# Technological Advancement in Tower-Based Canopy Reflectance Monitoring: The AMSPEC-III System
## 2 Materials and Methods
### Data Processing  
The AMSPEC-III system records solar irradiance and canopy radiance simultaneously to the sensors viewing geometry, solar position, time of measurement and the webcam RGB image. The PTU movement was set to 10\({}^{\circ}\) horizontal steps, completing a full rotation every 30 min. At each horizontal location, four different vertical angles were measured, alternating between \(\theta_{v}\in\{48^{\circ};\,58^{\circ};\,68^{\circ};\,78^{\circ}\}\) and \(\theta_{v}\in\{43^{\circ};\,53^{\circ};\,63^{\circ};\,73^{\circ}\}\) every 15 min (half full rotation). A measurement in the solar plane was performed at the beginning of every 15 min cycle. A portion is not seen by the instrument due to obstruction from the scaffold tower, and depends on the tower size and how the instrument is installed on it. At SOBS, we excluded the measurements with \(\phi_{v}\) comprised between 145\({}^{\circ}\) and 180\({}^{\circ}\), and between 220\({}^{\circ}\) and 340\({}^{\circ}\) in order to exclude obstructed images. An example of a \(\sim\)165\({}^{\circ}\) observation cycle at SOBS is in Figure 2.  
By design, the use of normalized difference indices should compensate some of the differences in light sensitivity between the upward- and downward-looking channels (_i.e._ irradiance and radiance, respectively). However, light sensitivity depends also on data acquisition wavelength. For this  
Figure 1: _In situ_ photograph of the third generation Automated Multiangular SPectro-radiometer for Estimation of Canopy reflectance system (AMSPEC-III) taken at the Southern Old Black Spruce (SOBS) site. Figure 2: Image composite at SOBS over a \(\sim\)165\({}^{\circ}\) observation cycle. The photographs have been stitched from 57 individual observations (340\({}^{\circ}\)\(\leqslant\)\(\phi_{v}\)\(\leqslant\) 145\({}^{\circ}\)) using Microsoft® Research Image Composite Editor. reason, a calibration before and during data acquisition was performed using a Labsphere(r) diffuse reflectance target [37]. After considering the sensor's differences in sensitivity to light due to the individual photodiodes and fiber optics used in the system, for dual-channel radiometers the measured canopy reflectance (\(\rho\)) is defined as the ratio of canopy radiance and solar irradiance. Differences in light sensitivity can be corrected through a cross-calibration approach by measuring the reflectance of the standardized reference target [6]:  
\[\rho=\frac{L\cdot I^{\prime}}{I\cdot L^{\prime}}\]  
where \(L\) is the measured radiance of the canopy sensor, \(I\) is the simultaneously measured irradiance, \(L^{\prime}\) is the measured radiance of the control surface, and \(I^{\prime}\) is the irradiance at the time \(L^{\prime}\) was measured. Neither UDC nor JC provides an internal shutter mechanism to automatically correct for dark current (dc), defined as the electrical current that generated by thermal electrons in the photocathode of optical instruments [38]. For this reason, the acquired data can only be corrected in a post-processing step using manual measurements taken with both sensors completely covered from light. Using previous AMSPEC versions, [PERSON] _et al._[7, 8] demonstrated a relationship between the sensor's temperature (as measured by the internal thermometer of the spectro-radiometer) and the dc measured when blocking off the light from both sensors, thereby allowing an automated correction of this drift in the measured sensor radiance. For this study, we analyze simultaneous UDC and JC measurements at SOBS. In order to directly compare the two systems, the JC channels were spectrally resampled to UDC using the arithmetic mean of overlapping wavebands. We present radiance, irradiance and calculated reflectance over one sample day (1 September 2013) in three spectral channels (_i.e._, 559 nm; 660.3 nm; 809.7 nm) representative of green, red and NIR light, respectively. To avoid possible sky contaminations, all measurements with \(\theta_{v}>63^{\circ}\) were excluded from the dataset.