Changes In Spectral Reflectance Research Articles

Exploring the effects of bura over the northern Adriatic: CZCS imagery and a mathematical model prediction

Abstract This is the first attempt to test the results of wind-induced current modelling for the northern Adriatic (Kuzmic and Orlic 1987) using space-borne Coastal Zone Colour Scanner (CZCS) data. In particular, new remotely sensed empirical evidence has been sought for the previously studied effect of the spatially heterogeneous bura (gusty, katabatic wind of north-easterly direction) by observing changes in spectral reflectance of the northern Adriatic waters. To that end, an existing hydrodynamical model has been extended to include a simple two-dimensional random walk dispersion model. The CZCS measured radiance has been processed to estimate pigment concentration. Remotely sensed data corroborate mathematical model findings. Comparison of model-generated and sensor-collected information strongly suggests that bura-induced gyre is the mechanism responsible for particular spatial pattern change observed in the CZCS detected radiance and estimated pigment concentration.

Longterm changes of silicon photodiodes and their use for photometric standardization

A secondary standard silicon photodiode matched with a V-lambda filter was calibrated against primary standard, self-calibrated inversion layer silicon photodiodes, to achieve a high accuracy photometer, according to the new definition of the canadela (the photometric base unit). The measured several percent/year specular spectral reflectance change of the windowless primary standard photodiodes was eliminated by their repeated self-calibration. This self-calibration also eliminated the measured several tenth of a percent/year spectral response change of the secondary standard silicon photodiode. The secondary standard detector could be a nonunity quantum efficiency light detector. The spectral response calibration of the V-lambda matched detector of medium spectral mismatch (f(1)= 3.0%) against the absolute spectral responses of three self-calibrated photodiodes resulted in a standard deviation of 0.17% in luminous flux (lumen) calibration. Also illuminance (lux) and light intensity (candela) calibrations were derived from the above primary photometric calibration. It is shown that the V-lambda matched photometer with the above spectral calibration can be used for accurate photometric measurements for all kinds of light sources of known spectral power distribution.

Effect of Oxidation and Plume Formation on Low Power Nd-Yag Laser Metal Interaction

The effect of oxide formation and plume formation on laser energy absorption by metallic targets has been studied. The change in directional-hemispherical spectral reflectance of metallic targets during single-shot Nd-Yag laser pulse irradiation was measured using an integrating sphere under controlled environments of both oxygen and argon gas. The spectral transmittance of the plume formed over the targets was also measured using a He-Ne probe laser. The metal targets studied included A16061, Cu, 304 stainless steel, and low-carbon steel. Results obtained show that once a target is hot enough to form a vapor plume, in either argon or oxygen, absorption by the plume significantly limits the amount of laser energy available for absorption at the target surface. Prior to plume formation, the amount of laser energy absorbed by the target is determined by the target surface optical properties. Under the conditions of the present study (incident laser flux of the order of 106 W/cm2 over 0.5–1.0 ms), in the case of absorbing metals (αλ>0.3), such as stainless and low carbon steel, the intrinsic absorptivity of the metal is high enough that the effect of ambient gas (oxygen or argon) on absorptivity is insignificant. In the case of nonabsorbing metals (αλ<0.3), such as Cu and A16061, the intrinsic metal absorptivity is low enough that the ambient gas does have a significant effect on the target absorptivity. The formation of an oxide layer that occurs in an oxidizing environment generally has the effect of increasing the target absorptivity. The relative magnitude of the absorptivity enhancement due to oxide formation depends on the respective absorptivities of the oxide and metal. For metals that form oxides that are intrinsically absorbing in the solid state, such as Cu2O or CuO, the enhancement in absorptivity due to oxide formation can be as much as an order of magnitude. For metals that form oxides that are intrinsically nonabsorbing in the solid state, such as MgO or Al2O3, the enhancement in absorptivity due to oxide formation is more modest but still significant (40 percent). The enhancement in absorptivity in the latter case (MgO or Al2O3) is postulated to be associated either with a thin, absorptive, transition region composed of a mixture of metal and substoichiometric solid oxide just below target surface or with the melting of the oxide at the surface.

