## 2 Methods
### Sensor Design  
A sensor design that primarily used commercial off-the-shelf (COTS) components was desired. Use of standard plumbing fittings in opto-mechanical assemblies minimized machining costs and the need for specialized machining equipment. The same narrow-waveband interference filters were used as in [PERSON] _et al._[14] that feature a 10 nm Full-Width Half Maximum (FWHM). One filter was centered at 532 nm and the other at 568 nm. The electrical circuit of the instrument is based on the TSL-230r (Texas Instruments Advanced Opto-Electronic Solutions Inc., Dallas, TX, USA) programmable light-to-frequency converter that combines a 10 \(\times\) 10 silicon-photodiode array with a current-to-frequency converter onto a single integrated circuit (IC) [18]. The sensitivity of the IC can be adjusted by setting the photosensitive area to portions of the 10 \(\times\) 10 array, allowing irradiances in the range 0.01-10,000 \(\upmu\)W\(\cdot\)cm\({}^{-2}\) to be converted to a frequency signal. The TSL-230r was chosen for its low cost, while the frequency output provides ease of read-out by low-cost microcontrollers, and facilitates ease of separation of noise and electromagnetic interference that commonly requires much attention in analog to digital conversion. The IC is packaged in an 8-lead Small Outline IC (SOIC) package for surface mounting. Signals from the TSL-230r were read by an ATMEGA 328p (Atmel Corp., San Jose, CA, USA) 16 MHz microcontroller, that was programmed using the Arduino framework, and was set to integrate frequencies over 500 ms windows. The frequencies were sent wirelessly to a base station using Zigbee modules (Digi International Inc., Minnetonka, MN, USA) [19, 20]. The base station was a small Linux-based computer that was connected to the research site's server, providing Internet access. While the entire system consumes only about 4 W, it was decided to supply the system (including base computer and sensor nodes) with line power, to avoid complexities around battery operation and charging. To reduce energy consumption, the sensors can be put into a cyclic-sleep mode of arbitrary length (e.g., hours, days), where each sleep cycle takes approximately eight seconds. It was anticipated that sunflecks could adversely influence sensor readings and to prevent such adverse effects, a sampling pattern was chosen that was out of phase with reported frequencies of sunflecks that, for temperate forests, has been reported on the order of seconds to minutes [21]. The sensor was programmed to sleep for eight sleep cycles (64 s), followed by a series of three samples, each spaced by one sleep cycle (8 s). Using the average and range of each series of three samples, an indication of representativeness of the sensor reading was obtained and when the difference between any two samples exceeded an order of magnitude or more, the previous series of three samples was deleted. In addition, the sensors can be programmed to remain asleep for prolonged periods of time when irradiance values are below a certain threshold, e.g., at night. This option was not used due to availability of line power at the site and the knowledge gained from receiving data, irrespective of sensor readings, needed to assess the functioning of the wireless data link. Figures 1 and 2 show a schematic of the circuit design, and housing of the sensor, respectively.  
The sensor head was created from a 40 mm nominal pipe coupler. A small printed circuit board carrying the photosensitive ICs was mounted onto a 40 mm DELRIN rod of 15 mm in length that had two 12.5 mm holes centered on the photodiode arrays. Following [PERSON] _et al._[14] narrow-bandpass interference filters and Teflon\({}^{\circledR}\) diffusers were installed inside the drilled holes, while stray light arising from the perimeters of the filters was blocked using flat rubber washers. These parts were held into place using thermoplastic glue to avoid the precipitation of solvents onto the optical and photosensitive parts.