Portable Spectroradiometer Research Articles

Assessing Canopy Spectral Reflectance of Hybrid Bermudagrass under Various Combinations of Nitrogen and Water Treatments

Remote sensing technology was tested as a non-destructive tool to assess the nitrogen (N) and water status of hybrid bermudagrass 'Tifway' [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy]. Ten combinations of water (based on leaching fraction: LF) and N treatments including cyclic and steady treatments were imposed on 20 plots (6.1 × 6.1 m) from October 2001 to October 2002 and January 2003 to June 2003. Each plot contained a non-weighing lysimeter which was used to assess the weekly water balance. Leaf xylem water potential, canopy temperature, tissue moisture content (TM), chlorophyll meter index, visual color ratings, and tissue nitrogen concentration (TN) were assessed twice per month. Canopy spectral reflectance measurements were also acquired twice per month using a portable spectroradiometer mounted on a movable cart. Analysis of variance (ANOVA) showed that spectral reflectance at individual wavelengths, vegetation indices, and soil-plant parameters separated based on N, LF and/or N × LF interaction (P < 0.05). Prediction equations for individual wavelengths and vegetation indices were generated from plant and soil parameters that included TN (Nitrogen Content index (NC): R2 = 0.73), TM (R555 : R2 = 0.66), or TM × TN (Normalized Difference Vegetation Index (NDVI) : R2 = 0.71). Multiple regression analysis also produced significant predictive models for four plant variables based on a selected number of wavelengths (between 3 and 9): canopy color (R2 = 0.48), TN (R2 = 0.56), TM × TN (R2 = 0.43), and chlorophyll meter index (R2 = 0.78). Normalized difference vegetation index (NDVI) showed a strong threshold relationship with canopy color rating (R2 = 0.64, P < 0.001, n = 194) and a consistent positive curvilinear relationship with the chlorophyll meter index (R2 = 0.75, P < 0.001, n = 200). The Water Band Index (WBI) significantly correlated with TM (R2 = 0.76, P < 0.001) and N content index (NC) correlated with TN (R2 = 0.45, P < 0.001) for single day measurements, but not for the larger data sets. Results from this experiment demonstrate that canopy reflectance measured with a ground-based spectroradiometer has potential for improving turfgrass N and irrigation management under field conditions, which could lead to significant N and water savings on large turfgrass areas such as golf courses and sports fields.

Portable Fourier spectroradiometer with an uncooled photodetector

This paper presents a portable Fourier spectroradiometer with resolution 8 cm−1, operating in the 7-14-µm range, with an MG-32 uncooled single-element pyroelectric photodetector. This Fourier spectroradiometer with uncooled photodetector is intended for remote determination of pollutant vapors in the open atmosphere, as well as for methane leaks on gas pipelines.

The influence of canopy, sky condition, and solar angle on light quality in a longleaf pine woodland

Light transmittance estimates under open, heterogeneous woodland canopies such as those of longleaf pine (Pinus palustris Mill.) forests report high spatial and temporal variation in the quantity of the light environment. In addition, light quality, that is, the ratio of red to far-red light (R:FR), regulates important aspects of plant development including stem extension, specific leaf area, and seed germination. We conducted two experiments to document sources of variation in R:FR (using a LI-COR 1800 portable spectroradiometer with a cosine-corrected light sensor) in a 70- to 90-year-old natural longleaf pine woodland in southwest Georgia, USA. The first experiment compared instantaneous measurements of R:FR over a 3-day period (March) with annual estimates of canopy transmittance (using gallium arsenide phosphide photodiodes) across the range of observed overstory abundance. The second experiment examined the effect of wiregrass cover (above or below), sky condition (blue sky or overcast), and solar angle (four sampling periods between October and March) on R:FR using a multifactorial repeated measures design. We found that (1) R:FR was significantly (p &lt; 0.0001) and strongly (R2 = 0.72) related to annual estimates of canopy transmittance (percent photosynthetic photon flux density, %PPFD); (2) R:FR and %PPFD showed significant negative relationships with increasing overstory stocking (R2 = 0.20, p = 0.028 for R:FR, and R2 = 0.87, p &lt; 0.0001 for %PPFD); and (3) R:FR decreased with increasing solar angle from maximum zenith for the study site under blue skies, was greater under overcast skies (0.84 blue sky vs. 1.18 overcast sky), and decreased under wiregrass (Aristida stricta Michx.) canopies (1.10 above vs. 0.98 below).

