Analysis and application of multiframe superresolution processing for conventional imaging systems and lenslet arrays

Low-cost compact sensors for ultrasmall systems are a pressing need in many new applications. One potential solution is a shallow aspect ratio system using a lenslet array to form multiple undersampled subimages of a scene on a single focal plane array, where processing techniques then produce an upsampled restored image. We have investigated the optimization and theoretical limits of the performance of such arrays. We have built a hardware simulator and developed algorithms to process imagery similar to that of a full lenslet imaging sensor, which allowed us to quickly test optical components, algorithms, and complete system designs for future lenslet imaging systems.     

Patterning of multilayer dielectric optical coatings for multispectral CCDs

The development of optical sensors for spacecraft applications requires that all components be as lightweight as possible. One method to reduce the weight of a multispectral optical system is to eliminate beamsplitting optics and multiple detectors by patterning a filter array directly onto a CCD. However, techniques commonly used in the production of these filter arrays result in decreased image resolutions. This can greatly impact the performance of sensors used for applications such as planetary probes. To address this issue, we have studied the patterning of multilayer dielectric optical coatings in a small scale, two dimensional array, which will allow development of a four color sensor with a resolution one-half that of monochromatic sensors (compared to one-fourth or less for a four color striped array). We have developed ion milling techniques for the preparation of optical filter arrays which are patterned on a scale as small as 7.5 μm, enabling each pixel of a CCD to have its own associated filter. This paper presents details of the fabrication of these multispectral arrays, and discusses problems associated with pixel-sized filters.     

A Simulator for X-ray images

A simulator for X-ray images is presented based on a virtual X-ray source and a virtual human body obtained from tomographic slices. In the simulator it is possible to modify the tube potential, the anodic current, the exposure time, the filtration and some geometric parameters such as source-skin distance, orientation and field size. The virtual body consists of a three-dimensional voxel matrix in which CT numbers for each point of the body are stored. The interactions of X rays passing through the body are evaluated using the pencil beam technique. The image is obtained by computing the dose absorbed by the detector and converting it into optical density using a proper response function. The image spatial resolution is limited by the voxel size. The influence of each parameter on the image quality can be observed interactively. The dose absorbed in each point of the body is an important parameter obtained as output of the simulator.     

Design and assessment of microlenslet-array relay optics

Recent progress in micro-optics fabrication and optical modeling software opens the opportunity to investigate how microlenslet-array-based compact relay systems can be designed and assessed. We present various optical configurations that include an appropriate baffle computation to eliminate ghost images, followed by an analysis of image quality. The investigation shows the existing trade-off between compactness of the system and a tiling effect observed in the corresponding image, an effect we refer to as lensletization. To yield meaningful optical modeling results, we provide insight into ray-tracing optimization while ensuring a sufficient signal-to-noise ratio. The results show that, given no discernable lensletization, the most compact configuration to image gray-scale images is the 5f-based system. Finally, simulations of the imaging of gray scale and color bitmaps through microlenslet arrays are demonstrated for the first time to our knowledge.     

Wavefront sensor architectures fully embedded in an image sensor

Several architectures of wavefront sensors have been developed since the rise of adaptive optics. In all cases, optical elements are placed in front of image sensors. This makes the sensor quite bulky, expensive, and sensitive to optical misalignment. I propose two novel architectures fully embedded in the image sensor that require no additional optical element. The sensor can be placed directly in the beam to analyze, leading to small, easy to use, and cost-efficient systems. The two architectures are described before testing by simulation of their ability to sense the wavefront distortion and their sensitivity to signal-to-noise ratio.     

TESSA—A new greedy heuristic for facilities layout planning

TESSA is a heuristic for determining which facilities should be adjacent in a planar layout. Once the adjacencies are known the block plan can be constructed by existing techniques. TESSA overcomes problems with earlier heuristics for determining adjacencies as it does not require planarity testing nor does it restrict the type of layout produced. The algorithm is polynomial in time and produces good quality solutions, almost all of which are above 90% of the (often unattainable) upper bound.     

Binary spatial amplitude modulation of continuous transverse modal electric field using a single lens for mode selectivity in multimode fiber

The demand for portable real-time optical applications such as medical optical sensors and optical transceivers instigate the need for compact optical designs. The aim of this paper is to demonstrate a new, compact design for binary spatial amplitude modulation in a mode-selective transmitter in a multimode fiber, adapted from microscopy. Results show that it is possible to retrieve the original continuous-amplitude transverse modal electric field from the binary amplitude-modulated image using a single lens. The retrieved image is in good agreement with the original image.     

