IAC ETH Goniospectrometer: A Tool for Hyperspectral HDRF Measurements

Abstract

This work presents a new field goniospectrometer developed at the Institute for Atmospheric and Climate Science (IAC) of the Swiss Federal Institute of Technology (ETH; Switzerland). The goniospectrometer was built to study the hemispherical directional reflectance factor (HDRF) of snow, but can also be applied to other surfaces with moderate surface roughness. The IAC ETH goniospectrometer measures HDRFs with high spatial resolution. The goniometer is exclusively built of straight parts, thus ensuring a high pointing accuracy. The two robotic arms are controlled automatically with step motors, whereby the step size can be defined by the user. With the default grid size of 15° in zenith and azimuth, the time needed to collect one complete HDRF dataset is 11 min, corresponding to a change of less than 4° in solar zenith and azimuth angles. The spectrometer comprises two probes. The first probe is equipped with a 3° foreoptic and is used for taking a spectrum of the reflected radiance; the second is placed on a tripod, has a 2π foreoptic, and simultaneously records a spectrum of the incoming irradiance. Both probes measure in the spectral range from 350 to 1050 nm, with a resolution of approximately 3 nm at around 700 nm. The performance of the new goniospectrometer was tested at the Greenland Environmental Observatory Summit Station (72°35′N, 34°30′W, 3203 m ASL) during the summer of 2004.