional profile reconstruction. Because of the effects of environmental noise and phase changes caused by surface reflection, existing WLSI algorithms have problems in measurement accuracy and measurement speed. Addressing these problems, this paper proposes a fast template matching method to determine precisely the zero optical path difference (ZOPD) position in the WLSI. Due to the uniform shape of the interference signals, a template interference signal can be obtained in advance by performing a least-square fitting or Fourier interpolation on an interference signal of one pixel. In the method, the ZOPD position is initially obtained by the centroid method. Then, the ZOPD position is determined by a precise matching process through moving the template interference signal on the measured interference signal. Through the two-step processes, the ZOPD position can be obtained precisely with much less time. The method was simulated and verified through the measurement of a spherical surface, a 1.8-µm-height standard step and a flip-chip substrate. The experimental results show that the proposed algorithm can achieve both high precision and fast measurement.

A robust surface recover algorithm based on random phase noise correction for white light interferometry

White light scanning interferometry is a very important technique for nanometrology. In this paper, a robust white light interference signal processing algorithm using random phase noise correction method is proposed. By analyzing the phase response of the correlgram mathematically, the phase noise is modelized by the combination of random noise and systematic deviation. The random noise can be corrected by assembling the least square result of phase information. As a result, the relationship between phase distribution and surface height is established in a more accurate way. The simulations show that the novel algorithm is capable of achieving robust measurements with standard deviation one tenth comparing to the scanning step length errors, and has an extremely high noise tolerance 14.25 nm with scanning step length 20 nm. In the experiments, a step height standard (VLSI, 182.7 ± 2.0 nm), a protected aluminum mirror and a drive shaft cover of aircraft engine were tested, where the repeatability error for the step height standard is 0.5 nm, the roughness repeatability of the mirror is only 0.03 nm, and the roughness repeatability of the cover is 5.14 nm, which proves the accuracy and robust of the algorithm.

Light-assisted drying for anhydrous preservation of biological samples: optical characterization of the trehalose preservation matrix.

Protein-based drugs have been developed to treat a variety of conditions and assays use immobilized capture proteins for disease detection. Freeze-drying is currently the standard for the preservation of proteins, but this method is expensive and requires lengthy processing times. Anhydrous preservation in a trehalose amorphous solid matrix offers a promising alternative to freeze-drying. Light assisted drying (LAD) is a processing method to create an amorphous trehalose matrix. Proteins suspended in a trehalose solution are dehydrated using near-infrared laser light. The laser radiation accelerates drying and as water is removed the trehalose forms a protective matrix. In this work, LAD samples are characterized to determine the crystallization kinetics of the trehalose after LAD processing and the distribution of amorphous trehalose in the samples. These characteristics influence the long-term stability of the samples. Polarized light imaging revealed that LAD processed samples are stable against crystallization during low-humidity storage at room temperature. Scanning white light interferometry and Raman spectroscopy indicated that trehalose was present across samples in an amorphous form. In addition, differential scanning microcalorimetry was used to measure the thermodynamic characteristics of the protein lysozyme after LAD processing. These results demonstrate that LAD does not change the properties of this protein.

Short-term impact of microimplant-assisted rapid palatal expansion on the nasal soft tissues in adults: A three-dimensional stereophotogrammetry study.

ObjectiveThe aim of this study was to evaluate changes in the nasal soft tissues, including movements of landmarks, changes in linear distances, and volumetric changes, using three-dimensional (3D) stereophotogrammetry after microimplant-assisted rapid palatal expansion (MARPE) in adult patients.MethodsFacial data were scanned using a white light scanner before and after MARPE in 30 patients. In total, 7 mm of expansion was achieved over a 4-week expansion period. We determined 10 soft tissue landmarks using reverse engineering software and measured 3D vector changes at those points. In addition, we calculated the distances between points to determine changes in the width of the nasal soft tissues. The volumetric change in the nose was also measured.ResultsAll landmarks except pronasale and subnasale showed statistically significant movement on the x-axis. Pronasale, subnasale, alar right, and alar left showed significant movement on the y-axis, while all landmarks except subnasale showed significant movement on the z-axis. The alar base width, alar width, and alar curvature width increased by 1.214, 0.932, and 0.987 mm, respectively. The average volumetric change was 993.33 mm3, and the amount of increase relative to the average initial volume was 2.96%.ConclusionsThe majority of soft tissue landmarks around the nasal region show significant positional changes after MARPE in adults. The nose tends to widen and move forward and downward. The post-treatment nasal volume may also exhibit a significant increase relative to the initial volume. Clinicians should thoroughly explain the anticipated changes to patients before MARPE initiation.

