Special issue-Precision optical metrological testing

Dual-wavelength interferometric diagnosis of double-pulse laser induced aluminium plasma
Zhang Chuhui, Lu Jian, Zhang Hongchao, Gao Lou, Xie Zhijian
2022, 51(2): 20210892. doi: 10.3788/IRLA20210892
[Abstract](200) [FullText HTML] (109) [PDF 1396KB](31)
Double-pulse laser-induced plasma has broad application prospects and development space in laser processing, detection of element, material removal and other fields, so it is of great significance to diagnose it. The time evolution law of plasma was obtained by two-wavelength interferometry, to study the effect and mechanism of plasma induced by delayed double-pulse laser. A dual-wavelength interference diagnosis system based on the Mach-Zehnder interferometer was established. It had ability to acquire interferogram of the double-pulse laser-induced plasma. By processing and analyzing the interferogram, the evolution law of plasma electron density with the delay time of double-pulse laser was obtained. The result shows that the effect of the second pulse laser on the plasma electron density first enhances and then weakens with the prolongation of the delay time between double-pulse laser. Among them, when the delay time of the double-pulse laser is 10 ns, the enhancement effect on the plasma electron density is the strongest, when 30 ns, the average electron density in the central region can reach 6.49×1019 cm−3, which is 26% higher than that of single-pulse laser-induced plasma with the same energy. Meanwhile, the effect of delay time on the mechanism of secondary pulse laser was studied. The research results provide a reference for the optimization direction of double-pulse laser-induced plasma.
System error calibration for Φ300 mm vertical Fizeau interferometer based on liquid reference
Ma Zhiyao, Chen Lei, Zheng Donghui, Ma Haiying, Li Ruokun, Huang Chen, Hu Chenhui
2022, 51(2): 20210880. doi: 10.3788/IRLA20210880
[Abstract](200) [FullText HTML] (71) [PDF 2214KB](28)
In high-precision interference detection, the calibration of interferometer system errors was important. According to the structural characteristics of the vertical Fizeau interferometer, the liquid reference plane was used as a reference to compensate and calibrate the self-weight deformation and clamping deformation of the reference flat crystal, and calibrate its system error. Theoretically, the curvature of the liquid surface and the radius of the earth were the same, which can be regarded as a plane reference to calibrate the system error of the vertical structure interferometer. For the Φ300 mm vertical Fizeau interferometer, the influence of different liquid viscosity, thickness, interference cavity length and temperature was studied, and a reliable liquid reference plane reference was constructed. Through the liquid reference plane, the interferometer was guided to refer to the installation and calibration of the reference flat crystal, and its system error was compensated. The accuracy of the interferometery reached 0.035λ, which was better than λ/25. In order to further verify the reliability and accuracy of the liquid reference plane, repeatability experiments were carried out, and the liquid reference plane of Φ400 mm and Φ450 mm were used for comparison test. The deviation of the two calibration results was better than λ/100 (6 nm). The reliability and accuracy of the liquid reference plane were verified.
Review of design methodology for starting-point of freeform surface imaging optical system
Xu Ningyan, Chen Lu, Huang Jing, Zou Yutong, Yuan Qun, Gao Zhishan
2022, 51(2): 20210852. doi: 10.3788/IRLA20210852
[Abstract](354) [FullText HTML] (103) [PDF 1863KB](81)
Freeform surface provides more degrees of freedom for optical system and stronger capability of aberration correction to break the limitation of traditional surface characterization and system structure, which helps engineer to achieve large field of view, large aperture, miniaturization, and lightweight optical system design while improving the imaging quality. A favorable starting-point of freeform surface imaging optical system design can release the potential of freeform surface to correct aberration and improve the efficiency of system design. Comparing to coaxial optical systems, few examples can be referred to for freeform surface imaging optical system and theory for freeform is not perfect yet. Therefore, how to construct and solve a favorable starting-point of freeform surface imaging optical system is one of the frontier issues in the field of optical design. Based on the research experience of author team for many years, this review summarizes the current design methods of freeform surface imaging optical system design, and divides them into disturbing coaxial system method, directly solving method, and expending field or aperture method and stitching and fusion method, according to the construction principle of freeform surface. Finally, the problems to be solved in the design starting-point for freeform surface imaging optical system are analyzed and summarized.
