Acousto-optical tunable filters (AOTFs) based on interaction of light and ultrasound in uniaxial birefringent crystals are widely used in imaging spectroscopy applications due to random spectral access, narrow controllable bandwidth, compactness, overal ease-of-use and image trasmittance capability. The spectral transmission of AOTF-based spectral imagers is usually characterized for paraxial light beam, but AOTFs inherently have non- uniform spatio-spectral transmission, so the central wavelength of the transmission window varies with the angle of incident light. We demonstrate that the spatio-spectral characteristics of acousto-optic (AO) interaction may be described either by the dependency of ultrasound frequency on the incident light angle for a given wavelength or by the dependency of wavelength on the incident angle for a given sound frequency. These dependencies are derived from the phase matching condition and are determined by the refractive indices, sound phase velocities and the AO diffraction geometry. We experimentally estimate the appearance of this specific spatio-spectral structure on the images acquired by AOTF-based imagers based on both collimating and confocal (telecentric) schemes and show that the variations of central wavelength and spectral bandwidth are noticeable for collimating setup even in the commonly used small field of view. The results of the study may be applied in design of AOTF- based spectral imagers and image processing algorithms.
We address a multi-spectral holographic imaging of optically transparent objects widely used in biomedical studies and industrial inspection. Instead of complicated optomechanical systems based on several light sources, our setup is based on acousto-optic tunable filter (AOTF) coupled with a broadband light source, which enables fast and highly effective spectral selection within a wide wavelength range. The filtered light enters the Mach-Zehnder interferometer and produces the interference pattern captured by camera and processed in the spatial frequency domain to extract amplitude and phase information about the inspected sample. We compare successive acquisition of several single-wavelength holograms and two single-shot modes for obtaining wavelength-multiplexed holograms using either sequential tuning to several wavelengths during the exposure time of the sensor or multi-frequency AOTF driving. The effectiveness of AOTF-based setup for multi-spectral digital holographic microscopy in all modes is demonstrated by the quantitative phase imaging of red blood cells smear specimens. The results of the study may be helpful to choose the optimal mode based on several criteria, such as required spatial and spectral resolution, field of view and image acquisition time, as well as for the optimization of AOTF-based setups for digital holography.
The industrial endoscopic remote visual inspection and endoscopic minimally invasive procedures in medicine provide the wide range of visualization and measurement techniques for hard-to-reach objects’ inner surfaces inspection. In common practice, the three-dimensional visualization, three-dimensional geometrical measurement and spectral imaging techniques are realized independently in different specialized endoscopic devices. Simultaneous implementation of these techniques in a single versatile endoscopic system may increase the efficiency of the inspection and diagnostic procedures. We propose a combined approach to multispectral stereoscopic endoscopic imaging. The prototype of an endoscopic probe able to carry out remote three-dimensional geometrical measurements as well as spectral visualization and measurements is demonstrated.
KEYWORDS: Calibration, Endoscopy, Modulation transfer functions, Imaging systems, Cameras, 3D metrology, Image quality, Stereoscopy, Prisms, RGB color model
We address the problem of non-contact geometrical measurements of hard-to-reach objects that is an important task in various industrial and medical applications. We have developed two small-size prism-based systems for simultaneous acquisition of stereoscopic images by a single sensor. For a correct mathematical description of these systems, we use a ray tracing camera model based on a vector form of Snell’s law. We demonstrate that using the chessboard calibration target allows simultaneous geometrical calibration and image quality assessment. We show that the appearance of chromatic aberrations in RGB images caused by the prism may be significantly reduced by applying separate rectification procedure to each color channel. Experiments confirm that the developed optical systems provide high image quality and the software provides high precision of three-dimensional (3D) geometrical measurements. Described systems may become the basis of small-diameter endoscopic probes for various applications.
We consider the problem of 3D imaging spectroscopy on base of a pair of acousto-optical tunable filters (AOTFs). In such a system, quality of stereoscopic spectral images is the key factor influencing the accuracy of three-dimensional (3D) measurements and shape reconstruction. We analyze the image quality in a dual-channel AOTF-based spectrometer and proposed the optical scheme, which is free of image blur and has only aberrations of image distortion and drift. They can be eliminated by means of geometrical calibration of the imager. The optical characteristics of the system prototype are presented. The theoretical analyses and calculations of image aberrations are confirmed by experiments. The devices based on the dual-channel AOTF-based stereoscopic system may be effectively used for various 3D imaging spectroscopy applications.
