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Evaluation of thermally-induced change in sharpness of automotive cameras by coupled thermo-mechanical and optical simulation
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Structural, Thermal, and Optical Performance (STOP) analysis is important for understanding the dynamics and for predicting the performance of a large number of optical systems whose proper functioning is negatively influenced by thermally induced aberrations. Furthermore, STOP models are being used to design and test passive and active methods for the compensation of thermally induced aberrations. However, in many cases and scenarios, the lack of precise knowledge of system parameters and equations governing the dynamics of thermally induced aberrations can significantly deteriorate the prediction accuracy of STOP models. In such cases, STOP models and underlying parameters need to be estimated from the data. To the best of our knowledge, the problem of estimating transient state-space STOP models from the experimental data has not received significant attention. Similarly, little attention has been dedicated to the related problem of obtaining low-dimensional state-space models of thermally induced aberrations that can be used for the design of high-performance model-based control and estimation algorithms. Motivated by this, in this manuscript, we present a numerical proof of principle for estimating low-dimensional state-space models of thermally induced aberrations and for characterizing the transient dynamics. Our approach is based on the COMSOL Multiphysics simulation framework for generating the test data and on a system identification approach. We numerically test our method on a lens system with a temperature-dependent refractive index that is used in high-power laser systems. The dynamics of such a system is complex and described by the coupling of thermal, structural, and ray-tracing models. The approach proposed in this paper can be generalized to other types of optical systems.
To enable future autonomous driving, failure free operation is a requirement for safety relevant electronic and mechanical components. The paper investigates the health indicator (HI), change in sharpness (CIS), used for condition monitoring (CM) of optical modules of automotive stereo cameras. Driven by the cyclic thermo-mechanical stress, strain and displacement, the sharpness of the optical imaging system can change. Different root causes for change in sharpness (CIS) are identified and the contribution of the different failure modes for CIS are investigated. Failure modes are separated in two categories: Changes in the distance between focal and image plane by displacement of the image plane (failure mode - package) and changes in the lens stack of the objective (failure mode - objective). By combining DOE and accelerated life testing the contribution of different stress conditions, i.e. vibrations and temperature cycling, was quantified and compared. Strong evidence was found that vibration induced aging has no influence on optical performance of the objective. Moreover, the comparison showed that vibration induced CIS can be neglected compart to CIS induced by thermo-mechanical cycling. To investigate CIS as a health indicator for the entire camera sensor the specification and development of a novel in situ measurement system for CM of camera sensors is described.
This paper investigates the most common sources for Change in Sharpness (CIS) of an automotive camera system. Deformation related to mismatches in Coefficient of Thermal Expansion (CTE) and creep in the BGA-Solder interconnect has been identified as most important source of CIS. This paper focuses on the development of a Zero- Hour model for a later planned extensive thermal cycling simulation investigating CIS over system life time. The Model is based on a simplified camera module, simulated in the thermal operations window of the system. The Zero-Hour model includes the curing process of the adhesive, which accounts for residual deformation in the components during assembly. Using this as a baseline, temperature cycling was simulated to investigate creep in the solder interconnects.
All lenses have optical aberrations which reduce image sharpness. These aberrations can be reduced by deconvolving an image using the lens point spread function (PSF). However, fully measuring a PSF is laborious and prohibitive. Alternatively, one can simulate the PSF if the lens model is known. However, due to manufacturing tolerances lenses differ subtly from their models, so often a simulated PSF is a poor match to measured data. We present an algorithm that uses a PSF measurement at a single depth to calibrate the nominal lens model to the measured PSF. The calibrated model can then be used to compute the PSF for any desired setting of lens parameters for any scene depth, without additional measurements or calibration. The calibrated model gives deconvolution results comparable to measurement but is much more compact and require hundreds of times fewer calibration images.
The camera lenses that are built into the current generation of mobile devices are extremely stressed by the excessively tight packaging requirements, particularly the length. As a result, the aspheric departures and slopes on the lens surfaces, when designed with conventional power series based aspheres, are well beyond those encountered in most optical systems. When the as-manufactured performance is considered, the excessive aspheric slopes result in unusually high sensitivity to tilt and decenter and even despace resulting in unusually low manufacturing yield. Qbfs polynomials, a new formulation for nonspherical optical surfaces introduced by Forbes, not only build on orthogonal polynomials, but their unique normalization provides direct access to the RMS slope of the aspheric departure during optimization. Using surface shapes with this description in optimization results in equivalent performance with reduced alignment sensitivity and higher yield. As an additional approach to increasing yield, mechanically imposed external pivot points, introduced by Bottema, can be used as a design technique to further reduce alignment sensitivity and increase yield. In this paper, the Q-type polynomials and external pivot points were applied to a mobile device camera lens designed using an active RMS slope constraint that was then compared to a design developed using conventional power series surface descriptions. Results show that slope constrained Q-type polynomial description together with external pivot points lead directly to solutions with significantly higher manufacturing yield.
We thoroughly and critically review studies reporting the real (refractive index) and imaginary (absorption index) parts of the complex refractive index of silica glass over the spectral range from 30 nm to 1000 μ m . The general features of the optical constants over the electromagnetic spectrum are relatively consistent throughout the literature. In particular, silica glass is effectively opaque for wavelengths shorter than 200 nm and larger than 3.5 – 4.0 μ m . Strong absorption bands are observed (i) below 160 nm due to the interaction with electrons, absorption by impurities, and the presence of OH groups and point defects; (ii) at ∼ 2.73 – 2.85 , 3.5, and 4.3 μ m also caused by OH groups; and (iii) at ∼ 9 – 9.5 , 12.5, and 21 – 23 μ m due to Si―O―Si resonance modes of vibration. However, the actual values of the refractive and absorption indices can vary significantly due to the glass manufacturing process, crystallinity, wavelength, and temperature and to the presence of impurities, point defects, inclusions, and bubbles, as well as to the experimental uncertainties and approximations in the retrieval methods. Moreover, new formulas providing comprehensive approximations of the optical properties of silica glass are proposed between 7 and 50 μ m . These formulas are consistent with experimental data and substantially extend the spectral range of 0.21 – 7 μ m covered by existing formulas and can be used in various engineering applications.
