Anne-Sophie Poulin-Girard

Influence of camera calibration conditions on the accuracy of 3D reconstruction.

For stereoscopic systems designed for metrology applications, the accuracy of camera calibration dictates the precision of the 3D reconstruction. In this paper, the impact of various calibration conditions on the reconstruction quality is studied using a virtual camera calibration technique and the design file of a commercially available lens. This technique enables the study of the statistical behavior of the reconstruction task in selected calibration conditions. The data show that the mean reprojection error should not always be used to evaluate the performance of the calibration process and that a low quality of feature detection does not always lead to a high mean reconstruction error.

Optical testing of panoramic lenses

Here, we report for the first time the optical testing of modern 1.3, 5, and 8 megapixel panoramic lenses available on the market for security and surveillance applications. Our first test focused on the measurement of the image mapping and more particularly on the inverse of the instantaneous field of view expressed in pixels/degree (IFOV−1). This kind of measurement provides a valuable comparison parameter for the user community. The second test was a measurement of a modulation transfer function (MTF) system. All tested lenses were coupled with the same camera in order to measure the MTF system using the edge response. While this measurement is not a measurement of the MTF of the lens, it provides a basis for comparison between the different lenses. Our measurement shows that the IFOV is more robust than the MTF to manufacturing and assembly errors. This provides robustness to the unwrapping image post-processing. The MTF measurements show that all the tested lenses have an on-axis MTF of 35% at Nyquist frequency over 2 (Ny/2) for a 2.2 μm pixel pitch.

Dedicated testing setup for panoramic lenses

Panoramic imaging is of growing importance in many applications around the world spurred by the development of digital imaging. Panoramic lens characteristics are unique and their careful characterization can be a challenge. For example, the price to pay for a large field of view in this type of lens is high distortion in the image. For vision applications like security or inspection, a precise knowledge of the distortion introduced by panoramic lenses is essential to produce natural unwrapped views to the operator. Of special concern is the image quality which must be uniformed over the entire field of view because all directions are equally important. In addition, two hemispheric images can also be stitched together to create a complete spherical image. For these reasons, we have developed a dedicated setup to study the distortion and the image quality produced by panoramic lenses. The test setup is made of a 75-cm radius cylindrical structure with targets placed on it. Using referenced equally-spaced targets, we obtained the radial image mapping curves for various azymuthal angles, allowing us to calculate the full-field resolution map. Also, transition targets were used to find field-dependent spatial frequency where the MTF is 50%. We tested four different panoramic lenses, two panomorph lenses and two fisheyes. For each lens, we discussed the experimental resolution and MTF curves and compared some of those results to theoretical design data.

Study of camera calibration process with ray tracing

Camera calibration is essential for any optical system used to obtain 3D measurements from images. The precision of the 3D depth estimation relies on an appropriate camera model and the accurate estimation of model parameters. These parameters are sensitive to environmental conditions and it is well established that a vision system should be calibrated in operating conditions. This is not always possible since the calibration process is often tedious and time-consuming. Unfortunately, the use of poorly estimated calibration parameters for 3D reconstruction and measurements may lead to suboptimal performance of the system and inaccurate depth estimation. This paper presents a technique using an existing camera model and optical design software to perform calibration simulations. This virtual calibration technique allows for a study of the impact of environmental conditions on the calibration parameters. Using this procedure, it is also possible to predict the statistical behavior of the calibration parameters considering the chosen fabrication processes and tolerances. It can assist vision scientists in the choice of the optical system that best meets the requested precision of the 3D reconstruction. This technique could eventually be integrated in the lens design process to create more reliable optical systems that could be calibrated and used in a range of environmental conditions with a very small variation of their calibration parameters.

Outreach facilities within a research center

Worldwide, volunteers from student associations and non-profit organizations carry out outreach activities with high school students in their classrooms. Most of the time, these activities highlight optical phenomena but do not provide information about the reality of researchers in companies and universities. To address this issue, Université Laval’s OSA and SPIE student chapters set up a demonstration laboratory dedicated to outreach, located in a research center. In this paper, we list the advantages of this type of facility as well as the steps leading to the creation of the laboratory, and we give an overview of the demonstration laboratory.

Developments in modern panoramic lenses: lens design, controlled distortion, and characterization

Almost every aspect concerning the design of modern panoramic lenses brings new challenges to optical designers. Examples of these include ray tracing programs having problems finding the entrance pupil which is moving through the field of view, production particularities due to the shape of the front lenses, ways of tolerancing these systems having strong distortion, particular setups required for their characterization and calibration, and algorithms to properly analyze and make use of the obtained images. To better understand these modern panoramic lenses, the Optical Engineering Research Laboratory at Laval University has been doing research on them during the past few years. The most significant results are being presented in this paper. Controlled distortion, as in commercial panomorph lenses (Immervision), is used to image a specific part of the object with more pixels than in a normal fisheye lens. This idea is even more useful when a zone of interest vary in time with dynamically adjustable distortion as in a panoramic locally magnifying imager. Another axis of research is the use of modern computational techniques such as wavefront coding in wide-angle imaging systems. The particularities of such techniques when the field of view is large or with anamorphic imagers are considered. Presentation of a novel circular test bench in our laboratories, required to calibrate and check the image quality of wide-angle imaging system, follows. Another presented setup uses a laser and diffractive optical elements to compactly calibrate wide-angle lenses. Then, a discussion of the uniqueness in tolerancing these lenses, especially the front elements due to the large ratio between lens diameter and entrance pupil diameter, is included. Lastly, particularities with polarization imaging and experiments of triangle orientation detection tests before and after unwrapping the distorted images are briefly discussed.

