Spectral Reflectance Parameters Research Articles

Compositional and spectrochemical analyses of Cr-spinels in the Sittampundi Anorthosite Complex, Southern India: Implications for remote observation of spinels on the Moon

The spectroscopic techniques such as Laser Raman, Fourier Transform Infrared (FTIR), and hyperspectral have been used widely to understand the mineral chemistry and crystal structure and identify the functional phases and organic molecules in geological materials on Earth and other planetary bodies. The present study used these spectroscopic techniques combined with X-ray Diffraction (XRD) and Electron Probe Micro Analysis (EPMA) to understand the spectral-compositional relationships of the Cr-spinel (Chromian spinel) present in chromitite bodies associated with Sittampundi Anorthosite Complex (SAC), southern India. The bands/lenses of Cr-spinel are found as layers (few centimeters to 6 m thick) intercalated with anorthosites and clinopyroxenites of the SAC. The cumulate Cr-spinels of the study area exhibit compositions ranging between Al-chromite and Cr-spinel. Fe-, Al-rich Cr-spinel is a characteristic of the SAC with Cr2O3 content ranging from ~32–37 wt% and Cr# (Cr/[Cr + Al]) in the 0.44 to 0.53 range. The XRD spectra of these Cr-spinels have shown characteristic peaks corresponding to its constituent phases, with the highest peak at 36.11°. The observed longward shift in the Raman A1g peak (~705–714 cm−1) is likely to be caused by the substitution of Al3+ in the spinel structure. The Raman A1g peak position near 705 cm−1 in the spectra is attributed to the coexistence of (Mg, Fe) in the tetrahedral site and (Al, Cr) in the octahedral site. The broader and stronger 2 µm band position in the hyperspectral data is at relatively shorter wavelengths than typical Cr-spinels due to enhanced Al content (Al2O3 ~ 25 wt%) in the SAC samples. The 2 µm band position is observed to have a longward shift with increasing Cr2O3 and Cr# abundances and a corresponding shortward shift with enhanced Al2O3 content in Cr-spinels. The linear relationship between the 2 µm band position and Cr/Al abundances indicates that this absorption band is significant in distinguishing Cr-spinels from Al-spinels. Based on spectral and compositional resemblance, Fe- and Al-rich Cr-spinels in SAC are considered as probable terrestrial (functional) analogues for similar lunar spinel compositions given that evidence-based correlation of intricate processes involved in their formation under the lunar and terrestrial conditions. The present study demonstrates the approach of applying spectrochemical characteristics of terrestrial analogue spinels for remote identification of lunar spinels and retrieving their compositional ranges from the spectral reflectance parameters.

Photosynthetic performance of Antarctic lichen Dermatocarpon polyphyllizum when affected by desiccation and low temperatures.

Lichens are symbiotic organisms that are well adapted to desiccation/rehydration cycles. Over the last decades, the physiological background of their photosynthetic response-specifically activation of the protective mechanism during desiccation-has been studied at the level of photosystem II of the lichen photobiont by means of several biophysical methods. In our study, the effects of desiccation and low temperatures on chlorophyll fluorescence and spectral reflectance parameters were investigated in Antarctic chlorolichen Dermatocarpon polyphyllizum. Lichen thalli were collected from James Ross Island, Antarctica, and following transfer to a laboratory, samples were fully hydrated and exposed to desiccation at temperatures of 18, 10, and 4°C. During the desiccation process, the relative water content (RWC) was measured gravimetrically and photosynthetic parameters related to the fast transient of chlorophyll fluorescence (OJIP) were measured repeatedly. Similarly, the change in spectral reflectance parameters (e.g.,NDVI, PRI, G, NPCI) was monitored during thallus dehydration. The dehydration-response curves showed a decrease in a majority of the OJIP-derived parameters (e.g., maximum quantum yield of photosystem II photochemistry: FV/FM, and performance index: PI in D. polyphyllizum, which were more apparent at RWCs below 20%. The activation of protective mechanisms in severely dehydrated thalli was documented by increased thermal dissipation (DI0/RC) and its quantum yield (Phi_D0). Low temperature accelerated these processes. An analysis of the OJIP shape reveals the presence of K-bands (300μs), and L-bands (80μs), which can be attributed to dehydration-induced stress. Spectral reflectance indices decreased in a majority of cases with an RWC decrease and were positively related to the OJIP-derived parameters: FV/FM (capacity of photosynthetic processes in PSII), Phi_E0 (effectiveness of electron transport), and PI_tot (totalperformance index), which was more apparent in NDVI. A negative relation was found for NPCI. These indices could be used in follow-up ecophysiological photosynthetic studies of lichens that are undergoing rehydration/dehydration cycles.

