Simulation method for large subwavelength devices in infrared imaging systems and comparison with experiment

Infra Red (IR) imaging systems have various applications in military and civilian sectors. Most of the modern imaging systems are based on Infra Red Focal Plane Arrays (IRFPAs), which consists of an array of detector element placed at focal plane of optics module. Performance of IRFPAs operating in Medium Wave Infra Red (MWIR) and Long Wave Infra Red (LWIR) spectral bands are strongly affected by spatial and temporal Non-Uniformity (NU). Due to difference in the photo response of detector elements within the array, Fixed-Pattern Noise (FPN) becomes severe. To exploit the potential of current generation infrared focal plane arrays, it is crucial to correct IRFPA for fixed-pattern noise. Different Non-Uniformity Correction (NUC) techniques have been discussed and real-time performance of two-point non-uniformity correction related to IR band is presented in this paper. The proposed scheme corrects both gain and offset non-uniformities. The techniques have been implemented in reconfigurable hardware (FPGA) and exploits BlockRAM memories to store the gain and offset coefficients in order to achieve real-time performance. NUC results for long-range LWIR imaging system are also presented.

Optical imaging usually suffers from aberrations that are induced by various structures when imaging biological samples. Usually aberrations degrade the imaging system performances by broadening the point spread function (PSF). Unexpectedly we show that in spatially incoherent interferometry like full-filed optical coherence tomography (FFOCT), the system PSF width is almost insensitive to aberrations. Instead of considering the PSF of a classical imaging system such as a microscope, we specifically pay attention to the system PSF of interferometric imaging systems for which an undistorted wavefront from a reference beam interferes with the distorted wavefront of the object beam. By comparing the cases of scanning OCT with spatially coherent illumination, wide-field OCT with spatially coherent illumination and FFOCT with spatially incoherent illumination, we found that in FFOCT with spatially incoherent illumination the system PSF width is almost independent of the aberrations and only its amplitude varies. This is demonstrated by theoretical analysis as well as numerical calculations for different aberrations, and confirmed by experiments with a FFOCT system. It is the first time to the best of our knowledge that such specific merit of incoherent illumination in FFOCT has been demonstrated. Based on this, the signal level is used as metric in our adaptive optics FFOCT system for retinal imaging. Only the main aberrations (defocus and astigmatism) that are dominating in eye are corrected to improve the signal to noise ratio and the high order aberrations are skipped. This would increase the correction speed thus reducing the imaging time.

Diagnostics are crucial to the operation of all particle accelerator facilities. They provide a means of optimising beam parameters and monitoring beam quality for users. However, for the next generation of accelerators, operating in extreme parameter spaces, most current beam diagnostics will fail; either through material failure or lack of resolution. This thesis presents three new diagnostic concepts to address this imminent diagnostic need. These novel diagnostics are supported by a simulation-based algorithm, successfully benchmarked throughout this work. All work has been conducted by the Author unless otherwise stated and referenced. The first technique measures sub-micron transverse beam profiles using optical transition radiation (OTR). Simulations were used to design a new imaging system, which was then installed at KEK (Japan). OTR images were then collected for a range of beam sizes. Comparisons were then made with simulations to successfully achieve sub-micron beam size measurements. Validation of these results was found in the agreement across different imaging systems for fixed beam parameters, providing a self-consistency check for the algorithm in lieu of alternate corroboration. The next diagnostic is a bunch length monitor for ∼<100 fs particle bunches; based on imaging the spatial distribution of coherent transition radiation (CTR) in the THz regime. Simulations demonstrated a relationship between image width and intensity, with bunch length. A THz imaging system was then installed at MAX IV (Sweden). CTR images were taken for a range of compressions. Maximum compression was successfully identified by monitoring relative changes in the properties of the images. Bunch lengths were then acquired from these images through comparison with simulations. Finally, the optical performance of a Digital Micro-mirror Device (DMD) was assessed. Image resolution produced by a DMD was quantified by measuring the Point Spread Function (PSF) of an imaging system, which was capable of remotely replacing a DMD with a planar mirror. The PSFs were measured using the sub-micron focus of a laser. Results show no measurable alteration to a PSF with the introduction of a DMD. This provides a foundation for the development of future diagnostics centred around this device. This thesis provides several validated proof-of-principle experimental results and supporting simulations. Future work has been identified for each topic, only possible following the results presented here, which will provide an array of novel next generation diagnostics.

