A low-cost field and laboratory goniometer system for estimating hyperspectral bidirectional reflectance (original) (raw)
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Acquisition of bidirectional reflectance data using the Swiss Field-Goniometer System (FIGOS)
… of EARSeL symposium …, 1995
Most natural objects expose a non-Lambertian behaviour, i.e. the reflectance characteristics vary with changing illumination and viewing geometry. Numerous models have been developed to describe bidirectional reflectance effects and to involve them in the preprocessing of remote sensing data. However, only a few ground reference data is available to validate these models, and much of it is derived from laboratory experiments. In order to obtain bidirectional reflectance factor (BRF) data of naturally illuminated targets a transportable field-goniometer system (FIGOS) has been developed. It is operated together with a GER-3700 spectroradiometer. The goniometer consists of an azimuth full-circle and a zenith semi-arc of 2 m radius each. It enables to observe a target in the centre of the hemisphere from any desired viewing direction. In a field-campaign the bidirectional reflectance of a plane meadow is measured over the hemisphere within 15 minutes in a resolution of 15° and 30° in zenith and azimuth direction, respectively. A Spectralon panel measured periodically during the BRF-data acquisition allows for normalisation of the changes in atmosphere and solar irradiance. The resulting 66 BRF-data are used to model the bidirectional reflectance distribution function (BRDF) of the target. Special emphasis is given to the solar principal plane where the BRDF-effects are most pronounced. The obtained results clearly show the non-Lambertian reflectance characteristics of the target.
Calibration of BRDF Based on the Field Goniometer System Using a UAV Multispectral Camera
Sensors
The bidirectional reflectance distribution function (BRDF) is important for estimating the physical properties of a surface in remote sensing. In the laboratory, the BRDF can be estimated quickly and accurately using a goniometer, but it is very difficult to operate in the field. The purpose of this study was to evaluate whether estimating the BRDF with reasonable accuracy using an unmanned aerial vehicle (UAV) with a multispectral camera is possible in the field. Hemispherical reflectance was created from images taken using an UAV multispectral camera. The ground targets were four calibrated reference tarps (CRTs) of different reflectance, and the UAV was operated five times. Down-welling irradiance for reflectance calculation was measured in two ways: a sunlight sensor was mounted on a UAV, and a spectroradiometer with a remote cosine receptor (RCR) was installed on the ground. The BRDF was assessed through the anisotropy factor (ANIF) of the CRT reflectance derived from the colle...
International Journal of Remote Sensing, 2003
A portable goniometric device was used in the field to observe four natural surfaces: one composed of a mixed herbaceous species, one of an Alpine pratum, one of a Colza crop and one snow covered. Under stable environmental conditions 66 bidirectional reflectance factors were collected for each target. These data allowed the anisotropic behaviour of each surface to be studied and the actual albedo to be predicted. The differences between the values of albedo and the Lambertian-assumed values were all positive, ranging from 7.1% for the white snow target to 3.3% for the grassy surfaces.
2011
Recently, a laboratory measurement facility has been realized for assessing the anisotropic reflectance and emittance behaviour of soils, leaves and small canopies under controlled illumination conditions. The facility consists of an ASD FieldSpec 3 spectroradiometer covering the spectral range from 350-2500 nm at 1 nm spectral sampling interval. The spectroradiometer is deployed using a fiber optic cable with either a 1°, 8° or 25° instantaneous field of view (IFOV). These measurements can be used to assess the plant pigment (chlorophyll, xanthophyll, etc.) and non-pigment system (water, cellulose, lignin, nitrogen, etc.). The thermal emittance is measured using a NEC TH9100 Infrared Thermal Imager. It operates in a single band covering the spectral range from 8-14 µm with a resolution of 0.02 K. Images are 320 (H) by 240 (V) pixels with an IFOV of 1.2 mrad. A 1000 W Quartz Tungsten Halogen (QTH) lamp is used as illumination source, approximating the radiance distribution of the sun. This one is put at a fixed position during a measurement session. Multi-angular measurements are achieved by using a robotic positioning system allowing to perform either reflectance or emittance measurements over almost a complete hemisphere. The hemisphere can be sampled continuously between 0° and 80° from nadir and up to a few degrees from the hot-spot configuration (depending on the IFOV of the measurement device) for a backscattering target. Measurement distance to targets can be varied between 0.25 and 1 m, although with a distance of more than 0.6 m it is not possible to cover the full hemisphere. The goal is to infer the BRDF (bidirectional reflectance distribution function) and BTDF (bidirectional thermal distribution function) from these multi-angular measurements for various surface types (like soils, agricultural crops, small tree canopies and artificial objects) and surface roughness. The steering of the robotic arm and the reading of the spectroradiometer and the thermal camera are all fully automated.
