Spectral Aberrations Research Articles

An adaptive image fusion method for Sentinel-2 images and high-resolution images with long-time intervals

Sentinel-2 imagery has garnered significant attention in many earth system studies due to free access and high revisit frequency. Since its spatial resolution is insufficient for many applications, e.g., fine-grained land cover mapping, some studies employ fusion technique that combines high-resolution RGB images with Sentinel-2 multispectral images to improve the resolution of the latter. However, there are two issues in the existing image fusion methods. First, these methods usually assume that the time intervals between images are short (within several days), which is a strong assumption for large-scale high-resolution images and many real-world applications. Second, the spectral discrepancy between multispectral and RGB images could induce spectral aberrations in Sentinel-2 imagery upon fusion. To alleviate these issues, we propose an adaptive image fusion approach named S2IFNet, adaptively fusing images with long-time intervals (from months to years) and spectral inconsistency, thereby increasing the multispectral band resolution of Sentinel-2 imagery. Building on top of the feature extraction and fusion modules, we propose a spectral feature compensation module and a change-aware feature reconstruction module. The former alleviates the possible degradation of spectral attributes in Sentinel-2 imagery resulting from feature fusion. The latter integrates semantic and texture information to avoid adding fake textures caused by land cover changes over time. The experiments demonstrate that S2IFNet surpasses existing image fusion and reference-based super-resolution methods on synthetic and real datasets, yielding fusion results that are clearer and more reliable.

Investigations on the potential application of machine vision lenses for depth measurement by exploiting chromatic aberrations

Chromatic (spectral) aberrations are image imperfections that are disadvantageous for standard image processing tasks and are typically compensated through the application of different types of glass during lens design. The longitudinal chromatic aberration causes a relative unsharpness over different spectral channels. Since this error is corrected in most multi-chromatic lenses, this paper investigates to which extent the shift of the focal planes in a standard lens can be used specifically for image processing applications. Theoretical investigations of the longitudinal chromatic aberration are carried out. Based on this, conditions and a method to generate a 3D depth reconstruction out of different spectral channels are presented.

Programmable pulse shaping for time-gated amplifiers.

We experimentally demonstrate a novel use of a spatial light modulator (SLM) for shaping ultrashort pulses in time-gated amplification systems. We show that spectral aberrations because of the device's pixelated nature can be avoided by introducing a group delay offset to the pulse via the SLM, followed by a time-gated amplification. Because of phase wrapping, a large delay offset yields a nearly-periodic grating-like phase function (or a phase grating). We show that, in this regime, the phase grating periocidity defines the group delay spectrum applied to the pulse, while the grating's amplitude defines the fraction of light that is delayed. We therefore demonstrate that a one-dimensional (1D) SLM pixel array is sufficient to control both the spectral amplitude and the phase of the amplified pulses.

Detection of fresh palm oil adulteration with recycled cooking oil using fatty acid composition and FTIR spectral analysis

ABSTRACTThere is a growing concern over the food safety issue related to increased incidence of cooking oil adulteration with recycled cooking oil (RCO). The objective of this study was to detect fresh palm olein (FPO) adulteration with RCO using fatty acid composition (FAC) and Fourier-transform infrared spectroscopy (FTIR) spectral analyses combined with chemometrics. RCO prepared in the laboratory was mixed with FPO in the proportion ranged from 1% to 50% (v/v) to obtain the adulterated oil samples (AO). FACs for FPO, RCO, and AO were determined using gas chromatography equipped with a flame ionization detector (GC-FID). The compositions of most fatty acids in RCO lied within the normal ranges of Codex standard, except for C8:0, C10:0, C11:0, C15:0, trans C18:1, and polyunsaturated fatty acids (PUFAs), C20:5. PUFAs showed a consistent decreasing trend with increasing magnitude of change with respect to increasing adulteration level and thus might be a good indicator for detecting FPO adulteration with RCO. The evaluation parameters (coefficient of determination, root mean standard error) of the FTIR-partial least square (PLS) model of palm oil adulteration with recycled oil are R2 = 0.995 and 3.25, respectively. For FTIR spectral analysis, the distinct variations in spectral regions and aberrations in characteristic bands between FPO and RCO were observed. The optimized PLS calibration model developed from normal spectral of the combined region at 3602–3398, 3016–2642, and 1845–650 cm−1 overpredict the adulteration level. On the other hand, the discriminant analysis classification model was able to classify the FPO and AO into two distinct groups. Improvement of the principles of combined techniques in authenticating AO from fresh oil is beneficial as a guideline to detect adulteration in cooking oil.

Divergent groove gratings: wavelength scanning in fixed geometry spectrometers.

