The Improved Dual-view Field Goniometer System FIGOS

Abstract

In spectrodirectional Remote Sensing (RS) the Earth's surface reflectance characteristics are studied by means of their angular dimensions. Almost all natural surfaces exhibit an individual anisotropic reflectance behaviour due to the contrast between the optical properties of surface elements and background and the geometric surface properties of the observed scene. The underlying concept, which describes the reflectance characteristic of a specific surface area, is called the bidirectional reflectance distribution function (BRDF). BRDF knowledge is essential for both correction of directional effects in RS data and quantitative retrieval of surface parameters. Ground-based spectrodirectional measurements are usually performed with goniometer systems. An accurate retrieval of the bidirectional reflectance factors (BRF) from field goniometer measurements requires hyperspectral knowledge of the angular distribution of the reflected and the incident radiation. However, prior to the study at hand, no operational goniometer system was able to fulfill this requirement. This study presents the first dual-view field goniometer system, which is able to simultaneously collect both the reflected and the incident radiation at high angular and spectral resolution and, thus, providing the necessary spectrodirectional datasets to accurately retrieve the surface specific BRF. Furthermore, the angular distribution of the incoming diffuse radiation is characterized for various atmospheric conditions and the BRF retrieval is performed for an artificial target and compared to laboratory spectrodirectional measurement results obtained with the same goniometer system. Suggestions for further improving goniometer systems are given and the need for intercalibration of various goniometers as well as for standardizing spectrodirectional measurements is expressed.

Spectral Imaging From Uavs Under Varying Illumination Conditions

“…Recently, these methods are being extended for airborne sensors with rectangular image format (López et al, 2011), and also for UAVs (Honkavaara et al, , 2013a. Another extreme is the measurement of reflectance in field using goniospectrometers (Demicran et al, 2000;Schopfer et al, 2008;Suomalainen et al, 2009). These methods provide very accurate information of reflection properties of objects.…”

“…F(x,y) is the fraction of hemisphere visible to the sensor. To solve reflectance factor, information about diffuse light and other atmospheric influences are needed (Demicran et al, 2000;Richter and Schläpfer, 2002;Schaepman-Strub, 2006;Schopfer et al, 2008;Suomalainen et al, 2009).…”

Ground-Based Hyperspectral Characterization of Alaska Tundra Vegetation along Environmental Gradients

“…Instead of using the recorded irradiance directly, we used the irradiance spectra with the aim to interpolate the radiance measurement of the reference panel to the time of the target measurement. It is assumed that changes in the irradiance over the time period affect the radiance measurements of the reference panel to a similar degree [61]. This approach has already been used by various groups, but mostly with a sunphotometer with limited spectral bands for irradiance recording [61,62].…”

Changes in reflectance anisotropy of wheat crop during different phenophases

“…But estimation of BRDF from a satellite observation is difficult (Luo et al, 2005) and satellite sensors do not provide simultaneous reflectance in all viewing directions. Laboratory measurements provide a better control of the illumination conditions but the artificial light source shows a conical rather than directional geometry, leading to non-homogeneity of the illuminated area (Schopfer et al, 2008). The field measurement of bidirectional reflectance factor has the advantage that the target is left in its natural condition under natural illumination of the sun.…”

“…So, in situ measurements are essential for the validation of off nadir remote sensing image products and canopy BRDF models. With a view to the importance of field measurements, many bidirectional reflectance factor (BRF) sampling devices for field and natural illumination conditions have been designed by some research groups (Bourgeois et al 2006;Landis and Aber, 2007;Schopfer et al, 2008;Sridhar et al, 2009;Susaki et al, 2004). A majority of them resemble the well documented Swiss field goniometer system (FIGOS) (Sandmeier and Itten, 1999).…”