Spectral Imaging Experiments with Various Optical Schemes Based on the Same AOTF.

Spectral image filtration by means of acousto-optical tunable filters (AOTFs) has multiple applications. For its implementation, a few different optical schemes are in use. They differ in image quality, number of coupling components, dimensions and alignment complexity. To choose the optical system of AOTF-based spectral imager properly, many factors have to be considered. Though various schemes of acousto-optic (AO) filtration have been tested and discussed, their comparative analysis has not been reported up to now. In this study, we assembled the four most popular schemes (confocal, collimating, tandem and double-path) using the same AO cells and experimentally compared their main features. Depending on the application, each scheme may be the basis of compact cost-effective spectral imaging devices.

Optimization of prism-based stereoscopic imaging systems at the optical design stage with respect to required 3D measurement accuracy.

We address the optical design procedure of prism-based stereoscopic imaging systems. Conventional approach includes two sequential stages: selection of the hardware and development of the proper digital image processing algorithms. At each of these stages, specific techniques are applied, which are almost unrelated to each other. The main requirements to the imaging system include only the key parameters and the image quality. Therefore, the insufficient measurement accuracy may be revealed only after the prototype is assembled and tested. In this case, even applying complex time-consuming image processing and calibration procedures does not ensure the necessary precision. A radical solution of this issue is to include the measurement error estimation into the optical design stage. In this research, we discuss a simplified implementation of this approach and demonstrate the capabilities of optical design software for this purpose. We demonstrate the effectiveness of this approach by the analysis and optimization of a prism-based stereoscopic imager with respect to required 3D measurement accuracy. The results are meaningful for the development of 3D imaging techniques for machine vision, endoscopic and measurement systems.

Polarizer-Free AOTF-Based SWIR Hyperspectral Imaging for Biomedical Applications.

Optical biomedical imaging in short wave infrared (SWIR) range within 0.9-1.7 µm is a rapidly developing technique. For this reason, there is an increasing interest in cost-effective and robust hardware for hyperspectral imaging data acquisition in this range. Tunable-filter-based solutions are of particular interest as they provide image processing flexibility and effectiveness in terms of collected data volume. Acousto-optical tunable filters (AOTFs) provide a unique set of features necessary for high-quality SWIR hyperspectral imaging. In this paper, we discuss a polarizer-free configuration of an imaging AOTF that provides a compact and easy-to-integrate design of the whole imager. We have carried out image quality analysis of this system, assembled it and validated its efficiency through multiple experiments. The developed system can be helpful in many hyperspectral applications including biomedical analyses.

Single-volume dual-channel acousto-optical tunable filter.

In this paper, we present a novel approach to spectral stereoscopic imaging. It is based on simultaneous spectral filtration of two light beams with a tunable acousto-optical filter (AOTF) of original design. It does not require large crystals and complicated optical relay systems, because two beams diffract in the same volume of the crystal medium but at different angles. We show that this geometry can be composed of a common-type AO cell and two triangular prisms of the same material. We derive equations, which specify the prism angles ensuring the necessary orientation of beams trajectories inside the crystal medium as well as parallel propagation of input and output beams. Some angles were additionally optimized for aberrations minimization by means of ray-tracing simulation. Experimental testing demonstrates rather high quality of spectral images, which is necessary for stereoscopic reconstruction procedure. The proposed approach makes possible development of spectral stereo-imaging components based on different types of previously developed AOTFs.

Modeling and optimization of a geometrical calibration procedure for stereoscopic video endoscopes.

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 a geometrical calibration procedure, the accuracy of which affects the measurement uncertainty. We propose to perform an optimal choice of the calibration technique and the calibration target parameters using a computer simulation at the design stage. The effectiveness of this approach is demonstrated via the design of a self-developed miniature prism-based stereoscopic system. We simulated acquisition of 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 that 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 acceptable measurement accuracy using only two images. The results of the computer simulation were confirmed experimentally using the prototype of the stereoscopic endoscope. The proposed approach may be used to optimize calibration techniques and reduce the cost of calibration equipment for various stereoscopic measurement systems.

RGB laser based on an optical parametric oscillator for single-shot color digital holographic microscopy.

We report on the new technical realization of single-shot color digital holographic microscopy. For this application, we propose to use an original three-wavelength red, green, and blue laser based on the Nd:YAG active element with sequential parametric downconversion and upconversion of optical frequencies into red (634 nm), green (532 nm), and blue (451 nm) spectral intervals. This light source provides high-power short (∼10 ns) pulses and enables simultaneous formation of three color interference patterns in a two-path interferometer. Their registration by a color image sensor provides fast acquisition of phase delay distribution induced by the inspected object at three wavelengths without spectral tuning.

Optimization of stereoscopic imager performance by computer simulation of geometrical calibration using optical design software.

Stereoscopic imagers are widely used in machine vision for three-dimensional (3D) visualization as well as in non-destructive testing for quantitative characterization of cracks, delamination and other defects. Measurement capability in these systems is provided by a proper combination of the optical parameters and data processing techniques. Conventional approach to their design consists of two sequential stages: optical system design and optimization of calibration and image processing algorithms. Such two-stage procedure often complicates both the hardware and the software, and results in a time-ineffective design procedure and cost-ineffective solution. We demonstrate a more effective approach and propose to estimate errors of 3D measurements at the early optical design stage. We show that computer simulation using optical design software allows not only optimizing optical parameters of the imager but also choosing the most effective mathematical model of the system and the equipment necessary for calibration. We tested the proposed approach on the design of miniature prism-based stereoscopic system and analyzed the impact of various factors (aberrations, tolerances, etc.) as on the image quality, so on the quality of calibration and 3D measurements accuracy. The proposed joint design approach may be highly effective for various measurement systems and applications when both optical parameters and image processing algorithms are not defined in advance and are necessary to be optimized.

Acousto-optic tunable spectral filtration of stereoscopic images.

We propose a new technique for three-dimensional (3-D) imaging in arbitrary spectral intervals. It is based on a simultaneous diffraction of two divergent stereoscopic light beams on a single acoustic wave propagating in a uniaxial birefringent crystal. We discuss in detail this configuration of acousto-optic (AO) interaction, derive basic relations, and experimentally demonstrate the applicability of the proposed approach to 3-D spectral imaging. A stereo-imager of this type may be produced as an ultra-compact embeddable optical element, which is promising for many imaging applications.

Aberration analysis of AOTF-based spectral imaging systems.

Image aberrations caused by acousto-optic (AO) anisotropic diffraction in uniaxial crystals are discussed. For their analysis, we propose a simplified ray-tracing model of an AO crystal cell (AOC). With this approach, one can assign any configuration of AO interaction, any material and geometry of the crystal, and then estimate all conventional ray aberrations, such as spherical, coma, astigmatism, distortion, etc. The optimization procedure is demonstrated by the aberration analysis of three principal spectral imaging schemes based on AO tunable filters (AOTFs). The approach developed promises performance improvement of AOTF-based systems for high-quality spectral imaging and image processing.