Spectral Reflectance Properties of Crusted Soils under Solar Illumination

Reflectance of soil crusts has been studied under artificial light but not under solar illumination. Our objectives were to (i) compare reflectance from crusted and noncrusted soil surfaces under solar illumination, and (ii) explore the relationship between crust permeability and spectral signature. Two California soils were studied, Capay (fine, smectitic, thermic Typic Haploxererts) and Reiff (coarse‐loamy, mixed, superactive, nonacid, thermic Mollic Xerofluvents), with samples for the latter taken from plots under organic (ReiffOM) and conventional (ReiffCM) management. A laboratory rainfall simulator was used to form crusts which were sampled at different stages of development and permeability. A portable spectroradiometer was used to collect spectral data in direct sunlight for wet and dry crusted samples. Baseline spectra (albedo) of the dry samples were, in most cases, higher than spectra from the corresponding moist samples. Crusted samples exhibited higher baseline spectra compared with the noncrusted samples. The absorption feature at ≈1400 nm (related to lattice OH of montmorillonite) suggested accumulation of clay in the crust of the ReiffCM, and clay depletion from the crusts of the Capay and ReiffOM An inverse linear relationship existed between reflectance at many wavelengths and crust permeability at different stages of crust development. The reflectance at 1700 nm provided highly significant correlations with infiltration rate (IR) for all three soils, and reflectance at 2130 nm also had a highly significant correlation for the Capay. Our data suggest that spectral measurements of soil surfaces under solar illumination can differentiate between crusted and noncrusted surfaces and assist in evaluating the degree of crust development.

Potato Leafhopper (Homoptera: Cicadellidae) Varietal Preferences in Edible Beans in Relation to Visual and Olfactory Cues

Migratory adult potato leafhoppers, Empoasca fabae (Harris), have shown a preference for particular cultivars of edible beans, Phaseolus vulgaris L. Laboratory experiments were conducted to determine the role of visual and olfactory cues in these preferences. No preference for any particular cultivar of edible bean was detected in Y-tube olfactometer binary comparison tests: when selecting leaves on the basis of olfactory stimuli. Adult potato leafhoppers chose ‘Berna Dutch brown’ as frequently as ‘Stingray white’ bean and ‘EMP 419’. When adult potato leafhoppers selected leaflets of uniform shape and size from an aerial position in a Plexiglas chamber, ‘Berna Dutch brown’ was significantly preferred (54%) over ‘Stingray white bean’ (24%) and ‘EMP 419’ (22%). An external integrating sphere attached to a portable spectroradiometer was used to determine the wavelength reflectance of ‘Berna Dutch brown’, ‘Stingray white’, and ‘EMP 419’ bean leaflets. ‘Berna Dutch brown’ bean leaflets had higher percent reflectance readings in the green region of the spectrum and lower percent reflectance readings in the blue and yellow regions of the spectrum compared with ‘Stingray white’ and ‘EMP 419’. Although host-finding behavior may involve an interaction between visual and olfactory stimuli, the results of this study indicate that leaf color significantly influences host preference, whereas host odor does not.