Arsenic- and selenium-induced changes in spectral reflectance and morphology of soybean plants

Soybean (Glycine max) plants were grown in hydroponic solutions treated with high concentrations of either arsenic or selenium. Spectral reflectance changes in arsenic-dosed plants included a shift to shorter wavelengths in the long-wavelength edge of the chlorophyll absorption band centered at 680 nm (the red edge) and higher reflectance in the 550–650 nm region. These results are consistent with vegetation reflectance anomalies observed in previous greenhouse experiments and in airborne radiometer studies. The selenium-dosed plants contrast, exhibited a shift to longer wavelengths of the red edge and lower reflectance between 550 nm and 650 wh when compared with control plants. Morphological effects of arsenic uptake included lower overall biomass, stunted and discolored roots, and smaller leaves oriented more vertically than leaves of control plants. Selenium-dosed plants also displayed morphological changes, but root and leaf biomass were less affected than were those of arsenic-dosed plants when compared to control plants.

Spectral Characteristics of Tree Components of Balsam Fir and Spruce Damaged by Spruce Budworm

Abstract Spectral reflectances from 360 to 1100 nm at 5 nm intervals in 6 nm bands were measured with a laboratory spectrometer for samples of the components of balsam fir and white spruce trees defoliated by the spruce budworm. These components included current-year needles, 1-year-old needles, 2-year and older needles, budworm feeding debris, twigs, bark, and lichen. Current-year needles had significantly higher reflectances than older needles in the green and yellow parts of the spectrum. Feeding debris, which is visually red, had a smooth spectral curve of increasing reflectance with wavelength. The greatest difference between needles and feeding debris was in the short near-infrared wavelengths and at the chlorophyll absorption maximum near 670 nm. The reflectance of twigs and bark changed with age and exposure to weathering. Variability in their reflectance was large. There were no distinct narrow band features of twig or feeding debris reflectance, which would be characteristic of defoliation. Lichen is highly reflective, and its presence on branches of defoliated trees will greatly alter branch reflectance characteristics. Changes in spectral reflectance of trees throughout the various stages of a spruce budworm outbreak depend on a complex interrelationship of the reflectances of individual components which, in turn, are complex and changing. For. Sci. 35(2):582-600.

Changes in leaf spectral properties induced in barley by cereal powdery mildew

Changes in spectral properties induced in barley leaves by cereal mildew were evaluated to identify important regions in the 400–1100 nm spectrum which can be used to detect infected barley leaves. Five spring barley lines grown in greenhouses were inoculated with mildew. During a 20-day period the directional spectral reflectance of the adaxial surface of the first leaf blades was recorded as well as the g d.wt. cm −2 leaf area, % water, and total chlorophyll content of the leaves. 1) The spectral reflectance of control leaves and inoculated resistant leaves was very similar throughout the experiment. 2) No changes in spectral reflectance from inoculated leaves were evident within the first 3 days following inoculation. 3) Six days after inoculation, the susceptible lines showed significantly higher reflectance in the visible (422–712 nm) wavelength region. 4) Ten days after inoculation, the susceptible lines showed significantly higher spectral reflectance between 400 and 1100 nm compared to control plants. 5) The differences in near infrared reflectance between control and infected plants observed were small and occurred several days later than changes in the visible region. 6) The differences in reflectance of blue and red wavebonds between control and inoculated plants were highly correlated to the cholorophyll content of infected leaves, r = 0.89 and 0.95, respectively. 7) Significant changes in spectral relectance of single leaves infected with mildew occur earlier in the visible region of the spectrum (400–706) than in the near infrared part of the spectrum.

