Data Processing Methods

The Licor Inc. (LI), Photosynthetically Active Radiation or PAR sensor consists of a high stability silicon photovoltaic detector mounted in a weatherproof aluminum case beneath an acrylic cosine correction diffuser. A visible bandpass interference filter is used in combination with colored glass filters to limit the spectral response to between 400 and 700 nanometers. The azimuthal response accuracy is listed by the manufacturer at better than 1% over 360° at 45° elevation angle. The instrument is calibrated against a National Bureau of Standards lamp, and the manufacturer states the uncertainty in the calibration is ± 5%. The instrument is cosine corrected up to 80° zenith angle. LI recommends recalibration at least every two years and states that the instrument is stable to within 2% / year.

The signal of the LI PAR Sensor is processed according to the procedure recommended in the LI-COR Radiation Sensors Instruction Manual. The instrument is connected to the Yankee Environmental System's (YES's) Data Acquisition System (DAS) using a millivolt (mV) adapter provided by LI. The purpose of the mV adaptor is to convert the direct output of the detector from units of μ amperes / (1000 &mu mol s-1 m-2) to a signal with units of mV/(1000 μ mol s-1 m-2) by insertion of a 604 ohm resistor. LI provides a calibration constant (k) for each sensor given in units of μ amperes / (1000 μ mol s-1 m-2). Additionally, there is a board gain introduced by the amplifier in the YES DAS with a value of 1 / 340 that multiplies the output of the signal. Thus the final formula for processing the LI PAR sensor is:

PAR (μ moles / m² s) = (mV / 340) * (1000 / (k * 0.604)), where k is the calibration constant