Research on 3-D vision measurement technology based on line structured light rotating scanning and laser stripe repair
Ji Yunjing, Du Siyue, Song Yang, Li Zhenhua
2022, 51(2): 20210894. doi: 10.3788/IRLA20210894
[Abstract](233) [FullText HTML] (99) [PDF 1525KB](55)
Non-contact 3-D vision measurement is widely used in industrial manufacturing quality inspection. Aiming at the application scenario of industrial metal parts detection, a 3-D vision measurement scheme based on line structured light rotating scanning and laser stripe repair was proposed. Firstly, through the computer vision technology based on line structured light projection, the line structured light rotating scanning vision subsystem was designed, and the industrial camera, line structured light plane and rotating scanning central axis were calibrated; Then, aiming at the problem of missing data in the low gray area of the collected laser stripe image, a laser stripe center line extraction algorithm based on adaptive gray enhancement of the missing area was proposed, which effectively repaired the line structured light projection stripes of the tested parts; At the same time, using the line structured light 3-D vision measurement scheme proposed in this paper, the accuracy of the measurement system was evaluated by reconstructing the surface point cloud of the standard bat and calculating the diameter and spacing of the two balls. The accuracy of the measurement system was better than 0.06 mm; Finally, the shape of the outer contour of the metal hub was measured, and the maximum radius of the outer contour of the hub was calculated through the repeatability experiment. It is verified that the repeatability error is better than 0.03%. The experimental results show that this method can realize the 3-D measurement of industrial metal parts without damage, high efficiency and high precision, and make up for the defects of the contact 3-D measurement method.
Application of binary search and compressive sensing for rapid detection of defects inside laser ultrasound
Sun Qiang, Dai Lunan, Ying Kaining, Ni Chenyin
2022, 51(2): 20210810. doi: 10.3788/IRLA20210810
[Abstract](197) [FullText HTML] (93) [PDF 2057KB](34)
Laser ultrasonic inspection technology has a broad application prospect in the field of nondestructive testing due to its non-contact, high sensitivity and high spatial resolution characteristics. However, its practicality is limited by the long scanning time required for large area sweeping at high spatial resolution. To address the above problems, a binary search method was proposed to improve the detection speed, and a compressed sensing algorithm was used to represent the detected laser ultrasound signal as a linear weighted combination of wavelet bases, and finally the entire range to be measured was restored from the less real laser ultrasound signal obtained by binary search. Further, a laser ultrasonic scanning detection device for internal defects was built, the laser excitation of ultrasound was achieved by using a pulsed laser, the non-contact detection of ultrasound was achieved by Doppler vibrometer, and a fast detection of internal defects by laser ultrasound based on binary search and compression perception was achieved by moving the sample at a fixed excitation detection distance. The technique proposed in this paper not only has the characteristics of laser ultrasound such as non-contact, high sensitivity and high spatial resolution, but also can improve the detection efficiency. The experimental results show that it takes 6 min to determine the defect location on a 120 mm×30 mm×8 mm aluminum plate, compared with 14 min for point-by-point sweeping, which shortens the time required for in vivo defect localization
Measurement of translucent coating thickness based on pulsed thermography
He Tao, Shen Zhonghua, Chen Fei, Chen Li
2022, 51(2): 20210890. doi: 10.3788/IRLA20210890
[Abstract](171) [FullText HTML] (92) [PDF 1752KB](26)
To measure the thickness of translucent coating quickly and accurately, the method based on pulse thermography was proposed. A simplified theoretical semi-infinite model of pulsed infrared heating conduction for translucent coating and a two-layer physical model of pulsed infrared heating for translucent coatings were established. The theoretical analysis and numerical results show that the thickness of the translucent coating is linearly proportional to the peak time of the surface temperature in logarithmic coordinates, through which the thickness of the translucent coating can be derived without spraying black ink on the surface of the sample to avoid the effect of translucency. A translucent coating specimen with continuously varying thickness was fabricated and tested with a flash thermographic system. The result of the measured thicknesses is within 5% of the actual values. It demonstrates that the technique has the potential for fast and non-contact measurement of the thickness for translucent coatings.
Design of multi-channel LED constant current driver in radiometric calibration light source
Li Haixing, Sui Xiubao, Yao Zheyi, Chen Qian, Gu Guohua
2022, 51(2): 20210902. doi: 10.3788/IRLA20210902
[Abstract](141) [FullText HTML] (53) [PDF 1755KB](40)
In order to deal with the influence of the current stability controllability of LED array in integrating sphere radiometric calibration light source system on the spectrum matching degree at the opening of integrating sphere, a multi-channel, high-precision and high-stability LED current driving circuit was designed. This circuit was a voltage controlled constant current driving circuit, which can realize the linear control of LED array driving current by means of analog dimming. It took FPGA as the control core, read and wrote the AD5371 chip register through SPI interface, and realized high precision control of LED driving current through AD5371 digital-to-analog conversion circuit. The temperature control of LED array was realized based on circulating liquid refrigeration equipment and sink refrigeration base. The experimental results show that the current of LED array can be continuously and linearly adjustable in the range of 0-1 050 mA, and the current adjustment accuracy can reach 0.14% of the range. When the LED lamp holder temperature is controlled at 10 ℃, the LED output light spectral stability is 0.2%.