Large View Angle Research Articles

Vicarious Calibration of Orbiting Carbon Observatory-2

Vicarious calibration methods use well-characterized surface sites to complement other on-orbit radiometric calibration techniques. Since 2009, NASA’s Orbiting Carbon Observatory-2 (OCO-2) and Japan’s Greenhouse gasses Observing SATellite teams have conducted annual campaigns at Railroad Valley, NV, USA, for this purpose. These sensors pose special challenges due to their large footprint sizes and view angles. OCO-2 sweeps the playa surface during a targeted overpass of the test site, and records data at a number of viewing angles. The smallest of these is selected for processing, thereby minimizing the off-nadir correction. Surface reflectances at nadir are recorded by the field team, and the Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance product is used to provide the small, off-nadir correction. Another MODIS product, the Level 1B top-of-atmosphere radiance product, is used to validate the results and to provide input into the OCO-2 calibration uncertainty estimate. From 11 experiments, the ratio of radiances reported by the OCO-2 Level 1B data product to those from the field campaigns is 1.01, 1.04, and 1.01 for the three OCO-2 spectral bands. These analyses validate the data product absolute calibration, to within the 5% requirement. The need for executing these experiments will be of continued importance to OCO-3. This sensor has an on-board calibrator that provides a dark signal and lamps for response trends but does not have the on-board solar-diffuser present on OCO-2, and thus cannot track degradations relative to the Sun.

An Optical Interference Suppression Scheme for TCSPC Flash LiDAR Imagers

This paper describes an optical interference suppression scheme that allows flash light detection and ranging (LiDAR) imagers to run safely and reliably in uncontrolled environments where multiple LiDARs are expected to operate concurrently. The issue of optical interference is a potential show-stopper for the adoption of flash LiDAR as a technology of choice in multi-user application fields such as automotive sensing and autonomous vehicle navigation. The relatively large emission angle and field of view of flash LiDAR imagers make them especially vulnerable to optical interference. This work illustrates how a time-correlated single-photon counting LiDAR can control the timing of its laser emission to reduce its statistical correlation to other modulated or pulsed light sources. This method is based on a variable random delay applied to the laser pulse generated by LiDAR and to the internal circuitry measuring the time-of-flight. The statistical properties of the pseudorandom sequence of delays determines the effectiveness of LiDAR resilience against unintentional and intentional optical interference. For basic multi-camera operation, a linear feedback shift register (LFSR) was used as a random delay generator, and the performance of the interference suppression was evaluated as a function of sequence length and integration time. Direct interference from an identical LiDAR emitter pointed at the same object was reduced up to 50 dB. Changing integration time between 10 ms and 100 ms showed a marginal impact on the performance of the suppression (less than 3 dB deviation). LiDAR signal integrity was characterized during suppression, obtaining a maximum relative deviation of the measured time-of-flight of 0.1%, and a maximum deviation of measurements spread (full-width half-maximum) of 3%. The LiDAR signal presented an expected worst-case reduction in intensity of 25%.

Unmanned aerial vehicle oblique image registration using an ASIFT-based matching method

Low-altitude unmanned aerial vehicles (UAV) are widely used to acquire aerial photographs, some of which are oblique and have a large angle of view. Precise, automatic registration of such images is a challenge for conventional image processing methods. We present an affine scale-invariant feature transform (ASIFT)-based method that can register UAV oblique images at a subpixel level. First, we used the ASIFT algorithm to collect initial feature points. Positions of the feature points on corresponding local images were then corrected using the weighted least square matching (WLSM) method. Mismatching points were discarded and a local transform model was estimated using the adaptive normalized cross correlation algorithm, which also provides initial parameters for WLSM. Experiments show that sufficient feature points are collected to successfully register, to the subpixel level, UAV and other images with large angle-of-view variations and strong affine distortions. The proposed method improves the matching accuracy of previous UAV image registration methods.

