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When it comes to monitoring crop health, aerial images taken from planes can only tell you so much. This is where a new type of spectral sensor comes into play, as it is directly mounted on the underside of a plant’s leaf.
This device was developed by Ko-ichiro Miyamoto, Kaori Kohzuma, and their colleagues at Tohoku University in Japan, designed for some “sentinel plants” in different areas of each field. It consists of two parts. The sensor head itself adheres to the underside of the leaf without obstructing sunlight to the plant. This head is connected via cable to a nearby control device, which includes a lithium battery, Arduino microprocessor, antenna, and other electronics.
Two LEDs on the sensor head briefly illuminate the underside of the leaf at intervals. A photodiode located between the two LEDs analyzes the spectrum of the reflected light. The diode can also read data while the LEDs are off to measure sunlight passing through the leaf from above.
By subtracting the second measurement from the first, the spectrum of the LED light absorbed by the leaf can be determined. This reading, in turn, is used to determine the current color of the leaf, which is an indicator of plant health (and season).
The sensor accesses a cloud server via a cellular network to process sensor data and provide it to users.
The algorithm of this system was trained on about 90 different colored leaves collected from 30 plant species.
In tests of this technology, some sensor units were installed on the leaves of golden birch trees growing in the experimental garden of the University of Tokyo. Over a two-week period, as the leaves’ color naturally changed from green to yellow or brown, the sensors automatically read data every two hours.
Subsequent analysis of the sensor readings found them to be very close to the readings obtained by people using handheld spectrometers during the same period. However, manually obtaining readings using the latter method takes significantly more time and labor in large-scale agricultural environments.
The manufacturing cost of each prototype device is “tens of dollars,” but this figure could drop significantly if commercial production is done in factories.
Kohzuma stated, “This low-cost sensor is an ideal tool for accurately monitoring plant health and stress through leaf color and light reflection data. Its low cost makes it possible to deploy multiple sensors in different locations, creating a network for simultaneous monitoring at many sites.”
A paper on this research was recently published in the journal Sensing and Bio-Sensing Research. Scientists from MIT, North Carolina State University, and Iowa State University are also researching their own plant-worn stress detection sensors.
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