Sensors, 2007
The design and calibration of a new hyperspectral Compact Laboratory Spectro-Goniometer (CLabSpeG) is presented. CLabSpeG effectively measures the bidirectional reflectance Factor (BRF) of a sample, using a halogen light source and an Analytical Spectral Devices (ASD) spectroradiometer. The apparatus collects 4356 reflectance data readings covering the spectrum from 350 nm to 2500 nm by independent positioning of the sensor, sample holder, and light source. It has an azimuth and zenith resolution of 30 and 15 degrees, respectively. CLabSpeG is used to collect BRF data and extract Bidirectional Reflectance Distribution Function (BRDF) data of non-isotropic vegetation elements such as bark, soil, and leaves. Accurate calibration has ensured robust geometric accuracy of the apparatus, correction for the conicality of the light source, while sufficient radiometric stability and repeatability between measurements are obtained. The bidirectional reflectance data collection is automated and remotely controlled and takes approximately two and half hours for a BRF measurement cycle over a full hemisphere with 125 cm radius and 2.4 minutes for a single BRF acquisition. A specific protocol for vegetative leaf collection and measurement was established in order to investigate the possibility to extract BRDF values from Fagus sylvatica L. leaves under laboratory conditions. Drying leaf effects induce a reflectance change during the BRF measurements due to the laboratory
Sensors, 2009
The design, operation, and properties of the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO) are presented. FIGIFIGO is a portable instrument for the measurement of surface Bidirectional Reflectance Factor (BRF) for samples with diameters of 10 -50 cm. A set of polarising optics enable the measurement of linearly polarised BRF over the full solar spectrum (350 -2,500 nm). FIGIFIGO is designed mainly for field operation using sunlight, but operation in a laboratory environment is also possible. The acquired BRF have an accuracy of 1 -5% depending on wavelength, sample properties, and measurement conditions. The angles are registered at accuracies better than 2°. During 2004 -2008, FIGIFIGO has been used in the measurement of over 150 samples, all around northern Europe. The samples concentrate mostly on boreal forest understorey, snow, urban surfaces, and reflectance calibration surfaces.
Journal of Geophysical Research, 1999
Hyperspectral bidirectional reflectance distribution function (BRDF) data of Konza prairie grassland acquired in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) on the ground with two SE-590 instruments and remotely with the airborne advanced solid-state array spectroradiometer (ASAS) are analyzed and compared to BRDF data of dense ryegrass obtained in the laboratory and field with the European goniometric facility (EGO) and the Swiss field-goniometer system (FIGOS). The soil underlying the relatively sparse Konza prairie grass disturbed the spectral BRDF effects of the vegetation components. After a correction of the soil influence based on the bidirectional canopy gap probability, the Konza data from SE-590 and ASAS sensors showed a consistently strong dependence of spectral BRDF effects from nadir reflectance as was observed in the EGO and FIGOS data. BRDF effects were inversely related to reflectance intensities, low reflectances being associated with pronounced BRDF effects and high reflectances with low BRDF effects. This relationship is due to multiple scattering effects and is influenced by the canopy optical properties and architecture parameters such as leaf area index (LAI), leaf angle distribution (LAD), and the gap fraction. The BRDF data of the Konza prairie grass from both ground and aircraft measurements showed a strong relationship between LAI and spectral BRDF variability. Reflectance data with high spectral resolution in the red edge range from 675 to about 900 nm wavelength, acquired from the two viewing directions with maximum and minimum reflectance intensities proved to be useful for deriving vegetation canopy architecture characteristics from hyperspectral BRDF data. BRDF data with high spectral resolution from the airborne ASAS sensor and from planned commercial remote sensing satellites are therefore an ideal testbed for a further exploration of this promising approach. bution function (BRDF) [Middleton,
An improved goniometer system for calibrating field reference-reflectance panels
Remote sensing of …, 1993
F~ld reference-reflectance panels need an initial els was restricted to well-equipped laboratories calibration and periodic recalibration to ensure until Jackson et al. (1987) reported details of a valid field reflectance data. The field calibration field method that offered a relatively quick, simmethod proposed by is an pie, and inexpensive calibration procedure. The affordable means to accomplish this provided that procedure has two advantages over conventional a field goniometer system is available. Design and laboratory methods. First, the method utilizes the construction details for such a system are desame irradiance source (the sun) and geometry as scribed, used in field reflectance measurements. Second, the required equipment (radiometer and data logger) is the same as that used for field measure-* Paper No. 9822, J. Ser., Nebraska Agric. Res. Div. and Hsia, 1981) was the standard surface used by Address correspondence to Elizabeth A. Walter-Shea, Dept. Jackson et al. (1987). The key component of the of Agricultural Meteorology, Univ. of Nebraska, P.O. Box 830728, calibration procedure is an apparatus that allows Lincoln, NE 68583-0728. Received 17January 1992; revised 3O May 1992. a panel to be held in a particular zenith and