The application of a dedicated device to acquire bidirectional reflectance factors over natural surfaces (original) (raw)
Related papers
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.
Polarised bidirectional reflectance factor measurements from vegetated land surfaces
Journal of Quantitative Spectroscopy and Radiative Transfer, 2009
The bidirectional reflectance factor and degree of linear polarisation of selected snow, soil, and gravel types have been measured using Finnish geodetic institute field goniospectropolariphotometer (FIGIFIGO). It was observed that with all measured samples the degree of linear polarisation is weakest 0230 backwards from nadir, turning 1-5% negative in the backward direction, and growing larger (5-50%) in the forward and Brewster directions. Polarisation was found to be inversely but non-linearly proportional to reflectance. In addition, a wavelength-dependent trend was found to exist in some data, but in general, the wavelength dependence was smooth. Dry old snow polarises clearly more than melting snow or new snow. White gravel polarises somewhat similarly to snow in visible region, and black gravel resembles snow in infrared.
Canadian Journal of Remote Sensing, 2006
The derivation of the bidirectional reflectance distribution function (BRDF) from earth-surface features provides a more complete basis for the estimation of both surface composition and physical structure than is possible using single-angle reflectance measurements. With the increase in number of remote sensors capable of multi-angular earth observation and the increased use and sophistication of canopy BRDF models, the need for field and laboratory BRDF data and validation has correspondingly grown to ensure proper calibration and improved understanding of the directional reflectance properties of earthsurface features. Goniometers are specialized devices for near-surface measurement of bidirectional reflectance factors and are typically used in the field but can also be used in controlled laboratory settings. Current goniometer systems are often prohibitively expensive, however, and are not always robust for use in different field environments or the laboratory. This paper presents the University of Lethbridge Goniometer System (ULGS) as a low-cost, flexible, and capable alternative for estimating BRDF in a variety of field and laboratory situations. The ULGS uses an analytical spectral devices full-range hyperspectral spectroradiometer sensor (ASD-FR, 350-2500 nm) to acquire 217 unique angular hemispherical measurements (-60°to +60°z enith angle over the full 360°azimuth with an angular resolution of 10°in both dimensions). The system is manually operated, reducing the significant overhead (cost, design, and weight) associated with computer and robotic control systems. BRDF results are presented as example applications for three different field and laboratory targets: (i) a Lambertian calibration panel, (ii) a field grassland site in a river valley coulee, and (iii) laboratory-based measurements of a moss sample from Fluxnet-Canada. It was concluded that the ULGS hyperspectral goniometer is capable of estimating the directional reflectance properties of non-Lambertian structured canopies. Compared with other goniometer systems, the ULGS is considerably more accessible due to its much lower cost and feasibility to construct and its portability and versatility, yet it sacrifices little in terms of the quality and speed of BRDF data acquisition.
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.
Measurements of the bidirectional reflectance of snow at fine spectral and angular resolution
Proceedings of the 70th Annual Western Snow Conference, available online at http://www. westernsnow conference. org/2002/PDF/2002PainterAndDozier. pdf, 2002
This paper presents measurements of the bidirectional reflectance factor (BRF) of snow made at fine spectral and angular resolution with the Automated Spectro-Goniometer (ASG) for a range of solar zenith angles (θ0= 40-50) and snow textures (surface grain size= 80-240µm) on two days. Measurements of the stratigraphy of snow texture and density accompanied each day's suite of measurements. The BRF for fine grain, faceted snow exhibited a local backscattering peak at the view zenith near the solar zenith angle, ...
Reflectance and polarization characteristics of various vegetation types
Light Scattering Reviews 9, 2014
An important factor for reducing the amount of data acquired by Earth Observation projects and for collecting more accurate knowledge on the Earth, universe, and environment is to provide a set of reliable references by measuring various known terrestrial and planetary targets. This requires strong technological bases and careful gathering of information about land surfaces and vegetation via remote sensing technologies supplemented by Earth surveying techniques. Many satellites and aircraft are observing Earth with a variety of sophisticated instruments (e.g. . While the quality and quantity of the remote sensing information are constantly growing, the need and demand for support studies, such as the collection of ground references, are growing even faster. At present, it is already possible to distinguish vegetation from non-vegetated surfaces. Ongoing research concentrates on getting more quantitative measurements of different forest types, tree species, understory types, biomass, diseases, phases of growth, and crop estimates. As an application, these data support land-use or climate change studies, monitoring and responding to natural disasters, including fires, floods, earthquakes, and storm damage. In addition to improved performance and resolution, new types of sensors are being introduced. These include lasers, spectral imagers and, multi-directional cameras. Polarization is already used as a valuable tool for many purposes.
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.
About the importance of the definition of reflectance quantities-results of case studies
2004
In the remote sensing user community there is a lack of consistency in definitions and properties of reflectance quantities. On one hand, more recent satellite programs such as NASA's MODIS and MISR sensors take into account the directional dimension of the different reflectance products. On the other hand, many published studies still remain unspecific on the reflectance quantities they are based on, or do not follow common definitions. One example is the term 'albedo' assigned to significantly differing products. This fact makes it difficult and confusing to evaluate and compare published results. Our contribution briefly summarizes basic reflectance nomenclature articles. The main aim is to quantify differences of reflectance products to stress the importance of adequate usage of reflectance definitions and quantities. Results from the comparison of directional-hemispherical reflectance versus bihemispherical reflectance and bidirectional reflectance factors versus hemispherical-directional reflectance factors are shown. We exemplify differences of these quantities using modelling results of a black spruce forest canopy and snow cover, as well as selected biome-specific MISR reflectance products of the year 2001. The presented case studies can only give an insight into the dimension of the problem. The actual differences in the reflectance products of a remotely sensed surface depend on the atmospheric conditions, the surroundings, topography, and the scattering properties of the surface itself. Never the less the presented results are urging the user community to be more specific on the application and definition of reflectance quantities.
Remote Sensing, 2018
Rugged terrain, including mountains, hills, and some high lands are typical land surfaces around the world. As a physical parameter for characterizing the anisotropic reflectance of the land surface, the importance of the bidirectional reflectance distribution function (BRDF) has been gradually recognized in the remote sensing community, and great efforts have been dedicated to build BRDF models over various terrain types. However, on rugged terrain, the topography intensely affects the shape and magnitude of the BRDF and creates challenges in modeling the BRDF. In this paper, after a brief introduction of the theoretical background of the BRDF over rugged terrain, the status of estimating land surface BRDF properties over rugged terrain is comprehensively reviewed from a historical perspective and summarized in two categories: BRDFs describing solo slopes and those describing composite slopes. The discussion focuses on land surface reflectance retrieval over mountainous areas, the difference in solo slope and composite slope BRDF models, and suggested future research to improve the accuracy of BRDFs derived with remote sensing satellites.