Passive Illumination Research Articles

Passive imaging through dense scattering media

Imaging through non-static and optically thick scattering media such as dense fog, heavy smoke, and turbid water is crucial in various applications. However, most existing methods rely on either active and coherent light illumination, or image priors, preventing their application in situations where only passive illumination is possible. In this study we present a universal passive method for imaging through dense scattering media that does not depend on any prior information. Combining the selection of small-angle components out of the incoming information-carrying scattering light and image enhancement algorithm that incorporates time-domain minimum filtering and denoising, we show that the proposed method can dramatically improve the signal-to-interference ratio and contrast of the raw camera image in outfield experiments.

The impact of crustal scattering on body-wave reflection interferometry by cross-correlation and multidimensional deconvolution

SUMMARY Previous studies indicate that scattering may pose a trade-off for the performance of seismic interferometry (SI) applications for retrieving body-wave reflections of a target reflector. While it has been demonstrated that a higher scattering strength of the overburden improves the Green’s function estimated by cross-correlation SI, other theoretical and empirical studies showed that multiple scattering also gives rise to more artefacts. The implications of this trade-off are analysed in this numerical study for a lithospheric scenario with varying crustal scattering strength and passive illumination conditions. In this scenario, we apply SI by cross-correlation to elastodynamic responses to double-couple sources to reconstruct virtual Moho primary reflections. We include multidimensional deconvolution (MDD) methods in the analysis to investigate whether scattering-induced artefacts affect MDD methods in a similar way as was shown for the cross-correlation method. Our results show that there indeed exists a trade-off between the quality of the virtual primary reflection of the target that can be obtained by SI and the scattering strength of the overburden. Furthermore, we find that the full-field MDD method proves to be most resilient to the negative effects of multiple scattering for all illumination conditions and scattering strengths analysed.

A far-field super-resolution imaging method based on phase constructive

Super-resolution imaging under passive illumination is always a crucial issue to be addressed. Because the wavefront of an incoherent light field is not easily detected, it is hard to achieve super-resolution imaging by synthetic aperture under passive illumination. In this work, we proposed a super-resolution method based on phase constructive. We use a quadrature phase-shifting algorithm to retrieve the wavefront from a series of correlation patterns and reconstruct the super-resolution image with a wavefront synthesis algorithm. A simulation result verifies the effectiveness of our proposed method with a USAF-1951 target as the object.

Hydrothermal Evaluation of Vernacular Housing: Comparing Case Studies of Waste PET Bottles, Stone, and Adobe Houses

This work shows the humidity and thermal evaluation of different case studies of houses. The analyses focused on room prototypes and houses built in different regions of Querétaro, México. Three different climatic zones were included with the evaluations of houses and prototypes. The assessments of internal and external parameters in residential buildings are also shown. The internal parameter measurements were done with hydro-thermometers and thermography images. Six structures were evaluated, one of the conventional housing materials and five of alternative or vernacular materials. The predicted mean vote (PMV) and the predicted percentage dissatisfied (PPD) were determined using the average temperature and relative humidity values, and they were considered to be dynamic because they can change depending on many factors. The six dwelling insulating properties were associated with their construction materials, design, location, and other factors. The six houses have many differences, but considering their behavior in their location, the two floor porous stone house got the best results. On the contrary, the flagstone house with a gabled roof of galvanized sheets had a cold perception despite many advantageous elements, failing to provide comfort in such a cold location. Contrasting these with other adobe constructions, the analyzed one had low thermal insulation, which was explained by its characteristics. PET and adobe uninhabited prototypes had medium to cold perceptions. The PET prototype has excellent and adjustable insulations, both on temperature and RH, considering the passive illumination and ventilation of its glass bottle skylights.

