Characterization of the displacement response in chromatic confocal microscopy with a hybrid radial basis function network

Optical path optimization of chromatic line confocal displacement sensor for high resolution and wide range

This study introduces the optical path-optimized dual-grating chromatic line confocal imaging (DG-LCI) technique for high-resolution and wide-range surface topography measurements. Chromatic line confocal imaging (LCI) finds extensive applications in high-speed 3D imaging of surface morphology, roughness analysis in industrial production, and quality inspection. A key advantage of LCI is its ability to achieve a large depth of focus, enabling the imaging system to measure a wide range in the Z direction. However, the challenge lies in the trade-off between the measurement range and resolution. Increasing the measurement range reduces the resolution, making it unsuitable for precise measurements required in industrial processing. Conversely, enhancing the resolution limits the measurement range, thereby sacrificing the advantage of LCI systems' broad measurement capabilities. Addressing this limitation, we propose a dual optical path dual-grating structure using a simplified and ingenious optical path optimization design. This design overcomes the challenge of sacrificing the millimeter-level measurement range while simultaneously improving the resolution. Rigorous simulations and experiments validate the effectiveness and validity of our proposed method.

Measurement of thickness and refractive index of transparent material synchronously based on chromatic confocal sensor

A defined refractive index is essential to measure the thickness of transparent materials with a chromatic confocal sensor (CCS). To overcome this limitation, a new measuring model is proposed by configuring a motor to drive the CCS for movement and placing a reflector behind the sample. This innovative approach enables the measurement of thickness and refractive index of transparent material synchronously through geometric calculations based on peak signals from different surfaces. Experimental results show that the model can achieve an average thickness measurement deviation of ±0.4µm and an average refractive index measurement deviation of ±0.005, making it highly suitable for industrial applications in thin film manufacturing sectors such as new energy vehicles, flexible displays, biomedicine, and more.

Design and Research of Chromatic Confocal System for Parallel Non-Coaxial Illumination Based on Optical Fiber Bundle

Conventional chromatic confocal systems are mostly single-point coaxial illumination systems with a low signal-to-noise ratio, light energy utility and measurement efficiency. To overcome the above shortcomings, we propose a parallel non-coaxial-illumination chromatic-confocal-measurement system based on an optical fiber bundle. Based on the existing single-point non-coaxial-illumination system, the optical fiber bundle is used as the optical beam splitter to achieve parallel measurements. Thus, the system can yield measurements through line scanning, which greatly improves measurement efficiency. To verify the measurement performance of the system, based on the calibration experiment, the system realizes the measurement of the height of the step, the thickness of the transparent specimen and the reconstruction of the three-dimensional topography of the surface of the step and coin. The experimental results show that the measuring range of the system is 200 μm. The measurement accurcy can reach micron level, and the system can realize a good three-dimensional topography reconstruction effect.

Improved calibration method of a four-quadrant detector based on Bayesian theory in a laser auto-collimation measurement system

Reliable and accurate calibration for a four-quadrant detector (QD) is a prerequisite for high-accuracy laser auto-collimation measurements. However, the calibration accuracy is limited largely by the non-linearity of QD, especially for large-range detection. To address this issue, an improved calibration method of QD based on Bayesian theory in laser auto-collimation measurement is proposed in this paper. First, the non-linearity problem of QD is analyzed, and for accurate calibration of QD, a high-precision identification model based on Bayesian theory is presented. An analytical expression between the output signal of QD and the position of the laser spot is established, and then a calibration system with laser drift compensation to avoid influences from the laser source as a datum is constructed. A series of experiments is conducted to verify the performance of the improved calibration method. The results reveal that the improved method can effectively enhance the calibration accuracy of QD and reduce the residuals in root mean square error by 86% compared to the 15-order polynomial fitting over a detection range of ±1mm. The comparison experiments also demonstrate that the proposed calibration method has advantages over the conventional method in terms of accuracy and robustness.

Asymmetry robust centroid localization in confocal microscopy

We present a centroid algorithm with asymmetry-robust error compensation for the peak position localization of asymmetrical axial response signals in confocal microscopy. Compared with the state-of-the-art algorithms, which are usually developed for symmetrical signals, our asymmetry robust centroid algorithm is found to have much smaller localization bias and higher precision for an asymmetrical confocal signal in numerical simulations and experiments.

Channeled imaging spectropolarimeter reconstruction by neural networks

Snapshot channeled imaging spectropolarimetry (SCISP), which can achieve spectral and polarization imaging without scanning (a single exposure), is a promising optical technique. As Fourier transform is used to reconstruct information, SCISP has its inherent limitations such as channel crosstalk, resolution and accuracy drop, the complex phase calibration, et al. To overcome these drawbacks, a nonlinear technique based on neural networks (NNs) is introduced to replace the role of Fourier reconstruction. Herein, abundant spectral and polarization datasets were built through specially designed generators. The established NNs can effectively learn the forward conversion procedure through minimizing a loss function, subsequently enabling a stable output containing spectral, polarization, and spatial information. The utility and reliability of the proposed technique is confirmed by experiments, which are proved to maintain high spectral and polarization accuracy.