Endoscopic imaging systems are widely used for non-invasive diagnostics of inner tissues for biomedical applications as well as for non-destructive testing of various hard-to-reach industrial objects. Conventional white light endoscopy often does not provide contrast visualization of the inspected objects and quantitative characterization of their differences. In these cases, spectral imaging techniques may be helpful to increase the effectiveness of endoscopic inspection. Acoustooptic (AO) filtration of light is a very promising technique for implementing spectral imaging capabilities. In this paper, we discuss the peculiarities and benefits of conjugating AO tunable filters (AOTF) with rigid borescopes, flexible fiberscopes and video endoscopes. We analyze and compare imaging and spectroscopy capabilities of two alternative technical implementations: AOTF-based tunable light sources and AOTF-based add-on imaging modules. The results of this analysis are confirmed by multiple experiments.
KEYWORDS: Calibration, Computer simulations, Cameras, 3D modeling, 3D metrology, 3D acquisition, 3D image processing, Ray tracing, 3D image reconstruction, Endoscopy
Stereoscopic video endoscopes are widely used for remote visual inspection and precise three-dimensional (3D) measurements in industrial and biomedical applications. The reconstruction of 3D points from the corresponding image points requires calibration procedure which accuracy affects the measurement uncertainty. We propose to perform an optimal choice of the calibration technique and the calibration target parameters using the computer simulation at the design stage. The effectiveness of this approach is demonstrated via the design of self-developed miniature prism-based stereoscopic system. We simulated acquisition of the calibration and measurement data using optical design software. The conventional calibration technique requiring many positions of the flat target with arbitrary displacements and rotations was compared with another one, which uses the translation stage to provide pure translation of the target. We analyzed the impact of the translation uncertainty, the number of positions, the number of targets and the uncertainty of image point coordinates on the uncertainty of calibration parameters and 3D measurements. We have shown that the second technique could provide the same calibration accuracy as the first one with less number of images. The results of computer simulation were confirmed experimentally using the prototype of the self-developed stereoscopic endoscope. The proposed approach may be used to optimize calibration techniques and reduce a cost of calibration equipment for various stereoscopic measurement systems.
Stereoscopic imagers are widely used tools for precise three-dimensional (3D) characterization of various objects in industrial and biomedical applications. Narrow-band spectral imaging significantly increases capabilities of these devices, i.e. allows to analyze spatial distribution of spectral properties as well as to achieve higher image contrast, lower optical aberrations and, therefore, to improve geometrical measurements accuracy. Using spectral stereoscopic images for 3D measurements requires a proper choice of a number, positions and width of spectral bands used for calibration and interpolation of the calculated parameters. The experimental determination of these parameters does not guarantee the optimal choice and may be difficult to implement and time-consuming for a large number of spectral bands. In this paper, we demonstrate that the optical design software can be effective for the computer simulation of calibration, comparison of mathematical models and assigning spectral calibration parameters. We show the possibility to optimize the parameters of multi-spectral geometrical calibration to ensure the required measurement accuracy provided by the stereoscopic system on the stage of its optical design before manufacturing via the design of self-developed prism-based imager. Computer simulation allowed us to compare two camera models and various spectral options (conventional white-light as well as arbitrary number, positions and width of spectral channels) applied to calibration procedure. The results of computer simulation are confirmed by multiple experiments. Proposed approach may be used for estimation of 3D measurements errors caused by image noise, tolerances of optical components, temperature variations and other factors.
