Licor PAR Calibration

Last Updated: January 21, 2015

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's angular-cosine response is close to unity up to a zenith angle of 80°. Licor 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. LI provides a calibration constant (k) for each sensor given in units of amperes / (1000 moles s-1 m-2). The PAR values are provided on the UVMRP Data Access web page in units of micro moles per square meter per second.