Surface reflectance of sea ice and under-ice irradiance in Kongsfjorden, Svalbard

Initial results from a field experiment on fast ice in Kongsfjorden, Svalbard, in March 2002 are presented. We measured surface reflectance and under-ice irradiance using an advanced, portable spectroradiometer sensitive in the visible and near-infrared parts of the electromagnetic spectrum, i.e. 350-1100 nm. Under-ice irradiance (UV-A, UV-B and photosynthetically active radiation [PAR]) was measured down to depths of 7.5 m by vertical profiling using a six-channel radiometer. We also present model results of wavelength-dependent transmittance of radiation through a combined snow and sea ice layer for various thicknesses of snow. Model results show that the snow and sea ice is more transparent for solar radiation in the PAR region (400-700 nm) than at shorter and longer wavelengths. This is confirmed by the field measurements. Even very thin snow layers on top of the sea ice efficiently prevent solar radiation from penetrating the snow–sea ice system. For example, a 5 cm thick snow layer reduces under-ice irradiance in the PAR region with a factor of about 10. Measurements of under-ice UV irradiance show that both UV-A and UV-B irradiance is reduced with a factor of more than 10 at depths of 7.5 m below the ice compared to at the ice-sea water interface.

Surface reflectance of sea ice and under-ice irradiance in Kongsfjorden, Svalbard

Initial results from a field experiment on fast ice in Kongsfjorden, Svalbard, in March 2002 are presented. We measured surface reflectance and under-ice irradiance using an advanced, portable spectroradiometer sensitive in the visible and near-infrared parts of the electromagnetic spectrum, i.e. 350-1100 nm. Under-ice irradiance (UV-A, UV-B and photosynthetically active radiation [PAR]) was measured down to depths of 7.5 m by vertical profiling using a six-channel radiometer. We also present model results of wavelength-dependent transmittance of radiation through a combined snow and sea ice layer for various thicknesses of snow. Model results show that the snow and sea ice is more transparent for solar radiation in the PAR region (400-700 nm) than at shorter and longer wavelengths. This is confirmed by the field measurements. Even very thin snow layers on top of the sea ice efficiently prevent solar radiation from penetrating the snow–sea ice system. For example, a 5 cm thick snow layer reduces under-ice irradiance in the PAR region with a factor of about 10. Measurements of under-ice UV irradiance show that both UV-A and UV-B irradiance is reduced with a factor of more than 10 at depths of 7.5 m below the ice compared to at the ice-sea water interface.

Flower Color, Hummingbird Pollination, and Habitat Irradiance in Four Neotropical Forests1

ABSTRACTPollinator visual systems differ considerably among broad groupings such as bees, bats, and birds, and it has been proposed that factors shaping the diversity of flower color in tropical plants include differences in pollinator perceptual abilities. Within the pollinators of the Neotropics, one major difference between taxa is that hummingbirds perceive color well across a broad range of wavelengths from 300–660 nm whereas most bees perceive color well over a narrower range spanning 300–550 nm. Thus, hummingbirds can see red and other long‐wavelength reflection much better than bees. Another factor that could potentially influence flower color in tropical forests is the difference in light availability among habitats such as gaps, canopy, and forest understory. I conducted a survey of floral color in four Neotropical forests using a portable spectroradiometer to provide an unbiased measure of color reflectance. The primary pollinator agents and light habitats of each plant species were classified using primary literature or accounts of direct observations by experts. Flower color was not influenced by differences in light availability between open and closed habitats but was influenced by pollinator visual systems. Specifically, insect flowers reflected across a broad range of the spectrum but hummingbird flowers reflected mostly long‐wavelength light (typically median wavelength &lt;585 nm). Thus, hummingbird‐pollinated flowers are not tuned specifically to hummingbird color sensitivity but instead may decrease conspicuousness to bees and other insects that have poor visual sensitivity to long‐wavelength color.

Spectral irradiance profiles of suspended marine clay for the estimation of suspended sediment concentration in tropical waters

Reflectance spectra of water containing varying concentrations of organic and inorganic sediments isolated from the coastal waters of Singapore were measured using a portable spectroradiometer under controlled experimental conditions and natural sunlight. The effect of different sizes of sediments on the spectral profile of water was also investigated. In the presence of organic sediments, maximum reflectance of water was observed over a broad band between 440-580 nm, with a peak reflectance at about 550 nm, followed by two smaller peaks at 600-645 nm and 665-690 nm. In contrast, inorganic sediments produced a distinct band peak between 595 and 690 nm. For both sediment types, characteristic features in the infra-red region include a reflectance trough at 754 nm and peak at 814 nm. The empirical colour ratios, OD550-OD754 and OD 595-OD754, were used to estimate the organic and inorganic sediment concentrations respectively. In the case of organic sediments, a power function was found to fit the data well ( R 2 =0.89-0.98), whereas a linear fit was found for inorganic sediments ( R 2 =0.53-0.86). In general, decreases in particle size resulted in overall increases in spectral reflectance.

