Photosynthetically Active Radiation Measurement

Photosynthetically Active Radiation (PAR) represents the fraction of sunlight with a spectral range from 400 to 700 nm, usually expressed in µmol (photons) m^-2 s^-1, though it is also expressed in microEinsteins. Increases in PAR enhance the light reactions of photosynthesis before reaching a saturation point. Beyond that saturation point, any further increase in PAR will not result in more light-fixation. The photosynthetic response to different levels of PAR varies with plant species and leaf position. Shade plants and shaded leaves harvest PAR more efficiently at low light levels but less efficiently at high light levels compared with sun plants and sunlit leaves.

In any region, the amount of photosynthetically active radiation changes seasonally and varies depending on time of day. This can drive morphological changes in plants, causing plants to flower or to become dormant. Measuring terrestrial PAR can help determine the light levels in a greenhouse, natural area, or growth chamber. These measurements are useful in agronomic and agricultural research.

LI-COR's Solution for
Measuring PAR:

The LI-190 quantum sensor in conjunction with either the LI-1400 Datalogger or LI-250A Light Meter can be used to measure Photosynthetically Active Radiation. The LI-191 Line Quantum Sensor can be used to measure average PAR over 1m lengths.

For information on measuring underwater light, view our Underwater PAR application page.