OverviewMeasure Photosynthetically Active Radiation in Plant Canopies
The LI-191 Line Quantum Sensor spatially averages PPFD over its one-meter length.
During photosynthesis, plants use energy in the region of the electromagnetic spectrum from 400-700nm (1, 2). The radiation in this range, referred to as Photosynthetically Active Radiation (PAR), can be measured in energy units (watts/m²) or as Photosynthetic Photon Flux Density (PPFD), which has units of quanta (photons) per unit time per unit surface area. The scaled units most commonly used are micromoles of quanta per second per square meter (µmol/sec/m²).
Measuring PAR within a plant canopy can be very difficult because of the non-uniformity of the light field. When PAR is measured with a small diameter quantum sensor such as the LI-190 Quantum Sensor, intensity can vary 10-fold between sunflecks and shadows, requiring a large number of readings to get an accurate average. The LI-191 Line quantum Sensor reduces the number of individual readings required because it effectively averages PPFD over its one-meter length. One person can quickly make plant canopy PPFD measurements in many plots in a short period of time.
Rather than using multiple detectors linearly arranged over its one-meter length, the LI-191 uses a one-meter-long quartz rod under a diffuser to conduct light to a single, high-quality quantum sensor. Its response is shown below.
There are two advantages to this design. First, the sensor has a very good quantum response, unlike sensors using inexpensive gallium arsenide detectors with only an approximation of the ideal quantum response. Second, it is much easier to maintain calibration on a single quantum sensor than on multiple (up to 80) individual gallium arsenide detectors.
References
1. Biggs, W.W., A.R. Edison, J.D. Easton, K.W. Brown, J.W. Maranville and M.D. Clegg. 1971. Photosynthesis light sensor and meter. Ecology 52:125-131.
2. De Wit, C. T. 1965. Photosynthesis of leaf canopies. Agric. Res. Rep. No. 663. Institute for Bio. and Chem. Res. on Field Crops and Herbage. Wageningen, The Netherlands.
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Features
Sensor lead terminates with a BNC connector for use with a LI-COR light meter or data logger. Sensor delivers a microamp current output.
Type SA sensors can be used with non-LI-COR devices requiring a millivolt signal, with the use of the appropriate Millivolt Adapter. This adapter mates with the sensor’s BNC connector and terminates in bare wires. Resulting output is in millivolts, and is proportional to the sensitivity of the sensor to which it is connected.
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