Detailed studies of light in forest canopies typically require a large number of light sensors. However, the high cost of commercially available sensors can make such studies very expensive. This report describes the construction and testing of a practical, rugged, and inexpensive sensor for measuring photosynthetic photon flux density. Detailed instructions are provided for assembly and calibration. The sensor was made from a gallium arsenide phosphide (GaAsP) photodiode held within a protective casing of acrylic and aluminum. The LI-190 Quantum Sensor (LI-cor Inc.) was used as a standard for comparison. The stability of the GaAsP sensor compared favourably with the LI-190 during a 24-month open-sky test. Results from nine GaAsP sensors exhibited signal drift of 3-23%, and from four LI-190 quantum sensors of 5-18.5%. The GaAsP sensors generally drifted by <6% over a single season except for one of the Type 1 sensors (#42) that displayed intermittent signal drift. Open-sky daily calibration coefficients of GaAsP Types 2, 3, and 4 showed that about 95% of the data points were within 2-4% of the mean for June to September 1998, and within 3-10% of the mean for January to November 1999. Data variability almost doubled during the wetter months and Type 1 (#41 and #42) sensors were particularly affected. GaAsP (Types 2 and 3) and LI-190 sensors exhibited similar signal drift beneath a canopy as in the open-sky test. Daily calibrations made beneath the canopy were variable over the season, suggesting that care should be taken in making in situ calibrations. Field testing beneath a birch canopy indicated that, compared to a quantum sensor (LI-190), one GaAsP sensor had a spectral response error of <7% in the densest canopy. Recommended sensor care includes minimizing contact with humidity, frequent calibration (pre- and post-season), frequent field checks, and regular maintenance, including diffuser cleaning. Low cost may make these sensors desirable for studies that require a large number of sensors.
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Updated July 24, 2015