Atmospheric Correction of Multispectral Satellite Images Based on the Solar Radiation Transfer Approximation Model

A model of the solar radiation transfer in the Earth’s atmosphere is developed for explicit calculations of the transfer characteristics of the atmosphere (brightness coefficients, transmission, and spherical albedo) in the visible and near-IR atmospheric windows depending on the surface air pressure (or terrain elevation), aerosol optical parameters, and angles indicating the position of the Sun and the satellite system with respect to the Earth’s surface. The model is based on spectrally dependent approximations obtained using the atmospheric radiative transfer code DISORT. The effect of altitudinal stratification of the atmospheric optical parameters on the accuracy of underlying surface spectral albedo retrieval from the brightness coefficients at the top of the atmosphere is investigated. An algorithm is developed for radiometric correction of multispectral satellite images based on the model suggested. Approbation of the algorithm is performed using MERIS (Medium Resolution Imaging Spectrometer) data. It is shown that accounting for optical properties of the atmosphere and adjacency effects in processing satellite images substantially improves the visibility of the underlying surface and renders their spectral reflectance parameters.

Diagnosing ozone stress and differential tolerance in rice (Oryza sativa L.) with ethylenediurea (EDU)

Rising tropospheric ozone concentrations in Asia necessitate the breeding of adapted rice varieties to ensure food security. However, breeding requires field-based evaluation of ample plant material, which can be technically challenging or very costly when using ozone fumigation facilities. The chemical ethylenediurea (EDU) has been proposed for estimating the effects of ozone in large-scale field applications, but controlled experiments investigating constitutive effects on rice or its suitability to detect genotypic differences in ozone tolerance are missing. This study comprised a controlled open top chamber experiment with four treatments (i) control (average ozone concentration 16 ppb), (ii) control with EDU application, (iii) ozone stress (average 77 ppb for 7 h daily throughout the season), and (iv) ozone stress with EDU application. Three contrasting rice genotypes were tested, i.e. the tolerant line L81 and the sensitive Nipponbare and BR28. The ozone treatment had significant negative effects on plant growth (height and tillering), stomatal conductance, SPAD value, spectral reflectance indices such as the normalized difference vegetation index (NDVI), lipid peroxidation, as well as biomass and grain yields. These negative effects were more pronounced in the a priori sensitive varieties, especially the widely grown Bangladeshi variety BR28, which showed grain yield reductions by 37 percent. EDU application had almost no effects on plants in the absence of ozone, but partly mitigated ozone effects on foliar symptoms, lipid peroxidation, SPAD value, stomatal conductance, several spectral reflectance parameters, panicle number, grain yield, and spikelet sterility. EDU responses were more pronounced in sensitive genotypes than in the tolerant L81. In conclusion, EDU had no constitutive effects on rice and partly offset negative ozone effects, especially in sensitive varieties. It can thus be used to diagnose ozone damage in field grown rice and for distinguishing tolerant (less EDU-responsive) and sensitive (more EDU-responsive) genotypes.

