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NASA Builds Chlorophyll Measurement System

Photonics Spectra
Aug 1998
Kevin Robinson

Researchers at the John C. Stennis Space Center have developed an optical system for measuring the levels of chlorophyll in plant leaves that can provide an early warning of plant stress. The system is a test bed for both an optical system, designed by engineer Bruce Spiering, and a chlorophyll detection algorithm, created by scientist Gregory Carter.
Carter's early algorithm tests used a system that frame-grabbed images using three separate cameras. One problem with such a system is that the images must be processed and registered in a laboratory.
Spiering's optical system, which is mostly an assembly of off-the-shelf components, uses a telescopic lens, a collimating lens, bandpass filters, three focusing lenses and three Sony XC75 charge-coupled device cameras. This assembly eliminates the problems of registration because all of the cameras see the same image transmitted through the primary lens.
Spiering said his goal was to build a multispectral imager that would use only one optical setup with primary lenses that could be changed easily. The plant chlorophyll application was an ideal setting for testing such an optical train. "You want to have co-registered images," he said. "You don't want to have to realign the optics whenever you change the target setup."

Looking for green
The chlorophyll detection device measures the ratio of 700-nm reflectance to 840-nm reflectance. Chlorophyll absorbs 700-nm light, so high absorption and low reflectance mean a healthy plant. The second wavelength, 840 nm, is used as a reference band because its reflectance is not affected by chlorophyll. The third channel is available for a standard video image, but Spiering said that it is not always necessary.
The researchers have made two instruments using Spiering's optical system and Carter's algorithm: One is a handheld unit that measures the reflectance and displays a numerical ratio; the other, a video system, creates an image of the chlorophyll distribution in a plant's leaf. To be useful for diagnosing plant stress, the values displayed by the handheld unit must be compared with previously calculated values for healthy and sick plants of the same species, Spiering explained.
The researchers' next step is to take the device into the air where it can estimate the health of forests by measuring chlorophyll in tree canopies.

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