Radiometric Cross-Calibration of GF-4 PMS Sensor Based on Assimilation of Landsat-8 OLI Images

Earth observation data obtained from remote sensors must undergo radiometric calibration before use in quantitative applications. However, the large view angles of the panchromatic multispectral sensor (PMS) aboard the GF-4 satellite pose challenges for cross-calibration due to the effects of atmospheric radiation transfer and the bidirectional reflectance distribution function (BRDF). To address this problem, this paper introduces a novel cross-calibration method based on data assimilation considering cross-calibration as an optimal approximation problem. The GF-4 PMS was cross-calibrated with the well-calibrated Landsat-8 Operational Land Imager (OLI) as the reference sensor. In order to correct unequal bidirectional reflection effects, an adjustment factor for the BRDF was established, making complex models unnecessary. The proposed method employed the Shuffled Complex Evolution-University of Arizona (SCE-UA) algorithm to find the optimal calibration coefficients and BRDF adjustment factor through an iterative process. The validation results revealed a surface reflectance error of <5% for the new cross-calibration coefficients. The accuracy of calibration coefficients were significantly improved when compared to the officially published coefficients as well as those derived using conventional methods. The uncertainty produced by the proposed method was less than 7%, meeting the demands for future quantitative applications and research. This method is also applicable to other sensors with large view angles.

Dynamic measurement of first-order spatial derivatives of deformations by digital shearography

This paper presents a simple spatial phase shift shearography based on the Michelson interferometer. A novel digital shearography set-up with a large angle of view, which is based on a 4f system is demonstrated. In the system, the Michelson interferometer is used as a shearing device to generate a shearing distance by tilting a small angle in one of the two mirrors. In fact spherical wave fronts become plane after going through the first lens in 4f system. Tilting the mirror in the Michelson interferometer also generates spatial carrier frequency. Sinusoid fitting method is applied to evaluate the phase. This system can generate a phase map of shearography by using only a single image. The effects of shearing angle, the choice of algorithm and comparison result are discussed in detail. The theory and the application are presented.

A Comparison of MODIS/VIIRS Cloud Masks over Ice-Bearing River: On Achieving Consistent Cloud Masking and Improved River Ice Mapping

The capability of frequently and accurately monitoring ice on rivers is important, since it may be possible to timely identify ice accumulations corresponding to ice jams. Ice jams are dam-like structures formed from arrested ice floes, and may cause rapid flooding. To inform on this potential hazard, the CREST River Ice Observing System (CRIOS) produces ice cover maps based on MODIS and VIIRS overpass data at several locations, including the Susquehanna River. CRIOS uses the respective platform’s automatically produced cloud masks to discriminate ice/snow covered grid cells from clouds. However, since cloud masks are produced using each instrument’s data, and owing to differences in detector performance, it is quite possible that identical algorithms applied to even nearly identical instruments may produce substantially different cloud masks. Besides detector performance, cloud identification can be biased due to local (e.g., land cover), viewing geometry, and transient conditions (snow and ice). Snow/cloud confusions and large view angles can result in substantial overestimates of clouds and ice. This impacts algorithms, such as CRIOS, since false cloud cover precludes the determination of whether an otherwise reasonably cloud free grid consists of water or ice. Especially for applications aiming to frequently classify or monitor a location it is important to evaluate cloud masking, including false cloud detections. We present an assessment of three cloud masks via the parameter of effective revisit time. A 100 km stretch of up to 1.6 km wide river was examined with daily data sampled at 500 m resolution, examined over 317 days during winter. Results show that there are substantial differences between each of the cloud mask products, especially while the river bears ice. A contrast-based cloud screening approach was found to provide improved and consistent cloud and ice identification within the reach (95%–99% correlations, and 3%–7% mean absolute differences) between the independently observing platforms. River ice was also detected accurately (proportion correct 95%–100%) and more frequently. Owing to cross-platform compositing, it is possible to obtain an effective revisit time of 2.8 days and further error reductions.

Statistical Analysis of Periodic Oscillations in LASCO Coronal Mass Ejection Speeds

A large set of coronal mass ejections (CMEs, 3463) has been selected to study their periodic oscillations in speed in the Solar and Heliospheric Observatory (SOHO) missions Large Angle and Spectrometric Coronagraph (LASCO) field of view. These events, reported in the SOHOLASCO catalog in the period of time 19962004, were selected based on having at least 11 height-time measurements. This selection criterion allows us to construct at least ten-point speed distance profiles and evaluate kinematic properties of CMEs with a reasonable accuracy. To identify quasi-periodic oscillations in the speed of the CMEs a sinusoidal function was fitted to speed distance profiles and the speed time profiles. Of the considered events 22 revealed periodic velocity fluctuations. These speed oscillations have on average amplitude equal to 87 kms(exp -1) and period 7.8R /241 min (in distance-time). The study shows that speed oscillations are a common phenomenon associated with CME propagation implying that all the CMEs have a similar magnetic flux-rope structure. The nature of oscillations can be explained in terms of magnetohydrodynamic (MHD) waves excited during the eruption process. More accurate detection of these modes could, in the future, enable us to characterize magnetic structures in space (space seismology).

