More info about this article: http://www.ndt.net/?id=22672 Performance Improvement of a 3D Stereo Measurement Video Endoscope by Means of a Tunable Monochromator In the Illumination System Alexander S. Machikhin 1,2,3, Alexey V. Gorevoy 1,3, Vladislav I. Batshev 1,3, Demid D. Khokhlov 1,3, Valentin A. Kaloshin 2 and Alexey M. Perfilov 2 1 National Research University Moscow Power Engineering Institute, Russia 2 NPO ENERGOMASH named after Academician V.P. Glushko, Russia 3 Scientific and Technological Center of Unique Instrumentation RAS, Russia Abstract Industrial video endoscopes are the main tool for remote visual inspection and quantitative characterization of the defects on the hard-to-reach surfaces. We propose a technique to increase the effectiveness of existing stereo measurement videoscopes based on a tunable monochromatization of the illumination. Selection of a proper narrow band (NB) of light spectrum allows high contrast visualization of the defects and better precision of its geometrical parameters measurement. In this paper, we demonstrate experimentally that camera calibration under wide-band () light may lead to significant inaccuracies if NB illumination is used for 3D measurements. As a result, it is necessary to calibrate the system either in a certain NB, where the inspection is the most effective, or in a few NBs with further interpolation of calibration parameters for operation in any selected spectral band. In this case, calibration and image processing software of the existing video endoscopes have to be modified. The results may be useful for the development of new remote visual inspection tools for fluorescent penetrant inspection and analysis of local spectral properties. 1. Introduction Nowadays, video endoscopes are the main tool for remote visual inspection (RVI) of inner cavities in industrial objects (engines, steam generators, heat exchangers, etc.). It is a mandatory procedure at the stages of their manufacturing, assembling, and testing. Endoscopic probe can be delivered to the inspected area through small-diameter holes and capture high resolution images to effectively solve the main problems of nondestructive testing (NDT), i.e. to assess the condition of the surface, to reveal and classify existing cracks, flaws and contaminations, and to evaluate their geometrical parameters (1,2). There is a few methods for endoscopic measurements (stereo, shadow, multi-point, phase) implemented in modern video endoscopes in terms of both hardware and software. The mostly used approach is the utilization of a miniature prism-based optics and matching two images of a surface obtained from two different points. Calibrating such a system and processing the captured images using stereoscopy algorithms allow one to calculate the 3D coordinates of object points and carry out geometrical measurements (3). In practice, there are applications when contrast visualization of the defect is possible only using fluorescence, Raman or other spectral imaging techniques. It can be implemented via spectral filtration of the illumination irradiated by the light source. Creative Commons CC-BY-NC licence https://creativecommons.org/licenses/by-nc/4.0/
For example, fluorescent penetrant inspection (FPI) requires selection of ultraviolet light to ensure good contrast between the glow emitted by the penetrant in the defected areas and the unlit surface of the material (4). For the material identification, the reflectance spectra in the wide range is needed (5). In this case, it is especially important to analyze both local spectral properties of the object and spatial distribution of these properties. Therefore, spectral imaging systems are very informative and multifunctional instruments for real-time analysis in these applications. Tunable monochromatization may be used for consequential collection of spectral images and obtaining reflectance spectrum in each image pixel. Such systems based on AO tunable filters (AOTFs) additionally provide fast spectral access, as well as high spectral and spatial resolution (6). Compact design without moving or adjustable elements facilitates development of portable spectrally tunable monochromators compatible with endoscopic light sources (7). Combination of both features in a video endoscope, i.e. registration of stereoscopic images and selection of illumination wavelength, can significantly improve its detection (Fig. 1) and measurement capabilities in various applications. It may be implemented using two components described above: prism-based optical system and AOTF-based monochromator (Fig. 2). Figure 1. Stereoscopic image obtained by a video endoscope under (left) and NB at 486 nm (b) light Inspected surface Stereo tip adapter Probe with integrated lens Spectral stereoscopic images data Image processing Measurements Light guide AOTF light source Figure 2. A concept of a video endoscope with a prism-based stereoscopic tip and AOTF-based light source The practical usage of this approach requires some modifications of the software. Conventional geometrical calibration procedure is implemented for white-light illumination limited only by spectral width of the light source and sensitivity of the sensor (8). Such calibration is not optimal for NB light and may limit the achievable measurement accuracy due to inevitable chromatic aberrations of the optical system. In this paper, we experimentally demonstrate that calibration in the light leads to significant errors of the geometrical measurements of the defects illuminated by NB light and discuss possible solutions of this problem. 2