The point spread function (PSF) of imaging system with coded mask is generally acquired by practical measure- ment with calibration light source. As the thermal radiation of coded masks are relatively severe than it is in visible imaging systems, which buries the modulation effects of the mask pattern, it is difficult to estimate and evaluate the performance of mask pattern from measured results. To tackle this problem, a model for infrared imaging systems with masks is presented in this paper. The model is composed with two functional components, the coded mask imaging with ideal focused lenses and the imperfection imaging with practical lenses. Ignoring the thermal radiation, the systems PSF can then be represented by a convolution of the diffraction pattern of mask with the PSF of practical lenses. To evaluate performances of different mask patterns, a set of criterion are designed according to different imaging and recovery methods. Furthermore, imaging results with inclined plane waves are analyzed to achieve the variation of PSF within the view field. The influence of mask cell size is also analyzed to control the diffraction pattern. Numerical results show that mask pattern for direct imaging systems should have more random structures, while more periodic structures are needed in system with image reconstruction. By adjusting the combination of random and periodic arrangement, desired diffraction pattern can be achieved.

The properties of UWFC (Ultra Wide-Field Camera) astronomical systems along with specific visual data in astronomical images contribute to a comprehensive evaluation of the acquired image data. These systems contain many different kinds of optical aberrations which have a negatively effect on image quality and imaging system transfer characteristics, and reduce the precision of astronomical measurement. It is very important to figure two main questions out. At first: In which astrometric depend on optical aberrations? And at second: How optical aberrations affect the transfer characteristics of the whole optical system. If we define the PSF (Point Spread Function) [2] of an optical system, we can use some suitable methods for restoring the original image. Optical aberration models for LSI/LSV (Linear Space Invariant/Variant) [2] systems are presented in this paper. These models are based on Seidel and Zernike approximating polynomials [1]. Optical aberration models serve as suitable tool for estimating and fitting the wavefront aberration of a real optical system. Real data from the BOOTES (Burst Observer and Optical Transient Exploring System) experiment is used for our simulations. Problems related to UWFC imaging systems, especially a restoration method in the presence of space variant PSF are described in this paper. A model of the space variant imaging system and partially of the space variant optical system has been implemented in MATLAB. The “brute force” method has been used for restoration of the testing images. The results of different deconvolution algorithms are demonstrated in this paper. This approach could help to improve the precision of astronomic measurements.

The increased requirement for visible light/infrared dual band imaging systems with compact structure, suitable field of view (FOV), high resolution coverage and without significantly complicates the optical design is increasing. Such systems are particularly desirable for unmanned aerial vehicle (UAV) remote sensing applications where increased levels of target reconnaissance, surveillance, precision target location and designation are required in cost-effective and light weight systems. Therefore, in this paper, a high resolution Electro-optical/infrared (EO/IR) sensor to simultaneously operate in the visible band and short wave infrared band (SWIR) band using ZEMAX software are presented. It has a common aperture using common refractive system and the spectrum is separated using a dichroic beam splitter. A long wave infrared (LWIR) imaging system based on uncooled microbolometer focal plane array (FPA) detector with pixel size equal to 17µm is presented. The results of the optical systems design indicated that our proposed approaches meet the technical performance requirements for high image performance and compact size. This system can be applied in UAV remote sensing applications.

A long standing debate has existed on the utility of Mid Wave Infrared (MWIR) vs Long Wave Infrared (LWIR) remote-sensing imagery for Precision Agriculture applications. We have conducted a combined qualitative and quantitative, multi-field, aircraft remote-sensing study to show that LWIR measurements often result in more actionable for day-to-day farm management applications. LWIR imagery when processed with SaraniaSat's nonlinear, weak-signal detection algorithms is an effective predictor of crops stress and ultimate harvest yield. It is also shown that both processed LWIR and MWIR imagery contain actionable, crop-stress information that is absent in the conventional Normalized Difference Vegetation Index (NDVI) that is currently, widely used for Precision Agriculture. Finally, at a simulated 10m ground resolution (satellite remote sensing) it is shown, once again, that LWIR is superior and more actionable.

The Point Spread Function (PSF) is one of the key indicators characterizing the signal transfer characteristics of an imaging system. Edge method is applicable to calculate the PSF of the remote sensing imaging systems for its easy implement and robust noise-resistant ability. In this paper, a Double-Knife-Edge method is proposed to recover the degraded images using a precise estimated PSF of the imaging system. The exact motion-blur direction is estimated by image differentiation firstly. Two orthogonal edges, one of which is in the same direction as the main motion-blur, are picked up from the candidate edges via Hough transform and employed to obtain edge spread functions (ESF). Derived from these ESFs, a more accurate PSF is used to deconvolute the degraded image by an image restoration algorithm based on total variation (TV) deconvolution which is capable of suppressing the artifacts and noise. The experiment results show that this algorithm is adaptive and efficient to reconstruct remote sensing images, and the reconstructed image has better PSNR, MSE and MTF than the original degraded image.

Biomimetic staring infrared imaging omnidirectional detection technology

: This dissertation presents both analytic and processing techniques for various radar imaging systems. A two dimensional system classification method, which is very general and hence applies to the special case of radar imaging systems as well, is proposed to assist in understanding the structure and describing the limitations of 2-D systems. Once a given system is identified with the simplest possible class, the specific techniques can be directly utilized to process the data or reconstruct the images. Following a review of radar imaging principles, several coherent radar systems are analyzed and experimented upon. They include synthetic aperture radar (SAR) ground mapping, imaging of an aircraft target from turntable data, and imaging of a flying aircraft target. In each case the point spread function (PSF) of the imaging system is derived or estimated. Physical considerations are then incorporated in mathematical PSF's to categorize the imaging systems according to the aforementioned system classification principle proposed.