A see-through holographic head-mounted display with the large viewing angle

A novel solution for the large view angle holographic head-mounted display (HHMD) is presented. Divergent light is used for the hologram illumination to construct a large size three-dimensional object outside the display in a short distance. A designed project-type lens with large numerical aperture projects the object constructed by the hologram to its real location. The presented solution can realize a compact HHMD system with a large field of view. The basic principle and the structure of the system are described. An augmented reality (AR) prototype with the size of 50mm×40mm and the view angle above 60° is demonstrated.

Fabrication of Lens With Large View Angle Through Droplet Evaporation for High-Power Light-Emitting Diodes

A simple and low-cost method for fabricating lens with large view angle was proposed based on droplet evaporation for high-power light-emitting diodes (LEDs). In such a method, an immiscible and vaporizable droplet was deposited on the top surface of a liquid silicone lens. By applying a heat load, the silicone was cured and the droplet was evaporated simultaneously. Then, lenses with craters were fabricated for large view-angle emission of LEDs. In experiments, the diameter and depth of the crater on the lens surface were controlled by adjusting the volume of droplet. As a result of the variation of carter size, light intensity distributions (LIDs) of LED modules were obviously modulated. Compared with traditional spherical cap lens, the view angles of the maximum light intensity of the fabricated lens varied from 0° to ±54°. Meanwhile, the maximum light intensity increased by 17%.

Radiometric cross-calibration of Gaofen-1 WFV cameras using Landsat-8 OLI images: A solution for large view angle associated problems

Abstract Four wide-field-of-view (WFV) instruments are onboard the Gaofen-1 (or GF-1) satellite, providing a combined swath of ~ 800 km. However, observations with large view angles pose new challenges for radiometric cross-calibration with a simple image-based method when using Landsat-8 Operational Land Imager (OLI) data as a reference. Based on radiative transfer modeling, a novel radiometric cross-calibration method has been proposed in this study to solve large view angle-associated problems. The Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol products were used to simulate the top of atmosphere (TOA) signal of the reference and target instruments, and the unequal bidirectional effects were corrected using MODIS bi-directional reflectance distribution function (BRDF) products. Extensive validations with both satellite data and in situ measurements revealed an uncertainty of ~ 8% for the newly produced cross-calibration coefficients when they were used to calibrate TOA reflectance for both close-nadir and off-nadir instruments. The improvements are discernable when compared with the official provided coefficients and that were derived using the image-based cross-calibration method. This study demonstrated not only the usefulness of Landsat-8 OLI data in sensor radiometric calibration but also the impressive accuracy of the MODIS BRDF and aerosol products in radiative transfer simulations. The proposed method can be used in the future to monitor and correct potential radiometric degradations of the GF-1 WFV instruments, and it also can be easily extended to other similar satellite missions to conduct radiometric cross-calibrations.

Multiple-exposure colloidal lithography for enhancing light output of GaN-based light-emitting diodes by patterning Ni/Au electrodes.

Ni/Au electrodes with single, twined and triplet hole array patterns light-emitting diodes have been fabricated by multiple-exposure colloidal lithography. It is found that 45.6%, 83.6% and 15.5% improvement in light output at 350 mA has been achieved by patterning Ni/Au electrodes with single, twined, triplet hole arrays. In addition, patterned Ni/Au LEDs possess much larger view angles than non-patterned ones due to scattering effects of light around the holes, especially for triplet hole array patterned Ni/Au LEDs. Our proposed method for fabricating multiple holes structure would be very promising to improve light output power of LEDs when using advanced electrodes.

The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly.

The glasswing butterfly (Greta oto) has, as its name suggests, transparent wings with remarkable low haze and reflectance over the whole visible spectral range even for large view angles of 80°. This omnidirectional anti-reflection behaviour is caused by small nanopillars covering the transparent regions of its wings. In difference to other anti-reflection coatings found in nature, these pillars are irregularly arranged and feature a random height and width distribution. Here we simulate the optical properties with the effective medium theory and transfer matrix method and show that the random height distribution of pillars significantly reduces the reflection not only for normal incidence but also for high view angles.