Flower Color, Hummingbird Pollination, and Habitat Irradiance in Four Neotropical Forests1

ABSTRACT Pollinator visual systems differ considerably among broad groupings such as bees, bats, and birds, and it has been proposed that factors shaping the diversity of flower color in tropical plants include differences in pollinator perceptual abilities. Within the pollinators of the Neotropics, one major difference between taxa is that hummingbirds perceive color well across a broad range of wavelengths from 300–660 nm whereas most bees perceive color well over a narrower range spanning 300–550 nm. Thus, hummingbirds can see red and other long-wavelength reflection much better than bees. Another factor that could potentially influence flower color in tropical forests is the difference in light availability among habitats such as gaps, canopy, and forest understory. I conducted a survey of floral color in four Neotropical forests using a portable spectroradiometer to provide an unbiased measure of color reflectance. The primary pollinator agents and light habitats of each plant species were classified...

Impact of nitrogen and environmental conditions on corn as detected by hyperspectral reflectance

Indices derived from hyperspectral reflectance spectra have the potential to be used as indicators of environmental stress in crops. This study uses canopy-scale, ground-based measurements of hyperspectral reflectance to demonstrate the temporal patterns in corn development under imposed fertility (N rate) and environmental (water availability) stresses. In 1998, two large areas in a 30-ha corn ( Zea mays, L.) field near Ottawa, Canada (45°18′N, 75°44′W) were supplied with 99 and 17 kg N ha −1, while the balance of the field received the recommended rate of 155 kg N ha −1. Reflectance measurements were taken nine times using a portable spectroradiometer at georeferenced locations within these areas. Individual reflectance-based indices demonstrated the relative differences between application rates and identified both nitrogen and water stresses at various times in the growing season. No single index was able to describe the status of the corn crop throughout the season. Canonical discriminant analysis provided accurate classification of samples by N rate during early, mid, and late season conditions with overall success rates of 70%, 88%, and 93%, respectively. A shift in importance from green-based derivatives to red-based derivatives was noted from mid to late season and attributed to the natural reduction in green pigments as the crop entered senescence. Canopy-scale photochemical reflectance index (PRI) was shown to be correlated with canopy radiation use efficiency (RUE). Mid-season water stress affected the relationship. Multiple years of data are required to demonstrate robust relationships between hyperspectral indices and corn ecophysiological status because of the interaction between environmental and nutrient stresses. Identifying areas of fields sensitive to weather-induced stresses will allow better management of N application. Timing the collection of hyperspectral image data at early and late vegetative phase could enhance precision agriculture by allowing supplemental nutrient application, identifying stress patterns and aid in yield forecasting.

Field reflectance of snow/ice covers at Terra Nova Bay, Antarctica

This paper studies the relationship between reflectance and physical characteristics of snow/ice surfaces in Antarctica, to support remote sensing monitoring of this environment. Field data were acquired on different targets selected at Terra Nova Bay, austral summer 1998. In each session of measurements the following data were collected: reflectance in the spectral range 350-2500 nm by a portable spectroradiometer; snow data including temperature, grain size and geometry, surface layer morphology, vertical profile of the snow pack, snow density and water content. The analysis of field data revealed that there are significant changes in surface reflectance properties, related to grain size and inclusions, and that a detailed characterization of snow metamorphosism is highly important for data interpretation. Meaningful reflectance variations were also observed after field data were re-sampled at Landsat 7 ETM spectral intervals.