Surface quality evaluation of non-sintered powder layers in Selective Laser Sintering by 3D scanning

In Selective Laser Sintering, powders of various materials are deposited layer by layer in order to form 3D objects of complex geometries via sintering or melting by a concentrated laser source. The evaluation of the quality of the non-sintered powder layer is of paramount importance, because it is among the parameters that influence the mechanical properties of the finished part, such as tensile strength and density. In order to monitor the quality of the non-sintered powder layer various methods have been suggested that are either invasive resulting in the destruction of the layer, or unable to provide an accurate measure of the layer’s quality, notably estimation of surface roughness and layer thickness distribution. In this paper, these disadvantages are tackled by a method of optical evaluation of powder surface quality by means of 3D white light scanning. The process is fast, non-invasive and quantitative. A case study is provided to prove its applicability in a prototype SLS machine. The details of the method are explained and indicative surface roughness measurements are given, followed by ANOVA concerning contribution of recoating cylinder rotary speed as main control factor of the surface roughness of the compacted powder layer.

Measuring the Refractive Index and Its Two-Dimensional Distribution of Picoliter Microfluidics With the Scanning White Light Interference Microscopy Method

Measurements on refractive index and its distribution of microfluidics are widely applied in the field of biological detection, chemical analysis, material researches, medical diagnostics, and lab on a chip. Since the volume of the microfluidics is ultra-small, such as nanoliters or even picoliters, measuring the refractive index of microfluidics is challenging. We here presents an effective method to measure the refractive index and its two-dimensional distribution of picoliter microfluidics by the SWLI (scanning white light interference) microscopy technology. This method has the advantages of simplicity, low cost, non-contact, wide measurement range and high precision. We conduct the experiment to measure the refractive index of 2.4 picoliters liquid. The resolution is 10−4RIU (refractive index unit), and the relative error is 10−4 compared with the Abbe refractometer. We obtained the relationship between refractive index and concentration of glucose solution. In addition, we can measure the 2D (two-dimensional) distribution of RI (refractive index) of liquids. We can observe the liquid interface between deionized water and liquid paraffin clearly. Besides, we can observe the process of solute dissolution in the liquid and the change of refractive index.

Estimation of root inclination of anterior teeth from virtual study models: accuracy of a commercial software

BackgroundThe aim of the study was to assess the accuracy of commercially available software in estimating anterior tooth root inclination from digital impressions of the crowns of the teeth.Subjects and methodsFollowing sample size calculation and application of inclusion and exclusion criteria, 55 anterior natural teeth derived from 14 dry human skulls were selected. Impressions were taken and plaster study models were fabricated. Plaster models were scanned using the high-resolution mode of an Ortho Insight 3D laser scanner. The teeth on the digital scans were segmented and virtual roots were predicted and constructed by the Ortho Insight 3D software. The 55 natural teeth were removed from the dry skulls and scanned using the Identica extraoral white-light scanner in order to calculate their actual root angulation. The teeth were scanned twice, once to acquire the crown and the cervical part of the root, and a second time to acquire the remaining part of the root, including the apex. The two scanned segments were joined in software by superimposing them along their common part. The accuracy of the digital models generated by the Ortho Insight 3D scanner in predicting root angulation was assessed by comparing these results to the corresponding measurements of the 55 natural teeth. The long axes of the tooth models obtained from the software prediction and the scanning of the actual teeth were computed and the discrepancy between them was evaluated. The error of the methods was evaluated by repeating the measurements on 14 teeth and showed an acceptable range.ResultsThe predicted tooth angulation was found to differ significantly from the actual angulation, both statistically and clinically. The angle between the predicted and actual long axes ranged from 2.0 to 37.6°(average 9.7°; median 7.4°). No statistically significant difference was found between tooth categories.ConclusionsFurther investigations and improvements of the software are needed before it can be considered clinically effective.

