Why are leaves green? The process by which plants grow is called photosynthesis. The energy for photosynthesis comes from the sun. When sunlight falls on leaves, they absorb energy across the visible spectrum, a little in the near-infrared (IR) and into the near-ultraviolet (UV) except (mostly) for green light. Green light is reflected and transmitted, with a peak transmission around 550 nanometers, which is the answer to our original question. The part of the sunlight spectrum absorbed by plants, and used in photosynthesis, is called photosynthetically active radiation (PAR). The ability to monitor the local and global variability and long-term trends of PAR is essential for understanding the health of vegetation, including the health of all agricultural crops, and the effects of weather and climate on vegetation.
Commercial instruments for measuring PAR use photodetectors whose response is tailored (using filters) to match the average response of plants to sunlight. Basically, such instruments seek to provide a uniform response across the visible part of the light spectrum, from 400 to 700 nm. For most schools, these detectors are too expensive to be used as part of a regular atmospheric monitoring program. However, it is possible to use LEDs as spectrally selective light detectors -- "surrogates" for more expensive detectors. The effectiveness of this approach has already been demonstrated with the LED detectors already used in GLOBE sun photometers, water vapor instruments, and UV-A radiometers.
Figure 1 shows the transmission of green leaves and also the spectral response of some blue and red LEDs. (All figures are from: F. M. Mims III, Five years of photosynthetic radiation measurements using a new kind of LED sensor. Photochemistry and Photobiology, 77, 30-33, 2003.) These detectors respond in the near-UV and red, where leaves absorb light. Therefore, the output from such detectors can be correlated with PAR.
|Figure 1. Leaf transmission and the spectral response of blue and red LEDs used as detectors.|
Figure 2 shows the correlation of a linear combination of the output signals from LEDs that respond to light in the blue and red part of the spectrum to the output from a commercial PAR detector from LI-COR (a widely used commercial instrument). Although there is some "noise" in these data, it is clear that the relationship is a linear one, with a correlation (r2) of about 0.97. Both detector systems are affected by atmospheric conditions, but in different ways (because their spectral response is different). Therefore, it is not surprising that there is not complete agreement between the two measurements.
|Figure 2. Correlation between LI-COR PAR detector and an instrument using blue and red LED detectors.|
Figure 3 shows that, for long-term monitoring of PAR, the two-LED instrument performs just as well as the much more expensive commercial product. Both show the seasonal cycle in PAR, which is much larger than the differences between the two instruments. There is some evidence from Figure 3 that the LED-based instrument underestimates high values of PAR, assuming that the LI-COR instrument is accepted as a standard measurement for this quantity. Overall, however, it is clear that this relatively inexpensive and very rugged LED-based instrument is an extremely valuable tool for monitoring PAR.
|Figure 3. Four-year monitoring of PAR with commercial and LED-based instruments.|