Solar-absorbing energy storage materials demonstrating superior solar-thermal conversion and solar-persistent luminescence conversion towards building thermal management and passive illumination

Nowadays, building energy consumption accounts for more than 50% of the total energy consumption. Exploiting advanced solar energy strategy is of great significance to achieve the building energy saving by spontaneously providing energy for a building. Herein, novel solar-absorbing energy storage materials constructed by solar-thermal conversion material, phase change material gel and persistent luminescence material are proposed to efficiently utilize the full spectrum of renewable solar energy towards the building thermal management and passive illumination. A flow chemical synthesis strategy is designed to continuously prepare the solar-thermal conversion material N, S-co-doped conjugated polybenzobisthiazole towards the robust solar energy harvesting (solar absorbance of 94.1%) and considerable thermal energy production (solar-thermal conversion efficiency of 88.3%). The liquid phase blending and sol–gel transition are proposed to fabricate the phase change material gels demonstrating high latent heat of 192.12 Jg−1 and self-supporting capacity, which can store the heat generated by N, S-co-doped conjugated polybenzobisthiazole and release the heat in the night. The persistent luminescence materials can absorb the ultraviolet–visible light in the solar spectrum in the daytime and emit the multicolour luminescence in the night. Taking advantages of the synergistic effect of the functional components, the proposed solar-absorbing energy storage materials demonstrate full spectrum utilization of solar energy (total solar absorbance of 91.93%) towards the building thermal management and passive illumination with a solar-light conversion efficiency of 1.65% and a solar-thermal conversion efficiency of 10.52%. The solar-absorbing energy storage materials create a state-of-the-art alternative for the next-generation energy saving buildings.

Automatic pupil detection and gaze estimation using the vestibulo-ocular reflex in a low-cost eye-tracking setup

Automatic eye tracking is of interest for interaction with people suffering from amyotrophic lateral sclerosis, for using the eyes to control a computer mouse, and for controlled radiotherapy of uveal melanoma. It has been speculated that gaze estimation accuracy might be improved by using the vestibulo-ocular reflex. This involuntary reflex results in slow, compensatory eye movements, opposing the direction of head motion. We therefore hypothesised that leaving the head to move freely during eye tracking must produce more accurate results than keeping the head fixed, only allowing the eyes to move. The purpose of this study was to create a low-cost eye tracking system that incorporates the vestibulo-ocular reflex in gaze estimation, by keeping the head freely moving. The instrument used comprised a low-cost head-mounted webcam which recorded a single eye. Pupil detection was fully automatic and in real time with a straightforward hybrid colour-based and model-based algorithm, despite the lower-end webcam used for recording and despite the absence of direct illumination. A model-based algorithm and an interpolation-based algorithm were tested in this study. Based on mean absolute angle difference in the gaze estimation results, we conclude that the model-based algorithm performed better when the head was not moving and equally well when the head was moving. With most deviations of the points of gaze from the target points being less than 1° using either algorithm when the head is moving freely, it can be concluded that our setup performs fully within the 2° benchmark from literature, whereas deviations when the head was not moving exceeded 2°. The algorithms used were not previously tested under passive illumination. This was the first study of a low-cost eye-tracking setup taking into account the vestibulo-ocular reflex.

Scoring Cercospora Leaf Spot on Sugar Beet: Comparison of UGV and UAV Phenotyping Systems.

Selection of sugar beet (Beta vulgaris L.) cultivars that are resistant to Cercospora Leaf Spot (CLS) disease is critical to increase yield. Such selection requires an automatic, fast, and objective method to assess CLS severity on thousands of cultivars in the field. For this purpose, we compare the use of submillimeter scale RGB imagery acquired from an Unmanned Ground Vehicle (UGV) under active illumination and centimeter scale multispectral imagery acquired from an Unmanned Aerial Vehicle (UAV) under passive illumination. Several variables are extracted from the images (spot density and spot size for UGV, green fraction for UGV and UAV) and related to visual scores assessed by an expert. Results show that spot density and green fraction are critical variables to assess low and high CLS severities, respectively, which emphasizes the importance of having submillimeter images to early detect CLS in field conditions. Genotype sensitivity to CLS can then be accurately retrieved based on time integrals of UGV- and UAV-derived scores. While UGV shows the best estimation performance, UAV can show accurate estimates of cultivar sensitivity if the data are properly acquired. Advantages and limitations of UGV, UAV, and visual scoring methods are finally discussed in the perspective of high-throughput phenotyping.

