Differential Confocal Measurement Method Research Articles

A normal tracking differential confocal measurement method for multiple geometric parameters of hemispherical shell resonator with a common reference

This paper proposes a normal tracking differential confocal measurement method for the inside and outside surface profiles, shell thickness uniformity, and central asymmetry of inside and outside surfaces of the hemispherical shell resonator (HSR). A differential confocal technique with high-transmittance focusing ability is used to measure a single point on the inside and outside surfaces of the HSR. The normal alignment measurement technique is used to accurately measure the inside and outside surfaces and shell thickness of the HSR with a common reference in one measurement process. The HSR is step-rotated to synchronously collect information on the inside and outside surfaces, and using the differential confocal sensor to measure the different normal-section profiles. The experimental results indicate successful measurement of HSR central asymmetry. The repeated measurement accuracy for the inside and outside surface profiles and thickness uniformity is better than 30 nm.

Differential confocal measurement of microstructure surface topography based on centering error optimization and wavelet threshold denoising

This paper proposes a precise differential confocal measurement method based on centering error optimization and wavelet threshold denoising (DCM-COWD), which can realize non-contact measurement of microstructure surface topography with high precision and efficiency. Firstly, the influence of pinhole diaphragm size and offset on axial response curve is analyzed through multiple experiments to obtain the optimal pinhole diaphragm diameter and offset of the differential confocal measurement system. Secondly, a probe centering method based on grating rotation successive approximation is designed, in order to reduce the centering error of 3D topography measurement during the spiral scanning process. Thirdly, a translation invariant (TI) wavelet threshold denoising algorithm based on median filtering and whale optimization (WOA) is proposed, which can improve the signal-to-noise ratio (SNR) and reduce the root mean square error (RMSE), and effectively suppress the Pseudo-Gibbs phenomenon of discontinuous points. Finally, the surface topography of a standard sample is measured based on DCM-COWD system. The measurement results show that the center error is within 0.75 μm and the maximum deviation is within 1.45 %. The measurement results show high accordance with the commercial white light interferometer, and the processing speed is improved by 4.17 times. The proposed method has great application potential for the precise and efficient measurement of 3D surface topography.

Differential confocal measurement method of linear scanning based on dual linear array camera

针对目前微观三维表面形貌测量技术中存在检测效率和测量范围较小的问题，提出一种基于双线阵相机的线扫描差动共聚焦三维形貌测量方法。采用线扫描光源，利用线扫描拼接算法，分别合成焦前和焦后图像；再利用差动算法获得样本位于测量区域的差动图像，结合预先刻度的轴向响应曲线，完成样本的三维形貌还原，实现大范围高效测量。实验结果证明：基于双线阵相机的线扫描差动共聚焦三维形貌测量方法在相同时间内测量范围和测量效率分别是白光干涉仪的16.48倍和6.59倍，并且该方法在测量过程中不需要停顿，只需实现一次对焦，就可以对样品进行连续不间断的扫描检测。研究成果为满足智能制造中在线在位、实时高效、大范围测量的检测需求提供依据。

Differential confocal measurement for surface topography with microstructures based on spiral scanning and wavelet filter.

This paper proposes a novel spiral-scanning laser differential confocal measurement method (SSLDCM) for fast and precise measurement of surface topography with microstructures. Spiral plane scanning is used to eliminate frequent acceleration and deceleration problems in traditional raster-scanning differential confocal measurement systems and helps to keep the measuring process efficient and stable. To solve the problem of uneven sampling distribution during spiral scanning, a variable sampling rate method is adopted to distribute the sampling points at equal intervals, which would help to reduce the time of the 3D imaging process. A denoising method based on an adaptive wavelet threshold is proposed to filter the existing noise during the measuring process. An experimental measurement platform based on SSLDCM is constructed, and the axial response curve is tested and analyzed. The linear region range of the experimental platform reaches 13 µm, and the slope is about 164.15 mV/µm. In addition, the measurement results of a silicon wafer specimen by SSLDCM show good consistency with a commercial high-precision microscope, and the largest deviation is less than 2.71%. The SSLDCM has great potential to be used in various noncontact surface measurement applications with high efficiency and accuracy.