2. Experimental setup In our experiments, we used video endoscope Mentor Visual IQ (GE Inspection Technologies) with forward view stereo tip adapter. The probe has a diameter 6.1 mm and a 1/6" CCD image sensor with 440000 pixels. A prism-based stereo adapter provides two registration channels with the fields of view 55 /55. For calibration and tests, we used a сhessboard pattern with a grid of 25 25 markers. Due to sufficient range of distances and magnifications, three samples of the calibration target with 0.5 mm (smallsized), 1 mm (middle-sized) and 2 mm (large-sized) distance between markers (the size of chessboard square) have been manufactured. The distal end of the endoscope was fixed on a mechanical stand that allows adjustable movement along z-axis and rotation around y- and z-axis. The axis of the probe was approximately coincident with the z-axis of the stand. Another mechanical stand was placed in front of the probe to mount a calibration target. To imitate spectrally tunable illumination, we used 4 NB interchangeable glass filters with maximal transmission at 434 nm, 503 nm, 672 nm and 704 nm located in front of white-light LED source (Fig. 3). Full width at half maximum of each filter is about 10 nm. The built-in light source of the video endoscope was switched off during the experiments. We captured the calibration set of images placing calibration targets at the distances from 8 to 40 mm from the probe and rotating them around horizontal and vertical axes about 30 using the same positions under LED illumination and with each filter. Next, we calibrated the video endoscope using two different camera models: the conventional pinhole model and the ray-tracing one (8). To compare the efficiencies of the models correctly, we used the same set of test-object images. The coordinates of the test-object grid points in the images were determined automatically (3,8). 3. Data analysis Figure 3. Scheme of experimental setup To check the effectiveness of calibration in narrow spectral bands, we used the translation stage to shift the distal end of the endoscope along z-axis and captured images with 1 mm step to obtain the test set of images in the range from 8 to 16 mm. The plane of calibration target was set approximately perpendicular to the axis of the probe. Then we made the 3
same linear measurements (the distance between chessboard corners in xy-plane and the shift along z-axis between the consecutive images) in and in each of NBs using every set of calibration parameters we obtained. Tables 1 and 2 show the calculated mean values of absolute difference between real and measured distances obtained at each combination of spectral ranges used at calibration and measurement stages. The data corresponds to the medium-sized calibration target at the 15 mm, so the nominal value of the measured distances is 1 mm. The minimal error is achieved when the same spectral range is used at both stages (the diagonal cells of the tables shown in green). If the calibration parameters are applied to the measurements under NB illumination (the rightmost column), the inaccuracies may reach ~20% of the nominal value (for measurement at 434 nm). However, the calibration can be successfully used for the measurements at 503 nm, as this wavelength is close to the maximum of the sensor sensitivity. The spectral ranges for 672 nm and 704 nm filters are very close, so the calibration parameters for these filters can be used interchangeably. The corresponding cells with low error values are shown in yellow. We have chosen the 15 mm distance to the target for this example as the upper bound of the range used in practice where both camera models still provide similar inaccuracy values (3,8). Table 1. Mean value of absolute difference between real and measured values of the 1 mm distance in xy-plane Ray-tracing model Pinhole model Measurement 434 nm 503 nm 672 nm 704 nm 434 nm 503 nm 672 nm 704 nm 0,27 0,27 0,28 0,29 0,19 0,20 0,06 0,37 0,36 0,39 0,38 0,21 Table 2. Mean value of absolute difference between real and measured values of the 1 mm distance along z-axis Ray-tracing model Pinhole model Measurement 434 nm 503 nm 672 nm 704 nm 434 nm 503 nm 672 nm 704 nm 0,23 0,25 0,25 0,19 0,37 0,31 0,07 0,39 0,33 0,18 0,09 0,18 0,04 0,09 The experimental results have confirmed the theoretical conclusion that applying calibration parameters is not optimal if NB illumination is used for the measurements. For accurate quantitative characterization of the defects, it is necessary to calibrate the 4
system either in a certain NB, where the inspection is the most effective, or in a few NBs with further interpolation of calibration parameters for operation in any selected spectral band. Therefore, the calibration and image processing software of the existing video endoscopes has to be modified. 4. Conclusion One of the ways for contrast visualization of the defects is narrowing the spectral band of light reflected from them. Using this feature for stereoscopic measurements may raise the efficiency of the RVI system significantly. NB imaging provides a reduction of chromatic image aberrations and, therefore, better accuracy of stereo matching. To provide this, the system has to be calibrated not in the white-light illumination, but in those NBs where measurements are supposed to be carried out. Usually these bands are selected in order to produce contrast patterns of physical, chemical and other properties of the defects on the surface of the inspected objects. The results of our experiments may be useful for the development of new RVI tools for spectral detection and quantitative characterization of low-contrast defects, for example, based on AO spectral filtration. Acknowledgements The Russian Science Foundation (project #17-19-01355) financially supported the work. References 1. P. Lorenz, The science of remote visual inspection (RVI): Technology, applications, equipment, Olympus Corp., 1990. 2. P. Mix, Introduction to Non-destructive Testing: A Training Guide, John Wiley & Sons, 576 p., 2005. 3. A. Gorevoy, A. Machikhin, D. Khokhlov, V. Batshev, V. Kaloshin and A. Perfilov, Applying a ray tracing model of an optoelectronic system to improve the accuracy of endoscopic measurements, Russian Journal of Nondestructive Testing, 53(9). pp. 660-668, 2017. 4. L. Cartz, Nondestructive testing: radiography, ultrasonics, liquid penetrant, magnetic particle, eddy current, ASM International, 229 p., 1995. 5. C. Chang, Hyperspectral imaging: Techniques for spectral detection and classification, Springer Science & Business Media, 370 p., 2003. 6. A. Goutzulis and D. Rape, Design and fabrication of acousto-optic devices, Dekker, 523 p., 1994. 7. A. Machikhin, V. Batshev, O. Polschikova, D. Khokhlov, V. Pozhar and A. Gorevoy, Conjugation of fiber-coupled wide-band light sources and acoustooptical spectral elements, Proc. SPIE, 10592, 105920I, 2017. 8. A. Gorevoy and A. Machikhin, Optimal calibration of a prism-based videoendoscopic system for precise 3D measurements, Computer Optics, 41(4). pp. 536-546, 2017. 5