A new geometric scheme translates a diffraction grating along the straight central groove of an exponentially curved pattern. Lit by a stationary incident beam, the two-dimensional pattern scales isotropically, scanning wavelength without change to any angles, macroscopic distances, curvatures or aberrations. This is exemplified by a new class of self-focused grating monochromator, analyzed by rigorous light-path expansion and numerical raytracing. All spectral aberrations in pure meridional powers (including defocus, coma and spherical aberration) cancel for any choice of angular deviation, magnification and translation range. The residual mixed powers yield Δλ/λ=10-3~10-5 in the soft x-ray for plane and concave gratings at grazing incidence. Over the visible spectrum, Δλ/λ~10-4is shown for plane gratings mounted at Littrow and at normal incidence in reflection or transmission.

Suppressing sampling noise in linear and two-dimensional spectral simulations

We examine the problem of sampling noise encountered in time-domain simulations of linear and two-dimensional spectroscopies. A new adaptive apodization scheme based on physical arguments is devised for suppressing the noise in order to allow reducing the number of used disorder realisations, but introducing only a minimum of spectral aberrations and thus allowing a potential speed-up of these types of simulations. First, the method is demonstrated on an artificial dimer system, where the effect on slope analysis, typically used to study spectral dynamics, is analysed. It is, furthermore, tested on the simulated two-dimensional infrared spectra in the amide I region of the protein lysozyme. The cross polarisation component is investigated, particularly sensitive to sampling noise, because it relies on cancelling of the dominant diagonal spectral contributions. In all these cases, the adaptive apodization scheme is found to give more accurate results than the commonly used lifetime apodization scheme and in most cases better than the gaussian apodization scheme.

Impact of temporal and spectral aberrations on coherent beam combination of nanosecond fiber lasers

The effects of aberrations on coherent beam combination (CBC) of nanosecond fiber lasers are analyzed in this paper. These effects include imperfect overlap of the pulses, spectral phase shift, and nonlinear phase shift, which are induced by optical path difference and self-phase modulation. A typical system of CBC of two lasers is calculated in detail and design guidelines are given.

Automatic Correction of Spectral Aberrations in EELS

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Control of spectral aberrations in a monochromator using a plane holographic chirped grating

A method aimed to minimize the impact of spectral aberrations in a monochromator is proposed in which the spectrum of the source of radiation under study is scanned by the rectilinear translation of a plane chirped grating. The chirped grating, which has a spatially variable groove spacing, is used to diffract and to spectrally focus the radiation. Imaging properties of the chirped grating were analyzed in order to develop the expression of the aberration coefficients of the system and the expression of the width of the instrument line shape due to aberrations. The optimal rectilinear trajectory required to operate the monochromator without significant spectral aberrations in measurements has been obtained numerically and tested in the laboratory. Experimental measurements of the emission spectrum of a seven-wavelength helium-neon laser are presented, as well as the sensitivity of the monochromator performance to different geometrical parameters.

Frequency response and systematic errors in STI measurements

The paper examines two fundamental problems when measuring and verifying the STI of sound systems. These relate to the overall accuracy of the measurement platform in use and the fact that STI does not correctly account for system frequency response and spectral aberrations under certain conditions. As has previously been reported by the author, STI appears to significantly overestimate the intelligibility of limited bandwith sound systems under reverberant, high S/N ratio conditions. Further evidence and word score comparisons for a number of different frequency response conditions, measured under several reverberant situations are presented. The results indicate that errors of up to 30 comparisons with low-reverberant field and anechoic conditions are also made. It is also shown that the measurement platform itself and sound source characteristics can affect the resultant error. Tests, carried out under highly controlled conditions, are reported that enable the causes of the potential errors to be established. The implications of reported limitations in STI and STIPa, when assessing the potential intelligibility of a sound system or acoustics of a space, are discussed.

Role of processing geometry in SAR raw data focusing

Synthetic aperture radar (SAR) systems require that a focusing operation be performed on the received backscattered echoes (raw data) to generate high-resolution microwave images. Either due to platform attitude instabilities, or to Earth rotation effects, the SAR raw data may be acquired in geometries, i.e., with the radar beam directed with an offset angle (squint angle) from the broadside direction. This research investigates the impact of the focusing operation carried out on squinted raw data acquisitions performed by SAR sensors operating in the stripmap mode. To this end the 2D frequency SAR processing approach is generalized with respect to conical, i.e., nonorthogonal, reference systems. This allows analysis of the geometric, spectral, and phase aberrations introduced in the images by the chosen processing geometry with respect to the acquisition, and identification of the focusing procedure that minimizes these aberrations. The whole theory is validated by experimental results carried out on simulated data. Moreover, the extension of this analysis to the interferometric case where these aberrations may have a significant role is also investigated.