A measurement method for three-dimensional inner and outer surface profiles and spatial shell uniformity of laser fusion capsule

We propose a novel laser differential confocal measurement method with high spatial resolution for the simultaneous measurement of 3D inner and outer surface profiles and spatial shell uniformity of capsules. The zero-crossing point of characteristic curve of a laser differential confocal sensor (LDCS) exactly corresponds to the focal point; therefore, the proposed method performs high-precision and fast fixed-focus measurements on the vertexes of the surfaces of the capsule along its radial direction using the sensor. Then through the precision rotation measurement of the circumference trace and multi-directional meridian contour measurement of the capsule around the horizontal axis, the simultaneous high-precision measurement of 3D inner and outer surface profiles and spatial shell uniformity of the capsule can be achieved. To the best of our knowledge, the proposed method is the first to achieve high-precision and non-destructive measurement of 3D inner and outer surface profiles and spatial shell uniformity of the capsule simultaneously, and it provides a novel technical approach for achieving high-precision and non-destructive measurement of geometric parameters of ICF capsules.

High-precision laser differential confocal measurement method for multi-geometric parameters of inner and outer spherical surfaces of laser fusion capsules.

We propose a well-integrated, high-efficiency, high-precision, and non-destructive differential confocal measurement method for the multi-geometric parameters of the inner and outer spherical surfaces of laser fusion capsules. Based on the laser differential confocal measurement system with high tomography fixed-focus ability and high spatial resolution, the proposed method is used to perform the fixed-focus trigger measurement of the outer vertex, the inner vertex, and the spherical center of the capsule. From the rotation measurement around the Y-axis and the transposition measurement around the Z-axis, the inner and outer diameters, the three-dimensional inner and outer profiles, the shell thickness uniformity, and the shell non-concentricity of the capsule are measured with high precision and no damage. To the best of our knowledge, this is the first method to achieve the high-precision measurement for the multi-geometric parameters of the capsule inner and outer spherical surfaces with the same instrument.

Laser differential confocal measurement of the outer surface profile of a laser inertial confinement fusion capsule

A new laser differential confocal measurement method with a high axial resolution and insensitivity to surface inclination is proposed for high-precision measurement of the outer surface profile of a laser inertial confinement fusion (ICF) capsule. This method uses an eccentricity automatic adjustment system to rapidly adjust the capsule’s centre to be aligned with the centreline of the rotary shaft. The capsule is then rotated by a high-precision rotation system, and the laser differential confocal measurement system (LDCS) radially measures the geometrical coordinates of each sampling point position in order to obtain the outer surface profile data of the ICF capsule. The proposed method achieves high-precision, non-contact, automatic, and rapid measurement of the outer surface profile of an ICF capsule. Theoretical analyses and preliminary experiments indicate that for the proposed method the measurement repeatability for the outer surface profile of an ICF capsule can reach 20 nm.

Approach to higher spatial resolutions in a laser probe measurement system using a phase-only pupil filter

Phase-only pupil-filtering differential confocal measurement, a new approach, is proposed to improve the spatial resolution of a laser probe measurement system (LPMS) for ultraprecise measurement of microstructural workpieces. The proposed approach uses a lateral superresolution pupil filter for sharpening the main lobe of the Airy spot to improve the LPMS lateral resolution and uses the differential confocal measurement method to improve the LPMS axial resolution, thereby improving the LPMS spatial resolution. In addition to improving the spatial resolution, linearity, and antiinterference capability of a confocal measurement system, the proposed approach can be used for bipolar absolute measurement and improvement of the measurement range. Experimental comparison and analyses indicate that the lateral resolution of a phase-only pupil-filtering differential confocal system can be improved by 50% over that of an LPMS with the same parameters, and a lateral resolution better than 0.27 µm and an axial resolution better than 3 nm can be achieved when the wavelength of the incident beam is =632.8 nm, the numerical aperture of the measuring lens is NA=0.65, and uM=4. It is therefore concluded that the phase-only pupil filtering differential confocal measurement method is a new approach to a higher spatial resolution of LPMSs and can be used for ultraprecise measurement of surface microcontours and microdimensions.