Photosynthetically Active Radiation (PAR) – Principles of Environmental Measurement Lecture 4

Photosynthetically Active Radiation (PAR) – Principles of Environmental Measurement Lecture 4

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Mark Blonquist discusses photosynthetically active radiation (PAR) and PAR meters in part 4 of 9 in the ICT International and Apogee Instruments lectures that took place at the Hawkesbury Institute for the Environment campus of the Western Sydney University.

00:22 QUANTUM SENSOR (or PAR Meters)
I’m going to talk about quantum sensors, sometimes called PAR sensors (photosynthetically active radiation sensors). These sensors measure photosynthetically active wavelengths, typically defined as 400 to 700 nanometers (nm). The same wavelengths that the human eye sees are the same wavelengths that drive photosynthesis in plants. It is measurement that is often used in plant research because it tells us how much radiation is available or how many photos are available to make photosynthesis happen.

There are a lot of different radiation sources: the Sun, different electric lights, and LEDs. LEDs are becoming more and more popular in plant growth. They are often used in different color treatments to see what effect they have on a plant growth and morphology.

-PAR is radiation that drives photosynthesis; typically defined as 400 to 700 nm.
-Photosynthetic photon flux density (PPFD) is the number of photons in the photosynthetically active range that strike a surface.
-Quantum sensor or PAR meter is a sensor that measures PPFD.

05:05 PPFD vs. YPFD
This graph shows the difference between photosynthetic photon flux (PPF) and yield photon flux (YPF). In the 70’s, McCree took a leaf and measured the photosynthetic response of the leaf at each wavelength of light. He found that plant leaves respond a little bit more to blue radiation than green, and they are most sensitive to red; these measurements are YPFD. It is important to remember that the data measured was in a laboratory with plants grown under controlled conditions in low light environment and where of a single leaf not a complete plant. Rather than weighting PAR by the response McCree found we assume that all photons between 400 and 700 nm are equally efficient at photosynthesis, this is called the defined plant response to photon and is the universally accepted definition of PAR; these measurements are of PPFD.

Quantum sensors provide the sum of all the radiation between 400 and 700 nm that is incident on the detector as a single PPFD measurement. The spectral response of the sensor is trying to match the defined plant response to photons the best it can.
Spectroradiometers can be used to give you a spectral measurement at every wavelength between 400 and 700 nm as well as the PAR value. The spectroradiometer is able to give the spectrum of the light in addition to PAR.

Here we show the spectral response of Kipp and Zonen, Sky Instruments, Licor, and Apogee’s quantum sensors compared to the defined plant response to photons. Apogee has two types of quantum sensors. The SQ-110 is the sensor we have sold for years and is still available. The SQ-500 is the new sensor with a better spectral response.

Because no sensor has a spectral response that is identical to the defined plant response to photons the sensors have errors when they are used for light sources that are different than the light source used to calibrate the sensor. These errors are called spectral errors and we can calculate them and correct for them as long as we know the spectral response of the sensor, the spectral output of the light source used to calibrate the sensor, and the spectral response of the light source we are measuring. 15:42 shows a table of data where sunlight on a clear day is used as the calibration source. You can see the errors the sensors would then have if they were used to measure other light sources.

Quantum sensors can be inverted to measure reflected radiation from a plant canopy. If you have a measurement of incoming radiation and a measurement of reflected radiation, it tells you how much of the photosynthetic radiation is absorbed by the plants at the surface.

Oftentimes, we put quantum sensors underneath plant canopies to measure how much of the photosynthetic radiation is transmitted through the plant canopy. A line quantum sensor is just like a regular quantum sensor except for there are multiple sensors along the line. The reason that you want multiple sensors when you make under canopy measurements is that the light environment underneath the plant canopy is not uniform. The line quantum sensor works really well to measure under a plant canopy because you get an average along the length of the line of radiation measured in multiple locations.

More info on Apogee quantum sensors at

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