With the recent transformation of the automotive industry towards electrified and partially automated driving, the responsibility for reliable environmental perception moves from the driver to the vehicles advanced driving assistance systems (ADAS). As a consequence, the respective sensor systems are becoming more and more safety critical, as well as a major cost driver in modern vehicles. A large portion of automotive sensors consist of imaging devices like mono and stereo cameras for object detection, as well as currently emerging, non-scanning solid-state LiDAR (Light Detection And Ranging) sensors. Within this work, we investigate key parameters to monitor the thermo-mechanical state of the sensor optics and its impact on imaging quality during vehicle operation. Therefore, the intrinsic calibration parameters and modulated transfer function of a commercial, indirect time-of-flight camera were measured under varying operating temperatures in order to uncover major correlations. By doing so, it could be shown that thermally induced deformations within the optical path and the corresponding loss of sharpness can – to a certain extend – be predicted by accurate camera calibration. Hence, this work aims to motivate precise in-field camera re-calibration to continuously monitor the sensors optical system condition.
This article presents an analysis of thermal influences on the image acquisition process of an electronic camera. It is shown that temperature changes lead to thermal expansion of the mechanical camera components and thus to a displacement of the camera sensor and/or to a displacement of the center of projection. This change in the imaging geometry leads to changing camera parameters which have to be adjusted if the camera is used for high accurate measurements. The slight change in the imaging geometry can be determined by an analysis of the optical flow of a static calibration pattern. The dependency between temperature change and the change of the camera parameters can be modeled using methods from linear time invariant system theory. Once the model is determined for a specific camera it can be used for adjusting the camera parameters according to the current temperature and thus increasing the accuracy of an optical measurement device.
The refractive index n is known to vary with the temperature. Thus the optical path length and the focusing properties of transmissive optical elements may change with the ambient temperature T. Since there was no reliable dispersion formula of the temperature coefficient dn/dT the manufacturers of optical glasses published data on dn/dT averaged over certain intervals ( +i). These data are available for a small number of discrete wavelengths )k only. To calculate numerically n as a function of T for wavelengths other than ZAk one has to inter polate and to integrate over different intervals of T with the respective average data of dn/dT. In order to avoid this awkward procedure and to reduce the necessary data used for numerical calculations a dispersion formula for dn/dT is developed on the basis of a simple model. This formula has been tested and applied successfully to experimental data of dn/dT both as a function of wavelength ) and of temperature T for more than 140 different opti cal glasses in the visible spectral region and in the range of 400C T 800C. For 22 glasses data are available also for an extended range of temperature _1000C T 1400C. Even in this case the dispersion formula can be applied successfully. Moreover this formula can be integrated easily as a function of T to give immediately the increment of the refrac
Lens distortion is one of the main factors affecting camera calibration. In this paper, a new model of camera lens distortion is presented, according to which lens distortion is governed by the coefficients of radial distortion and a transform from ideal image plane to real sensor array plane. The transform is determined by two angular parameters describing the pose of the real sensor array plane with respect to the ideal image plane and two linear parameters locating the real sensor array with respect to the optical axis. Experiments show that the new model has about the same correcting effect upon lens distortion as the conventional model including all the radial distortion, decentering distortion and prism distortion. Compared with the conventional model, the new model has fewer parameters to be calibrated and more explicit physical meaning.
To determine the impact of spherical and aspheric foldable intraocular lens (IOL) tilt and decentration on optical quality after cataract surgery in an intraindividual comparative study.
Department of Ophthalmology, Goethe-University, Frankfurt am Main, Germany.
Randomized implantation of a spherical IOL (Sensar AR40e) was performed in 1 eye and of an aspheric IOL with negative spherical aberration (Tecnis Z9000) in the contralateral eye. Three to 4 months postoperatively, the wavefront was measured and higher-order aberrations (HOAs) were calculated for virtual pupil diameters of 3.5 mm and 6.0 mm. Tilt and decentration of the IOLs were measured using Scheimpflug photography. The tilt and decentration, HOAs, and best corrected visual Strehl ratio of the optical transfer function (BCVSOTF) calculated from the wavefront aberration were compared. The effect of tilt and decentration on HOAs and optical quality was assessed using multiple regression analysis.
The mean optic tilt was 2.89 degrees +/- 1.46 (SD) for the spherical IOL and 2.85 +/- 1.36 degrees for the aspheric IOL. The mean optic decentration was 0.19 +/- 0.12 mm and 0.27 +/- 0.16 mm, respectively. No significant intergroup differences in IOL tilt or decentration were found. Tilt and decentration did not significantly affect the BCVSOTF with either IOL.
The amounts of tilt and decentration of both IOLs were not large enough to cause deterioration of optical quality. Thus, when IOL tilt and decentration are within normal limits, they do not compromise the correction of spherical aberration by the aspheric IOL.
Research of zernike fitting algorithm in finite element process
- X.-F Song
- L Li
- Y.-F Huang
- S.-Y. Lu
X.-F. Song, L. Li, Y.-F. Huang, and S.-Y. Lu, "Research of zernike fitting algorithm in finite element process," in
2011 International Conference on Optical Instruments and Technology: Optical Systems and Modern Optoelectronic
Instruments, Beijing, Beijing, China, 2011, 81971K.