地面反射波谱与卫星磁带记录数据间关系模拟的初步探讨──以冬小麦和土壤为例

根据冬小麦和土壤地面反射波谱测试数据，计算了在卫星高度上与卫星磁带数据相对应波段的辐亮度值，对ＮＯＡＡＡＶＨＲＲ和ＴＭ某些通道的差值绿度植被指数ＤＶＩ、归一化绿度植被指数ＮＤＶＩ和比值绿度植被指数ＲＶＩ的分析，从理论上证明了目前采用ＴＭＤＶＩ＿（４，３）提取冬小麦种植面积和ＮＯＡＡＮＤＶＩ＿（２，１）区分植被和土壤背景的有效性。同时在冬小麦种植面积和长势监测方面提出了一些新建议。     

Comparative analysis of urban reflectance and surface temperature

Urban environmental conditions are strongly dependent on the biophysical properties and radiant thermal field of the land cover elements in the urban mosaic. Observations of urban reflectance and surface temperature provide valuable constraints on the physical properties that are determinants of mass and energy fluxes in the urban environment. Consistencies in the covariation of surface temperature with reflectance properties can be parameterized to represent characteristics of the surface energy flux associated with different land covers and physical conditions. Linear mixture models can accurately represent Landsat ETM+ reflectances as fractions of generic spectral endmembers that correspond to land surface materials with distinct physical properties. Modeling heterogeneous land cover as mixtures of rock and/or soil Substrate, Vegetation and non-reflective Dark surface (SVD) generic endmembers makes it possible to quantify the dependence of aggregate surface temperature on the relative abundance of each physical component of the land cover, thereby distinguishing the effects of vegetation abundance, soil exposure, albedo and shadowing. Comparing these covariations in a wide variety of urban settings and physical environments provides a more robust indication of the global variability in these parameter spaces than could be inferred from a single study area. A comparative analysis of 24 urban areas and their non-urban peripheries illustrates the variability in the urban thermal fields and its dependence on biophysical land surface components. Contrary to expectation, moderate resolution intra-urban variations in surface temperature are generally as large as regional surface heat island signatures in these urban areas. Many of the non-temperate urban areas did not have surface heat island signatures at all. However, the multivariate distributions of surface temperature and generic endmember fractions reveal consistent patterns of thermal fraction covariation resulting from land cover characteristics. The Thermal-Vegetation (TV) fraction space illustrates the considerable variability in the well-known inverse correlation between surface temperature and vegetation fraction at moderate (< 100 m) spatial resolutions. The Thermal-Substrate (TS) fraction space reveals energetic thresholds where competing effects of albedo, illumination and soil moisture determine the covariation of maximum and minimum temperature with illuminated substrate fraction. The dark surface endmember fraction represents a fundamental ambiguity in the radiance signal because it can correspond to either absorptive (e.g. low albedo asphalt), transmissive (e.g. deep clear water) or shadowed (e.g. tree canopy shadow) surfaces. However, in areas where dark surface composition can be inferred from spatial context, the different responses of these surfaces may still allow them to be distinguished in the thermal fraction space.     

Measurement of solid fat content by ultrasonic reflectance in model systems and chocolate

A series of confectionery coating fat (CCF) and cocoa butter (CB) dispersions in corn oil were prepared and tempered to a range of temperatures (10, 12 and 15 °C and 26, 27.5 and 29 °C respectively) to produce samples with solid fat contents (SFC) between 0 and 9%. The ultrasonic velocity in the CCF samples increased with increasing SFC and decreased with temperature, however the CB samples were too attenuating to allow velocity measurements. The ultrasonic reflectance at the surface of both sets of dispersions decreased linearly with SFC but at the same level of solids was less for CB than CCF suggesting some dependence on other aspects of fat structure. The microstructures and thermal properties of the fat systems are compared in an attempt to understand these structural differences. Finally ultrasonic reflectance is shown to respond to changes in a chocolate melt during tempering.     

Estimation of soil clay and calcium carbonate using laboratory, field and airborne hyperspectral measurements

This paper examines how reflectance spectrometry used in the laboratory to estimate clay and calcium carbonate (CaCO₃) soil contents can be applied to field and airborne measurements for soil property mapping. A continuum removal (CR) technique quantifying specific absorption features of clay (2206 nm) and CaCO₃ (2341 nm) was applied to laboratory, field and airborne HYMAP reflectance measurements collected in 2003 (33 sites) and 2005 (19 sites) over bare soil sites of a few meters within the La Peyne Valley area, southern France. Nine intermediate stages from the laboratory up to HYMAP sensor measurements were considered for separately evaluating the possible degradation of estimation performances when going across scales and sensors, e.g. radiometric calibration, spectral resolution, spatial variability, illumination conditions, and surface status including roughness, soil moisture and presence and nature of pebbles.Significant relationships were observed between clay and CaCO₃ contents and CR values computed respectively at 2206 nm and 2341 nm from reflectance measurements at the laboratory level with an ASD spectrophotometer up to the HYMAP spectro-imaging sensor. Performances of clay and CaCO₃ estimations decreased from the laboratory to airborne scales. The main factors inducing uncertainties in the estimates were radiometric and wavelength calibration uncertainties of the HYMAP sensor as well as possible residual atmospheric effects.     