Joint Waveform and Receiver Design for Co-Channel Hybrid Active-Passive Sensing With Timing Uncertainty

We consider a hybrid active-passive radar system that employs a wireless source as a passive illuminator of opportunity (IO) and a co-channel active radar transmitter operating in the same frequency band to seek spectral efficiency. The hybrid system can take advantage of the strengths of passive radar (e.g., energy efficiency, bi-/multi-static configuration, and spatial diversity) as well as those of active radar (dedicated transmitter, flexible transmit beam steering, waveform optimized for sensing, etc.). To mitigate the mutual interference and location-induced timing uncertainty between the radar and communication signals, we propose two designs for the joint optimization of the radar waveform and receive filters. The first is a max-min (MM) criterion that optimizes a worst-case performance metric over a timing uncertainty interval, and the other a weighted-sum (WS) criterion that forms a weighted sum of the performance metric at each delay within the delay uncertainty interval. Both design criteria result in nonconvex constrained optimization problems that are solved by sequential convex programming methods. When timing uncertainty vanishes, the two designs become identical and admit a simpler solution. Numerical results are presented to demonstrate the performance of the proposed hybrid schemes in comparison with conventional active-only and passive-only radar systems.

Toggling between active and passive imaging with an omni-resonant micro-cavity.

While passive illumination schemes often utilize a broadband spectral acceptance, the performance of active illumination with a laser is improved by narrowband spectral filtering at the sensor. We present an experimental demonstration of an optical cavity structure that is capable of toggling between two performance limits: narrowband resonant and broadband omni-resonant transmission. To achieve omni-resonance without modifying the cavity, the incident optical field is pre-conditioned by associating each wavelength with a particular incidence angle that enables a broad continuous spectrum to resonate with the cavity. This strategy can help seamlessly combine passive and active illumination in the same system.

EyeSpyVR

Low cost virtual reality (VR) headsets powered by smartphones are becoming ubiquitous. Their unique position on the user's face opens interesting opportunities for interactive sensing. In this paper, we describe EyeSpyVR, a software-only eye sensing approach for smartphone-based VR, which uses a phone's front facing camera as a sensor and its display as a passive illuminator. Our proof-of-concept system, using a commodity Apple iPhone, enables four sensing modalities: detecting when the VR head set is worn, detecting blinks, recognizing the wearer's identity, and coarse gaze tracking - features typically found in high-end or specialty VR headsets. We demonstrate the utility and accuracy of EyeSpyVR in a series of studies with 70 participants, finding a worn detection of 100%, blink detection rate of 95.3%, family user identification accuracy of 81.4%, and mean gaze tracking error of 10.8° when calibrated to the wearer (12.9° without calibration). These sensing abilities can be used by developers to enable new interactive features and more immersive VR experiences on existing, off-the-shelf hardware.

Statistical analysis of polarization vectors for target identification

Polarization measurements can be used to differentiate objects of different geometry and material. Polarimetry has been used as a tool to differentiate between two composite geometries, each comprised of a model satellite bus, a solar panel, and a dish antenna. Each component demonstrates unique polarization signatures that can be obtained by analyzing the reflection of passive illumination. An approach for characterizing geometries using Stokes vector–vector space has been demonstrated to be very effective in differentiating between composite geometries representative of orbiting satellites. Our approach lends itself well to a statistical analysis that is applicable for a computational method to provide target identification of space objects.