MODELING OF SUGARBEET QUALITY VARIABLES FROM CANOPY SPECTRAL INDICES IN A FERTILITY TRIAL

Sugarbeet quality varies inversely with N fertility, while gross yield varies positively with N. Existing models of yield from remotely sensed data do not consider crop quality. A fertility trial was used as the basis for a canopy reflectance study to correlate quality variables to canopy characteristics. Sugarbeet canopy reflectance and radiance were measured with a portable spectroradiometer, and from airborne images of the trial plots. Canopy data were collected five times from early August to early October of 1999. Linear regression models were used to correlate canopy indices to sugarbeet quality variables on each date. Both a green NDVI and the conventional NDVI indices showed strong correlation to recoverable sucrose concentration in sugarbeet roots. Correlations to harvest quality in October were highest in spectral measurements taken in early to mid–August. Other quality variables including root concentrations of sodium, potassium, and amino–N were poorly correlated to spectral properties of the canopy. The results suggest that a single image of a field in August could provide spatial indication of variability in root quality.

Effect of wetness on turfgrass canopy reflectance

Wetness is a key factor in the development of various foliar pathogens on turfgrass. A spectroradiometric approach was used to measure dew and precipitation in short turfgrass (creeping bentgrass and bluegrass) canopies, because of the unavailability of sensors representing the leaf size and the need not to disturb such a short canopy environment. Measurements of reflectance, between 400 and 2250 nm with a field portable spectroradiometer, showed that the erectophile rough bluegrass (about 4 cm height) trapped more radiation within the canopy, hence lowering the reflectance, than the planophile homogeneous bentgrass (about 1 cm height). For both dew and rain-covered bentgrass canopies, reflectance increased in the visible (VIS), about 50 and 16%, respectively, and decreased in the middle-infrared (MIR), about 45 and 28%, respectively. However, changes in dew reflectance on the bluegrass canopy were mainly observed in the near-infrared (NIR) and MIR (9 and 19% decrease, respectively), as the reflectance in the VIS was almost unaffected (about 4% increase). The light precipitation affected the bluegrass reflectance by mainly decreasing NIR (765–960 nm) wavelengths. Relationships were generally weak for the dew-covered bluegrass reflectance, but a strong quadratic relationship at 1165 nm reflectance was found under rainy conditions. Strong quadratic relationships were found between dew-covered bentgrass and MIR/green and MIR/red ratios. The reflectance at 1165 nm ( r 2=0.95) showed the greatest response to increasing wetness either for dew or rainfall, suggesting that the reflectance at this wavelength may be used on a remote sensor to measure wetness on bentgrass canopies.

Remote sensing of photosynthetic-light-use efficiency of boreal forest

Using a helicopter-mounted portable spectroradiometer and continuous eddy covariance data we were able to evaluate the photochemical reflectance index (PRI) as an indicator of canopy photosynthetic light-use efficiency (LUE) in four boreal forest species during the Boreal Ecosystem Atmosphere experiment (BOREAS). PRI was calculated from narrow waveband reflectance data and correlated with LUE calculated from eddy covariance data. Significant linear correlations were found between PRI and LUE when the four species were grouped together and when divided into functional type: coniferous and deciduous. Data from the helicopter-mounted spectroradiometer were then averaged to represent data generated by the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). We calculated PRI from these data and relationships with canopy LUE were investigated. The relationship between PRI and LUE was weakened for deciduous species but strengthened for the coniferous species. The robust nature of this relationship suggests that relative photosynthetic rates may be derived from remotely-sensed reflectance measurements.