Stereo Light Probe

In this paper we present a practical, simple and robust method to acquire the spatially‐varying illumination of a real‐world scene. The basic idea of the proposed method is to acquire the radiance distribution of the scene using high‐dynamic range images of two reflective balls. The use of two light probes instead of a single one allows to estimate, not only the direction and intensity of the light sources, but also the actual position in space of the light sources. To robustly achieve this goal we first rectify the two input spherical images, then, using a region‐based stereo matching algorithm, we establish correspondences and compute the position of each light. The radiance distribution so obtained can be used for augmented reality applications, photo‐realistic rendering and accurate reflectance properties estimation. The accuracy and the effectiveness of the method have been tested by measuring the computed light position and rendering synthetic version of a real object in the same scene. The comparison with standard method that uses a simple spherical lighting environment is also shown.     

Responses of the reflectance indices PRI and NDVI to experimental warming and drought in European shrublands along a north-south climatic gradient

The aim of this study was to evaluate the use of ground-based canopy reflectance measurements to detect changes in physiology and structure of vegetation in response to experimental warming and drought treatment at six European shrublands located along a North-South climatic gradient. We measured canopy reflectance, effective green leaf area index (green LAIe) and chlorophyll fluorescence of dominant species. The treatment effects on green LAIe varied among sites. We calculated three reflectance indices: photochemical reflectance index PRI [531 nm; 570 nm], normalized difference vegetation index NDVI₆₈₀ [780 nm; 680 nm] using red spectral region, and NDVI₅₇₀ [780 nm; 570 nm] using the same green spectral region as PRI. All three reflectance indices were significantly related to green LAIe and were able to detect changes in shrubland vegetation among treatments. In general warming treatment increased PRI and drought treatment reduced NDVI values. The significant treatment effect on photochemical efficiency of plants detected with PRI could not be detected by fluorescence measurements. However, we found canopy level measured PRI to be very sensitive to soil reflectance properties especially in vegetation areas with low green LAIe. As both soil reflectance and LAI varied between northern and southern sites it is problematic to draw universal conclusions of climate-derived changes in all vegetation types based merely on PRI measurements. We propose that canopy level PRI measurements can be more useful in areas of dense vegetation and dark soils.     

FluorMODleaf: A new leaf fluorescence emission model based on the PROSPECT model

A new model of chlorophyll a fluorescence emission by plant leaves, FluorMODleaf, is presented. It is an extension of PROSPECT, a widely used leaf optical properties model that regards the leaf as a pile of N absorbing and diffusing elementary plates. In FluorMODleaf, fluorescence emission of an infinitesimal layer of thickness dx is integrated over the entire elementary plate. The fluorescence source function is based on the excitation spectrum of diluted isolated thylakoids and on the emission spectra of isolated photosystems, PSI and PSII, which are the main pigment-protein complexes involved in the initial stages of photosynthesis. Scattering within the leaf is produced by multiple reflections within and between elementary plates. The input variables of FluorMODleaf are: the number of elementary plates N, also called leaf structure parameter, the total chlorophyll content C_ab, the total carotenoid content C_cx, the equivalent water thickness C_w, and the dry matter content C_m (or leaf mass per area), as in the new PROSPECT-5, plus the σ_II/σ_I ratio referring to the relative absorption cross section of PSI and PSII, and the fluorescence quantum efficiency of PSI and PSII, τ_I and τ_II, that are introduced here as mean fluorescence lifetimes. The model, which considers the reabsorption of emitted light within the leaf, allows good quantitative estimation of both upward and downward apparent spectral fluorescence yield (ASFY) at different excitation wavelengths from 400 nm to 700 nm. It also emphasizes the role of scattering in fluorescence emission by leaves having high chlorophyll content.     

