A temperature-controlled spectrometer system for continuous and unattended measurements of canopy spectral radiance and reflectance

This paper describes the development of a fully automated system for collecting high-resolution spectral data over a forested footprint. The system comprises a pair of off-the-shelf spectrometers in a custom-built thermal enclosure with a fixed off-nadir downward (target)-pointing fibre and upward-pointing fibre for irradiance measurement. Both instruments sample simultaneously via custom-written and user-controlled software during all weathers and sky conditions. The system is mounted on a 25 m eddy covariance scaffolding tower, approximately 7 m from a Scots pine forest canopy. The system was installed at the University of Helsinki’s SMEAR-II Field Station in Hyytiälä in March 2010 and has been operating continuously through a joint programme between the Universities of Edinburgh and Helsinki. The system was designed to capture diurnal and seasonal variation in vegetation light-use efficiency and fluorescence through the capture and analysis of well-defined narrow spectral features, but its implementation would permit the extraction of further optical signals linked to vegetation biophysical variables, and provide a continuous data stream with which to validate satellite data products including vegetation indices such as the photochemical reflectance index (PRI) as well as spectral indicators of solar induced fluorescence.