## 1 Introduction
### Multi-Spectral Sensors  
We used multi-spectral sensors (SKR-1800 and SKR-1850A) from Skye Instruments Ltd, UK ([[http://www.skyeinstruments.com/](http://www.skyeinstruments.com/)]([http://www.skyeinstruments.com/](http://www.skyeinstruments.com/))) with two or four wavelength bands for narrow-band spectral sampling. Sensor characteristics are specified in Table 2. These sensors have photo-detectors made from gallium phosphide (GaP), gallium arsenide phosphide (GaAsP), or silicon, depending on wavelength. The sensors measured two green bands for computation of photochemical reflectance index (PRI), and red and near-IR bands for computation of normalized difference vegetation index (NDVI), with somewhat different wavelength specifications depending on the manufacturer's calibration. The SKR-series light sensors measure with a 25\({}^{\circ}\) FOV as a standard, and an additional cosine correction \begin{table}
\begin{tabular}{l l l l l l} \hline
**Manufacturer** & **Type** & \begin{tabular}{l} **Central wavelength** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{l} **Band width** \\ **(nm)** \\ \end{tabular} & \
\begin{tabular}{l} **Accuracy** \\ **(nm)** \\ \end{tabular} &
\begin{tabular}{l} **FOV of directional** \\ **sensor** \\ \end{tabular} \\ \hline SKYE & SKR-1850A & 530, 570, 650, 870 & 10 & \(\pm\)2 nm & 60\({}^{\circ}\) \\ SKYE & SKR-1800 & 650, 860 & 50 & \(\pm\)2 nm & 60\({}^{\circ}\); 25\({}^{\circ}\) (Norunda \\ SDEC & JYP-1000 & 550 & 260 & N/A & 180\({}^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 2: Specification of the sensor characteristics. Figure 2: Photograph of the Stordalen mite site on September 18, 2010, showing the spectral mast (centre) and an eddy covariance flux tower (right). The oblique sensor views the ground tundra cotton-grass with a 55\({}^{\circ}\) off-nadir angle. The flux tower is measuring the carbon flux, and is operated by [PERSON], McGill University, Montreal, Canada. diffusor was provided to enable hemispherical irradiance measurements. To achieve a wider FOV than 25\({}^{\circ}\) for the downward-looking sensors, custom-made 60\({}^{\circ}\) FOV collars were mounted on top of the cosine diffusors. However, this construction attenuated the signals, and at one site (Norunda forest) we had to use the standard 25\({}^{\circ}\) FOV to improve the signal-to-noise (S/N) ratio. Radiation sensors for reflectance measurement used in meteorology are generally mounted for viewing vertically towards the ground. For vegetation monitoring this may not be optimal for the following reasons: (1) the sensor may view the tower and the installations next to it; (2) the footprint size is generally small (for narrow-angle sensors/low tower); (3) the instrument may view an disproportionally large gap area between the trees; and (4) the ground right below the tower is sometimes disturbed by frequent access by observers. These problems can be avoided by directing the instrument off nadir, though these measurement results will differ from nadir observations due to anisotropic effects. However, several studies have demonstrated that off-nadir viewing might be advantageous for vegetation monitoring. For example, in agricultural crops, [PERSON] and [PERSON] [31] and [PERSON] _et al._[32] demonstrated improved estimates of chlorophyll and other agronomic traits, respectively, by measuring with field instruments in off-nadir directions rather than in nadir. Also [PERSON] and [PERSON] [33] found differences, but only when looking in the direct backscatter region. For other azimuths they did not see any differences when estimating fraction of vegetation cover and height of grass. [PERSON] and [PERSON] [34] investigated angular effects on estimation of canopy variables in forests using simulated and measured reflectance data. They found that discrimination of cover and leaf area index (LAI) was improved in off-nadir direction in comparison to nadir viewing.