Fiber optic white light interferometer for areal surface measurement

White light interferometers (WLI) are extremely powerful tools in the context of areal surface topography instrumentation. With lateral scanning mechanisms, larger objects with trans-scale structures are also measurable. However, the existing commercial WLIs are usually only applicable for off-line measurements in the laboratory. This paper presents a flexible WLI, in which the light source and the vertical scanning mechanism are separated from the interference probe using fiber-optic components. As a result, the interference probe is more compact, and portable for on-machine measurement. Meanwhile, a comprehensive algorithm that combines the improved gravity center method and phase-shifting technique is proposed for signal processing, which can balance computation time and noise robustness. The simulation results show the effectiveness by comparing with the traditional white light scanning interferometry methods. A 1000 nm step height standard was used as a test specimen to evaluate the proposed system and algorithm. The experimental data show that the measurement results are basically in good agreement with the calibrated value.

Evaluation of holding handheld scanner on muscle activity, heart rate variability, and model accuracy in industrial applications

PurposeThe capabilities of handheld scanners have been investigated previously in different applications, without considering its impact on the accuracy of model, heart rate variability (HRV), subjective measurements, and worker muscular activity. The present study aims to evaluate the impact of using two handheld scanners, elevation, and orientation of object that is scanned on the basis of aforementioned parameters. Design/methodology/approachTo achieve these objectives, electromyographic measurements of six muscles were obtained. Moreover, dependent variables include; NASA task load index (NASA-TLX) as a subjective measure, HRV, and accuracy of the model; while, the scanner-type (laser versus white light scanner), target orientation (0° versus 45°), and elevation (50 cm versus 80 cm table height) were the independent variables. FindingsThe results of the within-subject design indicated that a light scanner exhibited less accurate performance, lower cardiac cost, less subjective NASA-TLX, and less task execution time than the laser scanner. Originality/valueWhen using the laser scanner, to reduce muscle contraction and decrease the subjective NASA-TLX, the target elevation and orientation angle with respect to the horizontal plane must be 80 cm and 45°, respectively. However, when using the light scanner, to reduce muscle contraction and the subjective NASA-TLX, the target elevation and orientation angle with respect to the horizontal plane must be 50 cm and 0°, respectively. Furthermore, cardiac rhythms were less stressed when using the laser scanner at a 50 cm target elevation and light scanner at an 80 cm target elevation.

On the wear behavior and damage mechanism of bonded interface: Ceramic vs resin composite inlays

Advances in adhesive technologies have increased indications for the use of inlays. Decrease in the bonded interface integrity due to wear has been cited as the main cause of its failure. However, this process of interface degradation and the influence of inlay material on damage mechanism appear to be poorly understood. Thus, we aimed to compare the wear behavior and interface damage between ceramic and resin composite inlays bonded to enamel under sliding contact and use the experimental findings to support recommendation of the appropriate inlay material. Bonded interface specimens involving tooth enamel and either ceramic or resin composite inlays were prepared and subjected to reciprocating wear tests up to 5×104 cycles. The wear track profiles and morphologies were characterized after increments of cyclic sliding contact using white light interferometry and scanning electron microscopy, respectively. Optical microscopy was used to evaluate sub-surface cracks and their propagation within the samples. A finite element analysis was used to analyze the stress distributions of the bonded interfaces. Composite inlays showed higher wear depth than the ceramic in the early stage (N ≤ 5×102 cycles), while no significant difference was found at the later stage. For ceramic inlay a greater portion of the contact load was concentrated in the ceramic structure, which facilitated cracks and chipping of the ceramic inlay, with rather minimal damage in the adjacent interface and enamel. In contrast, for the resin composite inlay there was larger stress concentrated in the adjacent enamel, which caused the development of cracks and their propagation to the inner enamel. The restoration material could contribute to the stress distribution and extent of damage within enamel-inlay bonded interfaces. A tough ceramic appears to be more effective at protecting the residual dental tissue.