Novel Laboratory Design of Passive Enhanced Surveillance System of Resident Space Objects

The objective of this study is to present a novel experimental laboratory design for polarimetric exploratory data analysis for the detection and discrimination of dimly lit resident space objects in the presence of starlight or cluttered luminous objects, or in the shadow of the Earth. The experimental results of this study indicate that fusion of polarimetric exploratory data analysis and Stokes parameters lead to reliable figures of merit that can be used as powerful tools for image analysis, detection, characterization, and discrimination of dim targets, under passive illumination. The outcome of this study allows for the design of planned future sensor systems through experimentation, modeling, and simulation, enabling the more accurate assessment of sensor requirements for space-to-space and Earth-to-space sensing of dimly lit resident space objects as well as anticipated sensor detection and discrimination potential.

Digital phase correction of a partially coherent sparse aperture system.

A digital piston phase correction method is presented for partially coherent synthesis with active or passive illumination. An anamorphic pupil relay causes a linear shift of a two subaperture array between the entrance and exit pupils. This shift separates the subapertures' cross- and auto-correlations while retaining their common spatial frequency information. Digital analysis of these common frequencies finds the separation distance and piston phase error of the cross-correlations and enables lossless correction of phase error. Corrected images are diffraction limited. Partial coherence affects the contrast of each spatial frequency, causing optical path difference tolerances to relax as system bandwidth decreases.

The Optics of Anatomy and Light: A Studio-based Investigation of the Construction of Anatomical Images

As anatomical images of the human body increasingly circulate in the current visual and media culture, they belong not only within the domain of scientific enquiry but also exist in a creative field that helps to further define our human identity. Images that expose the inner materiality of the body are becoming naturalized in our everyday lives, as a result of the increased proliferation of medical imagery in various media. The context for this research project is provided by a contemporary investment in what we may term ‘the anatomical gaze’; one that reaches beyond the strictly defined discipline of anatomy. However, this work considers anatomical images at their inception to explore the role light plays in defining the anatomical gaze. Here, light is presented as an instigator rather than a passive illuminator of anatomical knowledge. The studio-based investigation poetically engages with some of the ways in which light impacts upon our negotiations and clarifies our imaginings of the human anatomical body. It also discusses how these experiences are articulated through images. The visual part of the research is comprised of a series of installations that hybridize old and new optical technologies, in ways that expose slippages and meeting points between different ways of visualizing anatomy. The magnification of sight and intensification of light enabled by various technological advancements have allowed us to see what previously remained invisible in the anatomical body. But, more importantly, these technical developments have also provided us with some new ways of conceptualizing these recently discovered bodily structures and their representations. For this reason anatomy is positioned in this research project as a topic that should not be, and in reality cannot be, insulated from cultural concerns. Indeed, anatomy underpins Western ideas of the body, identity and subjectivity, and it is important that it should remain open to critical investigation.

TARGET TRACKING AND RECEIVER PLACEMENT IN MIMO DVB-T BASED PCL

Recently many studies have been done on different issues of using multiple antennas at transmit and receive sides of a radar system in a MIMO (Multiple Input Multiple Output) configuration. In addition, for the electronic warfare‟s purposes, the utilization of available signals of the environment to detect the targets has gained considerable attention in recent years. Such passive radars use the transmitter of other purposes (e.g. the GSM or television stations) as their noncooperative transmitter. Moreover, the idea of using multiple of such illuminators to get the advantages of MIMO technology, besides the advantages of passive illumination, is new and attractive. An excellent candidate of such configuration is a DVB-T SFN (single frequency network). However, various obstacles and considerations appear when working with a MIMO DVB-T based passive radar system. In this paper, we consider two such problems: tracking multiple targets and choosing appropriate positions for the receive antennas in the surveillance region. First, based on the previously developed schemes to associate the receive data to the transmitters and targets, we propose a tracking algorithm to extract the true targets from the candidate targets. Then, we develop a procedure to place the receivers of such MIMO passive systems in such positions that result in better performance of the aforementioned association algorithm.

