Photosynthetically Active Radiation (PAR) sensor
Photosynthetically Active Radiation (PAR) sensor

LI-COR Respose to light

The LI-COR model LI-190SA Quantum Sensor measures photosynthetically active radiation (PAR) in the 400 nm to 700 nm wavelength range. The measurement is of Photosynthetic Photon Flux Density (PPFD), which has units of quanta (photons) per unit time per unit surface area, which for this sensor is in micromoles per second per square meter (µmol s-1m-2). LI-COR designed the quantum sensor to measure PAR incident on a flat plane. The sensor response approximates the photosynthetic response of plants. A silicon photodiode with an enhanced response in the visible wavelengths is used as the detector. Under the acrylic diffuser are colored glass filters that tailor the silicon photodiode response to the desired quantum response, plus an interference filter that provides a sharp cutoff at 700 nm to eliminate interference from the infrared region. This instrument does not have any internal heating system, and thus operates at ambient temperature (operating range: -40°C to 65°C; 0 to 100% RH). Error calculations indicate that under sun-and-sky radiation, and various natural or artificial light sources, the relative errors are less than ±5%. [Specifications and graph are from the LI-COR instruction manual.] The UVMRP uses the LI_190SA with the 2290 Millivolt Adapter. This adapter uses a 604 Ohm precision resistor to convert the LI-190 output from microamps to millivolts, to match the input parameters of the Yankee Environmental Systems datalogger.

The PAR is calibrated periodically by sending the instrument back to LiCor. The calibration information for each PAR instrument can be found under the Location Information, Deployment History.

The PAR sensor is connected to the UV-MFRSR datalogger, so its output is measured every 20 seconds and integrated into 3-minute averages, and its data is retrieved nightly in conjunction with the polling of the UV-MFRSR datalogger.