Latest developments of dynamic holographic three-dimensional display

Holographic three-dimensional (3D) display is a true 3D display technique, which can provide realistic image of a real object or a scene because holography has the ability to reconstruct both the intensity and phase information, i.e., the wave front of the object or scene. Therefore, it could allow the observers to perceive the light as it is scattered by the real object itself without any special eyewear, which is quite different from other 3D display techniques, such as stereoscopic displays and volumetric 3D displays. In this paper, the achievements and developments of the latest new holographic 3D displays are presented. Holographic 3D displays can be divided into static holographic 3D displays and dynamic holographic 3D displays. Here, we briefly introduce the principle of holographic 3D display technique and static holographic 3D displays, and focus on dynamic holographic 3D displays. Large-size, high-resolution and color static holographic 3D displays have already been successfully fabricated and applied in some areas, such as holographic 3D maps and holographic 3D images. However, dynamic holographic 3D displays based on both optical materials and spatial light modulators (SLMs) are still under research, which is a challenge to their applications. Some holographic researchers study the holographic 3D displays based on the SLMs for large-size and large view angle display, but it is difficult to realize them because of limitations of SLMs and there still needs much effort to solve these problems in SLMs. Other holographic researchers work on dynamic holographic materials, such as inorganic crystals, photorefractive polymer, photochromic material etc. The response time and diffraction efficiency are key factors to these materials. Compared with other holographic media, liquid crystals with super-fast response time (about 1 ms) have been reported, which makes it possible to realize video refresh-rate holographic displays. The achievements of dynamic holography, which are helpful for holographic 3D video applications, are presented. Recently, real-time dynamic holographic display has been obtained in super-fast response liquid crystal films, which makes it possible that large-size, high-definition, color holographic 3D video displayers are developed by using these liquid crystal films in the future.

A refined classification of SPEs based on the multienergy channel observations

AbstractWe have investigated characteristics of solar proton events (SPEs) and their association with other types of solar eruption using 42 SPEs observed with SOlar and Heliospheric Observatory/Energetic and Relativistic Nuclei and Electron detector from 1997 to 2012. A velocity dispersion analysis was performed to correctly estimate the onset times of proton flux increase at the solar vicinity. These SPE onset times were compared with those of associated flares, coronal mass ejections (CMEs), and interplanetary type II radio bursts. We found that (i) the proton flux of 13 SPEs (31%) increase during the flare X‐ray intensity is increasing, and the rest 29 SPEs (69%) show onsets well coincident with the first appearance of CMEs in Large Angle and Spectrometric COronagraph field of view. (ii) All flare‐associated SPEs show the flux enhancements starting from the lower energy, while the CME‐associated SPEs show the flux enhancements starting from either the higher or the lower energies. In the other events, the flux enhancement occurs simultaneously at all energies within 10 min. (iii) For the former, large flux enhancements occur in a short time, while the latter tend to show relatively weak and slow flux enhancements. Our classification uses two criteria, SPE onset timing relative to flares and energy‐dependent flux enhancement, unlike the conventional classification of SPEs based on whether the flux time profile is impulsive or gradual. Nevertheless our classification scheme refines the distinction between the flare‐associated SPEs and the CME‐associated SPEs in terms of the onset timing. Additional information on the proton acceleration as implied by the energy‐dependent patterns of flux enhancement is briefly discussed.

Light field moment imaging with the ptychographic iterative engine

The recently developed Light Field Moment Imaging (LMI) is adopted to show the stereoscopic structure of the sample studied in Coherent Diffractive Imaging (CDI), where 3D image were always generated with complicated experimental procedure such as the rotation of the sample and time-consuming computation. The animation of large view angle can be generated with LMI very quickly, and the 3D structure of sample can be shown vividly. This method can find many applications for the coherent diffraction imaging with x-ray and electron beams, where a glimpse of the hierarchical structure required and the quick and simple 3D view of object is sufficient. The feasibility of this method is demonstrated theoretically and experimentally with a recently developed CDI method called Ptychographic Iterative Engine.

Insect-Human Hybrid Eye (IHHE): an adaptive optofluidic lens combining the structural characteristics of insect and human eyes.