Spherical-grating monochromator with a variable-line-spaced grating for synchrotron radiation

An optical design for spherical-grating monochromators for application to synchrotron radiation is presented. High spectral and spatial performance is obtained with a spherical variable-line-spaced grating coupled to a spherical mirror with its tangential plane coincident with the grating's equatorial plane. The monochromator works without an entrance slit in an off-Rowland configuration with a fixed entrance arm and demagnification on the exit slit. The law for groove-space variation of the grating compensates for the main spectral aberrations; spectral focusing in an extended energy range is ensured by a slight change in the exit arm with translations of the order of a few tens of millimeters. The inclusion of a spherical mirror ensures focusing on a plane perpendicular to the plane of spectral dispersion. The ultimate resolution is limited by the slope errors of a single spherical surface. The layout is applied to the design of a high-resolution monochromator for the 1000-250-eV region.

Calcium determination in serum with stable alkaline Arsenazo III and triglyceride clearing

We describe an analytical procedure for determining serum calcium, using the ligand Arsenazo III in an aqueous alkaline medium. The choice of pH for the proposed technique differs from current procedures, which are for the most part carried out in a slightly acidic medium. An acidic medium avoids interference from magnesium, but is spectrophotometrically suboptimal for this pH-dependent reaction: the molar absorptivity of the Arsenazo III complex with calcium at acid pH is 13,787 L mol-1 cm-1, about one-half of that at a more optimal alkaline pH (26,574 L mol-1 cm-1). We have included a clearing technique in the reagent to avoid spectral aberrations from hypertriglyceridemic samples. alpha-Cyclodextrin absorbs the nonesterified fatty acids liberated from triglycerides by a microbial lipase. This modification may also be helpful for binding nonesterified fatty acids, which are known to interfere with calcium procedures by forming calcium soaps and thus preventing the reaction with intended ligands. The use of 8-hydroxyquinoline sulfonate as the magnesium-masking agent facilitates the use of alkaline pH. The less-water-soluble alternative, 8-hydroxyquinoline, commonly used as a precipitating agent for several methods, is difficult to solubilize in the alkaline reagent and tends to precipitate when complexed to magnesium. Finally, the use of alkaline pH results in a prolonged (> or = 6 weeks) shelf life for the reagent.

MATCHDB - A Program for the Identification of Phases Using a Digitized Diffraction-Pattern Database

The availability of automated powder diffractometers, APD, has revolutionized the collection of diffraction data and allowed many improvements in the analysis of these data. The biggest change is the ease of digitizing the diffraction trace rather than preparing it as a strip chart on paper from an analog recorder. When the data are collected properly, the trace is a digitized record of intensity versus 2θ. The trace is an accurate representation of the diffraction pattern containing the sample information along with the spectral and instrument aberrations.

New mounting for X-ray and extreme ultraviolet concave holographic grating

An entirely new mounting for concave holographic gratings for X-ray and the extreme ultraviolet (EUV) has been proposed and its properties have been investigated theoretically. Design parameters for recording a holographic concave grating for minimum spectral image aberrations have been calculated. It is found that this mounting is very suitable.

Hydroperoxysterols as a probe for the mechanism of cytochrome P-450scc-mediated hydroxylation. Homolytic versus heterolytic oxygen-oxygen bond scission.

The interaction of 20-, 23-, and 25-hydroperoxy derivatives of cholesterol with various heme proteins, including the cholesterol side-chain-cleaving enzyme, cytochrome P-450scc, was studied by means of product and spectral analyses. Quasi-Fenton homolytic decomposition via intermediate alkoxy radicals appears to prevail during nonspecific interaction. Highly stereospecific hydroperoxide-driven hydroxylations suggest the absence of free radical species and are interpreted as resulting from a heterolytic type of peroxide decomposition or, alternatively, homolytic decomposition assuming proximal base effect to stabilize a putative intermediate alkoxy radical. Spectral aberrations during the early stages of the latter interaction indicate formation of a ternary iron-peroxo-substrate complex. Decomposition of this complex results in multiple product formation suggesting that peroxide cleavage reverts to regular homolytic decomposition upon denaturation of the enzyme. The implications of these observations for the mechanism of hydroxylation and oxidative carbon-carbon bond scission during enzymic side-chain cleavage of cholesterol are discussed.

Variable line-space gratings: new designs for use in grazing incidence spectrometers

An account is given of a fundamentally different approach to the use of variable line-space gratings for grazing incidence spectrometers. In the new approach, the gratings are flat and they are placed to intercept the converging beam from a collecting mirror rather than the diverging beam from a slit. This results in (1) small aberrations over a wide instantaneous range in wavelength, (2) a modest required variation in line spacing across the ruled width, (3) a simultaneous minimization of both the spectral and image height aberrations, and (4) a completely stigmatic zero order image. The slitless arrangement common to these designs is very compact, having no additional length behind the focal plane of the collecting mirror. In-plane and conical fan designs are considered. One combination design, in the form of an echelle spectrometer, is discussed. The designs are considered to represent ideal candidates for moderate to high resolution spectrometers on such missions as the Far Ultraviolet Spectroscopic Explorer (FUSE, now Columbus) and the Advanced X-ray Astrophysics Facility (AXAF).