三轴紫外光学成像敏感器

设计了一种三轴紫外敏感器,解决了近地轨道大视场观测问题。通过非常规光学系统,即反射阵列组合式光学系统,将一圆型大视场分成八个子视场成像、叠加后经图像处理,为卫星提供三轴姿态信息和自主导航数据。三轴紫外敏感器标定精度为偏航小于0.05°,俯仰小于0.05°,滚动小于0.05°。研制成的工程样机,通过观星试验和利用星模拟器测试,三轴紫外敏感器可以观测到5等星;紫外波段敏感器可以作为是深空探测航天器重要敏感器之一。     

An automatic system using mobile-agent software to model the calculation process of a chemical vapor deposition film deposition simulator

We have developed an automatic modeling system for calculation processes of the simulator to reproduce experimental results of chemical vapor deposition (CVD), in order to decrease the calculation cost of the simulator. Replacing the simulator by the mathematical models proposed by the system will contribute towards decreasing the calculation costs for predicting the experimental results. The system consists of a mobile agent and two software resources in computer networks, that is, generalized modeling software and a simulator reproducing cross-sections of the deposited films on the substrates with the micrometer- or nanometer-sized trenches. The mobile agent autonomously creates appropriate models by moving to and then operating the software resources. The models are calculated by partial least squares regression (PLS), quadratic PLS (QPLS) and error back propagation (BP) methods using artificial neural networks (ANN) and expresses by mathematical formulas to reproduce the calculated results of the simulator. The models show good reproducibility and predictability both for uniformity and filling properties of the films calculated by the simulator. The models using the BP method yield the best performance. The filling property data are more suitable to modeling than film uniformity.     

Characterization of microscopic damage in composite laminates and real-time monitoring by embedded optical fiber sensors

First, a methodology for observation and modeling of microscopic damage evolution in quasi-isotropic composite laminates is presented. Based on the damage observation using both an optical microscope and a soft X-ray radiography, a damage mechanics analysis is conducted to formulate the stiffness change due to transverse cracking. Then, both energy and stress criteria are combined to provide a valid procedure to predict the transverse crack evolution. The theoretical prediction is found to agree well with the experimental results for the transverse crack density as a function of strain as well as stress–strain curves. Then, another methodology is introduced using two kinds of embedded optical fiber sensors to detect and monitor the transverse crack evolution in composite laminates. One is plastic optical fibers (POF), where the loss in optical power is generated by local deformation of POF due to transverse cracking. The other is fiber Bragg grating (FBG) sensors, where the local strain distribution within the FBG gage length due to transverse cracking alters the power spectrum of the light reflected from the FBG sensors. Embedded optical fiber sensors are found to be a powerful method to detect and monitor the transverse crack evolution in composite laminates.     

Integrated analysis and design of analog and digital processing in imaging systems: introduction to the feature issue

This feature presents themes arising from the Integrated Computational Imaging Systems topical meeting sponsored by the OSA in fall 2001. These themes revolve around joint design of digital and analog image processing to improve the specificity and efficiency of optical image sensors.     

An Artificial Flexible Visual Memory System Based on an UV‐Motivated Memristor

For the mimicry of human visual memory, a prominent challenge is how to detect and store the image information by electronic devices, which demands a multifunctional integration to sense light like eyes and to memorize image information like the brain by transforming optical signals to electrical signals that can be recognized by electronic devices. Although current image sensors can perceive simple images in real time, the image information fades away when the external image stimuli are removed. The deficiency between the state‐of‐the‐art image sensors and visual memory system inspires the logical integration of image sensors and memory devices to realize the sensing and memory process toward light information for the bionic design of human visual memory. Hence, a facile architecture is designed to construct artificial flexible visual memory system by employing an UV‐motivated memristor. The visual memory arrays can realize the detection and memory process of UV light distribution with a patterned image for a long‐term retention and the stored image information can be reset by a negative voltage sweep and reprogrammed to the same or an other image distribution, which proves the effective reusability. These results provide new opportunities for the mimicry of human visual memory and enable the flexible visual memory device to be applied in future wearable electronics, electronic eyes, multifunctional robotics, and auxiliary equipment for visual handicapped.     

Sun and aureole spectrometer for airborne measurements to derive aerosol optical properties

We have designed an airborne spectrometer system for the simultaneous measurement of the direct Sun irradiance and aureole radiance. The instrument is based on diffraction grating spectrometers with linear image sensors. It is robust, lightweight, compact, and reliable, characteristics that are important for airborne applications. The multispectral radiation measurements are used to derive optical properties of tropospheric aerosols. We extract the altitude dependence of the aerosol volume scattering function and of the aerosol optical depth by using flight patterns with descents and ascents ranging from the surface level to the top of the boundary layer. The extinction coefficient and the product of single scattering albedo and phase function of separate layers can be derived from the airborne measurements.     

Diagnosis based on reliability analysis using monitors and sensors

We develop a process for using monitors or sensors to optimize diagnostic decision trees (DDTs) generated for large systems. We present algorithms for optimizing the diagnosis process, which combines evidence data captured from monitors or sensors into the diagnostic tree generation process to produce DDTs. Since evidence data can be extracted from monitors and sensors, we developed a method for sensor modeling. Our method allows modeling monitors or sensors as an abstract layer on top of a systems fault tree model. This method of modeling allows the designer to graphically link monitors or sensors to the components that they monitor, without impacting the reliability analysis. We use a real system from the industry to demonstrate the practicality and effectiveness of our algorithms and methods.     

