A low-cost field and laboratory goniometer system for estimating hyperspectral bidirectional reflectance

Abstract

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 earth-surface 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° zenith 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.