28 July 2005
Crop Diagnosis With Polarized Light
by Kate Melville 
Researchers have uncovered a picture of plant health that is invisible to the naked eye by using rapid pulses of polarized light. The portable light source and detector can reveal over- or under-fertilization, crop-nutrient levels, and perhaps even disease. The tractor-mountable system works on corn, spinach and other crops by picking out minute differences in leaf color. The N-Checker (nitrogen-checker), as it has been dubbed, could help farmers determine in real time how much fertilizer to apply. This would decrease the cost of crop production, cut wastage and dramatically reduce the run-off responsible for algal blooms. Researcher Steve Finkelman said the technology revealed previously hidden attributes. "With our technology, we are able to easily see what is hidden from conventional instruments.
The system eliminates interference from light reflected at a leaf's surface and allows us to see light re-emitting from within." Finkelman explained that depending on the plant, leaves reflect, transmit and absorb varying amounts of light. Polarized light that enters a leaf's interior can lose its polarity and be re-emitted as depolarized light. The depolarized light reveals nitrogen content and other properties the sensors in the N-Checker can detect. Changes in nitrogen levels change the way light interacts with the molecules in the leaf, affecting the spectrum of light that re-emits from the plant. "Other devices use both red and infrared wavelengths," said Finkelman. "Those devices tend to be imprecise because they measure bulk chlorophyll content, which can result from a number of factors." He added that by using two specific, visible, red wavelengths, the N-Checker can differentiate among the several types of chlorophyll molecules and therefore reveal nitrogen-dependent plant health information. The N-Checker uses two red-light sources that cut down on sensor and polarizer costs and increase the system speed. The N-Checker can take 1000 measurements per second while moving at roughly 5 miles an hour. Source: Media release - National Science Foundation
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