Quantum (PAR) Sensors

typical spectrum response
Typical spectral response of LI-COR Quantum Sensors vs. Wavelength and the Ideal Quantum Response (equal response to all photons in the 400-700 nm waveband).

During photosynthesis, plants use energy in the region of the electromagnetic spectrum from 400-700 nm. The radiation in this range, referred to as Photosynthetically Active Radiation (PAR), is typically measured as Photosynthetic Photon Flux Density (PPFD), which has units of quanta (photons) per unit time per unit surface area. The units most commonly used are micromoles of quanta per second per square meter (µmol s^-1 m^-2). Plant scientists, horticulturists, ecologists, and other environmental scientists use LI-COR's Quantum Sensors to accurately measure this variable.

LI-190 Quantum Sensor

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With the LI-190 Quantum Sensor, accurate measurements are obtained under all natural and artificial lighting conditions because of the computer-tailored spectral response. Colored glass filters are used to tailor the silicon photodiode response to the desired quantum response. An interference filter provides a sharp cutoff at 700 nm, which is critical for measurements under vegetation where the ratio of infrared to visible light may be high. A small response in the infrared region can cause an appreciable measurement error. This sensor, developed from earlier work⁽¹⁾, was pioneered by LI-COR and has become the standard for PPFD measurement in most photosynthesis-related studies.

The LI-190 is also used in oceanography, limnology, and marine science as a reference sensor for comparison to underwater PAR measured by the LI-192 Underwater Quantum Sensor and LI-193 Spherical Underwater Quantum Sensor.

References

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.

The LI-191 Line Quantum Sensor

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The Line Quantum Sensor spatially averages PPFD over its one-meter length.

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.

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.