Continuous wavelet analysis for the detection of green attack damage due to mountain pine beetle infestation

Mountain pine beetle (Dendroctonus ponderosae Hopkins) is the most destructive insect infesting mature pine forests in North America and has devastated millions of hectares of forest in western Canada. Past studies have demonstrated the use of multispectral imagery for remote identification and mapping of visible or red attack damage in forests. This study aims to detect pre-visual or green attack damage in lodgepole pine needles by means of hyperspectral measurements, particularly via continuous wavelet analysis. Field measurements of lodgepole pine stands were conducted at two sites located northwest of Edmonton, Alberta, Canada. In June and August of 2007, reflectance spectra (350-2500 nm) were collected for 16 pairs of trees. Each of the 16 tree pairs included one control tree (healthy), and one stressed tree (girdled to simulate the effects of beetle infestation). In addition, during the period of June through October 2008, spectra were collected from 15 pairs of control- and beetle-infested trees. Spectra derived from these 31 tree pairs were subjected to a continuous wavelet transform, generating a scalogram that compiles the wavelet power as a function of wavelength location and scale of decomposition. Linear relationships were then explored between the wavelet scalograms and chemical properties or class labels (control and non-control) of the sample populations in order to isolate the most useful distinguishing spectral features that related to infested or girdled trees vs. control trees.A deficit in water content is observed in infested trees while an additional deficit in chlorophyll content is seen for girdled trees. The measurable water deficit of infested and girdled tree samples was detectable from the wavelet analysis of the reflectance spectra providing a novel method for the detection of green attack. The spectral features distinguishing control and infested trees are predominantly located between 950 and 1390 nm from scales 1 to 8. Of those, five features between 1318 to 1322 nm at scale 7 are consistently found in the July and August 2008 datasets. These features are located at longer wavelengths than those investigated in previous studies (below 1100 nm) and provide new insights into the potential remote detection of green attack. Spectral features that distinguish control and girdled trees were mostly observed between 1550 and 2370 nm from scales 1 to 5. The differing response of girdled and infested trees appears to indicate that the girdling process does not provide a perfect simulation of the effects caused by beetle infestation.It remains to be determined if the location of the 1318-1322 nm features, near the edge of a strong atmospheric water absorption band, will be sufficiently separable for use in airborne detection of green attack. A plot comparing needle water content and wavelet power at 1320 nm reveals considerable overlap between data derived from both infested and control samples, though the groups are statistically separable. This obstacle may preclude a high accuracy separation of healthy and infected single individuals, but establishing threshold identification levels may provide an economical, efficient and expeditious method for discriminating between healthy and infested tree populations.     

A distributed Bragg reflector porous silicon layer for optical interferometric sensing of organic vapor

In this study, we successfully demonstrated the rapid, sensitive, and reversible sensing of organic vapor using a distributed Bragg reflector (DBR) porous silicon (PS) layer. We fabricated the DBR PS layer on a p⁺-type silicon substrate and investigated its reflectance spectra before, during, and after exposure to the different concentrations of various organic vapors. When the DBR PS layer sample was exposed to methanol, acetone, ethanol, and isopropanol vapors, the maximum reflectance peak promptly shifted toward longer wavelengths by about 4.5, 23.2, 26.0, and 38.2 nm, respectively. We determined that the red-shift in the reflectance spectrum could be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the DBR PS layer. The DBR PS layer showed excellent sensing ability under the different concentrations and types of organic vapors. In addition, a slight hysteresis of the red-shift was observed during repeated exposure to organic vapors at different concentrations. After removing the organic vapors, the reflectance spectrum promptly returned to its original state.     

Sensitivity of spectral reflectance to variation in live fuel moisture content at leaf and canopy level

Wildland fires burn large areas of the earth's land surface annually, causing significant environmental damage and danger to human health. In order to mitigate the effects, and to better manage the incidence of such fires, fire behaviour models are used to predict, among other things, the likelihood of ignition, the rate of spread, and the intensity and duration of burning. A key input parameter to these models is the amount of water in the vegetation, described as the fuel moisture content (FMC). A number of studies have shown that vegetation indices (VI) calculated from red and NIR reflectances may be used to map spatial and temporal variation in FMC in a range of fire-prone environments, with varying degrees of success. Strong empirical relationships may be established between VI and FMC over grasslands, yet over shrublands and forests, the relationships are weaker. If FMC is to be estimated with greater accuracy and consistency than is currently achieved, it is necessary to fully understand the relative contribution that spatial and temporal variation in the various biophysical and geometrical variables make to reflectance variability at the leaf and canopy level.This paper uses a modelling approach to investigate the sensitivity of reflectance data at leaf and canopy level to variation in the biophysical variables that are used to compute FMC. At the leaf level, the results show that the sensitivity of reflectance to variation in leaf water and dry matter content, used to compute FMC, is greatest in the SWIR and NIR, respectively. Variation in FMC has no effect in the visible wavelengths. At the canopy level, the results show that the sensitivity of reflectance to variation in leaf water and dry matter content is heavily dependent upon the type of model used and the range of variation over which the variables are tested. In the longer wavelengths of the SWIR, the competing influence of variable leaf area index, fractional vegetation cover, and solar zenith angle is shown to be greater than that at the shorter wavelengths of the SWIR and NIR. Empirical relationships between the normalised difference water index (NDWI) and FMC are shown to be weaker than that with canopy water content. However, when the range of the variables under study is more limited, useful empirical relationships between FMC and remotely sensed VI may be established.