Acid mist and ozone effects on the leaf chemistry of two Western Conifer species

Seedlings of Jeffrey pine ( Pinus jeffreyi) and giant sequoia ( Sequoiadendron gigantea) were more susceptible to leaf chemical changes following exposure to acid mist (pH 3·4–2·0) or acid mist/ozone combinations, than to ozone alone (0·1–0·2 μl/litre), when plants were exposed to alternating doses of these pollutants for 6–9 weeks. Under acid mist treatment, leaves exhibited higher levels of nitrogen and sulfur, two elements applied in acid mist. In addition, levels of foliar sodium, and, in the case of giant sequia, potassium, as well, increased under acid mist treatment. Iron and manganese were also mobilized, resulting in significant increases in these elements in pine, and decreases in manganese in giant sequoia foliage. The acid treatment also reduced chlorophyll b concentrations in pine, and, to a less significant extent, in giant sequoia. Calcium, magnesium, barium and strontium were differentially accumulated in giant sequoia compared to Jeffrey pine. Under acid mist treatment, all of these elements (except strontium) declined in concentration in giant sequoia, with calcium showing the most significant trend. The more extensive changes in leaf chemistry induced by acid mist are consistent with earlier observations of significant changes in spectral reflectance of these seedlings after 3 weeks of fumigation. Limited foliage samples collected from these two species in 1985 and 1986 in Sequoia/Kings Canyon National Parks in the southern Sierra Nevada do not in themselves indicate any clearcut or severe effects of ozone alone on leaf chemistry of these species, but a mild influence of nitrate-laden acid deposition, possibly in combination with ozone, is consistent with the rise in nitrogen and lignin levels in Jeffrey pine on sites observed to have moderate visible injury symptoms. No firm conclusions about effects of pollutants on leaf chemistry in these field sites is possible without further study.

Relationship between discoloration and histological changes in leaves of trees affected by forest decline

The relationship between discoloration and histological changes induced by forest decline in leaves of Norway spruce ( Picea abies) and beech ( Fagus sylvatica) is analyzed. Discoloration of leaves by various degrees of damage is quantitatively measured by high resolution reflection spectroscopy. The cell structure of leaves is determined from microscopic images of histological cuts. Major significant changes in spectral reflectance as shown by t-value maxima are observed in the visible, but not in the near infrared region. The related cellular changes are hypertrophy of chloroplasts, reduction of chloroplast number, and formation of large isodiametrical hypodermal cells. These results may be helpful in assessing the forest decline from remote sensing data.

Vacuum weathering of sulfur: Temperature effects and applications to Io

We have conducted laboratory experiments on frozen sulfur to further study the vacuum‐weathering phenomenon discovered by Nash [1987]. In this report we discuss the effect of surface temperature on the initial transient and subsequent steady‐state behavior of the vacuum‐sublimation rate and the UV/VIS spectral reflectance of frozen sulfur. We show that surface temperature is a dominant parameter controlling the rate of vacuum weathering of sulfur, a finding consistent with the steep vapor pressure curve of sulfur. We set limits on the prodigious sublimation rates expected for sulfur from hot spots on Io's surface and further define the range of changes that occur in the spectral reflectance of freshly frozen sulfur. These results imply that any freshly‐frozen sulfur deposits on Io at typical Io hotspot temperatures should display major transient changes in color (and microtexture) in a matter of hours to days. These rapid changes in spectral reflectivity should be readily detectable by high time‐resolution synoptic observations of Io's spectrum in the UV/VIS by spacecraft optical instruments such as on Galileo and, in certain instances (e.g., during or immediately after large‐scale Io volcanic activity), by Earthbased telescopes. Such observations should be useful in determining the frequency of occurrence, surface location, and areal scale of any sulfur volcanic activity on Io.

Ground Cover and Weathering Effects on Reflectances of Three Crop Residues1

AbstractCrop residue, which is important in protecting soil from wind and water erosion, is not easily differentiated from bare soil and difficult to quantify using remote sensing techniques. A field study was conducted on Amarillo fine sandy loam (Aridic Paleustalfs) for the purpose of measuring the change in spectral reflectance due to varying concentrations and weathering of Texas panicum (Panicum texanum Buckl.) pearl millet [Pennisetum americanum (L.) K. Schum], and cotton (Gossypium hirsutum L.) bur residue. Reflectance was measured in seven wavelength intervals with a multiband radiometer (MMR). The greatest change in reflectance occurred between bare soil and 33% ground cover. Band MMR 2 gave the best separation between the crop residues. The MMR bands 1, 2, and 3 provided the best discrimination among fresh residue types; however, as weathering progressed after 240 mm of rainfall, reflectance in bands MMR 5, 6, and 7 was significantly different among residues. The ratio of MMR 5 divided by MMR 6 was < 1.0 for fresh crop residues compared with values > 1.0 after exposure to a 240‐mm rainfall. The largest relative change in reflectance during the weathering period among residues in decreasing order was pearl millet, Texas panicum, and cotton burs. Band MMR 4 showed the greatest relative reflectance change during 251 days of weathering. Detecting crop residue on bare soil with remote sensing techniques may be feasible if it is known whether the residue is fresh or weathered.