Image sharpness and beam focus VLSI sensors for adaptive optics

High-resolution wavefront control for adaptive optics requires accurate sensing of a measure of optical quality. We present two analog very-large-scale-integration (VLSI) image-plane sensors that supply real-time metrics of image and beam quality, for applications in imaging and line-of-sight laser communication. The image metric VLSI sensor quantifies sharpness of the received image in terms of average rectified spatial gradients. The beam metric VLSI sensor returns first and second order spatial moments of the received laser beam to quantify centroid and width. Closed-loop wavefront control of a laser beam through turbulence is demonstrated using a spatial phase modulator and analog VLSI controller that performs stochastic parallel gradient descent of the beam width metric.     

Transient Response and Fixed Pattern Noise in Logarithmic CMOS Image Sensors

Logarithmic CMOS image sensors are appealing for their high-contrast and high-speed response but they require postprocessing to achieve high-quality images. Previously published work has explained the fixed pattern noise (FPN) in these image sensors using a steady-state analysis. This paper explains how the transient response of the readout circuit may also contribute to FPN. Thus, the performance of these CMOS cameras may be optimized with a proper understanding of the transient response, which is explained here through modeling and simulation with some experimental validation. In particular, the gain variation of a logarithmic camera is shown to be caused primarily by premature digitization. As logarithmic and linear active pixel sensors use similar circuits, some results in this paper, e.g., an analysis of readout capacitance, apply equally to the latter.     

Partial analytical solution of a model used for measuring oxygen diffusion coefficients of polymer films by luminescence quenching

Many aspects of optical chemical sensor design would benefit from a better knowledge of the diffusion properties of the analyte in the polymer host. The response times of such sensors to a step change of analyte concentration are of vital interest for many applications of fast-responding sensors. Further, the diffusion properties govern their quenching behavior and their sensitivity. A method for determination of the diffusion constant of oxygen in polymers has been developed and used by several groups in the past. The underlying mathematical model for luminescence quenching by molecules of a gas in a single sensing layer on an impermeable support has not yet been completely derived in an analytical form and still uses tedious numerical methods. We present a partial analytical solution to the problem of modeling the time dependence of luminescence generated by in- or out-diffusion of a gaseous quencher in a polymer film in which a luminophor is immobilized and offer a suitable method to predict sensor response times.     

Design Considerations for Two CCD-based Star Sensors

Solid-state image sensors such as charge-coupled devices (CCD) are being considered as the detectors in future star sensors required for the high-accuracy attitude control of spacecraft. Such detectors are adequate in respect of their electro-optical properties, and offer a fundamental advantage in geometrical stability in addition to their obvious practical advantages over other types of image scanner. However, imperfections inherent in present semiconductor processing technology necessitate complex calibration procedures if their full performance is to be realized. Other properties, such as the small image formats, complicate the optical design. Measures taken to overcome such design difficulties must be considered in relation to their impact on the strict weight, size, power and reliability specifications imposed on star sensors. These problems are illustrated with reference to two specific requirements: for a general-purpose star sensor with 1 arc second accuracy and for a 10 to 30 arc second Polaris sensor.     

Noise-resilient estimation of optical flow by use of overlapped basis functions

Conventional techniques for the computation of optical flow from image gradients are used to formulate the problem as a nonlinear optimization that comprises a gradient constraint term and a field smoothness factor. The results of these techniques are often erroneous, highly sensitive to noise and numerical precision, determined sparsely, and computationally expensive. We regularize the gradient constraint equation by modeling optical flow as a linear combination of a set of overlapped basis functions. We develop a theory for estimating model parameters robustly and reliably. We prove that the extended-least-squares solution proposed here is unbiased and robust to small perturbations in the gradient estimates and to mild deviations from the gradient constraint. Our solution is obtained with a numerically stable sparse matrix inversion, which gives a reliable flow-field estimate over the entire frame. To validate our claims, we perform a series of experiments on standard benchmark data sets at a range of noise levels. Overall, our algorithm outperforms by a wide margin the others considered in the comparison. We demonstrate the applicability of our algorithm to image mosaicking and to motion superresolution through experiments on noisy compressed sequences. We conclude that our flow-field model offers greater accuracy and robustness than conventional optical flow techniques in a variety of situations and permits real-time operation.     

Information theoretical optimization for optical range sensors

Most of the known optical range sensors require a large amount of two-dimensional raw data from which the three-dimensional (3D) data are decoded and so are associated with considerable cost. The cost arises from expensive hardware as well as from the time necessary to acquire the images. We will address the question of how one can acquire maximum shape information with a minimum amount of image raw data, in terms of information theory. It is shown that one can greatly reduce the amount of raw data needed by proper optical redundancy reduction. Through these considerations, a 3D sensor is introduced, which needs only a single color (red-green-blue) raw image and still delivers data with only approximately 2-microm longitudinal measurement uncertainty.