Imaging systems using aspheric imaging lenses with complementary computation can deliver performance unobtainable in conventional imaging systems. These new imaging systems, termed Wavefront coded imaging systems, use specialized optics to capture a coded image of the scene. Decoding the intermediate image provides the "human-usable" image expected of an imaging system. Computation for the decoding step can be made completely transparent to the user with today's technology. Real-time Wavefront coded systems are feasible and cost-effective. This "computational imaging" technology can be adapted to solve a wide range of imaging problems. Solutions include the ability to provide focus-free imaging, to increase the field of view, to increase the depth of read, to correct for aberrations (even in single lens systems), and to account for assembly and temperature induced misalignment. Wavefront coded imaging has been demonstrated across a wide range of applications, including microscopy, miniature cameras, machine vision systems, infrared imaging systems and telescopes.

Wavefront coding involves the insertion of an asymmetric refractive mask close to the pupil plane of an imaging system so as to encode the image with a specific point spread function that, when combined with decoding of the recorded image, can enable greatly reduced sensitivity to imaging aberrations. The application of wavefront coding has potential in the fields of microscopy, where increased instantaneous depth of field is advantageous and in thermal imaging where it can enable the use of simple, low-cost, light-weight lens systems. It has been previously shown that wavefront coding can alleviate optical aberrations and extend the depth of field of incoherent imaging systems whilst maintaining diffraction-limited resolution. It is particularly useful in controlling thermally induced defocus aberrations in infrared imaging systems. These improvements in performance are subject to a range of constraints including the difficulty in manufacturing an asymmetrical phase mask and significant noise amplification in the digitally restored image. We describe the relation between the optical path difference (OPD) introduced by the phase mask and the magnitude of noise amplification in the restored image. In particular there is a trade between the increased tolerance to optical aberrations and reduced signal-to-noise ratio in the recovered image. We present numerical and experimental studies based of noise amplification with the specific consideration of a simple refractive infrared imaging system operated in an ambient temperature varying from 0°C to +50 C. These results are used to delineate the design and application envelope for which infrared imaging can benefit from wavefront coding.

The line spread function (LSF) is a classical figure of merit used to derive the modulation transfer function (MTF) of an imaging system. However this test cannot be directly applied to cameras based on an infrared focal plane array (IRFPA), as these sampled systems exhibit aliasing effects. ONERA has proposed a new technique called generalized line spread function allowing MTF assessment for sampled imaging systems. A testing of spatial frequency filtering becomes possible until at least twice the Nyquist frequency. The purpose of this paper is to show that this method is well-adapted to IRFPA cameras. In particular, the effects of bad a priori estimation of the characteristics of the camera are discussed. Secondly, we have carried on a laboratory experiment with a widespread infrared imaging system. The purpose is to demonstrate experimentally the advantages of this new method, not only in terms of performances but also in terms of simplicity of implementation and robustness. The infrared imaging system tested is a 512 by 512 Si:Pt IR5120C Mitsubishi camera equipped with a F/1.2 Angenieux lens.

In this paper the effects of scattering, with four atmospheric fog aerosols models, on the overall point spread function (PSF) of an imaging system are investigated.The PSF and radiometric calculations are then used to estimate the mean numbers of direct path and scattered photons detected by the imaging system in the visible and near infra-red (NIR) spectral bands of the electro-magnetic spectrum (EMS), for a horizontal near-ground imaging scenario.The image spectrum signal-to-noise ratio (SNR) is then used to estimate the noise effective spatial resolution of the imaging system which emphasizes the degrading effect of fog aerosol models on the spatial resolution of the imaging systems.

The precise matching of the optical system and the detector has attracted more and more attention to satisfy requirements of future infrared (IR) imaging systems, both for miniaturization and long-distance detection. Considering the trade-off between the spatial resolution and the sensitivity, this paper proposes a general optimization method, which can optimize parameters of the IR imaging system automatically by modifying λF/d. Firstly, on the basis of the trade-off between the spatial transfer characteristics and the spatial sampling characteristics, we establish a more accurate minimum resolvable temperature difference (MRTD) model, and analyze the coupling relationship between the optical system and the detector under the optimum MRTD. Secondly, we analyze the relationship between parameters of the IR imaging system and MRTD model, and propose a trade-off optimization approach of the IR imaging system. Finally, this paper verifies the rationality of the optimization method by optimizing CA-295 system. The trade-off optimization method proposed in this paper can not only satisfy the design requirement, but also reduce the size, weight, power consumption and cost (SWaP$) of IR imaging systems.