A Compact Laboratory Spectro-Goniometer (CLabSpeG) to Assess the BRDF of Materials. Presentation, Calibration and Implementation on Fagus sylvatica L. Leaves

Abstract

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 illumination source. Therefore, the full hemisphere could not be covered with one leaf. Instead 12 BRF measurements per leaf were acquired covering all azimuth positions for a single light source zenith position. Data are collected in radiance format and reflectance is calculated by dividing the leaf cycle measurement with a radiance cycle of a Spectralon reference panel, multiplied by a Spectralon reflectance correction factor and a factor to correct for the conical effect of the light source. BRF results of measured leaves are presented.

RPV Model Parameters Based on Hyperspectral Bidirectional Reflectance Measurementsof Fagus sylvatica L. Leaves

“…Moreover, data at the 0°/0° light/sensor position could not be retrieved due to shadow casting on the measured sample. Additional details about the measurement setup can be found in [16]. A set up of the measurements is given in Figure 1.…”

“…A Compact Laboratory Spectro-Goniometric device (CLabSpeG) [16] was used to measure the Bidirectional Reflectance Factor (BRF), of 12 sets of five Fagus sylvatica L. leaves each, for the wavelength range of 400 to 2,500 nm covering the hemisphere between nadir and 60°, with 30° and 15° angular steps in azimuth and zenith, respectively. This paper focuses on:…”

Hyperspectral Reflectance Anisotropy Measurements Using a Pushbroom Spectrometer on an Unmanned Aerial Vehicle—Results for Barley, Winter Wheat, and Potato

“…Reflectance anisotropy is commonly studied by performing multi-angular reflectance measurements using goniometers in laboratories under controlled conditions e.g., [12][13][14][15] or in the field under natural conditions e.g., [16][17][18][19][20][21]. Both laboratory and field goniometer measurements have their advantages and disadvantages [22].…”

Modeling and intercomparison of field and laboratory hyperspectral goniometer measurements with G-LiHT imagery of the Algodones Dunes

“…(1), factors of μ i and 1∕π are removed since our measurements involve reflectance factors (BCRF and HCRF), which are the ratio of radiance values obtained from the sediment sample and a Spectralon ® white reference, an approximately Lambertian standard. 4,7,8,10,12 Importantly, a geophysical parameter, the sediment fill factor ϕ (the fraction of volume occupied by particles), explicitly parameterizes the approximate IMSA solution since the nonlinear porosity factor K ¼ KðϕÞ is a function of the fill factor ϕ, and, for equant particles, it has the form 4,16 E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 3 ; 1 1 6 ; 6 1 6…”

“…The term "factor" signifies the referencing of sample radiance data to the radiance of a Lambertian standard. In our experiments, Spectralon ® panels, which approximate a Lambertian surface, 12 were used as the reference standard. These panels were calibrated by LabSphere.…”