## 2 Methods
### Data  
The sensor response to varying light intensity was assessed against a calibrated light source (model OL455, Optronics Laboratories, Orlando, FL, USA) that features a tungsten quartz-halogen lamp, a micrometer-controlled aperture that controls the luminance inside an integrating sphere, and a precision silicon detector-filter combination that measures the sphere luminance in foot Lamberts (fL). The calibrated light source provides for a constant color temperature, while the sphere luminance can be adjusted. The raw waveband data from the PRI sensor were recorded simultaneously with luminance  
Figure 1: Schematic diagram of the electrical circuit showing the light-sensitive sensors, along with logic circuit and the wireless transmitter. Figure 2: Sensor housing. A cable was used to power the instruments while a wireless link was used for communicating data. readings. From these luminance data, radiance was computed using a conversion table that was provided by the manufacturer, and then only for those tabulated wavelengths that were nearest to the centers of the PRI wavebands (_i.e._, no interpolation was computed to evaluate the black-body radiation at the waveband centers). To investigate instrument sensitivity within its field of view, the PRiAnalyze was placed under a goniometer that was supplied with a halogen light bulb, enabling incidence angles to be sampled from 0\({}^{\circ}\) to 70\({}^{\circ}\). Laboratory PRI measurements were acquired on 3 March 2015, at the Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, over (approximately) 60 \(\times\) 60 cm hydroponic grass turf (Fresh Patch, Camarillo, CA, USA). To accommodate the wide field of view of the PRiAnalyze, mirrors were used to laterally enclose the turf. To ensure that illumination was full spectrum within the narrow wavebands, and that the turf would not be exposed to excessive temperatures, a combination of halogen and LED lighting was used. Luminance was approximately 350 \(\upmu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), measured using an AccuPAR sensor (Decagon Devices, Inc., Pullman, WA, USA). To ensure that the temperature and light spectrum were constant, the light sources were turned on during the entire experiment. To acclimate the turf to darkness, turf was covered with a cardboard lid for 2 h and the lid was then suddenly removed while the PRiAnalyze measured radiance for about 45 min. This sequence of dark-adaptation and illumination was repeated to investigate if any potential reflectance changes were reversible. Field data were acquired between 6 March 2013 and 14 June 2013 from a scaffolding tower that provided access to a Douglas-fir canopy. The sensor was fixed using a wooden boom that was attached to the top of the scaffolding tower at around the 7-m height, and the sensor head was placed about 60 cm above a conifer whorl using a nadir field of view. This location was chosen to minimize the occurrence of sunflecks in the field of view, while the sensor was still easily accessible from the tower. Moreover, sensors at this height can more easily be mounted to stems, with potential to reduce installation costs. Data were periodically downloaded to the University of British Columbia and examined for irregularities. Eddy-covariance data, along with meteorological records of total photosynthetically active radiation (PAR), temperature, and relative humidity, were obtained from the Land and Food Systems Department (The University of British Columbia, Vancouver, BC, Canada) for the study site and period. EC-derived estimates of stand-level gross primary productivity, GPP, were calculated as daytime ecosystem respiration, \(R_{e}\), minus daytime Net Ecosystem Exchange, NEE. The daytime \(R_{e}\) was estimated using the exponential relationship between measured nighttime NEE and soil temperature at 2 cm depth [22] and NEE was calculated as the sum of above-canopy EC-measured CO\({}_{2}\) flux.