Thickness Measurement for Glass Slides Based on Chromatic Confocal Microscopy with Inclined Illumination

Chromatic confocal microscopy is a widely used method to measure the thickness of transparent specimens. In conventional configurations, both the illumination and imaging axes are perpendicular to the test specimen. The reflection will be very weak when measuring high-transparency specimens. In order to overcome this limitation, a special chromatic confocal measuring system was developed based on inclined illumination. This design was able to significantly improve the signal-to-noise ratio. Compared with conventional designs, the proposed system was also featured by its biaxial optical scheme, instead of a coaxial one. This biaxial design improved the flexibility of the system and also increased the energy efficiency by avoiding light beam splitting. Based on this design, a prototype was built by the authors’ team. In this paper, the theoretical model of this specially designed chromatic confocal system is analyzed, and the calculating formula for the thickness of transparent specimen is provided accordingly. In order to verify its measurement performance, two experimental methodology and results are presented. The experimental results show that the repeatability is better than 0.54 μm, and the axial measurement accuracy of the system could reach the micron level.

Characterization of a chromatic confocal displacement sensor integrated with an optical laser head

In this paper, the results of experimental characterization of an optical system consisting of a chromatic confocal displacement sensor integrated with an optical laser head are presented. As a result of integration, the part of the optical path of the displacement sensor was combined with the optical path of the laser beam. Consequently, a working distance of 250 mm for the optical system was achieved. The main aim of the characterization was to determine the calibration curve, as well as the application parameters of the system. The methodology for spectral data processing with particular emphasis on the effectiveness of various extraction algorithms is presented. The Lorentzian fitting was considered as the optimal algorithm for the optical system. Consequently, a large measuring range of 10 mm was obtained with perfect linear tendency of the calibration curve. The optical system was characterized by high accuracy equal to ±0.11% of the measuring range, as well as 1 µm resolution. Moreover, the functionality of the system was verified on materials commonly used in laser processing and additive manufacturing. Finally, the system was validated through a comparative measurement with a laser profiler.

Locally adaptive thresholding centroid localization in confocal microscopy

We introduce an iteration-free approach, based on a centroid algorithm with a locally adaptive threshold, for nanometer-level peak position localization of the axial response signal in confocal microscopy. This approach has localization accuracies that are near theoretical limits, especially when there is a small number of sampling points within the discrete signal. The algorithm is also orders of magnitude faster compared to fitting schemes based on maximum likelihood estimation. Simulations and experiments demonstrate the localization performance of the approach.

Two-dimensional spectral signal model for chromatic confocal microscopy

In chromatic confocal microscopy, the signal characteristics influence the accuracy of the signal processing, which in turn determines measurement performance. Thus, a full understanding of the spectral characteristics is critical to enhance the measurement performance. Existing spectral models only describe the signal intensity-wavelength characteristics, without taking the displacement-wavelength relation into consideration. These models require prior knowledge of the optical design, which reduces the effectiveness in the optical design process. In this paper, we develop a two-dimensional spectral signal model to describe the signal intensity-wavelength-displacement characteristics in chromatic confocal microscopy without prior knowledge of the optical design layout. With this model, the influence of the dimensional characteristics of the confocal setup and the displacement-wavelength characteristics and monochromatic aberrations of the hyperchromatic objective are investigated. Experimental results are presented to illustrate the effectiveness of our signal model. Using our model, further evaluation of the spectral signal can be used to enhance the measurement performance of chromatic confocal microscopy.

Adaptive modal decomposition based overlapping-peaks extraction for thickness measurement in chromatic confocal microscopy

Accurate overlapping-peaks extraction plays a critical role in chromatic confocal thickness measurement of ultra-thin transparent film. However, the current algorithms usually appear as a perceptible extraction error resulting from the disturbing influence among peaks in the process of fitting the spectral axial response signal (sARS) of the two measuring surfaces. In this paper, we propose an adaptive modal decomposition method to extract multi peaks for the ultra-thin materials. With this method, the sARS can be firstly decomposed into several sub-modes, which can be used to obtain the peak wavelength of each measuring surface by the existing single peak extraction algorithms, such as the centroid method and Gauss fitting method. Monte Carlo simulations and experimental tests demonstrate that the proposed algorithm has significant improvements over the existing nonlinear fitting algorithms in terms of peak extraction accuracy and precision.

High-dynamic-range areal profilometry using an imaging, dispersion-encoded low-coherence interferometer

This work presents the design and characterization of an approach for areal surface profilometry with sub-nm axial resolution. The developed approach is based on a low-coherence interferometer enhanced by an dispersive element to control the axial resolution and measurement range. Optical path differences are detected by an imaging spectrometer where equalization wavelengths are determined as a basis for fitting spectra. This enables the acquisition of surface profiles with a length of up to 1.5 mm without mechanical scanning where a minimal resolution of 0.1 nm in an axial measurement range of nearly 80 µm was achieved. The resolution calculation was based on the standard deviation of measured feature heights. In addition to the system design, its capabilities are demonstrated on samples such as height standards.