Field-of-view enhancement for commercially available active imaging camera using time-of-flight technology

Active imaging techniques have been, and will remain relevant as an integral part of the sensor network needed in mobile robotics navigation, most commonly with the use of LiDARs. For close range applications, time-of-flight (TOF) cameras are an alternative to more expensive LiDARs. The Kinect v2 (Kinect for XboxOne) has been used frequently as a range imaging camera in machine vision research because of its low cost and good performance. For mobile robotics applications moving at low speed, a larger field of view than the original (70° X 60°) would be desirable to increase awareness of the surroundings, and also detect other moving objects in the trajectory. Our main goal is therefore to increase the Kinect v2 field of view while keeping its main feature of depth measurements. A simple but effective solution consists in using a conversion lens to increase the ray collection angle before the rays enter the IR camera. Depth measurements remain possible from each pixel using TOF with the Kinect v2 modulated signal, provided the field of illumination matches the field of view. A trade-off has to be made between accuracy of depth measurement, optical performance and field of view/illumination enhancement. Therefore, we modified a Kinect v2 to characterize its optical performances and evaluate its relevancy for applications requiring accurate knowledge of the 3D surroundings. A field of view improvement of the Kinect v2 and other similar TOF cameras could offer an alternative at minimal cost for depth measurements in machine vision, specifically in mobile robotics research where object detection and mapping tasks are frequently carried out.

Study of the performance of stereoscopic panomorph systems calibrated with traditional pinhole model

Abstract. With their large field of view, anamorphosis, and areas of enhanced magnification, panomorph lenses are an interesting choice for navigation systems for mobile robotics in which knowledge of the surroundings is mandatory. However, panomorph lenses special characteristics can be challenging during the calibration process. This study focuses on the calibration of two panomorph stereoscopic systems with a model and technique developed for narrow-angle lenses, the “Camera Calibration Toolbox for MATLAB.” In order to assess the performance of the systems, the mean reprojection error (MRE) related to the calibration and the reconstruction error of control points of an object of interest at various locations in the field of view are used. The calibrations were successful and exhibit MREs of less than one pixel in all cases. However, some poorly reconstructed control points illustrate that an acceptable MRE guarantees neither the quality of 3-D reconstruction nor its uniformity in the field of view. In addition, the nonuniformity in the 3-D reconstruction quality indicates that panomorph lenses require a more accurate estimation of the principal point (center of distortion) coordinates to improve the calibration and therefore the 3-D reconstruction.

Engaging the optics community in the development of informative, accessible resources focusing on careers

Young people often have biased and pre-conceived ideas about scientists and engineers that can dissuade them from considering a career in optics. This situation is compounded by the fact that existing resources on careers in optics are not suitable since they mostly focus on more general occupations such as a physicist and an electrical engineer. In addition, the linguistic register is not adapted for students, and many of these resources are only available to guidance counselors. To create appropriate resources that will inform high school students on different career opportunities in optics and photonics, we sought the collaboration of our local optics community. We selected seven specific occupations: entrepreneur in optics, university professor, teacher, technician, research and development engineer, sales representative and graduate student in optics. For each career, a list of daily tasks was created from the existing documentation by a guidance counselor and was validated by an expert working in the field of optics. Following a process of validation, we built surveys in which professionals were asked to select the tasks that best represented their occupation. The surveys were also used to gather other information such as level of education and advice for young people wishing to pursue careers in optics. Over 175 professionals answered the surveys. With these results, we created a leaflet and career cards that are available online and depict the activities of people working in optics and photonics. We hope that these resources will help counter the negative bias against scientific careers and inform teenagers and young adults on making career choices that are better suited to their preferences and aspirations.

Passive Stereoscopic Panomorph System

In the last decade, wide-angle stereoscopic systems using fisheye lenses have been proposed but the compromise made to obtain a large field of view is low resolution and high distortion resulting in imprecise depth estimation of objects in a 3D scene. High and non-uniform distortion, especially in the azimuthal direction, is often considered as a weakness of panoramic lenses because it is sometimes difficult to compensate for by image processing. The aim of this paper is to present an alternative to existing stereoscopic panoramic systems by taking advantage of nonuniform distortion and anamorphosis in Panomorph lenses. There are many challenges related to this project such as the calibration of the system and the creation of a 3D depth estimation algorithm that suits the resolution of the different areas in the images. This paper presents different configurations of two Panomorph lenses within a stereoscopic device and a study of specific parameters to highlight their impact on the quality of 3D reconstruction of an object in a scene. Finally, an overview of future work is presented.