Real-time vision-based eye state detection for driver alertness monitoring

This paper presents a real-time vision-based system to detect the eye state. The system is implemented with a consumer-grade computer and an uncalibrated web camera with passive illumination. Previously established similarity measures between image regions, feature selection algorithms, and classifiers have been applied to achieve vision-based eye state detection without introducing a new methodology. From many different extracted data of 1,293 pair of eyes images and 2,322 individual eye images, such as histograms, projections, and contours, 186 similarity measures with three eye templates were computed. Two feature selection algorithms, the $$ J_{5} (\xi ) $$ criterion and sequential forward selection, and two classifiers, multi-layer perceptron and support vector machine, were intensively studied to select the best scheme for pair of eyes and individual eye state detection. The output of both the selected classifiers was combined to optimize eye state monitoring in video sequences. We tested the system with videos with different users, environments, and illumination. It achieved an overall accuracy of 96.22 %, which outperforms previously published approaches. The system runs at 40 fps and can be used to monitor driver alertness robustly.

Intelligent Lighting Control for Vision-Based Robotic Manipulation

The ability of a robot vision system to capture informative images is greatly affected by the condition of lighting in the scene. This paper reveals the importance of active lighting control for robotic manipulation and proposes novel strategies for good visual interpretation of objects in the workspace. Good illumination means that it helps to get images with large signal-to-noise ratio, wide range of linearity, high image contrast, and true color rendering of the object's natural properties. It should also avoid occurrences of highlight and extreme intensity unbalance. If only passive illumination is used, the robot often gets poor images where no appropriate algorithms can be used to extract useful information. A fuzzy controller is further developed to maintain the lighting level suitable for robotic manipulation and guidance in dynamic environments. As carried out in this paper, with both examples of numerical simulations and practical experiments, it promises satisfactory results with the proposed idea of active lighting control.

Availability of luminous flux below a bended light-pipe: Design modelling under optimal daylight conditions

To quantify an averaged directionality of light beams transmitted by hollow light guides the asymmetry parameter has been introduced which is computed from the luminous intensity solid or curve. In many situations, the light guides with upper part elbowed toward the Southern sky are able to collect more light than a corresponding straight pipe. However, under different sunlight and skylight conditions the internal reflections are more complex, resulting in a large variety of light-pipe efficiency. A more realistic prediction of the luminous intensity solid under manifold exterior daylight conditions is significant for the optimal design and installation of passive illumination systems using bended light guides. Here, the orientation of the elbow is crucial for an efficient utilization of daylight, especially during a clear day. This study documents that the maximum efficiency of the bended light-pipe is reached when its upper part is pointed nearby the sun position but directly towards the sun.

Systemic approach explored in the context of passive target tracking

For obvious reasons of cost, discretion and reliability costs, locating and tracking aerial targets under an electromagnetically completely passive paradigm, relying exclusively on illuminators of opportunity, is very appealing not only for military but also for civilian tasks. Such a passive radar system could exploit signals emitted by existing commercial television or radio stations or even satellite signals, such as the ones belonging to the global positioning system. This study considers target locating and tracking using a network of passive receivers and/or non-cooperative illuminators (a multi-static radar configuration) by making use of the Doppler shift only. A concept, the systemic approach, is used to combine and interpret information available from different sensors. Both the formalisation of the problem and the hardware and software implementation are presented. Implementation makes use of multi-component polynomial-phase signal models and genetic algorithms. For increased performance, implementation on field programmable gate array is envisaged.

Introduction

Imaging through the atmosphere is an important technology area, with applications in fields such as astronomy and remote sensing. For optimal results, many facets of signal collection and recovery must be understood. These facets include understanding relevant atmospheric effects, optimizing the signal collection system, and effectively processing the collected signals to obtain the desired imagery. Many different types of radiation can be used to obtain images through the atmosphere, including microwave, infrared, and optical. This radiation can be supplied by the object or natural sources (passive illumination), or it can be provided by the user (active illumination). This focus issue contains a number of invited papers describing imaging work being accomplished in the Advanced Optics and Imaging Division of the Air Force Research Laboratory, formerly the Optical Sensing Division of the Air Force Phillips Laboratory. These papers assume that optical or infrared radiation is being used, either actively or passively.