Recognition of reflection and illumination edges by the human visual system

A simple two-stage lateral-inhibition mechanism is described that permits edges due to changes in surface illumination and orientation to be distinguished directly in the retina from those due to changes in spectral reflectivity. It is based on the assumption that the spectral content of the reflected light changes acrpss a reflectivity edge, whereas it remains relatively constat across an illumination edge.

Spectral reflectance change and luminescence of selected salts during 2–10 KeV proton bombardment: Implications for Io

Radiation damage and luminescence, caused by magnetospheric charged particles, have been suggested by several authors as mechanisms for explaining some of the peculiar spectral/albedo features of Io. We have pursued this possibility by measuring the uv-visual spectral reflectance and luminescent efficiency of several proposed Io surface constituents during 2 to 10-keV proton irradiation at room temperature and at low temperature (120 < T < 140°K). The spectral reflectance of NaCl and KCl during proton irradiation exhibits the well-known F-center absorption bands at 4580 and 5560 Å. Na 2SO 4 shows a generalized darkening which increases toward longer wavelengths. NaNO 3 shows a spectral reflectance change indicative of the partial alteration of NaNo 3 to NaNo 2. NaNO 2 shows no change. The luminescent efficiencies of NaCl and KCl are ∼10 −4 at 300°K and increase by one-half order of magnitude at ∼130°K. The efficiencies of K 2CO 3, Na 2CO 3, Na 2SO 4, and NaNO 3 are 10 −4, 10 −4, 10 −5 and 10 −6, respectively, at 300°K and they all decrease by one-half order of magnitude at ∼130°K. These results indicate that magnetospheric proton irradiation of Io could cause spectral features in its observed ultraviolet and visible reflection spectrum if salts such as those studied here are present on its surface. However, because the magnitude of these spectral effects is dependent on competing factors such as surface temperature, incident particle energy flux, solar bleaching effects, and trace element abundance, we are unable at this time to make a quantitative estimate of the strength of these spectral effects on Io. The luminescent efficiencies of pure samples that we have studied in the laboratory suggest that charged-particle induced luminescence from Io's surface might be observable by a spacecraft such as Voyager when viewing Io's dark side.

Spectral reflectance changes accompanying a post-fire recovery sequence in a subarctic spruce lichen woodland

A sequence of burned surfaces aged 0, 1, 2, 25 and 80 years was investigated regarding changes in the spectral distribution of reflected light. Controls were introduced to isolate diurnal and seasonal effects. The results show gradually increasing reflectance with increasing age of burn. With the establishment of vegetation a new set of absorption and reflectance criteria are established substantially altering the spectral characteristics. The apparent effect of a mature forest canopy is ambiguous. Diffuse and overcast conditions reduce the reflectance for all surfaces. Further work is suggested to reinforce results for surfaces with low sampling replication.

The spectral reflectivity of ion-bombarded surfaces

A variety of effects are found to contribute to changes in spectral reflectivity found after bombardment with ions of gaseous species; they cannot be assigned solely to the formation of a gas layer. There may be a change in refractive index and thickness associated with a phase change (e.g., quartz); a decrease in refractive index and an increase of thickness by the introduction of microporosity (e.g., SiC, B4C, Al2O3); an increase in refractive index from the introduction of centers, etc. (e.g., SiC, MgO); an increase of refractive index and a decrease in thickness by compaction (e.g., vitreous silica); or a decrease in refractive index caused by changes in composition (e.g., alkali loss in glasses). Often the chromatic fringes are composites of different fringe systems arising in different parts of the illuminated area; and within any part of an area, there may be more than one cause. These effects are observed in the early stages of bombardment manifested by changes in spacing, phase, amplitude, and level of the chromatic fringes. When a gas layer forms (blistering) the large amplitude of the chromatic fringes associated with it usually dominates the spectral reflectivity at large dose, but this is not always the case (e.g., C1 glass).