Information depths of analytical techniques assessing whitefish otolith chemistry

The elemental composition of otoliths provides historical information on migration and provenance of fish. Due to the complex structure of the otoliths, the depth from which the elemental information originates has to be considered. In this work, three analytical methods often used to assess otolith chemistry are compared. The information depths are calculated for µ-XRF and PIXE and measured for LA-ICP-MS. The information depth in PIXE depends on the energy of the incident particles and on the element to be analysed while in XRF it depends mostly on the element to be analysed as the energy of the incident X-rays usually is high enough to excite atoms at depths of several hundreds of micrometres. If we assume that the otolith is exposed to X-rays from a Rh-tube (20.16 keV) about 50% of the detectable Sr(Kα) X-rays will be emitted from a depth ranging from 0 to 114 µm. In the case of PIXE with 3 MeV protons the corresponding range is 0–15 µm. The information depths in LA-ICP-MS were determined by measuring the depth of the laser-ablated craters or trenches that remained on the otolith surface. The depths measured with a scanning white light interferometer (SWLI) were found to be about 40 µm (spots) and 11 µm (trenches).The correlation between strontium concentrations obtained by spot analysis of whitefish otoliths with PIXE and LA-ICP-MS was excellent (92%), although different spot sizes were used. A comparison of strontium concentration profiles measured with µ-XRF and LA-ICP-MS showed that the higher information depth of µ-XRF in combination with the 52 degree angle of the incident X-rays smoothens out the sharp edges seen in the LA-ICP-MS profiles. Otoliths from whitefish captured in the Baltic Sea (n = 30) were analysed in this comparison of methods. Among these whitefish there are sea spawners, river spawners and stocked fish with different life history and different otolith chemistry.

Selective laser ablation of CFRP composite to enhance adhesion bonding

ABSTRACTIn this work, the effect of picosecond laser radiation on ablating carbon fiber-reinforced polymer (CFRP) composite surface to increase the adhesive bond strength was analyzed. The experiment was conducted using a picosecond laser of 3 W average power with 532 nm wavelength, working at the 20kHz frequency. Effect of process parameters (scanning speed, beam overlap, and number of passes) on surface treatment of CFRP was analyzed and optimized for maximum shear strength using response surface methodology (RSM). Single lap shear test was performed on adhesively bonded laser-treated specimens using the optimized process parameter (for maximum shear strength) and maximum roughness. Acoustic emission data was collected during the shear test to investigate and characterize the failure mechanism. The morphology of laser-treated surface and shear tested surface was explored using scanning electron microscopy (SEM) and white light interferometer (WLI).

Nanosecond laser texturing with hexagonal honeycomb micro-structure on Titanium for improved wettability and optical properties

Nanosecond laser provided an efficient and flexible solution for surface texturing on different metals. In this work, a hexagonal honeycomb model was designed on Titanium by laser processing. The surface texturing experiment was carried out by a nanosecond pulsed fiber laser with different scanning speeds. The Scanning Electron Microscopy and White Light Interferometer were used to characterize the surface morphology and roughness. Besides, the contact angle instrument and the Fourier Transform Infrared Spectrometer were used to obtain the effects of surface modification on wettability and optical properties of the Titanium. The wettability and optical reflectivity of the surface changed by controlling the side length of the hexagonal honeycomb model and the laser scanning parameters. The contact angle reached a maximum of 147.4°, the contact angle hysteresis was found to lie between 1.6° and 7.1°, and the optical reflectance for titanium surface decreased by more than 90% after laser processing. The approach presented in this study provided a feasible and efficient method to improve the wettability and optical absorption of titanium surface.