Insect compound eyes and human camera eyes are two exquisite optical systems created by nature. The compound eye boasts an angle of view (AOV) up to 180° thanks to its hemispherical arrangement of hundreds of prime microscale lenses. The camera eye, on the other hand, can change shape to focus on objects at various depths, yet accepts light within a smaller AOV. Imitations of either imaging system have been abundant but with limited success. Here, we describe a reconfigurable polymeric optofluidic device that combines the architectural merits of both vision mechanisms, featuring a large AOV (up to 120°) with adaptive focusing capabilities (from 0 to 275 diopter (D)). This device consists of bi-layered microfluidics: an array of millimeter-sized fluidic lenses is integrated into the top layer and arranged on an elastomeric membrane embedded within the bottom layer. The membrane can be deformed from a planar surface into a series of dome-shaped geometries, rearranging individual fluidic lenses in desired curvilinear layouts. Meanwhile, each fluidic lens can vary its radius of curvature for a monocular depth sensation. Such a design presents a new perspective of tunable optofluidics for a broad range of applications, such as robotic vision and medical laparoendoscopy, where adaptive focalization with a large viewing angle is a clear advantage.

Method study and uncertainty analysis of calibration coefficients validation based on the Inner Mongolia test site

Calibration coefficients validation is the foundation for ascertaining the sensor performance and carrying out the quantitative application. Based on the analysis of the differences between the calibration and validation, two calibration coefficients validation methods were introduced in this paper. Taking the HJ-1A satellite CCD1 camera as an example, the uncertainties of calibration coefficients validation were analyzed. The calibration coefficients validation errors were simulated based on the measured data at an Inner Mongolia test site. The result showed that in the large view angle, the ground directional reflectance variation and the atmospheric path variation were the main error sources in calibration coefficients validation. The ground directional reflectance correction and atmospheric observation angle normalization should be carried out to improve the validation accuracy of calibration coefficients.

Improved Mach–Zehnder interferometer-based shearography

Digital shearography based on Mach–Zehnder interferometer suffers from the disadvantage of a small angle of view due to the structure. A novel digital shearography set-up with a large angle of view, which is based on the combination of new CCD technology, a semiconductor laser, a Mach–Zehnder shear interferometer and a 4f system, is demonstrated. Independent control of the shear and the frequency of the spatial carrier can be attained with this set-up. In the optical arrangement, the imaging lens is placed in front of the Mach–Zehnder interferometer instead of behind it as in the traditional digital shearography. So the angle of view is no longer limited by the Mach–Zehnder interferometer. Phase extraction is obtained by both advanced sinusoidal-fitting method and Fourier transform method, which are applied to interference patterns with a spatial carrier in the primary fringes. The experimental implementation of the set-up and the results obtained with it are presented and discussed.

Soil moisture mapping over the central part of the Tibetan Plateau using a series of ASAR WS images

The wide swath (WS) and global monitoring (GM) modes of the Advanced Synthetic Aperture Radar (ASAR) provide for monitoring purposes a good compromise between the spatial coverage, spatial resolution and revisit time. Similar imaging modes will be a part of the Sentinel-1 SAR sensing system. A consequence of operating at the wider swath is, however, the rather large view angle range (about 16–43°), which needs to be corrected for if biophysical variables are to be retrieved from these data sets.A soil moisture retrieval algorithm based on the Integral Equation Method (IEM) scattering model is presented that explicitly simulates the backscatter as a function of the incidence angle instead of normalizing. This algorithm is applied to 150 ASAR WS scenes collected from April 2005 to September 2007 over the central part of the Tibetan Plateau. We selected three of these images covering the study area at different view angles during the winter season for the IEM roughness derivation. Subsequently, soil moisture is retrieved from the entire σo data sets via inversion of the IEM model with the obtained roughness parameters as input. The retrievals are validated against soil moisture measured from July 2006 to September 2007 at four sites representative for a 10×10km2 area. Further, the uncertainties following from assumptions made for the development of the algorithm are also quantified.

Enlarging the angle of view in Michelson-interferometer-based shearography by embedding a 4f system

Digital shearography based on Michelson interferometers suffers from the disadvantage of a small angle of view due to the structure. We demonstrate a novel digital shearography system with a large angle of view. In the optical arrangement, the imaging lens is in front of the Michelson interferometer rather than behind it as in traditional digital shearography. Thus, the angle of view is no longer limited by the Michelson interferometer. The images transmitting between the separate lens and camera are accomplished by a 4f system in the new style of shearography. The influences of the 4f system on shearography are also discussed.