Vegetation mapping of moss-dominated areas of northern part of James Ross Island (Antarctica) and a suggestion of protective measures

James Ross Island is a large (2500 km) island situated in the north-western part of the Weddell Sea, close to the northern tip of the Antarctic Peninsula. The island is rich in vegetation oases, i.e. areas covered by autotrophic organisms comprising a great variety of terrestrial algae, cyanobacteria, lichens and mosses. In this study, a GPS method was used to measure moss-dominated vegetation areas located at northern part of James Ross Island. Several small-scale (above 400 m2) areas were selected for the measurements of their shape, total area, boarder line, mean slope, and exposition as well as biodiversity of mosses. The areas of interest were located particularly in neighbourhood of the Big and Small Lachman lakes and northern slopes of Berry Hill mesa. Typically, the moss-dominated areas were supplied with melt water from neighbouring snow fields formed in previous austral winter season. Specimens of mosses were collected from each site so that dominating species forming community structure could be evaluated. Individual areas differed in species richness, however, Bryum pseudotriquetrum and Hypnum revolutum dominated most of them. Sanionia uncinata was found at the only one of the investigated areas and might be considered as rare species for James Ross Island. In the paper, shape location and area of moss-dominated spots at James Ross Island is reported and discussed in relation to morphological characteristics of particular sites as well as water availability during austral summer season. Potential of the data for geographical information systems (GIS), further application of unmanned aerial vehicle (UAV) in vegetation mapping, and spectral reflectance parameters measurements of Antarctic moss-dominated areas are discussed. For particular areas, protective measures are suggested to minimize human disturbance of the sites and provide a platform for a long-term study of biodiversity.

Computation of optimal metamers

A fast, accurate model for computing Logvinenko's optimal metamers is introduced. The spectral reflectance of an optimal metamer is uniquely determined from a class of metamers with common tristimulus values. A set of optimal metamers is useful for evaluating the color reproduction and gamut of object colors under different illuminants. In conventional methods, optimal metamers are calculated by interpolating the coordinates of precomputed optimal colors. This model optimizes the spectral reflectance parameters efficiently without switching between bandpass (Type I) and bandstop (Type II) optimal metamers, or requiring any stored optimal-color datasets. Some optimal metamer loci computed using MATLAB are presented.

A comparison of physiological indicators of sublethal cadmium stress in wetland plants

Physiological indices, including photosynthesis, chlorophyll fluorescence, adenylate energy charge (AEC) ratio, and leaf reflectance, were determined for Typha domingensis and Spartina alterniflora in response to increasing concentrations of Cd and compared with the growth responses of these species. Leaf expansion, the live/total ratio of plant aboveground biomass, and the aboveground regrowth rate after the initial harvests were significantly reduced with increasing Cd concentration in the growth medium. Of the four physiological responses measured, only photosynthesis and AEC responded to the Cd treatment before damage was visually apparent. Also, these indices were significantly correlated with leaf expansion rate and live/total ratio in most instances. Except at the end of the experiment, when the most stressed plants began to die, the Fv/Fm ratio was not significantly affected by the Cd treatment. The leaf spectral reflectance parameters showed no significant change during the entire treatment period. The significant correlation between the stress indicators and plant growth supported the findings that photosynthesis and AEC were the most responsive of the indicators tested, however, further research investigating other chlorophyll fluorescence and leaf reflectance parameters may demonstrate as well the value of these indicators in quantifying sublethal stress.

Image segmentation by object color: a unifying framework and connection to color constancy

A unifying framework is presented for algorithms that use the bands of a multispectral image to segment the image at material (i.e., reflectance) boundaries while ignoring spatial inhomogeneities incurred by accidents of lighting and viewing geometry. The framework assumes that the visual stimulus (image field) from a uniformly colored object is the sum of a small number of terms, each term being the product of a spatial and a spectral part. Based on this assumption, several quantities depending on the reflected light can be computed that are spatially invariant within object boundaries. For an image field either from two light sources on a matte surface or from a single light source on a dielectric surface with highlights, the invariants are the components of the unit normal to the plane in color space spanned by the pixels from the object. In some limited cases the normal to the plane can be used to estimate spectral-reflectance parameters of the object. However, in general the connection of color-constancy theories with image segmentation by object color is a difficult problem. The concomitant constraints on segmentation and color-constancy algorithms are discussed in light of this fact.