Development of an Index of Balsam Fir Vigor by Foliar Spectral Reflectance

The potential to measure indices of forest vigor from foliar spectral reflectance was assessed in a range of balsam fir conditions achieved through stand manipulations. In order of increasing vigor, treatments consisted of root pruning, control, thinning, and thinning in combination with fertilization. Concentrations of chlorophyll and nitrogen increased significantly from the low to high vigor plots, and shoot lengths increased by more than three times. Balsam fir shoots from each treatment were measured for reflectance under laboratory conditions by a portable spectroradiometer with a spectral range from 350 nm to 2500 nm. Reflectance decreased consistently with vigor throughout the spectral region analyzed. Most wavebands showed significant effects of the treatments, with the most significant wavebands in the region of chlorophyll absorption between 500 nm and 740 nm. Foliage age class and sample date also affected reflectance in these wavebands. Reflectance indices, defined as the ratio and normalized difference of the most significant waveband at 711 nm and the least significant waveband at 913 nm, were affected by the treatments ( P<0.0001). The normalized difference index correlated strongly with concentrations of chlorophyll a ( r 2⩾0.75), chlorophyll b ( r 2⩾0.66), nitrogen ( r 2⩾0.52), and shoots lengths ( r 2⩾0.69), thereby comprising a spectral index of balsam fir vigor that integrates several aspects of forest physiological condition.

The impact of a wind-roughened water surface on remote measurements of turbidity

Specular reflection from a wind-roughened water surface constitutes a serious problem in the interpretation of spectral data. The purpose of this research was to evaluate the impact of wind-induced specular reflection on remote measurements of turbidity in a large reservoir. A portable spectroradiometer with 353 channels between 400 and 900 nm was used to collect data at 23 sampling sites. Five reflectance spectra were generated for each site. Relations between turbidity and the mean reflectance, and its first derivative, were analysed. The main finding was that specular reflection had less impact on derivative spectra than on reflectance spectra.

Transformation of irradiance measurements into spectral actinic flux for photolysis rates determination

The actinic flux is the only radiometric quantity suitable for photolysisfrequency determination. It is derived from solar spectral irradiancemeasurements performed by a portable spectroradiometer in the 300–850nm wavelength range. The spectral irradiance is first divided into a directpart and a diffuse part, according to the atmospheric conditions, and thenconverted into the corresponding actinic flux quantity. As an intermediary,the ratio $$r_{dd} $$ of diffuse actinic flux to diffuseirradiance is calculated by the spherical harmonics radiative code withrespect to wavelength, solar zenith angle, surface albedo, and aerosolproperties.The results of extensive sensitivity studies of $$r_{dd} $$ asa function of the main atmospheric parameters are discussed and lead to theconclusion that aerosol optical depth is the major critical value for aprecise $$r_{dd} $$ determination. The global algorithm totransform spectral irradiance into actinic flux is finally applied forphotodissociation rate calculations by convolution of the obtained actinicflux spectra with the absorption cross-sections and quantum yields of themolecule studied. Photolysis rates of different atmospheric photooxidants havebeen measured with this spectroradiometric method during the summers of 1993and 1994 in Brittany and in Portugal. The ozone and nitrogen dioxidephotodissociation rates obtained present a good agreement with thej(O3) and j(NO2) actinometerresults, for the same experimental conditions in Brittany.

Estimation of ozone with total ozone portable spectroradiometer instruments I Theoretical model and error analysis

Inexpensive devices to measure solar UV irradiance are available to monitor atmospheric ozone, for example, total ozone portable spectroradiometers (TOPS instruments). A procedure to convert these measurements into ozone estimates is examined. For well-characterized filters with 7-nm FWHM bandpasses, the method provides ozone values (from 304- and 310-nm channels) with less than 0.4% error attributable to inversion of the theoretical model. Analysis of sensitivity to model assumptions and parameters yields estimates of ±3% bias in total ozone results with dependence on total ozone and path length. Unmodeled effects of atmospheric constituents and instrument components can result in additional ±2% errors.

Estimation of ozone with total ozone portable spectroradiometer instruments II Practical operation and comparisons

We used a microcomputer-controlled total ozone portable spectroradiometer instrument #21 (MTOPS21) to measure solar radiation at 298, 304 and 310 nm in Greenbelt, Md., during 1995. One day's ozone measurements from a Brewer instrument (B105) were used to calibrate the 304- and 310-nm channel ratios to a theoretical model. Total ozone estimates were then determined for the entire MTOPS21 data set. Differences between individual B105 and MTOPS21 ozone estimates show a 1% drop as solar zenith angles increase and depend on atmospheric attenuation and SO(2) variation at the ±2% level. Daily average values agree well (<0.5% average offset, 2% standard deviation).