A Study of Scratch Speed Effects on Ductile–Brittle Transition in Silicon

A unique rotational double-taper scratching setup is used to study ductile brittle transitions in single crystal (100) p-type silicon using a conical diamond tool at room temperature and scratching speeds ranging between 0.1 m/s and 0.3 m/s. In such a setup, transition from brittle to ductile occurs twice in a single-tapered scratch, during tool entry and tool exit. A well-defined way to determine critical depth of cut via linear crack density per unit crack length is proposed. The scratches were studied using scanning electron microscopy (morphology) and white light interferometry (depth measurements). A comprehensive study of critical depth of cut, compiled from the literature together with data from this study, with scratching speeds from very low to high shows that critical depth of cut decreases from very low scratch speeds to medium scratch speeds and then increases again at very high scratch speeds. An inference from this study is that diamond turning should be conducted at higher cutting speeds than being undertaken today to make use of larger critical depths of cut.

Vibration Compensated, Scanning White Light Interferometer for In Situ Depth Measurements in a Deep Reactive Ion Etcher

A scanning white light interferometer with a long working distance and active vibration cancellation was designed and tested to enable in situ depth measurements in a deep reactive ion etcher. A superluminescent diode is used as the light source, enabling high-intensity illumination of the sample inside the etcher’s vacuum chamber. An additional laser beam co-propagates with the white light beam and is used to sense changes in the optical path length difference by observing the beam’s interference signal on a photodiode. The photodiode signal is used as feedback to control the position of the interferometer’s reference mirror which is mounted on a piezo actuator. By locking the position of the reference mirror to a zero crossing of the laser interference signal, during a scan, the reference mirror moves in discrete multiples of the laser wavelength. This design choice has the added advantage of tying the stability of the scan distance and step size to the laser wavelength stability. The prototype instrument is capable of imaging over a 1 cm $\times1$ cm field of view with a 1- ${\boldsymbol \mu }\text{m}$ depth resolution, 20- ${\boldsymbol \mu }\text{m}$ lateral resolution, and 30-cm working distance. The feedback system reduces the amplitude of vibrations by more than 10 dB out to 1 kHz. [2018-0263]

Structuring of Bioceramics by Micro-Grinding for Dental Implant Applications.

Metallic implants were the only option for both medical and dental applications for decades. However, it has been reported that patients with metal implants can show allergic reactions. Consequently, technical ceramics have become an accessible material alternative due to their combination of biocompatibility and mechanical properties. Despite the recent developments in ductile mode machining, the micro-grinding of bioceramics can cause insufficient surface and subsurface integrity due to the inherent hardness and brittleness of these materials. This work aims to determine the influence on the surface and subsurface damage (SSD) of zirconia-based ceramics ground with diamond wheels of 10 mm diameter with a diamond grain size (dg) of 75 μm within eight grinding operations using a variation of the machining parameters, i.e., peripheral speed (vc), feed speed (vf), and depth of cut (ae). In this regard, dental thread structures were machined on fully sintered zirconia (ZrO2), alumina toughened zirconia (ATZ), and zirconia toughened alumina (ZTA) bioceramics. The ground workpieces were analysed through a scanning electron microscope (SEM), X-ray diffraction (XRD), and white light interferometry (WLI) to evaluate the microstructure, residual stresses, and surface roughness, respectively. Moreover, the grinding processes were monitored through forces measurement. Based on the machining parameters tested, the results showed that low peripheral speed (vc) and low depth of cut (ae) were the main conditions investigated to achieve the optimum surface integrity and the desired low grinding forces. Finally, the methodology proposed to investigate the surface integrity of the ground workpieces was helpful to understand the zirconia-based ceramics response under micro-grinding processes, as well as to set further machining parameters for dental implant threads.