Optimal calibration of a prism-based videoendoscopic system for precise 3D measurements

Modern videoendoscopes are capable of performing precise three-dimensional (3D) measurements of hard-to-reach elements. An attachable prism-based stereo adapter allows one to register images from two different viewpoints using a single sensor and apply stereoscopic methods. The key condition for achieving high measurement accuracy is the optimal choice of a mathematical model for calibration and 3D reconstruction procedures. In this paper, the conventional pinhole camera models with polynomial distortion approximation were analyzed and compared to the ray tracing model based on the vector form of Snell’s law. We, first, conducted a series of experiments using an industrial videoendoscope and utilized the criteria based on the measurement error of a segment length to evaluate the mathematical models considered. The experimental results confirmed a theoretical conclusion that the ray tracing model outperforms the pinhole models in a wide range of working distances. The results may be useful for the development of new stereoscopic measurement tools and algorithms for remote visual inspection in industrial and medical applications.The Russian Science Foundation (project #7-19-01355) financially supported the work. The authors thank A. Naumov, A. Shurygin and D. Khokhlov for continuous technical support

Reconstruction of three-dimensional surface structure of hard-to-reach objects using prism-based stereoscopic systems

Video endoscopes with stereoscopic prism-based optical systems are widely used for non-destructive testing and geometric measurements of hard-to-reach elements inside complex technical objects. The functionality of the existing devices of this type is limited to a great extent by the capabilities of the embedded software. In this paper, we present our software for the calibration of such systems and the processing of obtained stereoscopic images. It allows to reconstruct and process the whole array of three-dimensional coordinates of the observable object points, to compare it with the reference surfaces and to export the data to other mathematical software.The reported study was funded by RFBR according to the research project № 17-29-03469

Calibration simulation for stereoscopic optical systems using optical design software

To reconstruct three-dimensional (3D) structure of objects and measure their geometric parameters using stereoscopic imaging systems, it is necessary to implement a number of image processing algorithms. For higher system efficiency, the choice of these algorithms and mathematical models should be taken into account at the stage of optical system design. We demonstrate the capabilities of optical design software to perform computer simulation of geometrical calibration for stereoscopic systems. The simulation allows comparison of mathematical models used for 3D reconstruction and estimation of 3D measurements errors caused by tolerances of optical elements, temperature variations and other factors. Using this technique, we analyze the design of prism-based stereoscopic system and show that the proposed ray tracing camera model considering pupil aberrations provides higher measurement precision. The results of computer simulation are confirmed by experiments with the self-developed stereoscopic system

Multi-spectral image processing for the measurement of a spatial temperature distribution on the surface of a laser-heated microscopic object

In this paper, we demonstrate that combining a laser heating (LH) system with a tandem acousto-optical tunable filter (TAOTF) allows us to measure the temperature distribution (TD) across a laser-heated microscopic specimen. Spectral image processing is based on one-dimensional (1D) non-linear least squares fitting of the Planck radiation function. It is applied for determining the temperature T at each point (x, y) of the specimen surface. It is shown that spectral image processing using the 1D non-linear least squares fitting allows measurement of the TD of the laser-heated microscopic specimen with higher precision and stability than those of the conventional linear least-squares fitting of the Wien approximation of Planck’s law.The Russian Science Foundation (project #17-12-01535) financially supported the work

Processing of acousto-optic images at early diagnosis of the functional state of the developing biosystem

The task of studying the embryo development at an early stage is considered. For this, hyperspectral imaging using an acousto-optical tunable filter is proposed. Acousto-optic visualization of the early developmental stages of the loach Misgurnus fossilis embryo showed a regular change in the spectral characteristics of different image areas corresponding to the tissues and body fluids, depending on the functional state of the embryo. This is manifested in the regular arrangement of a discrete set of maxima in the optical absorption spectrum with a distance of 20 and 30 nm between them. Value 20 nm corresponds to the normal development of biological tissue. The appearance of the value 30 nm between the maxima of neighboring optical absorption bands indicates the development of pathological processes in the biosystem. Such deviations from the norm are characteristic of germinal tissues, and they are absent in the perivitelline fluid. Therefore, for the early diagnosis of the physiological state of a developing biosystem, special attention should be paid to optical absorption spectra, in which 30 nm intervals between the maxima of the neighboring absorption bands prevail

Resistance investigation of diamond-like carbon coatings to cyclic temperature changes

Optical elements used in outer space must be designed considering the effects of such factors as space vacuum, atomic oxygen in low Earth orbit, solar and space radiation, large temperature drops, gas release of spacecraft materials and structural elements, space dust and debris. In order to harden and protect mirror surfaces of optical elements from external factors, it has been promisingly applied diamond-like carbon coatings on their surface. These coatings are characterized by high strength and wear-resistant properties, in particular, high hardness, low friction coefficient, high wear resistance and chemical inertness. This leads to their widespread use in various fields of science and technology, including optical instrumentation. This paper presents the results of testing an aluminum mirror with a diamond-like carbon coating under the effect of cyclic temperature changes for determining their ability to withstand a rapid cyclic ambient temperature change, and specifically, to maintain optical and mechanical properties. © 2021 Institute of Physics Publishing. All rights reserved