Nowadays, stereoscopic devices are widely used for precise three-dimensional (3-D) measurements of hard-to-reach elements in industrial and biomedical applications. The most common approach for their implementation is the utilization of prism-based optical systems, which allows simultaneous acquisition of two images from different viewpoints on a single sensor. Normally, they are equipped with a wideband (WB) white-light source, but contrast visualization of the inspected object and, therefore, accurate quantitative characterization of its parameters, often requires narrow-band (NB) spectral imaging. We show that the standard geometrical calibration may lead to significant measurement errors when obtained using WB illumination and applied to NB images. As the criterion to evaluate the calibration error, we have chosen the difference between reference and measured lengths of the 1 mm segment along transverse x, y, and longitudinal z axes. If WB calibration data are applied to NB images, the measurement bias increases from the middle of the working spectral range to its edges and can reach significant values: up to 0.1 mm along x axis and 0.15 mm along z axis in 10 to 25 mm distance range. To overcome this, we propose the calibration and image processing procedures based on a proper choice of a few spectral bands for calibration and interpolation of the calculated calibration parameters. Results of multiple experiments using stereo video endoscope confirmed that the proposed technique allows a decrease in the measurement bias by three times in comparison to conventional WB calibration for all wavelengths of the visible range, which essentially improves the measurement accuracy. The impact of WB calibration on random errors of measurements and the quality of image rectification was also analyzed and shown to be insignificant.
We have developed a prototype of a miniature prism-based optical system for simultaneous acquisition of two stereoscopic images on a single image sensor. The scheme and optical characteristics of the system are presented. We show that after a proper geometrical calibration and image processing it is possible to calculate three-dimensional (3-D) shape of the inspected objects. The devices based on this system may be effectively used for 3-D machine vision applications and remote visual inspection.
Spectrally tunable illumination is widely used for colorimetry, spectroscopy, spectral-domain optical coherence tomography and other applications. Most of the swept light sources suffer either from a limited number of spectral channels or from a spatial noise and speckles. In this paper, we discuss an approach based on acousto-optic filtration of wide-band illumination. We show experimentally that acousto-optical tunable filter (AOTF) provides a unique collection of features: arbitrary spectral tuning, multi-bandpass mode and acoustic frequency modulation. It allows assigning a number, positions and width of the spectral channels. Results of multiple experiments show that the proposed technique may become the basis of the swept light sources.
Nowadays, the stereoscopic endoscopy is a widely used tool for precise three-dimensional (3D) measurements of hard-to-reach elements in industrial and biomedical applications. The most common approach for its implementation is the utilization of prism-based optical tips which allow to acquire two images from different viewpoints on a single sensor. Stereo video endoscopes are typically equipped with a wideband white light source, but contrast visualization of the inspected object and, therefore, accurate quantitative characterization of its parameters often requires narrow band spectral imaging. We show that the standard geometrical calibration may lead to significant measurement errors when obtained using white illumination and applied to narrow band images. In order to overcome this, we propose the new calibration procedure based on a proper choice of a few spectral bands for calibration and interpolation of the calculated parameters. Results of multiple experiments show that the proposed technique fosters the measurement accuracy increase.
Endoscopic instrumentation is widely used for diagnostics and surgery. The imaging systems, which provide the hyperspectral information of the tissues accessible by endoscopes, are particularly interesting and promising for in vivo photoluminescence diagnostics and therapy of tumour and inflammatory diseases. To add the spectral imaging feature to standard video endoscopes, we propose to implement acousto-optical (AO) filtration of wide-band illumination of incandescent-lamp-based light sources. To collect maximum light and direct it to the fiber-optic light guide inside the endoscopic probe, we have developed and tested the optical system for coupling the light source, the acousto-optical tunable filter (AOTF) and the light guide. The system is compact and compatible with the standard endoscopic components.
The paper is targeted to find the optimal mathematical model and the calibration algorithm for the industrial endoscope equipped with the prism-based attachable stereo adapter, which allows imaging from two different points by a single sensor. We consider the conventional calibration methods for the pinhole camera model with polynomial distortion approximation and compared them with the ray tracing model based on the vector form of Snell’s law. In order to evaluate each of the proposed models we have developed the software for the imitation of various calibration procedures using different types of calibration targets. We use the computer simulation to prove that the pinhole camera models, widely used in machine vision, are very limited for describing prism-based endoscopic measurement systems. Our analysis identified the main problems for these models, such as entrance pupil shift, non-homocentric beams and required number of coefficients for polynomial models and the iterative forward ray aiming for the ray-tracing model. The proposed technique is flexible and can also be used to test stability and convergence of the parameter estimation procedures and to compare calibration targets and strategies.
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